Design of bearing arrangements

Arrangement of bearings

Support of a shaft normally requires two bearings

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.

Locating/non-locating bearing arrangement

The non-locating bearing compensates for differences in distance

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.

Non-locating bearings

Suitable non-locating bearings

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.

Locating bearings

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.

Suitable locating bearings

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.

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).

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.

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

No adjustment or setting work with matched pairs of tapered roller bearings

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.

Locating/non-locating bearing arrangements = non-locating bearing Locating bearing: deep groove ball bearing Non-locating bearing: deep groove ball bearing Locating bearing: spherical roller bearing Non-locating bearing: spherical roller bearing Locating bearing: deep groove ball bearing Non-locating bearing: cylindrical roller bearing NU Locating bearing: spherical roller bearing Non-locating bearing: toroidal roller bearing Locating bearing: double row angular contact ball bearing Locating bearing: cylindrical roller bearing NU 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 Locating bearing: tapered roller bearing Non-locating bearing: cylindrical roller bearing NU Locating bearing: cylindrical roller bearing NUP Non-locating bearing: cylindrical roller bearing NU

Adjusted bearing arrangement

The “adjustment” process

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”.

Angular contact bearings and deep groove ball bearings suitable for adjusted bearing arrangements

Angular contact bearings support radial and axial forces

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.

Deep groove ball bearings are also suitable

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.

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

O or X arrangement

Two arrangements

In an adjusted bearing arrangement, an O or X arrangement of the bearings is essentially possible.

The contact cone apexes point outwards or inwards

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.

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.

The support spacing is larger in an O arrangement

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.

Adjusted bearing arrangement with angular contact ball bearings S = contact cone apex H = support spacing O arrangement X arrangement

Influence of thermal expansion in O and X arrangements

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.

X arrangement

If the shaft is warmer than the housing (T_W ＞ T_G ), 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).

Adjusted bearing arrangement with tapered roller bearings X arrangement R = roller cone apex S = contact cone apex

Temperature behaviour and thermal expansion in an O arrangement

The behaviour is different in an O arrangement. A distinction must be drawn between three cases here:

• If the roller cone apexes R coincide at a point, the axial and radial thermal expansion cancel each other out and the clearance set is maintained ➤ Figure
• If the roller cones overlap with a small bearing spacing, the radial expansion has a stronger effect than the axial expansion on the bearing clearance ➤ Figure: the axial clearance is reduced. This must be taken into consideration in the adjustment of bearings
• In the third case, the roller cones do not overlap with a large bearing spacing ➤ Figure. The radial expansion then has a weaker effect than the axial expansion on the bearing clearance: the axial clearance is increased

Adjusted bearings in an O arrangement, the roller cone apexes coincide R = roller cone apex S = contact cone apex

Adjusted bearings in an O arrangement, the roller cone apexes overlap R = roller cone apex S = contact cone apex

Adjusted bearings in an O arrangement, the roller cone apexes do not overlap R = roller cone apex S = contact cone apex

Sliding seat in the bearing ring to be adjusted

Sliding seat only permissible on the bearing ring with point load

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.

A sliding seat should fundamentally only be permitted on the ring that is subject to point load.

Elastic adjustment

Preloading using springs

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.

Bearing arrangement adjusted by means of spring washer Deep groove ball bearing Spring washer (spring preload) Cover

Floating bearing arrangement

Difference from the adjusted bearing arrangement: this does not give close axial guidance

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.

Suitable bearing types

A floating bearing arrangement is suitable for bearings that must not be adjusted

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.

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.

Floating bearing arrangements s = axial displacement distance (axial clearance) Deep groove ball bearing Spherical roller bearing Cylindrical roller bearing NJ

Radial location of bearings

Location of the bearing rings in a radial and tangential direction by fit, in an axial direction by form fit

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.

Criteria for selection of fits

Points to be observed in the selection of fits

The following must be taken into consideration in determining the fit:

• The rolling bearing rings must be well supported over their entire circumference in order to allow full utilisation of the load carrying capacity of the bearing
• The rings must not creep on their mating parts, otherwise the seating surfaces will be damaged
• The non-locating bearing must compensate changes in the length of the shaft and housing and one ring must therefore be axially adjustable
• Mounting and dismounting of the bearings should be possible without a large amount of work

Interference fits

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.

