Dynamic feed axis with ball screw drive: Reprint HVK

Dynamic feed axis with
ball screw drive
Thorsten Frank and Erich Lunz
INA reprint
September 1997
Dynamic feed axis with ball screw drive
Dipl.-Ing. Thorsten Frank and Erich Lunz
This paper describes the design
and development of a “Dynamic
feed axis with ball screw drive” and
the potential for innovative drive
mechanisms for machine tool axes
based on proven technology.
The system comprises a ball screw
spindle rigidly located at both ends
and a driven nut. The design features
and operating characteristics
achieved with the test device are
described.
2
1 Introduction
In recent years, the German machine
construction sector has been affected by
strong international competition [1]. As a
result, companies in the NC machine tool
sector are under constant pressure to
increase both the productivity and
machining quality of their machines.
In addition to reduced costs in the
production process, new machine
concepts are required which allow flexible
and cost-effective handling and interlinking functions.
Increasing both the machining speed and
machining quality puts high demands on
the dynamic and thermal behaviour of the
feed systems of modern machines.
This trend is supported by considerable
progress in the development of control
systems and dynamic drive motors [2].
In contrast, the mechanical components
in the drive are often the weakest link
when seeking improvements in dynamic
behaviour. The classical NC axis with a
locating/non-locating bearing arrangement is currently limited to a speed range
of 30 to 60 m/min and acceleration up to
10 m/s2 [3].
One possibility for overcoming these
technical limitations is the linear drive,
whose particular features offer a clear
increase in dynamic characteristics.
This system converts electrical energy
directly into motion without the use of
mechanical elements. Test drives of this
type have achieved acceleration values of
10 to 12 times acceleration due to gravity
and speeds up to 100 m/min [4].
In contrast to these general benefits,
however, drives of this type have lower
dynamic rigidity than electromechanical
feed systems, greater thermal sensitivity
to the machine structure, significant
sensitivity to load parameter fluctuations
and, in particular, significantly higher
system costs; the linear drive cannot be
seen as the ideal solution for all applications [3, 5]. It is therefore considered
mainly for light machining applications.
It is not feasible for the manufacture of
large, heavy components requiring high
machining forces.
Legend:
1. Polymer concrete bed
2. Locating housing
3. Linear guidance system
RUE 45, INA
4. Ball screw drive (40 x 40),
length 2 m, Deutsche Star
5. Bearing housing with double
row ball bearing ZKLF, INA
6. Motor flange and toothed belt
drive
7. Table for SIEMENS
synchronous servomotor (FT6)
and AMK asynchronous
servomotor (SKB 10)
Figure 1 Overview of test device, Institute for Machine Tools and Production Sciences (wbk) of
University of Karlsruhe
It is therefore desirable to develop a
system which is competitive in dynamic
terms and based on proven feed system
technology. In a joint project promoted by
the federal state of Baden-Württemberg,
established machine tool manufacturers
and component suppliers have worked
with the Institute for Machine Tools and
Production Sciences of the University
of Karlsruhe to design and build an innovative feed kinematic system comprising
a spindle rigidly located at both ends and
a driven nut.
2 Structure of test device of
“Dynamic feed axis” with
driven nut
In electromechanical feed axes, the rotary
motion of the servomotor is converted by
a ball screw drive to give linear table
motion. The classical NC axis design
comprises a driven feed spindle and a
nut fixed rigidly to the table.
The “Dynamic feed axis with ball screw
drive” is a variant on this design: the
spindle is rigidly clamped in two housings
and the nut is driven by a servomotor
(Figure 1).
The investigation considered two drive
variants. Figure 2 shows (left) a standard
servomotor driving the nut via a toothed
belt and (right) a direct drive system with
a hollow shaft servomotor. In both cases,
the dynamic motors of the latest design
provide 100 Nm for table acceleration
and achieve speeds up to 4000 min–1.
Figure 3 An important component of the test
device: the INA double row axial
angular contact ball bearing ZKLF
Furthermore, the table mass can be
increased in steps up to half a tonne.
Digital control systems with a linear scale
for positional feedback are used.
In order to achieve a system with high
dynamic characteristics, the complete
rotary drive system was designed in order
to achieve a compact structure which
would minimize the moments of inertia
(Figure 2). The bearing arrangement was
based on the INA double row axial
angular contact ball bearing ZKLF
(Figure 3) [7], giving rigid connection of
the nut with the table.
In order to allow accurate comparison of
the two drive variants, the same bearing
was fitted in the hollow shaft servomotor.
