US3643490A

US3643490A - Method and apparatus for investigating interfiber friction

Info

Publication number: US3643490A
Application number: US660044A
Authority: US
Inventors: Kenneth L Hertel
Original assignee: University of Tennessee Research Foundation
Current assignee: University of Tennessee Research Foundation
Priority date: 1967-08-11
Filing date: 1967-08-11
Publication date: 1972-02-22
Anticipated expiration: 1989-02-22
Also published as: DE1798036A1

Abstract

Interfiber friction information is obtained by measuring the energy dissipated when a fibrous assembly is loaded in shear. The specimen of fibers is mounted between parallel surfaces one of which is connected to a pendulum so as to be oscillated in a direction parallel to the surfaces. During relative oscillatory movement between the surfaces, fibers of the assembly move relative to each other and the work expended in overcoming friction is reflected in the decay in the amplitude of the pendulum oscillations.

Description

[151 Mesuse liiertell [54] MiE'iiIi-IHDD AND AiPlWtrliilA'iiiU 2,032,202 2/1936 Dennis.........................................73/7

HiF iEiUi 3,087,326 4/1963 MacDonnell.....

Primary Examiner-Richard C. Queisser Assistant Examiner-John Whalen [72] lnventor: Kenneth 1L. liliertel, Knoxville, Tenn.

Attorney-Burns, Doene, Sweclcer & Mathis [57] AiliS'llllR/tt'l'll lnterfiber friction information is obtained by measuring the [22] Filed:

[21] Appl. No.:

energy dissipated when a fibrous assembly is loaded in shear.

The specimen of fibers is mounted between parallel surfaces one of which is connected to a pendulum so as to be oscillated in a direction parallel to the surfaces. During relative oscillatory movement between the surfaces, fibers of the assembly move relative to each other and the work expended in over- [52] US. [51] lint. [58] Field ofiieareh coming friction is reflected in the decay in the amplitude of the pendulum oscillations.

[56] References Cited 7 UNTTED STATES PATENTS Claims, Drawing Figures 2,296,657 9/1942 Wallace......................................,73/9

PAIENIEmwzz 1972 3,643 .490

sum 1 BF 2 ENTOR KENNETH L HERTEL BY /W a a 1.4

ATTORNEYS PAIENIEDFEB22 I972 SHEET 2 UF 3 SQUARE OF THE DIFFERENCE BETWEEN SUCCESSIVE AMPLITUDES X X/ am .w INVENTOR KENNETH L. HERTEL ATTORNEYS" METHOD AND APPARATUS EUR INVESTIGATING INTEIRTIIEER lFItlICTllUN BACKGRUUND OF THE INVENTION This invention relates to a method and apparatus for determining interfiber friction properties and more particularly to a method and apparatus for measuring the shear friction of an assembly of fibers.

For most solid bodies, friction properties can be dealt with satisfactorily in terms of coefficient of friction," defined as the ratio between the tangential force required to produce sliding of one surface upon another and the normal force pressing the two surfaces into contact. This is so because the coefficient of friction is nearly constant in connection with most hard solid substances. However, efforts to define the friction properties of textile fibers by reference to coefficients of friction have lead to disappointing results.

For most textile materials, the coefficient of friction is not a constant. It varies somewhat irregularly as load changes and a number of apparent inconsistencies have yet to be resolved. Moreover, the wide range of the valve of the coefficient of friction as a function of the load normal to the sliding surfaces greatly complicates attempts at analysis of fiber friction forces upon the basis of coefficient of friction. Yet friction is important to textiles both from the standpoint of the finished product and in connection with the problems of processing the fibers.

SUMMARY OF THE INVENTION Accordingly, it an object of this invention to provide a method and apparatus for determining a meaningful indication of the frictional properties of fibers.

It is another object of the invention to provide a method and apparatus for measuring the energy dissipated in the shear motion of a fiber aggregate.

It is a further object to provide a method and apparatus for determining interfiber friction and internal structural stress characteristics of fiber assemblies.

