Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
The test rig for the nylon polymer vessel fairlead material is large, Figures 10 and 11. The PET mooring rope is periodically tensioned over a nylon polymer section by a hydraulic arm, first without PFPE lubrication. A ‘Block’ of tension events is applied through the hydraulic actuator, varying up to 150kN, a series of major and minor tension, computer controlled, events for 60 minutes. The rope will move over the central fixed sheave steadily, or not. The block of tension events causes extreme ‘stick/slip’ behavior to occur, Figure 11, with very loud, sudden, noise associated with each slip, i.e., a very loud ‘bang’, with the rope pattern impressed into the nylon polymer surface. The PET rope/nylon polymer surface ‘dry’ friction coefficient is 0.36.
A lubricated nylon sheave section allows the PET rope to flow smoothly over the fairlead sample with no ‘stick/slip’ and no associated noise. There is a considerably reduced wear scar, Figure.12, with a friction coefficient of 0.09.
Figure 13. Very Little Stick/Slip, Low Level of Noise. Rope flows Smoothly over Sheave. Considerably Reduced Wear Scar with Lubricated Nylon Polymer Sheave, Friction Coefficient of 0.09
The Future for Polymer Tribology: There is a need for more effective lubricant additives for polymers, the criteria being:
- the additives must withstand initial chemical attack from the catalyst in casting methods, must also withstand high shear and high temperatures in injection moulding or extrusion,
Figure 10. Plan of Mooring Rope ‘Stick-Slip’ Test Rig
- the lubricant additives must have a long operating life, low volatility and a high resistance to oxidation/degradation,
- the lubricant additives must be compatible with human contact, food contact, according to applications.
Conclusion: The tribology of polymer/metal ad polymer/polymer contacts is more complex than metal/metal or metal/ceramic tribology because of the relative softness of polymers compared to metals. Polymer tribology does not follow the established Laws of Friction and a PV function locus should be determined for each new polymer application. Design conditions must stay within each polymers PV Envelope.
Friction coefficients and wear can be reduced for polymers by using internal fillers, external or intrinsic lubrication. Internal fillers must be softer than the polymer matrix. External lubrication can reduce friction coefficients, down to 0.045, dependent on the lubricant. Internal lubricants reduce friction coefficients to < 0.10, with specialised fluids down to 0.04. ‘Stick/Slip’ can be addressed using specialised intrinsic and external lubricants.
Figure 11. PET Mooring Rope Stretched over Nylon Polymer Sample
Acknowledgements: I am very pleased to acknowledge the contributions of my colleagues at Nylacast, Ade Meshach and Preet Bahra, and discussions, collaboration and project work with Professor Martin Priest, Dr Richard Chittenden and Dr Christopher Dyson at the Universities of Bradford and Leeds, and Dr Isabel Ridge at Tension Technology International.
LUBE MAGAZINE NO.135 OCTOBER 2016 37
No.106 page 6
Figure 12. Effect of Extreme ‘Stick/Slip’ between PET Rope and Unlubricated Nylon Polymer Fairlead Section, ‘Loud Bangs’ with Each Slip. The Pattern of the PET Rope is Impressed into Sheave, ‘Dry’ Friction Coefficient of 0.36
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