Tight fit necessary

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.

Materials other than cast iron or steel

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.

Higher loads

Higher loads, especially shocks, require a fit with larger interference and narrower geometrical tolerances.

Bearing seat for axial bearings

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.

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.

Contact surfaces of the mating parts

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.

Conditions of rotation

Point or circumferential load

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 point load, a loose fit is also possible

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.

For circumferential load, a firm bearing seat is necessary

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 possible creep or slippage of a bearing ring can only be effectively prevented by a firm bearing seat.

Differentiation between point load and circumferential load

Condition of rotation	Example	Schematic	Load case	Fit
Rotating inner ring, stationary outer ring	Shaft with weight load		Circumferential load on inner ring and point load on outer ring	Inner ring: tight fit necessary Outer ring: loose fit permissible
Constant load direction	Shaft with weight load
Stationary inner ring, rotating outer ring	Hub bearing arrangement with significant imbalance		Circumferential load on inner ring and point load on outer ring	Inner ring: tight fit necessary Outer ring: loose fit permissible
Load direction rotates with outer ring	Hub bearing arrangement with significant imbalance
Stationary inner ring, rotating outer ring	Passenger car front wheel track roller (hub bearing arrangement)		Point load on inner ring and circumferential load on outer ring	Inner ring: loose fit permissible Outer ring: tight fit necessary
Constant load direction	Passenger car front wheel track roller (hub bearing arrangement)
Rotating inner ring, stationary outer ring	Centrifuge, vibrating screen		Point load on inner ring and circumferential load on outer ring	Inner ring: loose fit permissible Outer ring: tight fit necessary
Load direction rotates with inner ring	Centrifuge, vibrating screen

Recommended fits

Shaft and housing tolerances

ISO tolerance classes

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.

Shaft and housing fits for rolling bearings D = nominal bearing outside diameter d = nominal bearing bore diameter tΔDmp = deviation of mean bearing outside diameter (in accordance with ISO 492) tΔdmp = deviation of mean bearing bore diameter (in accordance with ISO 492) Zero line Housing Shaft Clearance fit Transition fit Interference fit

Recommendations for shaft and housing tolerances

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.

Objective: the best overall solution

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.

Tolerance classes for cylindrical shaft seats (radial bearings)

Condition of rotation1)	Bearing type	Shaft diameter		Displacement facility Load	Tolerance class2) of shaft
		mm
		over	incl.
Point load on inner ring
	Ball bearings, roller bearings	All sizes		Inner ring easily displaced	g6 (g5)
	Ball bearings, roller bearings	All sizes		Inner ring not easily displaced, angular contact ball bearings and tapered roller bearings with adjusted inner ring	h6 (j6)
	Needle roller bearings	All sizes		Non-locating bearing	h6 (g6)3)
Circumfer­ential load on inner ring or indeter­minate load direction
	Ball bearings
		‒	50	Normal loads4)	j6 (j5)
		50	100	Low loads5)	j6 (j5)
		50	100	Normal and high loads6)	k6 (k5)
		100	200	Low loads4)	k6 (m6)
		100	200	Normal and high loads7)	m6 (m5)
		200	‒	Low loads	m6 (m5)
		200	‒	Normal and high loads	n6 (n5)
	Roller bearings
		‒	60	Low loads	j6 (j5)
		‒	60	Normal and high loads	k6 (k5)
		60	200	Low loads	k6 (k5)
		60	200	Normal loads	m6 (m5)
		60	200	High loads	n6 (n5)
		200	500	Normal loads	m6 (n6)
		200	500	High loads, shocks	p6
		500	‒	Normal loads	n6 (p6)
		500	‒	High loads	p6
	Needle roller bearings
		‒	50	Low loads	k6
		‒	50	Normal and high loads	m6
		50	120	Low loads	m6
		50	120	Normal and high loads	n6
		120	250	Low loads	n6
		120	250	Normal and high loads	p6
		250	400	Low loads	p6
		250	400	Normal and high loads	r6
		400	500	Low loads	r6
		400	500	Normal and high loads	s6
		500	‒	Low loads	r6
		500	‒	Normal and high loads	s6

1. Condition of rotation ➤ Table.
2. The envelope requirement Ⓔ ➤ Figure applies.
3. For easy fitting.
4. C₀/P ＞ 10.
5. C₀/P ＞ 12.
6. C₀/P ＜ 12.
7. C₀/P ＜ 10.

