GB2068272A

GB2068272A - A device for applying a force

Info

Publication number: GB2068272A
Application number: GB8102100A
Authority: GB
Original assignee: INST ZA METALOREJESHTI MACHINI
Current assignee: INST ZA METALOREJESHTI MACHINI
Priority date: 1980-01-24
Filing date: 1981-01-23
Publication date: 1981-08-12
Also published as: CS34081A1; DE3102235A1; CH651488A5; CS241960B1; BG31534A1; FR2474375A1; SU1164003A1; SE8100274L; HU189490B; FR2474375B1

Abstract

A device for applying a force, such as a clamping device, comprises an electric motor (2) which, when energised, causes rotation of a nut (8) which in turn causes linear displacement of a spigot (9) and of an output element (16). When the output element (16) meets a resistance and is arrested, a resilient assembly (15) is compressed, and relative movement takes place between a rod (17) carried by the output element (16), and a tube (18), carried by the spigot (9). This relative movement is detected by a transducer (20) which, when the movement reaches a predetermined value, delivers a signal which causes the motor (2) to be de-energised. The device enables a predetermined clamping force to be achieved and maintained substantially constant, or varied in a predetermined manner. The device may be used on machine tools. <IMAGE>

Description

SPECIFICATION A device for applying a force This invention relates to a device for applying a force, particularly, although not exclusively, to a clamping device for use with machine tools.

In a known clamping device, an electric motor applies a clamping force through a reduction gear and an adjustable slipping clutch. The rotary motion of the motor is converted to linear displacement by a screw and nut mechanism. A resilient device, operable in both linear directions, is provided for energy accumulation.

In such a construction, the slipping clutch is subject to constant wear, which results in variation in the clamping force. Similarly, since the motor is continuously operating during clamping, the components of the device other than the clutch are also subject to constant wear.

According to the present invention there is provided a device for applying a force, comprising an electric motor which is connected to a mechanism for converting the rotational output of the motor into linear displacement of an output element, the direction of displacement of the output element depending on the direction of rotation of the motor, a resilient assembly being provided which undergoes strain when displacement of the output member in either direction is resisted, detector means being provided for detecting such strain and for de-energising the motor when the strain reaches a predetermined value.

With a construction in accordance with the present invention, the stopping of the motor is directly dependent on the deformation of the resilient assembly and consequently on the magnitude of the force applied. The automatic stopping of the motor can be achieved for both directions of displacement of the output element, for example when it is extended to apply a clamping force, and when it is retracted to release the clamping force. The magnitude of the force required to stop the motor may be different for the two directions.

Thus, on retraction of the output element, the magnitude of the force required to stop the motor may be lower than that required when applying the clamping force, so reducing the time required for the release operation and avoiding excessive wear on the components.

In one cycle of operation, it is possible to vary the magnitude of the clamping force applied in accordance with a preset sequence, and it is also possible to incorporate an automatic feed-back in order to maintain the clamping force at a constant value, or to vary it in a predetermined manner.

The resilient assembly may or may not be accommodated in a housing of the device. In some cases, there may be advantages in accommodating the resilient assembly in the housing. For example, when mounting on the rotating spindle and using a tube for a transition rod to the clamping chuck, the hole of the spindle is vacated and part of the machined workpiece can be introduced into it.

For a better understanding of the present invention and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a sectional view of one embodiment of an electromechanical clamping device; and Figure 2 shows a partially sectioned view of another embodiment of an electromechanical clamping device.

The device shown in Fig. 1 comprises a housing 1, in which is mounted an electric motor 2 having a hollow spindle 3 which is supported in flanges 4 and 5 fitted within the housing 1. A planetary reduction gear 6 is accommodated within the housing 1 at one end, the reduction gear 6 being coaxial with the motor 2. The output member, i.e. the planet carrier 7, of the reduction gear 6 is rigidly secured to a nut 8, which is axially supported in the housing 1 by two thrust bearings 13. The nut 8 cooperates with a screwthreaded spigot 9 which is prevented from rotating by cooperation with a third flange 10 which closes the end of the housing 1. The nut 8, the spigot 9 and the third flange 10 operate as a mechanism for converting rotation to linear displacement.The spigot 9 has a through-bore from which extends a tube 18, which passes all the way through the hollow shaft 3 of the motor 2 and projects from the shaft 3. A cap 19 is seated on the projecting portion of the tube 18, and a transducer 20 is attached to the cap. The cap 19 is axially movable in a guide 21 attached to the non-rotating part of the current-supply unit 22 of the electric motor 2.

The transmission of motion from the shaft 3 to the planetary reduction gear 6 is effected through a toothed portion 11 machine on the end of the hollow shaft 3, which meshes with the planet wheels 12 of the reduction gear 6.

The spigot 9 is attached to an energy accumulator 26 comprising a cylindrical housing 14, inside which there is provided a nonrotating mandrel 16. A resilient assembly 15, comprising a set of spring washers, is located on the mandrel 16. The assembly 15 is disposed between two discs 27 and is retained on the mandrel 16 by a nut 28. The housing 14 is connected by a screw thread to the spigot 9. The mandrel 16 is provided with a rod 17 extending within the tube 18, and terminating adjacent the sensor of the transducer 20.

