chain pull calculations

7/22/2019 Chain Pull Calculations

1/3966 engineering excellence w w w . r e n o ld . c o m

CONVEYOR CHAI N DESIGNER GUIDE

S E C T I O N 4


2/39w w w . r e n o ld . c o m engineering exce llence 67

D e s i g n e r G u i d e

4

In t r o d u c t i o n

Selecting the right chain for a given application is essential to

ob tain long se rvice life. This guide ha s b een d eveloped for use

with Renold conveyor chain to help in specifying the right chain

a nd lubrica tion for your con veyor sys tem. The s ignifica nce of the

Renold conveyor chain design is empha sised, followe d by

guidance on s election proce dure. Detailed des criptions are g iven

of the various methods of applica tion in a variety of mecha nica lha ndling prob lems a nd unde r wide ly varying cond itions . The

supporting material includes various reference tables and

statistics.

From the pyramids to the railway revolution, muscle-power of men

and animals has moved goods and materials, but throughout

history, machines, however primitive, have played some part,

beco ming mo re and mo re versa tile.

Within the immediate pa st, mecha nica l handling ha s e merged as

a manufacturing industry in its own right, of considerable size and

with co untles s a pplica tions . This is a co nse q uence o f itscoverag e, which now rang es from the simplest store conveyor

sys tem to the largest flow line production layouts, a nd a lso

includes the movement of personnel by lifts, esc alators a nd

platforms.

Amongs t the most w idely used types o f handling eq uipment are

conveyors, elevators a nd s imilar a ss emblies. These ca n take

many forms, e mploying as their bas ic mo ving medium both

metallic and non-metallic components or a mixture of the two.

For the great majority of applications Renold conveyor chain in its

many variations, when fitted with suitable attachments, provides a

highly efficient propulsion a nd/or ca rrying me dium, ha ving ma ny

advantages over other types. Roller chain has been employed as

an efficient means of transmitting power since it was invented by

Hans Renold in 1880. Later the principle was applied to conveyor

cha in giving the s ame ad vantag es of precision, hea t-trea ted

compo nents to resist wea r, high strength to weight ratio a nd high

mechanical efficiency.

Renold conveyor chain is made up of a series of inner and outer

links. Each link comprises components manufactured from

ma terials b es t suited to their function in the ch ain; the va rious

pa rts a re sho wn in Figure 1. An inner link cons ists of a pa ir of

inner plates which are pressed onto cylindrical bushes, whilst on

each bush a free fitting roller is normally assembled. Each outer

link has a pair of outer plates which are pressed onto bearing

pins and the ends of the pins are then rivetted over the plate.

From the foreg oing, it will be se en tha t a leng th of cha in is a

series o f pla in journal be a rings free to a rticulate in one plane.

When a cha in a rticulates under load the friction b etwee n pin and

bush , whilst inherently low bec a use of the smo oth finish o n the

com ponen ts, will tend to turn the bus h in the inner pla tes a nd

simila rly the be a ring pin in the o uter plate. To p revent this the

bush and pin are force fitted into the c ha in plates. Close limits o f

ac curac y a re applied to the diameters of plate ho les, bushes

an d b ea ring pins, resulting in high to rsiona l security a nd rigidity

of the ma ting c omponents. S imilar stand ards of acc urac y a pply

to the pitch of the holes in the cha in plates .

To e nsure o ptimum we a r life the pin an d b ush a re harde ned . The

bush o utside d iame ter is ha rdened to co ntend with the loa d

ca rrying pressure and gea ring ac tion, b oth of which are imparted

by the chain rollers. Chain roller material and diameter can be

varied and are s elected to suit applica tiona l conditions; guidance

in roller selection is given on page 73. Materials used in chain

manufacture conform to closely controlled specifications.

Manufacture of components is similarly controlled both

dimensiona lly a nd w ith regard to hea t trea tment.

For a g iven pitch size of transm iss ion c ha in, there is norma lly agiven breaking load. However, conveyor chain does not follow

this convention. For each breaking load, conveyor chain has

multiple pitch s izes a vailab le. The minimum pitch is go verned b y

the need for adequate sprocket tooth strength, the maximum

pitch b eing dictated by p la te a nd g eneral cha in rigidity. The

normal maximum pitch can be exceeded by incorporating

strengthening bushes between the link plates, and suitable ga ps

in the sprocket teeth to clear these bushes.

CHAIN TYPES

There a re two ma in types of conveyo r chain - hollow bea ring pin

and solid bearing pin.

H o l l o w B e a r i n g P i n C h a i nHollow pin conveyor chain offers the facility for fixing attachments

to the outer links using bolts through the hollow pin and

attac hment, this method of fixing being s uitable for use in mos t

normal circumstance s. The a ttachments ma y be b olted up tight

or be he ld in a free manne r. Bo lted a ttachments sho uld o nly

span the outer link as a bolted attachment spanning the inner

link wo uld impa ir the free a rticulation o f the cha in.

RENOLD

Fig. 1




4

S o l i d B e a r i n g P i n C h a i nSolid bearing pin chain, while having exactly the same gearing

dimensions in the BS series of chain as the equivalent hollow pin

chain, i.e.pitch, inside width and roller diameter, is more robust with

a higher breaking load and is recommended for use where more

arduous conditions may be encountered.

D e e p L i n k C h a i nHollow and solid pin chain has an optional side plate design

known as deep link. This chains side plates have greater depth

than normal, thus providing a continuous carrying edge above the

roller periphery.

INTERNATIONAL STANDARDS

C onveyor chain, lik e transmission chain, can be manufactured to a

number of different international standards. The main standards

available are:

B r i t i s h S t a n d a r d - B SThis standard covers chain manufactured to suit the British market

and markets where a strong British presence has dominated

engineering design and purchasing. The standard is based on the

original Renold conveyor chain design.

I S O S t a n d a r dC hain manufactured to ISO standards is not interchangeable with

BS or DIN standard chain. This standard has a wide acceptance in

the European market, except in G ermany. C hain manufactured to

this standard is becoming more popular and are used extensively

in the Scandinavian region.

CHAIN ATTACHMENTS

An attachment is any part fitted to the basic chain to adapt it for a

particular conveying duty, and it may be an integral part of the

chain plate or can be built into the chain as a replacement for the

normal link.

K A t t a c h m e n t sThese are the most popular types of attachment, being used on

slat and apron conveyors, bucket elevators etc. A s shown in F ig.

2 they provide a platform parallel to the chain and bearing pin

axes. They are used for securing slats and buckets etc. to the

chain. Either one or two holes are normally provided in the

platform, being designated K 1 or K2 respectively. K attachments

can be incorporated on one or both sides of the chain. For the

more important stock pitches where large quantities justify the use

of special manufacturing equipment, the attachments are

produced as an integral part of the chain, as shown in Fig. 2(a).

Here the platform is a bent over extension of the chain plate itself.

O n other chain or where only small quantities are involved, separate

attachments are used, as shown in Fig. 2(b). These are usually

welded to the chain depending on the particular chain series and

the application. A lternatively, ( see Fig 2(c) ) , K attachments may be

bolted to the chain either through the hollow bearing pins, or by

using special outer links with extended and screwed bearing pin

ends.

(a) K 1 bent over attachment.

(b ) K 1 attachment, welded to link plate.

(c) K 2 attachment bolted through hollow bearing pin.

F A t t a c h m e n t sThese attachments as shown in Fig. 3 are frequently used for

pusher and scraper applications. They comprise a wing with a

vertical surface at right angles to the chain. They can be fitted to

one or both sides and are usually secured by welding. Each wing

can be provided with one or two holes, being designated F1 or F2

respectively.

(a) F1 attachments welded to link plates on one or both sides of

the chain as required.

(b) F2 attachments welded to link plates on one or both sides of

the chain as required.

S p i g o t P in s a n d E x t e n d e dB e a r i n g P i n s

Both types are used on pusher and festoon conveyors and tray

elevators, etc. Spigot pins may be assembled through hollow

bearing pins, inner links or outer links. When assembled through

link plates a spacing bush is necessary to ensure that the inside

width of the chain is not reduced. G apping of the sprocket teeth is

necessary to clear the bush.

Solid bearing pin chains can have similar extensions at the pitch

points by incorporating extended pins. Both spigot pins and

extended pins, as shown in Fig. 4, can be case-hardened on their

working diameters for increased wear resistance.

(a) Spigot pin assembled through outer or inner link.

(b ) Spigot pin bolted through hollow bearing pin.

(c) Extended bearing pin.

RENOLDRENOLD

RENOLD

RENOLD

RENOLD

RENOLD

RENOLDRENOLD RENOLDRENOLD

RENOLD

RENOLD

RENOLD

RENOLD

c

Fig. 2

Fig. 3

Fig. 4

a b

ca b

a b




4

S t a y b a r sTypes of mechanical handling equipment that use staybars are

pusher, wire mesh, festoon conveyors, etc., the staybars being

assembled in the same manner as spigot pins. When assembled

through link plates a spacing bush and gapping of the sprocket

teeth are necessary.

The plain bar-and-tube type shown in Fig. 5 has the advantage that

the staybar can be assembled with the chain in situ by simply

threading the bar through the chain and tube. The shouldered bar

type has a greater carrying capacity than the bar-and-tube type.

Staybars are normally used for either increasing overall rigidity by

tying two chains together, maintaining transverse spacing of the

chains, or supporting loads.

(a) Staybar bolted through hollow bearing pin.

(b) Staybar assembled through outer or inner link.

G A t t a c h m e n t sAs shown in Fig. 6 this attachment takes the form of a flat

surface positioned against the side of the chain plate and parallel

to the chain line. It is normally used for bucket elevators and

pallet conveyors. When the attachment is integral with the outer

plate then the shroud of the chain sprocket has to be removed to

clear the plate. G Attachments are normally fitted only to one

side of the chain.

(a) G 2 attachment outer plate.

(b ) G 2 attachment, welded or rivetted to link plate.

