Bearing Master | Learning about Bearing Speed and Limiting Speed

The limiting speed of a bearing is the limit value at which it can rotate continuously without generating the frictional heat that causes burns.

The rotational speed of a bearing is mainly limited by the temperature rise caused by the frictional heat inside the bearing, and when the rotational speed exceeds a certain limit, the bearing will not be able to continue rotating due to burns and so on.

The limiting speed of a bearing is the limit value at which it can rotate continuously without generating the frictional heat that causes burns.

Therefore, the limiting speed of a bearing depends on the type, size and accuracy of the bearing, as well as on various factors such as the lubrication method, the quality and quantity of the lubricant, the material and type of the cage, and the loading conditions.

The limiting speed given by the bearing manufacturer is generally the highest speed that can be achieved under agreed conditions, e.g.:
Bearing tolerance of class 0;
Bearing clearance of group 0;
Bearing load of 10 per cent of the rated load;
Normal lubrication and cooling conditions;
Radial bearings are subject to radial loads only, thrust bearings are subject to axial loads only;
Outer ring temperature does not exceed 100 ℃, etc.

The speed range required for the application helps to determine what type of bearing to use.

The catalogues of most bearing manufacturers provide the limiting speed values of their products, and it has been proved that it is better to work at speeds lower than 90% of the limiting speed.

Selection of bearing type by rotational speed

A, ball bearings than roller bearings have higher limiting speed and rotary accuracy, so in high speed should be preferred to use ball bearings.
B, under the condition of the same inner diameter, the smaller the outer diameter is, the smaller the rolling body is, the smaller the centrifugal inertia force added by the rolling body on the foreign raceway during the operation, thus it is also suitable for working at higher speed.

Therefore, in high speed, it is appropriate to use the same diameter series in the outer diameter of the smaller bearings. If the bearing with a smaller diameter and the bearing capacity does not meet the requirements, and then installed the same bearing, or consider using a wide range of bearings.
C, the cage material and structure of the bearing speed has a great influence. Solid keep frame than stamping keep frame allows some higher speed, bronze solid keep frame allows higher speed.
Generally higher speed work occasions, the appropriate choice of deep groove ball bearings, angular contact bearings, cylindrical roller bearings;
In the lower speed work occasions, can choose tapered roller bearings.
Tapered roller bearings limit speed, generally about 65% of the deep groove ball bearings, cylindrical roller bearings 70%, angular contact ball bearings 60%. Thrust ball bearings have a low limiting speed and can only be used in low speed applications.

Factors affecting bearing speed

Factors that affect the limiting speed of a bearing include:
The magnitude of the applied load;
The direction of the force;
The type of lubricant, the amount of lubricant;
Bearing manufacturing accuracy i.e. the tolerance grade of the bearing and the size of the clearance of the bearing.

However, the most important factor is still the bearing material allowable operating temperature or lubricant.

The limiting speed of various types of bearings are listed in the “rolling bearing product samples” bearing size and performance table, they are respectively in the oil lubrication and grease lubrication conditions to determine the scope of application:
1、Standard (G) tolerance bearings;
2、Centripetal bearings bear only radial load, thrust bearings bear only axial load;
3, P less than or equal to 0.1C (C is the basic dynamic load rating of the bearing);
4、Rigid shaft and bearing housing;
5, lubrication and cooling conditions are normal.
When the bearing in the P is greater than or equal to 0.1C load conditions of operation, due to the raceway contact surface and rolling body contact stress increases, resulting in bearing operating temperature rises, the relative deterioration of the performance of the lubricant, so the bearing’s limit rotational speed will be reduced accordingly.

Selection methods for rolling bearing fits

When we are engaged in daily maintenance or mechanical design, reasonable and correct selection of bearing fit is crucial. Correct selection of the bearing fit has a great deal to do with ensuring the normal operation of the machine, improving the service life of the bearing and making full use of the bearing’s load carrying capacity.
Whether the collar is rotating. When the inner or outer ring of the bearing work for the rotating circle, should be used slightly tight with the size of the surplus should make the surface in the working load does not occur “crawling”, because once the crawling occurs, with the surface to be worn out, resulting in sliding, the higher the speed of the ring, the more serious the wear and tear.
Load type. Bear local load rings should be selected loose transition fit or clearance with a small fit, in order to allow the friction torque between the ring raceway to drive the ring transposition, so that the ring force uniformity, prolonging the service life of the bearings.

Withstand cyclic loads of the collar should be selected with the interference fit or tighter transition fit, the size of the surplus, in order not to make the collar and shaft or shell hole with the surface of the phenomenon of crawling as a principle. Withstand swing load, the requirements of its fit and cyclic load or slightly looser.

Other factors affecting bearing fit

Influence of operating temperature. When the bearing is working, due to frictional heat and other heat sources, the temperature of the collar is higher than the temperature of its mating parts. The thermal expansion of the inner ring will cause it to loosen with the journal, while the thermal expansion of the outer ring will cause it to tighten with the shell hole.
Influence of rotational accuracy and speed. For bearings with large loads and requiring high rotational accuracy, clearance fits should be avoided in order to eliminate the effects of elastic deformation and vibration. For some light load bearings of precision machine tools, in order to avoid the influence of the shape error of the shaft on the accuracy of the bearing, the clearance fit is often used. It is generally believed that the higher the rotational speed of the bearing, the tighter the fit should be.
Conditions for mounting and dismounting bearings. Considering the convenience of mounting and dismounting of bearings, it is appropriate to use a looser fit, which is especially important for large and extra-large bearings used in heavy machinery. If it is required for easy mounting and dismounting and tight fit, it can be used as separating type bearings, or bearings with tapered inner ring, tight sleeve and unloading groove.

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