Differences in cross roller guides

Precision cross roller linear guides, there are different types available. But what are the differences between them? And what criteria are important to take into account while selecting linear guides? 

This article will help you make the best selection by taking a deep dive into the characteristics of cross roller type linear guides.  

General information about the cross roller guide

For most design engineers, cross roller guides are known as precision rail guides with flat cages, a common choice for use in precision applications with limited stroke lengths, typically less than 1 meter. The main function of the linear bearing is to support a load with low friction in a straight-line motion. 

The reasons for using precision linear guides are numerous, but their most obvious benefits over other types of guides are:

  • Very low and uniform coefficient of friction

  • Almost stick-slip free and smooth operation

  • Compact dimensions

  • High load capacity and high rigidity

  • No backlash / high running accuracy

  • Low wear and high reliability 

Linear rail guides are available in different product types and designed with specific features for the applications.

In some applications, specifications such as very low-friction movement, high system rigidity, or limited space are important. Because of different requirements, we present in this article three types (i.e., RSD, RSDE, and RNG) of crossed-roller linear guides.

Note that there is another type of precision linear guide with needle roller cages offering the highest load ratings. Needles are cylindrical rollers that are small in diameter relative to their length, in fact, the length of a needle roller is at least 2.4 times its diameter. However, in this article, the needle bearings are not included and the focus is on the linear guides which are available with different crossed-roller cages. These linear guides with needle roller cages are called N/O linear guides or M/V linear guides according to the shape of the rails. Read here more about these guides.

What is a cross roller linear guide?

The cross roller linear guide consists of 2 pairs of two rails with a roller cage in between. The rollers in the cage are used to transport the load while minimizing wear and friction. You can see a brief explanation in the video.

Types of crossed-roller precision linear guides

Since PM started in 1966 with the manufacturing of ball and cross roller guide type RSD, the product has become a great success in various challenging precision applications such as XY-positioning stages.

Since its introduction, several versions of the crossed-roller bearing have been developed, each with specific features that we will explain in more detail below.

Today, it is impossible to imagine precision applications without crossed-roller guides. We distinguish the following types of precision linear bearings which can be fitted with crossed-roller cages:

  • Cross roller and ball type RSD

  • High-load cross roller guide type RSDE

  • Compact cross roller guide type RNG

Below we look more in detail at the differences between the guides.

Cross roller guide type RSD

The RSD linear guide was the first crossed-roller linear guide which was developed by PM, it is established to be the standard in the precision industry. This RSD series can be used either with ball cages or crossed-roller cages.

RSD-type linear guides are designed to support low up to medium loads. Cages are available in many designs for balls and cylindrical rollers, in different materials (including plastic) to offer the best solution for various applications. The linear bearings are available in ten sizes starting with a roller diameter of 1.5 mm up to 24 mm.

A quick note on ball cages: ball cages excel in light-load applications with low friction and smooth running. The ball contact surface to the raceways is limited to one point. With two guide rails and two raceways per rail, it makes four raceways and four contact points per ball for bidirectional load capacity. 

Linear guides fitted with crossed-roller cages offer high load ratings, long operational life, and good stiffness due to the line contact of the cylindrical rollers to the rail raceways.

Main features of RSD linear guides:

  • Can be fitted either with ball or crossed-roller cages

  • The widest choice in cage designs

  • Excellent support for low – medium loads

  • Widest range of sizes and lengths

  • Compact design

Main features of cross roller type RSD guides:

  • Can be fitted either with ball or crossed-roller cages

  • The widest choice in cage designs

  • Excellent support for low – medium loads

  • Widest range of sizes and lengths

  • Compact design

High-performance linear guide type RSDE

Cross roller linear guide type RSDE is the next step in performance. This precision linear guide is only available with crossed-roller cages and comes in four industry sizes; 3, 4, 6, and 9mm. The RSDE-type is 100% interchangeable with RSD cross roller linear guide type but offers considerably higher load ratings and other features to boost positioning accuracy and life.

RSDE linear guides are fitted with high-density crossed-roller cages. These cages are made from POM or PEEK polymer materials. The rails are designed with an optimized contact geometry between the roller and the raceway, resulting in better performance in medium-high load applications.