L A t t a c h m e n t sThese have some affinity with the F attachment, being in a similar

position on the chain. A familiar application is the box scraper

conveyor. As shown in Fig. 7 the attachments are integral with the

outer plates, being extended beyond one bearing pin hole and

then bent round. The attachments can be plain or drilled with one

or two holes, being designated L0, L1 or L2 respectively. They can

be supplied on one or both sides of the chain. With this type of

attachment the chain rollers are normally equal to the plate depth,

or a bush chain without rollers is used.

L2 attachments on both sides of the outer link.

S a n d P u s h e r A t t a c h m e n t sThese are normally used on dog pusher conveyors. A s shown inFig. 8 the S attachment consists of a triangular plate integral with

the chain plate; it can be assembled on one or both sides of the

chain, but may also be assembled at the inner link position. S

attachments are intended for lighter duty, but for heavier duty a

pair of attachments on one link is connected by a spacer block

to form a pusher attachment. This increases chain rigidity and

pushing area.

(a) S attachment outer plate; assembled on one or both sides

of chain as required.

(b) Pusher attachment.

D r i l l e d L i n k P l a t e sPlates with single holes as shown in Fig. 9(a) are associated with

the fitting of staybars or spigot pins. Where G or K attachments

are to be fitted then link plates with two holes as shown in Fig.

9(b) are used. Where attachments are fitted to inner links then

countersunk bolts must be used to provide sprocket tooth

clearance.

O u t b o a r d R o l l e r sThe main reasons for using outboard rollers are that they

increase roller loading capacity of the chain and provide a

stabilised form of load carrier. As shown in Fig. 10 the outboard

rollers are fixed to the chain by bolts which pass through hollow

bearing p ins. O utboard rollers have the advantage that they are

easily replaced in the event of wear and allow the chain rollers to

be used for gearing purposes only.

C h a i n J o i n t sC onveyor chain is normally supplied in convenient handling

lengths, these being joined by means of outer connecting links.

This can be accomplished by the use of any of the following:

No. 107 No. 69

RENOLD

RENOLD

RENOLD

RENOLD

RENOLD

RENOLD

RENOLD

RENOLD

O uter link used for rivetting

chain endless. It is particularly

useful in hollow bearing pin

chains where the hollow pin

feature is to be retained.

Bolt-type connecting link

with solid bearing pin. Loose

plate is a slip fit on the bearing

pins and retained by self

locking nuts.

Fig. 5

Fig. 6

Fig. 7

Fig. 8

Fig. 9

Fig. 10 Outboard rollers.

Fig. 11

a b

a b

a b

a b




4

A d v a n t a g e s o f R e n o l d C o n v e y o rC h a i n

These c an be s ummarised a s follows :-

a . Large bearing areas and hardened components promote

ma ximum life.

b. Low friction due to a s mooth finish of the components.c. The inclusion of a cha in roller and the high s treng th to

weight ratio e nab le lighter cha in se lection a nd lower pow er

consumption.

d. The use o f high g rad e ma terials ensures reliab ility on

onerous a nd a rduous applications.

e. The fa cility to obta in a variety of pitches w ith each cha in

breaking s treng th and a variation in attac hment types

provides adaptability.

f. The accuracy of components provides consistency of

operation, accurate gearing and low sprocket tooth wear.

The la tter is pa rticularly impo rtant in multistra nd sys tems

where equal load distribution is vital.

BASIC REQUIREMENTSTo ena ble the mos t suitab le chain to be se lecte d for a pa rticular

application it is necessary to know full applicational details such

as the following:

Type of co nveyor.

Conveyo r centre distance a nd inclination from the horizontal.

Type o f chain attac hment, spa cing a nd me thod o f fixing to the

chain.

Number of chains a nd cha in speed .

Details of conveying attachments, e.g. weight of slats, buckets, etc.

Desc ription of ma terial ca rried, i.e. w eight, size a nd q uantity.

Method of feed and rate of delivery.

S e l e c t i o n o f C h a i n P i t c hIn general the largest stock pitch possible consistent with correct

operation s hould b e used for any a pplica tion, since ec onomic

ad vantag e results from the use of the reduced number of chain

components per unit length. Other factors include size of bucket

or slats etc., cha in roller loa ding (see P ag e 73) and the nec ess ity

for an acceptable minimum number of teeth in the sprocketswhere space restriction exists.

CHAIN PULL CALCULATIONS

The preferred me thod o f calculating the ten sion in a co nveyor

cha in is to co nsider each s ection of the conveyor that ha s a

different op erating c ond ition. This is pa rticula rly nec es sa ry where

cha nges in direction o ccur or where the load is no t consta nt over

the whole of the conveyor.

For uniformly loaded conveyors there is a progressive increase in

cha in tens ion from the oretica lly zero a t A to a ma ximum a t D. This

is illustrated graphically in Fig. 14 where the vertical distances

represent the c ha in tension o ccurring a t pa rticular points in thecircuit, the s umma tion o f which gives the to tal tension in the cha in.

Thus, in Fig. 14 the ma ximum pull a t D co mprise s the sum of:

(a) Pull due to chain and moving parts on the unload ed side.

(b) Extra pull required to turn the idler wheels and sha ft.

(c) Pull due to chain and moving parts on the loa ded side.

(d) Pull due to the load being moved.

If it is imagined that the chains are cut at position X then there

will be a lowe r loa d pull or tens ion a t this po sition tha n a t Y. This

fact is s ignifica nt in the pla cing o f ca terpillar d rives in co mplex

circuits a nd a lso in a ss ess ing tens ion load ings for automa tic take-

up units.

This principle ha s b een us ed to a rrive a t the eas y referenc e

layo uts a nd formulae (Pa ge 80 - 81) to which mos t conveyor and

elevator a pplica tions should co nform. Where conveyors do not

easily fit these layouts and circuits are more complex then see

page 82 or consult Renold Applications Department for advice.

FACTORS OF SAFETYChain manufacturers specify the chain in their product range by

breaking load. Some have q uoted averag e breaking loads , some

have quo ted minimum breaking loa ds depend ing upon their level

of confidence in their product. Renold always specify minimum

breaking loa d. To o btain a des ign working loa d it is ne ces sa ry to

apply a factor of sa fety to the breaking load and this is an a rea

where confusion ha s a risen.

As a genera l rule, Renold sug ges t that for most applica tions a

factor of sa fety of 8 is us ed,

Working Loa d = Brea king Loa d

8

On lower breaking strength

cha in a so ft circlip retains the

connecting plate in position

on the pins, the connecting

plate b eing a n interference fit

on the bearing pins.

A modified version of the

bolt-type connecting link.

The c onne cting pins a re

extended to permit the

fitment of attachments on

one side of the chain only.

Travel

C

B

Y

D

Driver

Total chain tension

XA

For 4,500 lbf s eries cha in o nly,

circlips are fitted to both ends

of hollow connecting pins.

Simila r to No. 86 b ut a llows

attachments to be bolted to

both sides of the chain.

Fig. 12

Fig. 14

Fig. 13

No. 58 No. 86

No. 11 No. 85




4

On first inspection, a factor of safety of 8 seems very high and

suggests that the chain could be over-selected if this factor is

applied.

If, howe ver, w e e xamine the situation in deta il, the following po ints

arise:-

1. Most chain side plates are manufactured from low or medium

carbon steel and are sized to ensure they have adequate

strength and resistance to shock loading.

2. These s teels ha ve yield strengths that vary from 50%to 65%of

their ultima te tens ile strength. This mea ns that if chains a re

subjected to loa ds of 50%to 65%of their breaking loa d, then

perma nent pitch extens ion is likely to oc cur.

3. It is the tendency to over-select drive sizes just to be sure the

drive is adequate, and the motors used today are capable of up

to 200%full loa d torq ue o utput for a sho rt period.

4. The conseq uences of this a re that a chain confidently selected

with a factor of safety of 8 on breaking load is in effect operatingwith a factor of sa fety of a s low as 4 on the yield o f the ma terial,

and 2 when the pos sible instantaneous overloa d o n the drive is

considered, and this is without considering any over-selection of

the motor nominal power.

5. A further cons ideration when a pplying a fac tor of safety to a

chain application is the chain life.

The tens ion applied to a cha in is ca rried by the pin/bus h interface

which at the chain sprockets articulates as a plain bearing.

Experience has shown that, given a good environment, and a clean

a nd w ell lubrica ted c ha in, a be a ring press ure of up to 24N/mm2

(3500 lb/inch2) will give an a cce ptab le p in/bus h life. A sa fety factor o f

8 will give this bearing pressure.

In a nything other than a clean well lubrica ted e nvironment the factor

of sa fety should be increased , thus lowering the bea ring pressure, if

some detriment to the working life of the chain is to be avoided.

Ta ble 1 gives a ge neral guide to the app ropria te sa fety fa ctors for

different a pplica tions .

Ta b l e 1 - F a c t o r s o f S a f e t y

CLEANLINESS/LUBRICATION

TEMPERATURE/LUBRICATION

In all the listed applications and conditions, the increase in factor of

sa fety is a pplied with the object o f low ering the pin/bus h be a ring

pressure to improve the chain life.

CHAIN LIFE

There a re a numb er of factors a ffecting the life of a c ha in in a

pa rticular environme nt.

a . The load on the chain and therefore the bearing

pressure between the pin and the bush.

The de sign of co nveyor chain is such tha t at the c alculated

working load of the chain (relative to the breaking load)

then the bea ring pressure between the pin a nd the bus h will

be a t a ma ximum o f 24N/mm2

(3500lb /in2

) for a clean well

lubrica ted environme nt.

This press ure should be red uced for anything

less than clean, well lubricated conditions and this is

allowed for by increa sing the factor of safety as show n in

table 1.

b. The characteristics of the material handled, i.e.

abrasiveness, etc.

So me ma teria ls a re extremely a bras ive and if the ma teria lcannot be kept away from the chain then the bearing

pressure must be reduced to less en the effect of the

ab rasion. It is po ss ible to improve the abra sion resista nce o f

chain components by more sophistica ted heat treatments at

extra cos t but the usua l wa y of ensuring a n a cce ptable life is

to reduce the bearing pressure. See page 98 for the abrasive

cha racteristics of ma teria ls. In so me instance s it is p oss ible

to use block cha in to improve c hain life, see pa ge 88.

c . Corrosion.

