An investigation of friction clutches

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AIV INVESTIGATION OF FRICTIONCLUTCHES

BV

WILLIAM HERBERT ALMY

THESIS

FOR THK

DECREK OF BACHELOR OF SCIENCE

IN

MECHANICAL ENOINEERING

COLLEGE OF ENGINEERING

UNIVERSITY OF ILLINOIS

1911

nil

UNIVERSITY OF ILLINOIS

THIS IS TO CERTIFY THAT THE THESIS PREPARED UNDER MY SUPERVISION BY

ENTITLED

IS APPROVED BY ME AS FULFILLING THIS PART OF THE REQUIREMENTS FOR THE

DEGREE OF 05 O-C^-c/o-Or 0^ (U/c/V^C^

Instructor in Charge.

Approved: '^r

^d>^^2^A^ HEAD OF DEPARTMENT OF^ ^^?^^^^^^^^^:^

UlUC

J

COITTEITTS

Pag-e

IIITRODITCTIOIT 1

CMPTER I

DESCRIPTION OF CLUTCH 3

Section View of Clutch, Fig. 1 4

Photographic Details, Figs. 2, 3, 4, and 5 5

CHAPTER 11

DE SCRIPT I 0:J of APP.CRATUS 7

Diagram of Apparatus, Fig. 6 8

CHAPTER III

lilSTHOD OF TESTING ' 10

Photographs of Clutch Calibration Apparatus, Figs. 7 ,8, & 9. 11

Photograph of Clutch Test in Operation, Fig. 10 11

Clutch Calibration Data 12

Clutch Calibration Curve 13

CHAPTER IV

FORIvIULAE 15

Sample Calculations 18

CBAPTSR YPage

DISCUSSION ai;d COIICLUSIOITS 19

Requirements for Clutch Testing machine 23

Data and Curves from Tests of Discs Furniehed by:

Havana Manufacturing Company 24

Diamond State Fibre CQmpany 27

E.F.Houghton & Company 28

Wilmington Fibre Specialty Company 35

Continental Fibre Company 42

Photographs of Apparatus used in Tests 43

1

AIT HIVE ST I gAT I ON OF FRICTION CLUTCEP]S

INTRODUCTION

In choosing this subject for investigation the writer was

influenced by an interest in the subject of friction and its ap-

plications, and by the fact that a manufacturer was willing to

furnish a number of clutches for test purposes.

Since this is the first work of the kind attempted at the

University difficulties arose in collecting the required mechan-

ism and in finding suitable power for its operation.

The present investigation can be but a meager introduction

to the work that may be done at a future time when proper facili-

ties are at hand.

In making these tests the following lines of investigation

were attempted:

To determine the capacity of a line of clutches in pounds

at one foot radius

(a) Yihen the load is applied after the clutch is

set by throwing in the lever.

(b) YOieiL picking up the load by throwing in the

lever.

To determine the force necessary at the lover handle to

transmit any load up to the rated capacity.

To determine the actual axial pressure on the discs cor-

£

responding: to the knov/n effort applied at the lever handle.

To determine the actual coefficient of friction of the

several materials of which the discs are made when used in con-

tact v;ith cast iron.

To determine the apparent coefficient of friction on the

basis of torque at one foot radius and the axial force exerted

on the tapered sleeve by the clutch lever.

To compare the different materials for friction surfaces

used in these tests.

It is expected that some points will come up which will

prove of value in a subsequent design of a universal clutch test-

ing machine

.

Of the clutches usually found in factories the following

types or modifications of them are most common:

(a) Plane disc.

(b) Cone.

( c ) Rim and shoe

.

3

CiL\PT5R I

DE SCRIPT I PIT OF CLUTCH

The clutch used in these tests was furnished by the Havana

manufacturing Company, Havana, Illinois. It is a stock clutch

except that the same pulley was used for the several size's of

friction discs, which were twelve, fourteen, sixteen and eighteen

inches in diameter. This adaptation did not affect in any par-

ticular the normal action of the clutches and made it possible to

test four sizes in one, simply by changing the discs.

Referring to the section view. Fig. 1, and to the photo-

graphs, Figs. 2 to 5, a cast iron plate A having a long hub B is

keyed rigidly to the shaft. A pulley C having circular plates

cast integral with it on each end of the hub rotates freely on

the hub B. A circular cast iron plate D having a rim E on its

ribbed side is keyed to the hub B at F after t]je pulley C is in

place. This plate is free to move with the pulley axially but

must revolve with 3. The end of the hub 3 is threaded to take

the collar G which carries the two levers H and II. A^hen this

collar is screwed into place the step K on the end of the lever

nearest the pin, which holds it in place, is in contact against the

rim E. It is obvious that any motion of the ends of the levers

H and H a?/ay from the shaft centre will cause the disc D to press

against the pulley C which in turn presses against the plate A.

