The process for selecting a polymer bushing isnt that difficult, but it does involve quite a bit of information about the bushing application before you begin.
In this blog post, we are going to look at the information you need to have on hand to select an appropriate polymer bushing, beginning with determining what type of bushing you need.
Types of Bushings
Bushings serve to support shafts, constrain motion along one or more axis, and reduce the friction between contact surfaces. The first step in selecting an appropriate polymer bushing involves deciding what type of bushing you need which is usually based on what types of loads the bushing needs to support. To that end, there are three types of bushings:
- sleeved bushings
- flanged bushings
- thrust bushings
Sleeve bushings (also known as sleeve bearings) have the simplest design. They look like a simple sleeve that may or may not be split. The split makes installation much easier and can reduce the effects of dimensional changes resulting from changes in temperature. Sleeve bushings work well with rotating, linear, or oscillating shafts but only support radial loads.
Flange bushings are much like sleeve bushing but they have a flange at the base of the bearing. The flange usually acts as a locator to help position the bearing or secure it. The support radial loads and, because of the flange, can also support axial loads. Like the sleeved bushings, they are also available in a split design.
Note that flange bushings are going to be more expensive than sleeve bushings primarily because manufacturing them is more expensive — whether it’s injection molding, which will require more complex dies — or machining, which will require a significant amount of additional material to achieve the final shape.
Split bearings, whether sleeve or flanged, work very well for rod and piston applications or situations where a press-fit is not required.
Thrust bushings look just like a simple washer. They are designed to support thrust loads only and are used to prevent metal-to-metal contact.
Bushing Selection Process: Dimensions
You start the bushing selection process by determining the bushing dimensions. One thing you need to determine is the radial clearance needed between the bushing and the shaft, known as the bushing internal clearance. This helps you to specify the inner diameter and outer diameter of the bearing sleeve.
If you are specifying a flange bearing, you will need the flange length (thickness of the flange) and the flange diameter (outer diameter of the flange).
The length of the bushing is also needed, although for a thrust bushing it might be called the thickness.
Bushing Selection Process
The next step is selecting a bushing involves specifying the expected load to estimate the load bearing pressure, P, that will be distributed over the projected area of the bushing. The load includes direct external loads and other forces such as:
- non-concentric axial loads
- side-loads
- deflection forces
- the weight of what the bushing is supporting
The projected area is calculated differently based on whether the bushing is primarily carrying an axial load (length of bushing x inner diameter of bushing) or a thrust load (contact area). The pressure, P, is then equal to the expected load divided by the projected area of the bushing.
The running surface speed, V, is key to wear conditions for the bushing and is used with pressure, P, to determine the dry pressure velocity (PV) of the application. The PV of the application is then compared to the dry running PV limit of potential polymers to determine the appropriate material for the bushing.
Additional Considerations
Here are some additional questions you should ask when trying to specify the best bushing for your application:
- What is the shaft material and surface finish of the shaft?
- Will lubrication will be present?
- What is the expected operating temperature?
- Is the bearing faced with abrasive, erosive or chemically aggressive conditions?
Conclusion
Bushings are a key component in just about every mechanical design you see today, and it is critical to gather the correct information when specifying a bushing. This includes types of loads that the bushing will need to support, expected loads, dimensions, surface speed of the shaft involved, shaft surface finish and hardness, operating temperatures, and information about the environment. Careful consideration of these factors will help to ensure that you have the most appropriate bushing for your application.{{cta(‘a5e30473-329a-40fe-ac61-b969aae4441f’,’justifycenter’)}}