Operating temperature key to polymer-steel friction, research finds

Stenter chains incorporating polymer on steel sliders are used for applications at high speeds and high temperatures, where output needs to be maximised. The polymer – often different types of polyaryletherketone (PAEK) – generally copes well with extreme conditions, but some stenter chains are known to experience sudden spikes in friction power consumption without any prior loss of output or other sign of an impending problem.

Now joint research published by the University of Bradford and ROCOL^® Lubricants has mapped how lubrication – or the lack of it – affects polymer components as the material passes through the glass transition phase. This is the point at which the polymer changes from the brittle, glass-like properties it has at lower temperatures to the softer, rubbery solid it becomes at high temperatures. Previous research has looked at the polymer’s behaviour below and above the glass transition phase, but only extrapolated as to what happens between.

“We put four different PAEK polymers through their paces in extreme conditions and found that, with lubrication, friction between the polymer and steel parts remained very stable,” says Martin Priest, Professor of Tribology at the University of Bradford. “But without lubrication, friction rose throughout the glass transition phase, eventually to a level that caused irreversible damage to the polymer.”

The team found that at lower temperatures, the polymer was eroded slightly by the steel to create a polymer transfer layer on the metal component. This creates a polymer-polymer contact which has very low friction and ensures the polymer component is not damaged, even without additional lubrication.

But, even under the same speeds and loads, a small temperature rise into the glass transition caused the polymer layer to increasingly lose its ability to support the load. A vicious circle begins: Increasing temperatures increase the friction which increases the temperature further, and so on, leading to a final failure as the polymer component is irreversibly damaged. As a result, a system that was stable at lower temperature became extremely unstable in the glass transition region. When lubricated with a model high temperature lubricant, friction remained stable for all four polymers even as the temperature continued to increase through the glass transition.

Product Development Engineer at ROCOL^® Lubricants, Dr Christopher Dyson, believes this research provides important lessons for manufacturers that use machinery with polymer-steel components

“Polymer-steel stenter chains that operate at high temperatures and high loads are generally lubricated, but many industry operators are not aware how lubrication stability in their machine can often be on a knife-edge,” says Dr Dyson. “Our findings explain why some systems respond predictably when lubrication becomes unsustainable, giving the operators time to react, whereas in other systems, those operating in the glass transition temperature range, the response can be unstable and severe. This highlights the importance of both optimised lubrication practice and a lubricant that can deal with these conditions to provide sustained performance.”

The research was conducted as part of a knowledge transfer partnership (KTP) that brings academic expertise to address an industry issue. The findings are published in the latest issue of Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology.