The effect of lubrication on the friction performance in pneumatic clutch actuators

Pneumatic clutch actuators in trucks transform the force applied on the clutch pedal into the force acting on the clutch. The actuator consists of an anodised aluminium cylinder in which a piston, with a rubber lip seal and a PTFE guiding ring mounted on it, slides. The system is lubricated with silicone-grease before assembly. Wear of the lip seal will lead to a deteriorated sealing capacity, and ultimately failure. A test setup is used to evaluate the effect of silicone grease amount on friction performance, PTFE transfer and rubber lip seal wear. Reciprocating sliding and vibration tests were performed. The surfaces of cylinders, guiding rings and lip seals were studied by SEM and ESCA. The friction level is highly dependent on the amount of grease in the contact area. During sliding, grease is pushed out from or dragged into the contact area, resulting in a fluctuating friction. Furthermore, even with grease lubrication, PTFE transfer to the aluminium surface occurs. A triple transfer from guiding ring, to aluminium surface, to lip seal, to another part of the aluminium surface was observed. Finally, although the unstable lubricating effect, it was found that the grease protects the lip seal from wear. (C) 2013 Elsevier B.V. All rights reserved.