Further guidance
Calculating belt tension from vibration frequency (optional technique)
The test methodology is based on the theory of vibrating strings.
Strumming imparts a momentary force to the belt. If the belt is under tension, this will cause it to vibrate at its natural frequency (i.e. the frequency occuring with free vibration rather than with a constant forced vibration).
The belt's natural frequency decreases with length and increases (non-proportionally) with the tension per unit mass. Therefore, the shorter the belt and the higher the tension, the higher its natural frequency. We can calculate the tension within a belt if we measure its natural frequency and know its length and mass. For a string, the general relationship is given by:
T = 4 x m x l^2 x f^2
Where:
T is the tension, in newtons
m is the mass per unit length, in kilograms per meter
l is the belt span, in meters
f is the natural frequency, in hertz
To measure the span, measure the distance between either the two belt and pulley contact points or the pulley centres.
For example, a belt of span 0.5 m, unit mass 0.001 kg/m, vibrating at 223.6 Hz has a tension given by:
T = 4 x 0.001 x 0.5^2 x 223.6^2 N
T = 4 x 0.001 x 0.25 x 50000 N
T = 50 N
As belts are more complex structures than strings, belt manufacturers use variations of the above equation. For example, they need to account for the width of the belt using the number of ribs or strands, or the mass per unit area (rather than mass per unit length). However, the underlying principle is the same. For this reason, you must follow the procedures, data and calculations specified by the belt manufacturer.
Faults, effects, and symptoms
All belts attached to the engine's crankshaft pulley must be properly tensioned and aligned to ensure auxiliary and timing systems can be driven efficiently.
Tension with a force above or below specification can cause excessive stress and strain or excessive movement of the belt, pulleys, and tensioners. Symptoms may be:
- Premature wear and reduced operating cycles.
- Belt squeal, glazing, and overheating resulting in contamination of pulleys, rollers and tensioners from belt deposits.
- Pulley, idler, or tensioner bearing noise (rumble or whine).
- Misalignment of the belt and/or components.
Other indications of actual or impending belt drive problems are:
- Oil/coolant contamination, fraying, splits or cracks
- Pulley and tensioner wear, alignment, free-play, or float issues
- Incorrect belt seating on the pulleys
- Incorrect belt specification
