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Guidance on Bearing Loads
Bearing loads capacity is critical to its service life and performance. Insufficient bearing load capacity can lead to premature wear, overheating and catastrophic failure, so it is critical to consider bearing loading when designing new applications or modifying existing applications, especially after bearing failure. This blog aims to introduce information on bearing loads and provide constructive suggestions for you to choose appropriate bearings.
Bearing load is the amount of force or pressure exerted on the bearing. In detail, the force is transferred from one bearing ring to the other through some or all rolling elements. Typically, the applied load is transferred to the shaft, then to the inner race of the bearing, and finally to the outer race. Bearings can withstand various loads, such as radial load, axial load, centrifugal load, etc. The magnitude and direction of bearing loads depend on various factors, including equipment weight, operating speed, acceleration, deceleration, shock, vibration, temperature and lubrication. Improper alignment, installation or maintenance can also affect bearing loads.
Bearing load form
Bearing loads in the traditional sense include axial load, radial load and overturning moment. Bearings bear many types of loads. Whether in operation or not, they will bear working loads, dead weight loads, slope loads, collision loads, temperature loads, etc. Below we introduce the manifestations of bearing load in detail.
Bearing radial load
Radial bearing loads are perpendicular to the axis of the shaft and act on the outer ring of the bearing. They are caused by the weight of the equipment or the forces of rotating parts. To calculate radial capacity, determine the weight of the supported components and the forces acting on them, distribute the load among the bearings, and ensure that the load calculated using the manufacturer’s specifications is within the maximum capacity of the selected bearing. If your application requires bearings to handle radial loads, radial ball bearings or low contact angle angular contact bearings would be a good choice.
Bearing axial load
Axial loads, also called thrust loads, act parallel to the axis of the shaft and act on the inner or outer rings of the bearing. They are caused by thrust or tension and can be unidirectional or bidirectional. They transmit force evenly to produce a balanced load distribution. Angular contact bearings with higher contact angles (approximately 25°) are good choices for axial load applications, however, for offset axial loads, moments act on the inner race, resulting in uneven load distribution on the bearing rolling elements. To calculate axial load capacity, consider bearing size, material and geometry, as well as load direction and magnitude. Manufacturers rate bearings based on standardized formulas and tests, and applications with high axial loads include pumps, automotive transmissions and compressors.
Bearing load capacity can be calculated using a variety of formulas and software programs, including bearing manufacturer’s catalogs, online calculators, and finite element analysis (FEA) simulations. The most commonly used formulas for radial and axial loads are:
Radial load capacity = ( C/P)^(1/3)x Fr, axial load capacity = (C0/P)^(1/2)x Fa.
Among them, C is the basic dynamic load rating, P is the equivalent bearing dynamic load, C0 is the basic static load rating, Fr is the radial load, and Fa is the axial load. In order to obtain accurate results when calculating the bearing load capacity, please seek Experts recommend or use a software program provided by the bearing manufacturer.
Bearing centrifugal load
Bearing centrifugal loads are generated by the rotational speed of the application, especially high-speed applications such as turbines and centrifuges. As the inner ring rotates the rolling elements, they move tangentially in a straight path, but the outer ring must force them to follow the arc of the bearing. This interaction creates centrifugal radial loads, and the maximum speed of the application is sometimes limited by the force it creates. Centrifugal load limitations.
Static load
Bearing static load refers to the maximum force or maximum moment that the bearing is subjected to in the radial, axial and other directions when the bearing is not rotating. Static load is an important parameter in bearing design. It is part of the basic load rating of the bearing and is also one of the important reference data for determining the service life of the bearing.
Dynamic load
The dynamic load of the bearing refers to the maximum load that the bearing bears during rotation. It is usually calculated using the dynamic equivalent load calculation formula. The magnitude of the dynamic load on the bearing directly affects the service life and performance of the bearing. Therefore, bearing manufacturers must consider the impact of dynamic load when designing and producing bearings. The calculation method of dynamic bearing loads depends on the type of bearing and the conditions of use. The dynamic equivalent load calculation formula is usually used, and the calculation method is as follows:
P = (Fr^2 + Fa^2)^0.5
Among them, P is the dynamic equivalent load; Fr is the radial load; Fa is the axial load. The values of radial load and axial load need to be calculated according to specific usage conditions, while the dynamic equivalent load is calculated comprehensively based on factors such as bearing geometry, material, deviation, and number of bearings.