These objects may be realized by applying an oscillating shear loading to a fiber aggregate and obtaining a measure of the energy transformed into heat through fiber-to-fiber friction. According to one embodiment, the assembly of fibers is mounted between a stationary plate and a plate connected to a pendulum. The pendulum then is set in motion and the amplitudes of successive oscillations are measured. The difference between adjacent amplitudes is a function of the work expended in overcoming friction, and a major portion of this friction is fiber-to-fiber friction in that instrument friction and internal friction in the individual fibers are relatively small. A plot of the squares of the differences in amplitudes of adjacent oscillations against the sums of such amplitudes yields a substantially straight curve the slope and intercept of which are statistically usable quantities characteristic of the fiber sample.

THE DRAWINGS The method and apparatus of the instant invention will be more easily understood by reference to the accompanying drawings in which:

FIG. I is a perspective view of one embodiment of the apparatus for practicing the invention in which a pendulum is used to impart oscillatory shear to a specimen of fibers;

FIG. 2 is a perspective view of a portion of the apparatus used to measure the amplitude of the oscillations of the pendulum of FIG. l; and

FIG. 3 is a graph showing typical curves obtained by the exercise of the present invention.

THE PREFERRED EMBODIMENT Referring to the drawings, a frame 110 is mounted on a base plate 12 of sufiicient weight to impart stability to the frame Ml. An upright M of U-shaped horizontal cross section carries a horizontal plate lib about 8 inches square and is movable within guide means I18 on an extension 20 of the frame 110. A screw 22 passes through one wall of the guide means lltl through the open side of the U-shaped upright M, and on a threaded hole in a vertically extending scale member 23 pivotally mounted on the base plate 112. A compression spring (not shown) surrounds the screw 22 between the wall of the guide Ill and the scale member 23 to urge the latter to the left (FIG. l) but the scale 23 may be moved to the right by tightening the screw 22. The U-shaped upright M is adjustably positioned on the upper portion of the scale member 23 and may be clamped in place thereon at the desired level by suitable means 2A. A fiducial mark near the lower extremity of the upright M is thus disposed adjacent markings on the scale 23 calibrated so that the thickness of a fiber specimen 2b positioned on the plate llb may be read directly.

An upper portion of the frame lltl carries a horizontal yoke 2% through which a pendulum 30 passes freely and upon the upper surfaces of which the pendulum is supported through a pair of knife edges 32 rigidly attached to the pendulum. When the pendulum is set in motion, it serves as a harmonic oscillator as energy is converted cyclically from potential energy to kinetic energy and back again to potential energy.

Balanced on a second pair of knife edges 34 rigidly attached to the upper extremity of the pendulum Bill is a cross arm 36 having a flat plate 3% rigidly mounted at one end thereof. The plate 3% is of the same size and configuration as the plate 16 and cooperates therewith in receiving the assembly of fibers 2b, the frictional characteristics of which are to be investigated. The cooperating surfaces of plates l6 and 3ft may themselves be roughened or may, for example, be surfaced with sheets of sandpaper or the like so as to reduce the slippage between the specimen of fibers under test and the

plates

16 and 35%.

At the end of the cross arm 36 opposite the plate 3% there is suspended an adjustable counterweight it) through which the cross arm 1% may be balanced on the knife edges 341, and a guide d2 on the frame It) restricts the amount of permitted pivoting movement of the cross arm. After a fibrous sample 26 is placed between the plates 11.6 and 38, the lower plate to is moved upwardly to cause the sample to be contacted by both of the

plates

16 and 38. Then a standard weight 4141 of, for example, one hundred grams is placed on the arm 36 above the plate 3% to compress the sample, and the lower plate lid is moved upwardly again to position the plate 33 in a horizontal plane. The leveling of the arm 36 is greatly facilitated by the presence of a sensitive spirit level 46 mounted on a midportion of the cross arm 3%.

If desired, a standard precompression treatment of the specimen may be accomplished as an adjunct to the loading of the specimen. For some materials, for example, it may be desirable to initially load the sample with a 200 gram weight 44 for a period of time and then replace that weight with a standard one hundred gram weight prior to the final leveling operation.

