5 Key Differences Between Crossed Roller Linear Guides and Recirculating Ball Linear Guides

In this article, we will explore the utilization of bearings in mechanisms to ensure precise guidance and positioning in a single direction, be it linear or rotational. However, it is crucial to ensure adequate stiffness in all other directions to effectively guide the mechanism. The focus of this article is to examine the disparities between Linear Crossed Roller Guides and Recirculating Linear Ball Guides, and determine the circumstances in which one proves advantageous over the other.

First of all, what is a linear crossed roller bearing and what is a recirculating linear ball bearing?

A linear crossed roller bearing consists mainly of 2 pairs of 2 rails with a roller cage between the rails (see next placed image) The rollers provide smooth guidance in the desired direction. The rollers are oriented in such a way that the bearing is stiff in all other directions. A recirculating linear ball bearing consists of a rail and 2 recirculation units that guide the balls of the bearing (see image). Due to the recirculation of the balls, the stroke of a recirculating linear ball bearing can theoretically be infinite.

When a comparison is made between two different bearing types, the important properties of a bearing must be known. Since PM bearings are mainly used for high-precision guiding and positioning the following properties are important:

  • Stroke, velocity, and acceleration

  • Moving resistance

  • Accuracy and repeatability

  • Stiffness

  • Load capacity

Stroke, velocity, and acceleration

Stroke, velocity, and acceleration vary per application. Both crossed roller guides and recirculating ball guides can be used for high-speed and high-acceleration applications. When a large stroke is needed, the recirculating ball guides become favorable over the crossed roller guide since crossed roller guides have a limited stroke.

When small or medium strokes are needed, these can both be provided by a crossed roller bearing and by a recirculating ball bearing. In the past medium strokes were difficult to achieve with crossed roller bearings. However, due to the development of outrunning cages, larger strokes can be achieved. PM offers linear guide sets with a standard stroke length, the sets can be selected on this page.

Moving resistance

Since cross-roller guides are mainly used for high-precision guiding and positioning, the moving resistance of the bearing is a critical factor. This is a critical factor since more resistance generally leads to less accuracy and repeatability. The resistance of a contact bearing depends on multiple factors. First of all, the friction that is present in bearings results in resistance. So, contacting surfaces with a relative velocity to each other causes resistance during movement. But this is not the only factor that causes resistance in a contact bearing.

To create stiffness in a contact bearing, the bearing must be preloaded or a certain external force must be applied on the bearing. This (preload)force causes small local (elastic) deformations within the bearing itself due to the Hertzian contacts. When the bearing is moving, the locations of these small deformations move as well. The movement of these deformations costs energy and therefore it causes resistance in the bearing. Another factor that causes moving resistance in a contact bearing is lubrication. Lubrication is applied to extend the lifetime of a bearing. However, the viscosity of the lubricant generally causes resistance in the bearing. More information about the use of lubrication in linear guides can be found here,

In terms of moving resistance, the linear crossed roller bearing is superior to the linear recirculating ball bearing. This is caused by balls that are most of the time in contact with each other. In the ball recirculating channel, this friction becomes even worse since the balls will have some play. Since they have some play in the channel, friction appears between the balls and the channel (see next placed image). The loss ratio caused by this friction can be calculated with the below-listed formula. This is not the case with linear crossed roller bearings. In linear crossed roller bearings, only the cage will cause friction with the rollers and the rails, but this causes relatively low resistance. 

Accuracy and repeatability

Linear guides are one of the most crucial factors for high-precision guiding and positioning. Both, linear crossed roller guides and linear recirculating ball guides have good accuracy and repeatability. When top-notch accuracy and repeatability are required, linear crossed roller guides are favorable over recirculating ball or roller guides.

With recirculation of the balls, the balls will be preloaded when they are in contact with the raceway. When the balls are recirculated, the balls are not preloaded. The loading and unloading of the ball will cause inaccuracies in dynamic and stationary accuracy. In crossed roller bearings this loading and unloading of rollers does not occur and will therefore result in more dynamic and stationary accuracy (see diagrams below). The crossed roller bearing also has superior repeatability compared to a circulating ball bearing. This is because friction in a crossed roller bearing is less, almost friction-free. This results in better repeatability.

 

Stiffness

In high-precision mechatronic systems, stiffness is of high importance. High stiffness leads to less deformation when forces are applied to the system. This is beneficial for the accuracy of the system. Besides that, stiffness plays a crucial role in dynamic system behavior. A higher stiffness leads to higher eigenfrequencies of the system. With higher eigenfrequencies, a higher control bandwidth can be used. This can be beneficial for the overall performance of the system.

The stiffness of contact bearings is generated by the contacts of the balls or rollers with the raceway. The contact area of a rail-roller-rail contact is much greater than a rail-ball-rail contact.

The contact area is a crucial factor for the stiffness of a Hertzian contact. Therefore, the stiffness of a rail-roller-rail contact is higher than the stiffness of a rail-ball-rail contact when the same load is applied (see image). Therefore, the stiffness of a linear crossed roller bearing is superior to the stiffness of a recirculating ball.

Load capacity

Load capacity is an important parameter of a bearing. In most applications, available space is limited. Therefore, it would be beneficial when a smaller bearing can be selected that still contains a sufficient load capacity.

The load capacity of a bearing is related to the stress appearing in the bearing when a load is applied to it. The stress concentration appearing in a rail-ball-rail contact is more intense than in a rail-roller-rail contact (see image). Resulting in the fact that a rail-roller-rail contact has more load capacity than a rail-ball-rail contact.

Therefore, a linear crossed roller bearing has more load capacity than a linear recirculating ball bearing.