➤ Figure shows bearing types that can be used as locating bearings and can support axial forces in both directions. The arrows in ➤ Figure to ➤ Figure indicate what task the axial location methods perform in the various types of mounting and types of bearing, such as axial location on both sides of the outer and inner ring of the deep groove ball bearing.

In order to prevent the bearing rings co-rotating, they are radially fixed by means of a tight fit. At the same time, the rings must be axially located in both directions so that they cannot undergo lateral creep. Axial creep cannot be prevented solely by a tight fit, especially not if a radial bearing must support large axial forces. For axial location, the bearing rings must therefore be connected by form fit to the shaft or the housing.

Bearingfit tolerance chart

Bearings mounted in an adjusted and floating arrangement can support axial load in one direction only; this also applies to single direction axial bearings. The axial forces are supported by shaft or housing shoulders, snap rings, covers etc.

Fitting is also made easier when using matched pairs of tapered roller bearings as locating bearings (313..-N11CA). They are matched with appropriate axial internal clearance so that no adjustment or setting work is required.

Outer ring not easily displaced, angular contact ball bearings and tapered roller bearings with adjusted outer ring, housing split

Non-locating bearings must only support slight axial forces. The axial location method only needs to prevent lateral creep of the rings. The simplest way of achieving this is by a tight fit. In the case of non-separable bearings, the rotating bearing ring has a tight fit. The other ring is axially retained by the rolling elements. ➤ Figure shows rolling bearings that can be used as non-locating bearings.

In order to achieve the required fit, the bearing seats and fit surfaces of the shaft and housing bore must conform to certain tolerances ➤ Figure and ➤ Table.

If the raceway fulfils the requirements for rolling bearing materials but its hardness value is less than 670 HV (58 HRC), the load on the bearing arrangement cannot be as high as the full load carrying capacity of the bearing. In order to determine the load carrying capacity, the basic dynamic load rating C of the bearings must be multiplied by the reduction factor fH and the basic static load rating C0r by the reduction factor fH0 ➤ Figure and ➤ Figure.

There are particular advantages in using angular contact ball bearings of the universal design. The bearings can be fitted in pairs in any O or X arrangement without shims. Angular contact ball bearings of the universal design are matched so that, in an X or O arrangement, they have a low axial internal clearance (design UA), zero clearance (UO) or slight preload (UL).

Locating bearing: four point contact bearing and cylindrical roller bearing NU (outer ring of four point contact bearing not radially retained)Non-locating bearing: cylindrical roller bearing NU

Interference fits lead to expansion of the inner ring raceway and contraction of the outer ring raceway. The resulting stresses occurring in the rings and the reduction in the radial internal clearance must be taken into consideration in the selection of the fit; see ➤ link and ➤ link.

In the O arrangement, the cones and their apexes formed by the contact lines (the contact cone apexes S) point outwards, in the X arrangement, the cones point inwards ➤ Figure.

The contact surfaces of the mating parts must be perpendicular to the axis of rotation (total axial runout tolerance to IT5 or better), in order to ensure uniform load distribution over all the rolling elements.

In some applications, the question of fits can only be resolved by a compromise. The individual requirements must be weighed against each other and those selected that give the best overall solution.

In the adjusted pair of single row angular contact ball bearings, the inner rings are clamped against each other so that they are not forced apart by the axial component of the radial force.

Ideal non-locating bearings are cylindrical roller bearings with cage of series N and NU or needle roller bearings. In these bearings, the roller and cage assembly can be displaced on the raceway of the bearing ring without ribs. All other bearing types, for example deep groove ball bearings and spherical roller bearings, can only act as non-locating bearings if one bearing ring has a fit that allows displacement. The bearing ring subjected to point load therefore has a loose fit; this is normally the outer ring.

➤ Figure shows the locating bearing for a conveyor sheave. In order that the bearing can be mounted and dismounted with ease, a withdrawal sleeve is used to locate the inner ring, which is pressed in using a hydraulic mounting method. The taper on the withdrawal sleeve is self-retaining. The end cap serves as a retainer only.

Bearing fits to shafts and housingsPDF

The shaft locating washer of the double direction axial deep groove ball bearing and the inner ring of the cylindrical roller bearing are axially clamped by means of an end washer. The axial deep groove ball bearing is adjusted clearance-free by means of the intermediate ring inserted with a fit.

Locating bearings must in general also support axial forces. For the axial location of bearing rings, form fit elements such as shoulders, snap rings, covers, caps, nuts etc. have proved effective.

