ee

‘

\\

Helicopter TransmissionSystems

He Wharekura-tiniFiaihautu olflotearoa

THE OPE NPOLYTECHNICOFNEW ZEALAND

555—3—7

V’ CONTENTS

Transmission Systems

Transmission Components

Clutches

Engine-driven Clutch

Freewheel Unit

Gearboxes

Types of Gears Used

Main Rotor Gearbox (Single-stage Reduction)

Main Rotor Gearbox (Two-stage Reduction)

Tail Rotor Gearbox

Driveshafts

Main Driveshaft

Tail Rotor Driveshaft

Driveshaft Couplings

Other Components

Intermediate Gearbox

Universal Joint

Rotor Brake

Oil Cooler Fan Unit

555/3/7

\»,

AIRCRAFT ENGENEERKNG

HELICOPTERS ASSIGNMENT 7

TRANSMISSION SYSTEMS

The arrangement of drive shafts, gearboxes, and clutches thattransmit the power developed by the engine to the rotor blades isoften called the transmission or transmission system and, sometimes,the power train. In this assignment, we shall discuss thetransmission components of various types of helicopter generallyused in New Zealand.

The type and location of the engine determines the transmissioncomponents used in the power train. The smaller piston—enginedmachines have the engine mounted immediately underneath the mainrotor drive shaft, which is approximately the mid-point of thecentre-of~gravity range of movement. Fuel is carried in saddletanks (Hughes 269 and Bell 47 e and J series) or within the fuselageimmediately below the engine (Hiller UH 12 E series). Passengersand pilot are carried in a small cabin forward of the engine, andcargo is carried on external racks or on a cargo hook underneaththe fuselage at the ideal c. of g. position. This arrangement meansthat the centre-of—gravity movement due to fuel usage is kept to aminimum and the helicopter itself has a compact shape.

The large piston—engined machines had the engine mounted inthe nose, the main rotor gearbox mounted above the passenger cabin,the fuel carried in a bath—tub section under the passenger cabin,and the pilot and copilot carried in a separate cockpit sited aboveand behind the engine. This arrangement provided a large cabin forpassengers and cargo and kept the centre—of—gravity movement due tofuel usage to a minimum.

3/8lI5é 555/3/7

_ 2 _

The gas turbine engine, because of its smaller size and mass,is usually mounted on or near the cabin roof close to the main rotorgearbox, and the fuel is carried in a bathtub section beneath thecabin. This arrangement gives a compact helicopter and a very goodseating position for the pilot.

Figure l shows the power—plant locations for four types ofhelicopter used in New Zealand. Note how the relatively heavypiston engines are installed equally about the centre line of themain rotor driveshaft. The much lighter turbine engines arepositioned so that roomy cabin and baggage spaces can be dispersedevenly about the mid centre of gravity position. This gives greaterpassenger comfort and easier cargo loading.

TRANSMISSION COMPONENTS

Each helicopter manufacturer designs components to suit hisown helicopter and, while the function and operating principle ofa component will be the same from one type of helicopter to another,its constructional details are usually very different. In thefollowing pages, we shall describe the components of the power train,using, where needed, two varieties of each component as examples.

The essential components of a transmission are

l. An engine-driven clutch (on piston-enginehelicopters),

2. A freewheel unit,

3. A main rotor gearbox,

H. A tail rotor gearbox, and

5. Driveshafts from the engine to the mainrotor gearbox and from the main rotor gearboxto the tail rotor gearbox.

555/3/7

_3..

?i‘Q1i>"':

’ f a_“;%=n§ \\l , MtLl=wi" P““ ‘ a. \

» ea "S\

—~——-—-- ,_al_-- r<=§;t we a

Sue

1_) !%i'$imE§#=§;m

-

\_

EL fig‘ »

/-‘ta,_ ._ .=-.-.-1.

I .

A ‘ , rf " " _' Q'1 ,_,., “ /’ \\

$FIG. l Engine locations

555/3/7 ‘

~n-.o'

_ 4 -

The engine-driven clutch is necessary only on piston-enginedhelicopters and, rarely used now, on turbine-engined helicopters,which employ a direct mechanical drive between the compressor andthe helicopter transmission. The clutch is fitted to allow theengine to start without turning the transmission at the same time.Two basic types of clutch are used. The first type engagesautomatically once the engine has reached a low rev/min, and thesecond type is controlled by the pilot, who can select clutchengagement or disengagement as he wishes. Advantages of the secondtype are that the engine can be started and warmed up and passengerscan enter and leave the helicopter with the engine running andwithout the rotors turning.

A freewheel unit, also called a one-way sprag clutch or,simply, the sprag clutch, is fitted between the engine (or theengine and clutch combination) and the rest of the transmission.See Fig. 2. It allows the main rotor to turn at a higher equivalentrev/min than the engine. This condition occurs during autorotationor when power is substantially reduced for a steep approach to alanding site. If this unit were not fitted, the main rotor wouldtry to drive the engine_during autorotation, which would cause arapid loss of main rotor rev/min and a very high and uncontrollablerate of descent.

The main rotor gearbox OP main transmission reduces the engine

rev/min to the low rev/min needed for efficient operation of themain rotor head and blades assembly. This gearbox often has anexternal drive for the tail rotor and drive—mount pads for agenerator or alternator, an hydraulic pump, and a tachometergenerator. The other components that may be driven from thisgearbox are an engine cooling fan, for piston engines, and an oilcooler fan for cooling the lubricating oil, which is circulatedthrough the gearbox by its own oil pump. The carcass of thisgearbox is often used to carry flight loads from the main rotorhead into the airframe structure and to provide rigid attachmentpoints for servo control units. The gearbox is vented to theatmosphere.

555/3/7

é

e

_ 5 _

The tail rotor gearbox changes the direction and rev/min ofthe tail rotor driveshaft to suit the requirements of the tailrotor. This gearbox consists of an input and an output gear,meshing in an oil bath inside a sealed casing, which is vented toatmosphere. The tail rotor assembly is mounted on the tail rotordriveshaft. The casings of this gearbox carry flight loads fromthe tail rotor into the airframe structure and provide attachmentsfor the tail rotor pitch—change mechanism.

Driveshafts run from the engine to the main rotor gearbox andfrom the main rotor gearbox to the tail rotor gearbox.

Alternatively shafts may run directly from the engine to boththe main rotor gearbox and to tail rotor gearbox. However thesystem is laid out, the main rotor is always mechanically connectedto the tail rotor so that one rotor cannot turn without the otherturning.

The drive shafting consists of aluminium alloy or steel tubingsupported on grease-lubricated ball or roller bearings. It isconnected to the gearboxes and the engine by drive couplings, whichprovide for some degree of malalignment due to movement of theengine on its flexible mounts and for the twisting and expansionand contraction of the airframe.

Some further transmission components that may be found inhelicopters are

l. An intermediate gearbox between the main rotorgearbox and the tail rotor gearbox,

2. A universal joint between the two gearboxes,

3. A rotor brake,

M. An oil cooling fan unit, and

5. An engine cooling fan used on piston—enginedhelicopters.

The intermediate gearbox, placed in the run of the drive

shafting between the main rotor gearbox and the tail rotorbox,changes the direction of the drive to the tail rotor. The gearboxconsists of an input and an output gear meshing in an oil bathinside a sealed casing, which is vented to atmosphere.

555/8/7

_ 5 _

The universal joint, or Hardy Spicer coupling, is used, like

an intermediate gearbox, to change the direction of the drivebetween the main and tail rotor gearboxes. This joint is not usedin the larger helicopters or where the change in direction is large.

The rotor brake, is usually fitted close to the main rotorgearbox on the drive shafting to the tail rotor gearbox. It providesa means of quickly stopping the rotors after the engine has been

shut down.

