linear motion devices - uva · humans can’t perform well enough. modern sensors and controls,...

1998 PT Design A257

T rends in the last fewdecades have led de-signers to rethink

concepts of industrial mo-tion. Companion gains incontrol theory and hard-ware allow schemes forcomplete motion systems not previ-ously possible. And motion technolo-gies from fields as diverse as nationaldefense and medical diagnostics areavailable for exploitation in otherfields. The primary effect is that to-day’s designers must consider not justindustrial “power transmission” alongshafts and through reductions, butalso the broader concept of industrialmotion and control. And linear mo-tion is an essential part of it.

Many modern processes call forunattended operation, exceptionalprecision, high throughput, flexibilityfor short runs, and total manufactur-ing integration. Often in such cases,humans can’t perform well enough.Modern sensors and controls, coupledwith diverse and precise linear androtary motion devices, combine to fillthe needs. Thus, designers must con-sider linear motion as an integral partof industrial motion and control.

The major componentry of linearmotion systems can be categorized as:

• Actuators.• Support systems (bearings).•Control systems and components.Many equipment manufacturers

supply complete systems that includeall major types of components.

ACTUATORSCommon linear actuation devices

for single-axis motion include, but arenot limited to:

• Various complex linkages suchas a walking beam or slider-crankmechanism.

• Gear rack and pinion set.• Plate or disc-cam drive with

fixed-axis follower.• Cylindrical-cam drive with fixed-

axis follower.• Chain, belt, or cable drive in the

the assembly to your ma-chine. Similarly, you canget an actuator that is syn-chronous-belt-driven forthe linear actuation, motor-ized, and with integralgeared speed reducer. You

can also get electrohydraulically actu-ated devices that use the precision ofdigital control with the high force of ahydraulic cylinder. You can get anysuch packages with integral controlhardware and software.

Screw jacksOne of the older conventional actu-

ation packages, the screw jack, comesin a single housing containing the in-put rotating shaft, the output linearmotion shaft (screw), all bearings,and the lubricant. The user connects amotor and the load attachment. Thereare three major types.

Machine-screw jack. In the com-mon machine-screw jack, Figure 1, ro-tation of the input (worm) shaft turnsthe worm gear and drive nut, whichconnects rigidly to the worm gear.The leadscrew (also called the liftingscrew or stem) is of acme or modifiedsquare-thread form. It is threadedthrough the drive nut and convertsrotary motion of the nut to linear mo-tion, if the screw is kept from turningwith the drive nut.

Rolling-element bearings supportthe input shaft and worm gear to mini-mize frictional loss. Thrust bearingssupport the load. The stem cover storeslubricant and helps protect the stemfrom damage and contamination.

Machine-screw jacks come in manystock sizes with load ratings from lessthan 1 to more than 250 tons. A jackcan mount stationary so the stem re-ciprocates, or an external nut can beused so the stem rotates and the nut(attached to the load) reciprocates.Most machine-screw jacks are self-locking. Thus, the load remains sta-tionary in the event of a power failure.

The major limitation of machine-screw jacks is low efficiency, typically

linear part of its path, with or withoutspecial attachments.

• Plastic drive tape.• Sliding-action leadscrew (usually

acme screw), with nut.•Ball-bearing leadscrew (ball-

screw) with nut. The balls recirculateinto and out of the load zone.

• Planetary roller screw, in whicha nut engaging several planetarythreaded rollers mounts on athreaded shaft. The threaded rollersalso engage the shaft. Axes of shaft,nut, and rollers are parallel. Threadson the nut and planetary rollers are ofthe same helix angle.

• Recirculating roller screw, inwhich a nut engaging several groovedrollers mounts on a threaded shaft.The rollers also engage the shaft. Dif-fers from the planetary roller screw inthat rollers recirculate into and out ofthe load zone.

• Skewed rollers on a rotating cy-lindrical rod (traction motion on athreadless rod).

• Fluid-power cylinder with direct-driving rod, rodless cylinder, or cablecylinder.

• Electric solenoid.• Electric linear motor, such as lin-

ear induction motor or linear step motor.

• Adapted electric rotary motor. Acommon type has no shaft. The rotoralso serves as the mating nut on aleadscrew with axis coincident withrotor axis.

Preceding listing order does not im-ply relative importance.

Many linear actuation devicescome as complete packages. For ex-ample, you can get a motorized ballscrew powered through spur or wormgearing, and fully self-contained sothat you need only mount and wire

LINEAR MOTION DEVICESACTUATORS.....................................................A257 BEARINGS........................................................A268 CONTROLS.......................................................A271 LINEAR MOTION DEVICES ADVERTISING .......A272

25% or less. Sliding between drive nutand screw generates heat. This heat-ing restricts duty cycles of machinescrews to 5 to 7 1/2 min/hr at full load.

Ball screw jack. If an applicationcalls for the advantages of a machine-screw jack, but needs a longer dutycycle or higher efficiency, a betterchoice may be a ball screw jack, Fig-ure 2, or a roller-screw jack. See alsodiscussions “Ball screws” and “Rollerscrews” which follow. The heart of aball screw jack is an assembly com-posed of a screw and nut, separatedby a recirculating series of balls. Theball screw jack works like a ball bear-ing and has similar life predictability.

Rolling friction of the ball screw,compared with sliding friction of themachine screw, generates little heat.This allows higher lifting speeds inthe ball screw jack and much betterefficiency, typically 92 to 95%. Thehigher efficiency reduces input powerrequirement to about two thirds thatof a machine-screw jack for a givenload. Besides higher efficiency thanmachine-screw jacks, ball screw jackshave a lower ratio of starting to run-ning torque. Because of low rollingfriction, many ball screw jacks can beback-driven. See the discussion onback driving under “Ball screws.”

Roller-screw jack. You can alsoimprove on a machine-screw jack’sadvantages by using a roller-screwjack. See the discussion under “Rollerscrews” which follows.

• You can include other torque-sens-ing devices into a jack or a motor to pro-tect the whole system from overload.

To select a screw jack, use a systemdesign manual having illustrations,column strength charts, power andtorque data, and sample calculations.

Screw-jackapplication.Figure 3 de-picts a typicallifting ar-r a n g e m e n t ,s o m e t i m e scalled a T-sys-tem. It showshow one motorcan power sev-eral synchro-nized liftingpoints by us-ing couplingsand right-an-gle gearboxes.Selection ofthe best ar-rangement foran applicationis usuallybased on space

availability and motoraccessibility.

Here are some guide-lines:

• Keep screw load di-rection parallel to thescrew axis as much aspossible.

• Keep the span between drivecomponents as short as practical.That keeps interconnecting shaftingshort and limits the chance of a criti-cal-speed problem.

• When necessary, use pillow-block supports, dynamically balancedshafting, or both, to help avoid criti-cal-speed vibrations.

• Select shaft cou-plings with highstrength-to-bore ra-tios (such as gear cou-plings) to minimizesystem inertia.

• Use three andfour-way miter boxeswhenever possible,to shorten intercon-necting shafts.

• Use limitswitches to restrictextremes of stemtravel, or connect ro-tary switches di-rectly to one unusedside of the double-extending inputshaft of a jack.