In order that a standard initial amplitude of the oscillation of the pendulum 3t) may be obtained, a latching device is carried by the frame in position to cooperate with a hardened pin 47 on the lower end portion of the pendulum 3b. Although the specific construction of the latching device is not of critical importance, there has been illustrated in FIG. 11 a hardened hook 4d pivotally mounted at A9 on a cross member of the frame 110. Suitable spring and stop means (not illustrated) may be provided for normally holding the hook 33 in a position to engage the pin A7. Then when the hook Aft is pressed downward, the pin 47 is released to permit free oscillation of the pendulum.

As shown more clearly in FIG. 2, a fan 50 having a hub 52 and a number of equally spaced radial arms 5% is rotatably mounted on base plate I2 beneath the pendulum hill. The fan may encompass less than or may be mounted at an angle to the vertical so that the pendulum 3h cooperates with the fan 5% during displacement from the vertical in one direction only. The upper surfaces 56 of the arms 54 may be curved slightly concave upward to correspond more nearly to the are through which the pendulum 30 swings, and grooves 58 are provided in these upper surfaces. Low inertia slides 60 of paper or the like are mounted in the grooves 58 for cooperation with a pusher element 61 on the lower end of the pendulum 30.

The arrangement of the fan 50 with respect to the pendulum 30 is such that any one of the fan arms 54 may be brought into position under and in alignment with the path of the pusher element 61 on the lower end of the pendulum 30. As the pendulum swings, the pusher element contacts the slide or target 60 immediately therebeneath and moves such target to a position corresponding to the outward extremity of the pendulums arc. The upper surface 56 of each of the fan arms 54 may conveniently be inscribed with a scale 62 so that the amplitude of the pendulum swing may be read directly from the position of the slide 60.

In practicing the invention it is frequently convenient to employ as the specimen 26 a stack of about one hundred 8-by-8 inch card webs forming a parallelepiped about inches thick. Such a fibrous assembly may be deformed substantially upon vibration of the pendulum without being disrupted. That is to say, the movement of one face of the assembly relative to the other may be sufficient to cause a significant amount of sliding motion of fibers relative to each other without breaking the assembly as a whole.

Although the dimensions of the components of the pendulum system may be varied, it is desirable that the maximum amplitude of oscillation of the plate 38 be compatible with the type of specimen 26 employed. For example, a maximum amplitude of about one inch will be found to be entirely satisfactory in connection with a sample of the type described in the preceding paragraph. In selecting dimensions, one should also keep in mind that the plate 38 should oscillate substantially horizontally and that the amount of energy dissipated in the sample should significantly affect the amplitude of the pendulum 30.

In operation, the fiber assembly 26 is inserted between the surfaces of the

plates

16 and 38 and the height of the lower plate 16 is adjusted by moving the upright 14 in its guide 18 and relative to the scale member 23. The weight 44 also may be applied at this time. When adjustment of the upright 14 has brought the plate 38 into a horizontal position (with the pendulum 30 at rest and in a vertical position), thickness of the specimen may be determined from the scale 23 at the lower end of upright 14.

The pendulum 30 is then moved to the right in FIG. 1 toward frame into cooperative relation with the hook 48 by which the amplitude of the initial oscillation is gauged. When the pendulum is released, it swings to the left on its initial oscillation. During the portion of the swing of pendulum 30 to the left of the vertical, an arm 54 of fan 50 is rotated into alignment so that on the return swing to the right the pusher element 61 contacts a target or slide 60 and pushes it to along the arm 54 to register the maximum amplitude of that particular swing of the pendulum. As the pendulum 30 swings back to the left of the vertical, a second arm 54 of the fan 50 is rotated into alignment and on the return swing of the pendulum 30 a second slide 60 is engaged and moved by the pusher element 61 to record the extremity of that swing. The procedure is continued until the amplitudes of a number of swings have been registered.

