The rollers of crossed cylindrical roller bearings are arranged perpendicular to each other and can withstand radial loads, bidirectional thrust loads and overturning moments , and have high rigidity. They are suitable for industrial robot joints or rotating parts, machining center rotary tables, Torque motor rotary table, etc. With the improvement of the performance of bearing processing equipment, the rotation accuracy of crossed cylindrical roller bearings can reach above P5, which can meet the needs of most users; The measurement data at the same measurement point in the same circle is consistent, which is reflected on the processing equipment, that is, the degree of consistency of the continuous measurement results obtained by processing the same batch of parts with the same processing program) puts forward new requirements, and the repeated positioning accuracy will directly affect the rotary table of the equipment. Repeated positioning performance, which in turn affects the machining accuracy of parts.
1 Structure of crossed cylindrical roller bearing
In terms of shape and structure, crossed cylindrical roller bearings can be divided into three types: 1) Inner ring integrated, outer ring separable, suitable for parts requiring high rotation accuracy of the inner ring; 2) Outer ring integrated, inner ring separable , suitable for parts requiring high rotation accuracy of the outer ring; 3) The inner and outer rings are integrated, suitable for parts requiring high rotation accuracy of the outer and inner rings, and the installation has almost no effect on the bearing performance. From the internal structure, it can be divided into two types: full complement rollers and spacer cages. Full complement rollers are suitable for low speed and heavy load conditions, and spacer cages are suitable for low friction torque and high speed conditions.
2 Common factors affecting the repeated positioning accuracy of crossed cylindrical roller bearings
2.1 Bearing Design Aspects
2.1.1 Isolation block structure
The roller arrangement of the crossed cylindrical roller bearing is shown in Figure 2. Adjacent rollers are separated by spacers, which can reduce the friction between the rollers and guide the movement of the rollers.
Commonly used spacer block structure, the projections of the axis lines of the two cylindrical grooves of the spacer block on the same plane are perpendicular to each other and parallel to the outer diameter surface of the groove. The isolation block has strong versatility, as long as the rollers have the same size, the same isolation block can be used, and the cost is low.
However, the axes O1A and O2B of the adjacent rollers of the crossed cylindrical roller bearing are not parallel to the radial plane O1O2C perpendicular to the axis of the spacer  (Fig. The lines are not parallel, but inclined at a certain angle. The actual contact state is point contact. The smaller the diameter of the center of rotation of the bearing, the larger the inclination angle, and the worse the contact state. In the working condition of high speed, the isolation block will be skewed, which will affect the rotation track of the rollers, resulting in poor rotation and affecting the repeated positioning accuracy of the bearing.
Optimize the design of the spacer block structure so that the axis of the cylindrical groove of the spacer block forms an angle with the plane where the spacer block is located, parallel to the axis of the roller, so that the contact state between the roller and the cylindrical groove of the spacer block changes from point contact to line touch. The improved isolation block is in good contact with the outer diameter surface of the roller, which improves the stability of the bearing rotation, and is suitable for equipment with high rotational speed and high requirements for repeated positioning accuracy of the bearing.
2.1.2 Bearing stiffness
Under the joint action of radial load, axial load and overturning moment, the inner and outer rings will produce radial and axial relative displacement and relative inclination, which will affect the repeated positioning accuracy of the bearing.
Bearing stiffness has radial stiffness, axial stiffness and angular stiffness. In order to reduce or eliminate the relative displacement of crossed cylindrical roller bearings under external forces, the bearings need to be preloaded. When designing crossed cylindrical roller bearings, the radial clearance is generally negative clearance, that is, the internal preload of the bearing is controlled by adjusting the radial clearance. Therefore, during design, the bearing stiffness should be calculated according to the maximum equivalent radial load (the combination of radial load, axial load and overturning moment) borne by the bearing, and the initial clearance of the bearing should be obtained to ensure that the bearing has the ability to resist deformation under the action of external load, thereby ensuring Bearing repeat positioning accuracy.
2.1.3 Total roller clearance
After the cross cylindrical roller bearing rollers are assembled with the spacer, there will be a gap in the circumferential direction. The gap value can be adjusted by replacing the spacer. However, due to the limitation of the structure and material of the spacer, the total circumferential gap cannot be completely eliminated. If the total clearance of the circumference is too large, the spacer and the rollers will be easily skewed during the rotation process, resulting in uneven bearing load; if the total clearance of the circumference is too small, the rollers will squeeze each other during the rotation process, and the wear of the rollers and the raceway will increase: In the end, it will cause the bearing vibration to increase, and the repeated positioning accuracy will be poor. According to practical application experience, for crossed cylindrical roller bearings with spacer blocks whose outer diameter is less than 300 mm, in order to avoid the increase of the total circumferential clearance due to bearing wear during operation, the design should be as small as possible under the premise of easy assembly. total clearance.
2.1.4 Fitting gap between plug and plug hole
For crossed cylindrical roller bearings with integrated inner and outer rings, the rollers and spacers need to be loaded sequentially from the plug holes machined on the outer diameter surface. The value of the fitting clearance between the plug and the plug hole is very important: if the fitting clearance is too large, the plug will be inclined in the hole after assembly, and abnormal noise may occur at the fitting of the plug and the hole when the roller rotates to the raceway surface, which will affect the rotation accuracy of the bearing , which in turn affects the repeat positioning accuracy of the bearing; if the fit gap is too small, the mating surface between the plug and the plug hole will be deformed after heat treatment, and the plug hole will easily be damaged when the plug is taken out from the plug hole of the outer ring. According to practical application experience, the roughness of the outer diameter surface of the plug and the inner diameter surface of the plug hole is required to be no more than 0.8 μm, and the fit clearance is 0.01-0.02 mm.
