INA reprint
November 2000
So You Want to Think Lean?
New High-Precision Bearing Unit for Cost-Effective Ball Screw Supports in the
Small Diameter Range
JŸrgen Hilbinger and Martin Schreiber
Thinking lean is important – but of course not in terms of the “technical padding” but rather when it comes to cutting the over-all costs for screw drive bearing supports. The fact that significant improvements can even be made to sophisticated technology has been demonstrated time and again. The following article treats this kind of innovation.
Ready-to-install complete solutions with extremely high load carrying capacity and accuracy like INA’s ZKLF bearing units (Figure 1) have been very successful in machine-building applications for several years now. The easy installation is the reason why customers from this industrial sector like the product so much. All you have to do is take it out of the box, bolt it on and you’re finished. Gone are the days where installation involved mounting separate bearings and seals as well as numerous components required to locate the unit.
1 Product Development
The 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 and technical advantages were to be made available for other applications. In bearing supports for drives in testing and measuring equipment, in small process machines, and machines for precision mechanics and electronics manufacturing for instance, low frictional torque and high accuracy are more important than load carrying capacity. Complex solutions involving separate bearings are often used to move linear shafts with low advancing forces and high precision. The extremely compact, ready-to-install bearing unit, series ZKLR represents an innovative system solution for applica-tions such as those mentioned above. And once again, the manufacturer’s philosophy was the primary concern in perfecting the development: a compact design that is more cost-effective for the customer. This was the same principle that was followed for the development of the ZKLF series mentioned above.
2 Design
Series ZKLR bearings (Figure 2) consist of two axially and radially clearance-free and preloaded precision deep groove ball bearings in X arrangement and a thin-walled flange housing. Based on over 40 years of experience in precision forming, developers came up with the idea that the housing can be manufactured using metal forming techniques. This cost-sav-ing technology allows all requirements to be met: the unit is lightweight, compact and highly accurate.
ZKLR 0624.2Z Mean value Standard deviation
Radial runout 1.2 µm 0.3 µm
Axial runout 4.6 µm 0.5 µm
3 Friction
A low bearing frictional torque as well as frictional torque fluctuations are important characteristics of the newly developed bearing. Non-contact sealing shields are used so that these advantages are not impaired.
The friction curves for ZKLR bearing units shown in Figure 4 correlate well with the curves for the appropriate screw drives. Sophisticated testing using various types of grease allowed the difference between breakaway torque and operating torque to be optimized.
High limit speeds are possible due to the low heat generation in the bearing. This means that a sufficient amount of speed reserves are available for the bearing supports in ball screw drives.
4 Accuracy
As mentioned above, design adjustments were made that allowed a more cost-effective solution. One item here though was not considered: accuracy. Measure-ments of the axial and radial runout for ZKLR bearing units have shown that running accuracy in the P4 - P5 range can be achieved when high-precision formed components are combined with precision ground bearing components (Table1).
5 Stiffness and Limiting Load
The stiffness values determined during the design phase were confirmed when measurements 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 a user-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 Star preloaded with apprax. 10% of Cdyn
ZKLR with grease ÒaÓ
ZKLR with grease ÒbÓ (standard grease) ZKLR with grease ÒcÓ
ZKLR with grease ÒdÓ
Speed [min–1]
Frictional torque [Nmm] Frictional torque [Nmm]
Speed [min–1]
Evaluation criteria INA-ZKLR Conventional solution
+ = good O = satisfactory - = unsatisfactory
No additional components
necessary (flange, cover) + + – + +
No additional seals
necessary + + + + + +
Only minimum design
space required + + O – –
No high requirements
of the mounting structure + + – – O
Most assembly errors
can be excluded + + – +
6 Installation and Maintenance
The main feature of this new
development is certainly the extremely easy installation. The bearing unit can be flange-mounted to a flat, milled or possibly even an unmachined adjacent structure without radial centering. This means 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 bearing has to offer is the low number of required components, which means that mounting errors can be avoided.
A completely new perspective on cost savings 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 a functional 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 in the optimum radial position, and the only thing 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 achieved by clamping the bearing with only two mounting bores.
That’s right. You can even save costs when it comes to locating the bearing on the shaft. Since the defined preload has already been integrated in the bearing, the preloading of the bearing typically necessary for other screw drive bearing supports during installation is eliminated entirely. No locknut is required. Depending on the load involved, this means that the expensive threads at the end of the lead screw can also be eliminated. A clearance-free clamp connection provides a sufficient means of locating the bearing unit on the shaft. Another advantage is the fact that the bearing’s “pot-shaped” diameter can be used to center the motor support. The high dimensional accuracy of the deep-drawn housing makes this possible.
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] Calculated Nr 1 (+x) Nr 2 (+x) Nr 3 (+x) Nr 4 (+x) Nr 5 (+x) Nr 6 (+x) ZKLR2060.2RS ZKLR1547.2RS ZKLR1535.2Z ZKLR0828.2Z ZKLR0624.2Z
For most applications, ZKLR units are maintenance-free. Relubrication is not required due to the large grease reservoir. In addition, the bearing unit housing is coated with INA’s special plating Corrotect®, which provides an
effective means of protection against corrosion for all exposed bearing surfaces.
7 Customer Benefits
Numerous application areas have demonstrated the tremendous potential that lightweight, optimally designed machine elements have. One example here is the automotive supplier industry. For the product described here, the approach incorporates the combination of 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 VSL INA test report VA 74456
About the Authors:
Martin Schreiber is a senior applications engineer responsible for production machines and systems at INA Wälzlager Schaeffler oHG.
Jürgen Hilbinger is an applications engineer responsible for production machines and systems at INA Wälzlager Schaeffler 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 ordered
Once again it has been shown that targeted cost optimization, the
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