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How to correctly measure a flat belt

Ordering a flat belt, some considerations:

1) Finding the correct length:

Most belt-drive machine tools have some sort of belt-tensioning mechanism. This can vary and might include:

  • One or more jockey pulleys that press on the back of the belt
  • A motor mounted on an adjustable hinged or sliding plate or slotted arms to provide a "rough" setting for the belt tension.
  • A "fine-setting" mechanism with an over-centre lever of some sort that allows the belt tension to be released, the belt moved from pulley to pulley and then re-tensioned. The mechanism usually includes some form of screwed rod with a turnbuckle or lock nuts to make the final setting easier to set and then secure.
  • Grinding machines sometimes have a weighted jockey pulley to keep the belt tight as the head rises and falls. To get the correct length, lift the head to its highest position and measure without any tension on the belt.
  • Sometimes, on very cheap machines, no tension adjustment arrangement is provided other than by moving the motor or drive unit backwards and forwards on the bench.

To make certian that you order the right length you'll need to ensure :

  • any jockey pulleys are slackened right off, or:
  • the drive and driven pulleys are brought as close together as possible - then moved apart by about 10% of the available range, or:
  • the screw-tension adjuster (or other device) is set to give the minimum tension - i.e. the two sets of pulleys are brought as close together as possible.

Measuring at the "shortest setting" will allow the maximum adjustment to be available to compensate for stretch as the belt settles in service.

  • For flat belts run a dressmakers' tape measure around the pulleys, or a length of tape or string that can then be laid flat on the bench to be measured.
  • For V-belts and round belts, measure the length with a piece of rope that just fits into the pulley groove and does not protrude more than a mm or two above it.

2) Efficiency

To work properly a flat belt must run over pulleys that are in line and held on shafts that are parallel to each other. It's essential that the pulleys have a slight dome across their width - this causing the belts to self-centre. If the pulleys are home-made they may lack this essential feature and the belts will wander from side to side or even throw off.
Another cause of belts wobbling or coming off is uneven wear. An old belt that is alternatively thinner and thicker may be worn unequally across its width - this causing the belt to move sideways as it wraps around the pulley. The solution is to use a new belt and check - very carefully and with patience - that everything is perfectly in line. Need more direct help? Phone: 01298-871633 or from overseas: +44-1298-871633