Plastic bearing image courtesy of cqbearing.com
What is a bearing?
Some products will require a sliding property (bearing) between two components, this can be linear or rotational and used in conjunction with shafts to restrict the friction caused by one part being directly in contact with another and transfer motion smoothly. In both cases, this can be achieved with plastic bearings.
In fact, plastic bearings are being used in an increasing number of applications as the number of specific engineering polymers available increases.
Bearings are incredibly common parts, with all kinds of uses including turning a car’s wheels on your car, as components of mechanical machines, for gate mechanisms, airplane parts, and more.
Advantages of plastic bearings
Plastic bearings have many advantages over conventional metal bearings, such as in corrosive environments where metal bearings cannot be used. Most engineering plastics used in bearings are self-lubricating eliminating the need for oils and other lubricants, plastic bearings are generally cheaper than metal ones, and they have a constant coefficient of friction over their lifetime. Another major consideration in switching from metal bearings to plastic is the weight saving that can be made.
Types of plastics used in plastic bearings
There are multiple engineering plastics for internal plastic mechanical parts that have characteristics that make them candidates for use in a bearing, however, there are, in reality, just a few that are predominantly used, so let’s have a look at them.
Now some comments on the properties of each:
Polyoxymethylene – POM (Acetal)
There are in fact two types of POM, the copolymer and the homopolymer where the copolymer has a slightly lower melting temperature by some 5 degrees centigrade at 338℉ (170℃), and a continuous operating temperature of around 212℉ (100℃) so this needs to be taken into consideration when selecting materials and the operating temperatures.
Other differences between the copolymer and the homopolymer acetals are, the copolymer will perform better when it comes to thermal cycling, has better impact resistance properties, will generally last longer in the same application, and has slightly better chemical resistance and a lower water absorption rate.
With a coefficient of friction of 0.32, acetal is an ideal candidate for sliding linear bearings.
Typical uses include:
- Hydraulic applications
- General mechanical mechanisms
- Construction machinery
Polyamide – PA (Nylon)
Nylon is one of the most widely used materials for simple bush-style bearings as it has a low coefficient of friction and is self-lubricating which eliminates the need for additional lubrication. It is also inexpensive in comparison to some other engineering plastics. Nylon has a good wear resistance which makes it a long-lasting material and reduces the need for maintenance and product replacements. As nylon has a high working temperature of around 180℉ (80℃), it is an ideal material for applications that operate at medium temperatures.
Mechanical properties can be improved by adding modifiers and fillers such as graphite and molybdenum disulfide, both of which give nylon improved wear resistance and a lower coefficient of friction and so increase the life of nylon bearings.
The coefficient of friction for nylon is around 1.0 making this ideal for bush-style bearings.
Typical uses include:
- Household appliances
- Furniture wheels
- Skateboard trucks
- Pumps
- Aerospace applications
Polytetrafluoroethylene – PTFE (Teflon)
PTFE is one of the best materials for plastic bearings as it has a very low coefficient of friction, is self-lubricating and has a very wide operating temperature range of -328℉ to 360℉ (-200℃ to 182℃). PTFE also has a very good chemical resistance making this a perfect material for applications that are exposed to extreme weather conditions and chemical solutions.
PTFE bearings work well in applications where they are idle for a long period of time and need to operate first time with no sticking due to their low coefficient of friction and excellent self-lubricating properties.
One downside to PTFE is the cost, it is comparatively high against other engineering plastics.
The coefficient of friction for nylon is around just 0.04 making this ideal for low rotational speeds and intermittent use where smooth consistent motion is required.
Typical uses include:
- Sluice gates
- Food processing machines
- Automotive industry
- Aircraft accessories
- Business machines
- Textile machinery
Polyether ether ketone – PEEK
PEEK is another excellent material for bearing applications that require a high-temperature operation, capable of a continuous operating temperature of 482℉ (250℃) without loss of performance. PEEK has excellent wear resistance and high self-lubrication making it ideal for applications that require low maintenance and extended periods of time between replacements.
PEEK also has a low moisture absorption rate which makes it a good candidate for applications that are underwater or within a high moisture content environment.
The coefficient of friction for nylon is around 0.20 making this ideal for low rotational speeds and intermittent use where smooth consistent motion is required.
One negative aspect of PEEK is the cost, because this is one of the premier engineering materials that have excellent all-round properties, it has a very high price of $44.00/lb.
Typical uses include:
- Automotive
- Aerospace and aircraft applications
- Food processing machines
- Oil and gas industry
- Medical and industrial machinery
Summary: which plastic to choose for your bearings?
When sourcing plastic bearings your choice will likely be dictated by the application the bearing is to be used for. There are roughly three categories of bearings:
- General use
- Bearings in lubrication
- Specialist
Here are the plastic types most suited to each application:
General use bearings
A good material for general use bearings is PA polyamide (nylon) which is a self-lubricating plastic and can run in dry applications without the need for lubricants. Nylon absorbs moisture and deforms thus losing its dimensional parameters, therefore, nylon bearings need to run in a dry environment. It is cheap and easy to manufacture. Nylon is perfectly OK functioning at room and slightly elevated temperatures, but should not be subjected to temperatures over 180°F.
Bearings in lubrication
If you need a bearing to function inside a wet environment or one that is constantly running in a lubricant, you should consider POM (acetal) as this has a much better water absorption resistance than nylon. In fact, acetal functions better in a lubricated state and should last a long time without services or replacement. As for the operating temperature, acetal beings should not exceed 180℉.
Specialist bearings
If you require a bearing to function in abnormal conditions, such as extreme temperatures, then you need to be looking at either PTFE or PEEK as these are both top-class engineering plastics with exceptional properties. Both these materials are capable of functioning at around the 500℉ range.
PEEK has a greater hardness value, whereas PTFE has a greater impact strength, both these materials are excellent options for bearings running in specialized equipment and in extreme conditions, however, as the cost is often a major factor in the final selection it should be noted that PEEK is $44.00/lb (at the time of writing in summer ’22) and PTFE a far more reasonable price of $5.00/lb.
