Lifespan Key for Bearings

Engineers have a world of choices in bearings. Among their first decisions is whether to go with metal or plastic. Here is some advice on that score from a manufacturer of plastic bearings.

How should engineers decide between metal and plastic bearings?

The first thing any engineer should consider is the lifespan of the application and what bearing is going to exceed that lifetime, preferably without maintenance and with minimal purchasing and operating costs. A bearing is one of the most inexpensive parts, yet ironically one of the most critical. It can shut down a machine if it fails. The second factor to consider is maintenance. Maintenance should be minimal or non-existent. The less a customer has to do once an application is up and running, the better.

Why not always use metal?

Moisture will corrode metal. Metal can't handle edge loading and dirt will scratch the thin Teflon lining or get caught and ruin the bearing. In the past, metal bearings have been used in applications they were not optimized for because they did the job adequately enough and there really weren't any other options. They may have performed well initially, but not over time. Things like corrosion and other long-term effects will eventually develop. Metal bearings often require regular maintenance and if it's not done consistently, the performance of the bearing will diminish considerably.

Why use plastic?

Plastic bearings are self-lubricating and maintenance-free, which ultimately leads to a lower cost over time. The plastic-based construction eliminates corrosion and delivers better vibration dampening than metal bearings. Plastic also can handle edge loading better than metal-backed bearings. Metal is not going to give, so it's either going to wear, create noise or, worst-case scenario, ruin the shaft. Plastic has high shock absorption and emits low noise.

What are the disadvantages of plastic bearings?

Extreme speeds, massive loads, and high temperatures are possible for the high-end polymer materials but are difficult to achieve with the more affordable plastic compounds. Therefore, metal often holds up better under accelerated life testing in the lab where these conditions are intentionally applied. However, the machine will never be exposed to these conditions in reality. This is true for any plastic component, whether it be bearings, gears, or pulleys. It's an issue that a lot of engineers don't think about.

Are plastic bearings considered proven technology?

Yes and no. Engineers who have used them usually did so as a last resort. But, once they did, many never go back. In the minds of others, plastic is a niche product. At the low end of the spectrum are the everyday plastic bearings manufactured at a local injection molder. On the high end are extremely expensive, specially designed plastic materials like Torlon or Vespel. Ninety percent of engineers are working on applications that fall somewhere in between.

Do engineers know enough about materials and materials processes out of college?

Yes, but all engineers need to understand the difference between a simple plastic, and an engineered high-performance polymer material.

Reach Blase at cblase@igus.com.