How to Determine Bearing Shaft and Housing Fit
A proper shaft and housing fit are vital for the life of your bearings. There are several types of shaft and housing fits. Several factors will determine the appropriate fit for your application. Do not skim through bearing fit selection. Poor design or the incorrect fit selection is setting the application up for continual failure.
With a loose fit, bearings will creep – the unintended movement from or within their mounting place – or become damaged. A loose fit can also lead to a cracked raceway from lack of support to the inner or outer ring. When a fit is too tight, the bearing will experience a drastic loss in efficiency, operating temperatures will climb, and ambient noise increases.
Selecting the correct bearing fit will minimize failures. Correcting the fit can also be very difficult, depending on the application. Typically this will require an entire teardown to access both the shaft and housing. Fixing an issue like this in the field can be next to impossible. Bearing fits are not glamorous, but getting them right will save you from frustrating repairs.
There are multiple factors to consider when selecting shaft and housing 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 outside diameter
- The application itself
While not as common of a factor, bearing and housing materials are also relevant when determining correct fits. Aluminum will expand more than steel will, necessitating different fits, even if all other variables remain the same. Before beginning the selection process, we’ll discuss further generalities. Let’s take a look.
Consider the Type of Bearing Fit
A clearance fit, or slip fit, always enable clearance between the bearing bore and shaft, or outer ring and housing. Installation is generally easy because of the added clearance. If the fit is too loose, you can run into problems where a bearing can creep or spin, either on the shaft or inside the housing, especially when vibration is present. This spinning will generate heat and result in premature bearing failure.
An interference fit is precisely the opposite of the clearance fit. In this fit, often referred to as a press-fit, there is interference between the bearing ring and its mating part. The bearing bore is smaller than your shaft diameter, or your bearing O.D. is larger than your housing bore. This interference will make the parts challenging to assemble, and they will typically require pressing into place, or use of a heat fit to allow for easier assembly.
As the name implies, a transition fit is somewhere between the two described above. Either a clearance or interference fit may occur depending on the actual sizes of the bearing bore and shaft, or outer ring and housing. The resulting fit will be dependent on the tolerances of two points of contact – either the bearing bore and the shaft or the housing and bearing O.D.
It will be unusual for a shaft and housing fit to be the same. One fit requires a clearance fit, the other, generally, the rotating ring, will require an interference fit. The rotating ring requires this interference fit because when applying the load to a looser fit, there would be slippage and a loss of efficiency, and eventually, surface damage or fretting corrosion. Often, vibrating or shaker-type applications vary from the above generality. Incorrect fits can cause premature bearing failure.
Steps for Determining Shaft Fit
Most general applications include inner ring rotation and a constant radial load. For these conditions, we recommend an interference fit between the shaft and bearing bore. The level of interference will increase for heavier loads. When shaft conditions are stationary, and the radial load is constant, a moderate clearance fit between the shaft and bearing bore is an option.
Selecting the Proper Housing Fit
Your housing fit can be different from your shaft fit, and often is. Like the shaft fit, many conditions determine which fit is best. Considerations include the rotation of the inner or outer rings, type of load, and how easily it should be to install or remove the bearing from the housing.
The primary factor to consider is the rotation of the bearing’s inner ring and its relationship to the radial load. The amount of the radial load will also influence the choice of fit. For indeterminate or varying load directions, avoid clearance fits. Additionally, apply a clearance fit for applications with an axially split housing to avoid distorting the bearing’s outer rings.
Applying Theory to a Sample Situation
Imagine you are installing a bearing in an electric motor application. What type of shaft and housing fit do you need? Let’s look at the application specifics first. In this situation, your inner ring will rotate, the applied load direction will be constant, and the outer ring is stationary.
Let’s get more specific and use the following measurements: the shaft is 20mm with a 6204 ball bearing (0.787402” x 1.85039”)(I.D. x O.D.). In this circumstance, you’ll need an interference fit on the shaft because the inner ring is the one rotating. A transition fit is most appropriate for the housing bore because it will allow for easier installation as well as displacement for removal.
By reviewing the table in our Axis ball bearing Catalog, note that the appropriate fits are a k5 shaft fit of .7875”-.7878” and a J7 housing fit of 1.850”-1.851”. Of course, this is only one example. For your application, you’ll need to have a general idea about what the fits should be beforehand. Once you know if you’re looking for an interference, clearance, or transition fit, the catalog can give specific dimensions based on the inside and outside diameters of the bearing in question.
Let the specifics of your application guide you to the best housing and shaft fit.
Before selecting a housing or shaft fit, look to the application for guidance. Our Axis catalog has all the information you’ll need to accurately determine the specified tolerance range recommendations appropriate for every application.
The friendly engineers at Baart are always ready with helpful tips and advice. Questions about your application? Contact us.