Despite the mass of the servomotor to be
moved, the total moment of inertia of the
prototype with a 2 metre spindle is
identical to that of a driven spindle the
same length. For a longer spindle, the
inertia gives an even more favourable
situation.
Figure 2 Drive variants: standard servomotor / toothed belt (left), direct drive with hollow shaft servomotor (right)
3
50
Rigidity of locating/non-locating arrangement, c a1
80
300
60
200
40
20
100
0,00
45
Acceleration**, a [m/s 2]
Rigidity of locating/locating arrangement, ca2
Rigidity ratio between locating/non-locating
and locating/locating arrangements
100
Rigidity ratio, ca1/ca2*100 [%]
Axial rigidity, ca [N/µm]
400
Servomotor SIEMENS FT-6
with ball screw drive (d=40mm, p=40mm)
Servomotor SIEMENS FT-6
with ball screw drive (d=40mm, p=20mm)
40
35
* Rotary and translational masses
** Based on maximum motor torque of 120 Nm
30
25
20
15
10
5
0,25
0,50
0,75
1,00
1,25
1,50
1,75
0
2,00
0
50
150
250
Figure 4 Rigidity of location/non-locating and locating/locating spindle
arrangement as a function of axis position with 2 m spindle
length
3 Increased productivity
through new kinematic
system
The characteristics of the new feed
kinematic system are well matched to the
high motor performance. Since the
spindle is located rigidly at both ends, the
axial rigidity of the complete system far
exceeds that of a spindle with a locating/
non-locating bearing arrangement.
Figure 4 shows the relationship between
axial rigidity and spindle nut position.
While the axial rigidity of a conventional
spindle is lowest at the non-locating end,
the rigidity with a locating/locating
arrangement is lowest in the centre of the
traverse range, minimizing the rigidity
variation over the whole length.
In terms of maximum traverse speed, the
driven nut gives clear advantages since it
avoids the problem of critical whirling
speed. Frequency analyses of the test
device indicate that the lower axial natural
frequency of the driven system is over
120 Hz. Even at high drive speeds, the
system always runs in the subcritical
range.
4
350
450
550
650
750
850
950
1050 1150
Moving mass*, m t [kg]
Spindle nut position, ls [m]
Figure 5 Calculated acceleration of dynamic feed axis at maximum
torque
The traverse speed is therefore limited
only by the maximum rolling speed of the
screw drive. The screw drives in the tests
were of 40 mm diameter and 40 mm
pitch, giving a traverse speed of
120 m/min at the maximum possible
speed of 3000 min–1. For a table mass of
100 kg, acceleration of 30 m/s2 was
achieved; no attempt was made to reach
the theoretical limit of 45 m/s2, in order to
preserve the operating life of the drive
(Figure 5).
Increasing the productivity of a machine
tool significantly increases the thermal
load, since almost all the energy is
converted into heat [8, 9]. The ongoing
increase in speed and velocity gives a
continuous increase in drive system
temperature.
The measurement curves in Figure 6
show the temperature equilibrium of the
feed axes for traverse speeds between
40 and 100 m/min. The highest temperatures occur in the area of the nut and
spindle, while significantly lower temperatures occur in the area of the INA bearing
arrangement, motor and guidance
systems. Similar values are obtained for
the direct drive and indirect drive variants.
When the table mass is increased to the
600 kg maximum, the temperature values
increase by 10 %.
The influence of the speed on spindle
elongation is shown in Figure 6. Thermal
expansion of the spindle due to heating
causes a large length change in the
spindle at high speeds. Pretensioning the
spindle to compensate for this expansion
would require forces of about 60 to
100 kN and is not therefore feasible.
Geschwindigkeit, v [m/min]
20
40
60
80
100
1200
80
70
800
60
50
600
40
400
Temperature, T [°C]
Expansion, δ [µm]
1000
Nut temperature in direct drive
Nut temperature in indirect drive
Spindle temperature in indirect drive
External INA bearing arrangement in indirect drive
Spindle elongation of indirect drive
30
Travel distance: 0,84 m
200
Acceleration: 15 m/s
Table mass: 100 kg
500
1000
2
1500
Measurement time: 120 min
Ground spindle 40x40
2000
20
2500
Speed, n [min–1]
Figure 6 Temperature and expansion curves at characteristic points of
feed axis with table mass m1 = 100 kg
In order to achieve constant operating
behaviour of the ball screw drive, a
preload adjustment device is required
which can compensate for thermal
expansion by briefly opening a simple
clamping device without reducing the
clamping location rigidity.
