Linear Motion & Assembly
Bosch Rexroth Corp.
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.
In metal-cutting machines, tool travel is typically short, so proper end supports can help raise speeds. However, screw “whip” can be a problem in highspeed, long-stroke applications
Whipping causes unwanted chatter and imprecise cutting. To overcome this, many manufacturers offer balls screws with a driven nut option, in which the nut rotates along a stationary screw. Coupled with a measuring device on the rail guide, nut drives can be as precise as linear motors, even with long strokes. And the cost of ball screws, even for hightech rotating driven-nut systems, is still considerably less than that of a linear motor.
In some linear-motion applications, such as Cartesian robots, the “machine” consists primarily of linear-motion components and an electric controller. These robots are often assembled from linear-motion modules or actuators, which may be used in one axis, or combined in two or three axes with simple mounting plates. Each module has a mechanism to guide motion and one to drive motion. In many cases, the module is one or more linear guides plus a ball screw mounted in an aluminum extrusion or on a steel base. (If precision is not critical, the drive is often a toothed belt.) So, with manufacturers combining a wide range of components inside different housings, engineers should be able find a module that suits their application.
Don’t forget the housing
Besides the internal ball screws and guides, the housing is also crucial when it comes to precision. The most widely used housing material is extruded aluminum, which can be made into long continuous lengths. And the mounting surface or number of supports, along with the load, determine if an aluminum housing will deflect. The housing and guide also affect rigidity. Some systems, for example use two ball rails in an aluminum extrusion to add stiffness.
Modules mounted on a steel base or in a steel housing can provide levels of travel accuracy aluminum extrusion-based housings cannot match. Steel can also be machined more precisely than aluminum, so it makes for flatter or straighter travel. Steel is also more rigid than aluminum, so when steel is used, full mounting support is not required.
In summary, engineers designing highly accurate linear-motion systems should use precise electronic systems and mechanical components. So factors such as component sizing, linear-guide and bearing design, ball-screw and nut options, and housing materials are just as critical to accuracy as high-tech controls.
For more on factors to consider when selecting linear-motion components, download the Lostped white paper at www.boschrexroth-us.com.