The oil cooling fan unit, sited close to the main rotorgearbox, is driven by the main driveshaft or the tail rotordriveshaft. This component provides a means of cooling the mainrotor gearbox lubricating oil.

The engine cooling fan, used on piston-enginedhelicopters,supplies air to cool the engine and the engine oil cooler. It canbe driven directly by the engine or from the main transmission.

The drive to this component is arranged so that it turns anddelivers air whenever the engine is running.

555/3/7

..7-.

Main - roior gear-box. WTT ~Frge -wheel um? 1- »Cluich

I Refer brake

\ .Di Drive.-shaft Universal dnve

Engine ~cao(:'ng - Engine 5€an

(a) Vertical piston engine

Main rotorgar-be-X

Fra.-wheel uni?_ O/ \

Drive. - shaff Tai;- shsfi gear-boxClutch

El’\glfl€ I I

Engine-cooling Fan

(b) Horizontal piston engine

Main V070!‘gear ~ box

Drive. 5hafl'\‘Free -whul unit

E\ Oil cooling uni? \Tail roinrgear -box

Engine

(c) Gas turbine engine

FIG . Power trains2

555/3/7

O

Tai I rotorgearbox

-8-

SUMMARY

A clutch must be fitted if the engine is mechanicallyconnected to the power train. Thus, §§§_piston—enginedhelicopters have a clutch, but gas turbineengines usually do not.

A freewheel unit is fitted in all power trains so thatthe engine can turn more slowly than the main rotor.This ensures that the main rotor cannot drive the engine.

Gearboxes are used to change the direction of the drive,to speed it up, and to slow it down to suit the needs ofthe main and tail rotors.

Ancillaries can be driven from the main rotor box and fromthe driveshafts.

PRACTICE EXERCISE A

State whether each of the following statements is trueor false:

l. A freewheel unit allows a piston engine to bestarted without all of the power train turning.

2. The main rotor gearbox increases the rev/min ofthe engine to suit the requirement of the mainrotor. .

3. "Sprag clutcH'is another name for a freewheel unit.

4. The only drives taken off the main rotor gearboxare the rotor tachometer and the tail rotor.

5. To provide a quick warm up, the engine cooling fanturns only when the main rotor turns.

6. A clutch is fitted to allow the engine to bestarted without the transmission being turned atthe same time.

7. An intermediate gearbox is situated between theengine and the main rotor gearbox.

8. The tail rotor always turns when the main rotorturns.

9. The freewheel unit is fitted in the power trainbetween the engine and the main rotor gearbox.

l0. Drive couplings provide a rigid connection betweenthe drive shafts and gearboxes.

(Answers on page 43)

555/3/7

_ 9 _

CLUTCHES

Engine-driven Clutch

Two main types of engine-driven clutch are used. They are

l. The centrifugal, and

2. The belt drive.

The large piston-engined helicopters having powers of 600

b.h.p. and upwards used an oil-operated hydro-mechanical clutchvery similar to an automotive fluid flywheel. The lower—poweredpiston-engined helicopters use either the centrifugal or the belt-type clutch, the belt type being particularly reliable and troublefree.

Centrifugal clutch: As the name suggests, the clutch isoperated by centrifugal force. Splined to a drive ring bolted tothe engine crankshaft is a spider, and freely pivoting on thisspider are four massive steel shoes with brake lining materialriveted to their outer surfaces. On the input shaft of the mainrotor gearbox is splined and secured a sturdy steel clutch drum,which encircles the spider and shoes assembly. Immediately theengine is turned, the shoes are flung outward by centrifugal force,contact the clutch drum, and slowly start to turn the main andtail rotors. As the engine fires and its rev/min build up, theshoes are held more firmly against the clutch drum until, at about70% engine rev/min, the clutch drum rev/min become nearly thesame as those of the engine. No further increase in engine rev/minwill decrease the remaining slippage. The engine rev/min arenow reduced for a moment and then slowly increased and, from thetime the engine rev/min equal those of the clutch drum, no furtherslippage takes place and the drive between the engine and the restof the transmission system is complete.

The reason for the initial constant slippage is that thecoefficient of sliding (kinetic) friction is slightly less thanthat of static friction. Once the shoes are slipping, as theymust at initial startup, they will want to keep slipping. when wereduce the engine rev/min below that of the clutch drum and then

555/3/7

_ 10 l

restore them, a time comes when both rev/min are the same (theclutch drum slowing down due to friction and drag in thetransmission and the engine rev/min increasing). At this moment,the higher value of static friction takes effect and no more slippageoccurs.

Figure 3 shows a centrifugal clutch assembly. The drivefrom the engine is through the splines(1HJ of the spider (5),the four heavy clutch shoes (8), and into the drum (l). The splines(13) of the drum engage on the splines of the transmission lowersun gear, and the complete clutch assembly is secured to thisshaft by a large nut. The hearing (15) locates and supports thespider centrally in the drum.

1. Clutch drum2. Bolt - bearing retainer l ..3. Retainer u©u A \4. Bearing _ ~5. Spider “ a6. Lock ring 7. Pivot pin8. Clutch shoe assy

‘ O ‘ ‘ 9,~$_,/\

9. Bushing10. Shoe

F ’ 6~<i “ts‘<" " 0§\ewst

11. Plate12. Lining

i w 1 .

ixé13. Clutch drum drive splines14. Spider drive splines15. Drum - spider bearing

,-.\ ,.\

lb

c:3'//a;& [:3 E: -._..

,/T’» »)>»>¢

Illlllllllllll \\§,' \ 4 "" g\ fig g >

lllllll U U 11 |

\\\\\\\\\\\\\\\\\\\\\\&§\\\\\\\\\ \ \\\w.\§\\\\\§Q\\\§\\\§\\\\\\\\\

""' ~ \ ' e ~ w ' I§-

FIG. 3 Centrifugal clutch

555/3/7

_ 11 _

This clutch is sprayed with engine oil to cool the finneddrum and to lubricate the clutch shoe pivots and the drum-spiderbearing. The oil supply is part of the transmission lubricationsystem and is tapped from the engine lubrication system. Itflows all the time that the engine is running.

Belt clutch: This type of clutch consists of a drivingpulley connected to a driven pulley by a slack belt or belts.To make a positive drive between the two pulleys, the slack belt

is tensioned by a lever-operated jockey pr idler pulley. Thistype of drive is mechanically efficient, and if the diameters ofthe driving and driven pulleys are different, a reduction or anincrease in rev/min of the driven pulley can be effected.

The belt clutch used on one type of helicopter has eightbelts to take the drive from the engine-driven pulley to theslightly larger diameter~driven pulley so that the first stage ofreducing the engine rev/min to those needed by the rotor head ismade in the clutch. The belts are tensioned by a pilot-controlledlinear actuator connected to the idler pulley arm by a cable andspring assembly, the final tension on the belts being determinedby the tension applied through a cable_and spring assembly. Thelinear actuator is electrically driven and controlled by a three~way guarded switch in the cockpit marked engage, hold, and release,and its travel is determined by two internal limit switches. Thewiring circuit is arranged so that the engine starter motor cannot

be energised with the clutch engaged, as could happen if the enginehad been shut down with the clutch engaged.

Figure H shows a belt—driven clutch assembly with its eightmatched V belts. When the clutch is engaged, the linear actuatorcontracts and pulls the idler pulley assembly downward, compressingthe springs in the clutch-control spring assembly. These springsdetermine the pressure exerted by the idler pulley on the V beltset. when the clutch is released, the linear actuator extendsand tension comes off the clutch control spring assembly. To helpthe idler pulley remove its pressure on the V belt set, a returnspring is fitted to its pivot shaft.

555/3/7

e?\.

