Linear motion takes shape

Robert Lipsett
Engineering Manager
Danaher Motion
Wood Dale, Ill.

Design engineers considering linear guides often struggle with which basic shape to choose: round or square. 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 techniques and economies of scale let square rails compete with round in a broader range of applications. Cost may still be a factor, though other metrics come into play, including load capacity, stiffness, accuracy, smoothness and geometry.

Square or profile rails target such applications as machine tools needing high-load capacity (typically about 20 kN for a 25-mm size), good stiffness (on the order of 1 to 4 μm/kN at 13% preload, for example), and high accuracy (3 to 10-μm/m parallelism between the bearing guide and rail). Trading ball bearings for cylindrical rollers can roughly double load capacity in the samesized package.

Profile rails get their performance, in part, through precise grinding of the rail ball tracks. The race 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 race. As a result, profile-rail bearings are roughly a factor of five stiffer under load than a round-rail assembly with its convex ball and shaft surfaces. Square rails can be preloaded from 3 to 13% of rated dynamic load. Heavy preloads apply an initial deflection to the bearings and cut deflection under load.

With load capacity, stiffness and accuracy all favoring profile rail, some engineers may look no further. However, round-rail designs can offer several advantages over profile rails, one of which is the ability to run smoothly when mounted to less-than-perfect surfaces, defined here as a flatness error greater than 150 μm/m. Profile or square-rail designs are especially sensitive to flatness errors that can cause binding, high drag, and up to 50% shorter life. 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 concrete factory floors. Because a ball bushing is free to rotate about a round shaft, one rail can sit at a slightly different elevation than the other. Height differences of 1 to 2 mm can be accommodated when rails locate 300 mm or more apart, for example.

Bearing races on most ball bushings are designed such that each linear bearing can rotate up to 0.5° in pitch and yaw to handle flatness or alignment errors. The pillow blocks that constrain a ball bushing as well as the rail supports are typically of aluminum. The flexibility of aluminum lets the assemblies handle up to 25-μm/m parallelism error between shafts. This is important because numerous factory-automation applications mount linear guides to a welded tubular frame.

Drawn tubing and sheet metal typically serve as mounting surfaces for OEM applications. A round rail, in these conditions, is simpler to install and runs more smoothly than a square rail.

Round linear bearings are capable of spanning gaps 12 to 24 shaft diameter, making them useful in a range of applications from gantry systems in factory automation to pick-and-place modules in DNA sampling machines. 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. Precision of the device depends only on the accuracy of the end-support mounting.

By comparison, square and profile rail are not designed to span gaps or be end supported. However, a single rail can support moments in all directions, eliminating the problem of aligning two rails on divergent surfaces. A single profile-rail bearing generally handles light loads within the load and moment capacity of the bearing, though not all profile rails are designed to operate in this manner.

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 spec’ing a profile rail, be sure to check its moment rating and stiffness under expected moment loads.

Design flexibility is often an important consideration as well. For example, linear-guide components may be modified for better function or to fit in an allowed space. Profile-rail bearings are more compact than round ballbushing bearings of the same load rating. But it is generally easier to modify a round-rail system than a square one. Round shafting can be end supported and used as a structural member in a larger assembly. And round shafts can accept diameters, flats, drilled holes both on center and radially. Ball bushings can be fit to a bore integrated directly in an adjacent component or installed in an aluminum or steel pillow block. Pillow blocks ease bearing mounting and modifications.

In other applications, smoothness is a key factor. There is a general lack of data in the linear-guide industry about the smoothness qualities of different designs. Of the two basic types, ball-bushing bearings (round rail) tend to operate more smoothly than profile rail. In fact, the difference can typically be felt by hand. The point contact of convex surfaces between balls and rail on round types minimize scuffing. Ball bushings typically run with 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 this is not objectionable, though for instrument-grade bearings, round may have an edge. Also, the simple seals on round bushings tend to be more reliable and add less drag than the wiper on profile rails. Profile-rail wipers must conform to the ball tracks for a good seal, a feature that may double drag compared to round linear bearings.

Another option for certain light-duty applications is nonball profile and round bearings. Here, high-performance bearing-grade polymers bond to aluminum and other substrates to replace rolling elements. Round-rail designs, for example, handle about 20% of the load of ball bushings. The friction coefficient of the polymer designs is about 0.05 to 0.25. Such bearings work particularly well in harsh environments where particulates would damage even a well-sealed ball bushing.

Polymer bearings also endure impact loads that would damage ball-bearing systems. And they run quieter for applications where ball recirculation noise in a ball bushing or profile-rail bearing would be a problem. The polymer distributes load over a large area so there is no need for hardened- steel shafting as with a ball bushing. Stainless-steel shafts can be used, which is suitable for clean-room semiconductor and medical applications, as well as in harsh environments such as food processing.

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Danaher Motion, danahermotion.com