Basics of Design Engineering: Bearings

April 20, 2000
Air bearings for linear applications, segmented bearings that can be installed into hard-to-reach places, and thermoplastic bearings molded with integral pulleys or gear teeth are just a few of the recent developments from bearing manufacturers.

Edited by Paul Dvorak, , Kathleen Franzinger


Ball-rail systems

Star ball-rail systems are for machine tools and industrial robots needing rolling-element linear guides. The systems from Rexroth Star, Charlotte, N.C., are

available in up to five accuracy and four preload classes, each with high load-carrying capacity and rigidity. The compact systems come in eight industrystandard sizes and offer the same load rating in all four main-load directions. Individual components can be replaced at any time.

Circle 654



CW Bearing produces medium to high-volume bearings for OEMs. Bore sizes

range from 0.04 to 0.75 in. The company manufactures in China to ISO and ABMA standards. CW Bearing USA, Charlotte, N.C., oversees the distribution to the U.S., and can provide PPAP docu-

ments, CPK sampling, and

material certifications upon customer request.

Circle 655


A series of miniature profile guideways make precision motion control economical for tightly packaged applications. The Minirail series is a family of robust, two-row miniature guideways made from stainless steel. An advanced ball-retainer design simplifies assembly and

The minirail guideways come in 7, 9, 12, and 15-mm sizes. Available lengths range from 360 to 950 mm. Two preload and accuracy classes are available. Product dimensions conform to DIN 645-
2. Single-piece prices start at $75 (one MN9 carriage and one 195 mm rail).

maintenance, reduces total implementation cost and increases versatility.

Applications for minirail guideways include automated printed-circuit-board assembly, microelectronics, semiconductor production, metrology, optics positioning, medical and pharmaceutical packaging, and analytical process equipment.

The guideways are made of corrosion-resistant, throughhardened stainless steel, making them suitable for demanding applications.

This information provided by Schneeberger Inc., Bedford, Mass.

For more information Circle 667

Brochure highlights hydrodynamic-pressure bearings

Anew brochure, from Quality Bearings & Components, New Hyde Park. N.Y., features a

uniquely constructed series of

hydrodynamic-pressure bearings that weigh 30% less and cost less than conventional ball bearings. The brochure, identified as 8607, is

available from the company upon request.

Hydrodynamic-pressure bearings provide long-lasting, high-speed, quiet operation and are suitable for light applications, such as compact fan motors, paper feeders in copy machines and printers, laptop computers, and micromotors. The bearings, identified as BBSPLN-L Series, are formed by stacking "petalshaped" laminated aluminum-silicon

alloy plates that are housed in a stainless-steel case. The case has a polyurea-thickened grease containing antioxidant and anticorrosion additives. The inner faces of the laminated plates act as the bore of the assembled bearing.

When the shaft rotates, a fluid between the shaft guide and the shaft generates pressure and maintains a clearance of 7 to 10 m. The seven bearings in the series have IDs ranging from 1 to 6 mm and a maximum speed range from 6,000 to 20,000 rpm.

Circle 656

Online catalog sizes bearings and components

It doesn't take long to figure out after collecting several catalogs on CD ROM that if these things were online, you wouldn't need a 50-CD changer for the engineering department, or to hire someone to keep the stack updated. One online catalog

Selecting one of the bearing categories lets users download a PDF file of available sizes. Another page lets users download component drawings at no cost after they register for an ID.

from WM Berg Inc. (, East Rockaway, N.Y., can alleviate the problem. The initial screen asks for a productpart number or one from a competitor. If that is not available, users can select from one of about 29 categories, such as assemblies, bearings, belts, clutches, electromechanical components, and gears. Bearings has 13 categories.


A universal cam follower called Hexalube includes a lubrication port in its hexagonal head. The handy location makes it possible to lubricate it in tight spots. In addition, the universal design can eliminate the need to stock screwdriver slot and unsealed cam followers. The manufacturer expects most users to reduce their inventory on stud cam followers by 50% using the new universal line. The complete line is available in OD sizes from 0.75 to 7 in.

The line of Hexalube universal cam followers is dimensionally interchangeable with other needle-bearing cam followers. The universal cam follower is available in eccentric and crowned versions, and with standard and heavyduty studs.

This information provided by Roller Bearing Company of America, West Trenton, N.J.

