VISTA Technology Knowledge: Preload of Double Row Cylindrical Roller Bearing in Spindle Unit

Hot tags: double row cylindrical roller bearing(1), spindle unit(1)

Discuss the ideal value of bearing preload and how to realize it in structure after determining the preload value.

 

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Modern machine tools are increasingly developing in the direction of functional components. As an important functional component, spindle unit plays an important role in machine tools. In the design and use of high rigid spindle unit, there are two problems to be solved about the preload of roller bearing. One is how ideal the preload of bearing is, and the other is how to realize the structure after determining the preload value. This paper tries to discuss these two problems.

 

1. Ideal preload of bearing

The preload of double row cylindrical roller bearing is realized through the inner ring taper hole with 1:12 taper. When the inner ring moves on the cone neck of the main shaft under axial force, the inner ring will swell, causing the diameter of the roller envelope circle to increase. Difference between the diameter D2 of the roller envelope circle and the diameter D1 of the outer race after installation =  D2 - D1 is the radial preload of this bearing (referred to as preload), as shown in Figure 1. > 1, the bearing is in the pre tightening state, < 0, the bearing is in clearance.

 

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Fig. 1 preloading of double row cylindrical roller bearing

 

The preload of double row cylindrical roller bearing will affect the bearing stiffness of spindle unit, dynamic characteristics of spindle and bearing temperature rise. Theoretical analysis and test show that the stiffness of roller bearing is 0.5% of load It is directly proportional to the power of 1, and the preload has little effect on the stiffness, but an appropriate amount of preload can improve the contact state between the roller and the raceway, so as to improve the bearing stiffness. For NN3020K bearing, the preload exceeds 5μm under the external load of 12,000N, the bearing stiffness is basically constant; From the point of view of vibration resistance, it is considered that when the bearing bears normal radial load, the preloading state of the non stressed roller and the raceway is the best state. For NN3020K bearing, take the radial load as 8000N, then △ 0= 4.2μm. The experimental results show that a small preload has an obvious effect on the improvement of the dynamic stiffness of the bearing, but it has no obvious effect when it exceeds a certain preload value; The elastic contact deformation between the roller inside the bearing and the raceway increases. When the deformation energy is converted into heat energy, the bearing temperature rises. Therefore, the greater the preload, the higher the temperature rise. For NN3020K bearing, the preload exceeds 8μm when the rotating speed is between 1500 ~ 2500 r / min. The bearing temperature rise will increase significantly.

 

Based on these three aspects, the ideal preload of NN3020K bearing is 5 ~ 8μm.

 

2. Realization of ideal preload in structure

2.1 measurement of clearance (or preload)

There are two methods to measure the clearance. The first is to use the special double row cylindrical roller bearing roller envelope diameter measuring instrument and its supporting standard ring. Measure the diameter D2 of the roller envelope circle after the bearing is installed on the shaft outside the box, and measure the inner diameter D1 of the outer ring after installation in the box hole, that is, the preload of the roller is obtained. Different specifications of instruments are used for bearings with different diameters. In the absence of special instruments, the second method can be used, that is, using the spring scale to measure the roller deformation to obtain the bearing clearance. The principle is: before measurement, let the front end of the main shaft be affected by the weight G (see Figure 2), the main shaft assembly is located at the lower part of the box hole, and the roller at the lower part of the bearing has undergone certain elastic deformation under the gravity of the weight G and the main shaft assembly. Directly above the action line of weight G, the spring scale has an upward tension F on the main shaft. If F is continuously increased, the deformation of the roller at the lower part of the bearing is reduced, and the whole spindle assembly is lifted upward. A micrometer probe is installed at the front end of the spindle close to the bearing to measure the lifting amount S of the spindle. When F is greater than the sum of the gravity of the weight G and the spindle assembly, S will jump, which indicates that the spindle component has left the lower part of the box hole and began to act on the upper part of the box hole. Continue to increase F, the upper roller of the bearing begins to deform, and S continues to increase. The relationship curve between F and S is shown in Figure 3. The distance between two parallel lines along the S axis is the radial clearance of the bearing.

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Fig. 2 measuring roller bearing clearance with spring scale

 

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Fig. 3 Relationship between F and S

 

2.2 realization of radial preload of bearing in structure

The radial preload of the bearing is realized structurally by ensuring the distance B between the end face of the bearing inner ring and the shaft shoulder (see Fig. 2). Before determining the value of B, the proportional coefficient K between the axial movement of the bearing inner ring and the corresponding change of radial preload must be known (the theoretical value is 12:1). The recommended values are shown in Table 1. Also know the initial preload of the bearing △0 and the corresponding B0. At B, B0, △0and K, the relationship satisfies the formula (B - B0) /△- △0) = K, so B0, △0, K and △can get the required B = B0 + K△- △0). If the bearing is in the initial state with clearance, then △ 0 takes a negative value (i.e. negative preload).

 

2.3 initial preload △0 and determination of initial axial dimension B0

Since the groove width B between the end face of the bearing inner ring and the shaft shoulder is difficult to measure and the measurement accuracy is required to be high, it cannot be determined first △. Then measure the corresponding B0, but only determine B0 first, and then measure the corresponding B0, △0can be determined by machining a sleeve with a width of B0 and installing it between the bearing inner race and the shaft shoulder. This sleeve is called fixed pre tightening sleeve. When preload   When adjustment is needed, it can be completed by adjusting the width B of the fixed pre tightening sleeve. For the convenience of replacement, the fixed pre tightening sleeve is made into two half rings, so that it is not necessary to remove the bearing and other parts from the main shaft when replacing the pre tightening sleeve. In practical application, a sheath is added outside the fixed pre tightening sleeve to prevent the two half rings from flying off under the action of centrifugal force.

 

More about VISTA High-quality Double Row Cylindrical Roller Bearing:

Vista double row cylindrical roller bearings are available in a variety of designs with cylindrical or tapered holes.

 

Double Row Cylindrical Roller Bearing 

 

Double row cylindrical roller bearing is suitable for heavy radial load because of its high rigidity and bearing capacity. These bearings are designed for spindles that require high rigidity, most commonly lathe spindles.

 

As a Cylindrical Roller Bearing manufacturer, Vista also supplies the following series Cylindrical Roller Bearing for the customers: single row Cylindrical Roller Bearing, three row Cylindrical Roller Bearing,  four row Cylindrical Roller Bearing.

 

Cylindrical Roller Bearing 

 

Cylindrical roller bearings are available in a wide range of designs, series, variants and sizes. The main design differences are the number of roller rows and the inner/outer ring flanges as well as cage designs and materials.

 

The bearings can meet the challenges of applications faced with heavy radial loads and high speeds. Accommodating axial displacement (except for bearings with flanges on both the inner and outer rings), they offer high stiffness, low friction and long service life. 

 

Cylindrical roller bearings are also available in sealed or split designs. In sealed bearings, the rollers are protected from contaminants, water and dust, while providing lubricant retention and contaminant exclusion. This provides lower friction and longer service life. Split bearings are intended primarily for bearing arrangements which are difficult to access, such as crank shafts, where they simplify maintenance and replacements. 

 

 


2022-05-17

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