The radial internal clearance applies to bearings with an inner ring before the bearing is fitted. It is defined as the amount by which the inner ring can be moved in a radial direction from one extreme position to the other in relation to the outer ring, Figure 1.

In accordance with DIN 620-4, ISO 5 753, the radial internal clearance is divided into groups, see Figure 1 and table.

For bearings without an inner ring, the enveloping circle F_w is used. This is the inner inscribed circle of the needle rollers in clearance-free contact with the outer raceway, Figure 2. The enveloping circle for unfitted bearings is in the tolerance zone F6 (except for drawn cup needle roller bearings). Deviations for F6 and F8, see table.

The operating clearance is determined on a fitted bearing still warm from operation. It is defined as the amount by which the shaft can be moved in a radial direction from one extreme position to the other, Figure 3.

The operating clearance is derived from the radial internal clearance and the change in the radial internal clearance as a result of interference fit and thermal influences in the fitted condition.

The operating clearance value is dependent on the operating and installation conditions of the bearing, see also section link.

A larger operating clearance is, for example, necessary if heat is transferred via the shaft, the shaft undergoes deflection or if misalignment occurs.

An operating clearance smaller than CN should only be used in special cases, for example in high precision bearing arrangements.

The normal operating clearance is achieved with internal clearance CN or, in larger bearings, predominantly with C3 if the recommended shaft and housing tolerances are fulfilled, see section Design of bearing arrangements, link.

The operating clearance is derived from:

The radial internal clearance is reduced due to the fit as a result of expansion of the inner ring and contraction of the outer ring:

The expansion of the inner ring is calculated as follows:

For very thin-walled housings and light metal housings, the reduction in the radial internal clearance must be determined by means of mounting trials.

The contraction of the outer ring is calculated as follows:

The radial internal clearance can alter considerably if there is a substantial temperature differential between the inner and outer ring.

A larger radial internal clearance should be used for shafts running at high speeds, since adequate thermal compensation between the bearing, shaft and housing does not occur in this situation.

Δs_T can be significantly higher in this case than for continuous operation.

The axial internal clearance s_a is the extent to which one bearing ring can be displaced in r­elation to the other, without load, along the bearing axis­, Figure 4.

With various bearing types, the radial internal clearance s_r and the axial internal clearance s_a are dependent on each other. Guide values for the correlation between the radial and axial internal clearance are shown for some bearing types in the table.

For deep groove ball bearings, the calculation of the axial internal clearance is shown in the following example:

• s_a = 13 · 10 μm = 130 μm

INA and FAG rolling bearings fulfil the requirements for fatigue strength, wear resistance, hardness, toughness and structural stability.

The material used for the rings and rolling elements is generally a low-alloy, through hardening chromium steel of high purity. For bearings subjected to considerable shock loads and reversed bending stresses, case hardening steel is also used (supplied by agreement).

In recent years, the improved quality of rolling bearing steels has been the principal factor in achieving considerable increases in basic load ratings.

The results of research as well as practical experience confirm that bearings made from the steel currently used as standard can achieve their endurance limit if loads are not excessively high and the lubrication and cleanliness conditions are favourable.

With special bearings made from HNS (High Nitrogen Steel), it is possible to achieve adequate service life even under the most challenging conditions (high temperatures, moisture, contamination); these are supplied by agreement.

For increased performance requirements, highly corrosion-resistant, nitrogen-alloyed martensitic HNS steels are available such as Cronidur and the newly developed steel Cronitect.

In contrast to Cronidur, Cronitect has nitrogen introduced into the structure by means of a surface layer hardening process.

In terms of corrosion resistance, wear resistance and fatigue strength, both steels are considerably superior to conventional corrosion-resistant steels for rolling bearings, see also TPI 64, Corrosion-resistant products.

Ceramic hybrid spindle bearings contain balls made from silicon nitride. These ceramic balls are substantially lighter than steel balls. The centrifugal forces and friction are significantly lower.

Hybrid bearings allow very high speeds, even with grease lubrication, as well as long operating life and low operating temperatures.

The following table shows suitable materials and their use in bearing technology.

The most important functions of the cage are:

• to separate the rolling elements from each other in order to minimise friction and heat generation
• to maintain the rolling elements at the same distance from each other in order to ensure uniform load distribution
• to prevent the rolling elements from falling out where bearings can be dismantled or swivelled out
• to guide the rolling elements in the load zone of the bearing.

Rolling bearing cages are subdivided into sheet metal and solid section cages.

These cages are predominantly made from steel and for some bearings from brass, Figure 6. In comparison with solid section cages made from metal, they are of lower mass.

Since a sheet metal cage only fills a small proportion of the gap between the inner and outer ring, lubricant can easily reach the interior of the bearing and is held on the cage.

In general, a sheet steel cage is only included in the bearing designation if it is not defined as a standard version of the bearing.

These cages are made from metal, laminated fabric or plastic, Figure 7. They can be identified from the bearing designation.

Solid section cages made from metal are used where there are requirements for high cage strength and at high temperatures.

Solid section cages are also used if the cage must be guided on ribs. Rib-guided cages for bearings running at high speeds are made in many cases from light materials such as light metal or laminated fabric in order to achieve low inertia forces.