T0 mum \®®ACTUAJOR

IDLER PULLEYA§SY

ofw

DRIVEFROM ENGiNE

D.

/51 “’ es’‘:5’\

CLUTCH CONTROLSPRING ASSY

‘AR

ireclionrelation

’l">

\BELT DRIVEFRAME

of

ii

\ lE,will

-12-

am, tX 1'

\

.xrJ’m‘“<<<W\/i' ~‘*2

up x,‘' .'>""'—_—_'067

c._f—"#“~\§\<% /\<‘=9\

60.._/

»

/"T Ii;

Q @

@

V*BEl.T SH

~l

:§“::* \* 1 5»?. . . x .

STRUT ASSY

FIG. 4 Belt clutch

555/3/'7

1?)

‘Zcu/mo ASSY

D)

i T’€l:_-___.__li%.__

e-A» ,/l»\/3/

\ , \SrROLLER

-13-

All three pulleys are mounted on grease-packed heavy-dutyball bearings, and the upper pulley contains, within its hub, asprag clutch (freewheel unit). The belt-drive frame is steadiedat its bottom end by a strut assembly from the airframe, andthe complete belt drive assembly is enclosed by a detachableguard assembly. Any heat generated during clutch engagement andduring engaged running is dissipated by the free movement of airwithin the guard assembly.

Freewheel Unit

Two types of freewheel unit are used. They are

l. The cam and roller bearing type, usually called afreewheel unit,and

2. The sprag type, called a sprag clutch.

The word sprag means "a device to prevent a vehicle fromrunning backward". This suggests that a sprag will allow forwardmovement but will lock and prevent rearward movement.

Figure 5 shows the arrangement of driving member, drivenmember, and sprags in their retainer. Engine power is applied tothe driving member, which causes the sprags to try to roll to anupright position. This action locks the driving and drivenmembers together and completes the drive from the engine to thetransmission. To help the sprags in this action, a circular springis fitted to hold them in light contact with the driving anddriven members.

When the engine is throttled back, the driving memberimmediately slows down and the driven member turns faster thanthe driving member. The sprags roll away from their lockingcontact with the two members, and the transmission freewheels.

555/3/7

_ 14 _

l ----— SPRAGS/

1,' “‘%“\‘Avi‘/

‘Z

‘I’ ‘mo swamps HOLDspans: IN PO6|Tl0N1

1;’1 RETAINER’ . p '

V, . , V ':Q_~—

DRIVING MEMBER SPQKGQ

:2-"I-;'> .r \ A-f%:EE€S!!:§‘!!E§¢g’ AI, ” /sguz

.\C5::::§> ////€;;;;E;:\\\\ml

DRIVEN MEMBER

Homvmo, _'rn:_:v~m:e1.mo,

FIG. 5 A sprag clutchO

Figure 6 shows a cam-and-roller type freewheel unit indiagram form. The driven member has a series of cams cut on itsouter surface, and the driving member has a smooth inner surface.Between the two is a set of rollers with one roller to each cam.Engine power is applied to the driving member, which, as it turns,carries the rollers forward until they lock by wedging betweenthe cams and the driving member‘s inner surface.

When the engine is throttled back, the driving memberimmediately slows down, the rollers run down the cams to unlock thedriving and driven members, and the transmission freewheels.

555/3/7

. 9‘ DRIVING MEMBER

‘#0? 0 ODRHNNG

FIG. 6

The roller

FREEWHEEUN6

A cam and roller unit

s may be located in a ca eg or they may be free.One manufacturer uses a spring and nylon pack at each roller tolightly bias the rollers to the locked position.

"9

V ) smscwnm \ r _\, 1ASSY HUB =e, u 1

. rt.) A. ..;i~i. Q/' \ \/ ,4 1' gm

WASHRif

FIG 7 A s

///MI

‘.4

,4“\

--nu

Q/'\s@ $v\_,n@.i'..©."

p %. prag clutch installation

555/3/7

,-- j";\,_ ‘- - CLUTCH LOCKOSEAL: \\. Q

i. an WER @ _ Avx... pg I QFORWARDCAP fifilz <§§_ ll RACE ;* $ , \-

@* ’a\ \"¢:/ 1 "

mo LLJA @.r’€?;9@ Q

e

_ 15 _

Figure 7 shows a sprag clutch assembly and its fitment intoa helicopter transmission. In this installation, the sprag clutchforms the hub of the belt—driven clutch upper pulley and makes acompact and easy—to—service assembly.

This unit functions by the metal-to-metal contact of thesprags with the driving and driven members. To prevent damage tothese parts during freewheeling, that is, when they are slidingover each other, the unit is filled with an oil, which is checkedfor quantity and changed at intervals specified by the helicoptermanufacturer. The aft cap has removable sealed bolts, whichprovide access into the unit for oil filling and draining.

In the sectional view shown in Fig. 8, the drive comes from

the drive belts into the upper pulley, through the driving anddriven members of the sprag clutch, and into the input pinionshaft. The input pinion shaft is part of the main rotor gearbox,and the tail rotor driveshaft is splined on the aft end of thisshaft. Thus, the tail rotor driveshaft is directly connected to themain rotor gearbox, and the main and tail rotors can turn withthe upper pulley and engine stationary.

_ \\

/uvvsaPULLEY FRAME CLAMP

(BEARING CAP)

\\\\N

//.\\:

ix/“/'\?""i‘lilf.a§!h£‘!!F!//....‘@"‘\‘9//.1\‘AI4/1 ' I\\

%§§

vtri.A

SPRAG CLUTCH‘\ AFT BEARING

KL . ' Ila“ Ari O*RlNG 0_R,NG4", p 4 SPUNED DRIVElg P}? p SLEEVE7 \\\\‘@7/S ‘;_t.§:»~:*;"; .z77z'7'€*-:3‘x\I _,_. .... .,._.» Z .¢!Qa%&&c““,la““esssq<. 2

- fl.

Q»FORWARDO-RlNG

l_<.......-.....__V,

é §\;.~=“ “*5i '“:.l'£'ii:"i,::".e-./i / r

We Biiiiiie "i F§lY””m§ i“ MTHNWNNi i Mm

‘“““"""‘ DRIVE SHAFTRETAINING NUT

lNPUTPlNlON’/if FORWARD HUB X ii ii '"“' ?'§L;;‘§; f1

FIG. 8 Sectioned view of sprag clutch and pulley

555/3/7

-17..

The cam-and-roller type freewheel unit shown in partsection in Fig. 9 takes the drive from the engine through theflanged ring gear (l), the drive head and gear wheel (2) the

5

rollers (M), the driven cam (5) and, finally, on to the stub

shaft (6). The unit is connected between the stub shaft (6) andthe main rotor gearbox by two flexible couplings and a robustdrive shaft.

.¢D so QD

\\\

. Drive head and gear / iwheel Q ’

3. Roller retainer - ° ' a

c> / _ abQ. Flanged ring gear Q ‘r

\~5- - 154. Roller @\ \ ~ "

ifin ‘/'ft?“ jwi’unis we ca?-13'‘T4-»“Nb-*1Q

5|aEu

. - g5. Driven cam g6. Stub shaft 97. Cap screw A 3)", Q8. Lobe of cam ll 4"‘ -"i~'i.C-

I l-1

- J._ ~ ’

.....1

i..'_ ....,_.i,/

s/,__.- ‘Mk

"r-\i p c,

6 9

9 T“iie M

li\\.\\ ' ' '“"

FIG. 9 A cam—and-roller freewheel unit

The unit is sealed and contains its own supply oflubricating oil, which is checked for quantity and changed atspecified intervals through the hole normally blanked by the capscrew (7).