A further possibility for a rigidly tensioned
spindle is the use of a hollow spindle with
internal cooling by an agent. Variations in
torsional rigidity can effectively be ignored
and, due to the stationary spindle,
no eccentric mass effect should be
assumed. The spindle coolant circuit can
be integrated in the machine coolant
system. Long term studies [10] showed
a reduction in differential spindle
temperature from 32 to 4 K.
Figure 7 Investigation of dynamic behaviour of direct drives
3.1 Positional accuracy with the aid
of dynamic drive investigations
In addition to high acceleration and
traverse speed, the development
programme also has the objective of
optimized transmission behaviour of the
feed kinematic system with positional
control (Figure 7).
In order to give the required motion,
the motion axis must conform to the
specified positional guidance values with
the minimum possible distortion. In an
actual system, this can only be achieved
in approximate terms due to the intrinsic
dynamics of the positional control with
its energy conversion system and
mechanical components. The objective is
therefore to achieve system behaviour
which allows transmission of high
acceleration forces to the feed kinematic
system without causing significant
mechanical vibrations. This is a basic
precondition for achieving adequate path
accuracy even at high feed rates [11, 12].
In order to determine the transmission
behaviour, the skip response of the
speed control circuit was recorded on the
table using an acceleration sensor.
The table mass can be increased by
steps up to 600 kg. Figure 8 (top of
page 6) shows the system response to
a disruption due to a nominal value skip
from 0 to 10 m/min, corresponding for
example to forces in milling or turning
with interrupted cut.
The ripple times of the direct drive are
about 80 m/s and therefore in an
acceptable range for machine tools.
Increasing the table mass from 100 kg
to 600 kg does not lead to an increase
in times. The indirect drive gives even
shorter ripple times.
The excellent dynamic behaviour of the
system can be attributed to the fact that
the transmitted torques of the driven nut
are absorbed on both sides of the
spindle. Since the spindle is clamped at
both ends, this reduces the torsional
tendency of the spindle. The high axial
rigidity of the system allows the motor
torque to be converted directly into axial
acceleration without excessive energy
storage by spindle torsion. Furthermore,
the system has good damping with
maximum values of about 0,04 in the
central area of the spindle travel.
5
20
-640,000
Position, X [mm]
Acceleration, a [m/s 2]
10
0
-10
-20
Actual position
Nominal position
-645,000
-30
-40
-650,000
Ripple curve with table mass of 100 kg
Ripple curve with table mass of 600 kg
-50
0
20
40
60
80
100
120
140
160
180
200
0,30
0,33
Figure 8 Skip response of direct drive
For a defined positioning process, this
means a significant reduction in travel
time without a reduction in accuracy.
This is shown in Figure 9 by a positional
skip with accurate stop for a distance of
400 mm and a speed of 100 m/min.
These characteristics are also favoured
by the significant increase in the natural
frequencies of the complete system
compared to conventional axes. With a
single frequency of 170 Hz, the direct
drive shows clear single-mass vibration
behaviour. The axial dynamic rigidity of
the preloaded spindle is almost
independent of the table position.
Single-mass vibration behaviour offers
good controllability of the axis, which can
be further improved by use of an
appropriate control system. In addition to
its high axial natural spindle frequency of
130 kHz in the lower range, the indirect
drive has other natural frequencies which
can be attributed to the toothed belt.
This frequency can be favourably
influenced by adjusting the preload of the
toothed belt or the rigidity of the vibration
chain.
The dynamic drive investigations showed
that, due to the rigid arrangement,
the new NC motion axis type has good
dynamic behaviour, not only for small
table masses but also for larger masses,
one of the main weak points of the linear
motor.
6
0,35
0,38
0,40
0,43
0,45
0,48
0,50
0,53
Time, t [s]
Accurate stop [v = 100 m/min]
Time, t [ms]
Disruption behavior [nominal value skip v = 10 m/min]
Figure 9 Positional behavior at a (NC) = 15 m/s2, Kv = 4000/min,
table mass of 410 kg and a travel distance of 400 mm
4 Increased speeds through
modified bearing technology
4.1 Bearing arrangement of feed
spindle nut
The improved dynamics of the whole
drive system obviously requires further
development of the individual components.
The bearing arrangement of the spindle
and nut, comprising a preloaded, double
row axial angular contact ball bearing has
been further developed to give a clear
increase in performance of these proven
bearings. In particular, the individual
components of the bearing have been
modified to allow higher speeds.
