The invention relates to a chuck, in particular a jaw chuck for gripping crankshafts
and comprising an axially moveable center point that is guided in a chuck body and jaws moveable
radially relative to a chuck longitudinal axis.
A chuck of this type has already been disclosed in DE 91 13 879. The jaws here are
moved by a hydraulically powered piston.
Movement of the center point is frequently effected by a hydraulic assembly. In this
case a piston chamber can be provided to move the center point in the piston in order to actuate the
jaws, thereby creating a space-saving double-acting piston configuration.
When crankshafts are produced, the crankshaft workpieces are face-milled at their
end faces. Then a center hole is formed in each end face whose function is to allow precise axial
centering of the crankshaft for subsequent radial machining. The crankshaft is typically clamped
between two center points B one being fixed and the other being axially moveable. Using a
hydraulic arrangement for moving the center point makes it very difficult to meet the requirements
for environmentally friendly energy-efficient technologies. With this in mind, it becomes necessary
to seek new approaches for its axial actuation.
The object of this invention is therefore to provide a chuck and a method of
operating a chuck of the above-described type where the gripping parts function successfully
without hydraulic components.
This object is achieved by the chuck according to the invention where an
electric motor is provided as the actuator for axially moving the center point.
The advantage of the invention is that the electric motor moves the center point
electrically. In contrast to a binary hydraulic approach, this enables the center point to be
moved axially precisely, continuously, and with energy efficiency. During manufacture of the
crankshaft, the electric motor is operated accordingly so as to center the center point on the
end face of the crankshaft, after which the jaws that radially secure the crankshaft can be
clamped against it.
It has been found especially advantageous to provide a spindle drive for axial
movement. This provides a simple way of converting rotation of the electric motor to
straight-line movement of the center point.
A useful approach is to provide the spindle drive between a rod driven by the
electric motor and the center point. This rod can be used to obviate the need to connect the
electric motor directly to the spindle drive. This allows the distance between the electric
motor and the center point to be spanned, with the result that the electric motor can be
mounted so as to be physically offset from the parts of the chuck body that rotate when the
chuck is in use.
A preferable approach is that a spindle is provided that is detachably connected to the
center point. This spindle moves axially when the electric motor or the rod rotate, thereby at the
same time driving the center point. Whenever the associated components are due for replacement
after the spindle drive has worn out, this spindle can be replaced without having to swap out the
entire center point.
It is furthermore advantageous to provide an elastic element between the spindle and
the center point. This elastic element is used to prevent the center point from striking the end face of
the workpiece with the full force of the electric motor and thereby damaging this workpiece.
An especially preferred approach has been found whereby the rod is supported in a
bearing provided in the chuck body. If the distance between the rod and the electric motor is very
large, a small imbalance in the rotation of the electric motor can cause a large radial component in
the rotation at the end of the rod facing the center point. The rod is supported in the rod holder in
order to compensate for this radial component and to ensure reliable axial guidance.
It is furthermore advantageous to provide a gear transmission, preferably a planetary
transmission between the electric motor and the rod. This allows increased torque to be applied that
acts on the rod.
It is also advantageous to provide a threaded tube shaft to actuate the jaws, which
hollow rod can be driven by an electrically operated gripping motor. This also enables actuation of
the jaws without the use of hydraulic components. Accordingly, with electrical actuation the center
point can first axially center a crankshaft workpiece that is subsequently secured radially by the
jaws.
It is especially advantageous to provide a harmonic drive between the threaded tube
shaft and the rotor of the gripping motor, comprising a nonrotating splined steel sleeve, a driven flex
spline, and an inner gear that is coupled to the threaded tube shaft so as to transmit torque. This
enables the high rotational speed of the motor to be converted to a lower rotational speed of the
threaded sleeve, thereby allowing the jaws to be moved with very high precision.
It has been found advantageous to provide a screw drive on the threaded tube shaft
and on the means guided in the chuck body for moving the jaws. This allows the structure extending
essentially parallel to the chuck longitudinal axis body to be moved axially in order to move the
jaws, so that actuating wedges can be provided on the sides facing the jaws, the actuating wedges
pressing the jaws radially inward or outward,
It-is also advantageous for the rod to pass through the threaded tube shaft without
contacting it. This enables the actuator of the jaw to be designed in a very space-saving manner.
An especially advantageous embodiment is characterized in that a spindle rod is
provided between the center point and the electric motor, this rod being axially moveable and
detachably connected by friction to the center point. This allows the spindle rod to be disconnected
as required from the center point. When the chuck is being used, the electric drive does not have to
rotate along with the chuck; the setting of the center point is nevertheless maintained.
It has been found advantageous to design the center point as several parts, namely a
holder for the spindle rod, at least one elastic spring element, and a tip. Due to the elastic spring
element, this configuration also ensures that the center point does not strike the end face of a
workpiece to be clamped with the full force of the electric motor.