Tolerance classes for cylindrical shaft seats (axial bearings)

Load	Bearing type	Shaft diameter		Operating conditions	Tolerance class1) of shaft
		mm
		over	incl.
Axial load
	Axial deep groove ball bearings	All sizes		–	j6
	Axial deep groove ball bearings, double direction	All sizes		–	k6
	Axial cylindrical roller bearings with shaft locating washer	All sizes		–	h8
	Axial cylindrical roller and cage assemblies	All sizes		–	h8
Combined load
	Axial spherical roller bearings	All sizes		Point load on shaft locating washer	j6
	Axial spherical roller bearings	‒	200	Circumferential load on shaft locating washer	j6 (k6)
	Axial spherical roller bearings	200	‒	Circumferential load on shaft locating washer	k6 (m6)

1. The envelope requirement Ⓔ ➤ Figure applies.

Tolerance classes for bearing seats in housings (radial bearings)

Condition of rotation1)	Displacement facility Load	Operating conditions	Tolerance class2) of bore
Point load on outer ring
	Outer ring easily displaced, housing unsplit	The tolerance grade is determined by the running accuracy required	H7 (H6)3)
	Outer ring easily displaced, housing split	The tolerance grade is determined by the running accuracy required	H8 (H7)
	Outer ring not easily displaced, housing unsplit	High running accuracy required	H6 (J6)
	Outer ring not easily displaced, angular contact ball bearings and tapered roller bearings with adjusted outer ring, housing split	Normal running accuracy	H7 (J7)
	Outer ring easily displaced	Heat input via shaft	G74)
Circumfer­ential load on outer ring or indeter­minate load direction
	Low loads, outer ring cannot be displaced	High requirements for running accuracy: K6, M6, N6 and P6	K7 (K6)
	Normal loads, shocks, outer ring cannot be displaced	High requirements for running accuracy: K6, M6, N6 and P6	M7 (M6)
	High loads, shocks (C0/P ＜ 6), outer ring cannot be displaced	High requirements for running accuracy: K6, M6, N6 and P6	N7 (N6)
	High loads, severe shocks, thin‑walled housing, outer ring cannot be displaced	High requirements for running accuracy: K6, M6, N6 and P6	P7 (P6)

1. Condition of rotation ➤ Table.
2. The envelope requirement Ⓔ ➤ Figure applies.
3. G7 for housings made from flake graphite cast iron, with bearing outside diameter D ＞ 250 mm and temperature differential between outer ring and housing ＞ 10 K.
4. F7 for housings made from flake graphite cast iron, with bearing outside diameter D ＞ 250 mm and temperature differential between outer ring and housing ＞ 10 K.

Tolerance classes for bearing seats in housings (axial bearings)

Load	Bearing type	Operating conditions	Tolerance class1) of bore
Axial load
	Axial deep groove ball bearings	Normal running accuracy	E8
	Axial deep groove ball bearings	High running accuracy	H6
	Axial cylindrical roller bearings with housing locating washer	‒	H9
	Axial cylindrical roller and cage assemblies	‒	H10
	Axial spherical roller bearings	Normal loads	E8
	Axial spherical roller bearings	High loads	G7
Combined loads, point load on housing locating washer
	Axial spherical roller bearings	‒	H7
Combined loads, circumferential load on housing locating washer
	Axial spherical roller bearings	‒	K7

1. The envelope requirement Ⓔ ➤ Figure applies.

Tables of fits

Clearance, transition and interference fits for shafts and housing bores

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.

