Timing Belt Maintenance and Belt Failure

There are many advantages to a timing belt: positioning accuracy, speed, low maintenance, and high efficiency. In order to keep your belts in good working order there are some basic maintenance and installation procedures that should be followed. If your belt does fail prematurely what do the modes of failure indicate? Has your belt been exposed to contaminants, has it been crimped, is your alignment off?

Proper belt tension and alignment is critical for belt life, without which a belt will fail prematurely and cause downtime. To tension the belt, a take up system is required. Take-up can apply tension to the toothed side (inside), or to the outside (back side) of the belt. Check with the belt manufacturer to find the minimum bend radius for the belt you have selected. The inside and back side minimum bend radius will be different so double-check before designing.

Once you have selected your method of tensioning and know the center distance, number of teeth on each pulley, power, and operating speed, you can consult with the belt manufacturer to determine the proper tension for the belt. They will provide you with four figures:  new belt, old belt, and the maximum and minimum tension for each. The new belt and old belt figures will differ due to belt break in or seating into the pulley, as the belt sets into the pulley the tension required will decrease. A belt is considered broken in after 8 hours of use, which, in industry, is considered an old or used belt. Now that you know the correct tension you must apply the tension to the belt. There are two good ways to set belt tension; the first method, a spring plunger, while effective and low cost, is not the most accurate method of measurement. The second method uses vibrations similar to a guitar tuner, which is more costly, but also the most accurate way to measure tension.

Plunger tensioning:

Below is a typical spring tension tool – there is an O-ring on the shaft of the plunger that will move in order to indicate the force, given a certain amount of deflection.

Once you know the correct force and deflection from the manufacturer, use a straight edge or a string as a point of reference and the ruler gaige on the deflection side of the tool to determine how far you need to move the belt. The deflection the gauge will return a force value using the O-ring indicator on the force gauge shaft. Use your take-up to adjust your belt accordingly.

Sonic tensioning:

By inputting system parameters into a sonic tension device it can use frequency to calculate tension. Consult the device’s manual for details regarding its operation. Simply pluck the belt like a guitar string and place the microphone over the belt to take a reading as shown below. Once the reading is taken adjust the tension accordingly.

Alignment during belt installation is essential to maintaining belt life. There are two types of misalignment; angular and parallel; each type is pictured. The method of adjusting alignment will depend on how your machine was designed. Use a string or a straight edge to make sure that 4 points on both sides of the pulleys will come in contact with the straight edge; if there are not 4 points of contact the pulleys will require adjustment.

Angular misalignment:

Parallel misalignment:

If your belts are wearing prematurely, analyzing the failure modes is important so that the system can be repaired properly.  Below are the most common forms of failure and how to correct them.

Misalignment:

One side of the belt shows more wear than the other side. Look for cracked teeth or a single frayed edge; tooth radius cracks are also possible. The alignment is not set correctly in this situation; the pulleys must be realigned according to the method outlined above.

Crimp Failure:

Crimp failures happen when the belt is folded over creating a stress concentration in the belt. You can observe an even break in the middle of the belt almost as though the belt has been cut. To fix this issue replace the belt with a new one, being sure that the new belt was stored properly and has not been folded over. Avoid using tools like screwdrivers to remove belts.

Fit failure:

If there is excessive wear in the land area (valley) of the belt, typically the cord will be exposed. This is due to an over tensioned belt or excessive temperature. If the belt is over tensioned, consult the belt manufacture to determine the correct tension and adjust accordingly. If the temperature is too high, causing pulley expansion resulting in higher belt tensions, a lower installation tension may be required.

Shark toothing is another indication of fit failure, in this case the belt is too loose. Notice that one side of the teeth is more worn than the other giving it a shark tooth appearance. Again contact the manufacture and determine the correct tension.

Environmental:

The belt will show excessive wear or cracks. View the back of the belt for cracking or an uneven break across the belt. This occurs due to environmental contaminants, chemical exposure, or high temperature. If this is the case a belt may not be the best solution or a non-rubber belt may be required. A guard can be placed over the belt to prevent particle contaminants like metal, dust, or other abrasives from causing premature failures.

Now that you understand how to properly set up a belt system and how to trouble shoot one you should be able to extend the life of your belts and reduce down time.  Good luck setting up your belt and pulley system.