Between the plates are interposed free discs of suitable friction

» » » »

^ ^ ^ ^

6

material. The outv/ard motion of the small levers H and H is ac-

complished by the lever handle which moves the tapered sleeve 11.

7

CHAPTER 11

DE SCRIPT I on OF APPARATUS

In Fig. 5 page 8 is shown diagramatically the forra of ap-

paratus used in testing this clutch. The shaft hangers were

bolted to a rectangular frame of five inch hy eight inch timbers

across which two four inch by eight inch timbers were laid and

bolted by 7/3 in. tee bolts to the channels provided in the floor.

To tighten the belt all that was necessary to be done was to

loosen the bolts and drive the frame over a little and tighten up

the bolts again. The tightener pulley shown in the photograph on

page 44 was used on account of the high concrete foundation under

the belt wheel of the Ideal engine from which the power was de-

rived. The function of the tightener placed as it v/as on the

tight side of the bait was to keep the belt from touching this

foundation. On the left hand side of the clutch pulley » Fig. 6,

the water inlet and the water scoop are shown.

The clutch lever was fulcrumed by a forged bracket to the

adjacent hanger in such a position that the lever was at right

angles to the shaft when the levers H and H were just about to

pass the highest point of the angle on the sleeve. This insured

right angle forces.

The Ideal engine mentioned above is rated at sixty horse-

power at three hundred and twenty- five revolutions per minute.

The steam pressure being higher than that at which the engine is

rated, it was possible to get more than sixty horse-power.

r/ e. e

The prony brake was built of wood in laminations, the two

halves being held together by long bolts on waich the nuts were

in the form of hand wheels. The lever arm of the brake war> made

of 1-5/8 in. pipe and had a convenient length of five feet three

inches from the center of the shaft to the knife edge.

The scales were of the platform type, having a capacity of

500 pounds. The instruments used consisted of a spring scale of

150 pounds capacity for use in reading the force required to

throw in the clutch lever, and a tachometer for taking readings

of revolutions per minute at any instant.

The pulley was flanged so as to allow the cooling water to

be held in the rim. The heat was so great that some means had to

be provided for scooping the water out as fast as it came in thus

preventing the clouds of steam v;hich at first enveloped the ap-

paratus .

The shaft was two and fifteen-sixteenths inches in diameter.

The position of the old keyway and the shortness of the shaft

made it necessary to have the clutch between the bearings. Every

time new discs were put in it v/as necessary to slip off the belt

and raise the clutch, shaft, and drive pulley from the bearings

and move them to one side to be taken apart. Such difficulties

would not occur in case of a carefully designed apparatus..

10

CHAPTER III

METHOD OF TECTIIICt

Three men were necessary for making the tests:

(1) One handled the engine and operated the clutch

lever.

(2) One observed the revolutions of the clutch shaft.

(S) One took the prony brake readings.

The operation v,'as as follows: After the clutch lever was

set by pulling the handle by means of the spring scales, the

prony brake was gradually applied until it was finally set or

locked, thus causing the clutch to slip. At the instant slipping

occurred simultaneous readings of revolutions and scale beam were

taken. After the clutch slipped the transmitted load dropped off

and readings were again taken and the brake released. This was

continued until the torque w-is too great for the engine to slip

the clutch after 77hich- the brake was tightened and the lever

thrown in to determine the pick up capacity.

In order to determine the actual pressure on the discs cor-

responding to the known effort applied at the lever handle, the

clutch proper without the pulley was put in a Riehle testing ma-

chine as shown in Fig. 7, page 11. The collar P, Figs. 8 and 9

.

which is integral with the short shaft takes the upward thrust of

the hub B, collar G. and disc" A. These move upward due to the

operating lever on the tapered sleeve LI. The actual pressure

that otherwise would be between the discs is thus easily measured

11

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14

on the scale beam of the testing: machine. The curve shown on

page 13 gives the disc pressures corresponding to the efforts re-

quired at the lever handle.

15

CHAPTKR IV.

FORMULAE

HORSE P07/SR--The horse power transmitted by the prony brake

used in these tests is given by the follov/ing formula:

IT T> - ^( 1 )

" 1000

^

in v/hich F= force measured on the scales.

