Why is Shaft Seals by Size Better?

I realise, after no less than 50 years taking bikes and complete engines apart and (after the first couple of years....) successfully putting them back together again, I've only ever bought oil seals as either a manufacturer's part or by the markings.

Now, as part of my fascination with stone age machinery, I have an old Chinese sidevalve 750 flat twin sidecar outfit, a Chang Jiang CJ750. It runs actually very nicely despite being even more primitive than my Harley. It needs immediately some fork seals, and I found a couple in its huge tool kit. Sadly they are old and hard with rusted springs, and have a numerical code which seems to relate only to a part number.

What I know is that the fork tubes are 40mm, and the overall size of both seals is 47.5mm. Internally they are 38mm- 2mm under the size of the tubes. So my questions are , is this normal, or do I buy new seals based on the shaft size? I can order the real thing from China, but it will take time and seals from say Timken are likely to be better quality. And no doubt there will be seals in the dripping final drive, the weeping gearbox etc etc.......

I'm very fond of it already. I usually greet it in the voice of a Bond villain, 'Ah so, Mr Chang will see you to your doom Mr Bond.....'.

With competitive price and timely delivery, KDIK sincerely hope to be your supplier and partner.

Nitrile Butadiene Rubber (NBR, nitrile)

NBR, also known as nitrile rubber or nitrile, is the most popular material for an oil seal because of its good resistance to many oils and greases, such as mineral grease and hydraulic oil. Depending on their composition, synthetic oils and greases, such as those based on glycol, can damage NBR rubber materials. Depending on the amount of glycol, a PTFE lip seal may be the best choice. NBR is also unable to cope with contact with acids and solvents. The rubber is suitable for oil and grease at temperatures from -35 °C to 100 °C.

Most ERIKS oil seals, such as the types M, MST, R and RST, are made of NBR as standard.

Fluorine rubber (FKM, Viton&#;)

FKM or FPM, which is in well-known brand Viton&#;, can withstand higher liquid temperatures of up to 180 ˚C. FKM is highly resistant to strong acids and bases, as well as to synthetic oils and greases. Glycol-based oil and grease, however, can also damage FKM.

Because of the higher temperature resistance of FKM, this material is also chosen for applications where higher speeds play a role, which raise the temperature at the sealing lip considerably. Usually, using FKM will result in a longer life than using NBR. This compensates the higher price of FKM compared to NBR, as an FKM does not have to be replaced as frequently. The low temperature resistance of standard FKM is limited to -15 ˚C.

For more information, please visit Shaft Seals by Size.

Polytetrafluoroethylene (PTFE, Teflon®)

PTFE, which is used in the well-known brand Teflon®, is less commonly used, but it is the preferred material for specific rotating seals in the chemical, food and pharmaceutical industries. This material is notable for having a very low frictional resistance and the best chemical resistance. It can also withstand a very wide range of temperatures in these types of seals; -80 ˚C to 200 ˚C. The shafts on which oil seals with PTFE lips are used require a harder and finer finish. Something like an axle sleeve can also be used to meet this requirement.

EPDM

EPDM oil seals are less common. They are used in solvent, hot water and steam applications, EPDM resists low temperatures down to -50 °C and UV radiation well. Some types of EPDM are also suitable for higher temperatures up to +150 °C. EPDM oil seals are usually available upon request.

VMQ (silicone)

VMQ, also known as silicone, is also used for oil seals, but this is less common because the mechanical strength of VMQ is low and this material has poor wear-resistance This makes it less suitable for dynamic applications, but it can withstand fairly low and high temperatures from -60 °C to 200 °C. Many types of VMQ are also suitable for contact with pharmaceutical and food products, so VMQ is an option worth considering. VMQ oil seals are usually available on request.

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