Bearing clearance introduction and selection
7/10/2017 4:09:05 PM
Bearing clearance is also known as bearing clearance. The so-called bearing clearance refers to the amount of movement when the bearing is fixed to one of the inner ring or the outer ring when it is not mounted on the shaft or the bearing housing, and then the radial or axial movement of the bearing clearance is not fixed. . According to the direction of movement, it can be divided into radial clearance and axial clearance. The size of the running clearance (called the working clearance) affects the rolling fatigue life, temperature rise, noise, vibration and other properties of the bearing.Definition
1. Radial clearance: non-preloaded state, bearing with radial load, its radial clearance G is: in any radial direction, when there is no external load, the outer ring is from a radial direction with respect to the inner ring. The eccentricity limit position, the arithmetic mean of the radial distances moving to the opposite extreme positions.
2. Axial clearance: Non-preloaded state, bearing capable of bearing axial load in two directions, its axial internal clearance G is: when there is no external load, one ferrule is opposite to the other ferrule The average of the axial distance of an axial limit position to the opposite extreme position. 3. The clearance of the bearing under different conditions: The bearing will change correspondingly under different conditions. Specifically, it can be divided into: 1) Original clearance: The original clearance of the bearing refers to the bearing after the complete set. Installed in front of the machine, in a free state of play. In fact, the original clearance is difficult to know without passing the measurement. Therefore, the original play is often replaced by a test play. The test clearance is the clearance data obtained by the instrument under the condition of applying the measured load under the inspection state, strictly speaking, the original clearance of the bearing is not the same, but in general, the difference between the two readings It is not so large that it can be replaced with each other without causing much error. 2) Effective clearance: The effective clearance or working clearance refers to the actual clearance existing in the bearing under the condition that the bearing is installed under the load and reaches a certain temperature rise after being installed in the main engine. Obviously, the effective clearance is smaller than the original clearance. 4, the role of bearing clearance and requirements: The existence of clearance in the bearing is to ensure that the bearing can be flexible and unblocked operation, but at the same time also required to ensure that the bearing runs smoothly, the bearing axis does not significantly settle, and the load bearing rolling element The number is as much as possible. Therefore, the bearing clearance has a great influence on the dynamic performance (noise, vibration and friction) of the bearing and the rotation accuracy, and the bearing capacity of the service life (wear and fatigue).
The internal clearance of the bearing refers to the total distance (radial internal clearance) or the total distance of axial movement (axial internal clearance) of one bearing ring relative to the other bearing ring.
Working clearance refers to the clearance under actual operating conditions of the bearing.
The original clearance is the clearance before the bearing is installed.
The clearance value is divided into three groups according to the size, and one group is a basic group (or an ordinary group), a small clearance group (C2), and a large clearance group (C3, C4). Japan's NSK, NTN and other brands also have a special CM group (motor-specific clearance)
Add a few examples of everyday applications:
Under normal working conditions, the basic group should be preferred;
The large clearance group is suitable for the inner and outer rings with large interference, or the inner and outer ring temperature difference, deep groove ball bearings need to bear large axial load or need to improve the alignment performance, or need to increase the bearing limit speed and reduce Bearing friction torque and other occasions
The small clearance set is suitable for higher rotation accuracy, strict control of the axial displacement of the housing bore, and the need to reduce vibration and noise.
When measuring the clearance of the bearing, in order to obtain a stable measurement value, a specified measurement load is generally applied to the bearing.
Therefore, the measured value obtained is larger than the true clearance (referred to as the theoretical clearance), that is, the amount of elastic deformation generated by the measurement load is increased.
However, for roller bearings, since the amount of elastic deformation is small, it can be ignored.
The internal clearance of the bearing before installation is generally expressed in terms of theoretical clearance.
Bearing clearance selection
The amount of expansion of the ferrule caused by the interference fit or the clearance after shrinkage due to the interference fit when the bearing is mounted on the shaft or the casing is referred to as the "installation clearance".
The clearance after adding or subtracting the dimensional variation caused by the internal temperature difference of the bearing on the installation clearance is called "effective clearance".
The bearing is installed with the clearance when the machine is subjected to a certain load, that is, the effective clearance plus the amount of elastic deformation generated by the bearing load, so as to be called "working clearance".
When the working clearance is slightly negative, the fatigue life of the bearing is the longest, but the fatigue life decreases significantly with the increase of the negative clearance. Therefore, when selecting the clearance of the bearing, it is generally preferable to make the working clearance zero or slightly positive.
The radial clearance of a rolling bearing refers to the amount by which one ferrule is fixed while the other ferrule is moved from one extreme position to the other at a position perpendicular to the bearing axis. Whether the choice of bearing clearance is correct or not has a great influence on mechanical running accuracy, bearing life, frictional resistance, temperature rise, vibration and noise. If the choice of the radial bearing clearance is too small, it will make
The number of rolling elements under load increases, the contact stress decreases, and the operation is smoother. However, the frictional resistance increases and the temperature rise increases. On the contrary, the contact stress increases, the vibration is large, the frictional resistance is reduced, and the temperature rise is low. Therefore, according to the bearing conditions, it is very important to choose the most suitable clearance value. When choosing the bearing clearance, you must fully consider the following main factors:
(1) The tightness of the bearing with the shaft and the housing hole will result in a change in the bearing clearance value. Generally, after the bearing is installed, the clearance value will be reduced;
(2) During the operation of the mechanism, due to the different heat dissipation conditions of the shaft and the outer casing, a temperature difference is generated between the inner ring and the outer ring, which may cause the clearance value to decrease;
(3) Since the shaft and outer casing materials have different expansion coefficients, the clearance value may be reduced or increased.