555/3/7

-18..

l

SUMMARY

The two types of clutch in general use are

l. The centrifugal type, and

2. The belt type.

They are usually needed by and used with piston enginesonly.

The two types of freewheel unit in use are

l. The cam—and—roller type, and

2. The sprag type.

Both types perform the same function.

PRACTICE EXERCISE B

State whether each of the following statements istrue or false:

l. The centrifugal clutch starts to engage immediatelythe engine fires up.

2. When a belt-type clutch is engaged, the starter motorcannot be energised.

3. A sprag clutch locks in a clockwise direction, anda cam and roller clutch locks in a counterclockwisedirection.

4. Both types of freewheel unit are lubricated with oil.

5. Clutch—drive belts are unmatched and may bereplaced one at a time.

6. A centrifugal clutch is selected to engage witha linear actuator.

7. Air is circulated around the belt-type clutch todissipate heat.

8. The idler pulley assembly applies tension to thedrive belts.

9. A centrifugal clutch may be disengaged by runningthe engine at idle RPM.

l0. A sprag clutch performs the same function as a camand roller clutch.

(Answers on page 44)

_ 19 _

GEARBOXES

Types of Gears Used

In helicopter gearboxes, three types of gears are widely usedSee Fig. l0 (a), (b), and (c). The spiral bevel gear is alwaysused in preference to the straight bevel gear, and the spur gearis used more often than the helical gear.

Recently, a fourth type of gear has been developed by

Westland Helicopters. This is called a conformal gear, which iscombined with a helical pattern. See Fig. l0 (d), This gear willtransmit very high powers, is quiet in operation and, because ofthe high strength of the tooth section, a larger reduction inspeed can be had from a smaller driving gear than usual.

/Y1“ cm ,il||||||i||1||||1|11I1 y

“H \\\\n\ \\ n \||| \\ I * ‘(Q) 5pur Saar-s

"'//rm ////I/I/I]////W(ls) Heifcal gears

w-v<‘%fi?\1(<1) 5pi»~Q\ leave! gears Heiicai c0r\‘Fc9v"r*nc\i Sears

FIG. 10 Types of gears

555/3/7

_ 29 _

Main Rotor Gearbox(Single-stage Reduction)

A simple, compact, single~stage reduction main rotor gearboxis shown in exploded form in Fig. ll. It consists of a spiral bevelinput pinion (1) driving a spiral bevel ring gear (2) to give aspeed reduction of approximately M.5 to l. The ring gear iscarried on the drive shaft coupling (3), which is splined internallyat its top end for the mating external splines of the main rotordrive shaft. The driveshaft coupling is also splined internallyfor the lower driveshaft (H) to the oil pump and tachometerdrive (5). The input pinion and driveshaft coupling assemblies aremounted on heavy-duty tapered and plain roller bearings.

This gearbox is completely self-contained, with all partsand the oil supply being housed in the upper and lower housings (6)and (7). An oil pump (8) supplies oil at a low pressure to an oiljet at the top of the upper housing. This jet sprays oil on tothe driveshaft coupling upper bearing. Oil is also sprayed tothe disengaging teeth of the input pinion and bevel ring gear tocool and lubricate the teeth and it is fed to the aft bearings ofthe input pinion to ensure positive lubrication and cooling ofthose heavily loaded bearings.

Oil is added through a filler cap that incorporates a filterscreen. The oil level is checked by a ball-locked dipstick.

A magnetic drain plug and self-closing valve unit isinstalled in the lower housing. -All oil must pass over themagnetic plug, and so any magnetic material that may have beenworn from the gears and bearings is deposited on the plug. Thisplug can be easily removed for inspection and cleaning.

A removable fine mesh gauze filter is provided in the oilway to the input pinion aft bearing. -A screen (9) is fitted inthe centre of the coupling assembly (3) to prevent debris frompassing down the hollow main rotor driveshaft and into the gearbox.

555/3/7

@\

- 97. Lower housing8. Oil pump9. Filter screen

» 6). \~\

-21..

\ '\

{<1

lib».‘\

- §Q

\- _ _/-

\~Q8

er

.0‘

r__ 4»4

CZK;_

\e

Q

(0)11 _,, T

. A :

,2 ._ s,a1. input Pinion . ,_-.:*=““'*\\““‘ i

2. Bevel ring gear ' ;;r“»~ ‘ LN,3. Drive shaft coupling

' 4. Lower drive shaft ’ ,1 um %5. Oil pump and tachometer

dflve4"": .,6 Upperhoumn ¥§§:§

s\\\\i\i /.9)

\

v.--

Q.c 2.4‘ ll

555/3/7

/QJ‘ / 6 l

I

X ©\ fsa/Q

\ 1 l

~—@

2

0

41> be

FIG. ll Main rotor gearbox with single—stage reduction

-22..

Oil pressure and temperature are monitored by transmittingunits, which control warning lights in the cockpit. The completegearbox assembly is vented to atmosphere through a special fittingin the top of the upper housing. Air flow around the casings keepsthe assembly cool.

Figure 12 shows this gearbox assembled and in part section.

I £‘*'

, I mm was-'/"" omve sam-rwi ,2 .

, . .--OIL JET

:§:_“':§.__

\.\i*""“"Q

»ea?~<

'1 /It-I’“;~

'3|‘iZ!tII:I\._4*1\~,~,

,‘."'__E,21".

F".-1

Qu; , / _i.......s...__.. AW ;,,§5 '~ §3‘¥i?l$.9i"°‘

O‘ - — ~ illllliif ‘E 4- imam|I|jI| ' —i r

- _ ;n,f§:.-~

I-R1,.0%

\ ‘*2-‘“"1 lN|0N‘.55., »>= (INPUT)© . -' i; sum___ ___ _ ¢,&§g\_e_ RING cw:

" ' rill":F‘ _,, § N.» runs MESH FILTER(3Q. CAP SCREW

3": 1/~‘. "'1-. _5- 1.. ' 2. - . a _ .‘-“'3-'.:7l\‘~'1um 1"OIL PUMP mo ,:1 »§;__L';i_;; -§\“,.,,-,.-.\-. "Rm SH,“

'.-'

‘

mcnoumn DRIVE g coupunchi; ll MAGNETIC ua/am PLUG mo

5ELF~CLO5lN<5 VALVE

OIL PUMP DRIVERETAINER ASSY

FIG. 12 Assembled gear box in part section

The gearbox is braced to the helicopter centre frame by threestruts, and, through two shock mounts, it supports the drivingend of the engine.

The main rotor mast and the main rotor driveshaft are notthought of as part of the main rotor gearbox. However, they jointhe gearbox to the main rotor head, so we shall discuss them briefly

555/3/7

-23-

The main rotor mast is attached at its base to the upperhousing of the gearsecured by rivets to a cabin—section beam and by three support

box. At about its mid»height position, it is

struts to the centre frame. See Fig. l3.

L _ Wt‘ 4;;

MAIN ROTORDRIVE SHAFT

Qt; M / .RING NUTTHRUST BEA \~

THRUST BEARINGLOCKWASHERTHRUST BEARING

Ira? S\~r-»‘-‘~.~i»‘-;'_.._i u ._.f

2:1 .1 2*

ll.iwo

\\/\>swaur ATTACHMENT POINTS (3)

FRAME

:0‘?

