So You Want to Think Lean?
Jürgen Hilbinger and Martin Schreiber
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 overall 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.
2
1 Product Development
2 Design
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 applications 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.
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 thinwalled 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-saving technology allows all requirements to
be met: the unit is lightweight, compact
and highly accurate.
ZKLR units can provide clearance-free
support for radial loads and axial loads
from both directions (Figure 3). The
effective load ratings in the axial direction
are above the permissible loads on the
ball screw drive with the appropriate
nominal diameter. This automatically
allows load rating and envelope
conditions to be matched in the best
possible way.
Figure 1 Angular contact thrust ball bearing,
series ZKLF
Figure 2 Low-cost angular contact thrust ball
bearing, series ZKLR
Figure 3 Solution incorporating series ZKLR
angular contact thrust ball bearing
3 Friction
4 Accuracy
5 Stiffness and Limiting Load
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.
As mentioned above, design adjustments
were made that allowed a more costeffective solution. One item here though
was not considered: accuracy. Measurements 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).
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).
ZKLR 0624.2Z
Mean value
Standard deviation
Radial runout
1.2 µm
0.3 µm
Axial runout
4.6 µm
0.5 µm
Table 1: Running accuracy for ZKLR (excerpt from measuring cycle using 19 test samples)
Frictional torque [Nmm]
Frictional torque [Nmm]
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]
Speed [min–1]
Figure 4: Test results for the optimization of frictional torque
3
Radial load Fr [N]
F [N]
ZKLR2060.2RS
ZKLR1547.2RS
ZKLR1535.2Z
ZKLR0828.2Z
ZKLR0624.2Z
Calculated
Nr 1 (+x)
Nr 2 (+x)
Nr 3 (+x)
Nr 4 (+x)
Nr 5 (+x)
Nr 6 (+x)
Deflection [µm]
Axialload Fa [N]
Figure 5 Axial stiffness for ZKLR 0624.2Z
(excerpt from measuring cycle using
6 test samples)
Figure 6 Static limiting load graph for ZKLR
6 Installation and Maintenance
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 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.
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
++
–
+
Table 2
4
Assessment of requirements for the mounting structure and assembly for ball screw
support bearings
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 7 Aligning the bearing unit by turning the adjusting nut
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 manufacturing, and the philosophy behind
the product is “not as much as possible,
but only what is required.”
This means that the user has the following advantages:
• compact designs
• minimized maintenance
• high accuracy at very low costs
• minimum power through low frictional
torque
• 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
improvement of technical characteristics
and easy installation and maintenance
can all be combined in the best possible
way. This combination is a further
innovative step towards more productivity
at lower costs. INA’s new ZKLR bearings
represent still another advance in bearing
technology that will continue to support
the strong market position of electromechanical drives.
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.
5
91072 Herzogenaurach
Phone (+49) 91 32 82 - 0
Fax
(+49) 91 32 82 - 49 50
[email protected]
www.ina.com
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