A spindle drive is advantageously provided between the spindle rod and the rotor of
the electric motor, thereby also creating the ability to convert the rotation of the motor to straight-
line displacement of the spindle rod.
An especially advantageous approach is to provide a control unit to operate the
electric motor. This enables the center point to be rotated about its axis by the electric motor at the
same rotational speed as the chuck, with the result that this axis does not move out of the centered
position when the chuck is in operation.
It is also advantageous within the scope of the invention to provide a method of
operating a chuck having a center point that is axially moveable and guided within the chuck body,
an actuator that is detachably connected through an elastic element to the center point, an electric
motor including a sensor, and means for converting rotation into straight-line displacement, wherein
the means for converting rotation into straight-line displacement by the rotation of the electric motor
axially move the actuator while simultaneously driving the center point until a force is applied to the
elastic element during a relative motion of the actuator relative to the center point and the work
performed thereby is detected by a changing measured value at the sensor.
As a result, the measured value enables a determination to be made as to when the
center point has struck the end face of a workpiece to be machined in order to center it. The sensor
here is preferably in the form of an ammeter and measures current. The electric motor is always
operated at the same rotational speed, although the motor requires a variable amount of electrical
current depending on the resistance acting on the rotor. This change in the required current
correlates with the force being applied to the elastic element. The greater the current, the more
strongly the center point therefore presses against the workpiece.
The following describes the invention in more detail based on illustrated
embodiments shown in the drawing. Therein:
FIG. 1 is a longitudinal section through a first embodiment of the chuck according to
the invention;
FIG. 2 is a longitudinal section through a chuck as known in the prior art that includes
hydraulic components; and
FIG. 3 is a longitudinal section through another embodiment of the invention.
FIG. 2 shows a prior-art chuck that is a jaw chuck for gripping crankshafts, where
here movement of a center point 2 and jaws 4 is done hydraulically. The center point 2 that is
guided here in a chuck body 1 is formed with a ring piston 27 slidable in a piston chamber 26. This
piston chamber 26 is closed rearward by a gripping piston 28 that in turn is guided in a chamber 29.
It functions to displace jaws 4 radially relative to a chuck longitudinal axis 3, and is powered
hydraulically.
When gripping, the center point 2 here is first moved axially until it engages the end
face of a workpiece. In order to move the jaws 4, the gripping piston 28 is then actuated
hydraulically so as to press the jaws 4 radially against the workpiece, thereby griping it. In order to
release the workpiece, either the gripping piston 28 can be actuated simultaneously with the
centering piston 27, or the center point 2 is first moved axially and subsequently the jaws 4 are
released.
FIG. 1 shows an embodiment of the chuck according to the invention, also a jaw
chuck, that is especially well-suited for crankshafts, comprising the center point 2 that is axially
moveable and guided in the chuck body 1, and the jaws 4 moveable radially relative to the chuck
longitudinal axis 3. An electric motor 5 is provided to move the center point 2. A spindle 8 is
associated with the center point 2 and forms a spindle drive 6 for axially moving the center point 2.
An elastic element 9 in the form of a helical spring is provided between the spindle 8 and the center
point 2. It is not absolutely necessary for this to be a helical spring, however. It is also equally
possible to use other types of springs, such as, for example, disk springs, volute springs, annular
springs, or other spring types.
A rod holder 10 also provided in chuck body 1 rotatably supports a spindle rod 7. A
transmission 11 is provided between the electric motor 5 and the rod 7 in this embodiment.
A threaded tube shaft 13 that is drivable by an electrically operated gripping motor 12
is provided to actuate the jaws 4. The rod 7 passes through the threaded tube shaft 13 without
touching it.
A harmonic drive 14 is provided between the threaded tube shaft 13 and the rotor of
the gripping motor 12, comprising a non-rotating ring gear 15, a rotatably driven flex spline 16, and
a splined steel sleeve 17 that is connected to the threaded tube shaft 13 so as to transmit torque. The
rotor in the embodiment shown is connected through a belt drive 30 to a drive wheel 31 that drives
the flex spline 16. The splined steel sleeve 17 drives the threaded tube shaft 13 through a planetary
roller transmission 32. This planetary transmission 32 in the embodiment is biased by multiple
springs 33 that are arranged coaxially relative to chuck longitudinal axis 3, thereby ensuring a
reliable drive for the threaded tube shaft 13.
As is evident in the embodiment, the electric centering motor 5 and the gripping
motor 12 do not rotate with chuck body 1.
In order to move the jaws 4, a gripping plate 18 is provided between the threaded tube
shaft 13 and the elements 19 guided in chuck body 1 to move the jaws 4. The elements 19 for
moving the jaws 4 each have a gripping wedge 34 on the end opposite the gripping plate 18.
A control unit 24 is provided for operating the electric motor 5. During the
machining operation, the rotational speeds of the rod 7 and the threaded tube shaft 13 are preferably
matched to each other, and this corresponds exactly to the rotational speed of the chuck. What is
ensured here is that the jaws 4 and the center point 2 maintain their respective gripping and centering
positions when the chuck is operating.
