Articles of Interest

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Correct Tensioning of Drive Belts adapted by Keith Pickersgill

This applies to V-Belts, Poly-V Belts, Wedge Belts and to Poly-Flex Belts.
Correct belt tension is vital for long, satisfactory service life of any belt.
Under-tensioning leads to belt slippage causing rapid wear, whereas over-tensioning places excessive strain on the belt and bearings.
Between these two extremes is a reasonable range within which the belt will operate satisfactorily. Following these guidelines you will be able to find and set the appropriate level of tension on any drive.

Belt tension can be assessed by two means:
1. The Deflection Method or,
2. The Elongation Method

Belt Installation
• Pulley grooves should be in good condition, free from scores or sharp edges and should be dimensionally correct.
• Prepare the belt by marking two reference lines as required for checking by the Elongation Method.
• Mount the belt on the pulleys with no tension by moving the pulleys close enough together so that neither the ribs nor belt are damaged when fitting the belt. Arrange the belt so that both straight lengths between the two pulleys have the same amount of slack.
• Ensure the pulleys are in correct alignment before you procede. This can be done by placing a straight edge along the grooves of one pulley, ensuring they perfectly align with the other pulley. Both shafts must be parallel in every respect.
• Apply tension to the belt by increasing the centre distance (usually by means of a cam) while rotating the drive (easier if sparkplug is removed) to equalize tension on both sides.
• Tension until either of the following two criteria is met.
  1. Deflection Method
     The deflection force applied at mid-span produces a deflection equal to 16mm per meter of span distance (distance between centers of the pulleys), must fall within the range given in the table below. If the force required is too low, the belt is under-tensioned; if too high, the belt is over-tensioned.
     A new belt should be tensioned to the higher end of the range; and re-tensioned later to the lower end of the range.
  2. Elongation Method
     You should have two marks on the back of the belt, along one straight edge, approximately the same length as the free stretch of belt (not in contact with a pulley).
     Measure the distance between these two marks without any tension on the belt.
     Tension the belt until the marks have stretched apart a distance in accordance with the tables.
     A new belt should be tensioned to the higher end of the range; and re-tensioned later to the lower end of the range.
• The belt should run under load and observed for the first 30 minutes (on the ground, not in flight!) It may be necessary to re-tension during or after this period, as new belts will tend to stretch initially.
• Do not re-tension while the belt is hot - first let it cool down.
• Look for excessive wear of belt material forming black poweder around the area, caused by slippage resulting from under-tension.
• A good visual indication of tension is the slack side of the belt under peak load. If this fluctuates or bulges away excessively from the small pulley then the belt requires more tension. If it remains taut under peak load, it may be too tight.
• Beware of the spinning propeller during these operations and take all necessary precautions.
• When done, ensure the adjustment is securely tightened and cannot loosen by itself.