Abstract
The sealing performance of rotary shaft lip seals (below referred as »seal«) was investigated with view to interfacial phenomena.
Biodegradeable lubricants often cause higher wear than mineral based oils, and hence leakage. The root cause for this behaviour was supposed to be of interfacial nature. This lead to the approach of investigating surface tension properties of the seals components. Ten lubricants (polyglycols, poly-α-olefins, mineral based oils, silicone oils and esters), three elastomers (fluorocarbon rubber, nitrile butadiene rubber and acrylate rubber) and three shaft materials (steel, brass and glass) were investigated.
For all specimen the surface tension was measured. The surface tension was assumed to be splitted into a polar and a non-polar fraction. For the combination of each lubricant and each solid, the work of adhesion was calculated. This gave detailed information about the wetting behaviour of the lubricants.
For the lubricants some material values were measured: The dynamic viscosity, the specific density, the thermal conductivity and the specific heat capacity. These values were used to calculate dimensionless numbers of fluid motion. The onset of Taylor-Görtler vortices and thermocapillary instability could be estimated. For the onset of vortices, not only mechanical forces but also buoyancy forces (Bénard instability) were taken into account.
In numerous bench test runs, the following properties were investigated: The pumping rate and the frictional behaviour of elastomeric seals. In long term bench test runs, the wear behaviour was tested. For polytetrafluoroethylene-seals, leakage tests were done. The bench test runs were evaluated in terms of interfacial phenomena.
Themainresultsare:
• The work of adhesion between shaft and lubricant seems to be the main reason for the level of the seals pumping rate. Polyglycols with a high work of adhesion produced the highest pumping rates. • The work of adhesion between elastomer and lubricant plays a role as well. If this work of adhesion is smaller than the lubricants work of cohesion, the elastomer is not completely wetted. This results in high pumping rates and poor wearing behaviour. • At a critical duty parameter the seals lubrication condition changes. This change is attributed to thermocapillary instability at the border of the sealing zone. • Thermocapillary instability influences the seals pumping rate as well. This is due to temperature gradients from the sealing zone to its surrounding. For a high thermal conductivity of the shaft, this effect can be neglected. The achieved progress is an explanation of lubricant influence on the sealing performance of rotary shaft lip seals.
Links and resources
Tags
community