TAIL ROTORTRANSMISSION

y

fiexz,

/

muewM\\s :—; Z‘’ ii, M \

’(:;3 ,// Q I 1 ‘I $1-

' d %J§/”/1/

\ ( ‘\ /I/’ /\ /

I I

V -BELT D R IVECOVER HAT

O

' \\ ,. ////\ 1/" ‘ TAIL ROTOR

MMNRMOR is , ,°\wmsr p V V V

MAINTRANSMISSIQN

Q \.,_ = Ii I \‘ “Kg I <§§&4£ "ii?I \ ‘

0* 7 (9 iCENTERFRAME ,1’, Q5 !::\\—~)

\

WWW" §x 1 w'1% muwwa¥> TRANSMISSIONSTRUTS

-

o/\. J"fhq<_~\_

_‘,,,.»-v-as

FIG. l3 Gearbox and mast in relation to rest oftransmission

555/3/7

DRIVE SHAFT

LT DRIVEBELT DRIVE I‘ COVER SHELL

I\\ A K T sen DRIVE¥ I FRAMESTRUTASSY

e I ICENTER FRAMESECTION

_ Qu _

The main rotor driveshaft is housed in the mast. At itslower end, it engages with the driveshaft coupling of the gearbox.At its upper end, it carries the main rotor head assembly. Thisdriveshaft is held in the mast by a heavy-duty thrust bearing andthrust nut.

Lift and thrust forces in the rotor head pass from therotating driveshaft and thrust bearing, through the thrust nut,into the stationary mast. The forces are then led through thesupport struts and gearbox struts into the centre frame.

The complete assembly of gearbox and mast provides attachmentpoints for the cyclic and collective controls.

Main Rotor Gearbox(Two-stage Reduction)

Figure 1% shows a transmission with a two-stage speedreduction. The first reduction occurs between the main inputpinion (1) and the main input spiral bevel gear (2), with areduction of about 3 to 1 being obtained. The second reductionis between the planetary assembly (8) and the planetary ring gear(R), with a reduction of about 5 to l. The total reduction, theproduct of the two ratios, is about l5 to l.

The gear ratio of a planetary system may be found from theequation

. No. of teeth on sun gear + No. of teeth on ring—gearRatio = r rNo. of teeth on sun gear

The drive from the engine arrives at the main input pinionadapter (5): and passes through the first stage reduction gears (1)and (2) to the main input gear shaft (6). From the internal splinesin (6), the drive passes to the planetary sun gear (8), the planetgears and planetary ring gear (9) and (R), to the planetary spider(l0), and then through splines in (10) to the main rotor mast (ll).

555/3/7

iiiiuiiw

(’l i ,.

.- ®@\. @—*./® as); A

-- Xi@-»(<t.l.))'\ / Cg’

€ill’((

I‘;. If

€:£Q-*'.1’;

\§E§2/'|fiNln'

Ii/ .

Main input pinionMain input spiral bevel gearPlanetary assemblyPlanetary ring gearAdapter main input pinionMain input gear shaftSupport assembly, lower mast bearingii

Ml)?3iii

pi er“i Main rotor mast

“ Top case assemblyiIliIIlIiI? l 13 Thrust bearing and seal assembly1% I

. Fé_,- §K’@

ems

Accessory drive gearMain case assemblyOil pump assemblyDrag pin assemblyOil filler cap with breather and strainer

I

,1/@ @

‘K

€\\\\\./itv

“"v'<ag,

‘ \(

\... ...V \)vv 5*

~v;»,,~'/

GR?\

I6

G\._.. _\9

"\\J\ Z

a‘ \(Q / R

a111$\&\/

3% / Q!

Ww->1/\I --\

FT 111 Main rotor gearbox with two-stage reduction

_ 25 _

The main rotor mast is located at its lower end by the lowermast bearing support assembly (7) and is secured in the top caseassembly (12) by the mast thrust bearing and seal assembly (13).

Lift and thrust forces from the rotor head pass down the rotatingmast through the thrust bearing (13), and into the top case (12).From there, the forces pass into the airframe through two pylonsupport links that attach to each side of the main case assembly(15) ~— see position A for the left—hand pylon support attachment.

The gearbox can pivot fore-and-aft on the pylon supports.This movement is restrained by the drag pin assembly (17) ofFig. 15, which links the gearbox to a rubber isolation mountattached to the fuselage. A positive mechanical restriction isprovided for the gearbox movement if the isolation mount fails.This mounting arrangement tends to dampen or remove any residualvibration from the rotor head and thus provides a smooth flightfor the airframe.

® -

(fig ‘:ZQ;§;z;z£Z3 Zzzafih S

0%..0,‘ _

,- 1» ' Q‘ 3!?‘ 6 "“‘.'L;§:?..riiiiiiiiiiii " Q i,' .'\\\\\\\\VV\ I ._ A _ U 6

wzfifi? 3 - ,fil - ‘QEER 2: i‘ ' . i" Z é ., . ,

@ - ‘ ‘V . ,‘\ /®\\\\\\\\§ g;;,\'§,

" ““\ \\1"'2—?-'!;;\ ~.\:m 1

FIG. 15 Sectioned view of Fig. 14 assembled

1....\ .\\\\\\\,nEhIlfl-ikznmuwi

Ifig‘ In

I

‘E|\§"/~n¢

....@’'§="1W

O_VJ’

555/3/7

n 27 *

The accessory drive gear (16) drives a small oil pump forthe lubrication system. It also drives the rotor tachometergenerator and the hydraulic pump that powers the servo controlsused in the main and tail rotor controls. The hydraulic pumpunit and the tachometer are attached to the external mount faceof the oil pump (16).

See Fig. 1M for parts list.

Figure 15 shows the assembled gearbox in part section.It is clearly a compact and simple unit.

This gearbox has its own lubrication system, which it shareswith the freewheel unit mounted on the engine accessory case. Thesystem, shown schematically in Fig. 16, consists of a simplecircuit supplying pressure oil to jets in the gearbox and arestricted supply to the freewheel unit. All oil drains to thebottom of the main case assembly, which acts as the oil tank forthe system.

THERMOBYRASSVALVE

. as init) cootze-T T

FILLER cm» I _‘l"mwvmr ,

,2 E TRANSMBSON~,_ 7 I ,/_ASSEMBLYI

I 1. A ENGINE ACCESSORY3‘ ii CASE. .

K ..°“~ LEVEL I'll’ "JETN0‘ 2 FREEWHEELING5'6“ °I~A$5"\ fi gr UNIT ASSEMBLY

I

BYPA/25 JEO. IHEAD VAL .:_ Y ‘ASSEMBLY I1, I —Fl;__,v_ . __ in

¢-j I

---»-.<..~rHim, TEMP 2,1 ‘1' PRESS REGULATINGWARNING swx -. _.,,_‘ . V‘ 1, __ ‘ . , VALVE

W c :$:r:fi¢@&:r-- I . rt AIT~

LTHERMOBUL8 .,? l<~ :rre§+s¥*' <~§¥¥¥:**?$"" *“““**‘l*”l."'“*** *“2l~i :.-1 . ~11 G '1:-oi; 1- "OIL MONITOR -~' W l "‘ - .