The floating bearing arrangement is essentially similar in its arrangement to the adjusted bearing arrangement. While freedom from clearance or even preload is desirable when warm from operation in the latter case, floating bearing arrangements always have an axial clearance s of several tenths of a millimetre depending on the bearing size ➤ Figure. The value s is defined as a function of the required guidance accuracy such that the bearings are not axially stressed even under unfavourable thermal conditions.

For flame and induction hardening, steels to DIN EN ISO 683-17 must be used (e. g. C56E2, 43CrMo4) or DIN 17212 (e. g. Cf53).

The shaft and housing fits have a considerable influence on the bearing and operating clearance of the rolling bearing; this must be taken into consideration in determining the tolerances.

When deciding between an O and X arrangement, attention must also be paid to the temperature conditions and thermal expansions. This is based on the position of the roller cone apexes R. The roller cone apex R represents the intersection point of the extended, inclined outer ring raceway with the bearing axis ➤ Figure.

The raceways must be free of waviness and precision machined (grinding and honing); for design of raceways see product chapter.

Machining tolerances for shafts and housing bores are shown in ➤ Figure, ➤ Table and ➤ Table. The values are valid for solid steel shafts and flake graphite cast iron housings. In the table header, below the nominal diameters, are the normal tolerances for the bore or outside diameter of radial bearings (excluding tapered roller bearings). Below these are the deviations for the most important tolerance classes for mounting of rolling bearings.

The locating bearing in ➤ Figure must support high radial loads. When the adapter sleeve is tightened, this gives the bearing on the smooth shaft a tight fit, which prevents axial creep.

3 types ofbearing fits

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In accordance with DIN EN ISO 15787:2010, the hardening depth is the depth of the hardened surface zone at which there is still a hardness of 550 HV1. It is measured on the finish ground shaft and must correspond to the stated values, but must in any case be ≧ 0,3 mm.

Bearinghousing tolerance chart

The spherical roller bearing must, as a locating bearing, support axial forces from alternating directions. In this example, the inner ring is located by means of a withdrawal sleeve.

If the shaft is horizontal, a further adjustable bearing must be provided. This is particularly important in the case of high speeds. If the shaft is vertical, the opposing bearing can be omitted if the bearing is adjusted clearance-free by the load in all operating states.

If the bearing ring is stationary relative to the load direction (point load on the ring), no forces occur that could cause creep of the ring. A tight fit would be desirable here in order to give better support, but a loose fit is also possible since there is no risk that the ring will undergo creep. There is essentially a risk, however, that fretting corrosion will occur.

In gearboxes, a four point contact bearing is sometimes fitted directly adjacent to a cylindrical roller bearing to give a locating bearing arrangement. The four point contact bearing, without radial support of the outer ring, can only support axial forces. The radial force is supported by the cylindrical roller bearing.

The locating bearing A is formed by the tapered roller bearing pair clamped from within. Since spacer rings are arranged between the inner rings, the shaft nut can be tightened to a certain torque without leading to bracing of the bearing arrangement. The axial position of the pinion relative to the crown gear is set by means of shims at the time of mounting.

The cylindrical roller bearing NUP and deep groove ball bearing support alternating axial forces. Both rings must therefore be axially located on both sides.

In rolling bearings without an inner ring, the rolling elements run directly on the shaft, while in bearings without an outer ring they run directly in the housing bore. The shaft and/or housing bore must therefore be produced as a rolling bearing raceway; steels, surface hardness and hardening depth ➤ link.

The conditions of rotation indicate the motion of one bearing ring with respect to the load direction and are expressed as either circumferential load or point load ➤ Table.

For a floating bearing arrangement, almost all bearing types can be considered that must not be adjusted; examples ➤ Figure. Floating arrangements are thus possible with, for example, deep groove ball bearings, self-aligning ball bearings and spherical roller bearings; one ring of each of the two bearings (usually the outer ring) then has a sliding seat. In the floating bearing arrangement with cylindrical roller bearings NJ, length compensation is possible within the bearing.

A bearing ring that rotates relative to the load direction (circumferential load on the ring) will roll on its seat if a loose fit is present and will thus creep in a circumferential direction. If shock type load is present, the ring will slip. In both cases, there is a risk that the seats of the ring and mating part will be damaged by fretting corrosion and wear.

