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Belt Creep.

Posted by Mark Kelly (M) on February 2, 2006 at 20:25:57

Further down the page I posted with a link to an article on belt creep.

I've been through the article now and have found some interesting results.

Executive summary : Belt creep limits the speed stability of a belt drive system. Typical speed stability on-load for a belt sytem running open loop will be 0.2%.

The part of the article that really got me sitting up was this: "if the pulley is assumed to be rigid and the belt stretching is assumed to be isothermal, the adhesion condition implies (that) the belt must maintain a constant strain in the adhesion zone. This further implies that belt tension is also constant through this region, and thus (that) no frictional forces are supported by, or exerted on, the belt."

In other words if the belt didn't creep it could not transmit the driving force. This means that belt creep is an essential part of the belt drive system which cannot be engineered out. The authors go on and prove mathematically that in fact the supposed adhesion zone cannot exist, only creep zones do. Given this, the creep can be approximated as

Creep = T/r x A/E

Where T is torque transmitted, r is pulley radius, A is belt cross sectional area and E is the elastic modulus of the belt material for small strains. Thus a torque of 1mNm on a 10mm diameter pulley using a belt of 10 mm2 and made of rubber with a modulus of 50MPa will display 0.4% creep. If the torque reduces by half so does the creep so the speed change on load for a 0.5mNm load variation is around 0.2%. The 0.5mNm load variation is pretty typical of the stylus drag changes found on turntables.

Mark Kelly

• The paper.   (Open in New Window)

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