Machine Design

When precision bearings are exactly right for motion control

A special category of bearing fits the bill when long life and quiet operation are on the "must-have" list.

Randy Burchell
Dave Roth
Danaher Motion
Wood dale, Ill.

When self-alignment, bearings can handle three common types of alignment problems. When the bearing's outer race can yaw in the direction of the shaft, it can stay in contact with the entire lenght of the shaft. This produces maximum load capacity and smoothness. Pitch self-alignment binding due to misalignments from shaft deflection, house-bore alignment, and machine deflection. Reducing wear due to uneven load is the function of roll self-alignment.

The realities of today's business climate force manufacturers to face tougher challenges than ever before. Conveying, packing, sorting, and other machines often operate 24 hr a day, seven days a week. The need for faster product shipments has made manufacturers speed up their equipment. As a result, hardware wears faster, loses position accuracy, and generates more audible noise, conflicting with OSHA requirements for quieter work environments.

One way to meet these challenges is with precision bearings. Although no one type of bearing is specifically labeled "precision," some need little maintenance and offer features such as low noise and long life at high operating speeds. These features may boost cost about 10%, but the return on that investment is longer life. Depending on the type, precision bearings could last eight times longer than nonprecision types.

For example, material-handling equipment such as conveyors usually use rolling-element bearings, although some applications use tapered or cylindrical-bearing elements. Whether precision or standard, a rolling-element bearing consists of an inner ring, an outer ring, balls, and a retainer or separator, along with various housings and sealing mechanisms.

One factor that puts bearings in the precision category is the tolerances held throughout the manufacturing process. Bearings that would fall in the precision category are also able to run at high speed, usually between 5 to 10 fps, and eliminate bearing binding and chatter. Bearings manufactured more precisely usually have a coefficient of friction of 0.001 to 0.002. A low coefficient of friction also reduces stick-slip, which is another factor to consider when selecting precision-type bearings. For applications where acceleration is important, precision bearings can handle higher accelerations than standard bearings, with rates to 450 ft/sec 2 .

Simply increasing the speed of a motion system can push noise levels over OSHA limits, currently set at 80 dB. Several equipment manufacturers think OSHA may lower that requirement to 70 dB in the next few years. Thus, noise dampening is a priority on most new equipment.

Ordinarily, bearings are not considered silent components, but newer designs incorporate features that help keep down noise.

To help dampen noise, precision-type bearings generally have smaller ball elements. The smaller the ball, the less contact area is available among the balls and the less space between them, all of which helps reduce chatter and noise. Plus, much of the steel that's not used for load transfer is replaced by plastic such as Delrin. The ball-return portion of the bearing is one area where plastic is used instead of steel because the balls are not under load. The bearing's main steel components are usually the shaft, balls, and bearing plate.

In conveying equipment, for example, not every roller needs to have a precision-type bearing to reduce noise or lengthen life. Conveyor manufacturers often put precision bearings in about one-quarter to one-third of the conveyor rollers. This strategy can also be used on existing equipment, replacing old, worn rollers with units using higher-quality bearings.

Some bearings come with dual plates. This reduces the total number of plates compared to single-track products. Two plates bear the load concurrently, reducing the effect of minor variations in housing bore on load-carrying capacity and increasing bearing life.

Proper alignment of all the elements, both in the bearing case and in the equipment, are crucial to bearing life, low noise, and capacity. Three particular types of misalignment are pitch, or shaft angular deflection or misaligned housing bore; roll, or distributed load on the ball tracks; and yaw, or skewing between ball tracks and shaft. Precision bearings have inner tolerances that eliminate misalignment and reduce play among components. Hardened-steel bearing components react like elastic bodies under load. This means the bearing components flex as the load-bearing balls move though them. Over time, material fatigue can set in, and they eventually fail. The tolerances in precision-type bearings should be much higher than in other bearings, which aids in tolerance stack up.

Bearings installed on equipment often must accommodate either dynamic or static misalignment. This may arise from such factors as a warped roller shaft or a deflected head shaft on a loaded belt conveyor. Chatter is often an initial sign of misalignment in the system. Spherical roller bearings can accommodate some wobble while carrying full system load. Precision-hardened rings let a bearing find its proper position under load, which evens wear and lengthens life.

Self-aligning bearings compensate for misalignment from imperfections in housing-bore roundness and parallelism, deviations in flatness of mounting surfaces, imperfect system assembly, or deflection at load.

Maintenance is a big issue with motion equipment because typically, once installed and turned on, equipment tends to run until it fails. And most bearing failures are due to improper installation, lack of lubrication, and contamination. Lack of lubrication is probably the most common issue. Most precision bearings have a lubrication port for oil. But the design of medical equipment often tries to avoid periodic lubrication for sanitary reasons. Applications that cannot be lubricated periodically should use grease because it lasts longer than oil. Plus, grease tends to better dampen noise. Grease is generally used in high-load, low-speed applications, from 3 to 5 fps. Oil is recommended in high-speed applications.

Proper installation can also minimize maintenance. Do-it-yourself customers should pay attention to maintaining tight mountings and alignment, and eliminating vibration. All bearing manufacturers have written procedures and recommendations that should be followed to ensure correct shaft seating, mounting tightness, and unit alignment.

Alternatively, the bearing manufacturer can be brought in at the beginning of equipment design. Bearing manufacturers typically look at load, speed and life along with other critical factors. Also they can help with drive-system selection as well as other components.

In general, nearly any application can benefit from precision-type bearings. Precision bearings are increasingly used in medical and semiconductor applications, especially for pick-and-place equipment. As semiconductor chips get smaller, positional accuracy becomes more critical. And even in standard applications, OEMs are turning to precision bearings because they offer more extensive warranties or a differentiating factor from the competition.

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