bolt science report on torque-tension tests on lubrisilk ... · pdf filereport on fastener...

Report completed forAdvanced Lubrication Technology Inc.

Report completed by:William Eccles CEng BSc MIMechE

Bolt Science Limited(www.boltscience.com)

18th February 2008

Report onTorque-Tension

Tests on Fastenerscoated with

Lubrisilk Synthetic Grease

© Copyright Bolt Science Limited 2008

This document has been prepared by Bolt Science Limited in connectionwith an offer to supply services and is submitted on the basis of strictconfidentiality. Subject to any statutory requirement, the contents must notbe disclosed to third parties without the prior consent of Bolt ScienceLimited.

Report on Fastener Torque-Tension Tests using Lubrisilk Synthetic Grease

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Contents

Abstract and Introduction ................................................................................................................... 2

Test Method.......................................................................................................................................... 3

Results................................................................................................................................................... 4

Conclusion/Discussion.......................................................................................................................... 9

Appendix 1 Determination of the Coefficient of Friction.................................................................. 10Formula for the Coefficient of Friction.............................................................................................................10Nut Factor Determination ................................................................................................................................11

Appendix 2 Graphs of the results ...................................................................................................... 12

Abstract and Introduction

It was requested that torque-tension tests be conducted to determine the coefficient of friction of Lubrisilk#2 EP synthetic grease. The tests were conducted on M12 nuts, bolts and washers that had a zinc electro-plated finish. This type of finish is commonly applied to fasteners to provide some degree of corrosionprotection.

The friction coefficient was determined by measuring the clamp force produced by applying a tighteningtorque to the nut. A load cell was inserted into the joint to allow the clamp force to be determined. Thefollowing series of tests and results were recorded:

1. The primary test was carried out on fasteners where the threads and the nut and washer faces hadbeen coated with grease seven days prior to the test. A total of 10 samples were tested; the averagecoefficient of friction measured was 0.067 with a range of between 0.060 and 0.077.

2. An individual fastener was tightened (with grease on the parts that had been applied seven daysearlier) and then untightened and tightened again repeatedly 10 times without adding additionalgrease. This was to investigate the effect of re-tightening. The friction value was initially 0.070 reducingto 0.053 after 10 tightenings. On the second tightening the coefficient of friction reduced to 0.057 andthen remained reasonably constant.

3. A series involving five fasteners was tested using grease that had been applied just prior to the start ofthe test. From the 5 samples that were tested; the average coefficient of friction measured was 0.057with a range of between 0.050 and 0.063.

4. An individual fastener was tightened (with grease that had been applied just prior to the tests) and thenuntightened and tightened again repeatedly for 10 times without adding additional grease. This was toinvestigate the effect of re-tightening. The friction value on first tightening was 0.060 reducing to 0.050on the second tightening and 0.041 after the fourth tightening, it then remained reasonably constant forthe further six re-tightenings.

The grease was applied to the fasteners without them being de-greased i.e. the grease was applied to thefasteners in their ‘as received’ condition. (There was no noticeable oil or grease on the fasteners prior tothe Lubrisilk grease being applied.)

The Lubrisilk grease tested exhibits an exceptionally low coefficient of friction. Also, the repeated tighteningtests indicated that the Lubrisilk grease keeps the surfaces lubricated and is not displaced by the highpressures in the threads and under the nut face. Using the Lubrisilk grease resulted in the coefficient offriction reducing with re-use. It is uncertain why this should occur. The coefficient of friction for unlubricatedfasteners generally increases with re-use.

The placing of grease onto the fasteners several days prior to the test did not result in the coefficient offriction reducing as compared to fasteners being greased just prior to the test. The average value of thecoefficient of friction was slightly lower with fasteners greased just prior to testing compared to thosefasteners that had grease on them for several days. These tests were conducted on electro zinc platedfasteners. Whether a similar coefficient of friction will be exhibited on other finishes or on fasteners thathave no coating is uncertain.