1J= revolutions per minute of the brake pulley*

TAITGSI^TTIAL PULL--ghe tangential pull T at a distance of one

foot from the centre of the pulley is given by the following for-

mula:

T = F X 5.25 (2 )

The tangential pull at one foot radius may be referred to a

an equivalent pull at the mean radius of the disc. The product

of this new pull and the mean radius gives v/hat is knovm as the

moment of friction.

Letting L represent the mean diameter of the friction discs

M " " moment of friction.

n " " number of friction surfaces

P " " actual pressure on the discs.

Z' " " coefficient of friction.L/PDn

we have M = -/--g{ S )

.

For the case considered, n=2 . Substituting this value in

(5) and solving for P v/e get

in which Tj^ represents the tangential pull at the mean radius of

16

the discs, and has the following value

(5)

Combining (4) and (5). we may readily find a relation be-

tween P, T,/; , and D as follows

It is evident that if the axial pressure exerted upon the

discs corresponding to a given effort at the end of the clutch

lever, is known, the tangential capacity of any disc for any ma-

terial may be calculated, provided the coefficients of friction

of the different materials on cast iron are known. Under ideal

conditions the coefficient determined experimentally would give

accurate results in calculating capacities of disc clutches but

rarely if ever are ei^ch conditions met with; therefore, the coef'

fieient used must be slightly smaller than for the ideal case .

Time did not permit of making experiments in the physical

laboratory on the determination of the actual coefficients of

friction of the friction surfaces. Knowing the static coeffic-

ients^ the size of discs, and the pressure between them, probable

horsepower transmitted by a clutch may be calculated by solving

for li in formula (3) and using it in the well known formula for

horsepower , namely

It does not matter at what radius the tangential pull is

applied provided its product with that radius equals M. The

transmitted horsepower of a clutch after M is determined depends

directly upon the revolutions per minute.

(6)

mi(7)^" 65025

where M= moment of T^ in inch pounds

K« revolutions per minute of discs

17

In oalculatinp the tangential load that the clutoh will

pick up, the kinetic coefficient must be used instead of the

static coefficient. The friction of the pulley on its bearing

will help in picking up the tangential load but so slightly as to

be neglected in calculations.

18

SAI.!PLE CAXCULATIOHS

Given: data from page 25 line 2

R.P.M, of clutch pulley = 346

Force measured on scale beams ~ 100 pounds.

Substituting in formula (l)

100 X 346

= 34.6

From the above data find the tangential pull at one foot

from the centre of the pulley. Substituting the value of F in

formula (2)

The mean diameter of the 18" disc from which this data was

obtained is 11.875". A force of 57 pounds was necessary to set

the lever. On page 13 is a curve which gives the values of ?

corresponding to values of the force exerted at the lever handle.

Referring to this curve the value of P is found to be 5950 pounds

Substituting these values of P, T, and D in formula (6) and sol-

ving for/; ,

T = 100 X 5.25

= 525 pounds.

5950 X 11.375

= .0891

19

CHAPTER V

DISCUSSION AND COITCLUSIOITS

The results obtained throughout the tests are on the ?/hole

somewhat disappointing although several things of interest have

developed.

Some things connected v/ith the clutch mechanism may have

influenced its perfect operation. It was found that the pulley

did not run true on its hearing causing a lateral motion of the

rim; this lateral motion and the possibility that the cast iron

friction plates were not faced true may have caused the variation

noticed in the force required to operate the clutch lever. When,

the "brake was free, that is, the clutch lever thrown out, and the

engine running, the friction of the pulley on its hub-hearing and

the rubbing of the discs due to their closeness to each other

wou.ld keep the clutch pulley rotating. Ylhen the lever was thrown

in and out a number of times the force necessary at the lever

handle was found to be the same; but as soon as the brake was

tightened and the clutch made to slip a different force was found

necessary at the lever handle for each application of the brake.

The closeness of the discs mentioned above was caused by

the clamping of the levers on the small diameter of the operating

sleeve. If the tapered part of the sleeve were made longer this

trouble 7/ould be relieved. The levers which bear on the sleeve

should be made perfectly smooth and the friction plates should

also be smooth and show no defects in the faces. Before making

20

tests on a clutch, the discs should be "burned in" by running:

with the discs lightly rubbing and then gradually increasing the

pressure until the proper working conditions are arrived at. Care

in the above points will favorably affect the v/orking of the

clutch.

If the friction discs were made more like rings, that is,

were made with larger inner diameters the wear would be more uni-

form over the rubbing surfaces. The mean radius could be in-

creased in this manner and a more powerful clutch would result.

