Effect of shaft roughness on axial and radial seals at varying pressure and velocity

Radial and axial lip seals are used in a wide range of industrial applications involving rotating and reciprocating shaft as a way to retain lubricants, most often oil. Additionally, they provide a secure barrier against the access of external contaminating substances such as pollutants, powders, and gases. Predominant contemporary lip seals are made with mixtures of polymers, such as nitrile-butadiene (NBR), fluoroelastomer, silicone or polytetrafluoroethylene, frequently mixed with fillers, curing agents, pigments and antioxidants, depending on the field of application. The sealing mechanism is based on the contact between a thin portion of the lip surface and the shaft, where a mixed lubrication regime is created depending on different operating conditions; as put forward by various authors, a relevant aspect is the fact that the presence of micro asperities on the surface of the lip and on the surface of the steel shaft can promote the development of an elasto-hydro-dynamic lubrication regime, where pressure peaks and cavitation areas are spread in many microscopic spots along the circumferential contact area. This motivates our experimental study on the effect of shaft roughness on lip seals, in both cases of radial seals and axial seals, representing the two different types of relative motion between the surfaces. We designed and assembled two test beds, one for axial seals and another for radial seals. Both test beds are equipped with sensors and digital control systems. Experimental results show that the effect of roughness on the resisting force in axial seals is non-linear: increasing roughness initially gives a moderate improvement on friction, but excessive roughness gives high friction (and wear). The intermediate roughness value (Ra=1.19 μm) gave the lowest friction, at all sliding speeds.