Some materials are aggressive to normal steels and the

nature of the attack will be to reduce the side plate section

and therefore the chain strength, or cause pitting of the pin,

bush and roller surfac es.

The pitting of the surfac e ha s the e ffect o f red ucing thebea ring a rea of the compo nent and therefore increa sing the

bea ring pressure and wea r rate. The proces s w ill also

introduce (onto the bearing surfaces) corrosion products

which are themselves ab rasive.

Materials such as Nitrates will cause the failure of stressed

compo nents due to nitrate s tress cracking.

Pa ge 104 shows some materials tog ether with their corrosive

potential for various chain materials.

d . Maintenance by the end user is one of the most important

factors governing the life of a chain.

For the ba sic maintena nce mea sures required to o btain the

ma ximum us eful life from yo ur cha in cons ult the Insta lla tion

and Maintenance section.

LOAD

EXTENSION

MATERIAL

YIELD, POINT

P

O(a)

EXTENSION

PERMANENT

Fig. 15

Lubrication -30 / +150C 150 - 200C 200 - 300C

Reg ula r 8 10 12

Occa siona l 10 12 14None 12 14 16

Reg ula r 8 10 12 14

Occa s iona l 10 12 14 16

None 12 14 16 18

Lubrication Clean Moderately Dirty AbrasiveClean


7/39

BS Series

B S 13 13 25.4 0.13 0.14 0.16

B S 20 20 25.4 0.15 0.17 0.19

B S 27/B S 33 27/33 31.8 0.15 0.18 0.20

B S 54/B S 67 54/67 47.6 0.12 0.14 0.17

B S 107/B S 134 107/134 66.7 0.10 0.13 0.15

B S 160/B S 200 160/200 88.9 0.09 0.11 0.13

B S 267 267 88.9 0.09 0.11 0.13

B S 400 400 88.9 0.09 0.11 0.13

ISO Series

M40 40 36 0.11 0.12 0.14M56 56 42 0.10 0.12 0.14

MC 56 56 50 0.10 0.12 0.14

M80 80 50 0.09 0.11 0.13

M112 112 60 0.09 0.10 0.12

MC 112 112 70 0.09 0.11 0.13

M160 160 70 0.08 0.10 0.12

M224 224 85 0.08 0.09 0.11

MC 224 224 100 0.08 0.10 0.12

M315 315 100 0.07 0.09 0.11

M450 450 120 0.07 0.09 0.10

M630 630 140 0.07 0.09 0.10

M900 900 170 0.06 0.08 0.10

Chain Ultimate Roller Chain Overall Coefficient of Frictionc

Reference Strength Diameter(kN) (mm) Regular Occasional NoD Lubrication Lubrication Lubrication

F = 0.15 F = 0.20 F = 0.25

72 engineering excellence w w w . r e n o ld . c o m


4

ASSESSMENT OF CHAINROLLER FRICTION

In conveyo r ca lcula tions the va lue o f the coe fficient of friction of

the cha in roller has a cons iderable effect on cha in s election.

When a chain roller rotates on a supporting track there are two

aspects of friction to be considered. Firstly there is a resistance to

motion caused by rolling friction and the value for a steel roller

rolling on a stee l track is no rmally ta ken a s 0.00013. However this

figure a pplies to the periphery and needs to be related to the

roller diameter, therefore:

Coefficient of rolling friction =

0.00013 = 0.13 = 0.26

Roller radius (m) Roller radius (mm) Roller d iameter (mm)

Se cond ly a co ndition o f sliding friction e xists betwe en the roller bore

and the bush periphery. For well lubricated clean conditions a

coefficient of sliding friction F of 0.15 is used a nd for poo r lubrica tion

approaching the unlubricated state, a value of 0.25 should be used.Aga in this a pplies a t the bush/roller con tact fac es a nd nee ds to be

rela ted to their diameters.

Coefficient of sliding friction =

F x Roller bore (mm)

Roller diameter (mm)

Thus the o verall theo retica l coe fficient of c ha in rollers m oving o n

a rolled steel track =

0.26 + (F x Roller bore) (mm)

Roller diameter (mm)

In practice, a contingency is allowed, to account for variations in the

surfac e q uality of the trac king and other imperfections such as trac k

joints. The nee d for this is mo re evide nt as roller diame ters be co me

smaller, and therefore the roller diameter is used in an additional

part of the formula, which beco mes:

Overall coefficient of friction =

c = 0.26 + (F x d ) + 1. 64

D D

a nd simplified: c = 1.90 + F d

D

Where c = overall coefficient of friction for cha in.

F = bus h/roller sliding friction co efficient.

d = ro lle r b ore dia m ete r in m m.

D = ro lle r o uts id e dia m ete r in mm .

The formula is a pplica ble to a ny plain be a ring roller but in the

case of a roller having ball, roller or needle bearings the mean

diameter of the ba lls etc. (Bd), would b e use d a s the roller bore.

F is taken as 0.0025 to 0.005, the latter being assumed to apply

to mo st c ond itions . Thus overall coefficient of friction for a cha in

roller fitted w ith ba ll bea ring s a nd rolling on a stee l track:

c = 0.26 + (0.005 x Mean diameter of balls (mm)) + 1.64

Roller diameter (mm) Roller diameter (mm)

... c = 1. 90 + (0. 005 x B d)

D

The following ta ble s how s values for overall coefficient of friction

for standa rd conveyor cha in with standa rd rollers (c). Alterna tive

values ca n be ca lculated as ab ove if the roller diame ter is

modified from the standa rd shown.

OVERALL COEFFICIENTS OF ROLLING FRICTION

FOR STANDARD CONVEYOR CHAIN (c)

D

d

Chain Pull

Sliding Friction

Rolling Friction

Bush/Roller clearance(exaggerated)

Roller

Bush

Bd

Table 2

Fig. 16

Fig. 17

F




4

ROLLER SELECTION ANDROLLER LOADINGCONSIDERATIONS

R o l l e r S e l e c t i o n

Roller Materials

1. Unhardened mild steel rollers are used in lightly loa ded, clean

and well lubrica ted a pplica tions subject to occ as iona l use.

2. Hardened s teel rollers are used in the majority of applica tions

where a hard we aring s urface is required.

Note that through ha rdened s intered rollers a re standa rd on

BS cha in of 26 to 67KN breaking load . On all other BS a nd on

ISO c hain the sta nda rd hardened rollers are in case hardened

mild steel.

3. Ca st iron rollers are used in applications where some

co rros ion is likely a nd a mea sure o f self-lubrica tion isrequired.

4. Synthetic rollers, e.g. Delrin, nylon or other plas tics c an b e

used where either noise or corrosion is a major problem.

Please enquire.

R o l l e r S i z e s a n d T y p e s

1. Sma ll (ge aring) rollers are used for sprocket gea ring purpos es

only to reduce abrasion and wear between chain bush and

sproc ket tooth. Thes e rollers d o not project and c ons eq uently,

when not operating vertically, the chain will slide on the side

plate edg es.

2. Standa rd projecting rollers are used for most conveying

applica tions a nd a re designed to o perate smoo thly withoptimum rolling friction p roperties . They c reate a n a cc epta ble

rolling clearanc e a bove a nd below the cha in side plates .

3. Flang ed rollers a re used w here extra guidance is required or

where impos ed s ide load s w ould otherwise force the cha in

out of line.

4. Larger diameter rollers a re oc cas ionally used where the

greater diameter of the roller reduces wear by reducing the

rubbing velocity on the chain bushes and promotes smoother

running at slow speeds.

Thes e rollers ca n be either plain or flang ed in stee l, cas t iron

or synthetic material.

5. Most cha in ca n be supplied with ba ll bea ring rollers either

outbo a rd or integ ral. This spec ial des ig n option ca n bejustified by the selection of a lower breaking load chain in

many applica tions and a reduction in the drive powe r

required.

R o l l e r L o a d i n g ( B u s h / R o l l e rW e a r )

In the majority of cases a conveyor roller chain will meet

bus h/roller wea r req uirements if it ha s b een co rrectly se lecte d

using factors of sa fety on b reaking loa d. Doubt ca n arise w here

heavy unit loading is involved, which could cause the bearing

pressure between the cha in bush and roller to be excess ively

high, or where the chain speed may exceed the recommended

maximum. In such ca ses further checks have to be mad e.

B u s h / R o l l e r B e a r i n g A r e a s a n dB e a r i n g P r e s s u r e s

The bus h/roller bea ring a reas for sta nda rd BS a nd ISO se ries

conveyor chain are as follows :

Bush/Roller Bearing Area BS

Chain Reference Bearing Area mm2

B S 13 99

B S 20 143

B S 27 254

B S 33 254

B S 54 420

B S 67 420

B S 107 803

B S 134 803

B S 160 1403

B S 200 1403

B S 267 1403

B S 400 1403

Table 3

Bush/Roller Bearing Area - ISO

Chain Reference Bearing Area mm2

M40 232

M56 333

MC56 447

M80 475

M112 630

MC112 850

M160 880

M224 1218

MC224 1583

M315 1634

M450 2234M630 3145

M900 4410

Table 3 (Continued)

B e a r i n g P r e s s u r e

Normal maximum permitted bearing pressures for chain speeds

up to 0.5m/sec ., a nd in reaso nab ly clean and lubrica ted

applica tions are listed below:

Roller Bearing Pressure P

Material Normal Maximum

Mild s teel ca se ha rd ened 1.8N/mm 2

S inte re d s te el th ro ug h h ard ene d 1.2N/m m2

C a s t iron 0.68N/mm2

Table 4

The formula : b ea ring press ure P (N/mm2) = ro lle r lo a d R (N)

Bearing a rea BA (mm2)

is us ed first to che ck whether actual press ure exceed s the a bove

recommenda tion. If it does , or if the conveyor spe ed e xceeds

0.5m/se c, the cha in ma y s till be a cc epta ble if a lternative

conditions c an b e met. These d epend upon a comb ination of

bea ring press ure a nd rubbing s peed b etween b ush a nd roller,

known as the PVR value, and the deg ree of clea nliness and

lubrication on the application. If cleanliness and lubrication are

much better than averag e for example, higher bea ring pressures

and PVR values than normal ca n be tolerated. In order to ma ke

this judgement the following table is used, along with the formula:




4

Rubbing S peed VR (m/sec ) =

Cha in Speed (m/se c) x Bus h diame ter (mm)

Roller Diameter (mm)

Table 5

If the rubbing s peed is a bo ve 0.15 m/s, c a lcula te the P V value to

see if it is below the max value in the table. If the rubbing speed is

below 0.15 calculate the bearing pressure to see if it is below the

ma ximum g iven in the ta ble. If the s peed is be low 0.025 m/s it isbes t to us e rollers w ith a n o/d to bo re ratio o f 3 or highe r, or us e

ball bearing inboard or outboard rollers with the required load

capacity.

