By Coskun Mutlu
Product Line Specialist Linear Bearings & Guides
Thomson Industries, Inc
www.thomsonlinear.com thomson@thomsonlinear.com
If you have an application that requires guided straight-line movement with either side or moment loads to support on a carriage, it is important to consider whether round or square linear bearings best meet your needs. A square rail (also known as a profile rail) utilises an inner race, or rail, and an outer race which is the carriage. A round rail, also known as a ball bushing, has a shaft rail assembly with open bearings inside pillow blocks on the top. There are other technologies which may be considered, including bronze bushing, air bearing and cam follower, but these two are by far the most common and both have pros and cons depending upon the specific application requirements.
Fundamentally, profile rail products are well suited to high load applications and offer extremely high accuracy and stiffness, when supported throughout the length of the rail. The trade-off is that they require precise installation, and despite modern manufacturing techniques making them affordable they are generally a higher cost option. Round rails offer lower cost, easier installation, more flexibility and can be supported only at the ends but with some compromise on load and accuracy capability. In many applications, either technology may be used to good effect but how do you decide which is the best?
Round rail linear bearings are easy to install because they have a floating plate on top of the bearing which allows self-alignment of the rail to within ±0.5 degrees. For example, two rails can be installed in parallel 300mm apart and a height difference of 1 or 2 mm will be tolerated. This makes installation easier and less expensive because a less perfect mounting surface can be tolerated.
The flexibility of the round rail technology further extends to being able to customise the housing using different materials, mounting holes, size and shape to best suit the specific installation requirements. There is the possibility for the rail to only be supported at the ends and, because of the small area contact of the rolling elements with the shaft, there is very little drag. The simple design and flexibility of the round rail bearing can often make it the easier installation option at a lower cost than its square equivalent.
The profile rail bearing requires support along its full length but offers a very rigid, repeatable and accurate motion. In terms of linear motion, accuracy is defined as the variation of height during the movement. Where ultra-precision is required, a high grade three metre rigid square rail can provide up to ±0,003mm compared with ±0.025mm for the round rail equivalent. The bearings on the profile rail can be preloaded to varying amounts to provide less deflection and greater repeatability, although this can make installation more difficult and increase drag on the system. It’s important to consider that the accuracy is only as good as the mounting surface.
When selecting a linear motion solution, a number of parameters and questions need to be considered. The orientation of the system is a first consideration – vertical, horizontal, on a wall etc., along with the length of motion that is required. If only end support is possible for the system then a round rail bearing needs to be used. If continuously supported, the questions of required accuracy, repeatability and load may then determine whether profile or round rail is the better option.
Profile rails are often the best choice where there is a high load (in the range of approx. 20kN for a 25mm rail is typical) that requires high stiffness (in the order of 1 – 4 µm /kN with a 13% preload). The traditional application for profile rails is in machine tools where these factors are critical, but they are being increasingly used in factory automation, aerospace, packaging and medical devices. Although apparently a higher cost solution, new manufacturing techniques and economies of scale have reduced costs, and they also offer an economical alternative to multiple round rail bearings for total higher loads.
Round rail bearings offer greater flexibility and are more tolerant of less perfect installation conditions. As the housing is often manufactured based on the specific application needs, this technology offers a good solution in areas such as food and beverage where a hygienic solution is required. Although profile bearings can be preloaded, the amount of preload cannot be adjusted on site. In these applications, the round rails are more flexible as the pillow blocks can have adjusting slots to tighten down and adjust the gap between the shaft and the balls, increasing or decreasing the preload.
Standard round rails have a significantly higher profile, and although lower profile round rails are available, profile rails will offer a smaller overall installation envelope. Thomson invented the round rail anti-friction technology, and now manufactures the broadest standard product offering of mechanical motion control technology in the market, but it is worth noting that profile rails are produced in ISO standard-defined sizes and specifications such as height of the carriage and mounting hole positions are consistent across different manufacturers. Thomson has recently upgraded its profile rail customisation facilities in Wolfschlugen to keep up with demand for custom length profile rails, however if customisation is required this is generally easier to achieve with the round rails if they fulfil the other application requirements.
Both round and profile rail bearings can be adapted for reduced noise. For the round rail system nylon ball bearings can be used or, for a combination of low noise and higher loads, alternating nylon and steel balls. For the profile bearings ball spacers or ball cages can be used on the carriage. These stop the balls running into one another and not only reduce noise but also provide smoother running for the device. End seals and wipers can further be used to protect the bearings against site environmental conditions and prolong the life of the equipment.
Overall there are pros and cons with both technologies but both can offer reliable, low maintenance operation. The round rails are generally the lower cost option and are more tolerant of imperfect installation conditions and can be end mounted whereas the profile rails offer high accuracy for handling higher loads and offer a lower profile design envelope. In many applications both technologies are suitable and a designer has the flexibility to choose the technology which provides the best cost versus performance based on the specific parameters required for the installation. It should be remembered that machining, surface preparation and installation costs all need to be considered in deciding which is the solution is most economical. With multiple parameters and considerations, Thomson is committed to helping customers choose the correct components for each specification. Our online ‘Linear Guide Component Tool’ was developed to ensure the best system selection for any given application.