Bearing static load rating
The basic static load rating of a rolling bearing (radial Cor, axial Coa) refers to the equivalent phantom radial load or central axial static load when a certain contact stress is caused at the center of contact between the bearing rolling elements and the raceway under the maximum load application. . The static load rating is determined under assumed load conditions. For radial bearings, the static load rating refers to the radial load. For radial thrust bearings (angular contact ball bearings), it refers to the radial component of the load that loads the half-circle raceway in the bearing. For thrust bearings it refers to the central axial load. That is to say, the radial basic static load rating and axial basic static load rating of the bearing refer to the maximum load that the bearing can bear when static or rotating. The load-bearing capacity of deep groove ball bearings when stationary or slowly rotating (speed n≤10r/min) is the rated static load.
Bearing dynamic load rating
The rated dynamic load of the bearing is the constant radial load (constant axial load) that the rolling bearing can theoretically withstand. The basic rated life under this load is 100W revolutions. The basic dynamic load rating of the bearing reflects the bearing’s ability to withstand rolling fatigue. . The basic dynamic load ratings of radial bearings and thrust bearings are respectively called the radial basic dynamic load rating and the axial basic dynamic load rating, represented by Cr and Ca. The load-carrying capacity of deep groove ball bearings when rotating (speed n>10r/mim) is the basic dynamic load rating.
Fixed load
The resultant radial load acting on the bearing ring is borne by the local area of the ring raceway and transmitted to the opposite area of the shaft or bearing seat. This load is called a fixed load. The characteristic of a fixed load is that the resultant radial load vector is relatively stationary to the ferrule. Neither the ferrule nor the resultant radial load rotates or they rotate at the same speed and are considered fixed loads. Ferrules that bear fixed loads can use a looser fit.
Rotating load
The synthetic radial load acting on the bearing ring rotates along the circumferential direction of the raceway, and the load borne by each part in sequence is called rotational load. Rotational loading is characterized by the rotation of the resultant radial load vector relative to the ferrule. There are three situations of rotating load:
a. The load direction is fixed and the ferrule rotates;
b. The load vector rotates and the ferrule is stationary;
c. The load vector and the ferrule rotate at different speeds.
Oscillating loads and indefinite loads
Sometimes the direction and size of the load cannot be determined accurately. For example, in high-speed rotating machinery, in addition to the fixed-direction load of the rotor weight, there is also a rotating load caused by an unbalanced mass. If this rotating load is larger than the fixed load, If it is much larger, the resultant load will still be a rotational load. And if the rotating load is much smaller than the fixed load, the resultant load is an oscillating load. Regardless of rotating load or swing load, its magnitude and direction are constantly changing. Under variable working conditions, the load on some ferrules may be rotating loads, fixed loads, or swing loads. This type of load is called an indefinite load.
Oscillating loads and indefinite loads should be treated in the same manner as rotational loads in terms of fit. Too loose a fit will cause damage to the mating surface. The ferrule and shaft or seat hole that rotate relative to the load direction should choose transition fit or interference fit. The interference size is based on the principle that when the bearing is working under load, the ring will not “creep” on the mating surface on the shaft or in the seat hole. For heavy load applications, the fit should generally be tighter than for light load and normal load applications. The heavier the load, the greater the fit interference should be.
Working loading
When the bearing is working, it bears the sum of the weight of the machine itself and the weight of the heavy object, and slowly transfers the total weight to the bearing.
Temperature load
Mechanical equipment will generate a certain temperature during operation, and these temperatures must be absorbed by the bearings so that the bearings can withstand all temperatures.
Wind load
When the machine is working in the open air, the effect of wind load must be considered, including wind direction, rain, thunderstorms, etc. The above are only some of the loads borne by the slewing bearing device. In fact, the slewing bearing assembly must bear load to cope with all the weight and loads of the machine in operation. Under normal circumstances, the slew plate bearing itself has mounting holes, lubricating oil and sealing devices, which can meet the different needs of various types of hosts working under different working conditions.
Risk load
Unexpected and unpredictable load borne by the rotating bearing, lateral force, risk force, accidental violence, etc. Therefore, the selection of bearings must have a safety factor to ensure that it is foolproof.
Bearing minimum load
Rolling bearings are used to reduce friction in rotating machinery by removing as much sliding friction as possible from the system by using rolling friction with a lower coefficient of friction. However, even though rolling element bearings attempt to reduce the total friction in the system, the individual rolling elements within the bearing still require a certain amount of friction to roll rather than slide. This internal friction is created by applying load to the bearing. This load can be generated internally through preloading or through an externally applied load.
With many radial bearings, a certain amount of space is usually provided between the rolling elements and the raceway to allow for thermal expansion and prevent the bearing from seizing. “This internal clearance creates so-called loading and unloading zones within the bearing. As the shaft rotates, the rolling elements move in and out of the outer ring load-bearing zone. As the rolling elements move in and out of the loading zone, the speed of the rolling elements changes. If There is no minimum load on the rolling elements and acceleration into and out of the load zone can be very detrimental.