If the calculated bearing pressure or PV exceeds the guidelines

given in the tables then consider one of the following:

a. Use a larger chain size with consequently larger rollers.

b. Use larger diameter rollers to reduce the rubbing s peed.

c. Use outboard rollers, either plain or ball bearing.

d . Use bal l bear ing rollers .

e. If in doubt consult Renold.

STICK-SLIP

Stick-Slip is a problem that occurs in some slow moving

conveyor systems which results in irregular motion of the chain in

the form of a pulse. Stick slip only occurs under certain

conditions and the purpose of this section is to highlight those

cond itions to ena ble the problem to be recog nised and avoided.

For a co nveyor running a t a linea r speed of ap prox. 0.035m/se c

or less, one of the most often encountered causes of stick-slip is

over-lubrica tion o f the cha in. Too much oil on the cha in lead s to

the chain support tracks being coated with oil thus lowering R 1,

(Fig. 18). If any o f the other s tick-slip c ond itions a re prese nt then

R1 is insufficient to ca use the roller to turn a ga inst the roller/bus hfriction F a nd the roller slides along on a film of oil.

The o il film builds up be tween the bus h a nd roller a t the lead ingedg e of the press ure co ntact a rea a nd the resulting va cuumcond ition b etween the two s urfaces requires force to b reak itdown. If the chain tracks are coated with oil, or oil residue, then

this force is not immediately available and the roller slides alongthe track w ithout rotating. The va cuum then fails, either due to the

sta tic co ndition o f the bus h/roller surface s o r by the brea kdown o fthe dynamic film of lubricant on the track.

In either cas e the cha nge from the s liding sta te to rotation c aus esa pulse as the velocity of the chain decreases and then increases.

Once rotation returns then the c ycle is repeated c aus ing regularpulsations and variations of chain speed. Although the friction isinsufficient to c a use the roller to turn, friction is presen t a nd, overa period , the roller will develop a se ries of fla ts w hich will

compo und the problem.

The o ther fea tures that a re neces sa ry for stick slip to o ccur a re:

a . Lig ht loa ding - If the loa ding on the roller is very light then it is

ea sy for a vacuum co ndition to develop. Heavy loa ds tend to

break the oil film down on the chain tracks.

b. Irreg ular loading - If the cha in is loa ded at intervals, withunloa ded ga ps, it is po ss ible for the chain between the loads

to experience s tick slip d ue to ligh t loa ding.

P r e c a u t i o n s t o A v o i d S t ic k S l i p

1. Avoid s peeds in the critica l range up to a pprox. 0.035m/sec .,if pos sible.

2. Avoid irregular loa ding, if possible.

3. If it is not pos sible to a void the spee d a nd loading critica lity,then great ca re should be taken in system d esign:

3.1 Control the a pplica tion o f lubrica nt to a void track

contamination.3.2 If ligh t loa ds a re to be ca rried then c ha in rollers sh ould

be either larger than standard or be fitted with ball

be a ring s to low er the bus h/roller friction , F, or improvemechanical efficiency.

As a rough guide, where plain (not ball bearing) rollers are used,a ratio of roller diameter to bush diameter of 2.7:1 or greater

should eliminate stick slip at the critical speeds.

TRACKED BENDS

Where chain is guided around curves there is an inward reaction

press ure ac ting in the d irection o f the curve centre. This a pplieswhether the curved tracks are in the vertical or horizontal planes,

and , relative to the former, whe ther upwa rds or d ownw ards indirection. The load pull effect resulting from the c ha in tra nsversing

a curved section, even if this be in the vertical downwarddirection, is a lwa ys cons idered a s a positive value, i.e. s erving toincrease the chain load pull.

An analogy is a belt on a pulley whereby the holding or retainingeffect d epends upon the e xtent of wrap-around of the belt, andfriction b etwe en the belt and pulley.

Simila rly there is a definite rela tions hip betwe en the tension o rpull in the cha in a t entry and exit of the curve. Referring to thediagrams this relationship is given by:

P 2 = P 1ec

Where P 1 = Chain pull a t entry into bend (N)P 2 = Chain pull a t exit from bend (N)

e = Na pe ria n lo g a rith m b a s e (2. 718)

c = Coeffic ient of fric t ion between chain and track

= B end a ng le (ra d ia ns )

C as e ha rde ned 0.025 - 0.15 0.025 - 0.25 10.35 1.80

mild s teel over 0.15 over 0.25 use PVR = 1.55 us e P VR = 0.45

S inte re d thro ug h- 0.025 - 0.15 0.025 - 0.25 6.90 1.20

ha rd ene d s tee l o ve r 0.15 o ve r 0.25 us e P VR = 1.04 us e P VR = 0.30

Ca st iron 0.025 - 0.15 0.025 - 0.25 3.91 0.68

o ve r 0.15 o ve r 0.25 us e P VR = 0.59 us e P VR = 0.17

Bush/Roller Clearance

(Exaggerated)

Roller

Bush

F

od

W (kg)

F

R1

id

Fig. 18

Roller Rubbing speed Max. Bearing PressureMaterial VR (m/sec) P (N/mm2)

Very Good Average Very Good AverageConditions Conditions Conditions Conditions




4

The a bo ve formula a pplies whe ther the cha in is tra cked via the

cha in rollers or by the cha in plate edg es bea ring on s uitable

guide tracks . Ta ble 6 gives va lues o f ec.

Since high reaction loa dings c an be involved whe n neg otiating

bend sections it is usually advisable to check the resulting roller

load ing . This ca n be d one from the following formula whe re RL is

the loa d per roller due to the rea ction load ing at the b end s ection.

RL(N) = P 2(N) x Chain Pitch (mm)

Chain curve radius (mm)

The reac tion load ing value obtained s hould then be ad ded to the

normal roller loa d a nd the total ca n be compa red with the

permitted values discussed in the section on roller selection and

roller loading considerations.

There is a minimum rad ius w hich a ch a in ca n neg otiate without

fouling of the link plate edges. Relevant minimum radii against

each chain series are listed in table 7 on page 76, and it will be

noted that these will vary according to pitch, roller diameter and

plate depth.

P2

P1

P2

P1

P1

P2

VERTICAL PLANEDOWNWARDS

VERTICAL PLANE

UPWARDS

HORIZONTAL PLANE

MINIMUM TRACK RADIUS FOR LINK CLEARANCE

Values ofec for variable Values ofc

c ec c ec c ec

0.02 1.0202 0.25 1.2840 0.45 1.56830.04 1.0408 0.26 1.2969 0.46 1.58410.06 1.0618 0.27 1.3100 0.47 1.60000.08 1.0833 0.28 1.3231 0.48 1.6161

0.29 1.3364 0.49 1.6323

0.10 1.1052 0.30 1.3499 0.50 1.64870.11 1.1163 0.31 1.3634 0.56 1.82210.12 1.1275 0.32 1.3771 0.57 2.01380.13 1.1388 0.33 1.3910 0.58 2.22550.14 1.1505 0.34 1.4050 0.59 2.4596

0.15 1.1618 0.35 1.4191 1.0 2.71830.16 1.1735 0.36 1.4333 1.1 3.00420.17 1.1835 0.37 1.4477 1.2 3.32010.18 1.1972 0.38 1.4623 1.3 3.66930.19 1.2092 0.39 1.4770 1.4 4.0552

0.20 1.2214 0.40 1.4918 1.5 4.4817

0.21 1.2337 0.41 1.5068 1.6 4.95300.22 1.2461 0.42 1.5220 1.7 5.47390.23 1.2586 0.43 1.5373 1.8 6.04970.24 1.2712 0.44 1.5527 1.9 6.6859

2.0 7.3891

Table 6

CR

CR

Fig. 19




4

M i n i m u m T r a c k R a d i i f o r B S a n d I S O S e r i e s C h a i n

Table 7

BS 13 25.4 1.3 38.10 60

50.80 115

63.50 190

76.20 280

88.90 380

101.60 500

114.30 635

BS 20 25.4 1.3 38.10 90

50.80 160

63.50 255

76.20 375

BS 27 31.8 1.3 50.80 160BS 33 63.50 255

76.20 370

88.90 510

101.60 670

114.30 845

127.00 1050

139.70 1270

152.40 1500

BS 54 47.6 2.5 76.20 305

BS 67 88.90 425

101.60 560

114.30 720

127.00 890

152.40 1295177.80 1755

203.20 2300

228.60 2920

BS 107 66.7 4 101.60 295

BS 134 127.00 480

152.40 710

165.10 830

177.80 970

203.20 1280

228.60 1630

254.00 2020

304.80 2920

BS 160 88.9 5 127.00 185

BS 200 152.40 285BS 267 177.80 400

203.20 540

228.60 690

254.00 860

304.80 1250

381.00 1980

457.20 2870

BS 400 88.9 5 152.40 335

228.60 810

304.80 1470

381.00 2320

457.20 3350

609.60 5990

M40 36 1.3 63 13680 218

100 340125 530160 867

M56 50 2.5 63 7780 138

100 228125 368160 618200 978250 1540

MC56 50 2.5 80 163

M80 100 253125 393160 643200 1003250 1565

M112 60 4 80 106125 299160 506200 806250 1275315 2040400 3306

MC112 70 4 100 182M160 125 283

160 461200 718

250 1120315 1775

M224 85 5 125 222160 388200 628250 1003315 1615400 2628500 4128630 6576

MC224 100 5 160 593M315 200 953

250 1515315 2433400 3953500 6202

630 9875M450 120 6 200 304

250 505315 833400 1376500 2179630 3491800 5661

M630 140 7 250 537315 891400 1475500 2340630 3753800 6090

1000 9552

M900 170 8 315 653

400 1100500 1762630 2842800 4629

1000 7276

BS SERIES

Chain Ref. Roller Dia. Clearance C Pitch Track Radius Rmm mm mm mm

ISO SERIES

Chain Ref. Roller Dia. Clearance C Pitch Track Radius Rmm mm mm mm




4

MATCHING OF CONVEYOR CHAINAny application in which two or more strands of chain are required

to operate side by side may require the strands to be matched.