• Use a slip cou-pling between motor and jack inputshaft to provide overload protection.

A274 1997 Power Transmission Design

Figure 1—Typical machine screw jack.

Figure 2 — Typical ball screw jack.

Figure 3 — Typical screw jack systemarrangement.

1997 Power Transmission Design A275

Before selecting any jack equip-ment, determine:

• Number of lifting points.• Total load per jack.• Load direction (tension or com-

pression).• Speed at which load must move.• Total distance load must move.

Also, consider duty cycle and envi-ronmental conditions such as tempera-ture and contaminants. Next, using asystem design manual, make calcula-tions to determine input torque, speed,and power requirements. Then checkcolumn strength and select the jack.

Ball screwsThe ball screw jack is a specific use

of a more general linear actuation de-vice called a ball screw or ball-bearingscrew. The first ball screws appeared ahalf-century ago, but their popularitysurged in the last few decades asneeds for efficient mechanical linearactuation grew rapidly. Early modelswere seen as low-friction actuators inautomotive steering gear and similaruses. Sliding friction of threads on thenut and screw is replaced by rollingfriction of balls in approximately cir-cular-form threads on the nut andscrew, instead of Acme or modifiedsquare threads of the machine-screwactuator. A deflector forces the ballsout of the screw’s threads and into theball guide on the nut. The guide di-rects them diagonally back to the op-posite end of the nut and rechannelsthem to the screw’s thread, so the ballscirculate continuously. This self-con-tainment allows use of fewer balls.The ball guide can be an external tubethat mounts on the nut or, for morecompactness, an integral interior partof the nut.

Formed vs. machined threads.Most ball screws are of steel or stain-less steel. Threads are machined andground for high precision, or roll-formed for lesser precision and lowercost. In recent years, thread-formingtechniques have improved to wherebetter grade rolled-thread ball screwsapproach the precision of many ma-chined ball screws of several years ago.

Backlash. The precision of a ballscrew assembly depends strongly onthe amount of backlash in it. Backlashis the measured play between nut andscrew. Ball size and ball-and-ball-groove conformity dictate the amountof backlash. For ordinary non-

screw does add resisting torque. Insome applications such as numeri-cally controlled machines, in order tospecify the screw’s motor drive com-pletely, you must know preloadtorque in addition to load torque. Con-sult the ball screw manufacturer.

Life expectancy. A ball screw islike a ball bearing in many ways, andlife expectancy is based on similarprinciples. For a ball screw, life expec-tancy depends on:

• Applied force, including accelera-tion and friction loads.

• Number and length of loadstrokes.

For best life, the load should be ap-plied along the same axis as that ofthe ball screw. Side loads and loadsthat tend to overturn the nut causeuneven load distribution on the ballsand reduce life expectancy.

Life expectancy is based on a standardrating of 1 million in. of travel. Most ballscrew manufacturers offer charts to de-termine life in inches of travel for variousloads. For example, if the load is halvedfrom rated load, expected life increasesby about nine times.

Other factors affecting life rating arescrew and nut materials and hard-nesses. If hardness of a stainless steelball screw is reduced from, say, 56 to 45Rockwell C, then load capacity is re-duced to about 30% of what it had been.

Column load. Excessive compres-sion load on a ball screw may cause itto buckle. Column-load capacity of aball screw varies with length, type ofend mounting (fixity), and screw rootdiameter. In general, these parame-ter changes increase the tendency forthe screw to buckle as a column:

• Increasing axial load.• Increasing length-to-diameter

(slenderness) ratio.• Reducing end-support fixity.Another problem that axial load

can cause, unrelated to column buck-ling, is that load makes the ball screwlead change slightly. In most cases itis insignificant. However, for heavyload in tension or compression in ahigh-precision application, you mayneed to account for lead variation.

Installation. Misalignment canshorten life of a ball screw assemblyto a small fraction of the expected life.Avoid misalignment by makingmounting surfaces of the nut parallelor perpendicular to the ball screw cen-terline, or provide a gimbal housing ifappropriate.

preloaded ball screws, a common back-lash range is 0.002 to 0.013 in., de-pending on screw size and ball diame-ter. To avoid backlash, one nut can bepreloaded against another on thescrew so there is no play. Similarly,one ball circuit can be preloadedagainst another within the one nut. Insome assemblies, slots cut in the nutmake it act as a preloaded spring whenmounted, or a clamping device candraw it up against the screw thread.

Mountings. Critical speed of anylong shaft, including a ball screw, is aspeed at which it vibrates violently ina transverse direction. It results fromrotating-system unbalance. The mem-ber can have several critical speeds,all multiples or submultiples of onepredominating “first-order” speed. Amachine cannot operate long at criti-cal speed. Bidirectional thrust bear-ings at each end of a ball screw providegreatest support for critical-speed andstiffness considerations; bidirectionalbearings at one end with the other endtotally free, the least support.

Stiffness. A ball screw’s diameterand the number and size of its load-carrying balls determine assemblystiffness. Preload and end configura-tion can also affect stiffness.

When a screw mounts with single-direction thrust bearings at each endor thrust bearings at only one end andfreely at the other, then its springrate varies directly with its cross-sec-tional area at the root diameter; di-rectly with operating load; and in-versely with length. The spring rate ofthe ball screw system varies inverselywith the sum of the reciprocals of thespring rates of the screw, the nut, andthe bearings. Total system spring rateis always less than that of the mostcompliant member.

Drive torque. For rotary-to-linearmotion, ball screw assembly drivetorque is:

where: T = Torque input, lb-in. P = Operating load, lb L = Lead, in./rev E = Efficiency (about 90%)Torque values from the equation do

not account for drag or inefficienciesdue to mounting or to drive compo-nents, to wiper drag, or to torque dueto preload.

Preload torque. Preloading a ball

TPL

E=

2π


Many ball screws arerepairable. Commonproblems, such as loss ofrepeatability due to wearcan be fixed by regrind-ing the ball threadgrooves and then usinglarger balls in the ball-screw assembly. If a mi-nor crash bends thescrew a bit, it can oftenbe straightened to itsoriginal accuracyand returned toservice. Surfaceproblems such asspalling, brinelling,chipping, or feath-ering, Figure A,can be regroundand replated. All ofthese repairs mayprove more eco-nomical than re-placement with awhole newball-screw as-sembly.

To deter-mine when aball screwneeds repair,and approxi-mate howmuch repair,measure itsdiametralbacklash, orlash. Diametral lash (not axial back-lash) is a measurement that can betaken in the plant. The ball-screwassembly is placed in V blocks. Anengineer lifts the ball nut verticallyand with a gage, measures the playbetween the ball nut and screw. Fora preloaded assembly, a diametrallash of 0.0005 in. indicates a wearfactor of 50%. Minimal, or Level 1,repair is needed to bring the screwback into use. A lash of 0.0035 in.represents 80% wear and indicateseither a Level IV repair or a dead ballscrew. Similarly for nonpreloadedassemblies, a diametral lash of 0.009in. is 50% wear and needs Level 1and 0.015 in. is 80% wear and needsLevel IV or replacement.