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Test Method

The tests involved measuring the applied torque and the clamp force provided by the bolt simultaneouslyusing data collection equipment. All the tests involved applying a torque to the nut with the bolt head heldstationery. A load cell was inserted into the test joint so that the clamp force provided by the bolt could bemeasured. The load cell was connected to an indicator which in turn was connected to a 16 bit analogue todigital (A/D) converter. In a similar manner, the torque transducer was connected to a torque-tensionanalyser and then into the A/D converter. This was connected to a computer via an USB lead. Thecomputer was used to record the torque-clamp force values and to graph the results. The torque wasapplied to the nut via a hand wrench in a steady manner. The test arrangement is shown in figure 1.

Graphs of torque against preload (the clamp force provided by the bolt) could then be produced.Theoretically the plot of torque against bolt preload should be a straight line. Deviations from a straight linecan occur due to thread pitch variations, thread deformations and sampling error.

The basis of the test approach is in line with the standard DIN 946 (Determination of coefficient of friction ofbolt-nut assemblies under specified conditions). Software was used to analyse the data so that thecoefficient of friction and the nut factor could be calculated. Details of the calculation and the equationsused are shown in appendix 1.

Figure 1 Test Equipment Arrangement


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Results

A table presenting a summary of the results is presented below.

TestNo.

MaximumAppliedTorque(Nm)

MaximumBolt

Preload(kN)

Coefficientof Frictionµ

NutFactor

K

Age ofgrease onfasteners

Notes

1 50.7 42.39 0.066 0.0992 54.0 38.51 0.077 0.1123 51.0 46.43 0.060 0.0924 50.2 42.21 0.062 0.0955 50.4 37.69 0.072 0.1066 51.6 42.96 0.063 0.0957 51.9 38.92 0.070 0.1048 50.7 46.14 0.061 0.0939 51.6 45.19 0.064 0.09610 50.3 40.22 0.070 0.104

New

, pre

viou

sly

unus

ed, z

inc

elec

tro-

plat

ed n

ut, b

olt a

ndw

ashe

r us

ed fo

r ea

chte

st.

11 50.2 46.6 0.057 0.088 2nd tightening12 50.7 49.81 0.056 0.088 3rd tightening13 50.9 49.17 0.052 0.083 4th tightening14 50.6 49.97 0.049 0.080 5th tightening15 50.6 50.65 0.053 0.084 6th tightening16 50.3 50.07 0.054 0.085 7th tightening17 50.6 50.74 0.048 0.078 8th tightening18 52.3 50.46 0.057 0.089 9th tightening19 50.2 50.14 0.053 0.084

Gre

ase

appl

ied

to th

e fa

sten

ers

7 da

ys p

rior

toth

e te

sts.

Gre

ase

appl

ied

to th

e bo

lt th

read

, nut

bear

ing

face

and

nut

thre

ad a

nd w

ashe

r be

arin

gfa

ce w

hich

was

in c

onta

ct w

ith th

e nu

t.

Rep

eate

d tig

hten

ing

and

untig

hten

ing

ofth

e fa

sten

er u

sed

inte

st 1

0. N

o ne

wgr

ease

app

lied.

10th tightening20 50.7 47.36 0.052 0.08321 51.4 41.25 0.063 0.09522 50.9 45.45 0.050 0.08123 51.1 42.10 0.062 0.09524 50.1 43.13 0.060 0.092

New

,pr

evio

usly

unus

ed,

zinc

elec

tro-

plat

ed n

ut,

bolt

and

was

her

used

for

each

test

.25 50.4 50.97 0.050 0.081 2nd tightening26 50.9 55.38 0.045 0.075 3rd tightening27 50.1 57.18 0.041 0.070 4th tightening28 50.3 57.70 0.039 0.068 5th tightening29 51.1 57.96 0.041 0.071 6th tightening30 50.3 58.66 0.039 0.068 7th tightening31 50.1 57.64 0.038 0.067 8th tightening32 50.1 58.07 0.039 0.068 9th tightening33 51.6 57.63 0.041 0.070 G

reas

e ap

plie

d to

th

e fa

sten

ers

just

pr

ior

to

the

test

. G

reas

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, nu

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nd n

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Rep

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and

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ofth

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sed

inte

st 2

4. N

o ne

wgr

ease

app

lied.

