Belt tension

The purpose of this test is to measure the natural frequency of a belt (auxiliary or timing), as a means to check the applied tension.

How to perform the test

1. Ensure any belt manufacturer pre-test conditions are met (e.g. engine temperature or engine turning operations).
2. Take all necessary steps to prohibit the engine from rotating and cranking.
3. Identify and gain access to the belt test location, as specified by the manufacturer.
4. Measure and record the belt span. Use the distance between either the two contact points or the pulley centres.
5. Connect an NVH kit microphone to its interface box.
6. Connect the interface (use the Y-axis output, if an XYZ interface) to PicoScope Channel A.
7. Place and secure the microphone within 100 mm of the identified test location.
8. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
9. Start the scope.
10. Strum the belt, as you would a guitar string, in the middle of its span.
11. Your waveform will update with every strum and the belt’s natural frequency will be shown as the Frequency at Peak value within the Measurements table.
12. Strum the belt at least three times and check each time the Frequency at Peak value to ensure that the results are consistent.
13. Stop the scope.

Note

Shield the microphone to prevent erroneous readings, if background or wind noise is excessive.

Example waveform

Waveform notes

The “Scope 1” tab shows:

• A very rapid rise to a peak sound level when the belt is strummed.
• A gradual sound level decay for a period of 1 s.
• After the initial strum, the belt vibration settles at its natural frequency to give an almost pure, but decaying, sinusoidal sound level waveform.

The “Spectrum 1” tab shows:

• A series of peaks with diminishing amplitudes as frequency increases.
• The position of the first peak represents the natural frequency of the belt.

The Measurements table shows:

• A Frequency at Peak value, in hertz.

Further guidance

Calculating belt tension from vibration frequency

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 that occurs 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 ∙ m ∙ l² ∙ f_n²

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_n is the natural frequency, in hertz

For example, a string of length 0.5 m, unit mass 0.001 kg/m, vibrating at 223.6 Hz has a tension given by:

T = 4 ∙ 0.001 ∙ 0.5² ∙ 223.6² N

T = 4 ∙ 0.001 ∙ 0.25 ∙ 50000 N

T = 50 N

T = 4 ∙ 0.001 ∙ 0.25 ∙ 50000 

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.

The following webpage provides access to data and calculation methods from a variety of belt manufacturers:

Click here for belt manufacturer data and procedures.

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

GT890-EN