INA reprintNovember 2000

So You Want to Think Lean?Jürgen Hilbinger and Martin Schreiber

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So You Want to Think Lean?New High-Precision Bearing Unit for Cost-Effective Ball Screw Supports in theSmall Diameter RangeJ�rgen Hilbinger and Martin Schreiber

Thinking lean is important – but of course not in terms of the“technical padding” but ratherwhen it comes to cutting the over-all costs for screw drive bearingsupports. The fact that significantimprovements can even be madeto sophisticated technology hasbeen demonstrated time and again.The following article treats thiskind of innovation.

Ready-to-install complete solutions with extremely high load carrying capacity and accuracy like INA’s ZKLF bearingunits (Figure 1) have been very successful in machine-building applications for several years now.The easy installation is the reasonwhy customers from this industrialsector like the product so much. All you have to do is take it out ofthe box, bolt it on and you’re finished. Gone are the days whereinstallation involved mounting separate bearings and seals as wellas numerous components requiredto locate the unit.

1 Product DevelopmentThe fact that the bearing was so successful was reason enough to develop a low-cost design for small dimensions as an additional alternative to the proven ball screw support bearings. The same economical andtechnical advantages were to be madeavailable for other applications. In bearingsupports for drives in testing and measuring equipment, in small processmachines, and machines for precisionmechanics and electronics manufacturingfor instance, low frictional torque and highaccuracy are more important than loadcarrying capacity. Complex solutions involving separate bearings are oftenused to move linear shafts with low advancing forces and high precision.The extremely compact, ready-to-installbearing unit, series ZKLR represents aninnovative system solution for applica-tions such as those mentioned above.And once again, the manufacturer’s philosophy was the primary concern inperfecting the development: a compactdesign that is more cost-effective for thecustomer. This was the same principlethat was followed for the development ofthe ZKLF series mentioned above.

2 DesignSeries ZKLR bearings (Figure 2) consistof two axially and radially clearance-freeand preloaded precision deep groove ballbearings in X arrangement and a thin-walled flange housing. Based on over 40years of experience in precision forming,developers came up with the idea thatthe housing can be manufactured usingmetal forming techniques. This cost-sav-ing technology allows all requirements tobe met: the unit is lightweight, compactand highly accurate.ZKLR units can provide clearance-freesupport for radial loads and axial loadsfrom both directions (Figure 3). The effective load ratings in the axial directionare above the permissible loads on theball screw drive with the appropriate nominal diameter. This automatically allows load rating and envelope conditions to be matched in the bestpossible way.

ZKLR 0624.2Z Mean value Standard deviation

Radial runout 1.2 µm 0.3 µm

Axial runout 4.6 µm 0.5 µm

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3 FrictionA low bearing frictional torque as well asfrictional torque fluctuations are importantcharacteristics of the newly developedbearing. Non-contact sealing shields areused so that these advantages are notimpaired.The friction curves for ZKLR bearing unitsshown in Figure 4 correlate well with thecurves for the appropriate screw drives.Sophisticated testing using various typesof grease allowed the difference betweenbreakaway torque and operating torqueto be optimized.High limit speeds are possible due to thelow heat generation in the bearing. This means that a sufficient amount ofspeed reserves are available for the bearing supports in ball screw drives.

4 AccuracyAs mentioned above, design adjustmentswere made that allowed a more cost-effective solution. One item here thoughwas not considered: accuracy. Measure-ments of the axial and radial runout forZKLR bearing units have shown that running accuracy in the P4 - P5 rangecan be achieved when high-precision formed components are combined withprecision ground bearing components(Table1).

5 Stiffness and Limiting LoadThe stiffness values determined duringthe design phase were confirmed whenmeasurements were made (Figure 5). The axial rigidity for ZKLR 0624.2Z for instance is in the 20 N/µm range.Limiting load graphs are available for auser-friendly design and selecting the required bearing size (Figure 6).