The requisite case hardening depth CHD for case hardening steels is determined in accordance with ➤ Equation, while the requisite surface hardening depth SHD for steels for induction surface layer hardening is determined in accordance with ➤ Equation.

On a shaft supported by two radial bearings, the distances between the bearing seats on the shaft and in the housing frequently do not coincide as a result of manufacturing tolerances. The distances may also change as a result of temperature increases during operation. These differences in distance are compensated in the non-locating bearing. Examples of locating/non-locating bearing arrangements ➤ Figure.

Solutions proven in practice for individual bearing arrangements (locating bearing arrangement, non-locating bearing arrangement, adjusted/ floating bearing arrangement) and the axial location of bearing rings in certain bearing types are described below. Specific features of the individual bearing types are covered in the product chapters.

Due to the possibility of regulating the clearance, adjusted bearing arrangements are particularly suitable if close guidance is necessary.

Adapter and withdrawal sleeves are used if increased requirements are not made on the running accuracy of the bearing. For the seats, diameter tolerances corresponding to the IT grades 7 to 9 are possible, while the geometrical deviation can be 50% of this value.

In locating bearing arrangements, both bearing rings must always be abutted on both sides. The fasteners must be matched to the magnitude of the axial forces present.

Shaft tolerance forbearingfit

The tables ➤ Table to ➤ Table contain recommendations for the selection of shaft and housing tolerances that are valid for normal mounting and operating conditions. Deviations are possible if particular requirements apply, for example in relation to running accuracy, smooth running or operating temperature. Increased running accuracies thus require closer tolerances such as standard tolerance grade 5 instead of 6. If the inner ring is warmer than the shaft during operation, the seating may loosen to an impermissible extent. A tighter fit must then be selected, for example m6 instead of k6.

For bearings without an inner ring, the enveloping circle Fw ➤ Figure is used. This is the inner inscribed circle of the rolling elements in clearance-free contact with the outer raceway. The enveloping circle for unfitted machined needle roller bearings is in the tolerance class F6 and for drawn cup needle roller bearings in the tolerance class F8. Deviations for F6 and F8 ➤ Table.

Where axial bearings also support radial forces, such as in axial spherical roller bearings, fits should be selected in the same way as for radial bearings.

If materials other than cast iron or steel are used for the adjacent construction, the modulus of elasticity and the differing coefficients of thermal expansion of the materials must also be be taken into consideration to achieve rigid seating. For aluminium housings, thin-walled housings and hollow shafts, a closer fit should be selected if necessary in order to achieve the same force locking as with cast iron, steel or solid shafts.

The double direction axial deep groove ball bearing should be seen as a closed bearing group. The shaft locating washer is axially located on both sides, while the housing locating washers are each located on one side. In order that the ball and cage assemblies are guided correctly in the raceway grooves, the bearing is mounted clearance-free by adjustment of the housing locating washers.

The inner rings of the deep groove ball bearings have a tight fit on the journal and are abutted on the shaft shoulders. The outer rings have a sliding seat. A spring washer is fitted between the outer ring of the right hand bearing and the cover collar. The bearings are axially adjusted by the tensioned springs. This achieves particularly smooth running.

Rolling bearings must be located on the shaft and in the housing in a radial, axial and tangential direction in accordance with their function. In a radial and tangential direction, this occurs by means of a tight fit. However, this is only possible under certain conditions in an axial direction, therefore rolling bearings are generally axially located by means of form fit.

SKFbearing fits to shafts and housings

The example in ➤ Figure shows a bearing arrangement that is commonly used in small electric motors. The bearings are not subjected to high loads, the speed is in the moderate range. The radial load is small and only guidance forces must be supported in an axial direction.

The roughness of the bearing seats must be matched to the tolerance class of the bearings. The mean roughness value Ra must not be too high, in order to maintain the interference loss within limits. Shafts must be ground, while bores must be precision turned. For further information on this subject ➤ Table and product chapter.

In split housings, the joints must be free from burrs. The accuracy of the bearing seats is determined as a function of the accuracy of the bearing selected.

Adjusted bearing arrangements can also be achieved by preloading using springs ➤ Figure. This elastic adjustment method compensates for thermal expansion. It can also be used where bearing arrangements are at risk of vibration while stationary.

The angular contact ball bearing in ➤ Figure supports axial forces in one direction only. The bearing rings therefore only require abutment on one side each in accordance with the force pattern. The axial force component is supported by an additional bearing in a mirror image arrangement. Similar conditions are present in the cylindrical roller bearing NJ.