‘“ I """"""""" new PLUG a.DRAIN VALVE . , ,, , _ , - , , , , ,, MAQNETIQ Cmp

CARTRIDGE FILTER QIL PUMP ;é‘;,%;~ DETECTOR4.5-5 GPM _ W _ M Wm W —--> —DRAHVPLUG& " “' “' "G ~ .""* i“““*r"“" t “n

M new HPDé"1f3E¢T‘;)Rc ' 7 TO PILOTS PANEL GAGE J RE5TRiQTQR53. LOW PRESS. WARNING SW in Firriur;

FIG. 16 Main gearbox oil system

555/3/7

...I

-wmww-..i.~;-..,wu....,,».~1-

§9

‘Q

GHMMPHm30@_mapUGONOQHM®mGflflzPH._UH_m

\\_Y;_"‘;'\

6 \_pI_\\\\‘Ix?/I‘\)i

\\ __

‘Q_>_Ha_'__\‘_H’‘Q‘_ €\\X“ InbI‘

7.’1'?_i

0‘TQ_b_Q

zAW“figp//"_,_:____'”d4F_'_myfl"___/J,I_;p_‘____ _‘_ ‘_‘_

finA1,// p I

)\pm“;T/Q’_O Q‘

__\pi_\_hmVA‘ch_\,1/\\bK2Q5mV_,U‘T_A___\\1‘ppIV___m;H

6\Ap g

\P’Iv/I/I’/\y ___1/_

\_ _ pp ___y/”~I/"I,,$_w_m__“n____j\'_VfimH_I”if{NUI _

\IF”/’,_/V,M_/K__>_"fi>_:

p _Wm/J,‘_668__oX8gg/as/KM5J,_WW_"_%_%_|‘____'_zX0989L29=8_/p/7V_W‘%V/Iwzmgmmwcgw9%‘BE__E

_//hmmcgmémmm_

Q?/H'_p\_\_p_tgwtogH2

,5’;__.QIV_E5CE9:08=O_

_'‘___"‘_‘_‘K:mcmm>:_UCE

Q6cméwtog_U__m>>__om_

9___%8$25_E5_®2>>mm;_

X38%ByeEms3mcacm_w>_5_

Ems:59ENE_X86%L29ENE_

_ 29,-

The oil is cooled in a thermostatically controlled oilcooler, which is supplied with cooling air by a fan unit drivenby the tail rotor driveshaft.

Drain plugs, magnetic or electric chip detectors, temperatureand warning switches, inlet and outlet filters, and an adjustableoil pressure relief valve are used to control and monitor thesystem.

Oil is added to the system through a combined filler cap,breather, and strainer assembly, and the oil quantity is easilyseen through the oil-level sight glass.

Figure 17 shows the main gearbox in relation to the restof the power train. In this example, the drive from the freewheelunit goes forward into the main gearbox and aft to the tail rotorgearbox. Both drives are connected mechanically at the freewheelunit so that, when the main rotor turns, so does the tail rotor.

Tail Rotor Gearbox

The tail rotor gearbox is a simple gearbox with an input .gear and an output gear. A speed reduction/increase may bearranged between the two gears to suit the need of the tail rotor.The gearbox is usually mounted, with no vibration-absorbingdevices, directly on to the aft end of the airframe.

555/3/7

G

-39..

'//i ‘:9 £5 °

\ ROTATED ® p ‘/\° figV ~( / 0 I p Y ‘ I

Q

s\3’%C7 6).?

4.%l x\/T\(,\.Kg

I“ Aas

/b@"@ 2(eed ’ <%%§%E? \\

\ \> < ’

r Q‘ i

\

i If I J lav

1. Driven spline

S. Input bevel gear4. Output bevel gear5. Output gear shaft6. Ball bearing7. Ball bearing

...j;W

8. Roller bearing

10. Breather - filler assemblyl1. Liquid level plug12 Housing assembly13 Attachment points (four)14. Drain valve and chip detector

FIG. 18 Tail rotor gearbox

555/3/7

r l. A rotor tachometer, and

-31..

Figure 18 shows a tail rotor gearbox in exploded form. Thedrive from the tail rotor driveshaft passes through the drivenspline (l) into the input level gearshaft assembly (2). It thenpasses through the input and output bevel gears (3) and (M) tothe output gearshaft (5). The input shaft assembly is supportedon two ball bearings (6) and (7), and the output gearshaft issupported by a roller bearing (8) and a duplex ballthrust bearingassembly (9). The complete assembly is attached to the airframeby four studs at (13).

The gearbox contains its own oil supply, the gears andbearings being splash-lubricated by oil being thrown around asthe gears turn. The gearbox is filled through a breather/fillerassembly (10), with the oil level being easily seen through theliquid level plug window (ll). The oil is cooled by the air flowover the housing assembly (12). A drain valve and magnetic chipdetector (lq) is provided at the bottom of the housing to trap anymagnetic debris from the gears and bearings. This plug may beremoved for inspection without the oil being drained.

SUMMARY '

Main rotor gearboxes of modern helicopters have their ownoil supply and lubrication system.

The main rotor gearbox will be used to drive at least

5 2. An hydraulic pump.

i Gearbox casings may be used to dissipate heat to‘ circulating air.

Magnetic plugs are used in all gearboxes to trap magnetic ydebris.

555/3/7

- 32 _

PRACTICE EXERCISE C

State whether each of the following statements is trueor false:

l. The oil level in a modern gearbox is checked by adipstick or through a sight glass.

2. All main rotor gearboxes have at least two stagesof speed reduction between the input and outputshafts.

3. A gearbox is vented to atmosphere to prevent abuildup of air pressure inside the casings.

4. The number of teeth on the planet gears determinesthe speed reduction in a planetary gear system.

5. The output gearshaft of a tail rotor gearbox mustbe able to accept thrust from two directions.

6. A magnetic drain plug will trap all metallicparticles in the lubricating oil.

7. You can usually remove magnetic chip detectors froma gearbox without first draining the oil.

8. A tail rotor gearbox must give a speed reductionto the tail rotor.

9. A heavy duty thrust bearing is used to transferthe flight loads from the rotor head to a non-rotating part of the helicopter.

10. The tail rotor gearbox is usually directly attachedto the airframe structure.

(Answers on page 44)

DRIVESHAFTS

Main Driveshaft

The main driveshaft is a substantial but hollow shaft madefrom aluminium alloy or steel. It is attached at each end to theengine and main rotor gearbox by special couplings that allow somedegree of malalignment due to flexing of the airframe and movementof the engine on its mounts.

555/3/7

_ 33 _

Some main driveshafts are balanced during manufacture,and extra care must be taken with these shafts to ensure thatthey are fitted correctly. Item 3 of Fig. l7 shows a main drive-shaft with its couplings.

Tail Rotor Driveshaft

This assembly may consist of either one long driveshaftand its support bearings or two or more shafts coupled together.

These shafts may be made from steel or from aluminium alloy,and because they transmit far less power than the main rotor drive-shaft, they are of a much lighter construction. At each end ofthe shaft is a special coupling that allows for flexing of theairframe. Because of the length of the shaft, it is supported atregular intervals by bearings mounted in bearing hangers attachedto the airframe. These bearings are lubricated with grease.

Because correct alignment of this driveshaft is importantto the life of the shaft and the bearings, provision is made foradjustment of the bearing hangers. Thus, the shaft may be movedin any direction as desired to obtain an accurate alignment betweenits input and output ends. The shaft or shafts may be balanced,and they may also have to be assembled with a particular angularrelationship to the main rotor gearbox and the tail rotor gearbox.

Items(5)to(l2)of Fig.l7 show the assembly of the variouscomponents of a tail rotor driveshaft.

Figure l9 shows in some detail the tail rotor driveshaftof a light helicopter. This figure should be viewed with Fig. l7.Note that item (9) of Fig. l7 becomes item (l) of Fig. 19.

555/3/7

_ 3 L; ..

SEE vusw A"A

. i B31

., / \

Q ~ = ,’ \ """-' " U "_l_ \\\\\\\\\\x _Q '

_..'\..»...-- _.-_-> -———- ———

_[]._@ -:-._--. _ . . _ --‘ §\\\\\\\\\h/ 7/,” Q

‘ , l_ \ IA \ //_J is/\

-B

see DETAIL B

*'¥@A _ see DETAIL Bso-an

X

DETAIL A

E9 ailllli i

6"’\§' Qt

osmu. B

vuzw A"A

‘. II1*I’ ;

l.!.'

"beDETAIL C

1. Aft short shaft2. Forward shaft segments3. Aft shaft segments4.TaHrotorgearbox5. Steel disc assembly6. Splined adaptor

’ 7. Flange adaptor’ 8. Shaft adaptor

9.Beanng hanger10. Bearing11. Hanger bracket12.mputmnmnadamor

FIG. l9 Tail rotor driveshaft assembly

555/3/7

is \m “=11. T ."|@Q5 <5 l\<@/ @’>..