For ball-circle diameters rangingfrom 0.03934 to 0.04875 in., the ac-

ceptable lash is0.002 to 0.004 in.For diametersranging from0.5625 to 0.6250in., the acceptable

lash is 0.004 to 0.008 in. As noted, a ball screw with 80% or

more wear is likely irreparable. Fourrepairs is about the maximum forany ball screw. After that, bury theball screw with honors.

Levels of repair. When a ballscrew arrives at a repair facility, it isinspected and evaluated for the typeof needed repair. This process cantake up to three or four days.

In general, there are four levels ofcost-effective repair. While each suc-cessive level adds cost, this cost isstill less than a new ball-screw assembly.

All repair levels involve the samefour basic steps: inspect, clean, re-ball, and straighten.

• Level 1 (three days) repairs lossof repeatability due to wear. Balls

are the first to wear. Thus, new,larger balls are used at every level torestore preload and repeatability.The secret to proper ball replace-ment: for every 0.003 in. of wear, usea 0.001 in. larger ball. The screw isalso straightened because a bow aslittle as a thousandth of an inch canput excess moment on the ball nut,which can later result in failure.

• Level 2 (seven days) adds re-grinding of the ball nut to the stepstaken in Level 1. Ball-nut threadgrooves wear faster than the screwthreads because they are subject tomore ball travel.

• Level 3 (seven days) adds re-grinding of the ball screw threads,and as required, rebuilding of thejournal diameters with eutecticspraying and grinding them backto size.

• Level 4 (14 days) adds regrind-ing of the ball nut and ball screw.This level may cost 55% of a new as-sembly, but the repaired ball screwwill have a normal new-screw life.When the repair cost goes over 65%,buy a new assembly.

These four levels of repair are clas-sified by the most common repairsand do not cover all contingencies.

Excerpted from an article byThomson Saginaw Ball Screw Co., inthe June 1996 issue of PTD.

Get more life out of ball screws

Figure A —Common forms ofwear or damageon the threads ofball screws.

The ball screw on theright has seen many

hours of service, but itis more economical to

repair it than todiscard it. After

repair, it will look likethe assembly at left.

C — Before repair,brinnelling of ball

screw threads is seenin the foreground as

vertical “dashes.”

C — Before repair,brinnelling of ball

screw threads is seenin the foreground as

vertical “dashes.”

B — Chipped andbroken lands on thisball screw are notsevere enough toscrap it.

D — A close-up view of the inside of the

ball nut shows spalling of the twomiddle threads.


A common installation procedure:Mount the ball screw in its bearing ateither end; loosely connect the nut toits mounting; traverse the nut fromone end to the other so it can seek itsown center; then tighten the nutmounting. This tends to alleviate in-

preloaded, bearings, slides or rails,and frame. The user aligns and boltsdown the entire frame. You need notalign or preload any component. Youcan get motorized versions, too.

Back driving. The ball screw as-sembly has the capability of either the

duced side loads on the ball screw as-sembly. If torque from one end to theother is constant, there is no signifi-cant binding in the mounting system.

Several manufacturers now supplycomplete ball screw assemblies, in-cluding nut and screw properly

Lubricants maintain the low-fric-tion advantage of ball-screw assem-blies by minimizing rolling resistancebetween balls and tracks and slidingfriction between adjacent balls.Proper lubrication helps keep mostcontaminants out, which reduces thedamage foreign matter can cause.

Oil and grease provide corrosionprotection, but lubricant choice de-pends on the advantages and disad-vantages of each. Oil can be applied ata controlled flow rate directly to thepoint of need. It will clean out moistureand other contaminants as it runsthrough the ball nut and provides cool-ing. Oil disadvantages include:

• Possibility of excess oil contami-nating the process.

• Cost of a pump and metering sys-tem to apply oil properly.

Grease is less expensive than oil toapply and requires less frequent ap-plication, and it does not contaminateprocess fluids. Grease disadvantages:

• It is hard to keep inside the ballnut and has a tendency to build up atthe ends of ball nut travel, where it ac-cumulates chips and abrasive particles.

• Incompatibility of old greasewith relubrication grease can create aproblem.

Oil lubrication. Operating tem-perature, load, and speed determinethe oil viscosity and application ratefor an installation. If the oil is too vis-cous or if you use too much, heat maybe generated. If the oil is too thin oryou use too little, parts may not becoated adequately; friction and wearmay result.

The following guidelines are appro-priate for most applications, but if ex-tremes of temperature, load, or speedare involved, consult a lubricationspecialist.

The recommended nominal viscos-ity of the oil at 40 C is based on themean speed of the ball screw, its di-ameter, and the temperature at whichthe ball nut is likely to stabilize. Vis-

cosity is expressed in centistrokes (1cSt = 1 mm2/sec.) Various grades havebeen selected for standardization(DIN 51512).

To determine the nominal viscosity ofthe oil for an application, establish themean speed of rotation of the ball screwand, from it, the limiting speed, dnm, fac-tor. You also need the temperature atwhich the ball nut is likely to stabilize.

Mean speed of rotation accounts forthe ball screw’s duty cycle:

where:nm = mean speed, rpmn1, 2, 3 ... = speed for time q1,2,3 ..., rpmq1,2,3 = time at speed n1,2,3 ..., %

of totalFor typical applications, nm ranges

from 200 to 500 rpm.The dnm factor is given by:

where: d = ball screw nominal diameter,

mmTypical values of dnm range from

15,000 to 25,000 mm/min. Values ofdn to 100,000, where n is the maxi-mum speed of rotation, are becomingmore common, and in such cases thelower viscosity should be used if theoil selection guide indicates a grademidway between two adjacent viscos-ity curves.

Ball nut operating temperatureshould be about 20 C, however, it usu-ally stabilizes a few degrees abovescrew-shaft operating temperature. Ifyou can’t measure nut temperature,assume it to be 30 C for your initial se-lection of oil viscosity.

Required oil flow rate is a functionof:

• Number of ball circuits.• Ball-screw orientation.

• Operating environment.• Load.• Speed.• Judgments based on knowledge

of the application.In addition:• If the ball screw is horizontal,

add nothing to the flow rate, Qmin, toaccount for orientation; if vertical,add 25%.

• If the application is clean anddry, add nothing to Qmin to account forenvironment; if not, add 25%.

• If the screw is not subject to highloads or speeds, add nothing to Qmin toaccount for severe running conditions;if it is, add 50%.

Grease lubrication. Grease is notso widely used as oil for ball-nut lubri-cation, though it lubricates accept-ably. Speeds that are high for ballscrews are no problem for grease, sospeed is no criterion for selection.

One problem with grease: It tendsto be fed out of the nut and onto theball screws, accumulating at the ex-tremities of travel where it collectscontaminants. It must be replenishedregularly.

Grease is a complex subject.Greases consist of a mineral or syn-thetic oil, additives, and a thickeningagent such as lithium, bentonite, alu-minum, and barium complexes. Formost applications, use a grease with adrop point above 220 C, a service tem-perature range of -30 to 130 C, and alimiting speed factor (dn) above1,000,000. Such a grease is classifiedas HL91 Grade 2 (DIN 51818) and isbased on Mil-9-7711A.