Why is load important to bearings?
If a rolling bearing does not meet the minimum load, a number of conditions may occur that significantly shorten the bearing’s service life. Slippage that slides between rolling elements and raceways can destroy the lubricant film and cause smear damage. Smearing will not only damage the rolling surface but also cause the temperature to rise. The load is placed on a cage inside the bearing. Typically, cages are designed to prevent rolling elements from coming into contact with each other. However, when the minimum load is not met, i.e. when there is no traction force, the cage must now drive the rolling elements instead of the traction force of the raceway. This creates unexplained loads on the cage and can lead to premature cage failure.
Factors affecting bearing load
Materials, structure, manufacturing process, working load, rotation speed, temperature and lubrication conditions are the main factors affecting the internal load distribution of the bearing. During the use and maintenance of bearings, it is necessary to pay attention to changes in these factors and their impact on bearing life and operating stability to ensure normal operation of the bearings.
Structural factors
The structure of the bearing also has a certain impact on its bearing capacity. The structure of the bearing mainly includes inner and outer rings and rolling elements. Bearings with spherical rolling elements have a higher radial load capacity than bearings with roller-shaped rolling elements.
Manufacturing process
The manufacturing process is one of the important factors affecting the bearing capacity. Manufacturing processes include heat treatment, precision machining, assembly, etc. These directly affect the appearance quality and intrinsic quality of the bearing, and then affect the bearing capacity.
Workload
The working load of the bearing refers to the force and moment the bearing bears. It is one of the most important factors affecting the load distribution inside the bearing. The size and direction of the working load directly determine the stress and load distribution of various parts inside the bearing. When the working load of the bearing is uneven, the load distribution inside the bearing will be correspondingly uneven, which will cause local damage and fatigue fracture of the bearing.
Rotating speed
The rotation speed of the bearing refers to the rotation speed of the rolling elements inside the bearing. It is one of the important factors that affects the load distribution inside the bearing. The increase in rotational speed will increase the inertia force of the rolling elements inside the bearing, resulting in greater load on the bearing assembly. In addition, when the rotational speed is too high, the bearing material is prone to fatigue and overheating, which will also affect the load distribution inside the bearing.
Temperature
Temperature is another important factor affecting the load distribution inside the bearing. When the bearing is working, the temperature inside the bearing will rise due to the generation of friction and heat. When the temperature rises, the material properties of various parts inside the bearing will change, thus affecting the load distribution inside the bearing. If the temperature is relatively high, the polymer cage inside the bearing, the thermally stable temperature of the steel, seals, etc. The internal clearance of a bearing and its change with temperature can have a significant impact on the size of the load zone in the bearing. When there is a large temperature difference between the two ends of the bearing (i.e. the hot shaft and the cold housing), the internal clearance of the bearing will decrease. This will create higher loads and higher rolling friction within the bearings.
Lubrication conditions
Lubrication conditions are another key factor affecting the load distribution within the bearing. When the bearing is working, it needs lubricating oil or grease to maintain the lubrication state, thereby reducing the friction and wear inside the bearing. When the lubrication conditions are poor, local dry friction and heat accumulation will occur inside the bearing, which will lead to uneven load distribution inside the bearing and make bearing failure likely.
Bearings and load direction
Under normal circumstances, for pure radial load requirements, deep groove ball bearings or cylindrical roller bearings can be selected. And if it is a thrust ball bearing, it is only suitable for carrying a moderate amount of pure axial load. over, one-way thrust ball bearings can only bear bearing loads from one direction. If it is a two-way thrust ball bearing or a two-way thrust angular contact bearing, it can bear axial loads in both directions. For example, if the bearing is subject to combined radial and axial loads, then angular contact ball bearings or tapered roller bearings are usually used. And if it is a four-point contact ball bearing and a two-way thrust angular contact ball bearing, it can withstand the combined load in both directions.
If the load acts away from the center of the bearing, an overturning moment may occur. According to stainless steel bearing manufacturers, double-row ball bearings can withstand overturning moments, but it is recommended that you choose paired angular contact balls or paired tapered roller bearings. Face-to-face types are available, and back-to-back types are better. Of course, you can also choose crossed tapered roller bearings, etc. ,
Conclusion
When selecting bearings, application requirements, load type, speed, environment and temperature should be considered. Ball bearings are suitable for low to medium loads, while roller bearings are suitable for higher loads. Sliding bearings are suitable for low speed, high load machinery. By regularly Maintain bearings by inspecting, cleaning and lubricating them to ensure optimal performance and service life. Aubearing offers a wide range of bearings for different conditions and applications, providing high quality products and expert advice. For information, contact us.