Production of the raceways was
improved by modern manufacturing
technology, and considerable progress
was made by the use of lubricant
compositions matched to the specific
load spectrums.
Comprehensive tests at INA have
confirmed this progress, with the result
that a dimensionally identical bearing can
now be offered – with no increase in
costs – giving a 30% increase in speed
(Figure 10).
While the conventional bearing arrangement met the requirements of the test
device and the tests, it was known to
be close to the upper limits of the
performance range. The traverse speed
of 120 m/min and the 40 mm ball
screw lead require a bearing speed of
3000 min–1.
This speed was beyond the limiting
speed nG of 2400 min–1 given in the
catalogue for the bearing ZKLF 60145.2Z
then fitted in the test device. It was found
that this speed limit could be increased to
3000 min–1 with the modified bearing.
Where the traverse speed of 120 m/min
was previously outside the permissible
limiting speed, the new bearing is within
the permissible limit values.
4.2 Hybrid bearings
The use of ceramic balls as mentioned
earlier has also been brought to fruition.
On INA test rigs, a hybrid bearing
comprising steel rings and ceramic rolling
elements showed large increases in
speed, reaching speeds of 6500 min–1
without difficulty. The significantly lower
thermal expansion coefficient of
2,9 µm/m/K of the ceramic balls is of
positive benefit here. If the steel rings
(inner ring and larger outer ring) expand
by a certain amount due to increasing
temperature, the ceramic balls expand
by a smaller amount. This reduces the
preload in the bearing, reducing the
specific rolling element load and the
friction; from a bearing technology
viewpoint, a traverse speed of 260 m/min
could be achieved with a 40 mm pitch
ball screw.
This seems far-fetched of course, but it is
clear that there are no obstacles to
further progress in dynamic feed axes
with ball screw drive and that bearing
components with an appropriate level of
bearing technology are already available.
Hybrid special bearing 250%
with ceramic balls
Modified standard bearing
130%
Conventional 100%
standard bearing
Figure 10 Bearing arrangement of feed spindle nut with double direction axial angular contact ball bearing ZKLF. Speed increase with modified
bearing = 130 %, with hybrid special bearing with ceramic balls = 250 %. Fitting example according to wbk design.
5 Summary and prospects
Literature
The feed system developed at the
Institute for Machine Tools and Production Sciences opens up new prospects
for the development of new generations
of dynamic machines based on the
proven ball screw drive technology.
With traverse speeds up to 120 m/min
and acceleration up to 30 m/s2, this
technology represents an alternative to
linear drives in terms of equivalent
performance capacity. Furthermore, this
dynamic behaviour is not limited to small
table masses but is particularly suitable
for large table masses. It can therefore be
installed in existing machine designs with
relatively little additional work.
The continuing development of the INA
bearing arrangement and, perhaps most
important, the development of a ball
screw drive with a high speed rotating
nut promises significant performance
increases for this design principle in the
future.
[1]
[2]
[3]
[4]
[5]
[6]
Weule, H.:
Die Bedeutung der Produktentwicklung für den Industriestandort
Deutschland, VDI Entwicklung,
Konstruktion, Vertrieb, Jahresbericht
1997, Düsseldorf
Schmitt, Thomas:
Modell der Wärmeübertragungsvorgänge in der mechanischen
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[7] INA Wälzlager Schaeffler oHG:
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Herzogenaurach. 1992 [INA 92]
[8] Fischer, H.:
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thermischen Verhaltens von Bohrund Fräsmaschinen; Dissertation,
TU Berlin 1970
[9] Kersten, A.:
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[10] N.N.: Präzisionsmaschinenelemente,
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Corporation, Pr.No. GK030390 PME
[11] Zirn, O.; Weikert, S.; Rehsteiner F.:
Design and Optimization of Fast
Axis Feed Drives Using Nonlinear
Stability Analysis, Annals of Chirp
Vol.45/1/1996; Seite 363-367
[12] Pritschow, G.:
Zum Einfluß der Geschwindigkeitsverstärkung auf die dynamische
Bahnabweichung, wt-Produktion
und Management 86: 1996,
S. 337-341
About the authors:
Dipl.-Ing. Thorsten Frank is a scientist
at the Institute for Machine Tools and
Production Sciences (wbk) of the
University of Karlsruhe.
Erich Lunz is an application engineer in the
Industrial Sector Management for Machine
Tools of INA Wälzlager Schaeffler oHG,
Herzogenaurach (Germany)
7
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