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Antifriction linear-motion bearings.

Does your machine tool suffer loss of alignment and precision from wear of close-fit plain bushings or V-ways? It wouldn't It if it used modern components. Here's a report on the design and application of antifriction bearings. Certain types can handle loads up to 18, 000 lb per bearing.

Machine tools should have linear bearings with low coefficients of friction from 0.001 to 0.004 to provide high efficiency and low power requirements-allowing use of smaller drive units and power-transmission systems. There should be no vibration, stick-slip, or chatter-ensuring smooth operation and precise positioning.

Furthermore, bearings should have long life, low maintenance, and accurate alignment-without depending on an oil film. There's "no way" with conventional V-ways, but there's a viable alternative for all but the largest machine tools. That alternative is the linearmotion device that rides on antifriction roller or ball bearings. Such bearings have been around for a long time, but today they are improved, and there's an easier way to select the best model.

Organized assemblies

A significant development in the design and application of antifriction linear-motion bearings has been the introduction of the Systems Concept. It's an alternative to the conventional method of building and designing a linear-bearing system-a multi-step process involving identification and selection of the type of bearing or pillow block, proper size, matching inner race shaft), type of support, carriagetop design and other options.

In the new systems method, the engineer simply specifies functional linear-motion systems that are already pre-engineered and preassembled by the bearing manufacturer. They are shipped complete, ready to install by the customer.

Preassembled linear-motion systems are currently available in two general groups: (1) those systems that require some shaft alignment at installation time, and (2) systems that come with prealigned shafts (the bearing's inner race) and a compact carriage, ready for bolt-down installation right out of the shipping container.

In the group requiring shaft alignment, there are nine basic configurations. These are single-axis systems in three basic variations: Single shaft with two pillow blocks; parallel shafts, each with two pillow blocks; and parallel shafts with tabletop or carriage. Preassembled systems that are fully supported along their entire length to prevent shaft deflection are available in unlimited lengths for extended travel.

These systems, which require some shaft alignment at installation, are characterized by the use of standard precision antifriction bearings and related components that are precisely matched for critical dimensions and fit-up. For example, in Thomson systems, parts can be matched to within +/- 0.0005 ". This results in systems with tolerances up to 80 percent closer than those assembled onsite by customers using individual components. Sold aligned

Systems in the second group have parallel shafts bearing inner races) that are prealigned by the factory. They use a matched carriage of compact design and provide torsional resistance. They, too, are ready for immediate bolt-down installation, offering time and labor savings.

Within the prealigned-systems group, there are three basic types: An end-supported, compact Twin Shaft Web[Trademark] System; a continuously supported, low-profile Dual Shaft Rail[Trademark] System; and a heavy-duty, rigid [Trademark] Table System.

The Twin Shaft Web linear-motion system (two hardened-andground shafts connected along their full lengths aligned by a steel web) suits applications where the shafts are supported at the ends or used in a cantilevered arrangement. The web-utilized shafts improve system rigidity up to 16 times and provide torque-resistant linear motion in a compact package.

When webbed shafts are horizontally oriented (side by side), rigidity is improved by 75 percent over unwebbed systems; when vertically oriented (shaft over shaft), rigidity improves by 94 percent over unwebbed systems.

Dual-shaft rail systems (two hardened-and-ground steel shafts mounted parallel in alignment on a U-shaped aluminum base) provide low-profile packages for limited space. They install quickly and offer virtually deflection-free (less than 0.001 " under full-load rating) continuously supported shafts.

This system offers accurate 0.002 ipf in both travel planes), almost friction-free carriage travel under heavy downward, upward, sideward, and torsional loads. Its principal benefits are elimination of the need to align the shafts parallel for extralong travel length, and retention of 100 percent of load capacity in the inverted (upside-down) position.