2.1.5 Bearing usage
Crossed cylindrical roller bearings are mostly used horizontally and vertically: when using horizontally, the structure of the roller spacer spacer is usually used in the circumferential direction; The spacer block may fall and cause friction with the raceway, which will increase the friction torque of the bearing. In severe cases, it will cause the bearing to freeze, which will eventually affect the repeated positioning accuracy of the bearing. For vertical use, the roller spacer spacer structure can be used when the repeated positioning accuracy of the bearing is not high, and the integral cage spacer roller structure can be used for higher requirements.
2.2 Processing technology
If the burrs at the contact between the bearing oil hole or oil groove and the raceway are not cleaned, the burrs will have high hardness after heat treatment, and if the grinding process is not processed in time, the rollers will collide with the burrs when they rotate to this place, resulting in large vibration and poor rotation stability. Between the turning process and the heat treatment process, the process of cleaning the oil hole and the burrs around the lubricating oil groove should be added.
2.2.2 Raceway angle
The crossed cylindrical roller bearing is a linear raceway, and the angle between the two raceways is 90°. The raceway angle plays a decisive role in the bearing performance , and the requirement is 90°±2′. In practical application, it is found that if the raceway angle of one ring of the bearing is processed at 89°58′, and the raceway angle of the other ring is processed at 90°, a small included angle α will be formed between the raceways of the two rings after assembly (Fig. 5), an additional force will be generated at both ends of the roller, which will skew the roller and affect the repeated positioning accuracy of the bearing; on the contrary, if the raceway angle is processed according to the positive deviation, an additional force will be generated inside the bearing. During production and processing, the number of grinding wheel dressings should be strictly controlled in accordance with the equipment operating procedures to avoid the small raceway angle caused by failure to trim the grinding wheel in time after wear, resulting in poor contact between the rollers and the raceway, affecting the repeated positioning accuracy of the bearing.
width=218, height=151, dpi=110
2.2.3 Contact area between taper pin and taper pin hole
The crossed roller bearing ring end face plug and plug hole of the inner and outer ring integrated structure is processed with a 1:50 taper taper pin hole, and the plug is fixed by the taper pin. After the gap between the plug and the plug hole is matched, the taper pin hole is drilled. When matching drills, it is necessary to ensure that the contact area between the outer diameter surface of the tapered pin and the inner surface of the tapered pin hole reaches more than 80%. If the contact area is less than 80%, the roller may move to the plug when the bearing is subjected to a large impact load, which will affect the bearing. Bearing repeat positioning accuracy. Bearing parts will be deformed after heat treatment, and the toughness of the tapered pin will decrease after quenching together with the ring. Therefore, after rough grinding, the surface of the tapered pin hole and the plug hole is added to remove the scale, and the tapered pin is replaced at the same time. When replacing the taper pin, it is also necessary to carry out coloring inspection on the contact area between the outer diameter surface of the taper pin and the inner surface of the taper pin hole to ensure that the contact area is greater than 80%.
2.2.4 The difference between the two outer diameters or two inner diameters of the split structure
For crossed cylindrical roller bearings whose outer ring or inner ring is of split structure, it should be ensured that the outer diameters or inner diameters of the two rings are basically the same when fitting together, and at least the difference between the two inner diameters or the two outer diameters should be ensured. for the following requirements:
In the formula: Δd1mp, Δd2mp is the average size tolerance of a single plane of inner diameter; ΔD1mp, ΔD2mp is the average size tolerance of a single plane of outer diameter.
When the outer diameter or inner diameter of the two split rings that fit together is different from each other by more than 1/3 of the tolerance of the outer diameter or inner diameter, the two rings are easy to be misaligned after assembly, and the raceway is deflected, which affects the roller rotation state and Bearing repeat positioning accuracy.
2.3 Installation and maintenance
2.3.1 Fitting dimensions
For occasions where high repeatability of the bearing is required, the crossed cylindrical roller bearing generally adopts negative clearance (CC0 clearance), that is, there is a preload inside the bearing before installation . According to practical application experience, interference fit will increase the internal preload and frictional moment of the bearing, causing increased wear and affecting the service life of the bearing.
In order to avoid the interference fit from adding additional preload to the inside of the bearing, the inner ring and the shaft, and the outer ring and the housing generally adopt a clearance fit.
For the crossed cylindrical roller bearing with split structure of the outer ring or inner ring, two split outer rings or inner rings need to be pressed from the end face during installation. When installing the flange screws, in order to avoid the internal preload of the bearing due to the inconsistent tightening torque of the screws Uneven, should be tightened evenly and symmetrically with a torque wrench according to the number of screws. In the process of tightening the screws, it is necessary to rotate the inner ring or outer ring of the bearing at a constant speed, adjust the roller reset, and avoid the dead point when the bearing rotates. Bolts with a lower strength level are used, and the bolts are easy to loosen when the bearing is subjected to a large impact, and the internal preload of the bearing is reduced, which affects the repeated positioning accuracy of the bearing. It is recommended that the strength grade of the connecting screws of the bearing end cover be selected to be above grade 10.9.
Failure to replenish lubricating grease in time will result in poor internal lubrication of the bearing and increased wear, which will affect the repeated positioning accuracy of the bearing and reduce the service life of the bearing. Cross cylindrical roller bearings need to replenish grease regularly, and the grease replenishment cycle is 3 to 6 months .