For more information, Circle 657

Consider hybrid bearings to ward off current damage

bearing failure. A conventional solution minimizes risk by providing an effective insulation between the bearing shield and housing or between the shaft and bearing. A simpler and more economic solution consists of providing a current-insulated rolling bearing at one of the two bearing locations. Deep-groove ball bearings and cylindrical roller bearings have an oxide ceramic layer applied by plasma coating on the outer ring's outside diameter. Its faces have proved suitable under field conditions. The hard layers feature high electrical and wear resistance. Load-carrying capacity and service life of the bearings are identical to those of standardized bearings. They have the same dimensions, so retrofitting is possible.

In one application, the bearings on a special electric machine require current protection. Hybrid bearings with steel rings and ceramic balls provided an alternative to coated deep-groove ball bearings. The ceramic balls provide the current insulation.

Hybrid bearings offer several other benefits as well. For example, designers can increase speed, and reduce friction and temperature, especially with sealed hybrid bearings. Their longer grease life leads to a longer service life.

This information provided by FAG Bearings Corp., Danbury, Conn.

For more information, Circle 658

Rolling bearings in electric machines can be damaged by current passing through them.

Under the worst conditions, craters and fluting in the raceways cause premature

A few guidelines for fitting miniature bearings into assemblies

The operating characteristics of a system can be drastically affected by the way its ball bearings are handled and mounted. A bearing that has been damaged due to excessive force or shock loading during assembly, or is fitted too tight or loose may cause the device to perform below expectations. By following a few general guidelines during the design of mating parts and by observing basic precautions in the assembly process, the devices are more likely to perform as expected.

Assemblers should observe these guidelines:

Account for the effect of differential thermal expansion when establishing shaft or housing sizes. The partial table of recommended fits assumes stable operating conditions, so when thermal gradients are present, or dissimilar materials are being used, room temperature fits must be adjusted to attain the proper operating temperature.

When miniature and instrument ball bearings are interference fitted (either intentionally or as a result of thermal gradients) estimate the reduction in bearing radial play by 80% of the actual diametrical interference fit. This 80% figure is conservative, but it is a useful design guideline. Depending on materials involved, this factor will range from 50 to 80%. The following is an example of calculating loss of radial play:

Radial play of bearing = 0.0002 in.

Total interference fit = 0.0003 in. tight
80% of interference fit = 0.0003 (0.80) = 0.00024 in.

Theoretical resultant radial play of bearing = 0.00004 in. tight
Theoretically, this bearing could be operating with negative radial play. A bearing with excessive negative radial play is likely to fail early. However, the above calculation is for design only and does not take into account housing material, shaft material, or surface finish of the housing and shaft. As an example, if the finish of the shaft surface is rough, a part of the interference between the inner ring and shaft will be absorbed by the deformation of the shaft surface. This will reduce the overall interference fit, and thus, the radial play of the bearing will not be reduced as much as is shown in the calculation.

Preloading is an effective means of positioning and controlling stiffness because of the nature of the ball-raceway contact. Under light loads, the ball-raceway contact area is small so the amount of "yield" or "definition" is substantial with respect to the amount of load. As the load increases, the contact area (elliptically shaped) also increases and provides increased stiffness or reduced "yield" per unit of applied load.

This information provided by NMB Bearings, Chatsworth, Calif.
For more information, Circle 659

The upper curve in the chart shows a single-bearing deflection curve. When two bearings are preloaded together and subjected to an external thrust load, the axial yield rate for the pair drops because of the preload, the interaction of the forces exerted by the external load, and reactions of the two bearings. The lower curve shows that the yield rate for the preloaded pair is essentially linear. Miniature and instrument bearings are typically built to accept light preloads ranging from 0.25 lb to normally not more than 10 lb.

PTFE-composite bearing material

Meco 3000 is a PTFE-composite bearing material for chemical environments. The material from Woodex Bearing Co. Inc., Georgetown, Maine, features a low coefficient of friction, hightemperature capability, and high-compressive strength. Its lubricity is good for dry-running applications where lubricant contamination is a concern.

