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What is clearance? How to measure the clearance of rolling bearings? The so-called clearance of a rolling bearing refers to the maximum amount of movement of one ring in the radial or axial direction when one ring is fixed. The maximum amount of movement along the radial direction is called radial clearance, and the maximum amount of movement along the axial direction is called axial clearance. Generally speaking, the greater the radial clearance, the greater the axial clearance, and vice versa. According to the state of the bearing, the clearance can be divided into the following three types:: 1. Original clearance The clearance in the free state before the bearing is installed. The original clearance is determined by processing and assembly in the manufacturer. 2. Installation clearance is also called matching clearance. It is the clearance when the bearing, shaft and bearing seat are installed but not yet in operation. Due to the interference installation, either the inner ring is enlarged, the outer ring is reduced, or both, the installation clearance is smaller than the original clearance. 3. Working clearance: The clearance of the bearing when it is in working condition. When the inner ring is working, the temperature rise is the largest and the thermal expansion is the largest, which reduces the bearing clearance. ； At the same time, due to the effect of load, elastic deformation occurs at the contact point between the rolling element and the raceway, causing the bearing clearance to increase. Whether the bearing operating clearance is larger or smaller than the installation clearance depends on the combined effect of these two factors. Some rolling bearings cannot adjust the clearance, let alone disassemble. There are six types of these bearings, namely type 0000 to type 5000. ； Some rolling bearings can adjust the clearance, but cannot be disassembled. There are type 6000 (angular contact bearings) and type 1000, 2000 and 3000 rolling bearings with tapered inner rings. 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 are three types: type 7000 (tapered roller bearing), type 8000 (thrust ball bearing) and type 9000 (thrust roller bearing). These three types of bearings do not have original clearance. ； For type 6000 and type 7000 rolling bearings, when the radial clearance is adjusted smaller, the axial clearance also becomes smaller, and vice versa. For type 8000 and type 9000 rolling bearings, only the axial clearance is of practical significance. Proper installation clearance contributes to the normal operation of rolling bearings. If the clearance is too small, the temperature of the rolling bearing will rise and it will not work properly, causing the rolling elements to get stuck. ； If the clearance is too large, the equipment will vibrate greatly and the rolling bearing will make loud noise. The inspection method of radial clearance is as follows: 1. Feeling method 1. Turn the bearing by hand. The bearing should be smooth and flexible without any jamming. 2. Shake the outer ring of the bearing by hand. Even if the radial clearance is only 0.01 mm, the axial movement of the top point of the bearing is 0.10~0.15 mm. This method is specifically used for single row radial ball bearings. 2. Measurement method 1. Check with a feeler gauge to confirm the maximum load position of the rolling bearing. Insert a feeler gauge between the rolling element and the outer (inner) ring at an angle of 180°. The thickness of the feeler gauge with appropriate tightness is the bearing radial clearance. This method is widely used in spherical bearings and cylindrical roller bearings. 2. Check with a dial indicator. First set the dial indicator to zero, then lift up the outer ring of the rolling bearing. The reading of the dial indicator is the radial clearance of the bearing. The inspection method of axial clearance is as follows:: 1. Use the feeling method to check the axial clearance of the rolling bearing with your fingers. This method is used when the shaft end is exposed. When the shaft end is closed or cannot be checked with fingers for other reasons, you can check whether the shaft rotates flexibly. 2. Measurement method (1) Check with a feeler gauge. The operation method is the same as that of checking the radial clearance with a feeler gauge, but the axial clearance should be c=λ/(2sinβ) where c——axial clearance, mm ； λ——Thickness of feeler gauge, mm ； β——Bearing cone angle, (°). (2) Check with a dial indicator and use a crowbar to move the shaft so that the shaft is in two extreme positions. The difference between the dial indicator readings is the axial clearance of the bearing. However, the force applied to the crowbar cannot be too large, otherwise the housing will undergo elastic deformation. Even if the deformation is small, it will affect the accuracy of the measured axial clearance.