Selecting Bearings To Carry And Guide Loads

April 20, 2000
Track rollers use a large-diameter bearing surface to carry loads and a small-diameter bearing to guide the loads.

Track rollers use a large-diameter bearing surface to carry loads and a small-diameter bearing to guide the loads.

Four track rollers carry and guide a horizontal table. A similar setup mounted vertically can guide a lift mechanism.

When selecting bearings for linear-motion applications the factors to consider include load, speed, life, precision, tracks, and environment. A balance of these factors is often necessary to select bearings that will perform as required over the long run.

Load is dictated by the application. The load that the bearings must carry can be reduced by increasing the number of load-carrying bearings. In this way designers can fit several smaller bearings to increase the assembly's load capacity.

A rule of thumb helps account for radial loads and thrust loads. Although it varies by manufacturer, the following can be applied for determining resultant loads for roller bearings: When thrust loads are less than one-half of the radial load, select the bearing based on the applied radial load. When thrust loads exceed one-half of the radial load, select the bearing by summing the thrust and radial loads and using the total as the applied radial load.

Don't forget static, dynamic, and shock loads. Shock loads are intermittent high forces that occur in static or dynamic states. Select bearings based on the highest expected shock load.

Speed requirements and the load of the application will determine the life of the bearing. For example, when a roller bearing is used in a wheel, the rotational speed of the bearing can be reduced by increasing the wheel diameter. However, applications typically have limits on the maximum wheel diameter, so there is also a corresponding maximum speed.

Life of a bearing, using L10 values, is considered to be 20% of the average life of the bearing. The L10 rating is often misunderstood because the average term is not acknowledged. The L10 rating is a statistical number (hours of operation) of a bearing lot operating at a particular speed and load, where 90% of the bearings in that lot will not fail or fatigue. For example, if 100 bearings with an L10 life of 500 hr run at a given speed and load, it is expected that up to 10% of the bearings may fatigue or fail prior to the 500-hr period.

Precision of a linear-motion assembly is determined by the precision of the bearing and that of the track. Precise motion requires a machined track. Linear bearings need clean precision-ground tracks, thus they can only be used for precision motion. Roller bearings roll on sealed internal precision races, which lets them roll on machined or unmachined tracks. Therefore, rollerbearing linear-motion devices are capable of providing precision as well as nonprecision motion.

Environments in which bearings will operate are often neglected, which leads to premature bearing failure. Linear bearings in dirty environments fail from worn seals that allow contaminants on the bearing track. The ball bearings eventually seize. Once they have seized, they will damage the track, which will then need replacement.

Roller bearings are less susceptible to contamination because their seals ride on precision-machined internal surfaces, but the wheel surface rolls over the track and the track contaminants. With sufficiently hard wheel and track surfaces made from the proper materials, contaminants will not cause wear.

Two-Axis Track Rollers from APT have guide rollers integrated into the stud of the thrust roller. This provides a single device that carries and guides loads. Track rollers are specifically intended for medium to heavy linear-motion applications in dirty environments. They are suitable for precision and nonprecision applications. In addition, the company provides custom roller and track designs for linear-motion applications in a variety of industries, such as robotics, web winding, marine, and material handling.

This information supplied by A Plus Technology, Coram, N.Y.

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