Example for shaft fit, tolerance class j5

For the shaft ⌀ 40 j5 Ⓔ, ➤ Table gives an example of how to read the numerical values.

Example for housing fit, tolerance class K6

For the housing bore ⌀ 100 K6 Ⓔ, ➤ Table gives an example explaining the group of numbers.

Shaft fits

Example

Example: shaft ⌀ 40 j5 Ⓔ
Maximum material value	+6	18 10	Interference or fit clearance if the maximum material values are combined. Probable interference or fit clearance
Minimum material value	–5	5	Interference or fit clearance if the minimum material values are combined
Values in bold type in the group of three indicate fit interference, values in normal type indicate fit clearance.

Housing fits

Example

Housing ⌀ 100 K6 Ⓔ
Minimum material value	+4	18  6	Interference or fit clearance if the maximum material values are combined. Probable interference or fit clearance
Maximum material value	–18	19	Interference or fit clearance if the minimum material values are combined
Values in bold type in the group of three indicate fit interference, values in normal type indicate fit clearance.

Shaft tolerances for adapter sleeves and withdrawal sleeves are shown in ➤ Table.

Shaft tolerances for adapter sleeves and withdrawal sleeves

Nominal shaft diameter		Shaft tolerance
		h7/		h8/		h9/
mm		μm		μm		μm
over	incl.
3	6	0 –12	2,5	0 –18	2,5	0 –30	4
6	10	0 –15	3	0 –22	3	0 –36	4,5
10	18	0 –18	4	0 –27	4	0 –43	5,5
18	30	0 –21	4,5	0 –33	4,5	0 –52	6,5
30	50	0 –25	5,5	0 –39	5,5	0 –62	8
50	65	0 –30	6,5	0 –46	6,5	0 –74	9,5
65	80	0 –30	6,5	0 –46	6,5	0 –74	9,5
80	100	0 –35	7,5	0 –54	7,5	0 –87	11
100	120	0 –35	7,5	0 –54	7,5	0 –87	11
120	140	0 –40	9	0 –63	9	0 –100	12,5
140	160	0 –40	9	0 –63	9	0 –100	12,5
160	180	0 –40	9	0 –63	9	0 –100	12,5
180	200	0 –46	10	0 –72	10	0 –115	14,5
200	220	0 –46	10	0 –72	10	0 –115	14,5
220	250	0 –46	10	0 –72	10	0 –115	14,5
250	280	0 –52	11,5	0 –81	11,5	0 –130	16
280	315	0 –52	11,5	0 –81	11,5	0 –130	16
315	355	0 –57	12,5	0 –89	12,5	0 –140	18
355	400	0 –57	12,5	0 –89	12,5	0 –140	18
400	450	0 –63	13,5	0 –97	13,5	0 –155	20
450	500	0 –63	13,5	0 –97	13,5	0 –155	20
500	560	0 –70	16	0 –110	16	0 –175	22
560	630	0 –70	16	0 –110	16	0 –175	22
630	710	0 –80	18	0 –125	18	0 –200	25
710	800	0 –80	18	0 –125	18	0 –200	25
800	900	0 –90	20	0 –140	20	0 –230	28

The numbers printed in italics give guide values for the cylindricity tolerance t₁ (DIN EN ISO 1101) ➤ Figure.

Enveloping circle

For bearings without an inner ring, the enveloping circle F_w ➤ 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.

Enveloping circle Fw = enveloping circle diameter Rolling element Outer raceway

Deviations for the enveloping circle diameter

Enveloping circle diameter Fw		Tolerance class F6		Tolerance class F8
mm		Tolerance for enveloping circle diameter Fw		Tolerance for enveloping circle diameter Fw
		Upper deviation	Lower deviation	Upper deviation	Lower deviation
over	incl.	μm	μm	μm	μm
3	6	+18	+10	+28	+10
6	10	+22	+13	+35	+13
10	18	+27	+16	+43	+16
18	30	+33	+20	+53	+20
30	50	+41	+25	+64	+25
50	80	+49	+30	+76	+30
80	120	+58	+36	+90	+36
120	180	+68	+43	+106	+43
180	250	+79	+50	+122	+50
250	315	+88	+56	+137	+56
315	400	+98	+62	+151	+62
400	500	+108	+68	+165	+68

Dimensional, geometrical and running accuracy of mating parts

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.

