Machine Design

Consider the Whole System When Choosing Linear-Motion Components

Industrial designers who specify linear-motion systems for unusual manufacturing applications must understand and integrate multiple requirements. Although it may seem simple to select individual components, such as machine slides, drives, and linear bearings, engineers need to consider the complex interactions between components. The issue is especially relevant as automation engineers pack more speed and precision into smaller packages, and as smarter computers open new motion-control opportunities.

Oversizing linear-motion bearings, motors, and controls is an expensive mistake. Likewise, addressing inadequate specifications and making patchwork fixes along the way leads to “specification creep” as engineers beef up one component to fix another. In other words, substituting bigger, heavier motors on moving equipment achieves necessary speed but demands bigger, more costly bearings to carry the extra weight of the motor.

For “difficult” linear-motion applications, designers need a broad understanding of the system requirements during initial designs. Linear-motion requirements are spelled out in a convenient acronym, LOSTPED, which stands for load, orientation, speed, travel, precision, environment, and duty cycle. Difficult applications are classified as those that take one or more LOSTPED parameters to the extreme. Any time one or more of these parameters exceeds reasonable limits (which depend on the application), designers need to be on alert.

Load is the first parameter to consider. Careful analysis of the application, including orientation, speed, and travel will reveal the load that must be supported. Sometimes actual loads vary from the calculated load, so drive designers must consider intended use and potential misuse.

Orientation, or plane of travel, has dramatic implications on loads and the overall design of linear-motion systems. For instance, some bearings can carry inverted loads without difficulty but vertical or inverted slides can lose lubrication to gravity. Dry bearings under heavy loads burn out quickly. Pressure-lubrication systems let oil overcome gravity, and grease can lubricate moving parts better than oil in orientations in which gravity is a concern. Extended lubrication adapters with wicking reservoirs lengthen intervals between lubrication.

Speed and acceleration also impact actual loads for linear bearings and drives. Moving a 10-lb load 10 ft may be simple, but moving the same load the same distance with 10-g acceleration is not. Load, speed, acceleration, and deceleration help choose between a ball screw, belt, linear motor, or rack and pinion.

Travel length is also an important factor for linear-motion designs. For long runs, linear bearings must be parallel to prevent binding. Joints between rails must be ground flush to eliminate chatter. At the other extreme, short strokes may not allow proper lubrication in recirculating bearings, possibly causing fretting corrosion. A belt drive may seem the best option for long travel, but rapid deceleration may cause the belt to skip teeth. And long ball screws may have excessive whip, forcing designers to consider rack-and-pinion drives or costly linear motors.

In linear-motion systems, precision includes travel accuracy and final position. Mounting the most accurate bearing on an inaccurately milled aluminum base deforms the rail and compromises the precision of the entire system. Engineers must also consider overall system stiffness and deflection. Requirements vary greatly with the application. For example, inspection systems for computer hard disks demand micron precision and justify position encoders and closed-loop controls. Material-handling systems have less demanding requirements and need no costly feedback devices.

Environmental extremes, including temperature and dirt, impact linear-motion designs. Dirty or corrosive environments may require flexible shields or pressurized slides to keep contaminants out. Linear-motion systems in clean rooms may need covers to keep lubricants or other contaminants in.

Duty cycle, meaning how often the motion starts and stops, is also an important design parameter. Fast cycles with little settling time degrade positioning accuracy. In a recent difficult application, for example, a bar-code reader with a CKK compact slide had to move rapidly, settle, and move again. If the reader was still moving during the dwell period, it would have misread codes. The system engineers had to overcome the natural spring nature of the linear-motion system to satisfy the application.

This information supplied by Star Linear Systems, Charlotte, N.C.
For more information, Circle 630.

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