Angular contact bearings/precision spindle bearings

Technical information

Spindle bearings are angular contact bearings, with exceptional attention to outstanding running properties and speed values.

First some important characteristics of spindle bearings:

1. Contact angle α0 The contact angle is formed by a straight line drawn between the points of contact of the balls with the raceways and a plane perpendicular to the bearing axis. Externally applied loads are transmitted from one ring to the other along this line. The contact angle is designed into the bearing and changes during operation with the speed, the external forces and the difference in temperature between the inner and the outer ring. With increasing contact angle Limiting speed decreases Radial rigidity decreases Axial rigidity increases GMN manufactures spindle bearings with 15° and 25° contact angles. Other contact angles available on request.

2. Materials

Rings:
Basically GMN rings are made from vacuum degassed chrome steel 100Cr6, which is heat stabilized for temperatures up to 150° C (302° F). On request, an additional heat stabilization can be carried out for working temperatures between 150° C and 300° C (572° F). Bearings for working temperatures up to 500° C (932° F) are made of high temperature tool steel.

Balls:
Standard material for balls is vacuum degassed chrome steel 100Cr6. For the increase of speed value and lifetime, all bearings can be delivered with ceramic balls.

Further special materials are available on request.

3. Precision

The precision of a spindle bearing does affect the guiding properties as well as lifetime, especially with applications at max. speed.

The tolerances for dimensional, form and running accuracy of GMN high precision ball bearings are specified in international (ISO 492) and national standards (DIN 620). GMN high precision bearings are manufactured to precision class 4 and class 2 (P4 and P2) as well as ABEC 7 and ABEC 9. For special applications, e.g. vacuum pumps, gyroscopes as well as measuring engineering and optical systems, GMN manufactures bearings to the internal tolerance classes HG (high precision) and UP (ultra precision). Apart from the requirements mentioned, the tolerance classes contain additional selection criteria. All GMN high precision ball bearings are also available in compliance with the American AFBMA standards. The relationship between the various standards are explained below.

ISO	DIN	AFBMA
class 4	P4	ABEC7
class 2	P2	ABEC9

4. Preload and bearing arrangement

Basically, there are two types of preloading: Spring preload Characteristics: Insensitive to different thermal expansion of shaft and housing Suitable for very high speeds

The drawing shows a spindle where bearing 1 has a fixed location, whereas the outer ring of bearing 2 is free to move axially. The spring force acts on the outer ring of bearing 2 and results in a permanent preload of both bearings almost independent of speed and temperature factors. Care must be taken to ensure easy movement of the adjusted outer ring. Bearings preloaded in this way can be used up to the limiting speed of single bearings if oil lubrication is used.

The spring has to be arranged to be effective in the same direction as the external axial load.

Rigid preload Characteristics: Higher rigidity at radial loads Lower limiting speed compared to spring preload The magnitude of preload changes due to length variations as a result of temperature differences between shaft and housing. Distinct higher axial rigidity than with spring preload

With the spindle shown in the drawing both bearings are paired and mounted stationary in an axial direction. Bearings arranged like this have a defined axial pre-load. The sleeves shown in the drawing must be ground to identical lengths in one setting. GMN delivers the required bearing pairs with the necessary preload.

The change of the preload under operating conditions has to be considered.

Bearing arrangement: All bearing arrangements shown here after can be manufactured on request (depending on volume), or combined from universally matched bearings.

Back-to-back arrangement (DB)

Face-to-face arrangement (DF)

Tandem arrangement (DT)

Often, if a bearing is subjected to large loads or if a high degree of rigidity is required three or more bearings are used, assembled in sets shown here after. The bearing arrangements can be combined from universally matched bearings, or produced at sufficient lot sizes.

TDB

TDF

TDT

QDB

QDF

QDT

When using a single spring loaded bearing, a minimum preload must be observed to assure positive rotation of the balls and to prevent skidding. GMN application engineers will calculate the minimum preload for your application.

5. Accuracy of the associated components

The machining quality of the abutment surfaces and bearing seats are of great importance for running accuracy and low operating temperature of a bearing application. Reference values for form and position tolerances are available on request.

6. Bearing fits

The machining quality and the correct selection of the fits for the bearing seats are of great importance for a satisfactory operation of precision bearing applications.

For correct fit selection please contact our application engineers.

7. Lubrication

The correct choice of lubricant and method of lubrication is as important for the proper operation of the bearing as the selection of the bearing and the design of the associated components.

Grease lubrication

	Maintenance-free operation over long periods of time is desired,
	the maximum speed of the bearing does not exceed the speed factor n x dm of the grease,
	the heat generated is almost uniformly dissipated by the environment,
	low friction losses are required with bearings working under small loads and at high speeds.

Running-in period with grease lubrication
In order to obtain an optimum lubrication effect and grease life it is advisable to provide for a running-in period for bearings for high-speed applications. A better grease distribution and, at the same time, a low bearing temperature are thus achieved.

Grease manufacturer offer a multitude of greases suitable for high speeds. The n x dm factor is a criterion for the selection of the grease taking into consideration bearing size and operating speed.

D: Bearing outside diameter [ mm] d: Bearing bore diameter [mm] n: Bearing operating Speed [1/min]

Oil lubrication

	High speeds do not permit the use of greases,
	the lubricant must simultaneously serve to cool the bearing.

The most widely used lubricating methods are:

	Oil mist lubrication: The oil mist is produced in an atomizer and conveyed to the bearings by an air current. The air current also serves to cool the bearings and the slightly higher pressure prevents contamination from penetration.
	Oil-air lubrication (total loss lubrication): The oil is conveyed to the bearing in droplets by compressed air. The droplet size and the intervals between two droplets are controlled.
	Oil-jet lubrication (cooling lubrication): Considerable amounts of oil are carried through the bearing by injection, the frictional heat generated in the bearing is dissipated. The cooling of the oil is achieved e.g. with an oil-to-air heat exchanger.