The Pitfalls of Fixed Center Belt Drives
What are timing belts?
Timing belts are toothed belts used with toothed pulleys to transfer power and motion in small motion control system. Throughout this blog, I am referring to timing belts and timing pulleys such as that shown below, though the teeth on the pulleys and belts are not always shown.
What is a fixed center belt drive?
Small timing belt systems are sometimes designed without any means of belt tension adjustment. This type of system is called a fixed center belt drive.
In Fig. 2 there is no adjustment of the motor or drive screw positions so the distance between the pulleys is fixed. There is also not an idler pulley to adjust the belt tension.
Why are fixed center belt drives used?
- Designers who are inexperienced with toothed timing belts sometimes believe that belt tension is not important. They think the belt teeth will drive the system regardless of the belt tension (within reason). This fails to consider how the belt life is negatively affected by either too high a tension or too low a tension or how the system’s dynamic performance might suffer.
- Designers sometimes incorporate fixed center belt drives in an effort to save cost by simplifying parts. But this is usually a false economy because product assembly is often more difficult and can take longer, belt life and system dynamic performance can suffer, or higher precision components that cost more may need to be used.
- Designers sometimes believe fixed center belt drives simplify assembly, but it can often be more difficult to assemble the pulleys and belt if there is no way to slacken the belt during installation. Often the belt must be looped over a pulley flange or a loose pulley with the belt wrapped around it must be forced over a journal or shaft. If excessive force is used, then journals and shafts can be bent.
Figure 2: A Fixed Center Belt Drive
What is the primary pitfall of a fixed center belt drive system?
The primary pitfall of a fixed center belt drive system is failure to consider the effects of system tolerance stack up on the belt tension. Belts and pulleys are manufactured to industry accepted production tolerances. Belt lengths and pulley pitch diameters are not perfect. In any large population there will be a distribution of values. There are also limits to the accuracy that the pulley distances can be maintained on a production basis due to machining tolerances.
Excessively low belt tension can be created when a relatively longer belt is installed on relatively smaller pulleys with relatively smaller spacing. Conversely, excessively high belt tension can be created when the reverse occurs. Such variations can cause timing belt tensions at installation to vary by 3X or more due to the stiffness of the belts which often have fiberglass reinforcement.
Excessively high belt tension bends shafts and journals, decreases bearing life, and decreases belt life. Excessively low belt tension can also decrease belt life as tooth engagement will be non-uniform and belt ratcheting may even occur.
Two Common Methods of Belt Tension Adjustment:
Slotted mounting holes allowing the pulley distance to be varied.
Including an adjustable idler pulley or roller which may be inside or outside the belt loop.
Ensure the diameter of the adjustable idler pulley or roller meets the minimum diameter requirements of the belt manufacturer or belt life will be decreased or the system’s maximum speed may need to be decreased to preserve belt life.
Using a spring loaded idler pulley to obtain proper belt installation tension is acceptable as long as the idler can be locked down after installation. Dynamic spring loaded idler pulleys or rollers are not generally recommended for timing belt drives. If they are to be used, they should always be used on the slack (or non-load carrying) side of the drive. Most small motion control application run both directions so the slack side of the belt loop varies.
Sometimes fixed center belt drives can't be avoided. These recommendations will maximize the probability of success:
- Do not use a fixed center belt drive for power transmission drives. Excessive belt tension caused by the fixed center belt drive design plus the load on the belt due to the drive load could exceed the belt load rating causing very short belt life or belt breakage. Consider using a fixed center drive for only lightly loaded or motion transfer drives.
- Do not use a fixed center belt drive where high registration precision or high dynamic motion quality is required. High belt tensions can cause journals to bend causing system run-out and variations in system performance. Low belt tensions can cause non-uniform tooth engagement or even belt ratcheting which can impair system accuracy and repeatability.
- The pulleys’ center distance must be held as accurately as possible.
- Minimize pulley pitch diameter variation and belt length variation. Pulleys and belts for fixed center drives should be manufactured with a process that is capable of producing the required pulley pitch diameter and belt length accurately enough. Variation in part populations should be measured and known.
- Finally, the performance capability of the drive system should be verified by testing belts produced over their full length range, with pulleys varying over their full pitch diameter range, on drive systems representing the full potential center distance variation.