Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Polymer Wear Plates - Grade Selection and Key Advantages

As automation and industrialisation has evolved, the key objective for any mechanised system has always been twofold: higher speed with higher precision. While sensors, Servo motors, and computed aided software have allowed for ever increasing levels of precision, the issue of speed has always brought with it one unavoidable problem: friction.

In any system of moving parts, friction is the number one culprit keeping things from moving indefinitely and with minimal damage. Hence, methods to reduce friction have always played a vital role in ensuring the durability of mechanical systems.

One of the most common uses of polymers lies in the realm of friction reduction. The low thermal conductivity of polymers allows for better heat dissipation and lower wear outs. However, as this blog has always maintained – not all polymers were created alike. While most polymers can form an effective medium between moving metal parts, the issue of longevity narrows the list down significantly. Typically, a good wear material should have some or all of the following properties:

  1. Self-lubrication – especially needed for systems where oil cannot be used or where oils and other synthetic lubricants cannot easily be replaced

  2. Low coefficient of friction – this is paramount to ensure the smooth sliding materials that come in contact with the wear material

  3. Heat resistance – although not essential, some polymers are needed in environments where high temperatures would build up, even without excessive friction

  4. Robust - metals are many factors harder than even the strongest of polymers. A tough material is needed to last long term in an environment where metals are moving at high speeds.

In this regard, most of the common polymers – such as HDPE, LDPE, or PP – would be unable to take high wear loads for an extended period of time. PVC – which is stronger and has admirable strength – has the key drawback that the presence of chlorine in the material can corrode metal parts over time. Based on our experience, the following materials are the best suited among polymers for wear applications:

  1. POM (Delring/Polyacetal) – POM is not an immediate choice as a wear resistant material. However, it is inexpensive, mechanically strong, and capable of taking temperatures of up to 120°C. This makes it a comfortable choice in applications where the loads and RPMs are not very high, but where a spacer or bush between two metallic moving parts will allow for lower heat build-up.
    Further – the addition of PTFE fillers to POM (Delrin AF), allows for even lower coefficients of friction and higher loads as a result

  2. PA66 (Nylon 66, Nylon 6.6) – like POM, nylon is on the lower end of both tensile strength and coefficient of friction. However, PA66 bobbins and ferrules are excellent are taking friction in rotary applications. Again – the loads, temperatures, and RPMs cannot be very high, but the material is effective if used within the parameters specified.

  3. UHMWPE – although less known than some of the other polymers, UHMWPE rates one of the highest on out-and-out wear resistance. It would probably be on the top of any list of wear applications, but for the fact that it has a very low temperature rating. With the ability to only withstand 80-100°C continuously, UHMWPE is restricted to application in ambient temperatures. Still, it is very useful as a wear plate, or sliding plate, especially in railway applications, where the heat build up is limited. UHMWPE also forms a very effective medium as a bush or collar in low speed, high friction applications.

  4. PEEK – while usually too expensive to use in any regular applications the effectiveness of PEEK can be unparalleled when it is used properly. Specifically, HPV PEEK, which is PEEK with a blend of carbon, graphite, and PTFE, can endure high loads at high RPMs and do so in temperatures of up to 300°C.
    PEEK’s only drawback is that it is prohibitively expensive- costing 15-30X of any of the above polymers. But in situations where cost is not a factor, there are few polymers that can compare.

  5. PTFE (Teflon) – the most well know and versatile of wear materials, PTFE’s absurdly low coefficient of friction (as little as 0.03 against polished stainless steel) combines with an equally low thermal conductivity to offer a material that simply does not heat up even at high RPMs and loads. With a service temperature of 250°C, there is virtually no wear application where PTFE does not find consideration. While the material is relatively soft and therefore easily deformed, the addition of glass, carbon, and bronze fillers adds hardness, making the material suited to such diverse applications as:

    1. Piston bands in shock absorber struts

    2. Railway sliding plates

    3. Linear slideway bearing strips (Turcite/Lubring)

    4. Automotive wear plates

    5. DU Bearings/bushings

It should be mentioned that while PTFE certainly is the most popular choice, the wear resistance of UHMWPE still rates higher. Designers and engineers would do well to understand that if temperature is not an issue, UHMWPE is an excellent choice for any wear application. However, as most industrial and automotive systems operate at elevated temperatures, PTFE and PEEK are the most accepted and effective choices in wear applications.

Read More

1. PTFE Amplatz Sheaths - Specialised Tubing for Medical Applications

2. PTFE Wear Plates: Misconceptions and Applications for Heavy Equipments

3. PCTFE vs PTFE - A Comparison of Two Very Similar Polymers

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