Safety in Operation of Radial Shaft Seals - Investigations on Material/Lubricant Combinations

Safety in operation of radial shaft seals - Investigations on material/lubricant combinations

Introduction:

In a broad spectrum of applications, elastomer materials have shown themselves to be principally

suitable for providing reliable seals in engines, transmissions and hydraulic applications.

To be sure, there is a great deal of uncertainty in predicting the functional reliability of

radial shaft seals – firstly because the composition of lubricants for modern-day engines,

hydraulic systems and transmissions is constantly undergoing change due to increasing requirements

and, secondly, because there is hardly any well-founded knowledge on wear

processes and on temperature stressing in the contact zone composed of the radial sealing

lip and the shaft. For that reason, one of the objectives of the analyses carried out is – alongside

the evaluation of individual material/lubricant combinations – also the deduction of a test

process for assessing technical applicability.

Experimental:

NBR, HNBR, FKM and ACM materials (test plates and radial shaft sealing rings) were analyzed

in combination with polyalphaolefin (PAO)-based lubricants, including those with ester

content, as well as with polyglycol- and polyester-based lubricants. The approach taken included

endurance trials on a test rig, characterization of oxidative aging in the lubricant,

measurements of radial forces and temperatures under the sealing lip in continuous operation,

chemical and physical resistance tests including the use of lubricant additives and comprehensive

chemical analysis of what causes radial shaft seals to fail.

Summary of the results

In long-term testing, the seal rings made from the FKM material showed themselves to be

well suited in all lubricant combinations with the exception of a polyalphaolefin. Rings made

of HNBR material, on the other hand, were seen to perform inadequately with all oils tested.

During the investigations on HNBR and FKM materials, a sump temperature of 110 °C and

10 m/s sliding speed was applied. The failures in long-term testing correlate very with the

results of chemical analyses and a test of static oil-bath aging. Postcrosslinking and brittleness

were observed. Seal rings made of ACM likewise exhibited pronounced leakage rates

in long-term testing. Under a greatly reduced stress level of 80°C sump temperature and 5

m/s sliding speed, sulfur-crosslinked NBR material showed a good technical compatibility in

combination with a complex ester oil as opposed to a PAO (containing ester). The influence

of the lubricant additive was rudimentarily tested. The analyses lead to proposal of an accelerated

test procedure consisting of static preaging of the seal ring in an oil bath followed by a

dynamic endurance test. The chart below provides an overview of this testing sequence: