Benefits of plastic shafts include lower cost, weight reduction, the ability to be used in a corrosive environment, and in some cases the design of integrated features that can’t be achieved with a metal shaft design.
Key factors to consider when sourcing plastic shafts
We know that shafts need to have other components attached to them whether that be a drive gear or socket, a pulley for a belt drive, or a push fit bearing (plastic or metal). We also understand that some shafts will need to withstand high torsional stress and have high flexural strength so they do not bend.
As for dimensional stability, it can be assumed that most shafts need to have this attribute in order to fulfill the fit, form, and functional elements of the product specification. As we can see, all of these design criteria interact with each other and each has a defining role on the other.
When selecting a plastic material for a shaft, it is important to consider a number of key factors:
- Torsional stress – This is the shear stress produced in a shaft due to twisting. This is critical in any application for plastic shafts as it is assumed that the shaft needs to remain stiff and should not distort out of shape.
- Flexural strength – The amount the shaft can bend can be a critical aspect of the functionality of the product and could result in the total failure of a system.
- Dimensional stability – It is essential that the dimensions on the critical parts of the shaft are maintained and not affected by the environmental heat the shaft is operating in.
- Fit, Form, and Function – These aspects will ultimately drive the design of the shaft to ensure it meets the product specification, however, there are times when one of these aspects could compromise the others. This is where the designer needs to take everything into consideration while designing the plastic shaft.
Plastic composites are the key material for shafts
Because of the torsional and flexural forces a shaft has to withstand, the plastics generally used are classed as composites and are reinforced in different ways.
Plastic composites are plastics that have been modified by adding fibers, powders, strengthening additives, glass, carbon fiber, and in some cases metal additives, in order to improve their physical and mechanical strength.
Composites are where different materials are combined to make a new material which has vastly improved properties over the core plastic. A good example of this would be carbon fiber reinforced ABS.
The ABS would be the core plastic and the addition of carbon fibers enhances the mechanical properties to give a stiffer, stronger, more suitable material for the manufacturing of shafts.
Glass fibers are sometimes also used to enhance the mechanical properties of plastics giving them greater physical strengths in the same way.
Sourcing tips
The key point here is to review your composite options based on the shaft’s application and requirements. Two main factors usually come into the critical calculations, torsional strength and bending or deflection strength.
However, you’ll also take into consideration whether you need to save on weight or if you need a complex one-piece integral shaft design, too.