It was found that the outer areas of discs were scored and worn

more than the inner areas. One cause of scoring and failure of

the discs at the outer areas was due to the high speed. It might

be mentioned here that the scoring and tearing occurred only in

the case of fibre discs; the leather discs stood up well in this

res-oect but showed that tho speed corresponding to the pressure

was too great as evidenced by skin burning of the leather. The

conditions under which the discs v/ere tested are not ezactly the

same as are found in practice, for the capacity of a clutch for

picking up a load would be the load that the clutch v^ill pick

up from rest, ^iv© motion to, and finally bring up to speed. It

is clear that the velocity of slipping is a maximum only for an

instant at starting and decreases to ^ero -is the load is picked

up. In the present tests it was necessary to measure the load

transmitted when the discs slipped at maximum velocity. The time

required for taking readings was sufficient to allow excessive

heating of the friction media. It seems, therefore, that picking

up a load gradually and bringing the relative velocity of the

discs to zero would approach more closely to the conditions of

21

praotioe than do the conditions of these tests.

As has been stated, the force required at the lever handle,

when all adjustments remained unchanged, varied. The relation

between the force exerted at the lever handle and the tane-ential

load handled in transmission would be expected to vary approxi-

mately as much as the coefficient of friction of the materials

used. These actual coefficients vary as much as thirty percent

for the same materials. The relation between the force required

at the lever handle and the tangential load handled when picking

up a load should be fairly uniform, because the kinetic coeffic-

ient of friction is fairly well defined as a fixed value.

It was possible to plot curves from the major part of the

data obtained. From the data showTi on pages 27,41,& 42 no curves

could be constructed as the results varied too much. The curves

from the other data sheets though not clearly defined seem to

follow the path of an increasing ratio between the forces at the

end of the lever and the tangential loads handled. In two cases

pages 36 and 40 the curves seemed to be concave downwards.

If many more observations could have been made before the

discs became burned it is probable that a straight line might

have been substituted for the curves. However, the general ten-

dency of the curves shown is concave upwards which indicates an

increasing ratio between the tangential pull and the force on the

lever. The curve shown on page 13 is a straight line and cince

the curves under discussion are plotted with the same abscissae

it follows that the "increasing ratio" mentioned above is due to

elements outside of the clutch mechanism. An increase of the

coefficient of friction with the increase of pressure or an in-

crease of the coefficient of friction with the temperature are

likely causes.

The curves for leather discs on pages 29,21,22,2c 24 show how

near the leather discs come to picking up the same load that they

will transmit without slipping. A leather disc will pick up from

75 to 955^ of the load it will transmit without slipping. On the

other hand it was found that the fibre discs in general v/ill pick

up approximately 25/b of the maximum load transmitted.

For picking up a load at high speed and high pressure,

leather discs are undoubtedly superior to fibre. It is doubtful

whether or not leather discs will stand the destroying effect of

prolonged heat as well as fibres. Fibre dices v/hen used at prop-

er speeds and pressures will probably outwear leather discs. Fi-

bre discs do not change size or shape in use but leather discs

shrink considerably and usually more on one side than on the

other. The result of shrinking more on one side is to pull the

outer edge on that side against the friction plates and wear it

off. The accompanying figure shows the shape of the discs so af-

fected. Shrinkage amounted to as much as one inch in diameter

for an eighteen inch disc.

r/G. II

23

In the design of a universal clutch testing machine means

should be provided for high torque ,—about two thousand pounds at

one foot radius. The clutch shaft should be driven by gears or

through a gear set capable of several speed ratios. The power

should be derived from a variable speed motor of large enough

capacity to keep up the speed when the load is on. A higher

speed than can be obtained through the gears should be arranged

for by separate belted connection. The majority of tests require

low speeds but high speeds will be necessary for determining the

relation between speed, temperature^ and coefficient of friction.

The apparatus should be universal in respect to different sizes ,

of the same tj-pe of clutch and if possible with respect to dif-

ferent types.

The same apparatus for determining transmitting capacities

will answer for most types of clutches but for determining the

internal pressures that exist at certain settings of levers, etc.

a different machine will be necessary; i"t must permit of many

changes and adjustments to meet the different shapes and types

of clutches.

Some means should be provided for determining whether or

not the friction surfaces creep a little before finally slipping.

The brake should be so constructed that it may be relieved

quickly the instant that it grips the pulley. The trouble due to

heating will be relieved by such an arrangement when measuring

the load capacities in transmission. In conjunction v/ith the a-

bove device means should be at hand for measuring the eract prony

brake load at any moment without the necessity of having to bal-

ance a scale beam.

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