GB2405822A

GB2405822A - Power screwdriver with torque limiting device

Info

Publication number: GB2405822A
Application number: GB0419936A
Authority: GB
Inventors: Mike Voigt
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2003-09-11
Filing date: 2004-09-08
Publication date: 2005-03-16
Anticipated expiration: 2024-09-08
Also published as: CN100513085C; GB0419936D0; DE10341974A1; GB2405822B; CN1593851A; JP2005088190A

Abstract

The invention relates to a cut-out screwdriver 2 with a drive motor 8 , an assembly tool receptacle 16 driven by the drive motor 8 , an overlatching clutch 22 arranged in a drive train 18, 20 between the drive motor 8 and the assembly tool receptacle 16 and a cut-out device 26 for cutting out the drive motor 8 when the overlatching clutch 22 disengages as a result of reaching a pre-set torque in the drive train 18, 20. The cut-out device 26 comprises a sensor (36, fig. 2) which responds to an acceleration pulse generated when the overlatching clutch 22 disengages. The sensor (36, fig. 2) may be a piezoelectric sensor which generates a voltage, and may respond to an axial and/or a radial acceleration pulse.

Description

Cut-out screwdriver The invention relates to a cut-out screwdriver

according to the preamble of claim 1.

Prior art

Cut-out screwdrivers of the kind initially mentioned are used in industry, above all in the automobile industry, to produce screw connections with a predetermined tightening torque. A cut-out screwdriver of this kind is disclosed, for example, in DE 44 30 186 Al of the applicant. In the drive train between its electric drive motor and the receptacle for the assembly tool the known cut-out screwdriver has an over-latching clutch which disengages automatically when the predetermined tightening torque has been reached and thus effectively limits it. The known cut-out screwdriver is further provided with a cut-out device which cuts out and slows down its drive motor when the overlatching clutch disengages. In more recent cut-out screwdrivers of the applicant this cut-out device comprises a microswitch which is arranged in the area of the overlatching clutch and is actuated when the clutch -em 46- disengages via a switching bar, held as movable, as in the overlatching clutch of DE 44 30 186 Al. On actuation of this microswitch the drive motor is short-circuited by the cut-out device and thereby very quickly slowed down, thereby preventing exceeding of the set tightening torque.

However, it has been established that, owing to the high accelerations of more that 50 g occurring when the overlatching clutch disengages, in time damage of the microswitch and therefore failure of the cut-out screwdriver may arise.

Advantages of the invention The cut-out screwdriver according to the invention with the features of claim 1 has the advantage compared with this that it detects the disengaging of the overlatching clutch without contact via an acceleration pulse generated during the overlatching process and thus failures as a result of a mechanical switch actuation are avoided. As the acceleration pulse generated when the overlatching clutch is disengaged is on the one hand relatively strong, so it can be detected everywhere in the cut-out screwdriver, and on the other hand is composed of a radial and axial acceleration component, in relation to the axis of rotation of the cut-out screwdriver, the sensor serving to detect the acceleration pulse can additionally be arranged at any orientation and at any point, though preferably inside a housing of the cut-out screwdriver. Moreover, the acceleration pulses generated when the overlatching clutch disengages are in the range of more than 50 g and thus in the same range as the accelerations occurring in motor vehicle accidents, so it is possible to use an acceleration sensor developed to trigger motor vehicle seat belt retractor systems in the event of accidents and available at a reasonable price on the market.

In preferred configuration of the invention it is provided that the sensor is an active sensor which manages without its own current supply and on response generates a voltage signal which can be utilised by the cut-out device to cut out the drive motor after possible previous amplification.

A preferred example of an active acceleration sensor of this kind is a piezoelectric sensor in which the acceleration pulse generated when the overlatching clutch disengages effects a displacement of an acceleration mass conditional on inertia and a deformation of a piezoelectric crystal caused by the displacement, by which in turn a measurable voltage signal is generated.

A further preferred configuration of the invention provides that the cutout device comprises a microcontroller in which the voltage signal generated by the acceleration sensor is evaluated and if a predetermined threshold value is exceeded the drive motor of the cut-out screwdriver is cut out or slowed down.