Each succeeding oscillation will have a smaller amplitude than its predecessor. On each swing of the pendulum, the sample 26 is deformed, causing bending of the fibers and also fiber-to-fiber sliding at multiple contact points. It is significant, however, that most of the energy which goes into the bending of the fibers is not actually removed from the harmonically oscillating system but is in fact returned to the pendulum during the reverse movement thereof. Thus, when the amplitude measurements are made with respect to complete "back and forth movement cycles, fiber bending affects the results only to the extent that a very small amount of work is consumed in internal friction within the individual fibers. Instrument friction also represents a very small factor in the decay in amplitude of the oscillations of the pendulum, and for practical purposes changes in amplitude may be attributed to fiber-to-fiber friction within the sample 26.

A relation between change in amplitude and average friction force may be derived in the following manner.

As is well known, the energy of an oscillating pendulum may be expressed as n 1 n (n where A, is the amplitude of the n' vibration of the pendulum and K is an instrumental constant. The difference in the energy of successive swings is then The distance D through which the upper plate 38 moves for these two successive oscillations of pendulum 30 may be expressed as 2( n+ n+1) where K is the instrumental constant relating the arc through which pendulum 30 swings to the movement of plate 38.

Since the energy lost by the pendulum is also the average frictional force F multiplied by the distance D,

n n+ l 2( n+ n4-1) Equating (2) and (4), the average frictional force may be expressed as a( H n+1) where K =K,/4K

Although the amplitude data may be displayed in various ways, it has been found particularly advantageous to plot the squares of the differences between successive amplitudes against the sums of successive amplitudes, as indicated in FIG. 3. This type of curve was discovered to be substantially straight. Hence, the slope and intercept of this curve with the ordinate of the graph provide statistically usable values which are characteristic of the frictional properties of the specimen under test.

FIG. 3 shows curves representing values obtained from different fibers. It will be observed that the curves are well defined and that they distinguish unequivocally from each other. These specimens rank themselves in the order of their difficulty of hand-combing and processing.

Where large numbers of tests are to be made, it usually will be found unnecessary to actually record a whole series of amplitude values for each test and then to plot data on a chart as in FIG. 3. Practically useful results can be obtained for example when only the amplitude of the fourth or fifth oscillation is recorded. A sufficiently close approximation of the slope valve for the particular test can be obtained from the result of this measurement along because the amplitude of the initial oscillation of the pendulum will be a constant established by the apparatus configuration (e.g., position of hook 48, etc.

In many instances, it also is desirable to establish for the specimen being tested a characteristic value which is independent of the thickness of the specimen, that is, a value which would be substantially the same'for a plurality of specimens which differed from each other only in thickness. It has been discovered that such a value can be obtained by multiplying the specimen thickness, as measured on the scale 23, by a number representative of the slope characteristic for the specimen. For this purpose, it makes no fundamental difference whether the slope characteristic is determined from a curve such as that shown in FIG. 1 or from the numerical operation referred to in the preceding paragraph. For example, a highly useful and readily obtainable parameter is (A -M where A is the amplitude of the initial oscillation, A is the amplitude of the fifth oscillation of the pendulum, and T" is the square root of the thickness of the specimen.

Of course, where comparisons between samples are to be made, proper attention must be given to possible differences in the samples and/or test conditions. Different results may be obtained from tests on the same fiber where there are differences in such factors as sample mass and/or degree of compression, humidity, orientation of the fibers within the assembly, and the internal stress conditions of the fibers in the sample. For example, it has been found that the energy dissipated by oscillating a sample in shear is usually greater when the sample is tested immediately after the carding operation than when the sample is tested after there has been a sufficient time lapse during which relaxation of the stresses set up in the fibers by the carding operation may occur.

The processing to which the fiber has been subjected prior to testing in accordance with the invention may also have a significant effect on the results. For example, a specimen of fibers that have been subjected to repeated carding operations may be expected to have energy dissipating qualities different from a specimen made up of fibers that have been subjected to less sever treatments.

