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The Sum of the Parts

seals-gasketsWhile it might be the aim of many engine designers to minimise the chance of any oil leakage by dispensing with the humble rotating shaft seal, it may not always be possible or even desirable. At any point where there is a mechanical take-off in the form of a rotating shaft, an oil seal will need to be present and while race engine designers try to minimise these occurrences, for the rest of us wishing to improve existing OE equipment or re-engineer old engines, that option simply does not exist. However armed with a knowledge of the design parameters around which these seals perform best, there is no reason why an oil seal shouldn’t be anything other than 100% reliable. But in doing so it must be remembered that the seal itself is but only one part in a system.

In designing any rotating shaft seal, the primary concern must be to take into account the degree of misalignment between the surface of the shaft and the lip of the seal. This is the result of two main factors – the eccentricity of the housing holding the seal and the dynamic run-out of the surface of the shaft. For plain journal bearings, where the journal effectively rattles around within the shell bearing, this also needs to be accommodated, however small the clearance.

The eccentricity or perhaps more accurately, the shaft to bore misalignment (often referred to as STBM) is purely down to manufacturing. On older engines which have been rebuilt a number of times it may be advisable to check the concentricity of this against the crankshaft journal if this is possible. A special fixture may be necessary but in terms of the ultimate in reliability and minimising any potential source of unwanted friction, this eccentricity should be kept to a minimum. The more time spent minimising this factor and the less work the seal has to do, the greater the opportunity to reduce the radial loads and hence parasitic losses at the crankshaft.

The dynamic run-out (or DRO) of the shaft is, however, an altogether much more complex issue. Expressed as the ‘total run-out of a position on the outside diameter of the shaft as it is rotating’, this can be caused by a number of crank manufacturing related problems. Apart from the difficulties in machining a journal to be precisely circular, wear in the grinding machine itself, however slight, can introduce unwanted geometric shapes, while the dynamic effects of the crankshaft also have to be considered. Possible whirling and the effects of the firing of individual cylinders can create a ‘shaft shake’ condition. This can create an out of round movement of the journal, which also has to be absorbed by the seal. At speeds upwards of 18,000+ rpm, such effects if not properly controlled can have a significant effect. Furthermore, although the surface finish will have an effect on the sealing performance, the way in which the surface was machined can also make the difference between a seal that works and one that doesn’t. In particular, in order to produce the desired surface finish of between 0.2 to 0.4 Ra but also ensure there is no machining ‘lead’, journals should be finish ground using a ‘plunge’ grinding technique only. Failure to do this could introduce a slight ‘lead’ or scroll into the journal surface and actually pump oil out from behind the seal through the action of the scrolling.

While there are many other aspects to the design of oil seal systems, only if the geometry of the installation is correct, can it ever hope to succeed.

Written by John Coxon.

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