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Old 08-28-2010, 12:12 PM   #62
Tseta
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Joined: Nov 2008
Location: Finland
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Quote:
Originally Posted by gunnerbuck
The seal you used for a replacement is just a generic seal available at any bearing shop... The oem unit has the metal outer plate to make it a little more damage resistant...
To expand on this topic, I found a very good article: click here.

It is my understanding that the case of the seal is (almost) always made of metal to give the seal rigidity and strength. The common "generic" seal being discussed just has this metal case coated by rubber on all sides. For some reason, KTM has decided to use non-rubberized shaft seals on most of its LC4 engine shaft seal applications.

The article linked to above discusses to great detail the differences and pros and cons of the different designs and coating options for the seal's case.

Some highlights quoted below:

Quote:
Metal O.D. seals are economical and well suited for a variety of standard uses, including non-pressure fluid sealing and grease sealing. Metal O.D. seals have proven very effective when placed in steel and cast iron housings.
Quote:
Rubber O.D. seals are often used in applications where metal O.D.s will not work. For example, what if the housing in your application is aluminum rather than steel? A metal O.D. seal won’t be your best bet. The reason: differential thermal expansion of the metals in use. When heated, aluminum expands at roughly twice the rate of steel. Progressive expansion as a result of thermal cycling will decrease the interference (retention force) between a steel O.D. and an aluminum bore. Less retention force means the seal will be allowed to “walk” (move) within the housing. Leakage becomes a possibility.

...

At .010" to .050" thick, this rubber coating encapsulates the seal’s metal case and ensures good contact between the O.D. and the bore. In actuality, a rubber O.D. allows for a higher pressfit than a metal O.D. does, and less force is exerted on the housing. In addition, the rubber coating is capable of maintaining a tighter, more “reactive” fit during thermal expansion (and later contraction) of the aluminum housing. Rubber O.D. seals are also good in corrosive environments; the rubber coating shields the metal case.
Quote:
Though it offers many advantages, a rubber coated O.D. seal does have drawbacks. The rubber portion can be damaged during installation if proper lead-in chamfers are not built into the design. Care must also be taken due to a phenomenon known as springback. Springback is the tendency of a shaft seal with a rubber O.D. to unseat itself slightly following installation due to shearing stresses between the rubber and the housing bore.

...

Even if installation is perfect, excessive heat during service may cause the rubber coating to take a compression set, thus creating a leak path. In order to compensate for rubber’s higher coefficient of thermal expansion (compared to metal) and for the greatly reduced stiffness of the rubber O.D. (again, compared to metal), greater initial interference between the seal and the bore is required than when using metal O.D. seals.
By navigating on the site linked above, you can find out much more information about shaft seals principles, design, selection and such.

Cheers,

Tseta

Tseta screwed with this post 08-28-2010 at 12:51 PM
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