This would be to maintain the same fixed relationship between

handling lengths throughout the length of the chains.

Due to manufacturing tolerances on chain components actual

chain length may vary within certain limits. Thus, two strands of

any given pitch length would not necessarily have the same actual

overall length if chosen at random. Also, different sections along

any chain length may vary within the permissible limits and

therefore, even given identical overall lengths, corresponding

sections of random strands would be slightly out of register. These

displacements tend to become more pronounced with increasing

length.

CONVEYOR TYPES

The types of conveyors where this is likely to have the greatesteffect are:

1) Where chains are very close and tied together, i. e.

within approximately 300/500mm depending on breaking load.

2) O n very long or long and complex circuits.

3) Where load positioning /orientation at load or unload

is important.

PROCEDURE

The procedure used for matching conveyor chain is as follows:-

a) Each handling length is accurately measured and

numbered.

b) A list is produced ( for a two chain system) of chains,

e.g A and B, in which handling lengths placed opposite each

other are as near equal in length as possible.

c) This list will give a series of lengths in which A and

B are matched, A1 with B1, A2 with B2, etc.

d) The chains are then tagged with a brass tag

containing the appropriate identity, i.e. A 2 B4 etc and, where

required, the chain length.

ON-SITE ASSEMBLYWhen assembling the chain on site it is important that lengths A

and B are installed opposite each other as are A 1, and B1, etc.

ATTACHMENTSIt should be noted that chains can only be matched as regards the

chain pitch length. D ue to extra tolerances involved in attachment

positioning and holing it is not possible to match chains relative to

attachments.

SPROCKETS

Where chains have been matched, the drive sprockets should not

only be bored and keywayed as a set in relation to a tooth, as in a

normal conveyor drive, but it is recommended that a machine cut

tooth form is also used to ensure equal load sharing.

ACCURACY

In order to maintain the accuracy of matched chains it is important

to ensure equal tensioning and even lubrication of the chain set.




4

PUSHER CONVEYORS

Where cha in is use d w ith pusher a ttac hment plates , to move

loa ds along a sepa rate skid rail (e.g. billet trans fer conveyors),

then there will be an extra load in the chain due to the reaction in

the pushers.

This loa d ca n be c a lcula ted by the follow ing formula :

Reac tion Load Pull

P L = m W hu c

P

Where m = Coeffic ient Fric t ion, Load on S teel

c = Coefficient Friction, Chain Rolling

W = Loa d (N)

hu = Pusher Height from Chain Pitch Line (mm)

P = Cha in P itch (mm)

If there is mo re than one pusher and loa d po sition then the total

reaction load can be found by either multiplying by the total number of

loads or by assuming that the total load acts at one pusher.

This rea ction loa d pull sho uld then be a dd ed to the tota l cha in

pull Cp obta ined us ing layo ut B pa ge 80 and igno ring the term X

(side guide friction).

CONVEYING DIRECTLY ON CHAINROLLERS

In some a pplica tions , load s a re ca rried d irectly on the projecting

rollers of the chain, instead of on attachments connected to the

chain side plates. In this case the loads will travel at twice the

speed of the cha in.

Where high unit loa ds a re involved the rollers mus t be either

ca se hardened mild s teel or through ha rdened medium carbon

steel. For normal duty the tracks can be standard rolled sections

but for heavy unit loa ds hardened tracks ma y be necess ary.

Note: The roller ha rdness sho uld a lways be g reater than tra ck

hardness.

For a layout similar to the above, the chain pull can be

ca lculated as follows :

Where C p = Tota l cha in pu ll (N)

W = Weigh t o f mate ria l on conveyor (kg)

Wc = Weight of chain(s) and at tachments (kg/m)

L = Conveyor centres (m)

R1 = Coefficient of rolling friction between chain

roller and track

R3 = Coefficient of rolling friction between chain

roller and load

C = Chain overall coefficient of friction

d = Roller I/D (mm)

D = Roller O/D (mm)

Rolling friction R1 for a steel roller on a rolled or pressed steel

trac k is variab le b etween 0.051 and 0.13 depend ing on the trac k

surfac e cond ition.

The rolling friction betw een the roller a nd the load is a lso va ria ble

dep end ing upon the latter. For ma ny a pplica tions it is s ufficiently

accurate to take R3 as being 0.13.

SIDE FRICTION FACTORS

It must be a ppreciated that on a pron conveyors ca rrying loos e

materials, a nd w here static skirt plates are e mployed , the press ure

of material sliding against the skirt will increase the required load

pull of the c ha in.

This a dd itiona l pull is given b y the express ion:

2.25 x 104 G LH2 (N)

Wh ere H = the he ig ht of the m ate ria l (m )

L = the length of the loaded section of conveyor (m)G = a fa c to r d e pe nd in g up o n th e ma te ria l b e in g

hand led. - See pag e 79 tab le 8.

P

h u

m

CHAINPULL

TRAVEL

CHAIN PITCHLINE

W(N)

D

R1

W

R3

dCp

H

STATIC SKIRT PLATES

DRIVENC

B

W. kgTRAVEL

D

ADRIVER

L

Cp = 9.81 W x 2 + 1.64 + (2.05 x W x L x ) N( ( 2 + ) cx (1 + )) ccR1 R3D

Fig. 20

Fig. 21

Fig. 22

Fig. 22




4

Ashes , dry, 13mm a nd under 0.05 0.50

Ashes , wet, 13mm a nd under 0.02 0.60

Ashes , wet, 75mm a nd under 0.02 0.60

Cement, P ortla nd 0.09 0.70

Cement, c linker 0.08 0.70

Coa l, Anthra cite, nuts 0.04 0.50

Coa l, B ituminous , s la ck, wet 0.03 0.70

Coke, s ized 13mm 0.02 0.40

Coke, breeze, fine 0.03 0.70

G ra in 0.05 0.40

G ra vel, dry, screened 0.08 0.50

Lime, g round 0.04 0.40

Lime, pebble 0.07 0.50

Limes tone, crushed 0.14 0.90S a nd, dry 0.13 0.60

S a nd, da mp 0.17 0.90

S a nd, foundry, prepa red 0.07 0.90

S a wdus t 0.01 0.40

S tone, dus t 0.09 0.50

S tone, lumps a nd fines 0.10 0.70

S oda a sh (hea vy) 0.09 0.62

S odium ca rbona te 0.04 0.45

Wood, chips 0.01 0.40

Table 8

Values given are nominal and are for guidance only;

they are based on the materials sliding on steel.

METHODS OF SELECTION

1. Examine the diag rams A to K (pag e 80-81) and select the

layo ut nearest to the co nveyor under consideration.

2. Examine the formulae printed under the selected layout for

the co nveyor ch a in pull (Cp).

3. Identify and alloca te values to the elements of the formulae

by using the reference list opposite.

4. Calculate a preliminary chain pull using an estimated chain

mass .

5. Apply the correct factor of sa fety for the application from

Ta ble 1 pag e 71. If tempera ture a nd type of a pplica tion a ffect

your selection, then select the highest factor from other

relevant sec tions.

Cha in breaking load = Cha in Pull Cp x fac tor of sa fety.

6. For the chain breaking strength established in the

prelimina ry ca lculation, reca lcula te ma ximum c ha in pull Cp

using actual chain mass and check the factor of safety

obtained.

7. If load s are carried by the chain, then the roller capa cityshould be che cked - pag e 73.

8. Conveyor headshaft power may be calculated by using the

appropriate formula for K which will give the results in

Kilowatts.

Note: The po wer ca lcula ted is that req uired to keep the

co nveyor mo ving , not the mo tor size required. To s elect a

motor, a llowa nce s hould b e ma de for starting a ndtransmission losses.

9. Headshaft RPM can be calculated after se lecting a s uitable

size of drive sprocket.

RPM = V x 60

PCD x

where PC D = Pitch circle dia. o f sprocket (m).

10. Headshaft torque can be ca lculated as follows:

Torque = Cp x PC D (Nm)

2

REFERENCE LIST

C p = C ha in pull to ta l (N)

L = C entre d is tance (m) - head - to ta il-sha ft

Wc = Chain total mas s per metre (kg/m) including a ttachments

and fittings.

Wm = Mas s of loa d/metre (kg/m)

W = To ta l c a rrie d lo a d (kg )

T = C onvey ing capac ity (Tonnes/Hour)

V = C ha in s peed (m/s ec )

c = Co efficient of friction, cha in on s teel (sliding or rolling) -

se e Ta ble 2 pag e 72.

m = Coefficient of friction, load o n steel. See table 8 opposite.

= Loa d dens ity (kg/m3)

= Ang le of inclina tion (degree s).

G = S id e fric tion fac tor. S ee tab le 8 oppos ite .

