Why are the outsides of seals "ribbed."?

[H.G.P.]

As in rear main......Are they oil channels or other?

Thanks.

[slodave]

I think it is for the trannies pleasure. Sorry, couldn't resist. Dunno the answer.

Dave

[Hugh R]

Don't know about the outside, but the inside on many rear oil seals have angled striations so that they wipe the oil back into the crankcase. Kind of like a windshield wiper. Can't speak for pcar ones cause I've never had one apart.

[ianc]

Strength,

ianc

[jluetjen]

Can you show a picture of the ribs that you're talking about? Modern seals are surprisingly heavily engineered considering the relatively "simple" task that is being asked of them. "Race Engine Technology" just did a piece on them. To give you a sense of the factors involved...

1) Material: Nitrile Rubber (NBR), Polyacrylate Elastomers (ACM), Silicon Rubbers (SIL) and eventually Fluoroesastomer polymers (FKM, AKA Dupont's Viton) have all been used at one time or another. SIL and FKM are apparently the prefered material nowadays due to their exceptional high temperature properties, as well as being resistant to chemical attack (but not hot water BTW). Then there is Hydrogenated Nitrile Rubber (HNBR) which has higher temperature and chemical resistance capabilities compared to NBR. Finally, there are Fullyfluorinated perfuoroesastomer (FFKM) elastomers which are very expensive, but used in the most demanding applications because they are rated to temperatures of 315-320 degrees C and almost totally resistant to aggressive chemicals (including methanol).

2) Surface speeds (a function of the shaft's diameter and rotational speed): From 10 m/s for NBR up to 35 m/s for SIL and FKM. FFKM's apparently can get up in the range of 50 or 60 m/s.

3) The degree of misalignment of the shaft surface to the seal lip.

4) The amount of radial load, lip friction and the resulting wear of both the shaft and the lip. This will impact material, and sometime alternate materials such as PTFE will be used on the rubbing surface to reduce friction. This friction can sometimes be measured in kW per seal depending on the shaft speed.

5) The pressure/vacuum against which it is sealing.

6) The working temperature.

In addition to the material, there are other features which may be used:

A) Dust seals may be incorporated as a second lip on the outside of the seal. This will generally not touch the shaft, but instead be about .1-.2 mm away from the shaft.

B) The contact lip of the seal, in addition to any PTFE surface treatment, will be designed to support a band of oil about .2-.7 mm wide via capillary attraction.

C) The circular garter spring inside the lip which ensures that the lip stays in contact with the shaft under all but the most extreme conditions.

D) There is often a scroll imprinted lightly on the lip that returns the oil back to where it came. This scroll will be "handed" and do not take well to reverse rotation. I think that these are the ribs that you're referring to.

I hope that this helps.

[sammyg2]

To help seal on the outside diameter.

[H.G.P.]

Originally posted by sammyg2
To help seal on the outside diameter.

Then why the "ribs"? Friction-hold? Would the seal slide out otherwise? ..and if so, what could cause the outside "ribs"of the seal to move/slide?

Thanks