When purchasing rolling bearings, have you ever had such questions: the higher the bearing speed, the better the use effect? The faster the speed, the more expensive the price? Is the actual situation really like this? The following will explain in detail the core concepts of bearing speed and the key factors affecting bearing speed:
Every type of bearing has its own limiting speed, which is mainly limited by the temperature rise caused by internal frictional heating of the bearing. When the speed exceeds the critical value, the bearing will burn out due to overheating and fail to rotate continuously, and even seizure faults may occur. Therefore, we need to select the suitable bearing type according to the actual required speed and clarify the specific performance requirements of the bearing. In some application scenarios, the importance of other factors (such as low-speed operation, reciprocating oscillation, etc.) will far exceed the limiting speed.
The limiting speed of a bearing refers to the boundary rotational speed at which the bearing can rotate continuously by virtue of its own frictional heating without the risk of burnout. Therefore, the limiting speed of a bearing depends on multiple factors such as bearing type, size, precision, lubrication method, quality and dosage of lubricant, cage material and structure, and load conditions.
The limiting speed provided by bearing manufacturers is usually based on the following agreed conditions: the bearing tolerance grade is Class 0; the bearing clearance is Group 0; the bearing load is 10% of the rated load; the lubrication and cooling conditions are normal; radial bearings only bear radial loads, thrust bearings only bear axial loads; the temperature of the bearing outer ring does not exceed 100℃.
The speed range required by the application scenario is an important basis for determining the bearing type. In the product catalogs of most bearing manufacturers, the maximum speed value of each model is marked. Practice has proved that when the bearing operates below 90% of the limiting speed, its stability and service life are more guaranteed.
I. Selecting Bearing Types According to Speed
Give priority to ball bearings: Ball bearings have higher limiting speed and rotational accuracy than roller bearings, so ball bearings should be the first choice for high-speed operation scenarios (such as motors, high-speed transmission mechanisms).
Select bearings with a smaller outer diameter: For bearings with the same inner diameter, the smaller the outer diameter and the size of the rolling elements, the smaller the centrifugal inertial force exerted by the rolling elements on the outer ring raceway, making them more suitable for high-speed operation. Therefore, under high-speed working conditions (such as high-speed spindles), bearings with a smaller outer diameter in the same diameter series should be selected. If the load-carrying capacity of small outer diameter bearings cannot meet the requirements, multiple sets of the same bearings can be installed or wide-series bearings can be selected.
Pay attention to the material and structure of the cage: The cage has a significant impact on the bearing speed — the allowable speed of integral cages is higher than that of stamped cages, among which bronze integral cages have the highest allowable speed.
Bearing types suitable for high-speed scenarios: Deep groove ball bearings, angular contact ball bearings (such as motor bearings, automotive bearings, electric tool bearings);
Bearing types suitable for low-speed scenarios: Various roller bearings (such as cylindrical roller bearings, spherical roller bearings).
II. Factors Affecting Bearing Speed
The core factors affecting the limiting speed of bearings include: the magnitude of the applied load, the direction of the force, the type of lubricant, the dosage of lubricant, the manufacturing precision of the bearing, the bearing tolerance grade, and the bearing clearance. In addition, some lubricants, although excellent in anti-wear, anti-corrosion and other properties, may not be suitable for high-speed rotation scenarios due to insufficient high-temperature resistance.
In general, the limiting speed of a bearing is related to the bearing type, load, precision, size, lubrication method, clearance, cage structure and cooling conditions, but the most critical factors are the allowable operating temperature of the bearing material and the high-temperature resistance of the lubricant — if the material or lubricant cannot withstand the heat generated by high speed, the bearing will fail rapidly.
Special attention should be paid to the definition of "high-speed bearings":
The commonly referred to "high-speed bearings" refer to bearings with a linear speed of up to 60 meters per second. The classification of bearings into high, medium and low speed is based on the linear speed (the distance the rolling elements roll per unit time), rather than the rotational speed (revolutions per minute). For example, small bearings that can achieve tens of thousands of revolutions per minute (such as micro-motor bearings) may not be classified as high-speed bearings due to their low linear speed; while large bearings that can achieve several hundred revolutions per minute (such as large fan bearings) may be classified as high-speed bearings due to their high linear speed.
