Plain Polymer Bearings
In this article, we are going to look at the different types of plain (or plane) polymer bearings. Both spellings are correct, its only a matter of blogger preference. Plain polymer bearings is preferred for this article. Recall that the purpose of bearings in general is to:
- support shafts,
- reduce the friction between contacting surfaces,
- and to constrain motion along one or more axes.
There are different ways to classify plain bearings, and in this article we will look at one possible means of classification: sleeved, flanged, and thrust bearings.
Want even more Bearing Basics? Check out these articles from the Advanced EMC Technologies blog:
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- Breakout Torque for Ball Bearings
- No More Beating around the Bushing
This are also called plain bearings, plane bearings, and bushings. They are the simplest bearing design, and basically consist of a sleeve that provides support to the shaft. These bearings have been found to work well with linear, oscillating, or rotating shafts. Sleeve bearings are available as solid bearings or split bearings. The split can serve two purposes: to make installation easier, and to reduce the effects of expansion or contraction due to temperature changes.
Flange bearings look like sleeve bearings but have a flange, which is wider circle around the base of the bearing. These bearings can support both radial and axial loads. As plain bearings, they are available as solid bearings or split bearings. The flange portion of the bearing often acts as a locator to position and secure the bearing. There are rolling-element flange bearings available, also.
Thrust bearings were specifically designed to support thrust loads along the length of the shaft and prevent metal to metal contact. The simplest thrust bearings look like washers, and they usually do not require any lubrication. According to the eBearing glossary, the clutch bearing in a car is a specialized type of thrust bearing.
Specifying a Plain Bearing
There is certain information you need to have on hand to specify a bearing and select an appropriate polymer. One of the first things you will need to determine is the bushing internal clearance, which is the radial clearance between the sleeve and the shaft.
There are two types of clearance involved: initial internal clearance and operational internal clearance. The initial internal clearance is the clearance before the bearing is installed, and will always be greater than the operational internal clearance. This is due to factors such as thermal differences between contact parts and interference fits.
You will also need the bearing inner diameter (ID) and outer diameter (OD) of the bearing sleeve, as well as the overall length of the bearing. For a flanged bearing, you will need the flange length and flange diameter.
The loading (P) of the bearing is calculated as the load divided by the effective cross-sectional area on which it will be acting (this will usually be the length x inner diameter). For a flanged bearing, this cross-sectional area is (sleeve length + flange thickness) x inner diameter. The maximum load a plain bearing can carry at constant speed can be approximated as 1/3 of its compression strength.
The surface speed (V) of the bearing is the angular speed of the shaft x shaft diameter, resulting in units of sfpm (surface feet per minute). As discussed in a previous post, material is chosen based on surface speed (V), loading (P), and the PV factor. Other material considerations include the environment the bearing will be operating in, including operating temperature and chemicals it may come in contact with.
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