The typical setup of a belt and pulley system consists of two pulleys and a belt connecting them together. Some important terms are the pitch diameter, which is the diameter of the pulley; the center distance is the distance between the center of the two pulleys; the minimum wrap angle is a measure of how much of the smaller pulley is being gripped by the belt; the belt length is simply the length of the belt if it was cut and laid flat; and the pitch is the number of teeth per some length. For instance, a 3 mm pitch means 1 tooth for every 3 mm.
There are some general guidelines that are applicable to all timing belts, including miniature and double-sided belts:
- Systems should always be designed with ample reserve horsepower capacity. Use of overload service factors is important. Belts should be rated at only 1/15th of their respective ultimate strength. The pulley diameter should never be smaller than the width of the belt.
- Belts are, in general, rated to yield a minimum of 3,000 hours of useful life if all instructions are properly followed.
- Belt drives are usually a source of noise. The frequency of the noise level increases proportionally with the belt speed. The higher the initial belt tension, the greater the noise level. The belt teeth entering the pulleys at high speed act as a compressor and this creates noise. Some noise is the result of a belt rubbing against the flange, which in turn may be the result of the shafts not being parallel.
- The choice of the pulley material (metal vs. plastic) is a matter of price, desired precision, inertia, color, magnetic properties and, above all, personal preference based on experiences. Plastic pulleys with metal inserts or metal hubs represent a good compromise.
Also, some tips for a smooth-running system include the following:
- At least one pulley should be adjustable to allow for belt installation and tensioning.
- A minimum of six teeth in mesh and at least 60 degrees of belt wrap are recommended.
- Pretension belts with the proper recommended tension. This is important to maximize belt life and prevent ratcheting of the belt or tooth jumping.
- Make sure shafts and pulleys are aligned to prevent belt-tracking forces and belt edge wear.
- Don’t crimp belts beyond the smallest recommended pulley radius for that belt section.
- Select the appropriate belt for the design torque.
- Select the appropriate belt material for the environment (temperature, chemical, cleaning agents, oils and weather).
A final word about environmental considerations: Belt and pulley systems are suitable for use in a variety of environments. Special considerations may be necessary, however, depending on the application. A few of the more common environmental factors are:
- Dusty environments do not generally present serious problems as long as the particles are fine and dry. Particulate matter will, however, act as an abrasive resulting in a higher rate of belt and pulley wear.
- Debris should be prevented from falling into belt drives. Debris caught in the drive is generally either forced through the belt or results in stalling of the system. In either case, serious damage occurs to the belt and related drive hardware.
- Light and occasional contact with water (occasional wash downs) should not have any serious effects. Prolonged contact with constant spray or submersion results in significantly reduced tensile strength in fiberglass belts, and potential length variation in aramid belts.
- Light contact with oils on an occasional basis will not generally damage synchronous belts. Prolonged contact with oil or lubricants, either directly or airborne, results in significantly reduced belt service life. Lubricants cause the rubber compound to swell, break down internal adhesion systems, and reduce belt tensile strength. While alternate rubber compounds may provide some marginal improvement in durability, it’s best to prevent oil from contacting synchronous belts.
- The presence of ozone can be detrimental to the compounds used in rubber synchronous belts. Ozone degrades belt materials in much the same way as excessive temperatures. Although the rubber materials used in belts are compounded to resist the effects of ozone, eventually chemical breakdown occurs and they become hard and brittle and begin cracking. The amount of degradation depends upon the ozone concentration and duration of exposure.
- Rubber belts may not be suitable for use in clean room environments, where all potential contamination must be minimized or eliminated. Urethane timing belts typically generate significantly less debris than rubber timing belts. However, they are recommended only for light operating loads. Also, they cannot be produced in a static conductive construction to allow electrical charges to dissipate.
Filed Under: Motion Control Tips
