One-way bearings are used extensively in remote controlled model car transmission systems and are assemblies that contain small cylindrical hardened steel pins that allow free movement of the shaft they rotate on in only one direction. Bearing suppliers may also refer to them as clutch bearings. The design used in this article is not based on any particular companies product and serves only to demonstrate the principles involved.
One-way bearings use a relatively simple construction method consisting of just four parts.
The bearing illustration on the left shows a complete bearing and if you click on the links below you will see the main parts;
Casing - The outer part, which is usually a piece of steel tubing with the ends formed over to retain the inner components.
Cage - The inner part that is usually made from a hard plastic material. It functions as a retaining, limiting and locking mechanism for the parts shown below.
The remaining parts of the assembly are shown below left and are retained in the bearing by the cage;
A hard chrome plated steel pin that provides a smooth, consistent rolling surface for contact with the shaft.
A heat treated strip of spring steel that controls the position of the needle rollers.
The bearings are fitted into a housing, which is in turn mounted onto some type of driving shaft that needs to freely rotate in one direction, but transfer its rotational driving force in the other.
For the bearing to allow the housing to achieve this dual role is has to contain a mechanism that locks the bearing and stops it from being a bearing.
This is achieved by usung a cage that is designed in such a way as to allow the needle rollers to move sideways relative to the shaft. The cage also has a ramp/wedging mechanism built in to act as a brake/clutch for the needle rollers.
Figure 1 shows an end view of the inside of the bearing. It shows the position of the needle rollers when in use and is controlled using the buttons at the bottom.
Free - When the needle rollers are positioned on the left hand side they are resting against the springs, which control how far they can move in that direction. As there is no load on it in this position, the bearing spins freely.
Locked - When the needle rollers are positioned on the right hand side, they become lodged between the cage and the shaft, where they cannot rotate.
If you Play the animation above right you will see what happens in a situation where the drive through the bearing would be repeatedly engaging and disengaging, such as it would with the transaxle on a four wheel drive car.
It may be simpler, though, to look at a flattened out view of the internal working parts of a one-way bearing in figure 2, which shows the Free and Locked states of the bearing.
Although in use the needle rollers are rotating against the springs, the light load on the assembly leads to minimal loss of efficiency. As the diagram on the right shows, the contact area is minimal.
As with any bearing, correct lubrication is required for a long and trouble free working life. This type of bearing needs only a small quantity of very thin oil to operate.
The oil needs to be thick enough to provide sufficient lubrication, but thin enough to allow the locking action of the bearing to take place. Oils are sometimes available from dealers, but I find that the lubricating oil from a model railway supplier provides a widely available and cost effective solution.
Our Two Speed Transmission article is a good example of one-way bearings in operation, in which the initial driving force forces the needle rollers into the locked position and drives the first gears. Subsequently the greater rotational speed of the second gear allows the needle rollers to return to the free position.
Although there are different designs of bearing, including those with the springs arranged so that they act to release the needle rollers from the locked position, the operating principal (hopefully) explained in this article remains the same.