Joint SPI

A joint SPI is a term familiar of mechanics indicating a ring of radial sealing for revolving parts (case of the Vilebrequin S, camshafts, etc) or sliding (case of the fork of motor bike treated here). The origin of the term comes from the German company SPI which was the first to manufacture this type of joint. The ring is out of technical elastomer, it consists of a fixed case with force in the part fixes and of one or more machined lips very precisely which ensure the sealing by tightening against the moving part, this tightening being ensured by the famous spiral spring or the natural elasticity of material, possibly helped by the difference in pressure between the two faces of the ring. Any assembly of a roughly equivalent part, slightly of through, with back, or the least wound of the ring or the lip results in an escape which makes show these famous joined SPI of all the evils when it would be necessary to look after their installation and their conditions of use.

The longevity of the sealing rings to lips is such as it is not rare to see the metal range of a tree used with the site of the ring (see the paragraph devoted to the wear by abrasion in the wikilivre of tribology): in this case the replacement of the presumedly faulty ring does not correct obviously the problem! On the joints for revolving parts the sealing can be supplemented by grooves in spiral in front of the range, which move away oil from the ring. Certain English cars used only this last process with a Presse-étoupe to sponge the oil surplus, the stuffing box is now replaced by a ring with lips, even on the old cars.

The installation is generally carried out using a ring of adapted diameter and deposits it more delicate since one should damage neither the machined surface of the tree nor the housing of ring, also machined, using an extractor specialized or more often in screw autotapping machines taken in the case of the ring and being used as board puller.

Joint SPI of fork

The joint SPI ensures the sealing between the tube and the sleeve of the Fourche of the motor bikes. It prevents the Huile present in the sleeve from running along the tube. If the tube of fork is fatty, then joint SPI is damaged and it should be changed.

Many attempts were done to protect joints SPI from the premature wear which occurs when the tube of fork is clogged:

in the years 1970, a sliding metal mask hid the tube of fourche ; some as Harley-Davidson preserve this mask for esthetic reasons on the models of style rétro ;
the bellows of rubber was also tested, but it had the disadvantage of retaining dust and of worsening the problème ; there remains used on certain motor bikes of cross-country when the fork works more in mud than in the air ;
nowadays, one uses mainly, on the motor bikes of road, a small plate, which protects bottom from the visible part of the tube of the impacts of Insecte S or fine gravels, so that there remains always quite smooth. The longevity of these parts also largely improved following the development in the technologies and materials.

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