Experimental Analysis of Friction Forces of Hydraulic Rod Seals—Effect of Pressure, Sliding Speed, Sealing Type, and Different Rod Coatings

Abstract

In a hydraulic cylinder, forces are generated by high-pressure oil. To contain the pressurized oil inside the cylinder, tight sealing is needed. The sealing tightness causes friction that directly decreases the performance of a cylinder. Additionally, at very low operating speeds, the sealing friction can cause stick–slip phenomenon and worsen positioning accuracy. In this paper, friction forces of hydraulic rod seals made of polyurethane and bronze-filled PTFE were measured on three different rod coatings. Measurements were planned according to the standard ISO 7986 and were conducted in pressure points of 63, 110, and 160 bar in constant sliding speeds of 50, 150, 300, and 450 mm/s. The results show that the sealing choice has the biggest impact on the sealing friction: the tested polyurethane U-cup seal induces friction forces 1.8–8.5 times higher than the PTFE-bronze seal depending on the operating conditions. On average, the polyurethane seal induces forces 4.1 times the forces of the PTFE-bronze seal. For the same seal, changing the rod coating can change friction values up to 43% for the polyurethane and up to 27% for the PTFE-bronze seal. Therefore it is clear that sealing type and rod coating have a big impact on cylinder friction.