Although a single embodiment has been illustrated and described in detail, various other embodiments will readily suggest themselves to persons skilled in the art. For example, the illustrated. pendulum system is obviously just one example of various harmonic oscillators that may be used in carrying out the invention. Other modifications, such as different modes of evaluating the energy dissipated when the fibrous specimen is subjected to oscillating shear, will also be apparent. It is intended therefore that the foregoing be considered as exemplary only, the appended claims being intended to cover a broad range of equivalent constructions and methods as within the true spirit and scope of the invention.

What is claimed is:

l. A method of testing interfiber frictional behavior in a unitary fiber assembly having spaced apart faces and within which frictionally contacting fibers are capable of being moved relative to each other without disrupting said assembly comprising:

moving one of said faces of the fiber assembly back and forth relative to the other of said faces thereof in an amount and direction to load said assembly in shear without disrupting said assembly, said back and forth movements causing bending of individual fibers within said assembly and energy absorbing movements between frictionally contacting surface portions of fibers within said assembly, and

obtaining a measure of the energy absorbed by the fiber assembly during at least one back and forth movement of said one face relative to the other of said faces.

2. A method according to claim ll wherein the thickness of the fiber assembly is additionally measured.

3. A method of testing a unitary fiber assembly having spaced apart faces and within which frictionally contacting fibers are capable of being moved relative to each other without disrupting said assembly comprising:

moving one of said faces of the fiber assembly back and forth relative to the other of said faces thereof by an oscillating system in which energy is repetitively transformed from potential energy to kinetic energy and back again to potential energy, said movement being in an amount and direction to load said assembly in shear without disrupting said assembly and cause energy absorbing movement between frictionally contacting fibers within said assembly, and

4. A method according to claim 3 wherein said oscillating system is set in oscillation and then allowed to decay, and wherein amplitudes of a plurality of oscillations are measured to provide a measure of the energy dissipated by reason of movements within the fiber assembly.

5. A method according to claim 4 wherein the squares of the differences between successive amplitudes are plotted against the sums of successive amplitudes to provide a generally straight curve the slope of which is a statistically usable quantity characteristic of the interfiber friction properties of the sample.

6. A method according to claim 3 wherein the amplitude of initial oscillation of the system is predetermined and wherein a measure of the amplitude of a later oscillation is recorded.

7. A method of testing a fibrous assembly made up of multiple card web layers which comprises:

placing the fibrous assembly between a stationary horizontal surface and movable horizontal surface connected to a pendulum;

imparting an oscillatory motion to said pendulum sufficient to cause relative back and forth movement of fibers within said assembly without destroying said assembly; and

obtaining a measure of the amplitudes of a plurality of oscillations of said pendulum.

fl. A method according to claim 7 wherein the amplitude of the initial oscillation of the pendulum is predetermined and wherein the amplitude of a later oscillation is measured.

9. Apparatus for investigating properties of fibers comprisfirst and second surfaces disposed in a spaced parallel relation and being adapted to receive therebetween a specimen of the fibers to be investigated,

means for imparting an oscillatory movement to one of said surfaces, and

means for measuring the decay in the amplitude of the oscillatory movement of said one of said surfaces. fill. The apparatus of claim 9 wherein said surfaces are pro vided with means for reducing slippage between said surfaces and the specimen.

ill. The apparatus of claim 9 including means for determining the parallel relationship of said surfaces.

H2. The apparatus of claim ll wherein said means for impart ing an oscillatory movement to one of said surfaces includes a pendulum connected to said one of said surfaces.

llil. The apparatus of claim 112 wherein said means for measuring the decay in the amplitude of the oscillatory movement of said one of said surfaces includes a record medium and means carried by said pendulum for recording the maximum displacement of said pendulum from the vertical on at least one oscillation.