Among sliding bearings, hydrostatic bearings, hydrodynamic bearings, aerostatic bearings, liquid hydrodynamic bearings and other types all have structural characteristics suitable for high-speed rotation.
III. High-Speed Bearings vs Ordinary Bearings: Installation Fit and Adjustment of High-Speed Bearings
The installation fit and adjustment of high-speed bearings are significantly different from those of ordinary bearings, and special attention should be paid to fit precision and clearance control to cope with the special challenges brought by high speed and high temperature.
1. Selection of Fit and Clearance for High-Speed Bearings
When determining the fit and clearance of high-speed bearings, the following two points should be given priority: (1) The impact of temperature changes: from normal temperature to high-temperature environment, the size of the bearing will change due to thermal expansion, and the hardness may also decrease slightly due to high temperature; (2) The impact of centrifugal force: the centrifugal force generated by high-speed rotation will change the stress system of the bearing and even cause slight deformation of bearing components.
In short, under high-speed and high-temperature working conditions, it is much more difficult to maintain bearing precision and performance through the selection of fit and clearance than under ordinary working conditions. Specific attention should be paid to:
Control the amount of interference fit: To avoid raceway deformation after bearing installation, the interference fit amount of small angular contact ball bearings should not be too large — the centrifugal force generated by high speed and thermal expansion caused by high temperature may offset the normal pressure of the fitting surface, resulting in loose fit. If it is necessary to reduce the pressure on the fitting surface, the interference amount should be accurately calculated on the basis of comprehensively considering temperature and centrifugal force; it should be noted that the interference amount effective under normal temperature and constant speed conditions may fail when applied to high-speed bearings.
Special treatment for ultra-high-speed scenarios: If there are serious contradictions in the calculation results (usually only occurring in ultra-high-speed working conditions, such as the dmn value close to 3 million), a dual lubrication scheme combining "inner ring lubrication + hydrostatic lubrication" should be adopted to improve the high-speed tolerance of the bearing through forced cooling and lubrication.
Precise control of clearance: When determining the clearance of high-speed bearings, in addition to temperature and centrifugal force, the impact of thermal elongation of the shaft should also be included — the bearing is required to have the "optimal clearance" at the operating temperature, and this clearance needs to be achieved through the precise alignment of the ball groove centers of the inner and outer rings. Since high-speed bearings need to minimize relative sliding and internal friction, it is strictly forbidden to adjust the clearance by means of "axial relative misalignment of the inner and outer rings", otherwise wear and heating will be aggravated.
Impact of material characteristics: Bearing materials will become soft and easy to deform at high temperatures, and repeated temperature changes between "normal temperature and high temperature" may lead to permanent deformation. Therefore, when selecting the fit interference and clearance, sufficient deformation space should be reserved to avoid component seizure.
2. Requirements for Relevant Components of the Host Machine
High-speed bearings have much higher requirements for the precision and performance of host machine components than ordinary bearings, including:
Dynamic balance of the rotating system: The rotating system where the high-speed bearing is located (such as the shaft, impeller) needs to undergo precise dynamic balance to avoid the centrifugal force generated by the unbalance from aggravating vibration and wear.
Precision of the installation position: The dimensional accuracy (such as tolerance grade) and positional accuracy (such as coaxiality, perpendicularity) of the parts where the bearing is installed on the shaft and bearing housing bore need to be higher than general requirements — especially the coaxiality of the bearing (to ensure the concentricity of the inner and outer rings) and the perpendicularity of the shaft shoulder to the housing bore (or journal) (to avoid bearing eccentric load). In the design, thermal expansion space should be reserved in combination with the high-speed and high-temperature factors during bearing operation.
Design of the shaft support system: The shaft support system needs to meet the requirements of both "high rigidity" and "low mass" — high rigidity can reduce vibration, and low mass can reduce centrifugal force. To balance this contradiction, the following measures can be taken: improve the support rigidity by reducing the surface roughness and enhancing surface hardening treatment (such as quenching, nitriding); adopt a hollow shaft structure to reduce the system mass while ensuring the shaft strength.
