Sliding vs Rolling: Benefits of Plastic Linear Bearings

May 5, 2022 Shizu Yamaguchi

Prefer to watch the webinar? Here you go: https://www.youtube.com/watch?v=SLXX4kHNp5s

The topic of today's webinar is sliding versus rolling: the benefits of plastic linear bearings. We are going to discuss how to correctly implement plastic linear plain bearings into applications where you are likely more familiar with designing in recirculating ball bearings. Implementing plastic linear bearings is a way to help you improve the lifetime of your equipment, reduce costs and eliminate the need for grease from your applications.

This presentation is technical in nature and there are a number of key takeaways:

  1. How composite plastic bearings are made
  2. How what we call motion plastics or engineered plastics differs from simpler commercial plastics available on the market
  3. How they work
  4. How they differ from recirculating ball bearings
  5. How they can help reduce the maintenance and downtime associated with lubrication or lubrication-related failures in the field
  6. How they are able to offer cost reductions through self-lubrication
  7. How to properly implement plastic linear bearings into your designs
  8. Different application examples (so you can better understand when and where to use plastic bearings)

At igus, we aren't necessarily inventing new plastics, but we are developing new tribologically optimized, composite plastic compounds that are specially formulated for bearing applications.

A Note on Tribology

Tribology is a core competency that igus focuses on when creating motion plastics. We study mating surfaces, including the wear and the frictional characteristics of these systems. Basically, we try to develop materials that enhance plastics for bearings.

A lot of you are used to using parts that require oil and maintenance. If we can reduce oil costs and wear, we might be able to reduce the overall cost of your systems and applications.

Design of Composite Plastic Bearings

We start with base polymers. These polymers could be a POM or PA base. These polymers are responsible for the main mechanical properties of the bearing. It gives the core strength, carrying capability and the core temperature limits of the product.

To that, we add a matrix of solid lubrication. Sometimes that takes the form of PTFE fillers. There are other types of solid lubricants, sometimes silicone, that we add to the bearing materials. These lubricants are responsible for reducing friction in the application.

We also add strengthening fibers and reinforcement compounds into the base material, which not only help the load carrying capability, but they also help the wear resistance of the parts. This bearing itself is only one half of the equation in a bearing system. The other half of the equation is the shaft or the rail that the bearing is riding against.

Counter Phase and Washdowns

The counter phase occurs when transfer of the bearing material goes into the microfinish of the shaft.

What’s important to understand is that once the transfer is made, it can't be washed away. This is a core benefit of plastic parts. These parts work well in applications that are submerged or submitted to washdown environments.

In a lot of bottling lines, caustic washdowns take place. If you were trying to use bearings that required wet lubrication, as soon as they were sprayed with the washdown, the bearing lubricants would be leached from the system. This leaching means that in order to maintain the whole line, you would need to re-administer lubrication. Plastic bearings don't require this and are well-suited for applications that have chemical washdown or are submerged in water.

Geometry of Plastic Parts

A lot of people are used to seeing plastic bearings in a simple sleeve or flanged form. However, plastic linear bearings can also have features on the outer diameter that can help anti-rotation in applications. These features can keep the parts mechanically fixed in the axial direction.

Grooves are an important feature of the inner diameter (ID) of plastic bearings from igus. Grooves serve two purposes. One is that they are dirt channels. If your application is heavily contaminated by dirt, soil or particles, grooves can act as a channel for that media. Even more critically, they're able to reduce the amount of thermal expansion of the ID of the bearing towards the shaft. That means that we can produce parts to a tighter tolerance, with a tighter clearance in the application than you could with a standard sleeve bearing. A standard sleeve bearing, as it swells, would only have one way to go. That's towards the shaft, which could cause bearing failure. Therefore, the grooves on the ID are a critical feature of a plastic thermoplastic bearing.

How igus Plastics Compare to Generic Plastics

The above chart is a frictional com parison between generic plastics and of a composite plastic we call iglide J, which happens to be a linear bearing specialist from the igus line of motion plastics.

Note how the friction characteristics are different from the simpler plastics on the right such as PEEK or POM. You will see that iglide J has better friction on anodized aluminum and case-hardened steel, which is probably the most common linear bearing shaft material. The advances of tribology shine when compared to simple plastics, not only with friction but also with wear.

So, we've now discussed how plastic bearings are made, how the technology works and how they compare to simpler plastics. Now it's very important for us to compare them to the most common linear bearing systems available, which are recirculating ball bearings.

Plastic Linear Bearings vs. Recirculating Ball Bearings

There are frictional differences between plastic sliding bearings and linear recirculating ball bearings. Recirculating ball bearings have a coefficient of friction of 0.01, whereas plastic sliding bearings have a coefficient of friction between 0.16 and 0.25.