Solid section cages made from polyamide 66 are produced using injection moulding, Figure 8. As a result, cage types can generally be realised that allow designs with particularly high load carrying capacity. The elasticity and low mass of polyamide are favourable under shock type bearing loads, high accelerations and decelerations und tilting of the bearing rings in relation to each other. Polyamide cages have very good sliding and emergency running characteristics.

Cages made from glass fibre reinforced polyamide 66 are suitable for long term temperatures up to +120 °C.

When using oil lubrication, additives in the oil can impair the cage operating life. The interrelationship between the cage operating life, the long term temperature of the stationary bearing ring and the lubricant is shown in Figure 9. Aged oil can also impair the cage operating life at high temperatures, so attention must be paid to compliance with the oil change intervals.

A further means of distinguishing between cages is their guidance method, Figure 10. Most cages are guided by the rolling elements and do not have a suffix for the guidance method.

If guidance is by the bearing outer ring, the suffix A is used. Cages that are guided on the inner ring have the suffix B.

Under normal operating conditions, the cage design defined as the standard cage is generally suitable. Standard cages that may differ within a bearing series according to the bearing size are described in the product sections.

Under special operating conditions, a cage that is suitable for the specific conditions must be selected.

Rolling bearings are thermally stabilised such that, depending on the bearing type, they are generally dimensionally stable up to +120 °C (certain series up to +150 °C).

Operating temperatures above +150 °C require special heat treatment. Bearings treated in this way are available by agreement and are identified by the suffix S1, S2, S3 and S4 to DIN 623-1, see table.

Further temperature data in the product descriptions must be observed.

An operating temperature of +70 °C is regarded as a normal operating temperature. The additional information on temperature in the product descriptions must be observed.

The permissible temperature for sealed bearings is dependent on the requirements for the operating life of the grease filling and on the action of the contact seals.

Sealed bearings are greased with specially tested, high performance, high quality greases. These greases can withstand +120 °C for short periods. At long term temperatures of +70 °C and above, a reduction in the operating life of standard greases with a lithium soap base must be expected.

In many cases, adequate operating life values are only achieved at high temperatures through the use of special greases. In these cases, it must also be checked whether seals made from especially heat-resistant materials must be used. The operating limit of normal contact seals is +100 °C.

If high temperature synthetic materials are used for seals and greases, it must be noted that the particularly high performance materials containing fluoride may give off harmful gases and vapours when heated to approx. +300 °C and above. This may occur, for example, if a welding torch is used in the dismantling of a bearing.

High temperatures are critical especially in the case of seals made from fluoro rubber (FKM, FPM) or greases containing fluoride such as the rolling bearing greases Arcanol TEMP200 and greases to GA11. If high temperatures are unavoidable, attention must be paid to the valid safety data sheet for the specific fluoride-containing material, which can be obtained upon request.

Bearings are not resistant to corrosion by water or agents containing alkalis or acids but are often exposed to these corrosion-inducing agents. In these applications, anti-corrosion protection is therefore a decisive factor in achieving a long operating life of the bearings.

In principle, corrosion-resistant steels to ISO 693-17 can be used for components at risk of corrosion. These bearings have the prefix S. For higher requirements, the high performance steels Cronidur and Cronitect are available, see link.

In many applications, the special coating Corrotect^® is more cost-effective than corrosion-resistant steel.

Corrotect^® is an extremely thin, electroplated surface coating (coating thickness 0,5 μm to 3 μm). The coating is effective against moisture, contaminated water, salt spray and weakly alkaline and weakly acidic cleaning agents.

The advantages of the special coating Corrotect^® are all-round rust protection, including the turned surfaces of chamfers and radii, Figure 11. It also gives long term prevention of rust penetration beneath seals and smaller bright spots are protected against rust by the cathodic protection effect. In comparison with uncoated parts, operating life is significantly increased by the anti-corrosion protection. Uncoated bearings can be easily replaced by coated bearings of the same dimensions, there is no decrease in load carrying capacity, in contrast to corrosion-resistant steels. During storage, there is no need to use organic-based preservatives.

Before bearings with Corrotect^® coating are fitted, compatibility with the media should always be checked.

In order to reduce the press-in forces, the surface of the parts should be lightly greased, since the tolerances are increased by the thickness of the coating.

Unless stated otherwise, the tolerances for radial rolling bearings correspond to DIN 620-2 (ISO 492), the tolerances for axial rolling bearings correspond to DIN 620-3 (ISO 199), Figure 12.

The accuracy corresponds to tolerance class PN. For bearings with increased accuracy, the tolerances are restricted to values in the classes P6, P5, P4 and P2. Tolerance tables for the individual tolerance classes: see link to link.

The standardised tolerance classes are not applied to high precision bearings, which are produced to the tolerance classes P4S, SP and UP. These tolerances are listed in the product descriptions for the high precision bearings.

Measurement methods according to DIN 620-1 (ISO 1 132-2) are valid for the acceptance inspection of rolling bearings.

Further information on the measurement methods is given in TPI 138, Rolling bearing tolerances, Definitions and measurement principles. This TPI can be ordered via the Internet.