C.J. Hooke

The effect of fibre reinforcement on the friction and wear of polyamide 66 under dry rolling–sliding contact

Abstract An investigation into the effect of fibre reinforcement on the friction and wear of PA66 in rolling–sliding contact is reported. Three types of short fibre—aramid, carbon and glass—were examined with the composites running against identical materials in a twin disc machine. It was found that the aramid-fibre reinforcement did not significantly alter the friction of the matrix material. However, both the carbon-fibre and glass-fibre reinforcement reduced the coefficient of friction substantially. Wear of the aramid- and carbon-fibre composites was essentially linear with time and generally around ten times greater than that of the unreinforced material. The wear of the glass-fibre composite was complex with an initial period where the wear rate was similar to that of the unreinforced material. After a significant depth of wear had occurred the wear rate changed to a value similar to, but slightly higher than, that of the other reinforced materials. It appears that one of the major benefits of introducing fibre reinforcement, particularly glass, is that it reduces the coefficient of friction and hence allows the material to be used for higher duties without exceeding the softening point of the matrix. This increase in duty is, however, at the expense of an increased wear rate and shorter component life.

The friction and wear of polymers in non-conformal contacts

Abstract The behaviour of a number of polymers and polymeric composites in non-conformal contact is discussed. It is shown that friction-generated heat has a major influence on the materials performance and that, for example, the use of acetal in spur gears is controlled by the temperature rise. The wear behaviour of the materials examined showed a wide variation and in all cases was totally different from that found in conventional pin-on-disc tests. Of the materials examined, acetal appears the best of the unreinforced polymers and, in particular, nylon 66 was always inferior with higher wear rates and a tendency to form deep cracks in the surface. Of the composites, only the nylon-glass-fibre materials appeared superior to acetal and in these materials the wear process was complex. They appear to offer a somewhat increased load capacity over acetal but only where the required component life is below about 10 7 cycles.

Measurement and Prediction of the Surface Temperature in Polymer Gears and Its Relationship to Gear Wear

A design of a four square gear test rig that allows the wear of polymer and composite gears to be monitored continuously during operation is described. The wear behavior of three typical gear materials is examined and it is shown that the wear characteristics differ greatly. For Acetal there is a sharp rise in wear as the transmitted torque is increased, effectively limiting the torque that can be transmitted by an Acetal gear pair. This wear transition is shown to be associated with the maximum surface temperature of the gear reaching the melting point of Acetal

Wear behaviour of acetal gear pairs

Abstract The wear behaviour of polymer gear pairs has been little studied. Most wear studies have used pin-on-disc or polymer against steel tests. The non-conformal contact and high transient temperatures in polymer gear pairs make it essential to study wear under actual running conditions. This paper describes the design and operation of a novel test rig for measuring the wear behaviour of polymer gears continuously. Preliminary results are presented and comparisons made between failures in acetal and nylon. Wear of acetal gears is studied in more detail. It is shown that wear of acetal gears is a complex process. Optical and scanning electron microscopy (SEM) show the driving and driven gears to have different features on the worn flanks, with a ridge forming at the pitch line on the driven gear and a valley on the driving gear. The wear surfaces over the addendum and dedendum are also strikingly different. This is related to the direction of rolling and friction forces on each gear face. The limits to the use of acetal as a gear material are considered. At low torques, life is limited by wear and at high loads the maximum permissible surface temperature is a limiting factor.