M. The apparatus according to claim ll? including means for measuring the thickness of the specimen.

E5. The apparatus according to claim 14 including means for applying a standard compressive load to the specimen.

to. A method of testing interfiber frictional behavior in a specimen made up of multiple card webs stacked together in a unitary assembly having generally parallel external faces and an interior portion within which frictionally contacting fibers are capable of being moved relative to each other without disruption of the specimen as a whole comprising:

moving one of said faces back and forth relative to the other said face in a direction generally parallel to said faces to repetitively load the interior portion of said specimen in shear and in an amount insufficient to disrupt said specimen, said back and forth movements causing bending of individual fibers within said assembly and energy absorbing movements between frictionally contacting surface portions of fibers within said assembly, and

obtaining a measure of the energy absorbed by the specimen during at least one back and forth movement of said one face relative to said other face.

Claims

1. A method of testing interfiber frictional behavior in a unitary fiber assembly having spaced apart faces and within which frictionally contacting fibers are capable of being moved relative to each other without disrupting said assembly comprising: moving one of said faces of the fiber assembly back and forth relative to the other of said faces thereof in an amount and direction to load said assembly in shear without disrupting said assembly, said back and forth movements causing bending of individual fibers within said assembly and energy absorbing movements between frictionally contacting surface portions of fibers within said assembly, and obtaining a measure of the energy absorbed by the fiber assembly during at least one back and forth movement of said one face relative to the other of said faces.

2. A method according to claim 1 wherein the thickness of the fiber assembly is additionally measured.

3. A method of testing a unitary fiber assembly having spaced apart faces and within which frictionally contacting fibers are capable of being moved relative to each other without disrupting said assembly comprising: moving one of said faces of the fiber assembly back and forth relative to the other of said faces thereof by an oscillating system in which energy is repetitively transformed from potential energy to kinetic energy and back again to potential energy, said movement being in an amount and direction to load said assembly in shear without disrupting said assembly and cause energy absorbing movement between frictionally contacting fibers within said assembly, and obtaining a measure of the energy absorbed by the fiber assembly during at least one back and forth movement of said one face relative to the other of said faces.

7. A method of testing a fibrous assembly made up of multiple card web layers which comprises: placing the fibrous assembly between a stationary horizontal surface and movable horizontal surface connected to a pendulum; imparting an oscillatory motion to said pendulum sufficient to cause relative back and forth movement of fibers within said assembly without destroying said assembly; and obtaining a measure of the amplitudes of a plurality of oscillations of said pendulum.

8. A method according to claim 7 wherein the amplitude of the initial oscillation of the pendulum is predetermined and wherein the amplitude of a later oscillation is measured.

9. Apparatus for investigating properties of fibers comprising: first and second surfaces disposed in a spaced parallel relation and being adapted to receive therebetween a specimen of the fibers to be investigated, means for imparting an oscillatory movement to one of said surfaces, and means for measuring the decay in the amplitude of the oscillatory movement of said one of said surfaces.

10. The apparatus of claim 9 wherein said surfaces are provided with means for reducing slippage between said surfaces and the specimen.

11. The apparatus of claim 9 including means for determining the parallel relationship of said surfaces.

12. The apparatus of claim 9 wherein said means for imparting an oscillatory movement to one of said surfaces includes a pendulum connected to said one of said surfaces.

13. The apparatus of claim 12 wherein said means for measuring the decay in the amplitude of the oscillatory movement of said one of said surfaces includes a record medium and means carried by said pendulum for recording the maximum displacement of said pendulum from the vertical on at least one oscillation.

14. The apparatus according to claim 13 including means for measuring the thickness of the specimen.

15. The apparatus according to claim 14 including means for applying a standard compressive load to the specimen.

16. A method of testing interfiber frictional behavior in a specimen made up of multiple card webs stacked together in a unitary assembly having generally parallel external faces and an interior portion within which frictionally contacting fibers Are capable of being moved relative to each other without disruption of the specimen as a whole comprising: moving one of said faces back and forth relative to the other said face in a direction generally parallel to said faces to repetitively load the interior portion of said specimen in shear and in an amount insufficient to disrupt said specimen, said back and forth movements causing bending of individual fibers within said assembly and energy absorbing movements between frictionally contacting surface portions of fibers within said assembly, and obtaining a measure of the energy absorbed by the specimen during at least one back and forth movement of said one face relative to said other face.