Guide values for the geometrical and positional tolerances of bearing seating surfaces t1 = roundness tolerance t2 = parallelism tolerance t3 = total axial runout tolerance of abutment shoulders t4 = coaxiality tolerance

Accuracy of bearing seating surfaces

ISO fundamental tolerances

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).

Second bearing seat

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.

Housings

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.

Guide values for the geometrical and positional tolerances of bearing seating surfaces

Bearing tolerance class		Bearing seating surface	Fundamental tolerance grades1)
to ISO 492	to DIN 620		Diameter tolerance	Roundness tolerance	Parallelism tolerance	Total axial runout tolerance of abutment shoulder
				t1	t2	t3
Normal 6X	PN (P0) P6X	Shaft	IT6 (IT5)	Circumfer­ential load IT4/2	Circumfer­ential load IT4/2	IT4
		Shaft	IT6 (IT5)	Point load IT5/2	Point load IT5/2	IT4
		Housing	IT7 (IT6)	Circumfer­ential load IT5/2	Circumfer­ential load IT5/2	IT5
		Housing	IT7 (IT6)	Point load IT6/2	Point load IT6/2	IT5
6	P6	Shaft	IT5	Circumfer­ential load IT3/2	Circumfer­ential load IT3/2	IT3
		Shaft	IT5	Point load IT4/2	Point load IT4/2	IT3
		Housing	IT6	Circumfer­ential load IT4/2	Circumfer­ential load IT4/2	IT4
		Housing	IT6	Point load IT5/2	Point load IT5/2	IT4
5	P5	Shaft	IT5	Circumfer­ential load IT2/2	Circumfer­ential load IT2/2	IT2
		Shaft	IT5	Point load IT3/2	Point load IT3/2	IT2
		Housing	IT6	Circumfer­ential load IT3/2	Circumfer­ential load IT3/2	IT3
		Housing	IT6	Point load IT4/2	Point load IT4/2	IT3
4	P4 P4S2) SP2)	Shaft	IT4	Circumfer­ential load IT1/2	Circumfer­ential load IT1/2	IT1
		Shaft	IT4	Point load IT2/2	Point load IT2/2	IT1
		Housing	IT5	Circumfer­ential load IT2/2	Circumfer­ential load IT2/2	IT2
		Housing	IT5	Point load IT3/2	Point load IT3/2	IT2
	UP2)	Shaft	IT3	Circumfer­ential load IT0/2	Circumfer­ential load IT0/2	IT0
		Shaft	IT3	Point load IT1/2	Point load IT1/2	IT0
		Housing	IT4	Circumfer­ential load IT1/2	Circumfer­ential load IT1/2	IT1
		Housing	IT4	Point load IT2/2	Point load IT2/2	IT1

1. ISO fundamental tolerances (IT grades) in accordance with DIN ISO 286. Values for IT grades ➤ Table.
2. Not included in DIN 620.

Roughness of bearing seats

Ra must not be too high

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.

Roughness values for cylindrical bearing seating surfaces – guide values

Nominal diameter of bearing seat d (D)		Recommended mean roughness value for ground bearing seats Ramax
mm		μm
		Diameter tolerance (IT grade)
over	incl.	IT7	IT6	IT5	IT4
‒	80	1,6	0,8	0,4	0,2
80	500	1,6	1,6	0,8	0,4
500	1  250	3,21)	1,6	1,6	0,8

1. For the mounting of bearings using the hydraulic method, a value Ra = 1,6 μm must not be exceeded

Numerical values for IT grades

➤ Table shows numerical values for the ISO fundamental tolerances (IT grades) in accordance with DIN ISO 286-1:2010.