Graphite-andalloy bushings and bearings

Bushings and bearings made of Graphalloy, a graphite-andmetal alloy, work in hostile environments and handle temperatures ranging from cryogenic to 1,000F. Graphalloy from Graphite Metallizing Corp., Yonkers, N.Y., does not melt or cold flow, provides a low coefficient of friction, and resists galling when run with close tolerances against rotating metal elements. The material also maintains dimensional stability under load, when submerged, and at high temperatures. It is corrosion

resistant in most liquids including sulfuric acids and caustics. The self-lubricating bushings and bearings work in pumps, mixers, ovens, furnaces, dryers, and dampers.

Circle 661

Circle 660

Caged balls in linear bearings deliver high-speed, longer life

The SHS LM guide bearings feature caged-ball technology. The cage reduces ball-to-ball friction and heat for high-speed performance and increased product life. The technology drops torque values to onetenth of conventional full-compliment bearings. Without ball-to-ball contact, the bearings run quieter and have pockets for lubricant retention, which reduces maintenance.

A retainer in the bearing separates and aligns recirculating balls for a smoother and
more consistent
gliding motion.

Balls roll
through four
rows of
formed between
the LM rail and
block. The end
plates at both
ends of the block
let the balls

circulate through rolling tracks.

The linear bearings can be used in a wide range of applications, and are particularly suitable for machine tools. Each row of balls is arranged at a 45 contact angle for a four-way equal load rating in radial, reverse, and both lateral directions. The linear bearings provide high rigidity, good durability, and a self-adjusting capacity for uneven surfaces. Given an uneven load, the balls will not produce a differential slip. The result: good wear resistance and accuracy.

This information provided by THK America Inc., Shaumburg, Ill.

For more information, Circle 662

A closer look at caged balls

Grease circulates with the aid of a ball cage. The ballcirculating sections are made of resin molded in the block.

Grease pocket

The four-way equal load rating LM (linear motion) Guide uses a caged-ball technology for low noise, longer term maintenance-free operation at high speed.

Integrated motion module chops $500 from machine cost

Redesigning an ultrasonic wire-bonding machine with a ready-to-install linear module guarantees alignment of the miniature recirculating ball slide and Acme leadscrew drive. The prealigned miniature ball rail
and leadscrew are simply
bolted to the ma-
chine. The device
allows eliminating
alignment steps
and reportedly saves
about $500 per unit in assembly costs.

Orthodyne Electronics of Irvine, Calif., used the miniature recirculating ball slide and Acme leadscrew drive from Rexroth Star, on two robotic wire bonders to provide a compact feeder. The low-profile leadscrew and rail increases productivity and reduces

the footprint of the automated wire bonder. To further reduce cost and improve reliability, Orthodyne adopted a fully assembled motion-control module in place of individual components.

The bonder, used in the automobile

Orthodyne's Model 360C automatic wire bonder without a leadframe feeder has a 24 43-in. footprint. Vertical and horizontal actuators in tandem elevate full magazines to working height, push them through the work area, and out the front.

industry, has a four-axis rotary head that attaches aluminum wire leads to electronic devices for interconnects. When semiconductor manufacturers expressed interest in a high-volume production machine, Orthodyne engineers looked for a space-saving mechanism to feed leadframe magazines. "Floor space in electronic assembly areas is expensive," says Bob Babayan, vice president of engineering at Orthodyne. "So there's a lot of work creating a small footprint for assembly machines."

Semiconductor chips from 1 to 10-mm square are commonly bonded on leadframes packed in standard magazines. The automated-feeder concept called for loading magazines at the machine's load station. Leadframes are fed out of the magazine to the bond area. Wire-bonded leadframes are fed into empty magazines on the output side.

To minimize the impact on the machine design, Orthodyne engineers looked for a simple linear-motion system readily integrated with the existing wire bonder. They also wanted an affordable package that could reduce the overall footprint of the machine.

Engineers at Rexroth Star developed a linear-guide feed mechanism with what's called LOSTPED criteria: load, orientation, speed, travel, precision, environment, and duty cycle. A loaded magazine weighs about 7 lb.

With leadframes positioned for welding by the four-axis bonding head, the magazine feeder does not need high positioning accuracy. However, accurate alignment is critical to ensure smooth feeds without jamming. A stainless-steel leadscrew with
0.2-in. lead/rev provides the driving force for the recirculating ball slide. The Acme leadscrew has 0.0001-in. deviation per inch of travel and less than 0.005-in. backlash. A stepper motor turns the screw at about 10 rps to drive the carrier block at 2 ips. Electronic assembly environments are typically clean, so factory-sealed actuators need neither bellows nor special auxiliary seals. With relatively short strokes and assembly operations typically running 24 hr/day, the feeder mechanism must reliably withstand long duty cycles.

