Belt Tension Meter

The kinetic energy of components rotating about their axis can be transmitted to other components by means of gears, chains, cables or belts. Rope and belt drives are not form-fitting but frictionally connected to the pulleys. They enable high speeds with low-noise operation and do not require lubrication. The prerequisites for low-loss transmission of energy in belt drives are matched and correctly aligned belts and pulleys, perfect seating of the belt on the pulleys and correctly adjusted belt tension.

Belt drives are often equipped with built-in clamping devices. If necessary, the tension, which is too high or too low, can then be easily corrected. For this purpose, the manufacturers of machines and vehicles usually specify the permissible pretension values for each belt installed. With a special belt tension gauge, the pretension force on the belt can be checked at any time when the drive is at a standstill.

Belt tension or static pretension is the tensile force acting on the belt when the drive is not running. It is generated when the belt is tensioned on the pulleys. The belt length can change due to load, but also due to environmental influences, whereas the distance between the pulleys remains the same. Changing the belt length therefore has an effect on the pre-tensioning force and thus also on the efficiency of the drive and the service life of the belt.

On many belt drives, the tension can be easily increased or decreased by the built-in devices. Most types of belts need to be re-tensioned after one to four hours after the initial start-up of the drive. To do this, the belt tension is determined at standstill with the belt tension gauge, then increased and checked again with the belt tension gauge.

Excessively high pretension values lead to higher loads and greater wear on the belts and bearings. Too low pretension, on the other hand, causes the belt to oscillate, flutter or even slip, thus worsening the power transmission. Compliance with the specified pretension values ensures power transmission and the achievement of the maximum belt life. For this reason, the belt tension should be monitored regularly with the belt tension gauge and readjusted if necessary. The operating instructions provide recommendations for the frequency of checks. These should always be adapted to the specific operating conditions.

For uncomplicated determination of the belt tension, either the sag of the belt under compressive load can be measured mechanically at standstill or the vibration frequency of the belt after striking can be measured optically or acoustically.

Measuring principle, measured and calculated values
A mechanical belt tension gauge evaluates how much force is required to move the belt out of position by a defined amount or how far the belt can be moved when a defined amount of force is applied. Optical measuring probes record the natural frequency of the belt excited to vibrations with the pulsed light. The frequency of the vibrating belt is also determined with the acoustically operating devices.

The measured frequency is used in both the optical and acoustic measurements for comparison with the frequencies when the belt is correctly tensioned. If the span length of the oscillating belt section is known and the mass of the belt is known, the span force can also be calculated. Many models of the belt tension gauges offer input options for length and mass for this purpose.

Mechanical measurement requires contact at the exact location of the belt. Optical and acoustic measurements are non-contact, but the gauge probe must be placed close to the belt. For the frequency measurement, however, it is irrelevant at which point of the excited belt the vibration is detected.

Display and operation
The available devices also differ in terms of operability and readability. Some display only the measured value and others simultaneously also the calculated value of the pre-tensioning force and the underlying material data. With some models, limit values can be defined via the device menu. As long as the measured value is outside the limits, the belt tension meter then gives special optical or acoustic signals.

Memory and transmission options If extensive measurements are carried out, the results of which must be entered in logs and maintenance books, it is advisable to select a belt tension measuring device with a measured value memory. With some devices, the measured data can also be transferred directly via data interface.

Suitable accessories are often available for the belt tension gauges with optical or acoustic measuring principles. These include, for example:

• Frequency beater for generating a suitable oscillation,
• Tripod for setting up the measuring device,
• Magnetic holders for quick attachment to magnetic surfaces and
• Additional measuring probes for measuring points that are difficult to access.