IT grades and values

IT grade	Nominal dimension in mm
	over	–	3	6	10	18	30	50	80
	incl.	3	6	10	18	30	50	80	120
	Values in μm
IT01		0,3	0,4	0,4	0,5	0,6	0,6	0,8	1
IT0		0,5	0,6	0,6	0,8	1	1	1,2	1,5
IT1		0,8	1	1	1,2	1,5	1,5	2	2,5
IT2		1,2	1,5	1,5	2	2,5	2,5	3	4
IT3		2	2,5	2,5	3	4	4	5	6
IT4		3	4	4	5	6	7	8	10
IT5		4	5	6	8	9	11	13	15
IT6		6	8	9	11	13	16	19	22
IT7		10	12	15	18	21	25	30	35
IT8		14	18	22	27	33	39	46	54
IT9		25	30	36	43	52	62	74	87
IT10		40	48	58	70	84	100	120	140
IT11		60	75	90	110	130	160	190	220
IT12		100	120	150	180	210	250	300	350
continued ▼

IT grades and values

IT grade	Nominal dimension in mm
	over	120	180	250	315	400	500	630	800
	incl.	180	250	315	400	500	630	800	1  000
	Values in μm
IT01		1,2	2	2,5	3	4	‒	‒	‒
IT0		2	3	4	5	6	‒	‒	‒
IT1		3,5	4,5	6	7	8	9	10	11
IT2		5	7	8	9	10	11	13	15
IT3		8	10	12	13	15	16	18	21
IT4		12	14	16	18	20	22	25	28
IT5		18	20	23	25	27	32	36	40
IT6		25	29	32	36	40	44	50	56
IT7		40	46	52	57	63	70	80	90
IT8		63	72	81	89	97	110	125	140
IT9		100	115	130	140	155	175	200	230
IT10		160	185	210	230	250	280	320	360
IT11		250	290	320	360	400	440	500	560
IT12		400	460	520	570	630	700	800	900
continued ▲

Seats for adapter sleeves and withdrawal sleeves

Seat diameter tolerances for adapter sleeves and withdrawal sleeves

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.

Tapered bearing seats for radial bearings

Geometrical and positional tolerances of the shaft

Guide values for the machining of tapered bearing seats on shafts are shown in ➤ Figure, ➤ Table.

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.

Taper gauges

Schaeffler taper gauges can be used to check for adherence to the recommended tolerances.

Guide values for the geometrical and positional tolerances of tapered bearing seats B = bearing width SL = L · taper ratio (1:12, 1:30) tΔSL′ = taper angle tolerance t1 = roundness tolerance ➤ Table t6 = perpendicularity tolerance = 2/3 · t2; values for t2 ➤ Table z = recommended mean roughness ➤ Table

The tolerances for taper angle t_ΔSL relative to the bearing width B can be found in the table ➤ Table.

Taper angle tolerance of tapered bearing seats, relative to bearing width

Bearing width B mm		Taper angle tolerance tΔSL
		from		to
		Deviations
over	incl.	upper μm	lower μm	uper μm	lower μm
16	25	+8	0	+12,5	0
25	40	+10	0	+16	0
40	63	+12,5	0	+20	0
63	100	+16	0	+25	0
100	160	+20	0	+32	0
160	250	+25	0	+40	0
250	400	+32	0	+50	0
400	630	+40	0	+63	0

Determine t_ΔSL by means of interpolation

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

Interpolation of the taper angle tolerance

For a taper of length L, the taper angle tolerance t_ΔSL′ of the entire taper applies ➤ Equation.

Taper angle tolerance of the entire taper

Example of tolerance calculation

Given:

• bearing width B = 90 mm
• taper ratio 1:12
• taper length L = 100 mm

The tolerance t_ΔSL′/2 is thus 0 to +12 μm.

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

Taper slope

The nominal dimension for SL/2 is thus 4,166 mm; SL/2 = 4,166 +0,012/0.

The data can then be entered in the drawing as follows ➤ Figure.

Example of drawing entry for dimensional tolerances

Checking of a shaft

Measured values:

• d₁′ = 120 mm
• d₂′ = 128,345 mm

The taper slope is calculated from the measured values using ➤ Equation.

Taper slope

The value for SL/2 is thus within tolerance.