Patrick Scott
Danielle Collins
Linear Motion & Assembly
Technology Group
Bosch Rexroth Corp.
Charlotte, N.C.

Precise electronic motion controllers are certainly important in today’s advanced manufacturing environment. But to get the most out of the controllers, companies need mechanical systems that can take full advantage of their instructions. After all, linear-motion hardware executes the motion. Engineers should be aware of a few issues to get the most performance out of their machines.

You’re building a system

Precise motion calls for rightsized components to handle the load. Axial or torsional loading, for example, often requires wider or heavier-duty components than simple radial loading. But remember, you’re building a system, not just buying and assembling a collection of linear-motion components. For example, a frame design that’s too light will affect precision more than any guide component’s specified accuracy. And you also don’t want to overpay for heavy-duty components simply because you’re compensating for other weaknesses in the system.

In addition, it’s important to consider other design criteria, such as the environment the system will operate in, the angle at which loads are mounted, along with the required speed, travel distance, and duty cycle.

Accuracy depends on the linear guide, the trueness and flatness of its rail, raceways in the bearing through which balls or rollers travel, and a host of other factors. But for high-performance linear bearings, the most important factor is how smoothly balls recirculate in the runner block as it travels down the rail.

Applications at the high end

of the accuracy spectrum, such as gaging, coordinate-measuring machines, microelectronics, even metal cutting, can be degraded by small movements of balls in the recirculation chamber, or by just the carriage (or runner block) slightly pivoting about its axis. Any deflection or clearance at all reduces accuracy, as does any roughness in the recirculation of the balls, even when coupled with highly sophisticated motion controllers. Nonrecirculating linear systems such as cross-roller slides and air bearings often allow only a limited stroke or require complex air supplies and heavy, polished granite supports. A less-costly alternative would be accurate linear guides.

Accurate linear guides have smooth recirculation pathways that eliminate roughness at key transition points. Such guides allow for consistent performance, letting engineers compensate for minor deviations in control for near-perfect accuracy.

Ball screws are typically selected to drive motion in highperformance machines based on their rigidity, precision, and speed. And their ability to handle substantial axial loading often makes them a better choice than linear motors, particularly for cutting metal, wood, and stone.

Ball screws are manufactured in a wide variety of accuracy classes, letting designers select the one that best meets their needs. In addition, rolled ball screws now rival the performance of ground screws in a given accuracy class, thanks to advances in technology. So with rolled screws being more cost effective than equivalent ground screws, many designers are taking a new look at them, especially for applications that need a Class 5 screw, which is the highest level of precision rolling technology can provide.

As with runner blocks, ball recirculation inside the ball nut can affect precision. As a result, ball nuts have preload options that reduce play as the nut rotates around the screw. Preload can be generated by oversized balls inside the nut housing, using the so-called “double- nut” or “jam-nut” method, or by using a manufactured offset in the raceway spiral to change the angle of ball engagement (the “lead-shift” method). These three methods also minimize backlash between the nut and screw.