Bearings are an essential mechanical part that assists an object’s rotation movement in a machine. To be exact, bearings support the motor shaft or engine that rotates inside the machinery so that the machinery can move at extremely high speeds and carry loads with ease and efficiency. To best serve the purpose, bearings are highly engineered to offer excellent precision, reliability and durability. They also help reduce the noise and vibration when a shaft rotates. There are two basic types of bearings that are commonly applied: the ball bearings and the roller bearings.
Roller bearings can be further divided into subtypes. There are cylindrical roller bearings, tapered roller bearings, spherical roller bearings, and crossed roller bearings. These bearings are different structurally and serve for slightly different purposes. The cylindrical bearings are also called the needle roller bearings. They are commonly found in general machine application such as gearboxes. The tapered roller bearings are used for combined axial and radial loads in wheel applications. The spherical roller bearings can be seen on conveyors. The crossed roller bearings are designed for linear motion for friction reducing devices.
The anatomy of a crossed roller bearing as well as other types of bearings is similar. There are three fundamental parts of a bearing: the internal ring, the external ring and a ball set. The general configuration of a bearing is like a ring with the internal and external rings. The ball set is basically tiny metal balls that are placed between the internal ring and external ring. This setup allows the rotation force to be placed on the ball set, which is referred to as loading.
The two primary types of loading to a bearing are the thrust loading and the radial loading. The bearings that work with a radial load are the bearings that rotate or roll in accordance with the shaft. On the other hand, the thrust loading has to do with the angle that the force exerts. No matter which type of loading a bearing takes, it functions as to reduce friction and make the rotation smoother. A bearing can also protect the parts that support the shaft and make sure the shaft is positioned correctly.
The crossed roller bearings are also known as the crossed roller slide-way. They are the bearings that provide more accuracy, rigidity and weight-bearing capacity. This type of bearing can better support moment loads, radial forces and tilting loads than other types of bearing. Having these characteristics, the crossed roller bearings are made for high precision linear motion applications with short linear movements that require smooth motion. High levels of acceleration and deceleration are usually involved in such movements and the crossed roller bearings are durable enough to take the loads.
What makes the crossed roller bearings different from other types of bearings is that they use the cylinder shaped rollers that are more accurate and rigid mechanical linear components. The cylindrical rollers are wrapped between two parallel guides, the table and the bed. The rollers are the tiny metal balls and the table and the bed serve the role as the external and internal rings of a regular bearing. The roller balls are placed in the V-grooved raceways of the guides. The even rollers are mounted at ninety degrees to the odd rollers in the rails.
This V configuration of the rollers inside the crossed roller bearings provides a full line of contact rather than just the point of contact, which gives the bearing a broader contact surface and the capability to carry heavy loads. It also enhances the rigidity of the bearings so that they are less likely to deform while working and therefore they are more accurate. In addition, since this kind of bearing has the consistency of contact between the carriage and the base, the erosion is slower.
Taking a close look at the inner structure of a crossed roller bearing, there are cages in the rails to hold the roller balls in place. The cages are also called the retainers. The cages are typically made from two materials, metal or plastic. The metal cages have tabs on the carriage that fit into notches on the top and bottom of the rollers to keep them in place. How closely the rollers can be positioned between the neighboring rollers determine the bearing’s overall load carrying capacity. The metal cages are less expensive and can be used in vacuum environment such as the outer space.
The plastic cages allow the rollers to be positioned closer to each other and hence the bearing is granted with higher load carrying capacity. Generally speaking, giving the same bearing size, a crossed roller bearing with plastic cages has more rollers inside than that with metal cages. As a result, the crossed roller bearings with plastic cages can maintain the same load capacity and number of rollers while having a shorter length. A plastic cage is estimated to increase about thirty percent to fifty percent of contact area compared to a metal cage.
The limit to the crossed roller bearings is that, the rail assembly with this type of bearing has to be at least twice as long as the application’s travel length. The reason is that the two rails that contain the rollers move in opposite directions. In other words, when the table goes to the right, the bed goes to the left. Since the rails move in opposition to each other, the space required is twice the distance that the load carries.
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