Linear motion bearings are bearings that are made to move in a particular direction, and they come in a wide array of different designs. Some linear slides are motorized, others are not. Examples of the former include roller and XY tables as well as machine and dovetail slides. Non-motorized slides include dovetail, roller and ball bearing varieties, which allow for movement with a minimum of friction, whether the slide is powered manually or through resistance. Regardless of the bearing type — ball, dovetail, fluid, linear roller, magnetic — linear slides make linear motion possible. On the X and Y-axis, movement is provided for machine slides, stages and tables via linear motion bearings.
Due to advances, square rails and round rails are now similar in price.
Engineers faced with a linear-motion application often turn to pre-engineered linear bearings and guides. But linear guides come in two basic shapes – round and square – and designers often struggle with which type to use. Square or profile rails once cost significantly more than round because the former need extensive grinding that is inherent to the design. But recent manufacturing advances and economies of scale let square rails now compete with round in a broad range of applications.
The criteria for choosing one over the other are no different than choosing any other machine component: define system objectives, then consider factors such as load capacity, stiffness, accuracy, smoothness, size, and overall costs. The best place to start is to look at the fundamental qualities of each type of guide.
Square or profile rails are flat on the top and bottom, while the sides have a concave shape with ball-conforming races where bearings roll. They typically target applications such as machine tools that require high load capacity, stiffness, and accuracy. A profile rail’s performance comes from its inherent shape and through precise grinding of the rail ball tracks. The raceway on which the balls ride in the rail and carriage is only slightly larger in radius than the balls themselves. This geometry cradles the balls as they flatten slightly under load, which expands the contact area between them and the raceway and results in higher load capacity. Substituting cylindrical rollers for ball bearings can roughly double load capacity in a same sized package.
Square rails can be preloaded to between 3 and 13% of the rated dynamic load. Heavy preloads apply an initial deflection and cut deflection of the system when externally loaded. For example, a 13% preload gives stiffness on the order of 1 to 4 μm/kN. As a result, profile-rail bearings are roughly five times stiffer under load than a round-rail assembly with its convex ball and shaft surfaces.
A key advantage of square rails is their high positioning accuracy.
A key advantage of square rails is high positioning accuracy, especially useful in milling and grinding machine tools. They hold from 0.0002 to 0.001 in. over a length of 10 ft, with typical 3 to 10-μm/m parallelism between the bearing guide and rail. They also maintain this precision under moment loads; a single carriage and rail is often better suited for this than a round rail.
With load capacity, stiffness, and accuracy all favoring profile rail, some engineers may look no further. However round-rail designs, with a ball-bushing bearing riding on a precision shaft, can offer several advantages over profile rails. Primarily, round rails are generally less expensive. Additionally, square rails are more forgiving of misalignment and poor parallelism, which for many applications can be an issue.
Round-rail bearings also tend to operate more smoothly than profile rail. The point contact of convex surfaces between balls and rail on round types minimize scuffing. Ball bushings typically run with a slight clearance or a light preload about 1 to 2% of rated dynamic load, which also promotes smooth operation.
By comparison, profile-rail bearings with their high conformity and preloads may exhibit “notchiness” in operation. In most cases, that’s not a problem. Also, the simple seals on round bushings tend to add less drag than the wiper on profile rails. Profile-rail wipers must conform to the ball tracks for a good seal, which increases drag compared to round linear bearings.
Another advantage of round-rail bearings is the ability to run smoothly when mounted to less-than-perfect surfaces, often defined as a flatness error greater than 150 μm/m. Square-rail designs are especially sensitive to flatness errors that can cause binding and high drag that can cut life in half. Surfaces must be carefully prepared or the parts shimmed and adjusted during installation, adding to costs. In contrast, round rails can mount to welded tubular frames or directly to factory floors. Because a ball bushing is free to rotate about a round shaft, when used in pairs, one rail can sit at a slightly different elevation than the other and allow more variation in rail height than square-rail systems.
Round rails can hold a travel straightness of 0.01-in. for 10 ft. To attain this accuracy, they need to mount only at the ends, although many are supported at several points or along their full length. Round linear guides are capable of spanning gaps of 12 to 24 shaft diameters, making them useful in applications such as gantry systems and pick-and-place modules in factory automation equipment. The axis of motion is established entirely by fixing the two ends of the shaft; it doesn’t matter what the surface of the machine is like between these two points or whether there is one at all. The precision of the device depends only on the accuracy of the end-support mounting. Generally, round-rail installation is relatively easy and inexpensive.
By comparison, square profile rail are not designed to span gaps or be end supported. They are stiffer and more rigid but need straight, continuous support with tight requirements for flatness and parallelism.
A critical parameter for profile guides is running parallelism, which is in the several-micrometer range. Unless followed closely, the bearings bind or wear prematurely. To prevent such problems, installation guides painstakingly cover mounting-surface preparation, mounting tolerances, and rail parallelism. Installation data also encompasses rail vertical offset, vertical and lateral carriage offset, mounting- hole tolerances, bolt torque, and butt joints.