The tolerances are defined in the form of ISO tolerance classes to ISO 286‑1 and ISO 286-2. The designation of the tolerance classes, e. g. “E8”, comprises one or two upper case letters for housings or lower case letters for shafts (= fundamental deviation identifier, which defines the tolerance position relative to the zero line, e. g. “E”) and the grade number of the standard tolerance grade (this defines the tolerance quality, e. g. “8”). A schematic illustration of the most common rolling bearing fits is shown in ➤ Figure.

The locating bearing A is subjected not only to radial forces but also to axial forces of alternating direction. The axial forces are not very high and do not act in a shock type manner. For location of the deep groove ball bearing, rigid shoulders, covers, snap rings or other form fit elements are therefore normally used. The adjacent parts should require little production work and mounting and dismounting should be easy to perform.

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The guidance and support of a rotating machine part generally requires at least two bearings arranged at a certain distance from each other (exceptions: four point contact, crossed roller and slewing bearings). Depending on the application, a decision is made between a locating/non-locating bearing arrangement, an adjusted bearing arrangement and a floating bearing arrangement.

The non-locating bearing B must support radial forces only. The outer ring is clamped between the snap ring and cover, while the inner ring has a tight fit on the shaft.

The non-locating bearing B must support radial forces only. Due to the magnitude of the forces, both rings have tight fits. A snap ring in one annular slot of the outer ring securely prevents creep of the bearing to the left. The ribs of the bearing rings represent additional security against creep to the right. In order to prevent jamming of the bearing arrangement, the non-locating bearing must have axial clearance between the inner ring rib and the rollers.

Whether the inner ring or outer ring is adjusted depends on the degree of accessibility of the adjustment elements, such as nuts and covers. Since the bearing ring to be adjusted must be easily displaced, attention must also be paid in these considerations to the fits of the bearing rings.

➤ Figure shows the bearing arrangement of a heavy support roller. The bearings are subjected to high radial loads. In addition, a frictional force acts axially on the outside surface of the support roller. Close axial guidance is not required and, as a result, a floating bearing arrangement can be selected. In the course of this, the lateral movement of the outer rings is restricted by the contact in the housing. Both housings are split. The axial displacement distance s can be measured with the upper section removed.

The resulting support spacing H is larger in an O arrangement than in an X arrangement. An O arrangement should be used in preference if the component with small bearing spacing must be guided with the smallest possible tilting clearance or tilting forces must be supported.

Bearing fits to shafts and housingssizes

➤ Figure to ➤ Figure show possibilities for the location of bearing rings depending on the design of the bearing arrangement and the application.

The locating bearing in ➤ Figure is subjected to high axial forces in both directions and the shaft must be guided axially clearance-free.

In the spherical roller bearing and barrel roller bearing, as well as in the angular contact ball bearing pair, the outer ring is guided axially by the rolling elements. The inner ring of the barrel roller bearing is located on the shaft with or without an adapter sleeve. Location by means of an adapter sleeve secures the bearing against lateral creep.

In order to calculate the taper slope SL (nominal dimension), the taper length L is multiplied by the taper ratio (1:12) ➤ Equation.

In wheel bearing arrangements with a rotating outer ring in accordance with ➤ Figure, not only are high radial and axial forces present but tilting moments also occur. The outer rings have a tight fit. In these sorts of hub bearing arrangements, this is important due to the circumferential load acting on the outer rings. The axial clearance of the bearing group is set by means of the fixing nut, where the loosely fitted inner ring of the outer bearing undergoes displacement.

Axial bearings, which support axial loads only, must not be guided radially (with the exception of axial cylindrical roller bearings which have a degree of freedom in the radial direction due to flat raceways). In the case of groove-shaped raceways this is not present and must be achieved by a loose seat for the stationary washer. A rigid seat is normally selected for the rotating washer.

The degree of accuracy for the bearing seat tolerances on the shaft and in the housing, as well as the ISO fundamental tolerances, are shown in ➤ Table (DIN ISO 286-1:2010).

The hardness values apply to raceways, axial washers and shaft shoulders. Steels hardened by means of case, flame or induction hardening must have a surface hardness of 670 HV to 840 HV and an adequate hardening depth CHD or SHD.

If the shaft is warmer than the housing (TW ＞ TG ), the shaft expands more than the housing in an axial and radial direction. As a result, the clearance set in an X arrangement decreases in every case (assuming the following precondition: shaft and housing of same material).

