When lubricated rollers are run together they are separated by a hydrodynamically formed oil film. The thickness of this film has been measured by a capacitance method up to loads of 1000 Lb. per inch of face (1.76 x 10 3 dyn cm - 1 ) for conditions of pure rolling and for conditions of rolling with sliding such as exist at the contacts of gear teeth. It has been found, at low loads, that the film thickness varies inversely with load and proportionately with speed, as simple hydrodynamic theory suggests, but that the actual thickness is approximately one-half of the theoretical thickness. At higher loads, of greater practical importance, the film thickness is shown to be of the order 1u (-4x10 -5 in) which is greater than that predicted by simple theory. Experimental evidence is presented that this failure of the simple theory is due to the increased viscosity of the oil under pressure and to the deformation of the surfaces by the load. The film thickness, at practical loads, has been found to vary little with load, less than proportionately with speed but to vary greatly with the temperatures of the surfaces. Estimates of the temperature reached by the oil in its passage through the conjunction of the surfaces have been made from measurements of the electrical resistivity of the oil. This temperature, which depends upon load and the peripheral speeds and which may exceed 250° C, appears to have little influence upon the film thickness. From this it is argued that the film thickness is largely determined by the conditions, on the entry side of the conjunction, ahead of the region in which the viscous losses and heating of the oil become intense.
Measurements of friction with disk machines are described. The measurements have been made under conditions which fit them particularly for comparison with the theory of hydrodynamic lubrication of rollers and with the theory of frictional heating which has been developed in part III. The frictions due to rolling and sliding have been measured separately. It is shown that the frictional tractions T 1 and T 2 acting upon the surfaces of two disks can be expressed by — _ T _ where T R is the traction due to rolling and T s is that due to sliding. If the disk 1 runs freely in frictionless bearings T 1 must be zero and the disk runs in the condition T R = T s . A four-disk machine is described in which the central disk, since it has no bearings, runs in the above condition unless some external torque is imposed upon it. External torques were applied by a band brake and from the curves of traction against sliding thereby obtained both and the way in which T s increases with sliding speed were deduced. It is shown that in the elasto-hydrodynamic regime T R is independent of load and simply proportional to the thickness of the hydrodynamic film. It is also shown that this behaviour follows from the hydrodynamic theory and that the magnitudes of the rolling friction as predicted by theory and as deduced from experiment are substantially in accord. From experimental determinations of film thickness and from the linear increase of T s with sliding speed over the range of sliding speeds employed ( ~ 1 cm s- 1 ) values of the effective viscosity ( rjm) at the rolling point were deduced and are presented as a function of rolling speed {u as | ( mj +«2)). By effective viscosity is meant that constant viscosity throughout the conjunction of the disks which would give rise to the observed friction. It is shown that the values of so obtained when extrapolated to zero rolling speed are consistent with published results of experiments with dropping ball viscometers in high-pressure apparatus ( Pressure Viscosity Report 1953) both with respect to the effects of pressure and of temperature. But the more important feature of the results from the disk experiments is that they show the effects of pressure and temperature upon the apparent viscosity of the oil to diminish as the rolling speed is increased, i.e. as the time for which the oil is under stress diminishes. This behaviour is interpreted in terms of a Maxwellian fluid and the required values of the elastic modulus in shear are deduced. However, although the visco-elastic hypothesis accounts for the observations it is stressed that it cannot yet be taken as the definitive explanation. In a further series of experiments a two-disk machine adapted for the direct measurement of friction independently of bearing frictions was used to explore sliding speeds up to 400 cm s- 1 . In contrast with the previous measurements, at such speeds of sliding frictional heating has a major effect upon effective viscosity, for example, in a particular instance the introduction of 400 cm s- 1 sliding caused the effective viscosity to fall from ~ 3000 P at the rolling point to ~ 20 P. It is shown that the frictions and effective viscosities predicted by the theory of frictional heating (part III) and the measurements now reported are, in their larger aspects, substantially in accord. For example, in a particular instance theory predicted a coefficient of friction of 0-05 whereas experiment gave a coefficient of 0.03 and both experiment and theory show that as the sliding speed increases the friction rises to a maximum and then falls. But the theory of part III when applied to the experimental results leads to a value of the thermal conductivity of the oil of about half that to be expected from Bridgman’s work (1949). However, measurements in finer detail of friction up to a sliding speed of 30 cm s- 1 indicate that an intrinsic effect (i.e. an effect at constant temperature) of rate of strain upon viscosity exists. By taking this into account the anomaly with respect to thermal conductivity can be resolved. The experimental results show clearly that in a lubricating system of widespread type (e.g. ballraces, gears) a mineral oil exhibits distinctive dynamic characteristics which are of significance with respect both to friction and to the thickness of the hydrodynamic oil film. The comparison of experiment and theory also emphasizes the importance of the thermal conductivity of the oil in relation to friction and to the temperatures in the oil film.
This paper describes some experiments with disk machines in which the electrical resistance between the disks was measured at various stages of running. It also describes some experiments with mild steel disks run at such loads that considerable plastic deformation occurred. The resistance measurements illustrate the process of ‘running-in’ and show that a state of almost complete hydrodynamic lubrication is eventually reached. It has also been found that the surface temperatures of the disks have a very considerable influence upon the hydrodynamic film. The flow of material in the plastically deformed disks is described and its bearing upon the mutual accommodation of engaging tooth surfaces and their subsequent pitting is discussed. An explanation of the greater propensity to pit of surfaces with the lower peripheral speeds may be provided by one unexpected feature of the flow. It was also found that even at the high loads, which produce gross plastic distortion, the mild steel disks did not scuff, and this is attributed to the persistence of hydrodynamic lubrication even under such extreme conditions.
Continuous measurements of the force throughout impacts between metal cylinders and between a hard sphere and metal flats are described. The force has been measured piezo-electrically. Within their appropriate ranges the theories of elastic impacts due to St Venant and Hertz have been confirmed. For impacts so hard that the elastic regime preceding plastic deformation may be neglected, it is shown that the experimental results agree closely with force-time curves calculated, assuming that deformation is opposed by a constant pressure. This dynamic flow pressure is greater than the similarly defined pressure experienced in static tests and the new information given by the piezo-electric method shows that this difference cannot be accounted for satisfactorily in terms of forces of a viscous type.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.