Design

The outstanding characteristics of GMN ball bearings are the result of technically demanding quality characteristics which achieve maximum performance limits. Various measures in design, such as preload or multiple arrangements of bearings, counter performance limitations and increase the performance capabilities of bearings.

Preloading bearings

Preload is defined as a constantly acting axial force on a ball bearing which creates an elastic deformation in the contact area of the balls and raceways.

Axial force effect on the bearing

Performance optimization through preload

Installing ball bearings with stiff or spring preload optimizes many performance characteristics for bearing operation.

Reduced springing ensures the generation of a definable radial and axial rigidity (see diagram)
High running accuracy and workability even with changing loads
Reduced vibration and noise
Avoid slippage and friction in the rolling element contact at high speeds and high acceleration
Reduced sliding friction parts at high speeds (reduced contact angle change between inner and outer ring)
Increased load capacity (due to external loads and rotational speeds) with a long service life

Non-preloaded bearing pair

Preloaded bearing pair

Rigidity

Rigidity defines the amount of axial force effect [N] on the ball bearing, which causes a shift in the bearing ring by 1 μm.

Suitable preload increases bearing rigidity and supports the load carrying capability of the bearing against operating forces.

Preload, rigidity

Lifting force

Lifting force is the force at which the bearing becomes load-free through a central axial load on a bearing set.
If the external axial load exceeds the lifting force, …
… the balls and the raceways of the unloaded ball bearing are no longer in constant contact.
… the wear is increased by increasing sliding friction.

Spring preload

Design characteristics:

Bearing 1 (working side) is axially fixed in the housing, bearing 2 is arranged axially movably (fixed seat of the inner rings on the shaft)
The spring force on the outer ring of the bearing 2 ensures a constant preload for both bearings
The required spring preload is set by way of the spring travel (path-force function according to the spring characteristic curve)
For perfect preload results, a sufficient, axial mobility of the set outer ring on the floating bearing is required
The adjustment of the adjusting spring takes place in the direction of action of the external axial load
When using single bearings: <~>, untuned bearings can be used
When using bearings in tandem (<< ~ >>), bearings of the same type (L, M or S) ensure a uniform load distribution

Characteristics:

The preload – independent of speed and temperature – results exclusively from the spring force
The spring force results in an equal preload of the bearing and the thrust bearing
Thermal expansion of shaft and housing have no influence on preload
Spring-loaded bearing systems can have the highest speeds

Rigid preload individual bearings

Design characteristics:

Bearing 1 (working side) and bearing 2 are fixed axially and radially (fixed seating of the inner rings on the shaft or the outer rings in the housing)
The contact surfaces of the conversion parts on the inner and outer ring have the same length and are plane parallel
In order to achieve the predefined preload, a matching of the respective bearings is necessary

Characteristics:

Considerably higher axial as well as radial stiffness (compared to spring preload)
As the frictional heat increases as a result of increasing speeds, the preload increases and reduces the rotational speed limit (compared to the spring preload)
The theoretical speed limit can be calculated on the basis of the speed correction factors
Temperature differences between shaft (inner ring) and housing (outer ring) lead to preload changes due to thermal expansion
If the shaft has a higher temperature than the housing, the radial clearance in the bearing is reduced
Very high temperature differences and small contact angles can cause radial stress
At a small bearing distance, a temperature gradient from shaft to housing may cause an increase in the bias
With large bearing clearance, the temperature gradient from shaft to housing may cause a reduction in the preload
The change in the preload in the operating state must be taken into account during the design phase

For the complex calculation of the required bearing preload, GMN provides software solutions that deliver reliable preload results, using our long-term practical experience.

Rigid preload bearing sets

The arrangement of several bearings in so-called bearing sets increases load bearing capacity, rigidity and lifting force.

Thus the radial rigidity for all arrangements is:
at α = 15°: Crad ~ 6 · Cax
at α = 25°: Crad ~ 2 · Cax

Example: Bearing set with 3 bearings in a TBT arrangement

* Reference values for bearing pairs in O- or X-arrangement (see bearing data).
Operating-related influences (such as RPM, load) are not considered.

MULTIPLE ARRANGEMENT WITH 2 BEARINGS (BEARING PAIR)

With rigid bearing preload, specified bearing pairs in O, X or tandem arrangements offer an effective, cost-effective and technical solution for many applications.

O ARRANGEMENT (DB)

Pressure lines diverge in the direction of the bearing axis

Large support base (H) and high rigidity against tilting moments
Axial force absorption in both directions

Bearing pair in O arrangement

X ARRANGEMENT (DF)

Pressure lines converge in the direction of the bearing axis

Insensitive to escape errors
Reduced support base size and tilting rigidity
Axial force absorption in both directions

Bearing pair in X arrangement

TANDEM ARRANGEMENT (DT)

Parallel arrangement to load direction

Higher axial load capacity (factor 2) than single bearing
Both bearings have the same contact angle and are placed against a third bearing

Bearing pairs in tandem

MULTIPLE ARRANGEMENT WITH 3 OR MORE BEARINGS (BEARING SET)

With maximum requirements for system rigidity or high loads, X, O or tandem arrangements with 3 or more bearings provide outstanding performance characteristics.