However, a single profile rail can support moments in all directions, eliminating the problem of aligning two rails on divergent surfaces. Applications with a wide (about 300-mm) footprint may not work with a single guide because off-center loads can induce significant moments on the rail. Before specifying a profile rail, be sure to check its moment rating and stiffness under expected moment loads.
The most common of the linear bearing types are ball bearings.
The most commonly found, easily-available and typically-applied of the linear bearing types are ball bearings, despite the fact that they weren’t originally intended as such. As long as they’re not preloaded, the longevity of ball bearings is quite reliable due to their relative lack of friction. Since these bearings are pre-loadable, the tolerance they possess is very exact, and this prevents messiness and run-out.
Bearings can be reliable for a predetermined amount of time as long as they’re used correctly. Due to the contact between the respective metal surfaces of the shaft and bearings, however, the former is vulnerable to wear and eventual failure. Therefore, it’s crucial that bearings be replaced according to a set schedule, even if they’re in good condition. Otherwise, the shaft could get ruined.
In terms of application, ball bearings are most suitably applied to loads that are supported at one end. For motorized tools that necessitate rigid levels of tolerance, the most preferred bearing type is the ball bearing, which can also be used for processes that rely on manual movement. They’re even common in the operation of guard doors.
Selecting Linear Bearings
In order to determine which type of bearing is right for an industrial application, it’s crucial to know the nature of the application and how that impacts the material of a given bearing. Additional things to keep in mind include the prices and any further expenses that might arise if replacements are needed earlier than expected for any of the machine components. Further costs to consider include the possibility of lost productivity due to operation downtime.
Proper handling of bearings during the installation process is crucial for maximum performance. Shocks occurring at this time could lead to cracks and bends that could compromise the running elements. Bearings can also be damaged by external elements such as dust and debris, which can cause obstructions within the tracks. Harsh temperatures and weather conditions can also have a damaging effect. Corrosion, for example, can develop through exposure to heat or rain. Therefore, it is best to keep the bearings packed away in clean storage places when not in use.
Though some bearings manage to work when dry, a lack of lubrication is generally a prime cause for failure. Many users mistakenly assume, however, that the pre-applied manufacturer’s grease on newly purchased bearings is enough to last for the life of the product, but this is not the case. Proper lubrication on a regular basis is essential for maintaining the functions of bearings and to prevent metal-on-metal friction.
In applications that carry low loads, machine oils will suffice, and while such oils must be reapplied more frequently, they are good at draining out impurities throughout bearings. For applications that carry medium and higher loads, grease is the better lubricating option. Due to its consistency and higher viscosity, grease does not need to be reapplied as often as oil. Generally, lubrication must be reapplied either annually or for every 100 km of use. In that sense, it depends on application frequencies within a given period. Lubrication might even need to be reapplied with greater frequency if factors such as the environment have an impact on the bearings.
Inspections of bearings should be conducted on a regular basis. The easiest way to do this is manually, where you run your index finger across the shaft or rail. If the bearing happens to be lacking lubrication, reapply grease and oil.
The shafting should regularly be inspected for signs of corrosion. Indentions could cause interference with the alignment of the bearing. A shaft is more likely to fail if the applications consist of short strokes that barely exceed the length of the bearing. In the event that metal particles appear around the shaft or bearing, both might need to be replaced.
If you’re unable to repeat the same measurements under the same conditions on a consistent basis, it could be due to a number of errors, including faulty installation, carriage deflection or instability. For a more stable setup, it’s important to improve the stiffness, which can be done by switching to a different type of bearing. Repeatability can also be improved by increasing the size of the bearing, which could also help lower the possibility of deformation. Another way to make things more repeatable is to boost the stiffness as it applies to the mounting surface.
The frictional forces of a bearing can be brought down in order to reduce the amount of force needed to get a linear motion system moving. In order to cut down on push force, use recirculating element bearings in lieu of the plain contact variety.
In advance of making a final decision on a particular bearing type, it is wise to validate claims made by a given manufacturer via third party testing. Facilities that specialize in the science of interacting surfaces are good places to turn to for such requests because they are generally equipped with proper testing devices. This can provide you with the necessary assurance of a particular bearing and help maximize efficiency.
Choosing a Linear Bearing at NB
Nippon Bearing has been one of the world’s leading suppliers of linear motion products for over 75 years. Whether you need bearings for high or low-frequency applications, NB has the product for almost any type of linear motion.
Selecting linear bearings can often be a difficult process unless you know the specifics of an application as well as the range of conditions that could alter the performance of a given piece of machinery. Therefore, it is crucial to know how to select a linear bearing for your area of focus. At NB, we offer a collection of white pages that allow you to learn more about our products so that you may make an informed decision, whether you’re choosing a linear bearing or selecting rotational motion bearings.
To make the process of selecting motion bearings as easy as possible, we invite you to download our CAD files and learn more about NB products. This will help ensure the selection you make while choosing a motion bearing will be the most appropriate for your application in terms of load, frequency, speed, and direction. For even more information on how to select a linear bearing, watch our videos to see some of these products in motion.
With NB linear movement systems, you can expect the utmost in precision and performance with any application. Visit us online today to download our CAD files or to contact a representative for further information.