Traditional linear bearings would require a large, costly baseplate to anchor the rail. However, using the end plates as mounting points allows eliminating the baseplate and cutting total system cost by about 25%. The package measures 1.25-in. high 3-in. wide and requires about 20% less overhead space than comparable recirculating ball-and-rail actuators.

This information provided by Rexroth Star, Charlotte, NC.

For more information, Circle 663

Two preassembled linear-motion
systems from PIC Design,
Middlebury, Conn., have protective
bellows made of polyurethane-
coated nylon. The bellows protect
bearings and leadscrews from
harmful debris. Operating
temperatures range from –65 to
250F. System 9B, a Twin Track system, is recommended for vertical applications. It has a 0.75-in. suspended shaft and a 0.5-in.-diameter leadscrew. Five travel lengths from 2.9 to 15.4 in. are available. A second Twin Track system, called System 10B, incorporates supported rails that are integral to the base plate. Ten travel lengths ranging from 3.5 to 29.6 in. are available. Typical dynamic-load capacity is 300 lb for both systems. Circle 664


The next time you're ready to design a system using rolling-element bearings or replace worn bearings, give some thought to air bearings. They ride on air bushings and are interchangeable with rolling-element
designs that run
on round shafts.

Air bearings
have several ad-
vantages over
versions. For in-
stance, their
noncontact oper-
ation means the
shaft does not
have to be hard.

This allows us-

Air bushings at work

Stage can be a simple plate

Housing carries bushing

Moving stage

O-rings let bearings self-align and provide seal
for potting
in place


Lots of reasons.

For example:

• Zero static friction allows infinite resolution and high repeatability.

• Zero wear comes from noncontact operation and results in consistent machine performance and low particle generation.

• Straight motion is generated throughout the life of air bearings because they adjust to rough, irregular, or worn surfaces.

• Silent and smooth air-bearing operation almost eliminates noise and vibration. This is especially noticeable in sensitive machine output, such as scanner resolution.

• Higher damping in air bearings comes from their squeeze-film

damping effect. They are actually a fluid-film bearing. This produces higher dynamic stiffness and better controllability.

• Eliminating lubricants eliminates the problems associated with oil. In dusty environments, such as dry machining, ways are dry and bearings are self-cleaning because positive air pressure pushes dust away. In contrast, oil lubrication becomes a lapping slurry.

• High speed and acceleration results because there are no balls or rollers to slip at high acceleration.



Porous air bushing

Precision shaft

Air fitting

ing a wider variety of shaft materials.

Air bearings can provide rotary and linear motion. The bushings mount inside a bore or housing on O-rings that are compliant enough to let the bushings self-align. When high stiffness is required the bearings can be set in place using a precision potting procedure. Air bushings, housings, and end mounts are available for easy assembly of custom stages.

Although air bushings are commonly used in voice-coil linear-motor-driven stages, they are well suited in any motion application requiring low friction, oil-free operation, high speed, or unlimited service life. Stainless-steel or flush-chromed shafts are recommended to avoid rust.

This information supplied by New Way Machine Components Inc., Aston, Pa.

For more information, Circle 665

Cartridge provides lubricant for monorail guideways

A monorail accessory called the SPL Lubrication Cartridge supplies a continuous flow of lubricant to critical surfaces of the developer's profile linear guideways. For light to medium-duty applications, the
cartridge can replace
the central lubrication
system and
minimize the
scheduled relube
associated with
recirculating roller

attaches to one
or both sides of
a standard
roller carriage
and works in
any carriage
orientation. Seven lubricant
dispensing pads let oil flow from the cartridge
reservoir onto the guideway running surfaces, including the
top of the rail. This ensures the proper amount of lubricant to recirculating rollers and carriage end seals. A full cartridge is capable of dispensing the lubricant for more than 3,000 miles of travel.

This information provided by Schneeberger Inc., Bedford, Mass.
For more information, Circle 666

SPL lubrication cartridges work with any of the company's standard monorail carriages. It comes preassembled for new monorail carriages or separately for existing systems. Single piece prices start at less than $90.

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