Tech Talk Blog

Polymer gears are common place in many industries and applications. The advantages of a polymer gear include noise reduction, self lubricating features, dramatic weight reduction and cost savings. From paper mill gears to drive gears in copiers, polymer gears have been successfully used for years. But, everything you know about metal gear design gets thrown out the door with polymers. There are many different factors that have to be considered when designing gears out of plastics including thermal expansion and contraction, physical strength, moisture absorption and possible chemical exposure.

Typical polymers for gears are cast nylons, injection molded nylons, polyester and acetal. Various fillers help to strengthen the base polymers such as glass and carbon fiber, aramid fibers and other additives are used to improve lubricity. More recently high end polymers like PEEK and Torlon have been used to make high temperature gears or gears where exceptional strength is required. No matter what the material, designing polymer gears will require some extra thought and a change from the norm. Tooth profiles and overall height may need to be changed to accomodate bending forces. Contact conditions at the root of the tooth may need modifications from the norm. Flex strength of the polymer will definitely come into play so input and output torque requirements will need to be reviewed closely.

While there are a lot of advantages to polymer gears you can’t overlook the basic differences of steel gears and polymer gears. Material choice and adherence to design changes required to address the physical, thermal and wear requirements of the gear must all be looked at closely. Tri Star has several thermoplastic and thermoset materials used frequently for gears and our engineering department can help you in this process. Have more questions? Ask The Experts – they are bound to know something. Or check out our Video Learning Center for a deeper look.

The first part of the answer is we need to know what kind of Teflon. DuPont’s family of Teflon products includes a number of different materials. PTFE is the best know of this family and the answer to your question for this material would be NO! PTFE does not hold up well at all in gamma radiation. As an example, 4Mrads of exposure reduces the tensile strength of the PTFE to 2% of it’s original value.

A better fluoropolymer for gamma irradiation would be DuPon’ts Tefzel product which is known by the chemical moniker of ETFE. In vacuum or air irradiation, Tefzel maintains a high level of physical integrity after long exposures.

Another fluoropolymer that does well in gamma is PCTFE, formerly known as Kel-F. PCTFE has minimal property loss even after doses of 16-18Mrads of exposure. Rates of property loss in all main categories are less than 30% which is quite good for fluoropolymers.

TriStar Plastics has more information on different methods of sterilization and how it affects most medical grade polymers. Ask The Experts – you’ll still like them when they’re angry (but they won’t be)!

The life expectancy of filament wound composites like TriStar’s CJ product can be improved by understanding some of the typical failures. One consideration is the shaft material. In some applications where the motion is oscillating and at variable frequencies, the bearing can experience rapid wear. This is caused by small angle oscillation or small amplitude vibration and the resultant fretting corrosion. The wear debris that occurs can quickly become an abrasive that dramatically shortens life.

There are a couple of possible fixes for this problem. First, consider going to a 400 Series stainless pin. Testing by an independent company showed a 26 times improvement in wear life by making this simple change. The improvement was directly related to the elimination of the fretting corrosion that occurred on the 1215 carbon steel pin used before.

A second fix is improving the shaft finish and hardness. Another test showed that by going to a 50-55 Rc hardness on the shaft and improving the surface finish to 4-6 rms extended the wear life of an application from 500,000 cycles of life to over 1 million cycles.

One thing to look for if you are experiencing rapid wear of composite filament wound bearings is the debris. If the debris is a fine brown powder than the wear is normal. If the debris looks more like shavings or accumulates in large clumps on the shaft, then you may have a shaft material, surface finish or pin hardness issue that needs addressing.

That being said – CJ bearings are crucial for transportation and other roles. Find out more in our Shooting Star Archives!

A copolymer, though, is a polymer that has different numbers of repeating units. Copolymers are grouped according to the arrangement of the units in the polymer backbone.

The arrangement of the monomers in a copolymer is determined by the monomer types, the ratios between monomers, and processing conditions. Copolymers are the chemical mixture of two polymers in some ratio. Terpolymers are the chemical mixture of three different monomers like ABS. An engineer will polymerize monomers together to enhance strength, temperature resistance, or chemical resistance. Monomer forms can also be created through the parylene coating process.

If you still have a burning question, don’t hesitate to Ask The Experts!

A polymer is a very large molecule (macromolecule) composed of many small repeating molecular units (monomer). Polymers are formed from atoms that are capable of multiple covalent bonds. Such as the carbon atoms in ethylene CH2=CH2 molecule. Molecules with this type of bonding are said to be unsaturated. These compounds tend to keep this structure yet will readily react (under heat and pressure) to form more stable single bond structures; they will form a saturated compound. For example, ethylene will react to form polyethylene [-CH2-CH2-’]n . The [n] signifies the number of repeating units in the polymer backbone. This number can be from 1000 to ~300,000 units. The polyethylene material will have different properties based on the number of repeating ethylene monomer units.

From this simple compound, substitutions can be made to provide different properties. When one substitution is made the compound is a vinyl monomer. When two substitutions are made the compound is a vinylidene monomer. As more substitutions are made other compounds are created.

To recap, polymers are formed through chemical reactions under heat and pressure. Additives, ingredients, and conditions are designed to control how the polymer is formed and desired properties. This process is called polymerization. Polymerizing one kind of monomer will create a homopolymer as in polyethylene or polypropylene.

Visit TriStar to learn even more about polymers – or if you have a specific questions, Ask The Experts right away!

There are times when chemist / engineers need a little help remembering how to make a really cold bath…so here it is…