As a rule of thumb, replenish greaseat least every 800 hr. However, becauseconditions vary so widely, you shouldconfirm this interval by inspection, andreadjust if needed. For extreme condi-tions, such as dn values above 50,000,consult a lubrication expert.

Excerpted from an article by Thom-son Industries Inc., in the February1995 issue of PTD.

Selecting lubricants for ball screws

n n q n q

n q

m = ÷( ) + ÷( ) +

÷( ) +1 1 2 2

3 3

100 100

100 ...

dn d nm m= ( )( )

nut or screw rotating when a thrustload is applied to the other member ofthe assembly. However, not all ballscrews can back drive. The thread’shelix angle determines if the assem-bly can back drive. Generally, a ballscrew with a helix angle of 6 deg ormore will back drive; those of 4 to 6deg are marginal; those under 4 degprobably will not do so. Be aware that,in some situations in which you wouldnot expect back driving, continuousmachine vibration with the ball screwunpowered and unrestrained mightcause slow back driving.

In many situations, you would notwant the ball screw to back drive. Forexample, should power fail on a lift, itcould be disastrous for the load to runback. You must assure that either theball screw cannot back drive or, if itcan, that you provide a holding meanssuch as a spring-applied, electricallyreleased brake to prevent screw rota-tion on power loss.

Variations. Many variations inball screws and optional equipmentlet you adapt them to special require-ments. For example, hollow screwsare available for situations where lowsystem weight is important, such asin actuators on aircraft.

Options in seals, wipers, mountingarrangements, housings, preload de-vices, and similar characteristics helpalso. And you can get specialty sys-tems such as the bidirectional 1-pieceball screw of Figure 4. It lets two nutsmove in opposite directions simulta-neously, an advantage in applicationssuch as clamping devices and robot-ics. Also, a trunnion-mounted nut canbe helpful in some applications; a tele-scoping screw in others.

Roller screwsThe first roller screws appeared in

the early 1950s. However, only in thelast decade or so has their popularity

nipulators, and movement of prismsin laser measurement machines.

These curved linear systems con-sist of slides or races, and rings (360degrees of rotation) or segments ofrings (90 or 180 degrees of rotation).One type of slide uses opposing fe-male bearings with V-shaped outerrollers in a two-and-two arrange-ment. The bearings ride on a trackwith matching V-shaped rails. A car-riage plate on top of the two-and-twobearings is the mounting platform,Figure 6. Thus, the carriage assemblyeffectively runs on eight line-contactpoints on a track with varying circum-ferential diameters.

For a fixed segment of a ring, fixedcenter carriage plates are the mostpopular. A bogie carriage, Figure 7, isused around S-bends, slideways withdiffering bend radii, and curves wherelooseness in the movement betweenstraight and curved sections is not de-sirable. The bogie carriage runs onswivel bearings, which operate on aprincipal similar to that used in trainand tram bogies to negotiate bends inthe track.

increased significantly. Roller screwsare more costly to produce than ballscrews and they are applied mostlywhere application requirements ofload-carrying capacity, axial stiffness,linear speed, or acceleration and decel-eration rates are especially stringent.

Overall, roller screws are similar toball screws in preload configuration,backlash, lost motion, left-hand andright-hand thread, back driving, effi-ciency, torque, and power require-ments.

Figure 5 shows a planetary rollerscrew.

Linear slides and racesNot all linear motion applications

consist of straight lines. Some appli-cations require an oc-casional curve or thecircular motion ofpure radial move-ment, such as thatfound in tool changingmechanisms, mea-surement of turbineblades, rotating ma-


Figure 4—Bidirectional 1-piece ball screwneeds no joint to connect and synchronizeleft and right-hand screws.

Figure 5—Typicalplanetary roller screw.In this style, rollerscrews remain inconstant contact with thethreaded haft. In anotherstyle, roller screwrecirculate into and outof the load zone.

Opposing femaleV-shaped bearings

Track with male V-shaped rails

Fixed center carriage withtwo-and-two bearing support

Figure 6 — V-ball bearing systems use opposing female bearings with V shaped outerrollers in a two-and-two bearing support arrangement. A fixed center carriage uses thetwo-and-two arrangement to support the mounting plate. Two of the V bearings haveeccentric studs to facilitate adjustment.


V-ball bearing track systems arebest suited to light loads — directloads from 120 to 3,800 N (26.98 to854.38 lb) and moment loads from 0.6to 220 Nm (0.53 to 1,946.90 lb-in.) in alubricated system. Refer to manufac-turers’ tables for precise load han-dling capabilities.

Rings offer stability with supportas near to the load as possible. Agearcut rack on the outside or insidediameter of the register face of thering serves as the drive mechanism.

Linear slideways are available inlengths to 4 m; for longer lengths,slide segments are matched and but-ted together.

V-ball bearing systems can run dryor lubricated. Lubrication, throughlubricator blocks, can prolong systemlife by as much as 150%. Every time aring slide rotates it is wiped with oil,which is also imparted to the female Vof the bearing surfaces.

For rings or segments of rings, it ispossible to achieve circular motion withradial run-out no greater than 0.05 mmper 360 deg (pro rata over angle of seg-

nears an extreme of horizontal crankthrow, its velocity vector is nearlyparallel to your line of sight. Thepoint seems nearly motionless, thenmotionless, then it reverses direction.If the crank turns at constant angularvelocity, that projected action of thepoint is simple harmonic motion.Though simple harmonic motion isfairly easy to produce, it has high lin-ear acceleration at the extremes oftravel. High acceleration means highforce—which generally means in-creased tendency for machine-partwear or breakage, and for workpiece orproduct damage. Cams, bar-type link-ages, and similar devices can modifycrank-generated motions into profilesof lesser peak acceleration. “Cycloidalmotion” is such a profile.

Moreover, modern electric motorsand their controls modify displace-ment, velocity, and acceleration pro-files of mechanisms so that you canreadily get the best profiles for a pro-cess without danger to components.

For example, a cam-and-screwmechanism, Figure 8, adapts a con-stant-lead cam and a ball spline toprovide linear motion according topredetermined programs that the mo-tor and its control execute. The cammounts rigidly on two ball-splinebushings that can traverse the lengthof the spline shaft. The bushings arepreloaded against each other toprevent backlash.

The spline bushing can move axi-ally on ball tracks on the spline shaft,but it cannot rotate relative to theshaft. Thus if torque turns the splineshaft, the bushing turns with theshaft, causing the cam to rotate. A mo-tor-and-reducer package turns thespline shaft, which therefore turns thebushing and cam. The cam then actsmuch like it would in a typical rotaryindex drive except that the traditionalroles of cam and cam follower are re-versed: An axially mobile cam engagesa straight-line array of stationary camfollowers. The cam rotates throughthe row of followers to achieve linearactuation. The cam seemingly pulls it-self along the single-file row of stand-ing followers, contacting at least twoat any instant to prevent backlash.

