Sunday, November 24, 2019


Drive belt failures are one of the most common problems in industries. Premature failure of the belts is caused by improper installation, incorrect pulley alignment and improper tensioning of the belts. Hence, it is discussed here on how to properly install, align and tension the belts.

Belt Installation:
For easy installation, some people use screwdriver or pry-bar to push the belt(s) (known as rolling or prying) into the pulley grooves. This is done to not disturb the alignment. But, in doing so, the belts will get damaged. The highest quality belts are said to lose 50% of its initial tension during operation. So, if we are using improper methods like screwdriver/crowbar in pushing the belt, it will further reduce the tension, due to the plasticity of rubber. Also, it might damage the fiber cords inside the belt which leads to a lower belt life.
The proper method will be to bring the movable equipment closer to the fixed one, to make the installation of belts easier with hand. After placing the belts properly in the pulley grooves, bring back the movable equipment to the original position and adjust the belt tension.

Belt Tensioning:
To reduce the maintenance time, some people use "feel method" for belt tensioning. As per the study conducted by Bestorq, only 1% of such belts are tensioned correctly. Proper belt tension is an important factor in determining the belt life as well as the bearing life. High belt tension will induce greater stress on bearings and shaft, whereas low belt tension leads to excessive slip and temperature rise, both of which affect the bearing and belt life.
The belts can be tensioned properly using a pencil type tension gauge or by using a belt frequency meter. Pencil type tension gauge is the cheapest device for tensioning and can be purchased for approx. USD 40. The tension gauge uses force per deflection method to identify the belt tension. The recommended force for a belt type will be provided by the manufacturer or will be available on their website. The deflection for standard V-belt is generally calculated as 1/64 inches per inch of span length.
Allowed deflection for V-belts = (1/64 ") * Span length in inches

The calculated allowed deflection value is set in the pencil type tension gauge by moving the rubber washer. The small washer on the plunger indicates the force value. Initially, it is kept at zero. The gauge should be kept at the center of the span length, on the tight side of the belt and force is applied until the deflection value set in the gauge comes in-line with the other belt surface or in case of a single belt, the initial belt position marked by using a straight edge or other methods. This force value should be within the limits provided by the manufacturer.

By using a belt frequency meter, the values such as belt span and mass data (which can be availed from manufacturer) should be entered and the sensor is kept as close to the centre of the span. The longer edge of the sensor is kept parallel to the belt axis (along the belt span) and with a 1/2 inch gap from the belt. Tap the belt with some tool to make it vibrate. The meter will show the belt tension in units of force. In short spans, these instrument tends to give wrong readings. So, it is always good to check with a calibrated pencil-type tension gauge.

Pulley Alignment:
Pulley alignment is one of the major factors determining the belt life. Incorrect alignment leads to uneven tension across the belts and will lead to the flapping of belts with the lowest tension. The belts which flaps might turn-over and lead to failure. While the other belts have excess tension and these fail quicker than normal.
Generally, pulley alignment is done using straight edges for a smaller centre to centre distance or by using thread for a longer span. But the precise method will be to use laser alignment tools such as SKF belt alignment tool TKBA 40, Easy-Laser etc.
Always remember, the grooves are supposed to be aligned and not the sheave surfaces. Hence, it is always better to measure the rim thickness. If the thickness is different in both pulleys, then care must be taken to make the grooves aligned. Because of the same reason, it is called as belt alignment.

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