C = Conveyor width (m)

H = Ma te ria l he ig ht (m)

S = Bucket spa cing (m)

Vb = B uc ke t c a pa c ity (m3)

K = P ow er a t he ad sha ft (kW)

Wb = B uc ke t ma s s (kg )

X = E xt ra cha in pull d ue to s id e gu id e fric tion

[X = 2.25 x 104 G LH2 (N) - See page 78]

P B = Chain pull a t B (N).

s1 = (c x cos ) - sin

s2 = (c x cos ) + s in

sm = (m x cos ) + s in

Df = Dredg e fa ctor (spa ced bkts)(N) = 90 x Vb x

S

Dredge fac tor (continuous bkts)(N) = 30 x Vb x

S

J = Cha in sa g (m)

a = Id ler centres (m)

NOTE:m = Metres, N = Newtons, kW = Kilowa tts, kg = Kilograms.

} see appendix 3 page 104-105

Material Factor G m

(Side Friction)




4

Chain and material sliding

Cha in rolling a nd ma teria l sliding

Chain rolling and material carried

Vertical elevator

P B = 9.81 x Wc x L x s1 (N)

Cp = 9.81 x L [(Wc x s2) + (Wm x sm)] + P B + X (N)

P B = 9.81 x Wc x L x s1 (N)

Cp = 9.81 x L [(Wc x s2) + (Wm x sm)] + P B + X (N)

P B = 9.81 x Wc x L x s1 (N)

Cp = 9.81 x s2 [(Wc x L) + W] + P B (N)

Cp = 9.81 [(wb x L) + (Wc x L) + (L x Vb x )] + Df (N)

K =[(9.81(L/S x Vb x )) + Df] x V

1000

ss

(kW)

Cp = 9.81 x L [(2 .05 x Wc x c) + (Wm x m)] + X (N)

Cp = 9.81 x c [(2.05 x Wc x L ) + W] (N)

Cp = 9.81 x c [(2.05 x Wc x L) + W] (N)

Chain and material sliding

Cha in rolling a nd m a teria l sliding

Chain rolling and material carried

Chain sliding and material carried

Cp = 9.81 x L [(2.05 x Wc x c) + (Wm x m)] + X (N)

K =Cp x V

1000(kW)

K =Cp x V

1000(kW)

K =Cp x V

1000(kW)

K =Cp x V

1000(kW)

K =Cp x V

1000(kW)

K =Cp x V

1000(kW)

K =Cp x V

1000(kW)

LAYOUT A LAYOUT E

LAYOUT B LAYOUT F

LAYOUT C LAYOUT G

LAYOUT D LAYOUT H




4

SELECTION EXAMPLE

A continuous slat conveyor, 36 metre centres of head and tail

sprockets, is to ca rry bo xed products 650mm x 800mm, of mass

36kg ea ch. 50 boxes w ill be the maximum loa d a nd two cha ins

are required with K attachments at every pitch one side. 152.4mm

pitch c ha in is preferred a nd the m a ss of the sla ts is 15kg/m.

Operating cond itions are clean and well lubrica ted. Cha in spe ed

wo uld b e 0.45 m/se c us ing 8 too th sproc kets.

The e xample is o f cha in rolling a nd m a teria l carried , i.e. Layout C ,

page 80

It is first ne ces sa ry to ca rry out a prelimina ry ca lcula tion to a rrive

at a cha in size on which to ba se the final ca lculation. A rough

as sess ment of chain mas s ca n be done b y doubling the slat

mass, and for rolling friction a figure of 0.15 can be used.

Ma s s o f Lo a d o n C onve yo r = 50 x 36 = 1800 kg

Ma ss per Metre o f S la ts = 15 kg /m

Estimated Mas s o f Chain = 15 kg /m

Estimated Mass of Chain &Slats = 15 + 15 = 30 kg /m

Prelimina ry Cha in P ull = 9.81x c [(2.05 x Wc x L)+ W ] N

= 9.81 x 0.15 [(2.05 x 30 x 36)+ 1800 ] N

= 5907 N

Factor of sa fety for this a pplica tion is 8 (from ta ble 1 pa ge 71).

..

. Minimum Breaking Load Required = 5907 x 8 = 23628 N

2 per cha in

As a solid bearing pin chain is preferable for this application then

two s trands of 152.4 mm pitch B S se ries , 33000 N (7500 lbf)

breaking load cha in may b e suitab le.

F in a l C a l c u l a t io n

Cha in mas s + K3 integral attac hment one side every pitch

= 3.35 kg/m (from cha in ca talogue)

Ma ss of Bo th Cha ins = 3.35 x 2 = 6.7 kg /m

Ma ss of C ha in + S la ts = 6.7 + 15 = 21.7 kg /m

c = 0.15 - taken from table 2 page 72. (Regular lubrica tion).

Cp (Chain pull) = 9.81 x c [ (2.05 x Wc x L) + W ] N

Cp = 9.81 x 0.15 [ (2.05 x 21.7 x 36) + 1800 ] N

Cp = 5005 N

Fac tor o f S a f ety = B reaking load x 2 = 33000 x 2 = 13.19

Tota l cha in pull 5005

Thus the s election is co nfirmed.

It is now necessary to check the roller loading.

Box = 650 mm long. Load = mass x g (gravity) = 36 x 9.81 = 353 N

Load of Chain and S lats over 650 mm = 21.7 x 9.81 x .65

= 138 N

To ta l Loa d o n Rollers = 353 + 138 = 491 N

Numb er o f Ro lle rs S uppo rting Lo a d = 650 x 2 = 8.5

152.4

Loa d Per Roller = 491 = 58 N

8.5

Bearing Area of Roller (see table 3) = 254 mm2

..

. B ea rin g P re ss ure o f Ro lle rs = 58 = 0. 23 N/m m2

254

This is w ell below the a llow a ble ma ximum o f 1.2 N/mm

2

(see pa ge73 table 4 sintered steel) therefore the roller loading is acceptable.

C o n c l u s i o n

The s election for this a pplica tion w ould be 2 strand s o f 152.4 mm

pitch, 33,000N (7500lbf) brea king load BS se ries cha in with

sta nda rd sintered s teel rollers (Chain No. 145240/16) a nd K3 bent

over attachments one s ide every pitch.

Pow er required to d rive the co nveyor would be:

K = C ha in pull x C ha in speed = Cp x V kW1000 1000

= 5005 x 0.45 = 2.25 kW

1000

Note: This is the po wer required at the hea ds ha ft to keep theconveyor moving, not the motor size required. Allowance should

be ma de for starting a nd transmission/gea ring loss es when

selecting a drive motor.

Head sha ft RPM required using a n 8 tooth (398.2 mm PC D)

sprocket would be

RPM = C hain S peed (m/sec) x 60

PCD (m) x

= 0.45 x 60 = 21.6 RP M

0.398 x

Hea dsha ft Torq ue = Cp x P CD Nm

2

= 5005 x 0.398

2

= 996 Nm

L

Cp

J

W

DRIVE

Cp = 9.81 1.05 ( )

( )

+ ( x Wc x L) + ( x W)L x Wc + (Wc x J)2

(N)8 x J

K = 9.81 0.05 + ( x Wc x L) + ( x W)L x Wc + (Wc x J)2

x V (kW)8 x J 1000

c

c c

c

L

Cp

W

a

J

L

Cp

W

a

J

Cp = 9.81 1.05 x L a x Wc + (Wc x J) + ( x Wc x L) + ( x W) (N)2

8 x Ja ( )K = 9.81 0.05 x L a x Wc + (Wc x J) + ( x Wc x L) + ( x W) x V (kW)

2

8 x Ja ( ) 1000

c c

c c

LAYOUT J

Chain rolling, material carried. Return strand unsupported.

LAYOUT K

Chain rolling, material carried. Return strand on idlers.




4

CALCULATING COMPLEXCIRCUITS

For calculating chain pull Cp of complex circuits, which do not

conform to one of the layouts A to K (page 80), the followingmethod ca n be use d a s a guide, or Renold Applica tions

Department may be contacted.

On the circuit shown in fig. 24, loads are suspended from

stayb ars a t 304.8 mm (12") spacing a nd ca rried b y two c hains.

Eac h stayb ar ca rries 20 kg load. The loads are put on the

conveyor a t position H a nd unloade d a t the drive point. Cha in

sp ee d is 0.067 m /se c (13.2 ft/min). A 152.4 mm (6") pitch cha in is

to be use d running o n 12 tooth sprockets to ensure ad eq uate

clearance of the loads at each turn.

The chains are spac ed a t 1.5m centres a nd ea ch stayba r has a

mass of 3 kg. On all horizontal and inclined sections the chain is

supported on tracks a nd runs on its rollers. Ass ume occ as iona l

lubrication.

To ca lculate the ma ximum c ha in pull it is first nec es sa ry to

estimate a cha in mas s. This ca n be either an educa ted gues s, or

a typical chain mass from the Renold cata log ue, or by using a

guideline such as the staybar (attachment) mass.

For this e xample we w ill use the sta yba r mass .

Ma ss of s ta yba rs = 3 kg at 304.8mm spa cing

3 x 1000 kg /m

304.8

= 9.84 kg /m

Total estimated ma ss o f cha in + stayb ars

Wc = 9.84 + 9.84 = 19.68 kg /m

Mass of load

Wm = 20 kg a t 304. 8 m m s pa c in g

Ma s s of lo a d pe r me tre = 20 x 1000 = 65.62 kg /m

304.8

For initial c a lcula tion a ss ume co efficient o f friction c = 0. 15

..

. s1 = (c x cos 30) - sin 30

= (0.15 x 0.866) - 0.5

= 0.37

..

. s2 = (c x cos 30) + sin 30

= (0.15 x 0.866) + 0 .5

= 0.63

At the bend s ections it is nece ss ary to estab lish the bend fac tor ec.

c = 0.15

= 30 i.e . 0.524 rad ians

..

. c

= 0.15 x 0.524 = 0.0786

..

. ec = 1.082

To esta blish the tota l cha in pull it is neces sa ry to brea k the circuit into

co nvenient se ctions a s in fig. 24, i.e. A to R. Cha in pull in thes e

sections can be calculated separately and the values added together

sta rting a t the point of low es t tension which is immediately after the

drive sprocket.