A carriage housing mounts on andtravels with the cam, but does not ro-tate. Tapered roller bearings let thecam turn inside the housing but sup-port thrust load only; two linear bearingsystems over the actuator’s length carry

ment). Axial run-outis 5 microns.

For multiple car-riage track systems,the greatest re-peatability error isin the direction oftravel and is depen-dent on the play inthe drive mecha-nism. However, it ispossible to achieverepeatability with0.2 mm with a belt-driven system.

With all radialmotion, engineersmust consider cen-trifugal force. Theforce is proportionalto the square of thetangential velocity.Doubling the car-riage speed quadru-ples the force. It isalso inversely pro-portional to the ra-dius. Doubling theradius halves theforce. Often, thisforce will also causea moment load aboutthe carriage plate.

Excerpted from anarticle by Bishop-

Wisecarver Corp., in the August 1996issue of PTD.

Other mechanical actuators

As listed earlier, there are manylinear actuators besides screws.Among the oldest—still much used inspecific machines—is the slider-crankmechanism and its many variations.Perhaps in its eldest form it convertedrotary to linear motion to drive pumppistons. Later versions became popu-lar converting linear motion to rotarymotion on steam-engine-driven rail-road locomotives and paddlewheel-propelled ships. The simplest—theclassical Scotch yoke—converts ro-tary to linear motion in a characteris-tic linear-motion profile called simpleharmonic motion. Imagine lookingdown at right angles to the crank’saxis and following the motion of apoint on its circumference. As thepoint travels across the axis its entirevelocity vector is perpendicular toyour line of sight; the point seems tobe moving fastest. When the point

Bogie carriage

Ring segment

Figure 7 — A bogie carriage carries loads around S bends orslideways with differing bend radii. The bogie carriage runs onswivel bearings, which operate on a principal similar to thatused in train and tram bogies to negotiate bends in the track.

the weight of the cam, carriage, andpayload, and resist tipping moments.

Standard leads for such systemscan be up to five times more thanthose of ball screws.

Other types of mechanical linear ac-tuation include belt, chain, and cabledrives in their linear travel paths. Forexample, Figure 9 shows a linear actua-tor specifically for conveying (linearlypushing) products. The driving elements are wire-reinforcedpolyurethane synchronous belts with

and continue to seal. Magnetically coupled rodless cylin-

ders make slots and dynamic sealsunnecessary; the piston couples mag-netically to the external carriage.

Recent versions of these rodlesscylinders can now handle tipping ortransverse loads. Most vendors offerfully pre-engineered, out-of-the-box,bolt-it-down, hook-it-up systems. Op-tions include position sensors, posi-tion and velocity controls, end-of-stroke bumpers, shock absorbers, andother snubbing devices, brakes, ex-ternal and guides.

Electric linear motors

An electric linear motor makes con-version of rotary to linear motion un-necessary. Thus, linear motors caneliminate shafts, belts, and gears tominimize space, energy, and costs.For information on rotary motors, seethe Motors Products Department inthis handbook.

Linear motors generate forcerather than torque. Force to inertiaratio and stiffness — depending onthe type of linear motor — can be ashigh as 30:1, and to 0.9 million N/mmor 5 million lb/in., respectively. Someversions offer smooth motion towithin a fraction of a percent of theirnominal velocity, which can range be-tween 1 micron/sec to over 5 m/sec.Others can operate continuously, pro-vide 5 g (or higher) acceleration, andoffer a low settling time, in somecases, less than 50 msec.

Linear motors all have the samebasic structure. Imagine a rotary acor dc motor that has been sliced alongits axis and opened up flat, resultingin two sections. One section, the pri-mary, is a set of electrical coils embed-ded in a core (typically of steel, epoxy,or aluminum). The structure of thesecond section (the secondary) de-

pusher-cleat attachments.Many cable drives andpower-transmissionroller chain driveswork similarly withattachments. Manypositioning tables,single stages, and self-contained linear actuationsystems use the reinforced synchronousbelt as the linear-motion element.

The popularity of rodless cylinders,Figure 10, for linear motion applica-

tions is increasing,especially in appli-cations with spacerestrictions. It canchallenge mechani-cal and electricalactuators in manyuses, and productvariety has grownover the last fewyears.

The band cylin-der is a popularversion of the rod-less cylinder. The

cylinder wall has a slot running itsfull length. The slot lets the pistonconnect mechanically to an externalcarriage. A dynamic strip-type sealover the slot’s outer surface and an-other over its inner surface seal-incylinder pressure and seal-out con-taminants. Carriage travel opens theseal section beneath the carriage, butmaintains the seal. As the carriagepasses, the seals reseat on the slot


Figure 8—Cam-and-screw mechanism reverses usual roles of cam and followers. Asreducer output turns ball spline, constant-lead rotary cam engages cam followersstanding in a row. Positive cam pitch makes cam move axially, engaging succeedingfollowers and disengaging receding followers.

Figure 9—Wire-reinforced polyurethanesynchronous belts with attachmentsprovide linear motion in straight-lineportion of belt travel. Various cleats,pushers, or lugs provide contact. Manypositioning tables or stages use such adevice for linear motion. Withattachments, roller chain drives and cabledrives can work similarly. (Note: Notshown is the belt-driving mechanism,which can be various devices, such assprockets.)

Figure 10 —Rodless cylinder.

This is a band designwhich handles high, off-

center loads.


pends on the type of linear motor. Atypical air gap of 0.024 in. (0.6 mm)separates the two sections for non-contact force transmission.

While there are several types of lin-ear motors, many rely on the interac-tion of magnetic flux that producesforces on the moving and stationarymembers. For the voice-coil, dc force,and step-motor types, part of this fluxcomes from coil current. For the in-duction motor, ac excites a coil thatproduces flux. In turn, this flux inter-acts with flux produced by induction(like a transformer) and generates aforce proportional to the relativestrengths and distribution of the in-teracting fluxes.

Voice-coil motor. Constructedlike solenoids, these motors come inmoving-coil and moving-magnet con-figurations and produce more precisemotion than solenoids. Voice-coil mo-tors maintain high linearity betweenapplied current and developed force;operate efficiently; and, when builtwith high-energy magnet materials,develop high forces and accelerationrates, to 100 g in some versions, andcan oscillate at high frequencies.However, as with a solenoid, the sim-ple mechanical structure precludeslong strokes.

Single-axis actuators. Anothershort-stroke linear device is the sin-gle-axis actuator. Recent develop-ments have focused on the actuationdriver that provides the linear mo-tion. One driver is a polymer that ex-pands under applied current. Theother driver is an alloy material thatexpands when subjected to a mag-netic field.

The polymer-based actuator com-bines the small size of solenoids, theforces of hydraulic cylinders (to 500lb), and the proportional control ofelectric motors. One of its benefits isthat it lets engineers stay under the20-lb limit of traditional short-strokeactuators without buying customsolenoids. These actuators can oper-ate hydraulic valves and brakes, re-place a ball screw system, actuatecontrols in car engines, and controlrobotic gripper manipulators.