The cylindrical roller bearing NU is designed such that the inner ring can be displaced relative to the roller and cage assembly. For this reason, both bearing rings must also be secured against axial creep on both sides.

Deep groove ball bearings can also be used for an adjusted bearing arrangement; these are then angular contact ball bearings with a small nominal contact angle.

In the deep groove ball bearing, only the inner ring is located, while the outer ring is axially retained by the rolling elements.

If a lower axial force is present, a cylindrical roller bearing with cage of series NUP can also be used as a locating bearing.

Angular contact bearings support forces comprising a radial and an axial component. These are thus a combination of a radial and an axial bearing. Depending on the size of the nominal contact angle α, angular contact bearings are classified as radial or axial bearings.

Through hardening steels in accordance with ISO 683-17 (e. g. 100Cr6) are suitable as materials for rolling bearing raceways in direct bearing arrangements. These can also be surface layer hardened.

The bearing arrangement of a pinion shaft shown in ➤ Figure is subjected to high, occasionally shock type radial and axial forces. Due to the hypoid tooth set, precise axial adjustment of the pinion against the crown gear and rigid guidance are necessary.

The positional tolerances t4 for a second bearing seat on the shaft (d2) or in the housing (D2) are dependent on the types of bearings used and the operating conditions.

The vertical shaft in ➤ Figure is radially guided by a radial deep groove ball bearing and axially supported by an axial deep groove ball bearing. The snap ring functions with the disc spring to give preload and prevent lift-off when the working pressure is not directed downwards. There is some axial clearance between the disc spring when pressed flat and the snap ring. This gives easier mounting of the snap ring.

A double row angular contact ball bearing, for example, will give closer axial guidance than a deep groove ball bearing or a spherical roller bearing. A pair of symmetrically arranged angular contact ball bearings or tapered roller bearings used as locating bearings will also provide extremely close axial guidance.

Good support of the bearing rings on their circumference requires rigid seating. The requirement that rings must not creep on their mating parts also requires firm seating. If non-separable bearings must be mounted and dismounted, a tight fit can only be achieved for one bearing ring. In the case of cylindrical roller bearings N, NU and needle roller bearings, both rings can have tight fits, since the length compensation takes place within the bearing and since the rings can be mounted separately. With tight fits and a temperature differential between the inner and outer ring, the radial internal clearance of the bearing is reduced. This must be taken into consideration when selecting the radial internal clearance.

An adjusted bearing arrangement is generally constructed from two angular contact bearings (angular contact ball bearings, tapered roller bearings) in a mirror image arrangement ➤ Figure and ➤ Figure. The inner and outer rings of the bearings are displaced relative to each other until the required clearance or the required preload is achieved. This process is known as “adjustment”.

Bearingfit calculator

Tapered roller bearings and angular contact ball bearings are not suitable for a floating bearing arrangement, since these bearings must be adjusted in order to run correctly.

The pair of angular contact ball bearings forms a locating bearing in which the two single row bearings are adjusted against each other in mounting. For location on the shaft, readjustable fasteners, such as nuts, are suitable.

The local hardness must always be above the local requisite hardness, which can be calculated from the equivalent stress.

The balls in the axial deep groove ball bearing in ➤ Figure only roll correctly if the bearing runs clearance-free and with adequate minimum load.

An approximation value for determining the minimum hardness depth can be found in ➤ Equation. The reference value for the load present is the equivalent stress in accordance with the distortion energy hypothesis (DEH) as a function of the rolling element diameter Dw and the magnitude of the load.

For bearing widths with nominal dimensions between the values listed in the table, the taper angle tolerance tΔSL should be determined by means of interpolation of the upper deviations ➤ Equation.

In angular contact ball bearings and tapered roller bearings, the contact lines of the rolling element forces coincide at the contact cone apexes S ➤ Figure and ➤ Figure. In adjusted bearing arrangements, the bearing spacing is therefore defined as the spacing of the contact cone apexes.

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This information does not apply to super precision cylindrical roller bearings in machine tools (spindle bearing arrangement). For information on this subject, see the catalogue Super precision bearings SP 1.

The locating bearing guides the shaft in an axial direction and supports external axial forces. In order to prevent axial bracing, shafts with more than two bearings have only one locating bearing. The type of bearing selected as a locating bearing depends on the magnitude of the axial forces and the accuracy with which the shafts must be axially guided.