In operation, to product a desired clamping force in the direction to the right as seen in Fig. 1, the motor 2 is energised and the resulting rotational motion is transmitted via the planetary reduction gear 6 to the nut 9 which results in linear movement of the spigot 9 and the accumulator 26, together with the tube 18 and the rod 17. This linear movement continues until the mandrel 16 meets a resistance. When the mandrel 16 is arrested, continued displacement of the spigot 9 causes compression of the resilient assembly 15, while the tube 18 continues to advance relatively to the now stationary rod 17. This displacement is sensed by the transducer 20, and when the deformation of the resilient assembly 15 reaches a preset magnitude, which corresponds to the desired clamping force, the transducer 20 provides a signal which results in deenergisation of the electric motor 2.

For operation in the opposite direction, the electric motor 2 is energised for rotation in the opposite direction, resulting in linear displacement of the spigot 9 together with the housing 14 of the accumulator 26, until the clamping force is released. The spigot 9 then continues to be retracted, together with the accumulator 26, until the mandrel 16 is arrested by tensile forces acting on it. The retracting spigot 9 then compresses the resilient assembly 15, and the tube 18 moves relatively to the rod 17, this relative movement being sensed by the transducer 20, which ultimately gives a signal for the deenergisation of the electric motor 2.

In the modified construction shown in Fig.

2, the planet carrier 7 is integral with the nut 8. The resilient assembly 1 5 of the accumulator 26 is disposed between two supporting collars 24, accommodated between the two thrust bearings 13 which axially locate the nut 8. The supporting collars 24 abut respectively a shoulder 81 of the nut 8 and a retaining nut 25 screwed on to an elongate portion 82 of the nut 8.

The accumulator 26 is thus accommodated in the housing 1. The nut 8 is operatively connected to the sensor of the transducer 20 by the rod 17. In the embodiment of Fig. 2, the transducer 20 is mounted on the nonrotating part 23 of the current-supply unit 22.

The rod 17 is spring-loaded to ensure constant cooperation between the nut 8 and the transducer 20. In operation, the rotation of the electric motor 2 is transmitted via the planetary reduction gear 6 to the nut 8, which is thus rotated to cause linear displacement of the spigot 9 until it encounters resistance. The spigot 9 then stops, while the nut 8 continues to rotate, and so moves axially along the spigot 9, thus compressing the resilient assembly 15 between the two supporting collars 24. The rod 17 transmits the axial displacement of the nut 8 to the sensor of the transducer 20, and at a given magnitude of displacement, corresponding to the desired axial clamping force, the transducer 20 produces a signal to cause de-energisation of the electric motor 2.

For operation in the reverse direction, i.e. to release the clamping force, the electric motor 2 rotates in the opposite direction and the displacement of the rod 17 with respect to the sensor of the transducer 20 results in an analogous signal causing de-energisation of the electric motor 2.

Claims

1. A device for applying a force, comprise ing an electric motor which is connected to a mechanism for converting the rotational output of the motor into linear displacement of an output element, the direction of displacement of the output element depending on the direction of rotation of the motor, a resilient assembly being provided which undergoes strain when displacement of the output member in either direction is resisted, detector means being provided for detecting such strain and for de-energising the motor when the strain reaches a predetermined value.

2. A device as claimed in claim 1, in which a reduction gear is provided between the motor and the converting mechanism.

3. A device as claimed in claim 2, in which the reduction gear is a planetary reduction gear, the output of which is rigidly connected to the input of the converting mechanism.

4. A device as claimed in claim 3, in which the output of the planetary reduction gear is located axially within a housing of the device.

5. A device as claimed in any one of the preceding claims, in which the resilient assembly comprises at least two coaxial spring washers.

6. A device as claimed in any one of the preceding claims, in which the converting mechanism comprises a nut which is rotatable by the motor and a screwthreaded spigot which cooperates with the nut and is restrained against rotation.

7. A device as claimed in claim 6, in which the nut is located axially within a housing of the device.

8. A device as claimed in claim 7, in which a tube is connected to the spigot and extends through a hollow shaft of the motor, the detector means being mounted on the tube at the end away from the nut.

9. A device as claimed in claim 7, in which the output element is connected to the spigot by the resilient assembly, and in which, the output member and the detector means are operatively interconnected by a rod which extends through the tube, whereby the detector means is responsive to relative movement between the spigot and the output element.

10. A device as claimed in any one of claims 1 to 6, in which axial displacement of the nut in either direction is resisted by the resilient assembly.

11. A device as claimed in claim 10, in which the detector means is mounted on the housing, and in which the nut and the detector means are operatively interconnected by a rod which extends through a hollow shaft of the motor, whereby the detector means is responsive to axial displacement of the nut.

12. A device as claimed in any one of the preceding claims, in which the detector means comprises a displacement transducer.

13. A device as claimed in any one of the preceding claims, which is a clamping device.

14. A clamping device substantially as described herein with reference to, and as shown in, Fig. 1 or Fig. 2 of the accompanying drawings.

15. A machine tool including a clamping device in accordance with claim 13 or 14.