Rather than magnetics, it uses athermally reactive polymer. Heat issupplied by an electric heating ele-ment that consists of an electricallyand thermally conductive carbon-fiber or silicon-carbide grid. The heat

The rod responds to the applicationor withdrawal of current almost in-stantaneously. The expansion is pro-portional and repeatable. Theamount of force that the actuator sup-plies to linearly move an object de-pends on the size of the rod. A 12-mmdiam rod can exert at least 200 lb offorce. A 75-mm diam rod can exert atleast 9,000 lb. Commercial versionsof this actuator have available dis-placements in the thousandths of aninch to over 2 in.

Thus, these actuators are for appli-cations that need high speed and highforce, such as machining.

Linear induction motor. A lin-ear induction motor resembles a com-mon rotary induction motor that hasbeen split axially and rolled out flat.Its speed-thrust curve, Figure 13, re-sembles the speed-torque curve of astandard ac NEMA Design B rotarymotor. In operation, the moving partof the motor lags its synchronousspeed to develop thrust. Also, speeddepends on power-supply frequency.Efficiency is low except when the mo-tor operates near design rating.

expands the polymer that pushesagainst a piston, Figure 11. Asthe polymer cools, the piston re-tracts.

Actuation speed depends onthe heat sink temperature, ap-plied power, and applied load.Polymers are available to reactat temperatures from -50 to 625F. In some cases, the heat gener-ated in the application can beused to activate the actuator.

Polymer expansion is directly pro-portional to the received electric power.It can be precisely positioned anywherein its range of travel by varying theelectric power from 60 to 200 W. De-pending on heat sink temperature andthe weight of the load (minimum of 25lb), continuous power input is neededto maintain a specific temperature forthe piston to hold position.

Cooling occurs when electricalpower is reduced or removed from thedevice. The polymer is stiff, with abulk modulus (hydraulic stiffness) of1 million psi.

The other new linear actuator has adrive rod made of Terfenol-D, a mag-netostrictive metal alloy of terbium,dysprosium, and iron. It uses magne-tostriction, where an applied mag-netic field causes the material tochange its geometric dimensions.

The actuator has copper wire coiland permanent magnets housed inaluminum or stainless steel, Figure12. The copper wire is wound aroundthe drive rod. When current of 1.4 to3.4 A from an external power supplyis applied to the coil, it creates a mag-netic north-south orientation at themolecular level. This new orientationcauses the drive rod to lengthen asthe diameter shrinks. When currentis removed, the rod returns to its orig-inal shape.

Piston

Electricwires

Electric feedthrough Heater corePolymer core

Figure 11 — Cross-section of a solid-state actuator. Heat expands a polymer, whichpushes against a piston.

��

Drive rod

Coil

Coil

Permanent magnet

Figure 12 — The actuator consists of adrive rod made of the magnetostrictivealloy Terfenol-D, copper wire coil, Alnicopermanent magnets, and a magneticreturn circuit housed in aluminum orstainless steel.

The primary is a wound structure,much like a conventional motor sta-tor. The secondary is a metallic struc-ture, much like a rotor. Either struc-ture can be the moving part of a linearinduction motor.

The motor can operate directlyfrom line current with a fixed speedthrust characteristic. It can also oper-ate in an open loop from an ad-justable-speed source for adjustable-speed applications. And it can operate

stationary; the primary, with themoving part of the system.

The thrust produced in an ac linearmotor is approximately proportionalto the face area between the primaryand secondary parts. A modificationof the single-coil primary system is adual-coil system, in which a primarypart mounts at either side of a flatsecondary. In effect, that doubles theworking face area and thus the thrustof a similarly sized single primarysystem. In most applications, the dualprimaries are fixed and the secondarycoil is with the moving part of the sys-tem. Thus, you would consider first asingle-coil primary system for long-stroke applications; a dual-coil pri-mary for shorter-stroke, higher-thrust applications.

Linear force motor. Like the lin-ear induction motor, you can think ofa linear force motor as a conventionaldc motor that has been slit axially androlled out flat, Figure 15. It comes inmoving-magnet, moving-winding,brush, and brushless types, and withvarious core materials.

Brush-type motor is the least ex-pensive. Operating from direct cur-rent, each motor has a stationary coilassembly and moving magnets. Themotor cable does not move in this con-figuration. Velocity goes to 1 m/sec;and acceleration to 0.5 g. Above thesevalues there is excessive arcing andrapid deterioration of the brushes.These motors are excluded from cleanroom and vacuum applications.

Brushless, aluminum-core linear mo-tor operates from three phase power,with a moving coil and cable. In applica-tions requiring short travel lengths, thecoil can be stationary and the magnetsmoving. The core of the primary en-closes the windings in aluminum. Thereis no magnetic attraction between thetwo motor parts. Therefore, it may re-quire a double-sided magnet assemblyto close the magnetic circuit effectively.

as a servo systemwith a closed positionloop. Figure 14 showssuch a servomotorsystem in which thelinear measuring sys-tem could be an en-coder feeding positiondata back to an acvector drive. (Formore on vector drives,see the “AC vector”section in the Ad-justable Speed Drives

Product Department in this hand-book).

The linear induction motor in Fig-ure 14, sometimes called an asynchro-nous linear motor, is a single-coil mo-tor — the primary part (similar to astator in a rotary motor) holds thewindings. The secondary part (like arotor in a rotary ac motor) consists ofiron and short-circuit rods of copperor aluminum. The magnetic field inthe secondary is generated by the cur-rent induced by the moving and alter-nating magnetic field of the primary.In most cases, the secondary coil is


Figure 13—Typical speed-thrust curves oflinear induction, force, and step motors.

Figure 14—Typical ac linear servomotor. It could be powered by a vector drive thatreceives position and speed information from the linear measuring system.


This type of linear motor generatessmooth motion.

Depending on the moving load, it canaccelerate to 4 g’s, but can also developeddy currents at speeds over 1 m/sec. Itis ideal for vacuum applications.

Brushless, epoxy-core linear motoris also non-ferrous with its epoxy-based core. It too, provides smoothmotion. It has an advantage over alu-minum in that it does not experienceeddy currents at high speed. It haslow stiffness (typically 35,000 N/mm)at high coil temperatures (to 125 C)and it tends to give off gases in highvacuum environments.

Brushless, steel-core linear motoruses steel lamination in the mainbody of the primary to enclose the cop-per windings. This motor uses a sin-gle-sided magnet assembly and is airor water cooled for high duty cycle ap-plications. It functions well in appli-cations that require such duty cyclesand velocities up to 200 ips (5 m/sec).But this motor is subject to cogging. Itcan generate strong magnetic attrac-tion between the two motor parts,which must be accountedfor in the load carrying ca-pacity of the bearing sys-tem. Typical applicationsare machine tools wherehigh force, 5,000 lb (23,000N), may be required, or ingeneral automation wherespeed of several m/sec isneeded, 120 to 200 ips (3 to5 m/sec).

Moving-magnet linearmotors. This motor is aspecial case of a linear forcemotor. It uses an uncon-ventional magnetic circuitin a cylindrical armature,similar to the shape of hydraulic andpneumatic actuators, and a three-

phase wound statorassembly, Figure 16.The design helps its

speed approach that of voice coil actu-ators and is spatially efficient to ac-commodate an increase in the amountof magnet material and coil windingsin the actuator. Minimum magneticcircuit lengths and short air gaps aidthe efficiency and force capability.