_ 35 _

Driveshaft Couplings

Couplings are used to connect the driveshafts to theengine, to the gearboxes, and to each other. The amount ofpower being transmitted determines the size of the coupling.Whatever their size, all couplings have the ability to allow forsome small degree of malalignment and relative movement betweenone transmission component and another.

Three types of coupling in common use are

l. The rubber coupling,

2. The Thomas, or disc, coupling, and

3. The splined coupling.

These types are shown in Fig. 20, 2l, and 22.

____\ RECTANGULAR. 4- " sscueur4

\. Q Q \. 25:22:5 . _ RECTANGULAR. 5' ' ‘ secuun Q

a ;e';%.2:': ~ 2 g \ / VANES5 ,

\\ 3 r Q \\\\ ‘.iV igéifi ' ' Q jg?

~ aw ® s. ., Q

FIG. 20 Rubber couplings

The rubber coupling consists of two metallic plates withvanes formed on one face of each plate. The coupling is assembledwith the vaned faces towards each other and rubber blocksinterposed between the vanes.

The driving plate transmits the drive to the driven platethrough the rubber blocks, which are bonded to the vanes. Thistype of coupling can be used to transmit high powers.

555/3/7

_ 35 _

FLANGE,_._..A ‘5 QT \v

/ * ‘iiiFLANGE ' u ,

\ tail Ea sallillli-fa.,\\\\\_\_\}.\_\.<Q<\

O C)O O

STEEL DISC —GOUPL|NGCONSISTS OF A PA€K

or DISCS

O O 0 Ohf)

END VIEW OF SHAFT QSECTION A-A

FIG. 21 A Thomas, or disc, coupling

The Thomas, or disc coupling, consists of a pack of thinsteel discs. The steel used is made with its grain structurerunning in parallel lines, which makes the steel easier to flexalong the grain than at 90° to it. The pack is assembled withthe grain of each disc at 90° to that of the disc above and belowit. This ensures that flexing can take place evenly in alldirections.

The discs have a series of bolt holes in them and aresecured to opposing flanges by alternate bolt holes. Thus, if acoupling has six bolt holes at 60° to each other, it will besecured by three bolts at l20° to each other to one flange andby the other three bolts to the other flange.

555/3/7

-37..

(‘P on-7 C?)so X O ‘/ ,

®_'7 J F. ( . /’ i_ s»@>/.5, _é(,,,,,L ..3_ , _ _ .1 _

\e1.Dnveshafi,2.SeaL 3.CoupHng,sphencalspHneinnen 4.Coupfing,spheflcalspHne<nnen 5.Pkue,greasereuuner

FIG. 22 A splined coupling

The splined coupling shown in Fig. 22 consists of a steelinner spherical coupling (3) bolted to the driveshaft (l).Surrounding the inner coupling (3) is the steel outer spherical

coupling (Q), which is bolted to the drive flange of the engineor main gearbox. The coupling is packed with grease and is sealedby the plate (5), the seal (2), and two O rings. A spring isfitted inside the coupling to hold the driveshaft (l) in about _its mid travel. One of these couplings is fitted at each end ofthe driveshaft. This type of coupling is used to transmit highpowers.

Of the three types of coupling, the splined coupling permitsmost movement of the driveshaft, the shaft being able to slidein and out, tilt up and down, and move from side to side. Thistype of coupling is needed to connect the main driveshaft to amain rotor gearbox that is not rigidly attached to the airframe.

OTHER COMPONENTS

Intermediate Gearbox

This gearbox is used where an abrupt change in the directionof a tail rotor drive is needed.

Figure 23 shows an intermediate gearbox. It consists of twospiral bevel gears supported on heavy-duty roller bearings in ahousing containing a bath of oil. As the shafts turn, oil isthrown by the gears up the oil feed pipe to positively lubricatethe output shaft upper roller bearing. The other bearings are

555/3/7

_ 38 _

lubricated by splash oil. Fins on the main housing dissipateheat to atmosphere, and duct stream air around the gearbox.ThecanoilThe

assembly has an oil filter and breather, a drain plug, whichbe a combined drain valve and magnetic chip detector, and an

that is easily seen from outside the helicopterl b 'lt and is firmly attached to a strength-

level windowgearbox is rugged y ui

ened part of the airframe.

OIL FEED PIPE

INTERMEDIATE .HOUS 1 N6 '

.1§§igFgi;

*'I.5|1,'*§@z

\\OUTPUTHOUSING

0» ..(9

5

T' ‘. ' -‘ . 2 .'_'I

mm ll’: qp.- .. .: .: @ O 39

' L 5.3:"*W=+&;x- ~41IIoat-wuw QN Q

._ Er

7: ‘. Q;-_

" V..

\ .'._'.: ._ _ ._

INPUT _;g;.__'_<:I_-._-13;‘ ':nousmc ,.-,1=.:1;'-;_z-.'-;._-. , , ~ _ - _.$ .~» 1 :~ ~ .- ~ ..‘ .222. jfifitqg. _ -_ ._-. _l

uzsammun@- ywp.;¢ H .'_‘. 5 -‘ ‘.' :._-.-‘ryj-v.,‘.’,_"‘_\"' ~” "‘'- - . ‘ .-'

%§?gq‘ii|iI _‘fi4_ 3$p¢. ,; p_~ -, .'_ ..vafii w.:HHu‘ no '~nm-In ‘\\¢"-jg! '-»~-»-gP' 1gmmmH fur "'- .?flflMl BFI "$1; " . °""wAv

&%%%§q9w€Q%TP~~@iT; U1Ia-.. \0, ‘I '_- 3:: 1.I _

Q

$%Rg*Fg: )7;%,,

*2? \

1;'

‘T *olL FILLER AND ~BREATHER \

._

*9

sq o o .¥ Y" V__~‘_{-., INTERMEDIATE

v HOUSING

Jr“

'II‘ I it

5 __. . s wmoow ANDREFLECTING PLATE

FIG. 23 Intermediate gearbox

555/3/7

OIL LEVEL INDICAUNG

_ 39 _

Universal Joint

The universal joint, or Hardy Spicer coupling, may be usedin the tail rotor driveshafting where only a small change in thedirection of drive is needed. It is an efficient substitute forcostly intermediate gearbox. .

Figure 2H shows a typical universal joint.

~ \

i:f7j1=.\\\fi§'Y;-AF-'"""' ~\\\\<

///,5? \\/\ e.,is

\ .

RUNNION

FIG. 24 A universal joint

These joints are used only in the smaller helicopters.

The Rotor Brake

This component is not essential for the functioning of thehelicopter transmission and, for many helicopters, it is suppliedonly as an optional.kit. The rotor brake provides a safe methodof rapidly bringing the rotors to rest in an emergency. It maybe very desirable for certain types of surveying and for thesafety and peace of mind of passengers entering or leaving thehelicopter.

555/3/7

..L{.Q...

The rotor brake consists of an aircraft-type brake unit,a hydraulic accumulator or relief valve, a hand~operated mastercylinder/reservoir assembly, and connecting flexible hoses andpipelines. A rotor brake 0N warning light may be fitted to theinstrument panel. The oil used is normally a mineral hydraulicfluid.

The brake drum or brake disc is bolted to the main driveshaftor tail rotor driveshaft and is sited as close as possible to themain rotor gearbox. The housing containing the brake pads orbrake shoes is bolted to a reinforced part of the airframestructure. The capacity oi and the pressure in, the hydraulicaccumulator govern the maximum force that can be exerted by themaster cylinder. Alternatively, a relief valve bleeding excesspressure back to the master cylinder is used. The master cylinder/reservoir assembly is located within easy reach of the pilot andis usually operated by his left hand. Provision may be made forlocking the rotor brake in its ON position.

