Plastic bearings a design option

With the constant pressures on design engineers to provide cost savings without compromising quality, any new technology that can do this must be considered.

Plastic rolling element bearings are a relatively new concept which provide an alternative to the more traditional steel bearings. These bearings are generally molded and this process allows for more flexibility in terms of design and manufacture, turning a bearing into an assembly through the integration of any number of features such as gears, shafts, fixing clips and housings.

Sarnatech BNL are the leading exponents of this technology, designing and manufacturing products for major OEMs throughout the world. Their expertise in developing this new technology is based on the ability to analyze the application rather than simply the bearing or moving element within it. The more opportunities there are to offer the customer cost savings through parts reduction and reduced assembly time.

The business is very much project based with every customer having different requirements and requiring varying design solutions. Typically, when a project is introduced into the company it would be accompanied by all parts surrounding the bearing or moving element (mating parts) and an initial assessment would be made to see which components could be integrated within the assembly. There are a number of areas that need to be considered to ensure that the time and effort that goes into the project is fully justified. These "design considerations" include:

• Speed - It is important to understand the speed at which the bearing assembly will be run and included in this both the running time and the dwell time are important. Although excessively high speeds are generally prohibitive, there are other factors, particularly environmental, which influence how the product will perform at high speeds.
  

• Materials - There are a number of engineering polymers available for the manufacture of plastic bearings depending on the end requirements and environment. Chemicals, temperatures and radiation will all require particular consideration when a selecting a plastic material. Additionally, the balls used will be dependent on the environment. Stainless steel balls of varying grades, carbon steel, glass and polymer are all available to meet specific customer requirements.
  

• Environment - Various environmental situations can affect the performance of a bearing and consequently the design. Traditional steel bearings have problems running in liquids, chemicals and even in dry situations when contaminants such as paper dust are prevalent. These areas can be addressed when designing plastic bearings through the inclusion of specific ball types, raceway material and of course by the fact that plastic does not corrode. Additionally, plastic bearings with glass or plastic balls can be used in areas where a non-metallic component is required i.e. the medical industry. Another common technique in the medical industry is sterilization using high-energy radiation and the choice of materials in such an area is yet another design consideration.
  

• Temperature - is a further area that needs careful consideration at the design stage. It is fair to say that plastic bearings have generally been limited to maximum temperature levels of 100?C although this will change somewhat depending on the speed and local environment in which the bearing will work. However, there are now engineering polymers that have higher temperature limits and these can now be considered during the design process.
  

• Dimensional stability - It is a fact that thermal expansion and the absorption of fluids significantly affect most engineering polymers. These areas have to be considered during the initial design stage and any likely expansion can then be allowed for with the integration of special features on the design. The absorption of liquids into the polymer can cause both dimensional and performance changes and where this is likely particular care has to be taken in choosing the raceway.
  

• Size constraints - In the majority of cases, the objective is to injection mold the finished bearing tracks. Very high dimensional accuracy is required to produce an effective bearing and this generally means that the maximum size of a bearing is limited to 150 mm. Similarly, at the small end of the scale bearing raceways have to be sufficiently thick to allow satisfactory plastic material flow and to provide adequate support for the bearing loads. Typically, this means that the minimum bearing O/D is 11 mm.

• Loads - Clearly plastic bearings cannot withstand the same loads as steel bearings. However, there are various options available to increase the load capacity of plastic bearings. Increasing ball size and pitch circle diameter will improve load capacity that can also be increased by omitting the cage thus making the bearing a full complement bearing. Again, dependent on space available, the bearing can be designed to include two rows of balls instead of the more usual single row.

Once all these options are taken into consideration and a product is finally designed, a free of charge prototype can be produced and life tested in the application to prove it works or alternatively highlight some area which requires further consideration.

These are some of the areas to be considered during the design process of plastic rolling element bearings and assemblies and they clearly highlight the options open to Design Engineers. They say that for every problem there is a solution and the design options available through plastic rolling element bearings and assemblies clearly confirm this.