The cylindrical design aids acceler-ation, which ranges from 1,000 to1,500 ft/sec2. The working gap length,or circumference of the armature, islong versus the amount of material inthe armature. A 600 lb force outputversion demonstrated speeds to 90in./sec. The force/mass ratio was1,360 ft/sec2. Maximum velocity islimited by the drive voltage availableand the travel distance, Figure 17.

These motors can be synchronizedwith the electrical sequencing of thefield coils. This ability provides con-trolled velocity, acceleration, and po-sitioning without additional sensors.

They achieve high detent forcesthrough the permanent magnet de-sign and very small air gaps. Detentforces are the result of changes in thereluctance of the magnetic circuit inthe linear motor. Permanent mag-nets, even with the coils off, still pro-

duce flux. The armature seeks a posi-tion of least reluctance and attemptsto remain in that position. The forceneeded to move the armature, withthe coils off, is the detent force, whichcannot be completely eliminated inthese motors. It can be varied over arange of about 5 to 15% of the maxi-mum force capability.

Low detent motors are available.Reducing the detent involves thesame techniques used in dc motors toreduce cogging, i.e. use of smallerpitch for the coils and magnets, alarger pole width relative to pitch, alarger air gap, or skewing the statorpoles relative to the armature. Gener-ally, however, lowering the detentforce increases motor size and cost.These techniques also tend to reducethe force capability of the motor, re-

Figure 15—Typical linear force motor.

Figure 16 — The stator assembly (left)shows the windings that are sequentiallyactivated to move the armature (right). Theactuator, including its own weight, canaccelerate at a rate of 8,600 in./sec 2 whendriving a load of 23 lb. The stator is 9.0 in.OD 3 4.9 in. ID 35 in. long. The armatureis just under 4.9 in. in diameter 3 8 in. inlength. Armatures can be made to almostany length to accommodate the neededtravel distance.

84 in.

In position (in stop)

Out stopLinear motor Roller bearing

assembly 2X

Armature (piston)

Ø9.75 Ø9.0

Ø5.032 in. travel

Hall sensors

Roller bearing assembly, rotated 90°

48 in.

Figure 17 — This cylinder shaped linear actuator can develop 1,800 lb of force with atravel of 32 in.

quiring the use of a larger motor for agiven force.

These motors are suited to applica-tions requiring high acceleration, syn-chronized speed or acceleration, andhigh detent forces and reliability.

Linear step motor. This motor,Figure 18, provides the same incre-mental point-to-point precision as itsrotating counterpart.

A linear step motor has a toothed,magnetic pole structure on the stator(platen) and on the slider (forcer).Platen and forcer tooth structure al-most match. For example, the platenmay have 11 teeth in the same lengthin which the forcer has 10. By sequen-tially energizing two coils that oper-ate in conjunction with a permanentmagnet (oriented parallel to the axisof motion), the step motor can bemade to move in one-quarter-tooth-pitch increments. You can get ex-tremely fine resolution (to 25,000steps/in.) with microstep controls.

Linear step motors are well-suitedfor positioning applications requiringhigh acceleration and high-speed,low-mass moves. Motor systems offer:

• High throughput. Linear stepmotors are capable of speeds to 100ips, and low forcer mass allows fastacceleration.

• Mechanical simplicity.• Reliability. Few moving parts

and in some models, air bearings,make for long life and low mainte-nance compared with rotary systems.

• Precise open-loop operation. Lin-ear systems allow open-loop unidirec-tional repeatability to 1 micron(0.00004 in.).

• Small work envelope. Most linearstep motors need less space than

method is to provide not one flat sur-face, but two, butted at an angle toeach other to form one V-shaped way.A companion V-shaped way mateswith it. A variation of this techniqueuses a continuous V-shaped way withcompanion wheels or sheaves withmating circumferential cross sections.They roll on and are guided by theway. The wheels, however, make sucha system a rolling-element system.Dovetail ways are another variationof the V-shaped way.

By replacing the flat way with acylinder with axis parallel to the di-rection of motion and making thecompanion moving piece a cylindricalrod, you create a sleeve bearing. Now,however, you can use short, well-aligned sleeve bearings in series tosupport the linear-motion device.

Such a bearing looks like the sleevebearing used to support and guide ro-tary motion. However, it supports ax-ial motion, and the mechanics of mo-tion may differ. A major difference:the hydrodynamic oil-film “wedge”that develops between a rotatingshaft and a sleeve bearing above acertain minimum rotary speed isn’tthere when the shaft moves axially.Well-lubricated linear-motion sleevebearings can serve well at low speed.

Chief among the differences in lin-ear sleeve bearings are type of bear-ing material. Common bearingbronzes are often used, and so aregraphites. So, too, are metal sleevebearings with solid-lubricant inserts.Another type is the ceramic linearbearing—a metal sleeve coated with ahard ceramic. Solid plastic or metal-backed plastic sleeve bearings arealso in common linear-bearing use.

Rolling-element bearings. Youcan gain the advantage of lowerrolling friction and, thus, higherspeed capacity by substitutingrolling-element linear bearings forplain linear bearings, much like youcan with rotary bearings. For exam-ple, you can put rolling elements be-tween the simple flat-way plain mat-ing surfaces to reduce friction. Withproper restraining systems, the ele-ments could be balls or rollers. Like-wise for more complex systems suchas V and dovetail ways. Ball, roller,and crossed-roller systems are in ser-vice there. Also, the flat way bearingcan become more like a box beam withcam-follower bearings or similarbearings on two or more sides to guide

equivalent rotary conversion systems.• Long stroke. Travel length is lim-

ited only by platen length — and in-creasing length does not lessen per-formance.

• Multiple motion. By overlappingtrajectories, more than one forcer canoperate on one platen at one time.

BEARINGSAs with any other power transmis-

sion system, a linear motion systemmust be supported and guided. Gen-erally, moving parts exert some force,and the force must be resisted for thesystem to remain stable. That is thechief reason for any bearing: It mustbear a load. For information on bear-ings for rotary systems, see the Bear-ings Product Department in thishandbook.

Linear-motion bearings are ofmany types, some much like rotarybearings. One of the most common

ways to classify bearings is by type ofbearing-to-load contact:

• Plain bearings. Surfaces slide oneach other or on a lubricant film be-tween them.

• Rolling-element bearings. Ele-ments such as balls or rollers betweenmoving surfaces provide the lower re-sistance of rolling friction instead ofsliding friction.

Plain bearings. The simplest lin-ear-motion plain bearing to visualizeis the flat way, perhaps the oldest de-vice that lets one machine element,such as the bed of a planer, move eas-ily on another. However, it is also dif-ficult to make well, because nearlyperfect flatness is hard to maintainover a long distance. Early machineways were hand-scraped by crafts-men to remove high spots. The flatway must also have a means to keepthe payload from running off due toany transverse load. A common


Figure 18—Linear step motor works onsame principle as rotary step motor. Here,four sets of teeth on forcer are spaced inquadrature so only one set at a time linesup with any set of platen teeth.


a carriage along the length of thebeam. All these devices are used suc-cessfully in various applications, de-pending on precision, complexity, andcost constraints.

plain bearing.An example of a more complex

rolling element linear bearing is a“smart” bearing, Figure 21. Thesmart designation comes from thisbearing’s ability to recognize types ofmisalignment and to self-alignaround any or all of three axes inde-pendently. It can handle load capaci-ties without giving up the ability toabsorb torsional misalignment withno increase in stress levels.