Each type of section can be ca lculated a s follows :-

Vertica lly upwa rd

Pull = [(Wc + Wm) x L] x 9.81 (N)

Vertically downward

Pull = [(Wc + Wm) x - L] x 9.81 (N), i.e. negative

Horizonta l sec tion

Pull = [(Wc + Wm) x L x c] x 9.81 (N)

Inclined se ction

Pull = [(Wc + Wm) x L x s2] x 9.81 (N)

Declined section

Pull = [(Wc + Wm) x L x s1] x 9.81 (N)

For 180 sprocket lap

Pull = Tota l pull at en try x 1.05 (N)

For 90 sprocket lap

Pull = Tota l pull a t entry x 1.025 (N)

For bend section

Pull = Tota l pull at entry x ec (N)

Chain pull calculations for the example would be:

Section

A - Horizonta l S ection Cumula tive Tota l (N)

[(Wc + Wm) x L x c] x 9.81[(19.68 + 0) x 1 x 0.15] x 9.81 = 29N 29

B - 90 s procket la p

29 x 1.025 30

C - Ve rtic a lly d ow n

[(Wc + Wm) x -L] x 9.81[(19.68 + 0) x -3.6] x 9.81 = - 695N 0 (-665)*

D - 90 s pro cket la p

0 x 1.025 0

E - Ho riz on ta l s ec tio n

[(19.68 + 0) x 16.5 x 0.15] x 9. 81 = 478N 478 (-665)*F - 90 s procket la p

478 x 1.025 490

G - Ve rtic a lly up

[(Wc + Wm) x L] x 9.81[(19.68 + 0) x 1] x 9.81 = 193N 683

4 m

R Q

P

N2m

RADA B

3.6m

C

D

J

1m

UNLOAD

3m

10 m

MKI

L

H1m

F

15 m

E

16.5 m

DRIVE G

LOAD

Fig. 24 Sideview of conveyor circuit




4

H - 90 s pro cket la p

683 x 1.025 700

I - Ho riz onta l s ec tio n

[(19.68 + 65.62) x 15 x 0.15] x 9.81 = 1883N 2583J - 180 s procket la p

2583 x 1.05 2712

K - Ho riz onta l s e ctio n

[(19.68 + 65.62) x 15 x 0.15] x 9.81 = 1883N 4595L - 180 s pro cke t la p

4595 x 1.05 4825

M - Ho riz o nta l s e ctio n

[(19.68 + 65.62) x 10 x 0.15] x 9.81 = 1255N 6080N - Bend s ection

6080 x ec

6080 x 1.082 6579

P - Inc line d s ec tio n

[(Wc + Wm) x L x s2] x 9.81[(19.68 + 65.62) x 3 x 0.63] x 9.81 = 1582N 8161Q - Bend s ection

8161 x ec

8161 x 1.082 8830

R - Ho riz onta l s e ctio n

[(19.68 + 65.62) x 4 x 0.15] x 9.81 = 502N 9332 (-665)

..

. Total chain pull Cp = 9332 N *NOTE: The neg a tive figure is

igno red w hen e stab lishing

chain strength required.

Howe ver, this figure is ta ken

into a cco unt when c alculating

headshaft power or torque.

Using a ge neral safety fac tor of 8, then cha in breaking loa d

required wo uld be:

9332 x 8 = 37328 (N) per cha in

2

As a hollow be a ring pin cha in will be req uired for fitting the

staybars then 2 strands of 54 kN (12000 lbf) chain of 152.4

mm (6'' pitch) would be suitable. It would now be correct to

reca lculate the a bove using the ac tual mass of

54 kN (12000 lbf) chain and c from the friction fa ctors liste d

on page 72 table 2.

From cata log ue, Chain total mas s Wc = 4.89 kg/m per chain

For two cha ins = 9.78 kg /m tota l

To ta l m a s s of c ha in + s ta y ba rs = 9. 78 + 9. 84 = 19. 62 kg /m

Coeffic ient o f fric tion c = 0.14 (occa sional lubrication)

By reca lcula ting , the ma ximum c ha in pull would b e 8805 (N) with

negative value 665 (N).

S a fety Fa ctor = 2 x 54000 = 12.3

8805

This is q uite s a tisfa ctory.

Due to the bend s ections it is nec ess ary to chec k the imposed

roller load due to the bend and staybar loads.

Lo a d a t s ta yb a r = [20 + 3 + (9.78 x 304.8)] x 9.81 (N)

1000

= 255 N

Loa d per roller R = 255 = 127.5 N

2

B e a rin g a re a of ro lle r = 420 m m2

(Se e pa ge 12 table 3)

Bea ring press ure P = 127.5 = 0.3 N/mm2

420

Imposed load due to bend = Pull at exit (N) x Pitch (m)

Bend Rad (m)

On reca lcula ting , pull a t exit of top b end (Q) = 8337 N

..

.Impos ed loa d = 8337 x 0.3048 = 1271 N

2

Impo se d lo ad per ro ller = 1271 = 636 N

2

..

. Total roller loa d R = 636 + 127.5 = 763.5 N

B ea ring pres sure P = 763.5 = 1.82 N/mm2

420

As this series of chain has a sintered steel roller, maximum

allowa ble press ure P = 1.2 N/mm2

a t 0.5 m/se c.

Our figure of 1.8 N/mm2

is ab ove this figure but a s the cha in

spe ed is o nly 0.067 m/se c, the P VR value for a verag e c onditions

can be checked by the method shown on page 73-74.

Rubbing speed VR = Cha in spe ed (m/sec ) x roller bore (mm)

Roller dia (mm)

= 0.067 x 23.6 = 0.033 m/sec

47.6

..

. P VR = P re s s ure x Ru bb in g S pe e d

= 1.82 x 0.033 = 0.06

Maximum P VR for average condition for a sintered steel roller is 0.30.

..

. The sta nda rd roller is sa tisfa ctory.

..

. Use 2 strand s o f 54 kN (12000 lbf) chain, 154.2 mm pitch,

cha in no . 105241/16.

Power req uired a t hea dsha ft = C p x V kW

1000

= (8805 - 665) x 0.067

1000

= 0.55 kW

PC D (12 tooth) = 588.82 mm ..

. RP M = V (m /s e c) x 60

PCD (m) x

= 0.067 x 60

0.58882 x

= 2.2 RP M

Hea ds ha ft torq ue = (8805 - 665) x 0.5882 Nm2

= 2394 Nm




4

BUCKET ELEVATOR -DYNAMIC DISCHARGE

This system incorporates a series of buckets attached at intervals to

one or two chains as shown. M aterial to be moved is fed into theelevator boot by an inclined chute. The buckets then collect it by a

scooping or dredging motion. D ischarge relies on the velocity to

throw the material clear of the preceding bucket.

a. High speed with dredge feed and dynamic discharge.

b. M edium speed with dredge feed and dynamic discharge.

This type is particularly useful for handling materials not exceeding

75mm cube. M aterials having abrasive characteristics can be dealt

with, but a high wear rate of buckets and chain must then be

accepted. Versions with both single and double strand chain are

commonly used, the selection of the latter type depending on the

width of bucket required. Two chain strands are necessary if the

bucket width is 400mm or more. It is usual to operate elevators of

this type at a chain speed of about 1.25 to 1.5m/sec, but each

application must be considered individually in relation to achieving

an effective discharge, the latter being dependent on the peripheral

speed of the bucket around the head sprocket. O ther important

factors influencing discharge are the type of material, bucket shape

and spacing.

Feed chute angles vary with the materials handled but are generally

arranged at 45 to the horizontal. M aterial should be fed to the

buckets at or above the horizontal line through the boot sprocket

shaft. Where bucket elevators are an integral part of a production

process, it is usual to have interlocks on the conveyor and elevator

systems to avoid unrestricted feed to any unit which may for some

reason have stopped.

The selection of the correct shape and spacing of the buckets

relative to the material handled, are important factors in efficient

operation. Spacing of the buckets depends upon the type of bucket

and material handled, but generally 2 to 2.5 times the bucket

projection is satisfactory. Bucket capacities as stated by

manufacturers are normally based on the bucket being full, but this

capacity should be reduced in practice to about 66% or water level

to ensure that the desired throughput is obtained.

Solid bearing pin chain is essential for other than light, clean duty

application. C hain pitch is normally dictated by bucket proportions

and desired spacing. M ild steel case hardened rollers should be

used but where these are not required for guiding purposes, smaller

diameter gearing rollers of the same material are preferred.

Due to the high loadings which can occur during dredging,particular care is necessary in ensuring that the chain attachments,

buckets and bucket bolts are sufficiently robust to withstand these

loadings. Normally K 2 welded attachments are used; fig. 26

illustrates typical examples. This means that lower chain speeds can

be used to effect adequate material discharge speeds, as the

buckets operate at a greater radius than the sprocket pitch circle

diameter. Integral attachments are not recommended for this type of

elevator.

The selection of the head sprocket pitch circle diameter is related to

obtaining correct discharge as described later. G enerally the head

sprocket should have a minimum of 12 teeth, otherwise the large

variation in polygonal action which occurs with fewer numbers of

teeth will cause irregular discharge and impulsive loading. This will

result in increased chain tension, greater chain wear and stresses onthe buckets. Where the material handled has abrasive characteristics

and/or high tooth loadings exist, steel sprockets are necessary. For

extremely high engaging pressure the sprockets should have flame

hardened teeth.

To aid bucket filling the boot sprocket size should be the same as

that of the head sprocket. Where abrasive materials are involved

boot sprockets should be manufactured from steel. Irrespective of

size or material handled the boot sprocket teeth should be relieved

to reduce material packing between the tooth root and the chain.

(See page 45 Fig. 3).

C hain adjustment is normally provided by downward movement of

the boot shaft, and allowance should be made for this in the boot

design. C ertain materials handled by this type of elevator have a

tendency to pack hard, and therefore material in the boot should be

cleared before adjusting the chains to avoid fouling.

