Straight talk on linear motion

Aug. 1, 2000
The path to complex linear systems is rarely straight. When your designs start throwing you "curves," these techniques can help bring them back in line.

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.

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.

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Environmental extremes, including temperature and cleanliness, also affect linear motion system design. For example, water-cutting machines are often covered with garnet-filled swarf that can destroy unprotected bearings. Engineers facing such dirty, dusty, or corrosive environments should specify flexible shields or pressurized slides to keep contaminants out. Alternatively, linear systems in sensitive clean rooms may require covers to keep in lubricants or other contaminants.

The environment also influences the material make-up of linear components. A robot feeding a baker's oven may need steel bearings rather than plastic to counter sustained heat. Similarly, metal high-temperature bellows may be preferred over plastic shields in hot applications. Extreme heat or cold also dictates the type of lubrication used in linear bearings.

Limited space in the operating environment commonly packs linear components into smaller footprints. In some cases, space limitations will drive innovative designs. For example, to provide opposing motions in a diaper-packing machine, one design solution uses a single ball nut to drive two ball screws with opposing threads. The arrangement achieves the same result as a traditional drive with two motors, but it saves significant space.

Duty cycles – how often the motion starts and stops – strongly influence system specifications. Fast cycles with little settling time degrade positioning accuracy. In one difficult application, a barcode reader with a compact slide had to rapidly move, settle, then move again. If the reader was still moving during the dwell period, it could misread codes. Thus, the system engineers had to overcome the natural spring nature of this linear system to satisfy the application.

Rapid cycles, such as those found in automotive and other manufacturing applications, also generate heat that changes the dimensions of linear components, eroding precision. Therefore, designers will specify cooled ball screws in machine tools to guarantee consistently accurate parts.

Another consideration is the use of engineered subsystems or customer integrated components. Often, for best results and overall accountability, designers should work with a supplier capable of integrating components at this level.

Scott Spangler is an application engineer, and Kristina Kuehnel is product leader at Star Linear Systems, Charlotte, N.C.

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