Each double-track bearing plate in-

Perhaps thes i m p l e s tro l l ing -e le -ment linearbearing is thel i n e a r b a l lbearing. Fig-ures 19 and 20show a basicarrangement,with recircu-lating balls

that travel an oblong path. A linearball bearing has

three or more ob-long circuits ofballs. Each cir-cuit holds balls inone of it straightsides in rollingcontact betweenthe shaft and thebearing race. The

load rollsalong theballs in thispart of thecircuit. Ballsin the rest ofthe circuit re-circulatefreely in theclearance inthe sleeve. Aretainerguides theballs andkeeps themfrom fallingout if thebearing is re-moved fromthe shaft.

The retainermakes the linearball bearing aseasy to handleand install as a

Figure 20—Typical linear ball bearing.Retainer guides balls and keeps them fromdropping out should bearing be removedfrom shaft.

Figure 19—Principle of recirculatinglinear ball bearing.

Figure 21— A transparent version of theSuper Smart Ball Bushing bearing showsprovisions for high capacity and for pitch,roll, and yaw. Steady-state speed can beup to 3 m/sec; coefficient of friction, downto 0.001.

Recirculation tracks

Ring

Shaft side of bearing platewith double load track

Ring side of bearing plate

Figure 22 — Schematic axial cross section shows how bearingplate can self-align (“pitch”) about curved inner surface ofhardened ring.

Machine housing

Ring

Bearing plate

Balls

Shafting surface

Figure 23— Double-track bearing plates have ring-side curvaturethat lets them self-align (“roll”) to distribute load evenly on theirball tracks.

LoadRing inner surface Double track

Bearing plate

& balls

Figure 24 — Each bearing plate can turn about its geometric plancenter to prevent skewing relative to shaft surface (“yaw”).

Bearing plateCenter of rotation for bearing plate

dividually compensates for threetypes of misalignment:

• It compensates for shaft angulardeflection or misaligned housing bore,or pitch, Figure 22.

• It evenly distributes load on itstwo ball tracks, compensating for roll,Figure 23.

• It rotates on a radial axis to elim-inate skewing between ball tracksand shaft, or yaw, Figure 24.

Self-alignment minimizes friction,which holds performance and bearinglife and simplifies installation.

Life expectancy and shaft hardnessinfluence load capacity of a linearbearing-and-shaft combination. Cir-cumferential positioning of load-car-rying working tracks relative to ap-plied-load direction is important. Lifeexpectancy (travel life) is expressedas total inches of linear movement be-tween bearing and shaft. Manufactur-ers’ catalogs give normal and maxi-mum rolling load ratings. Generally,they are based on travel life of 2 mil-lion in. Table 2 and Figures 25 and 26show a way to compute expected life.

A linear ball bearing may form acontinuous cylinder around a shaft, orit may have an axial split that lets itbe preloaded on the shaft when the

linear ball bearing is the linear-and-rotary unit, made up of a sleeve thatholds a cylindrical array of balls incontact between the shaft and hous-ing. It permits individual or simulta-neous linear and rotary movement—a combination needed in manyapplications.

Precise, high-load, linear travel canbe gained in many applications by lin-ear roller bearings instead of ballbearings. Rollers pass over a flatchannel, rolling in line contact be-tween channel and load. At the end ofthe channel, rollers recirculate, run-ning unloaded behind the channel toreturn between channel and load.

Roller skewing could cause insta-bility and high friction. To prevent it,precise roller end guides, central sta-

bearing is forced intoan interference-fithousing.

Some linear ballbearings have “rollingkeyways.” Balls roll ina semicircular-cross-section axial groove toprevent rotation of theentire bearing unit onthe shaft. This ismuch like a ballspline, but with onlyone ball channel.

A variation of the


25

26

Figure 26 — Load correction factor forlinear ball bearings based on shafthardness.

Figure 25 —Load

correctionfactor for

linear ballbearingsbased onrequired

travel life.

Figure 27 — Cylindrical rollers providehigher load capacity than balls, becausethey are in theoretical line contact insteadof point contact. Linear roller way here isa rolling guide unit. Cylindrical rollers runon a track rail to get endless linear motionwhile circulating in a slide unit.


bilizer bands, or similar means areprovided.

Dynamic load capacity of linearroller bearings is generally quoted interms of L10 life for a given travel life,such as 10 million in.

CONTROLSSome sensing devices, such as prox-

imity switches, work equally well forrotary or linear systems. Others, suchas linear variable differential trans-formers, optical linear encoders, andforce transducers, are especially forlinear application.

Much control technology applicableto rotary motion systems applies aswell to linear systems. Digital micro-processing allows it. For example,programmable controls and computersoftware simplify motion control andpermit simultaneously controlled mo-tion in many degrees of freedom. Acase in point is a multiaxis position-ing system that includes rotary aswell as linear movement.

In many complex linear motion sys-tems today, quality of movement de-pends essentially on system stiffness

and resolution of the position-mea-suring equipment. For example, withthe ac linear servomotor shown inFigure 14, it is possible to positionwithin 61 increment of the encodersystem.

The linear measuring systemshown in Figure 14 is generic. It couldserve many kinds of linear motionsystems, and it could be:

• An optical linear encoder. Here ascanning unit consists of a lightsource, photovoltaic cells, condensinglens, and grating reticule. On a linearmotor, it usually mounts on the pri-mary unit. It moves relative to a lin-ear scale with line grating, producingsinusoidal signals. The controllercounts the resulting signals to estab-lish position and speed.

• An encoder system using cableand pulley. With an industrial rotaryencoder having a pulley on its shaftand a tension-controlled cable, youcan sense position and speed.

• Rack-and-pinion encoder system.Here, a gear rack transmits linearmovement. A pinion rotates as itsmeshing rack moves. A rotary en-coder couples to the pinion shaft and

provides position and speed informa-tion. Criteria for high accuracy arestraightness and evenness of rack andpinion.

Interferential optical encoder sys-tems are for applications demandingvery high accuracy and resolution.Laser interferometer measuring sys-tems can offer resolution of 0.01 mm.Common applications are inspectionmachines, extreme-precision machinetools, and wafer-slicing machines forsemiconductor manufacturing. Thesedevices require special consideration ofthe lasers’ environment, such as airtemperature, pressure, and cleanliness.

A growing tendency among linear-motion-device suppliers is to offercomplete systems as well as compo-nents for linear motion. You can getsome systems that include the actua-tor, its bearings, framework, driveand control, and sensors for controland safety limits. You need only boltthe system down and wire it up. Littleor no adjustment or tuning may beneeded.

For more about control compo-nents, see the Controls and SensorsDepartment in this handbook. ■

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