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On the separation of the bulk and surface components of rubber friction
On the separation of the bulk and surface components of rubber friction It has been coniinonl~~ accq~ted in t11c literature that tl~c separatic)n oi the bull; (deformation) and surface (adhesive) ct)mponents of rubber friction nla~’ be at‘co~iiplished bv providing a lubricant between the ntatin g surfaces at II)LVsliding ~peetls. The lubricant is assumed to suppress the surface contribution to fricticlrr and tbt~ resulting l~leasurell~~~nts11ave been attril~ute~~ to bulk losses. On this basis, ‘I’AHIIRI has demonstrated the ecltiivalcnce of lubricated sliding and rolling of a hard sphere on a rubber surface, and has cc.~-tcluded that the friction in both cases is due to Ii\xtcresis losses alone. JGxxnt studies2 have shown clearly, Imwever, that it is questionable whether the cases of rolling and Iubricated sliding can 1x1considered equivalent. Figure I shows the coefficient of friction measured on natural rubber at low sliding spted4, Tfle test
CL EL
WITH
0.1
0.82
0.05
0.1
0.2
SLIDING
on lubricatecl
Fig_ L. I;riction
on Inl~ricatctl
SLIDING
3
(it/se;)
rubber
VELOCI’I
i 0.5
VELOCITY
I, I:riction
Fig.
TEFLON
Y
with steel spherical
slider.
(f
ift,‘rec)
rul~bcr with
alun~inum
c~lindricalslitler.
(ICci. L)
was made with a well-polished spherical steel slider on which a lubricant has been initially sprayed. After placing a thin Teflon tape on the rubber, the coefficient of friction (under otherwise equal conditions) dropped considerably, as shown by the bottom curve in the same figure. However, even in the latter case the adhesive friction is still dominant in tile presence of a lubricant. The results of more refined tests using a cylindrical aluminum slider on rubber covered with Teflon are similar as shown in
59
SHORT COMMUNICATIONS Fig. 2. It is obvious
from these results that a more effective
friction is desirable, at least at low sliding speeds. HEGMON~ eliminated the adhesive component
suppression
of friction
of surface
effectively
by using
the apparatus shown in Fig. 3. A tape is placed between the slider and the moving rubber band. The slider is restrained by a force cell which measures the pull on the slider, while the tape is restrained separately. Ideally, the tape should be completely rigid in tension and at the same time perfectly flexible to wrap around the slider profile. Such a tape, if obtainable, would eliminate the adhesive component of friction from the force cell reading and permit hysteresis (which has been prestretched
to eliminate
losses to be measured.
further extension)
Teflon
tape
fulfils the requirements,
TAPE
Fig. 3, Use of vibrating tape to eliminate adhesion component. but it is extremely
difficult
to specify the amount
(Ref. 2)
of prestretch
and further
compli-
cations develop from load cell displacement. A pneumatic shaker interposed between the tape and its restraint was found to completely eliminate surface friction. The average force on the tape is now zero and the force cell records bulk losses only because of the indentation of the rubber band. A frequency of vibration of about 250 cjsec applied to the tape was found to be most effective. It is interesting to note that TABOR~ used a sliding speed of about which falls to the left of the curves shown in Fig. I and z and is therefore
I in./min probably
well below the adhesion peak. It is therefore appropriate to conclude that the adhesion component of friction in his lubricated sliding experiments is small although not negligible. This is confirmed by the fact that, whereas the rolling andlubricatedsliding of spheres on rubber give almost identical coefficients of friction for a range of applied loads, the lubricated sliding readings are consistently fractionally higher. When the speed of sliding of asperities relative to a fixed, lubricated rubber base is increased to values conveniently measurable in feet per second, elastohydrodynamic considerations assure the disappearance of the surface component of friction as shown by MOORER. The sliding speed is now in excess of that corresponding to the adhesion peak shown in Figs. I and 2, and it is sufficiently large to create hydrodynamic U’enr, 13 (r9G9) 58-60
CL EL
WITH
0.1
0.82
0.05
0.1
0.2
SLIDING
on lubricatecl
Fig_ L. I;riction
on Inl~ricatctl
SLIDING
3
(it/se;)
rubber
VELOCI’I
i 0.5
VELOCITY
I, I:riction
Fig.
TEFLON
Y
with steel spherical
slider.
(f
ift,‘rec)
rul~bcr with
alun~inum
c~lindricalslitler.
(ICci. L)
was made with a well-polished spherical steel slider on which a lubricant has been initially sprayed. After placing a thin Teflon tape on the rubber, the coefficient of friction (under otherwise equal conditions) dropped considerably, as shown by the bottom curve in the same figure. However, even in the latter case the adhesive friction is still dominant in tile presence of a lubricant. The results of more refined tests using a cylindrical aluminum slider on rubber covered with Teflon are similar as shown in
59
SHORT COMMUNICATIONS Fig. 2. It is obvious
from these results that a more effective
friction is desirable, at least at low sliding speeds. HEGMON~ eliminated the adhesive component
suppression
of friction
of surface
effectively
by using
the apparatus shown in Fig. 3. A tape is placed between the slider and the moving rubber band. The slider is restrained by a force cell which measures the pull on the slider, while the tape is restrained separately. Ideally, the tape should be completely rigid in tension and at the same time perfectly flexible to wrap around the slider profile. Such a tape, if obtainable, would eliminate the adhesive component of friction from the force cell reading and permit hysteresis (which has been prestretched
to eliminate
losses to be measured.
further extension)
Teflon
tape
fulfils the requirements,
TAPE
Fig. 3, Use of vibrating tape to eliminate adhesion component. but it is extremely
difficult
to specify the amount
(Ref. 2)
of prestretch
and further
compli-
cations develop from load cell displacement. A pneumatic shaker interposed between the tape and its restraint was found to completely eliminate surface friction. The average force on the tape is now zero and the force cell records bulk losses only because of the indentation of the rubber band. A frequency of vibration of about 250 cjsec applied to the tape was found to be most effective. It is interesting to note that TABOR~ used a sliding speed of about which falls to the left of the curves shown in Fig. I and z and is therefore
I in./min probably
well below the adhesion peak. It is therefore appropriate to conclude that the adhesion component of friction in his lubricated sliding experiments is small although not negligible. This is confirmed by the fact that, whereas the rolling andlubricatedsliding of spheres on rubber give almost identical coefficients of friction for a range of applied loads, the lubricated sliding readings are consistently fractionally higher. When the speed of sliding of asperities relative to a fixed, lubricated rubber base is increased to values conveniently measurable in feet per second, elastohydrodynamic considerations assure the disappearance of the surface component of friction as shown by MOORER. The sliding speed is now in excess of that corresponding to the adhesion peak shown in Figs. I and 2, and it is sufficiently large to create hydrodynamic U’enr, 13 (r9G9) 58-60