FIG. 3 shows another embodiment of the chuck according to the invention, where
here a spindle rod 20 is provided between the center point 2 and the electric motor 5, which spindle
rod is axially moveable and is detachably connected by a friction coupling with the center point 2.
The center point 2 shown here is of multipart design composed of a holder 21 for the spindle rod, at
least one spring element 22, and a tip 23. The spring elements 22 here are disk springs.
The spindle drive 6 is provided between the electric motor 5 and the spindle rod 20 to
move the spindle rod 20. In the embodiment shown, the spindle rod 20 is axially movable in a
housing 35.
List of reference numerals
1 chuck body
2 center point
3 chuck longitudinal axis
4 jaws
5 electric motor
6 spindle drive
7 rod
8 spindle
9 elastic element
10 rod holder
11 transmission
12 gripping motor
13 threaded tube shaft
14 harmonic drive
15 internal gear
16 flex spline
17 splined steel sleeve
18 gripping plate
19 means for moving the jaws
20 spindle rod
21 holder for the spindle rod
22 spring element
23 tip
24 control unit
25 hydraulic components
26 piston chamber
27 pteton
28 gripping piston
29 gripping piston chamber
30 belt drive
31 drive wheel
32 planetary roller transmission
33 springs
34 gripping wedge
35 housing
WE CLAIM:
1. A chuck, in particular a claw chuck for gripping crankshafts, comprising an axially
moveable center point (2) that is guided in a chuck body (1) and jaws (4) moveable radially relative
to a chuck longitudinal axis (3), characterized in that an electric motor (5) is provided to axially
move the center point (2).
2. The chuck according to claim 1, characterized in that a spindle drive (6) is
provided to axially move the center point (2).
3. The chuck according to claim 2, characterized in that the spindle drive (6) is
provided between a rod rotatable by the electric motor (5), and the center point (2).
4. The chuck according to claim 3, characterized in that a spindle (8) is provided that
is detachably connected to the center point (2).
5. The chuck according to claim 4, characterized in that an elastic element (9) is
provided between the spindle (8) and the center point (2).
6. The chuck according to one of claims 3 through 5, characterized in that the rod (7)
is supported in a rod holder (10) provided in the chuck body (1).
7. The chuck according to one of claims 3 through 6, characterized in that a
transmission (11), preferably a planetary transmission, is provided between the electric motor (6)
and the rod (7).
8. The chuck according to one of claims 1 through 7, characterized in that a threaded
tube shaft (13) is provided that can be driven by an electrical gripping motor (12) to actuate the jaws
(4).
9. The chuck according to claim 8, characterized in that a harmonic drive (14) is
provided between the threaded tube shaft (13) and a rotor of the gripping motor (12), comprising a
nonrotating ring gear (15), a flex spline (16) rotated by the rotor, and a splined steel sleeve (17)
coupled to the threaded tube shaft (13) so as to transmit torque thereto.
10. The chuck according to claims 8 or 9, characterized in that a gripping plate (18)
is provided between the threaded tube shaft (13) and elements guided within the chuck body (1) in
order to effect movement of the jaws (4).
11. The chuck according to one of claims 8 through 10, characterized in that the rod
(7) does not contact the threaded tube shaft (13).
12. The chuck according to claim 1, characterized in that a spindle rod (20) is
provided between the center point (2) and the electric motor (5), which spindle rod is axially
moveable and is detachably connected by a friction coupling with the center point (2).
13. The chuck according to claim 11, characterized in that the center point (2) is of
multipart design composed of a holder for the spindle rod (21), at least one spring element (22), and
a tip (23).
14. The chuck according to claims 11 or 12, characterized in that a spindle drive (6)
is provided between the spindle rod (20) and the rotor of the electric motor (5).
15. The chuck according to one of claims 1 through 13, characterized in that a
control unit (24) is provided that controls the electric motor (5).
16. A method of operating a chuck, in particular a chuck according to one
of claims 5 through 11 having a center point (2) that is axially moveable and guided within
the chuck body (1), an actuator that is detachably connected through a spring element (22)
to the center point (2), an electric motor (5) including a sensor, and means for converting
rotation into straight-line displacement, characterized in that the means for converting
rotation into straight-line displacement by the rotation of the electric motor (5) axially
moves the actuator while simultaneously driving the center point (2) until a force is
applied to the spring element (22) during movement of the actuator relative to the center
point (2), and the work performed thereby is detected by a changing measured value at the
sensor.
17. The method according to claim 16, characterized in that the sensor is provided
in the form of an ammeter and measures current.

ABSTRACT

The invention relates to a chuck, in particular a claw chuck for gripping
crankshafts, comprising an axially moveable center point (2) guided in the chuck body (1)
and jaws (4) that are moveable radially relative to the chuck longitudinal axis (3). An
electric motor (5) is provided to axially move the center point (2).