What Is a Crossed Roller Bearing?
A crossed roller bearing is a bearing in which the rollers are arranged orthogonally between the inner and outer rings.
Cylindrical rollers are arranged orthogonally and alternately at an angle of 90°, supporting loads from all directions simultaneously while maintaining high rotational accuracy. The crossed roller bearing is characterized by its high rigidity; in many cases, only one roller bearing is needed, whereas two average roller bearings are used.
Uses of Crossed Roller Bearings
Crossed roller bearings are used in robots and other components for various applications because of their high rigidity and space-saving capability. The following are applications of cross-roller bearings, taking advantage of their respective characteristics.
1. High Rigidity
Applications that take advantage of high rigidity include industrial and industrial robots. Specifically, welding robots that combine a variety of movements. It may also be used in the swiveling parts of machine tools.
2. Compact and Precise
Applications that take advantage of the characteristics of compactness and precision include humanoid robots that require lightweight, joint parts of robot suits for agricultural work, nursing care, and logistics, and measuring and medical equipment that require precision. Other applications include IC manufacturing equipment, which requires compact and precise movements.
Other applications include cutting-edge fields in the aerospace industry.
Principles of Crossed Roller Bearings
Unlike standard bearings, which have balls or rollers between the inner and outer rings, crossed roller bearings use cylindrical rollers arranged at 90° angles in alternating directions. This structure allows the bearing to support loads in a variety of directions. It can also support larger loads due to the increased contact surface area.
The principle is illustrated by using crossed roller bearings on a rotating table. To increase the moment rigidity of the table, two bearings are installed as far apart as possible to increase the distance between the points of action in the case of a standard bearing. On the other hand, if crossed roller bearings are used, it is possible to obtain very compact and high rigidity because of its sizeable stand-alone working point distance.
Other information on Crossed Roller Bearings
1. Precautions for Crossed Roller Bearings
A high-precision rotating mechanism requires attention to the bearing and the machining accuracy of the mounting parts and the assembly method.
Rigidity of mounting parts
In designing the housing and the push flange of crossed roller bearings, the rigidity of the parts, as well as the size and number of tightening bolts for the push flange, must be considered. Insufficient strength can cause deformation of the bearing or uneven roller contact inside, leading to premature failure or worsening rotational accuracy.
Housing Design
The housing should be designed so that the wall thickness is at least 60% of the bearing cross-sectional height. Also, if a threaded hole called a “pull-out tap” is machined for dismounting the bearing, the bearing can be dismounted without putting a load on it, preventing bearing damage during dismounting.
Design of push flange
The wall thickness of the push flange should be designed to be 50 to 120% of the bearing thickness, and the clearance between the flange and housing should be about 0.5 mm. Iron is the recommended material for the push flange.
Tightening Bolts
The bearing outer diameter dimensions determine the size and number of fastening bolts. For example, for bearings with outer diameters of 100 to 200 mm, the flange fastening bolts should be M4 to M8 in size, with a minimum of 12 bolts.
When installing the push flange, the order in which the bolts are tightened is also essential. To tighten the bearing evenly, tighten the diagonal screws little by little and assemble the bearing so that the tightening is even.
2. Pressurizing Crossed Roller Bearings
Crossed roller bearings can be pressurized similarly to standard ball bearings. However, since the rotational friction increases, it is necessary to calculate the rotational force.
Pressurization is usually applied by setting the radial clearance to a negative value. The recommended dimensional tolerance of the housing and shaft for mounting a pressurized bearing is g5/H7, and the fit should be set to avoid a tight fit. If the fit is tight, the internal stress will be high due to over pressurization, which may lead to bearing failure.