The third system in this group is the TX Table for heavy-duty precision machining, assembly, and inspection. It includes two hardenedand-ground steel shafts mounted on continuous steel shaft supports,-and four self-aligning antifriction linear bearings. Its shafts are precisionaligned at the factory, with predrilled mounting holes in the base. Self-aligning benefits

What's happening to the bearings themselves? There are new products here, too. For one thing, industry now has a self-aligning bearing for those high-load applications that require precision and extremely long life. Some have three times the load capacity, or 27 times the travel life, of conventional linear ball bearings.

Major benefits of the self-aligning feature are found in easier instalations.

Precision at high loads

There are two ends to the spectrum of antifriction linear bearings. At one end is a general-purpose recirculating ball bearing that has been used by industry for many years. It's characterized by very smooth movement, accurate travel, and the capability for rapid motion. At the other extreme is the recirculating linear roller bearing that travels on flat ways and provides extremely high rigidity. It offers high load-carrying capacity in a low-profile configuration, while operating at slow speeds. The bearing provides rolling contact between the flat way and the moving machine element. It consists of a bearing block around which rollers recirculate. Rollers on one side of the block contact the way and carry the load. The other side of this block has a return passage that permits the rollers to recirculate back to the load side.

One drawback, however, is the tendency for the rollers to skew. A number of modifications, including addition of close-fitting plates, have reduced skewing, but they have not been able to eliminate it at higher speeds, thus limiting the bearing's performance.

Because these bearings can take loads in only one direction, at least one bearing must be installed on each side of the rectangular way to resist side and pull-off loads. A typical installation would use 12 bearings, for instance, on a machine-tool carriage that has three bearings at each corner. Furthermore, exacting way alignment is required, and the bearings usually have to be individually shimmed into position-all of which greatly increase initial product and installation costs.

Another design, recirculating ball bearings on keystone ways, provides low friction, high load capacity, and rigidity in a compact silhouette. The high capacity comes from the combination of a ball-conforming race in both the bearing and shaft inner races.

The keystone system serves where high load capacity is required, but not as high as that specified for roller bearings on flat ways. By using a single rail and two bearings, it will support moderate overhanging loads and still operate smoothly as a single way structure.

Although the keystone design does provide good resistance to deflection under side and pull-off loads, the penalty is high rail cost. The rail must be ground to a complex shape and to extremely close tolerances. Also, it's difficult to replace the matched components because the individual parts are custom selected and are not usually interchangeable.

Limits of sliding ways

In conventional linear-motion devices, flat ways, V-ways, rectangular ways, and dovetail ways support the load. Each employs flat metal surfaces separated by a thin film of lubricant. Sliding ways can handle heavy loads and be extremely rigid, with high resistance to deflection.

Friction is reduced to an acceptable level in these systems as long as the film of lubricant is maintained between the two sliding members. But when the linear motion stops, even momentarily, the lubricant can be squeezed from between the sliding surfaces by the dead weight of the load.

Once a tear develops in the lubricant film and the machine restarts and movement resumes, metal-to-metal contact occurs between the two sliding members. This wear-causing condition remains until the lubricant is once again uniformly distributed between the sliding members.

Covering sliding surfaces of a saddle with wear-resistant plastic eliminates metal-to-metal contact and provides lower coefficient of friction, but this solution can be costly.

In most sliding-way systems, when the surfaces are not lubricated or are only partially lubricated, start-up forces are high because of increased friction. Similarly, this same friction causes stick-slip when the way motion begins, leading to nonuniform acceleration and deceleration during travel. But jerky movement is not acceptable when an accurate response to a computercontrol command is sought. Here, movement must be fast, smooth, and at a uniform rate of acceleration.

Sliding ways, themselves, are highly accurate systems, but this accuracy comes at a rather high price. The installation process is laborious and time consuming, because the two ways must be placed to exacting standards of neatness and parallelism. Another cost is the need for on-site pressurized lubrication to insure maintenance of the thin film of lubricant between the sliding surfaces of the way system.
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Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Title Annotation:includes related articles
Author:Bandrowski, John C.
Publication:Tooling & Production
Date:Jan 1, 1989
Previous Article:Managing technical data on the factory floor: new type of reporting system takes advantage of existing DNC networks.
Next Article:Measuring flat, straight, and tilt.

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