Engineers must consider many factors when specifying linear components for a motion system, especially if one or more reaches an extreme. Orientation, for example, can significantly influence the load handling ability of a system, as well as its lubrication needs.

What could be simpler than machine slides, linear bearings, and other linear components? Individually, they are fairly straightforward to apply. But combine them into multidimensional linear systems, and the level of design difficulty escalates.

Many designers respond by over specifying, which brings on other problems, including increasing cost of ownership. Calling for bearings that are more precise than needed, for example, can increase the expense of the entire system as well as upkeep.

Underspecifying, though, is even worse. Here, inadequate specifications and patchwork fixes breed "specification creep" as engineers beef up one component to fix another. Substituting a larger, heavier motor on moving equipment, for example, may provide the required speed but demands bigger, more costly bearings to carry the additional weight.

To avoid such pitfalls, designers need to look at the whole picture. They need to analyze such variables as load, orientation, speed, travel, precision, environment, and duty cycle. Any time one or more of these parameters exceeds reasonable limits, designers need to be on high alert.

Finding L O S T loads

Careful analysis of an application, including the expected orientation, speed, and travel, will reveal the load that must be supported. Sometimes, though, the actual load the design will experience will vary widely from the calculated load. To stay out of trouble, designers may need to analyze load in a broader context, anticipating for possible misuse as well as intended use.

Machine operators who use a linear bearing as a step or sit on a machined slide during a break are familiar stories. So too is the resulting compromised system operation.

Travel requirements affect whether applications with high deceleration forces need ball screws rather than belt drives. However, very long ball screws may force designers to look at rack-and-pinion drives or linear motors because of the potential for whip.

If such events are probable, then engineers may be able to make different design decisions without affecting other system needs. For example, using roller bearings on the linear bearing, which can carry heavy loads, instead of ball bearings may solve the additional weight problem without increasing cost or specification creep.

Another factor that affects loads and the overall design of a linear motion system includes orientation or plane of travel. Some bearings can carry inverted loads without difficulty. Vertical or inverted slides, however, can lose lubrication to gravity, and dry bearings quickly burn out under heavy loads. Solutions include pressure lubrication systems that help oil overcome gravity; grease, which usually lubricates moving parts in unusual orientations better than oil; and extended lubrication adapters with wicking reservoirs for bearing blocks.

Speed and acceleration are important factors in determining actual loads for linear bearings and drives. Moving a ten pound load ten feet may be simple, but moving the same load the same distance with an acceleration of 10 G is not. Load speed, acceleration, and deceleration determine, in part, whether a ball screw, belt, linear motor, or rack-and-pinion drive is the most appropriate choice to achieve a desired travel and accuracy.

Travel, whether long or short, can have farreaching consequences on linear motion systems. For long runs, linear bearings must be parallel to prevent binding. Joints between rails must be ground flush to eliminate railroad-like chatter. At the other extreme, short strokes may deny recirculating bearings necessary lubrication, making them subject to fretting corrosion.

A belt drive may seem the best way to achieve long travel, but rapid deceleration may cause the belt to skip teeth, compromising precision. Alternatively, a long ball screw with excessive whip may force designers to look at rack-and-pinion drives or linear motors.

P E D-estrian concerns

In linear motion systems, precision includes both accuracy of travel and final position as defined in all three axes. Requirements vary greatly with the application. An inspection system for computer hard disks, for example, demands micron precision and justifies position encoders and closed loop controls. A common material handling system with less demanding requirements, on the other hand, may reach position adequately without feedback devices.

Whatever the precision requirements, though, overall accuracy depends on the composite accuracies. Mounting the most accurate bearing on an inaccurate milled aluminum base will eventually deform the rail and compromise the precision of the entire system. Engineers must also consider overall system stiffness and deflection.

Continue on page 2