Usually the optimum working clearance value for the radial bearing is the basic group clearance value specified in the bearing clearance standard. The basic group clearance values apply to general conditions and should be preferred. Auxiliary group clearance values are available for radial bearing that cannot be used in special conditions. For example, the clearance values of the 3rd, 4th and 5th sets of deep groove ball bearings are suitable for the bearing and the shaft and the housing hole to be tighter than the normal fit or the mechanical parts with large difference between the inner ring and the outer ring. . In the case of a large inclination of the center of the shaft and the center line of the casing hole, and in order to increase the axial load capacity, increase the limit speed of the bearing, and reduce the frictional resistance of the bearing, the third, fourth, and fifth groups may also be used. Gap value. For shafts that require precise rotation or limited axial travel, bearings with a second set of clearance values (small clearance values) are generally used, and if necessary, a certain preloading "preload" is applied to increase the rigidity of the shaft.
The so-called rolling bearing clearance is the maximum amount of movement of one ferrule and the other rim in the radial or axial direction. The maximum amount of motion in the radial direction is called the radial clearance, and the maximum amount of motion in the axial direction is called the axial clearance. In general, the larger the radial clearance, the larger the axial clearance and vice versa. According to the state of the bearing, the clearance can be divided into the following three types:
Bearing clearance original clearance
The clearance in the free state before the bearing is installed. The original clearance is determined by the processing and assembly of the manufacturer.
Bearing clearance installation clearance
Also called the fit clearance is the clearance when the bearing and the shaft and the bearing housing are installed and not working. The installation clearance is smaller than the original clearance due to the interference installation, or the increase of the inner ring, or the reduction of the outer ring, or both.
Bearing clearance working clearance
When the bearing is in working condition, the inner ring temperature rises the most, the thermal expansion is maximum, and the bearing clearance is reduced. At the same time, due to the load, the rolling element and the raceway are elastically deformed, which makes the bearing clearance increase. Big. Whether the bearing working clearance is larger or smaller than the installation clearance depends on the combined effect of these two factors.
Some rolling bearings can't adjust the clearance, but they can't be disassembled. These bearings are available in six models, 0000 to 5000. Some rolling bearings can adjust the clearance, but they cannot be disassembled. There are 6000 type (deep groove ball bearings) and inner ring taper holes. Rolling bearings of type 1000, 2000 and 3000, the installation clearance of these types of rolling bearings will be smaller than the original clearance after adjustment; in addition, some bearings can be disassembled, and the clearance can be adjusted. There is 7000 type (angular contact ball). Bearing), 8000 (thrust ball bearing) and 9000 (thrust tapered roller bearing), these three bearings do not have original clearance; 6000 and 7000 rolling bearings, radial clearance is reduced, axial The clearance is also reduced, and vice versa, and for the 8000 and 9000 rolling bearings, only axial clearance has practical significance.
A suitable mounting clearance helps the rolling bearing to function properly. If the clearance is too small, the temperature of the rolling bearing will rise, it will not work properly, and the rolling element will be stuck; the clearance is too large, the equipment vibration is large, and the rolling bearing is noisy.
Bearing clearance check
The radial clearance is checked as follows:
Bearing clearance sensation
1. With the hand rotating bearing, the bearing should be stable and flexible without jamming.
2. Shake the outer ring of the bearing by hand. Even if the radial clearance is only 0.01mm, the axial movement of the uppermost point of the bearing is 0.10~0.15 mm. This method is specific to single row radial ball bearings.
Bearing clearance measurement
1. Check with the feeler gauge to confirm the maximum load position of the rolling bearing. Insert the feeler gauge between the rolling element and the outer (inner) ring which is 180°. The thickness of the locking feeler is the radial clearance of the bearing. This method is widely used in self-aligning bearings and cylindrical roller bearings.
2. Check with the dial gauge, first zero the dial gauge, then push up the outer ring of the rolling bearing. The reading of the dial gauge is the radial clearance of the bearing.
The method for checking the axial clearance is as follows:
1. Feeling method
Check the axial clearance of the rolling bearing with your fingers. This method is applied to the case where the shaft end is exposed. When the shaft end is closed or cannot be checked with a finger for other reasons, it can be checked whether the shaft is flexible or not.
2, measurement method
(1) Check with a feeler gauge, the method of operation is the same as the method of checking the radial clearance with a plug, but the axial clearance should be
c=λ/(2sinβ)
Where c - axial clearance, mm;
Λ——the thickness of the feeler gauge, mm;
Β——bearing cone angle, (°).
(2) Check with a dial gauge. When the shaft is tilted with the crowbar to make the shaft at two extreme positions, the difference between the dial gauge readings is the axial clearance of the bearing. However, the force applied to the crowbar should not be too large, otherwise the shell will be elastically deformed, and even if the deformation is small, the accuracy of the measured axial play is affected.