Figure 1 Angular contact thrust ball bearing,series ZKLF

Figure 4: Test results for the optimization of frictional torque

Figure 2 Low-cost angular contact thrust ball bearing, series ZKLR

Figure 3 Solution incorporating series ZKLR angular contact thrust ball bearing

KGT 8 x 2,5 Rexroth Starpreloaded with apprax. 10% of Cdyn

ZKLR with grease ÒaÓZKLR with grease ÒbÓ (standard grease)ZKLR with grease ÒcÓZKLR with grease ÒdÓ

Speed [min–1]

Frictional torque [Nmm] Frictional torque [Nmm]

Speed [min–1]

Table 1: Running accuracy for ZKLR (excerpt from measuring cycle using 19 test samples)

Evaluation criteria INA-ZKLR Conventional solution

+ = goodO = satisfactory- = unsatisfactory

No additional componentsnecessary (flange, cover) + + – + +

No additional seals necessary + + + + + +

Only minimum designspace required + + O – –

No high requirements of the mounting structure + + – – O

Most assembly errorscan be excluded + + – +

6 Installation and MaintenanceThe main feature of this new development is certainly the extremelyeasy installation. The bearing unit can be flange-mounted to a flat, milled or possibly even an unmachined adjacentstructure without radial centering. Thismeans that no additional components are required to locate the bearing in the mounting structure (Table 2). The expensive and time-consuming machining of bearing seat bores is also a thing of the past.

Another advantage INA’s new bearinghas to offer is the low number of requiredcomponents, which means that mountingerrors can be avoided.A completely new perspective on costsavings results from the fact that the bearing unit can be aligned during mounting by turning the adjusting nut.The position of the screw drive is the reference. The lead screw nut serves as afunctional element for bearing alignment.The bearing is first bolted on hand-tight,and the lead screw nut is rotated from

the opposite direction toward the bearing. In this way the bearing can be aligned according to the constraining forces of the reference on the flat mounting surface. The bearing is then inthe optimum radial position, and the onlything left to do is tighten the bolts. This prevents additional radial loads from distortion and provides a higher service life of the unit (Figure 7).Because the universal flange design incorporates two radiused cut-outs, a further cost reduction can be achievedby clamping the bearing with only two mounting bores.That’s right. You can even save costswhen it comes to locating the bearing onthe shaft. Since the defined preload hasalready been integrated in the bearing,the preloading of the bearing typically necessary for other screw drive bearingsupports during installation is eliminatedentirely. No locknut is required. Depending on the load involved, this means that the expensive threads at theend of the lead screw can also be eliminated. A clearance-free clampconnection provides a sufficient means of locating the bearing unit on the shaft. Another advantage is the fact that thebearing’s “pot-shaped” diameter can beused to center the motor support. The high dimensional accuracy of thedeep-drawn housing makes this possible.

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Figure 5 Axial stiffness for ZKLR 0624.2Z (excerpt from measuring cycle using 6 test samples)

Figure 6 Static limiting load graph for ZKLR

Deflection [µm] Axialload Fa [N]

F [N] Radial load Fr [N]

CalculatedNr 1 (+x)Nr 2 (+x)Nr 3 (+x)Nr 4 (+x)Nr 5 (+x)Nr 6 (+x)

ZKLR2060.2RSZKLR1547.2RSZKLR1535.2ZZKLR0828.2ZZKLR0624.2Z

Table 2 Assessment of requirements for the mounting structure and assembly for ball screw support bearings

For most applications, ZKLR units aremaintenance-free. Relubrication is not required due to the large grease reservoir. In addition, the bearing unithousing is coated with INA’s special plating Corrotect®, which provides an effective means of protection against corrosion for all exposed bearing surfaces.

7 Customer BenefitsNumerous application areas have demonstrated the tremendous potentialthat lightweight, optimally designed machine elements have. One examplehere is the automotive supplier industry.For the product described here, the approach incorporates the combinationof precision and cost-effective manu-facturing, and the philosophy behind the product is “not as much as possible,but only what is required.” This means that the user has the follo-wing advantages:• compact designs• minimized maintenance• high accuracy at very low costs• minimum power through low frictional

torque

Literature:INA publication VSLINA test report VA 74456

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About the Authors:Martin Schreiber is a senior applicationsengineer responsible for production machines and systems at INA WälzlagerSchaeffler oHG.Jürgen Hilbinger is an applications engineer responsible for productionmachines and systems at INA WälzlagerSchaeffler oHG.

Figure 7 Aligning the bearing unit by turning the adjusting nut

• large speed reserves• simple, quick and cost-effective

installation• simpler design of mating components• less paperwork involved since fewer

parts must be orderedOnce again it has been shown that targeted cost optimization, the improvement of technical characteristicsand easy installation and maintenancecan all be combined in the best possibleway. This combination is a further innovative step towards more productivityat lower costs. INA’s new ZKLR bearingsrepresent still another advance in bearingtechnology that will continue to supportthe strong market position of electro-mechanical drives.

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