Linear Modules and Cartesian Systems

How to Choose a Linear Module

Need help selecting the correct module? Use the table below to narrow your choices. The table lists some, but not all of the important factors to consider when selecting a module. For final sizing and selection of the correct module, please consult one of our distributors or our Application Engineering department.

Product	SOLUTIONS TABLE (1=GOOD, 3=BETTER, 5=BEST)
Product	PSK	CKK	CKR	MLR	MKR	MKK	TKK	SGK	SOK
Load Capacity	4	4	4	3	4	5	5	2	3
Thrust Capacity	3	4	3	2	2	5	5	4	4
Capable Stroke	2	3	4	5	5	4	3	2	3
Speed	3	3	4	5	4	3	3	3	3
Low Profile	5	5	5	4	4	2	3	3	3
Positional Accuracy	5	5	3	3	3	4	5	5	5
Travel Accuracy	5	3	3	3	3	3	5	2	2
Repeatibility	5	5	3	3	3	5	5	5	5
Low Cost	5	3	3	4	3	2	1	4	4

LOSTPED - Consider the whole system when choosing linear motion components.

Designers who specify linear motion systems for unusual manufacturing applications must understand and integrate multiple requirements.Machine Slide 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 concept development. Below you will find seven important factors in selecting linear products. These 7 factors can be easily remembered with the acronym LOSTPED.

Load is the first parameter to consider. Careful analysis of the application, including orientation, load moment and acceleration 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 reduce gravity effects with oil lubrication, and grease may be preferable to 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 not be a problem, but moving the same load the same distance with 322 ft/s2 (10g) acceleration may be more difficult. Load, speed, acceleration, and deceleration help choose between a ball screw, belt, linear motor, or rack and pinion drive.

Travel is the product of twice the stroke length and the total number of cycles anticipated before motion component replacement. For long strokes, linear bearings must be carefully aligned to avoid additional friction and bearing fatigue. Joints between rails must be carefully matched. At the other extreme, short strokes may not allow proper lubrication in recirculating bearings, possibly causing fretting corrosion. A belt drive may be a good option for long strokes at high speed, but acceleration must be controlled to avoid oscillation or even belt damage. Long ball screws may have critical speed problems. In some cases, it may be necessary to consider rack-and-pinion drives or costly linear motors.

Precision includes travel accuracy and final position. Mounting the most accurate bearing on an inaccurately milled 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 what proportion of time the system is operating, is an important design parameter. This affects the heating of the motor and possibly other motion components. This then determines the torque that can be safely applied. Care must be taken to consider the difference between short stops every cycle and long stops, perhaps overnight. Even though the overall duty cycle may be 25%, during use the duty cycle may be 90%. The latter figure will determine the allowable motor torque or allowable force for a linear motor.