O n long centre distance installations, guiding of the chain is

necessary to avoid a whipping action which can be promoted by

the dredging action. It is not always necessary to provide

continuous guide tracks, and common practice on say a 20m

elevator would be to introduce three equally spaced 2m lengths of

guide for each strand of chain.

Inclined elevators must have continuous chain guides irrespective of

the length of the elevator. The discharge sequence of a dynamic

discharge elevator is shown on Fig. 27.

TENSION TENSIONFEED

DRIVEDRIVE

BOOT

FEED

(a) (b)BOOT

RESULTANTFORCE

Y X

YX

Y

XRESULTANTFORCE

1

2

3

FrFg

Fc

Fg = Fc

Fg

Fc

Fr

Fig. 25

Fig. 27

Fig. 26




4

Newtons first law states that Every body will continue in its stateof rest or uniform mo tion in a straight line unless co mpelled tocha nge that sta te of rest or uniform motion.

The a pplica tion o f the first la w to a n elevator discha rge me a nsthat as an elevator bucket moves a round a cha in sprocket, at thetop o f the elevato r, then the ma teria l in the bucket w ill try toco ntinue in a straight line from a ny po sition o n the s procket. Thema teria l will therefore attempt to move in a pa th pa rallel to thetangent to the chain sprocket. At any point on the circular paththe material is attempting to move in a straight line and the onlyrestraining force is gravity acting vertically downwards.

At Position 1 (Fig. 28)

The g ravitationa l force Fg ca n be split into two c omponents:

a. Centripetal force towa rds the centre of the wheel which worksto ensure the material travels in a circular path. F c.

b. A component force at 90 to a., which presses the materialinto the bucket. Fr.

The ne tt effect o f the forces a t pos ition 1 is tha t the ma teria l isheld into the b ucket and no t allowe d to d ischa rge a t this p ointunless the speed of the chain is e xcessive. In that ca se thegra vitationa l force wo uld not provide s ufficient c entripeta l force toensure that the ma terial followe d the bucket path, a nd ma terialwo uld flow o ver the outer lip o f the bucket.


At pos ition 2 the g ravita tiona l force Fg and centripetal force Fc a rein line and for many materials handled it is the design objective toens ure tha t disc ha rge be gins to oc cur at this po int. To a chievethis the spee d of rotation sho uld be such that the gravitationalforce exactly balances the centripetal force required to maintainthe ma teria l in a circular pa th.

Given this situation the material is in effect weightless andimmed iately after the top c entre pos ition the ma teria l will beg in todischarge from the bucket, see fig. 30 for position 3.


At pos ition 3, w here the b ucket will completely discha rge, thegravitation force Fg can be resolved into two components Fr a ndFc . If the resolved component of Fc is not sufficient to ensure thatthe ma teria l will co ntinue o n a circular pa th then the ma teria l willdischarge on a tang ential path s ubject only to the effects of airresista nce a nd g ravity.

1. G ra vita tio na l fo rc e , Fg = mg2. C e ntrip eta l fo rc e , Fc = mvm

2

rm

Wh ere m = m a te ria l m a s s in th e b uc ke t, (kg )g = gravitation accelerat ion, i.e . 9 .81m/sec

2

.Vm = linea r velocity, chain speed - (m/sec ), at radius rmrm = radius of material from centre of wheel - (m).

3. At the ba lance poin t a t top cen tre mg= mvm2

(Pos ition 2) rm

4. ..

. Vm2

= rmg Vm = rmg

For heavy and coarse materials, such as coal or rock, it is the usualpractice to delay discharge until after the top centre position.

The formula th en is mo dified to :

5. Vm2

= 0. 7 rmg ..

. Vm = 0.7 rmg

This is the req uired linea r spee d for the ma terial a t a ra dius of rm

around a sprocket. It is no w pos sible to ca lculate the a ngle (seefig. 27 page 84) which is the angle at which the bucket willdischarge. For heavier materials s uch a s c oa l and rock this isusually a bout 40.

C e ntrip eta l fo rc e = m vm2

rm

Also the centripetal component of gravitational force at point ofdischarge. Fc = m g c os .

..

. mg c o s = m Vm2

rm

6. . . . cos = Vm2 where is the discha rge a ngle.

rmg

Note! It is us ual to ca lculate the discharge values at b oth the tipand the ba ck of the bucket. It is therefore nec ess ary to c alculatethe value o f cosfor both cas es using the respective values for rmand Vm.

When the material leaves the bucket it will have the tendency totravel in a straight line but will be immediately acted upon bygravity.

To d etermine the trajecto ry of the ma teria l it is then nec es sa ry toplot the path from this discharge point by resolving the initialvelocity into vertical and horizontal components.

The vertica l co mpo nen t will be :Vg = Vm cos (90-) = Vm Sin

and the horizontal component will be:Vh = Vm sin (90-) = Vm C os

RESULTANTFORCE

Y X

1

Fg

Fc

Fr

Y

X

2

Fg = Fc

Y

X

RESULTANTFORCE

3

FgFc

Fr

Fig. 28

Fig. 30

Fig. 29




4

At intervals of say 0.5 seconds it is now possible to plot the

trajectory of the material by using the following formulae:

7. Dv = Vgt + 1/2 gt2

8. Dh = Vht

Where Dv = vertical displacement (m) .Dh = horizontal displacement (m).

Vg = initial vertical velocity (m/sec).

Vh = initial horizontal velocity (m/sec).

t = elapsed time ( seconds) .

g = acceleration due to gravity, i.e. 9.81m/sec2

.

If the speed selection sets the discharge point at top centre then the

initial vertical velocity will be zero, and formula 7 becomes:

Dv = 1/2 gt2

Some elevator designs, in an attempt to economise on casing size,

use small head wheels and large bucket projections. In these cases it

is necessary to check the discharge characteristics for both the back

and the tip of the bucket rather than, as previously described, the

centre of gravity of the material.

In such designs it is possible to achieve a good discharge from

material at the tip of the bucket while material at the back of the

bucket has insufficient velocity and will fall back down the elevator,

and if the back of the bucket discharges correctly then the tip will

discharge before top centre. Such designs are false economy, as to

achieve an adequate capacity it is necessary to oversize the elevator

to compensate for the material recirculated by not discharging

correctly.

POSITIVE DISCHARGEELEVATOR

This type of elevator has a series of buckets fixed at intervals

between a pair of chains. T he method of pick-up on this type of

elevator is usually by direct filling of the bucket. The shape of the

buckets used, the angle back type, is such that they cannot dredge

efficiently and the material must be fed directly into them. A sensible

rule to ensure good filling is that the lowest point on the inlet chute

should be two bucket pitches above the highest position of the

tail/tension shaft. Some material will inevitably escape and fall into

the boot but this will be only a small proportion of the throughput

and the buckets can be relied upon to dredge this small amount.

The chains are mounted on the ends of the buckets as shown in fig. 31.

This arrangement facilitates the discharge operation which, due to theslow speed of the elevators, is accomplished by inverting the bucket,

the contents of which fall clear, or in some cases slide down the back of

the preceding bucket.

Because of their particular function, and consideration of space,

snub sprockets are smaller than the head and boot sprockets. O n

high capacity elevators they are subjected to impact tooth loading,

caused by the reaction load of the hanging chain and buckets

descending after negotiating the head sprocket. This tooth impact

will be greatest at the bucket positions, and therefore if, as is usual,

the buckets are spaced at an even number of chain pitches, the

snub sprocket should have an odd number of teeth to equalise

wear. C onversely, if the buckets are unevenly spaced relative to

chain pitch, an even or odd number of sprocket teeth can be used

providing the selected number is not divisible by the bucket spacing.

The recommended snub sprocket position is shown in fig. 32, the

distance E-F being the equivalent of an equal number of pitches

plus half a pitch; normally 4.5 are satisfactory. Snub sprockets

should be of steel with flame hardened teeth.

To find the best position for the snub sprocket on a particular

application it will be necessary to first set out the discharge

characteristics of the buckets relative to the elevator discharge chute

to ensure a full and clean discharge, bearing in mind that the

material will fall out of the buckets due to gravity alone, and

secondly to calculate the chain length between the head sprocket

and snub sprocket to ensure that the even pitch length plus a half

pitch rule is observed.

The method of calculation of this length is as follows:

C ompare this with the theoretical ideal where a = 360

N

where N is the number of teeth on the snub sprocket.

Length EF = BC

C alculate EF in pitches, i.e. EF

C hain Pitch

This figure should be X + 1/2 where X is a whole number of pitches.

The minimum value must be 41/2 chain pitches.

C G

F

B

D

E

A

R1

R2

O

AC = AD2

+ CD2

in v ADC

AB = R1 + R2

B C = AC2

- AB2

in v AB C

Sin = BC in v AB C

AC

S in = AD in v AC D

AC

AD = C G

= 90 - ( + )

Fig. 32

FEED

TWOBUCKETS

SNUBWHEEL

DISCHARGE

HEADWHEEL

TENSIONWHEEL

DISCHARGEPOINT OFBUCKET TIP

MATERIALTRAJECTORYENVELOPE

CALCULATEDPOINTS

DISCHARGE POINTOF BUCKET BACK

HORIZONTAL DISPLACEMENT m.AT 1 SECOND INTERVALS

VERTICALDISPLACE

MENT

Fig. 31




4

BOX SCRAPER CONVEYOR

D e s c r i p t i o n a n d c h a i n t y p e

The box scraper type of conveyor can use either a single strand or

two strands of chain. The general construction is that of an enclosed

box or trunking in which the chain is submerged in the material. The

conveying movement relies on the en masse principle, where the

cohesion of the particles of material is greater than the frictional

resistance of the material against the internal surface of the box.

Because of this feature, a remarkably large volume of material can

be moved by using flights of quite small depth.

In general a depth of material approximately equal to 2/3 of the

conveyor width can be successfully conveyed by the en masse

principle.

The bottom surface of the box which supports the material is also

used to carry the chain (fig. 33).

When conveying non-abrasive and free flowing material, such as

grain, a chain speed of up to 0.5-0.6m/sec is practicable. For

aeratable materials howeve

chain pull calculations

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