TECHNOLOGYCALLS THE TUNE,PAGE 74Unraveling mythsabout ball-screwaccuracy, page 88FRESH FROM THENATIONAL LABS:INNOVATIVE E-BEAMWELDING, page 94

JULY 8, 2004www.machinedesign.com

A PENTON PUBLICATION

CAD MAKES TEAMS MORE EFFICIENT, PAGE 84

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Misconceptions canlead to overspendingand premature failure.

Patrick ScottProduct Manager, BallscrewsBosch Rexroth Corp.Linear Motion and Assembly

TechnologiesCharlotte, N.C.

With so many different ballscrews on the marketand the attendant floodof data from manufac-turers, designers can

have a tough time sorting out what trulyaffects ball-screw performance. Informa-tion often focuses on whiz-bang featuresthat bring few real benefits, while moremundane but critical factors are missedcompletely.

To help cut through the confusion,here’s a look at four factors critical to suc-cessful ball-screw applications, as well assome related myths and misconceptions.

TRAVEL ACCURACYTypically, an engineer’s first and fore-

most concern is travel accuracy, which ispossibly why so many standards addressthis single topic. Chief among them areDIN 69051, ISO 3408, JIS B1191, andANSI-B5.48, which cover topics rangingfrom material specifications to geomet-ric tolerances.

The different norms can confuse theimportant issues. But common to all thespecs is that lead error is the most criti-cal indicator of travel accuracy. Lead er-ror refers to the travel error over a givenlength as the nut travels along a screw. Itis expressed in “mm per 300 mm” or“inches per foot” and determines a ballscrew’s accuracy rating.

According to convention, a lower rat-ing number means less lead error and,therefore, better accuracy. In otherwords, a Class 1 screw has a substan-tially higher accuracy than a Class 7screw. Of course, the high-accuracyClass 1 screw also costs more and maytake up to 10 weeks longer to produce.Designers should use accuracy ratingsas the starting point for overall slide ac-curacy, but also weigh performance re-quirements against lead time and cost.

MYTH #1: Accuracy dictates themanufacturing method.

This implies high-accuracy screwsmust be ground, not rolled. In fact, al-though many designers believe grindingis the only way to make high-accuracyscrews, no specification dictates a pro-duction method for a given ball-screwclass. The specs do, however, differenti-

ate between “Precision” and “Trans-port” screws. Although lead error repre-sents millimeters of error per 300-mmtravel segment, T-Class lead error canaccumulate over multiple segments. Incontrast, P-Class accuracy controls 300-mm lead error and limits accumu-lation over extended lengths.

In the past, rolled or cold-formedscrews traditionally fell into the T-Classarena, and only ground ball screws couldhold P-Class tolerances. Today, however,technological advances let some manu-facturers produce P-Class accuracies inprecision-rolled screws. Rolling hasevolved into a CNC-controlled, tight-tol-erance process capable of P3 accuracy,near-perfect roundness, and toleranceswell within DIN control limits.

The capability to produce screws ofmost any accuracy with either technique

MECHANICALEdited by Kenneth Korane

BALL-SCREW BASICS:Debunking the myths

Ball screws comein a myriad oftypes and styles.Look at factorssuch as accuracy,preloading, andlubrication to helpmatch the rightproduct to a givenapplication.

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offers significant benefits. Machinebuilders no longer have to pay premiumprices for top performance and tolerateexcessive lead times. This lets them offersuperior machines at a better price,while still making a fair profit.

NONSCREW FACTORSIn selecting components for a given

application, designers must also con-sider factors beyond the ball screw itself,such as accuracy and repeatability.

MYTH #2: Ball-screw accuracyequals axis accuracy.

On the surface, this seems true. But,accuracy and repeatability are not thesame, and other machine componentsaffect the total tolerance stack-up. Re-peatability is the screw’s ability to returnto a defined point, time after time, andmost designers agree it is most critical.

Many factors contribute to repeatabil-ity, including drive connections, guidemechanisms such as rails or shafting,and the machine structure itself. Itseems logical that poor performance ofsurrounding components can jeopard-ize both the accuracy and repeatability ofeven a “perfect” ball screw. Yet the pres-sure to cut corners and save money oftenresults in less than satisfactory machineperformance once testing begins. Then,the need to redesign a machine negatesany short-term savings, delays marketintroduction, and may add to mainte-nance or repair costs for end users.

Assuming the machine structure hasbeen properly designed, one can con-centrate on motion-related components.To optimize performance, eliminatingany and all lost motion — commonly re-ferred to as backlash — is critical. Typi-cally, most backlash is between the ballnut and screw, and a range of nut designshelps remove backlash by preloadingthe system. Preloaded ball nuts have nei-ther axial nor radial freedom. Instead,the ball nut is matched to the screw byadjusting a variable dimension. This canbe accomplished in several ways.

One preloading method uses a dou-ble-nut system. Double nuts wedge thenut bodies together against a spacerand lock them in place. (A simpler ex-ample of this would be a “jam nut”used on a nut-and-bolt mount. Thesecond nut wedges against the first andlocks it in place.)

Another preloading option is the lead-shift method. It uses a manufactured off-

set in the raceway spiral,shifting it a few micronshalfway down the nut. Thischanges the angle of en-gagement for the balls in asimilar fashion to the dou-ble nut.

