Proper shaft and housing adjustment is vital to bearing life. There are different types of shaft and housing adjustments. Several factors will determine the appropriate fit for your application. Do not touch the bearing adjustment selection. Poor design or incorrect fit selection is setting up the application for continued failure.
With a loose fit, the bearings will slip – the unintended movement to or from their mounting position – or be damaged. A loose fit can also cause cracks in the race due to lack of inner or outer ring support. When a fit is too tight, the bearing will suffer a drastic loss of efficiency, operating temperatures will increase, and ambient noise will increase.
Selecting the correct bearing setting will minimize failures. Fixing the fit can also be very difficult depending on the application. Typically this will require a complete disassembly to access the shaft and housing. Solving a problem like this in the field can be almost impossible. Bearing adjustments aren't glamorous, but getting them right will avoid frustrating repairs.
There are many factors to consider when selecting shaft and bearing fits for each application:
- Whether the inner ring is rotating or not
- What type of load the application is producing and its direction
- Bearing bore and outer diameter
- The application itself
Although not a common factor, bearing and housing materials are also important in determining proper adjustments. Aluminum will expand more than steel, requiring different adjustments, even if all other variables remain the same. Before starting the selection process, we will discuss more general information. Let's take a look.
Consider the type of bearing adjustment
The play coupling , or slip fit, always allows clearance between the bearing bore and the shaft, or the outer ring and the housing. Installation is generally easy due to the additional space. If the adjustment is too loose, you may encounter problems where a bearing may slip or rotate, either on the shaft or within the housing, especially if there is vibration. This rotation will generate heat and cause premature bearing failure.
Interference mating is the exact opposite of loose mating. In this fit, often called press fit, there is an interference between the bearing ring and its mating part. The bearing bore is smaller than the shaft diameter or the bearing outer diameter is larger than the housing bore. This interference will make parts difficult to assemble and will typically require pressing into place or the use of a heat setting to allow for easier assembly.
As the name suggests, a transitional adaptation is somewhere between the two described above. Depending on the actual dimensions of the bearing bore and shaft or outer ring and housing, play or interference fit may occur. The resulting fit will depend on the tolerances of two contact points: the bearing bore and the shaft or housing and the bearing outer diameter
It will be unusual for an axle and housing to fit together. One fit requires a loose fit, the other, usually the swivel ring, requires an interference fit. The slewing ring requires this interference fit because when load is applied to a looser fit, there would be slippage and loss of efficiency and ultimately surface damage or fretting corrosion. Often vibration or shaking type applications vary from the above generality. Incorrect adjustments can cause premature bearing failure.
Steps to Determine Tree Fit
Most general applications include inner ring rotation and constant radial loading. For these conditions, an interference fit between the shaft and the bearing hole is recommended. The interference level will increase for heavier loads. When shaft conditions are stationary and the radial load is constant, a fit with moderate clearance between the shaft and bearing bore is possible.
Selecting the Correct Housing Fit
Housing adjustment can be different from shaft adjustment, and usually is. Just like tree fit, many conditions determine what the best fit is. Considerations include the rotation of the inner or outer rings, the type of load, and the ease of installing or removing the bearing from the housing.
The main factor to consider is the rotation of the bearing's inner ring and its relationship to the radial load. The magnitude of the radial load will also influence the choice of adjustment. For indeterminate or variable load directions, avoid loose adjustments. Additionally, apply a loose fit for axially split housing applications to avoid distortion of the bearing outer rings.
Apply the theory to an exemplary situation
Imagine installing a bearing in an electric motor application. What type of shaft and housing coupling do you need? Let's take a look at the app's specifications first. In this situation, the inner ring will rotate, the direction of the applied load will be constant, and the outer ring will be stationary.
Let's be more specific and use the following measurements: The shaft is 20 mm with a 6204 ball bearing (0.787402” x 1.85039”) (ID x OD). In this circumstance, an interference fit on the shaft will be necessary because the inner ring is the rotating one. A transition fit is more appropriate for the housing bore because it will allow for easier installation and movement for removal.
Note that the appropriate adjustments are a k5 shaft adjustment of 0.7875″-0.7878″ and a J7 housing adjustment of 1.850″-1.851″. Of course this is just one example. For your application, you will need to have a general idea of what the adjustments should be in advance. Once you know whether you are looking for an interference, clearance, or transition fit, the catalog can provide specific dimensions based on the inner and outer diameters of the bearing in question.