10th tightening

Graphs for each test are presented in appendix 2. Graphs combining each test series onto a single chartare presented on the following pages. A summary of the test results is:

Test Series Averagecoefficientof friction

Minimumcoefficientof friction

Maximumcoefficientof friction

Tests with grease applied 7 days prior to the tests 0.067 0.060 0.077Tests with grease applied just prior to testing 0.057 0.050 0.063


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Figure 1 Summary of Test Results – Grease on fasten ers for 7 days


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Figure 2 Summary of Test Results for the repeated t ightening of a single fastener

Grease on fasteners for 7 days


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Figure 3 Summary of Test Results – Grease applied t o the fasteners just prior to testing


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Figure 4 Summary of Test Results for the repeated t ightening of a single fastener

Grease applied to the fastener just prior to testin g


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Conclusion/Discussion

The Lubrisilk grease tested exhibits an exceptionally low coefficient of friction. Lithium grease exhibits acoefficient in friction of the order of 0.13, molybdenum disulfide grease typically 0.09 compared to anaverage of 0.060 for the Lubrisilk grease after it had been on the fastener for seven days.

The repeated tightening tests indicated that the Lubrisilk grease keeps the surfaces lubricated and is notdisplaced by the high pressures in the threads and under the nut face. Using the Lubrisilk grease resultedin the coefficient of friction reducing with re-use. It is uncertain why this should occur. The coefficient offriction for unlubricated fasteners generally increases with re-use. For unlubricated electro-zinc platedfasteners, the coefficient of friction typically doubles (from 0.15 to 0.3) after five re-tightenings.

The placing of grease onto the fasteners several days prior to the test did not result in the coefficient offriction reducing as compared to fasteners being greased just prior to the test. The average value of thecoefficient of friction was slightly lower with fasteners greased just prior to testing compared to thosefasteners that had grease on them for several days.

These tests were conducted on electro zinc plated fasteners. Whether a similar coefficient of friction will beexhibited on other finishes or on fasteners that have no coating is uncertain.

William EcclesBolt Science Limited18th February 2008


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Appendix 1 Determination of the Coefficient of Friction

Formula for the Coefficient of Friction

The mathematical model relating the applied torque to the tension induced into the bolt isdescribed by:

2

2 cost

e n

F p dT D

µ µπ β = + + Equation 1

with eD2

o id d+= Equation 2

for metric threads, equation 1 can be simplified to:

20.15915 0.577352e n

t

DT F p d

µµ = + + Equation 3

where

T Total tightening torqueF Bolt preloadµt Coefficient of friction for the threadsd Nominal major diameter of the thread i.e. for a M12 thread d=12mmd2 The basic pitch diameter of the thread d2=d-0.6495pβ The half included angle for the threadsp Pitch of the threadµn Coefficient of friction for the nut face or bolt headDe The effective bearing diameter of the nut

The three terms inside the bracket in equation 1 represent the torque required to stretch the bolt, the torqueto overcome the friction between the bolt and nut threads and the final term is the torque needed toovercome friction under the nut face.

From equation 1, if the torque and preload values are known it is possible to determine the coefficient oftotal friction µtot by the formula

2

0.15915

0.577352

tote

Tp

FD

dµ

−=

+Equation 3

For the M12 thread used in these tests:

d2 The basic pitch diameter of the thread d2=d-0.6495p = 12 – 0.6495 x 1.75 = 10.863 mmp Pitch of the thread = 1.75 mmDe The effective bearing diameter of the nut = 15.0 mm

The coefficient of friction was calculated for every data point between 40% and 90% of the maximumpreload and the average determined.


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Nut Factor Determination

The nut factor is a combination of the coefficient of friction and the bolt/thread geometrical details. It is usedin a simplified formula to determine the tightening torque to be applied to a fastener, the formula is:

T = KFd / C

Where T = Tightening torque (Nm or lb-ft)K = Nut factorD = Nominal diameter of the bolt thread (mm or inches)C = A constant (1000 if the thread size is in mm and 12 if the thread size is in inches)

From the above formula, the nut factor K can be obtained using the following formula:

K = TC / (Fd)

The nut factor K was calculated for every data point between 40% and 90% of the maximum preload andthe average determined.


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Appendix 2 Graphs of the results


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