Plain bearing installation and tolerances

Poor tolerance selection can lead to a lot of issues such as premature wear, shaft damage, increased friction, reduced service life.

Using anything other than a flat press fit tool for the installation is likely to cause issues.

The problems that could arise if this is the case:

• weld-line cracking
• ovality
• oversized final ID
• excess clearance
• reduced bearing life

This is a massive issue. Measuring the bearing outside of the housing will measure incorrectly. The final ID only forms after having been press-fit into the H7 bore.

A lot of the time, the shaft is over looked in regards to the tolerances. The most common mistakes are:

• rough stock bar
• oversized shaft
• poor roundness
• wrong tolerance class
• bad surface finish
• all of these will mean too much play, increased wear, possible stick-slip and likely noise.

We advise you use a H7 housing bore tolerance for standard press-fit installation. This ensures the correct interference fit and allows the bearing inner diameter to form correctly after installation.

This is a common question! This is nominal. ID is achieved only after press-fit into a bore with a H7 tolerance. If, after press-fit the ID is still too small then possible causes can be:

• undersized bore
• centring pin use
• no chamfer
• soft housing material deformation

One of the most common reasons is that the housing bore is oversized or out of H7 recommended tolerance. Temperature expansion, excessive load, or soft housing materials can also reduce press-fit retention.

We recommend a minimum shaft tolerance of h9 for most standard iglidur applications. Final selection depends on factors such as required running clearance, load, and environmental expansion factors.

igus® uses a plug gauge test, also known as a "go/no-go test", to ensure that our bearings meet the specifications and function properly after installation.

First, the bearings are pressed into a test fixture. It is important to ensure that the bearings are installed without damage. A bevelled edge is recommended for this - ideally 25-30 degrees. It is also advisable to use a press with a flat press to press in the bearing. This is the most efficient installation method. It also ensures the integrity of the bearing. If you use a hammer, for example, the bearing can tilt during installation.

1.2 igus® recommends using a press to install a plain bearing.

After the bearing has been installed, the actual plug gauge test is carried out. A "Go" means that the bolt falls through the bearing under its own weight, while a "No-Go" means that the bolt does not fall through the bearing or remains stuck. As a rule, the plug gauges are graduated to 0.01 mm, so that it is possible to determine very precisely from which dimension the respective plug gauge hangs.

A plug gauge test is the quality check with the greatest possible accuracy because the bolt behaves like a shaft in a real application and reflects the narrowest cross section of the bearing. It is precisely this aspect that is usually decisive for the application. Plug gauge tests are particularly suitable for plastic bearings, as the irrelevant "unevenness" of the bearing caused by injection moulding is not taken into account. Later in operation, during the running-in phase, when the unevenness of the bearing and the shaft is smoothed out, an ideal sliding surface is created.
The quality control of a bearing can also be carried out using other tests; however, inaccuracies can occur when these methods are applied to plastic bearings. In particular, the use of gauges should be avoided. Gauges should generally only be used for cursory quality checks, depending on the degree of accuracy. Depending on the pressure exerted by the gauge on the measuring point, the measurement may be falsified. A plug gauge test is therefore much more reliable.

Depending on accessibility, the described test can also be carried out directly on the series component (i.e. not in a specially manufactured test fixture)

In standard cases, we have had very good experiences with instant adhesives (e.g. Loctite 401). With materials that are difficult to bond, such as iglidur® J, significantly better results are achieved with 2-component systems (e.g. Loctite 406 + Primer 770). We have had good experiences with epoxy resin systems (e.g. Hysol) for applications with increased temperatures.
Furthermore, it is important for every bonded joint that the workpieces are thoroughly cleaned and free of grease. This can be done with professional cleaners, for example, but also with simple quick degreasers. Roughening the contact surfaces also supports the bonding effect.
In general, bonding can only have a supporting effect and cannot completely replace the press fit.

iglidur®Plain bearings are press-fit bushings. The inner diameter is only set after pressing into the H7-housing bore with the corresponding tolerance. The press-fit interference can be up to 2% of the inner diameter. This ensures a secure press fit of the bearing. Axial or radial displacements in the housing are thus reliably avoided.
The hole in the housing should be manufactured to tolerance H7 for all bearings and be as smooth, even and chamfered as possible. Assembly is carried out with a flat press. The use of centring or calibration mandrels can damage the bearing and lead to greater clearance.

Bearing wear means material removal on the sliding surface, i.e. usually on the inner diameter of the bearing.

The clearance between the bearing and shaft is calculated from the tolerances of the bearing and shaft.

The actual starting clearance during commissioning is the difference between the measured actual inner diameter of the bearing and the measured actual outer diameter of the shaft. Wear on the bearing inner diameter leads to an increase in diameter and thus to an increase in clearance.
As iglidur® plain bearings do not have a layered structure and therefore the entire wall thickness is available as a wear zone, there is no wear limit specified by the bearing. Instead, the wear limit is determined by the maximum clearance permitted in an application. This can vary greatly depending on the application and user requirements. Precision control valves, for example, only allow a few hundredths of wear (and therefore an increase in play). In agricultural applications with shaft diameters greater than 50 mm, a clearance of significantly more than one millimetre is often uncritical.