The wear mechanisms of acetal in unlubricated rolling-sliding contact

The wear mechanisms of acetal (polyoxymethylene or POM) running against acetal in non-conformal, unlubricated rolling-sliding contacts have been investigated over a range of loads, rolling speeds and slip ratios. A wear and friction testing machine designed and built at Birmingham allows acceptable contact stresses for polymers and composites to be combined with a wide range of rolling speeds and slip ratios under well-controlled conditions. Based on the observed drastic changes in the wear rate with running conditions, wear has been divided into two regions: mild and severe. In the mild wear region, the wear rates are below 10−6 μm cycle−1 and are independent of rolling speed and slip ratio and vary approximately with the peak hertzian contact stress. Scanning electron microscopy examination shows that the wear appears to be at least partly due to mechanical fatigue of the surface. In the severe wear region, wear rates may exceed 10−4 μm cycle−1 and are very sensitive to operating conditions. The surface temperature is shown to be above 110°C and wear is caused by lateral cracking of the surface and subsequent tearing. n nThese results are compared with the wear behaviour of POM gear pairs and close correlation is shown to exist. It is shown that, provided the temperature limitation of the material is observed, POM gears and other non-conformal components should have a life of around 109 cycles.

The elastohydrodynamic lubrication of a cylinder on an elastomeric layer

Abstract In this paper the lubrication of a rigid cylinder sliding or rolling on an elastomeric layer which is bonded to a rigid substrate is examined. Results are presented for central and minimum film thicknesses for the complete range of operating conditions and for a wide range of values of the ratio of the semicontact width-to-layer thickness.

Considerations in the design of partially hydrostatic slipper bearings

The purpose of this study is to examine the design of hydrostatically balanced bearings as used in the slippers of high pressure axial piston pumps, and to outline a design procedure whereby the slipper behaviour, minimum film thickness and loss of high pressure fluid can be estimated. It is shown that for successful operation the slippers need to have small amounts of non-flatness on the running surfaces. In addition, good agreement between the measured and calculated film thickness is demonstrated.

Investigation into the effects of orifice size, offset and overclamp ratio on the lubrication of slipper bearings

This paper represents the results of an experimental investigation of the performance of hydrostatic slipper bearings in axial piston pumps and motors. The effect of clamping ratio, offset loading and orifice size on the behaviour of overclamped and underclamped slippers is outlined. It is shown that the slippers run satisfactorily with no orifice and have their greatest resistance to tilting couples and to minimum film thickness. The underclamped slippers and slippers with larger orifice diameter run with relatively larger clearance and tilt than those of overclamped slippers with no orifice. Cavitation tends to affect the slippers, especially at the rear of the slipper. Ineffective flood lubrication may be the cause and oil jet pressure must be maintained to prevent oil starvation.

Surface topography and wear mechanisms in polyamide 66 and its composites

A study has been made of the tribological behaviour of polyamide 66 (PA66) running against itself, in unlubricated, non-conformal and rolling-sliding contact. Tests were conducted over a wide range of loads and slip ratios using a twin-disc test rig. The wear and friction behaviour of unreinforced PA66 is dominated mainly by three major features: a critical slip ratio under a fixed load and running speed, macro-transverse cracks and a layer of film on the contact surface. Both the wear and friction properties of unreinforced PA66 can be improved considerably by filling with 20wt% PTFE, and the tribological mechanisms are changed significantly. This reinforcement prevents both the initiation and propagation of transverse cracks on the contact surfaces which occurred in the unreinforced material. It also decreases both the wear rate and the friction coefficient substantially. The 30wt% short glass-fibre reinforced PA66 also suppresses the transverse cracks from initiation on the surfaces. A thin film on the contact surfaces plays a dominant role in reducing wear and friction of the composite and in suppressing the transverse cracks. These results offer the prospect of enhanced applicability of PA66 in engineering components.

Slipper Balance in Axial Piston Pumps and Motors

The paper presents the results of an experimental and theoretical investigation of the effect of clamping ratio and orifice size on the performance of slippers in axial piston pumps and motors. Polishing of the running face to a slightly convex form appears to be essential for successful operation under all conditions. It is shown that slippers operate perfectly satisfactorily with the orifice blanked and, indeed, are generally most stable in this condition. Introduction of the orifice in under-clamped slippers increases the clearance. However it can also destabilise the slipper resulting in the slipper becoming sensitive to the effect of tilting couples. The effect on overclamped designs is more complex depending on the precise value of clamping ratio and on the width of the slipper land.