Tuesday, February 7, 2012

Belt Failures - Tensile Cord Crimp

A very common failure mode for a synchronous belt is when the tensile cord is "crimped", occurring when the tensile member is bent to a radius smaller than recommended by the belt manufacturer. Crimping the belt will damage the tensile members and reduce the structural integrity of the belt.

Definition and causes of a Gates Synchronous Belt crimp
Belt Crimp

Tensile cord crimp can happen by mishandling the belt, hanging the belt on a nail (or similar object), or by running the belt on a smaller than recommended pulley diameter. Another common cause of a belt crimp is when the belt ratchets over a sprocket groove. The instantaneous motion of a belt tooth exiting one groove and entering the adjacent groove can crimp the tensile cord in the area where the tooth jumped the groove.

After the tensile cord is crimped it loses tensile strength which the belt relies on to transmit the load. If the crimp is bad, the belt will fail exactly in spot where the tensile cord was damaged by shearing straight across the width of the belt in a clean break.

Example of a Gates Synchronous Belt crimp failure and ways to avoid it in the future.
Crimp Failure

Ways to avoid a crimp failure with a synchronous belt are as follows:
  1. Be careful when handling the belt, especially when installing the belt in a tight area.
  2. If storing the belt, do not hang it on a nail or similar object. Use a saddle or rack at least as large as the minimum diameter sheave or sprocket recommended for the belt cross section.
  3. Follow the manufacturer's recommended tension for the belt drive to ensure proper tension. If you notice that the belt is about to ratchet teeth or already has done so, increase the tension in the drive until the tooth has a tight fit with the sprocket groove.
  4. Follow the manufacturer's minimum recommended diameter specification for the belt cross section being used. Gates' recommendations for minimum pulley diameters can be found in the preventive maintenance manual at www.gates.com/catalogs.

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