Finally, the ball-selectmethod uses balls that areintentionally a few mi-crons larger than a perfectfit. Because the balls arelarger than the groove,they are forced into con-tact with all raceways ofthe nut and screw. Thisfour-point contact eliminates backlash.But only a few manufacturers can holdthe tight tolerances that both nut andscrew require in the ball-select method.

MYTH #3: The double nut is thebest preloading method.

While the double nut provides excel-lent preloading, for many applications aball-select preload offers almost thesame performance at considerablylower cost. A common misconceptionabout the double-nut method is that twonut bodies will double load capacity. Infact, each nut body takes load only inone direction. The opposing nut actuallydeflects out of load sharing during oper-ation so capacity does not double. Dou-ble nuts are also difficult to assemble.

Lead-shift nuts haveball-contact patternssimilar to double nutsbut typically have feweractive grooves. This re-duces load-carryingcapacity and, there-fore, life expectancy.

Ball-select preloadstrikes a compromisebetween these two op-tions. Oversized ballsmake each groove inthe nut an active oneand take loads in bothdirections.

Another lost-motionconsideration is de-flection. The Systemrigidity chart com-pares a double nut,lead-shift nut, and ball-select nut, all onscrews of increasinglength. The double nuthas only 5 to 10%

MECHANICAL

higher rigidity versus the ball-select nut,while the lead-shift version drops offdramatically. Taking the effects of screwlength into consideration, as shown inthe chart, the ball-nut styles have evenless differentiation. Thus, in many appli-cations, a single nut with ball-select pre-loading offers much the same perform-ance as double nuts, at substantiallylower costs.

LUBRICATIONEven a well-designed ball-screw as-

sembly that meets an application’s per-formance specs may disappoint in real-world operation, and lubrication failureis the number-one cause of prematurebreakdown.

System rigidity

Rigi

dity

, N/

m

Increasing unsupported screw length0

100

200

300

400

500

600

700

Lead-shift nut

Comparing preload optionsSTYLE DOUBLE NUT LEAD SHIFT BALL SELECTCapacity + – – + Dimensional – – + + Cost – + + Handling – + + Rigidity + + – – +

A brief comparison shows key advantages anddisadvantages of different preloadingmethods.

Double-nut ball screws have only 5 to 10%more rigidity than those using ball-selectnuts, and the advantage narrows atlonger screw lengths.

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All potentially have a severe impact onscrew performance and life. So it is al-ways a good idea to review the applica-tion carefully with the screw supplier’sengineers. They may have seen similarapplications and can help avoid mis-takes others have already made. They’llalso help ensure the right ball-screw andball-nut combination. ■.

Copyright © 2004 by Penton Media, Inc.

MECHANICAL

MYTH #4: Lubed-for-life screwassemblies guarantee 10,000 kmof travel.

One common misconception, sup-ported inadvertently by many manufac-turers’ claims, is that “lubed for life”means exactly that: No maintenance isever required.

Many publish test data on systems thatproduced 10,000 km of travel. But forusers, it is critical to know the test condi-tions and how they relate to real-worldoperation. In some cases, 10,000-kmresults are measured with no appliedloads in clean-room environments.

Real-world applications rarely runclean and unloaded. For a ball screw op-erating inside a machine tool, with heavyloads and high speeds, lubed-for-lifeclaims are dubious at best.

It’s important to understand how amanufacturer’s claims relate to an appli-cation. If test conditions are not easilyunderstood, or if the claims seem toogood to be true, get more information to

avoid disappointment in the end.When comparing products from dif-

ferent suppliers, always try to get enoughdata for an “apples-to-apples” compari-son. So-called “lubed-for-life” designsdo extend service intervals because theyseal in and maintain lubrication where itis needed, inside the ball nut. Effectiveseals also keep out contaminants. But“life” means different things to differentmanufacturers.

In the end, the most important point isto find the best match of capabilities,price, and lead times for the application.Usually, machine design and componentchoices determine how to get the highestperformance with the lowest total cost ofownership. Mistakes can be costly, eitherin redesign losses or, more importantly,poor machine performance.

These four factors are the most criti-cal, but other considerations can play arole as well. They include end-supportbearings, overall system rigidity, and op-erating environment, just to name a few.

MAKE CONTACT:Bosch Rexroth Corp., (800) 438-5983,

www.boschrexroth-us.com

PreloadingoptionsThis illustration shows 2D rep-resentations of different preloadmethods. In each, the diagonallines represent the direction inwhich force is applied to theballs.

The double-nut methodshows the lower edges of two nuthousings separated by a spacerplate. The spacer plate appliesforce to the nut housings which,in turn, apply force to the ballsin the screw shaft raceway.

In the lead shift method, theraceway in the nut body itself hasa manufactured offset to applyforce to the balls as the nut trav-els along the screw. (The offsetis shown as “lead + a.”) In theball-select method, the balls areslightly larger than the groove.

Ball-screw preload methods

DOUBLE NUT

LEAD SHIFT

BALL SELECT

Tension Tension

Tension Tension

Space plate

Nut B Nut A

Screw shaft

Lead Lead

Nut

Nut

Screw shaft

Screw shaft

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