Because of the improved contact geometry in combination with the high-density roller cage, this type of linear guide is popular in dynamic applications where high-precision repeatability is required.

The main features of RSDE crossed-roller linear guides are:

  • High load ratings for longer life

  • Improved stiffness to handle higher dynamics

These features are explained more in detail in the following sections.

High load ratings

As mentioned earlier, RSDE linear guides are designed for high-dynamic applications. Longer life or endurance can be achieved with higher load ratings. The higher load ratings are reached due to the very long contact length of the roller. Another main feature is the polymer roller cage design which retains more rollers, about +30%, than other cages. Due to these main features, the static and dynamic load ratings are higher compared to the commonly used linear bearings using steel cages.

The benefits of the improved load ratings are:

  • Able to carry higher loads than common linear guides

  • Longer life or endurance than common guides

  • Opportunity to reduce size resulting in smaller machine design footprint and cost reduction

Improved stiffness

This diagram shows the differences in stiffness (sometimes called rigidity). With stiffness, we mean to what extent an object deforms when you apply a force or load to it. The diagram shows the difference in stiffness with RSDE linear guides compared to RSD bearings with ball or crossed-roller cages. The RSDE linear guide series has a higher stiffness than designs with ball cages or commonly used crossed-roller cage bearings and is the best choice for dynamic applications.

This type of linear guide is preloaded after assembly. With preload, the backlash is removed and the precision and stiffness of the system are increased. Keep in mind that a higher preload also increases the internal load acting on the linear bearings and affects life. This means that the preload has to be set carefully at the right amount.

Stiffness is important for dynamic applications as it affects its ability to respond to external forces without excessive changes in its geometry or deformations. So, a high stiffness can handle higher settling times to improve the machine output.

The design offers limited protection against contaminants

In general, precision linear guides and sensitive to contaminants, such as fine dust and chips. Internal contamination may result in:

  • Non-smooth and inaccurate movement

  • Increased friction

  • Excessive wear

Linear guide type RSDE has a tight gap between a pair of rails. This tight gap offers some protection against contamination. In case of extreme contamination, additional precautions for instance covers and seals must be taken into account. Please consult PM in cases of extreme contamination.

Space-saving cross roller guide type RNG

RNG-type crossed-roller guide is PM's latest addition and is fully in line with today's trend towards increased performance and downsizing across the whole machine sector. It combines a space and weight-saving design with high load-carrying capacity. The figure shows the ratio in envelope space of a linear guide based on cages with 6 mm rolling elements. 

In this article, there are 3 main topics that were addressed:

  • Low friction movement

  • High stiffness or rigidity

  • Space-saving design

With a significant reduction in envelope size, the load carry capacity and stiffness values of RNG linear guide series are equal to that of the RSDE series. The RNG series reflects the experience gained by PM over half a century in the development of high-precision linear guides.

Anti-cage creep linear bearings

RSDE and RNG linear guide series can be supplied with a robust anti-cage creep solution (ACC) to eliminate cage creep, sometimes called cage slippage, and keep the roller cages in position.

The bottom of the V-groove is designed with a toothed rack and the roller cage includes a pinion which is retained in the middle of the cage. Both components fit perfectly together and as the rail moves the pinion runs over the rack and controls the cage.

ACC is the most reliable anti-cage creep solution offered in the industry can be used in every mounting orientation and enables high-dynamic motion. ACC is optionally available.

For more information about the anti-cage creep technology ACC click here

Linear guides for use in vacuum

Cross roller linear guides can be used in vacuum and in ultra-high vacuum applications. The guide rails can be manufactured from stainless steel and there are various cages available. Linear guides type RSD can be used with stainless steel roller cages, stainless steel end screws or end plates. 

RSDE and RNG linear guides can be fitted with roller cages made out of PEEK. The cylindrical rollers can be made from stainless steel or ceramic material Si3N4.

Anti-cage creep is available for use in (U)HV. Please contact us to receive more information about this and discuss the solutions.

Quick links to product pages and 3D CAD files