011 Cooler Fan Unit

Most main rotor gearboxes have their own lubricating oilsystems, and an essential part of this system for all but thesmallest helicopters is an oil cooler or a positive means ofcooling the oil.

The oil cooler is needed because a large amount of heat isgenerated by the meshing gears. Unless this heat is removed, itwill overheat metallic parts, destroy the gearbox oil seals andpackings, and impair the quality of the lubricating oil.

The oil cooler on a fixed-wing aircraft has cooling air ductedthrough its matrix, the airflow being obtained from the forwardspeed of the aircraft. The helicopter, however, cannot rely on itsforward speed to supply the airflow. AS when hgveringaitg airspeedis nil.

To provide a cooling airflow, a fan driven off the main rotorgearbox or the tail rotor driveshaft is used. Air from outside

S55/3/7

..L(.]_..

the helicopter is ducted to the fan and then from the fan to theoil cooler matrix. In some installations, the fan and coolermay be joined together as one assembly.

In Fig. 1?, an oil cooling fan unit is shown as item (8)and consists of a squirrel cage type impeller driven by the tailrotor driveshaft. The engine oil cooler is mounted on the fanunit, and cooling air is ducted to the main gearbox oil cooler(l3) and around the finned hydraulic system reservoir.

M/‘MNROTOR

GEAR BOX

<5 )5/

Q . ,

DRIVE\. PULLEY coummc T0 omvs\ sum ATTACHMENT~ g L

\\§ ~\\YiE2§L\O {\

MOUNTING BRACKET @\""@ ,‘_ R

BLOWER

O

\////

°‘\

4//0%'/I/4/0

//

I

/ 1/ </’oO °)‘{/

.F

" 4 _.‘l /\/

FANA§§§g ,/// ,. ~5:§@ \\

\ / Q\‘\ ~\/’ T \ ,

/I 1 / Q / \5 ,;@§§ ' _ TJT FAN MOUNTING BRACKET ‘ ’ / .§é§\ f

\‘3 ~ Q . INPUT saw Al Kip‘ f’J )/DRWEPULLEY '

\

1/4,]00

//I00

6

‘ \/ft / /' M l.t \ ,/ 1 ‘ mum§ \ » TRANSMISSION

““ INPUTC[I

V t 1,/‘W // owum/COUPLING sou

%\ U ‘ \” e- o H,

‘*9‘ I //\\€‘,) 1

, \\?,/ wg

cuncn °°\\\\\\\\\\v/ coupuwe

FIG. 25 Oil cooler fan unit

555/3/7

...L{.2..

The oil cooler fan unit in Fig. 25 is belt driven off themain rotor driveshaft. Cooling air enters through two ram airscoops, is ducted around the main rotor gearbox casings, andpasses into the fan unit. The fan unit outlet air is ductedthrough the engine oil cooler matrix and to various places aroundthe engine. The air flow over the main rotor gearbox casings keepsthe gearbox at a safe working temperature. Because of the fanunit, the cooling air flows while the helicopter is hovering.

SUMMARY

Driveshaft couplings allow for some malalignmentbetween transmission components.

An oil cooler fan unit is often used to supply theessential cooling air for transmission and engine oilcooling.

PRACTICE EXERCISE D

State whether each of the following statements is trueor false:

l. The main purpose of an oil cooler fan unit is tosupply cooling air for the engine andtransmission oil systems.

2. Tail rotor driveshaft bearings are lubricated withgrease.

3. Because of their strength, straight bevel gearsare always used in place of spiral bevel gears.

4. A rotor brake is fitted as close as possible tothe engine.

5. A Thomas coupling consists of a stack of thin steeldiscs.

6. A splined coupling will permit very little axialmovement of a driveshaft.

7. A universal joint is normally used in the tailrotor driveshaft run when a small change in thedirection of the drive is needed.

555/3/7

....2l3...

8. Of the Thomas, rubber, and splined couplings, onlythe splined coupling must be lubricated.

9. An intermediate gearbox is used when the directionof rotation of the tail rotor driveshaft must bechanged.

lO. A rotor brake system has a device fitted to preventexcessive pressures being applied to the brakedisc.

(Answers on page 44)

ANSWERS TO PRACTICE EXERCISES

EXERCISE A

Statements 3, 6, 8, and 9 are true.

l. False. A freewheel unit is fitted to allow the powertrain to keep turning when the engine stopsor slows down.

2. False. The main rotor gearbox decreases the rev/min ofthe engine.

H. False. Other components that may be driven by themain rotor gearbox are an alternator/generator and ahydraulic pump.

5. False. The engine cooling fan will turn whenever theengine is running.

7. False. The intermediate gearbox is situated betweenthe main rotor gearbox and the tail rotor gearbox.

l0. False. Drive couplings are designed to be fairlyflexible to allow for malalignment of power traincomponents.

555/3/7

_ nu _

EXERCISE B

Statements l, 2, H, 7, 8, and l0 are all True.

3. False. Both types of clutch can be designed to lockin either direction.

5. False. Clutch-drive belts are supplied as a matchedset and must be installed as a matched set.

6. False. A centrifugal clutch engages because of therev/min of the engine. A linear actuator is used totension the belts of a belt-drive clutch.

9. False. With the engine at idle RPM, enough centrifugalforce is generated to keep the clutch engaged.The only way to disengage a centrifugal clutch is tostop the engine.

EXERCISE C

Statements 1, 3, 5, 7, 9 and 10 are all True.

2. False. Main rotor gearboxes can have one or morestages of speed reduction.

H. False. The speed reduction is determined by thenumber of teeth on the sun and ring gears.

6. False. A magnetic drain plug will trap only magneticmaterial.

8. False. Depending upon the manufacturer, the tailrotor gearbox may give a reduction, an increase, orno change in rotational speed.

EXERCISE D

Statements l 2 5 7 8 and l0 are all True9 9 3 9 '

3. False. Spiral bevel gears are always used because theyare stronger and quieter in operation.

555/3/7

_ ug _

H. False. A rotor brake is fitted as close as possibleto the main rotor gearbox.

6. False. A splined coupling is the coupling used whenaxial movement of the driveshaft is needed.

9. False. An intermediate gearbox is used to change thedirection of the tail rotor drive.

TEST PAPER 7

A main rotor gearbox has its first stage of speed reductionthrough two spiral bevel gears with 25 and HO teethrespectively. Its second stage reduction is through aplanetary system whose sun gear has AH teeth, and ringgear, 1H3 teeth. Make a diagram of this gear train andcalculate the total speed reduction.

Sketch, in reasonable proportions, a roller-and-cam~typefreewheel unit. Show the power input going to the innermember the power takeoff from the outer member and the5 9power being transmitted in a clockwise direction.

State the differences in purpose and operation betweenan engine-driven centrifugal clutch and a one-way spragclutch.

What ancillary components could be driven by the main rotorgearbox? What ancillary component must be driven bythis gearbox? Give reasons for your answers.

Sketch a driveshaft and coupling assembly that would beused to join an engine to a main rotor gearbox that rocks ina fore and aft direction. Give reasons for your choice ofcoupling.

Why must an oil cooler fan unit be fitted to most helicopters?

State two reasons for having a rotor brake fitted in thepower train.

555/3/7

...L}§_

Draw a diagram of a power train for a piston-enginedhelicopter. Include in your drawing

(a) An intermediate gearbox,

(b) An oil cooler fan unit, and

(c) A rotor brake.

555/3/7

/‘*

.*.¢5

N-8,MW,

5