Plastic Gears and Their Advantages and Disadvantages

Plastic gears are increasingly replacing metal gears in many industrial applications. They offer some advantages that metal cannot: lightweight, non-rusting and quiet operation. They can withstand shock and vibration, as well as operate without lubrication. They are also injection molded allowing for low cost and large production runs. They also provide greater dimensional stability than metal gears in most cases.

While these are excellent points for plastic gears, there are some cautions that must be taken into account when designing a machine with them. Some of these include: their limited strength compared to metal, a tendency to hold heat and a higher rate at which they absorb moisture. To overcome these limitations, engineers must develop a true picture of environmental conditions in which the gears will operate and make sure they use the right materials for the job.

The most common engineering plastics for industrial gears are nylons (POM and acetal) and polyphenylene sulfide (PPS). These can be produced by machining, a process known as hobbing, or by injection molding. There are a variety of additives and fillers that can be added to these base plastics for increased toughness, strength and ductility, giving them different properties that can be used in various applications. These include molybdenum disulfide in nylons and acetals, graphite, colloidal carbon and silicone.

A key issue is that plastics tend to be more sensitive to thermal changes than metals. As the temperature of a plastic gear increases, it can experience swell and warp, and may eventually break. To minimize these effects, the proper lubricant should be selected. It is important to know that a lubricant’s chemical compatibility with the plastic should be considered as well. Some lubricants are safe to be used with most plastics while others will deteriorate the material.

Another issue is that plastic gears can sometimes be noisy and less accurate than their metal counterparts. This is because molded gears require precise molding to achieve good concentricity, tooth geometry and other qualities. Some gears, such as helical types, even need cored teeth in thicker sections to control mold shrinkage.

In the end, it is critical that engineers choose the correct plastic for their application and then design the gear with its operating environment in mind. This will help prevent misapplication that can lead to costly failures, or worse, lock a thermoplastic out of an opportunity.

For example, conventional wisdom often rules out plastics for wet operating environments because they can absorb moisture so readily. However, engineers should realize that they can often offset this effect by designing in swell or by moisture-conditioning the gears prior to shipping.

Another example is that some plastics will not work with short impact loads, which are not reflected on static data sheets. These issues can be resolved by incorporating a spring or other counterbalance, or by using a plastic with better mechanical properties than the one listed on the data sheet. This can be done with some engineering plastics such as unfilled, high-temperature nylon.

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