Arrangements with 3 bearings

Bearing sets in different arrangements

Arrangement with 4 bearings

Intermediate rings

Performance optimization through intermediate rings

One can achieve differentiated optimization of individual quality characteristics of paired bearings by installing intermediate rings (distance rings). The width of an intermediate ring is at least the width of an individual bearing.

Characteristics:

Increasing the support base (H) and increasing the radial rigidity
Optimization of heat dissipation
Improved bearing lubrication thanks to optimized oil feed and discharge

Intermediate ring width ≥ Single bearing width

Design characteristics:

Material: 100 Cr6, or similar, hardened (at least 45 HRC)
Care must be taken to ensure a good planar parallelism between the intermediate rings (see also the accuracy of the components).
The required parallelism of the outer and inner intermediate ring is ensured by planar grinding of both rings in one clamping operation.
In the case of bearing sets with intermediate rings (for example <||<||>||>), the spacer ring between the bearings is ground off with different pressure line trajectories and thus the pre-tensioning is coordinated.

Distance ring between different pressure line paths

Change of preload using intermediate rings

Intermediate rings provide a change to pre-tensioning for already coordinated ball bearings.
If the width of the shaft intermediate ring is less than the width of the housing…
… the preload in the O-arrangement increases
… the preload in the X array is reduced

You will find the required differential dimension of the intermediate rings for a change in the preload of a specific bearing in the technical information sheet “Change of pre-tension L-M-S” as well as further information in the download area.

Contact angle & coordination precision

_{Contact angle ⍺0}

The angle of the straight lines between the contact points: Inner ring raceway – ball – outer ring raceway and the radial level defines the contact angle.

The contact angle is determined depending upon the radial bearing clearance (bearing play) and osculation of the raceways.

Load transfers between both bearing rings are made over the contact points of the raceways with the balls.

Uniform load distribution on the individual bearings in bearing arrangements sets the same contact angle on all loaded bearings.

Standard contact angle C (15°) and E (25°) )

The contact angle is changed depending upon operation through …

… external forces
… internal forces
(Centrifugal force of inner ring and balls at high speeds)
… inner ring fits
… temperature differences from inner ring to outer ring.
Deviations of the contact angle cause changes to the bearing characteristics,
which influence bearing operation.

With increasing contact angles …

… the axial rigidity increases
… the maximum permissible speed decreases
… the radial rigidity decreases

Further contact angles are available on request.

Precision of the conversion parts

Guidance values for shaft adjustments and shape and position tolerances (DIN EN ISO 1101)

Optimization of the fit with high RPMs

With increasing RPM (from about n · Dm = 1.5 . 10⁶ mm / min.), the progressively increasing centrifugal force can cause a widening of the internal ring, and lead to functional impacts. For example:

Slipping of the inner ring at the contact with the shaft and at the contact surfaces
Frictional corrosion
Vibrations

In order to counteract the lifting of the inner ring, a stronger fit is recommended.

Correction factors for an oversized bearing design and bearing series:

SM 60..: 1
SM 619..: 1.10
KH 60..: 1.05
KH 619..: 1.15

Valid for solid shafts. For hollow shafts (50%): Correction factor = 0.8

Tensioning bearing sets together

Performance optimization through precision nuts

The use of precision nuts to clamp bearings (sets) supports an optimal utilization of the performance capacity of GMN high-precision ball bearings.

Tensioning together bearings using precision nuts

Design:

Careful installation with precision nuts prevents (interruption out, possibly follow up with a hyphen: micro-movements);
micro-movements cause contact corrosion.

Grind the sides of the nut at a right angle to the thread of the nut and the shaft to prevent tilting of the bearing or bending of the shaft (max 2 μm run-out tolerance)
Fix the precision nut on the shaft (against loosening)
Intermediate washers and bushes must be made parallel to the planes (max. 2 μm)

A sufficiently high axial clamping force fixes the bearings in the intended position and ensures the required preload, precision and rigidity of the bearing.

Installation:

Lightly lubricate the thread
Screw in the precision nuts with 2 to 3 times the TARGET tightening torque, then release them again and fasten them with the desired torque (compensation of temperature-dependent dimensional changes of the inner rings and seatings)
The required press banding of several bearings (axial) and the necessary overcoming of friction resistance when the bearings are pressed on the shaft (radial) are ensured by the 2 to 3 times primary (breakout) tightening torque

Values for clamping forces and tightening torques are experience-based indicative values and may differ depending on the application.