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Considerations in bearing selection

Considerations in bearing selection


The three main factors that must be considered in bearing selection are the limit speed, the required service life and the predetermined working load, which are the three performance requirements that must be met by the selected imported bearing.

The limit speed is the maximum allowable working speed of the import bearing, beyond this limit, the operating temperature of the import bearing will continue to rise, lubricant dry, so that the import bearing stuck, so the selected bearing's limit speed than the predetermined maximum working speed of the import bearing 10% higher is appropriate.Under the condition that the bearing type has not been selected, it is recommended to preorder the installation of the inlet bearing in the place where the shaft and seat hole diameter are half of the average diameter DM of the inlet bearing. Multiply dm by the predetermined maximum working speed n of the bearing, and the DMN value obtained shall not be 10/9 times higher than the limit speed of the selected inlet bearing.

But the limit speed of import bearing is not invariable, but is directly related to lubrication mode.For example, the limit speed of the bearing sample under the condition of oil lubrication is N0, the limit speed of the same bearing under the condition of grease lubrication is 0.8N0, and it is 11.5N0 under the condition of oil mist lubrication.Under the condition of fixed volume non-repeat circulation oil lubrication, the continuous operation condition for small tank is 0.85n0, and the intermittent operation condition for large tank is 0.95n0.With n0 value quite lubrication method is repeated with filtering and cooling pack of circulating oil lubrication, pay attention to not take this way of lubrication or other lubrication effect is the same way, cannot be imported bearing samples above n0 value as import bearing the actual limit speed, and should according to the specific way of lubrication, according to the above example will n0 multiplied by a constant proportion, select the number as the real limit speed of the bearing;Secondly, although the actual limit speed of imported bearings can be improved through the improvement of lubrication, but to take into account the cost of equipment and maintenance costs due to the improvement of lubrication, and lubrication equipment occupied space and other factors, a comprehensive balance of gains and losses, make the best choice.

Actually when considering importing bearing limit speed, not only should consider to import the type of bearing, sometimes bearing components must also be taken into account, such as retainers, form printing design and technological factors, also to the sliding friction within the imported bearing, storage and maintenance of lubricant, stability of the rolling element and so on, also sometimes becomes the key to import bearings can reach the limit speed of bearing life and reliability requirements are unified consideration, usually take the reliability coefficient was 90%, which means that in large Numbers in the imported bearings of the functioning of the same working conditions,90% of the imported bearings remain in normal condition when reaching the predetermined working life L10.If you want greater reliability, say 96%, you have to reduce the required bearing life, for example the L10 life to the L4 life.

Speaking for the main shaft bearing, the highest requirements is with the whole service life of the machine have the same or slightly higher, the minimum requirement is that life is longer than the spindle component overhaul period, and in fact, with both can satisfy the requirement of the host performance still not make the spindle system is too big and also more cost-effective comprehensive consideration on the economic plan, for most of the machine, there are successful precedents for reference.

It should be emphasized that the selection calculation should also consider the failure form of the bearing, as well as wear, lubrication failure and micro-abrasion, among which the wear life must be checked, other failure forms if involved must also be considered.

Note that import bearings can expect fatigue life and wear life not only with lubrication, and the installation of the imported bearing parts, imported bearings, bearings itself directly related to the cleanliness of the lubricant used three, also associated with the form and effect of sealing, the selected specified life must be provided with the bearing lubrication, cleanliness and sealing conditions, and improve these conditions can also improve the service life of the bearing of the actual.

When considering the load factor, it should be noted that the life of the inlet bearing is more sensitive to the load ratio and the speed. Therefore, for the multi-stage transmission shaft, if possible, it is advisable to put a large deceleration in the later series.For the main shaft bearing, if possible, proper deployment of outer load point and the distance between the imported bearing pivot, makes every effort to reduce the load of main shaft bearing, and strive to spread to the main shaft bearing the load on the smooth, avoid shock and vibration, this requires the installation of the imported bearing parts of the shape and location accuracy is high enough, imported bearing clearance and cooperate with the correct import bearing installation precision is good.

The more representative the actual load of the bearing is, the more correct the calculation result will be. Therefore, it is necessary to study the actual load of the bearing extensively and deeply.

If the inlet bearing not only bears a large load, but also stands still without rotation, this state is more unfavorable, small indentation will be formed between the rolling body and raceway and will soon develop into pitting corrosion, making the inlet bearing early failure.In this case, it is necessary to check whether the nominal static load of the selected import bearing can exceed the equivalent static load of the import bearing.

At the other extreme, it is just the opposite. The imported bearing runs at a high speed, but does not bear the load. In this way, the rolling body often slides on the racetrack instead of rolling, which results in the rapid rise of bearing temperature, decomposition and deterioration of lubricant, and the imported bearing will also lose its effect quickly.

In some cases, it is necessary to consider the axial and radial forces generated by the equipment itself, the weight of gear and other parts, even the weight of the shaft, the stress of relevant parts, and calculate them in the rear load borne by the import bearing, which is often easy to be ignored and missed.

In many cases, the actual load of the imported bearing increases significantly due to the impact of vibration. When the actual installation of the imported bearing has a large coaxiality error (wrong), the actual load of the imported bearing will also increase. Therefore, the value of the calculated load must be corrected.

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