Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
The reduction in friction coefficient for intrinsically lubricated nylons is dramatic, from 0.35/0.32/0.26 (according to measurement method) for plain, ‘dry’, nylon-6, to 0.14/0.16 for oil-filled cast nylon-6, to 0.08 for wax-filled cast nylon-6. Wax-filled nylon polymer slide/wear pads are used in the extending concentric masts of construction vehicles and cranes, Figure 7.
Figure 7. Wax-Filled Nylon Polymer Slide Pads, as Circled, used in the Concentric Extending Mast of a Construction Vehicle
Further reductions in nylon-6 friction coefficients are achieved by using a ‘nano-fluor’ oil, µ = 0.05 and using a PerFluoroPolyEther (PFPE) to further reduce the nylon-6 friction coefficient, µ = 0.04. The reductions in friction coefficient are increasingly costly as the required lubricants are expensive, even at low treat rates. Cost benefit analysis shows that the very low friction coefficient, intrinsically lubricated, nylon polymers are only viable for very demanding applications.
PerFluoroPolyEther Polymers – PFPE polymers have outstanding low friction performance. They are part of a large group of compounds which vary by structure and molecular weight range and can be liquids, waxes or thickened as greases. They are used extensively in fabric protection, particularly in outdoor clothing weather protection. PFPE’s can be extraordinarily expensive – but only a thin film is required for effective lubrication and, as their vapour pressure is very low, that film remains.
A very useful application of PFPE lubrication is between the upholstery squabs of passenger vehicles. The micro-relative, fretting, motion of the leather or polymer upholstery as the vehicle moves gives a ‘stick-slip’ action which emits noise as a squeak. Whilst unlikely to degrade the contacting surfaces, the noise is annoying, particularly for the purchasers of ‘up-market’ vehicles. A thin film of PFPE light wax applied between the contacting surfaces of the upholstery drastically reduces the ‘stick/slip’ effect to the point of eliminating the squeaking effect.
Static Friction Coefficients and Stick/Slip: as defined previously, the static friction coefficient applies to the initial effort required to start moving a body on a surface, µ’s
, and is
always greater than the dynamic coefficient. The effect is known as ‘stick/slip’ and is universally important, from micromachining
36 LUBE MAGAZINE NO.135 OCTOBER 2016 Figure 9. An Intrinsically Lubricated Nylon Polymer Fairlead Insert on a Vessel
No.106 page 5
surface velocity, particularly at very low relative velocities. The dynamic friction coefficient, µd
between the two friction coefficients decreases with increase in surface velocity until they merge. The ‘stick/slip’ effect is always present; it cannot be eliminated but can be substantially reduced by using a PFPE oil or grease as a lubricant or as a component in a lubricant formulation.
relative surface velocity but is always less than µs
‘Stick/slip’ affects the accuracy of micro-machining through to the safety critical mooring control of vessels such as LNG and crude oil tankers. For the latter, the ‘stick/slip’ motion of a vessel’s mooring ropes or tow ropes can lead to unpredictable and premature failure. The iron/steel surfaces through which the ropes pass, ‘the fairleads’, are often of poor quality with rust scale and old, scabrous, paint, accelerating degradation of a synthetic fibre rope, Figure 8. ‘Stick/slip’ of the ropes occurs as the moored vessel rolls and the mooring ropes are alternately tensioned and relaxed.
through to the slow amplitude motions of tethered large masses such as ships. The characteristics of ‘stick/slip’ are that the static friction coefficient, µs
also increases with decrease in relative , also increases with decrease in
. The difference
Figure 8. Steel Vessel Fairlead with Rusted and Worn Paint Surface
A solution is to provide a smooth, non-corroding surface such as a nylon polymer insert into the fairlead, Figure 9, which drastically reduces ‘stick/slip’ and extends the life of the ropes.
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