The acceleration sensor can have an orientation of such a kind that it responds only to one axial or radial acceleration pulse generated when the overlatching clutch disengages, but is preferably mounted in such a way that it detects both radial and axial components of the acceleration of the cut-out screwdriver.

The acceleration sensor preferably responds to an acceleration pulse of 50 g or more, the voltage signal generated by the sensor coming to advantageously more than 0.5 V. Drawings The invention is explained in greater detail below in an embodiment example using the associated drawings.

Fig. 1 shows a side view in section of a cut-out screwdriver with overlatching clutch.

Fig. 2 shows a basic illustration of an acceleration sensor as part of a cut-out device for cutting out the cut-out screwdriver when the overlatching clutch is disengaged.

Fig. 3 shows a diagrammatic illustration of voltage pulses of the acceleration sensor or axial acceleration pulses of the housing of the cut-out screwdriver when the overlatching clutch disengages.

Description of the embodiment example

The cut-out screwdriver 2 illustrated in the drawings serves to produce screw connections which are to be screwed down at a predetermined maximum tightening torque. Cut-out screwdrivers 2 of this kind are used particularly frequently in the automobile industry, where a multiplicity of screw connections with the same maximum tightening torque is normally to be produced on each automobile.

As best illustrated in Fig. 1, the cut-out screwdriver 2 substantially consists of a housing 4 with a handle part 6, an electric motor 8, accommodated in the housing 4, with control electronics 10 and a reduction gear 12, an accumulator pack 14, latched detachably to the lower end of the handle part 6, for supply of the electric motor 8 with direct current independent of the mains, an assembly tool receptacle 16 projecting above the housing 4 for inserting an assembly tool (not illustrated), an overlatching clutch 22, which is arranged in the drive train of the cut-out screwdriver 2 between an output shaft 18 of the reduction gear 12 and a driving shaft 20 of the assembly tool receptacle 16 and is disengaged if a presentable torque between the output shaft 18 and the driving shaft 20 is exceeded, and also a manually actuatable switch 24 for switching on the electric motor 8 and an automatic cut-out device 26, operating without contact, for cutting out or slowing down the electric motor 8 dependent on the torque when the overlatching clutch 22 disengages.

The overlatching clutch 22 substantially consists of a clutch ring 28, which is axially displaceable on the driving shaft 20 connected as fixed against rotation to the tool receptacle 16 and is pressed backwards in the direction of the outlet shaft 18 by a biased helical compression spring 30. Between two opposite front faces of the clutch ring 28 and the output shaft 18 is arranged a plurality of latching balls 32, which engage in a corresponding number of trough-shaped receiving pockets in the two front faces and before the preset tightening torque has been reached take care of the power transmission from the output shaft 18 to the clutch ring 28 and therefore to the driving shaft 20.

On its front end facing away from the clutch ring 28 the helical compression spring 30 is supported against a setting ring 34 on the front end of the driving shaft 20, which can be adjusted from outside, i.e. displaced in the axial direction on the driving shaft 20, by means of a suitable setting tool (not illustrated) to change the bias of the helical compression spring 30 and therefore to change the pre-set tightening torque of the screw connection to be produced. Closer details on a possible construction of the overlatching clutch 22 are to be found in the initially mentioned DE 44 30 186 Al, to which reference is made in this connection.

With increasing torque in the drive train 18, 20 the clutch ring 28 is displaced against the force of the helical compression spring 30 more and more in the axial direction forwards away from the output shaft 18, the latching balls 32 moving increasingly further out of the receiving pockets until they finally move abruptly into the adjacent receiving pocket of the clutch ring 28 or the output shaft 18 in each case if the tightening torque predetermined by the respective bias of the helical compression spring 30 is exceeded. During this overlatching process the overlatching clutch 32 is disengaged and transmission of the torque from the output shaft 18 to the driving shaft 20 is interrupted. Simultaneously, during the overlatching process a considerable radial and axial acceleration pulse discernible to the operator is generated in the cut-out screwdriver 2, which can be detected without contact, i.e. non- mechanically, with the aid of an acceleration sensor and utilised for activating the cutting out or slowing down of the electric motor 8.

Fig. 2 shows an example of an acceleration sensor 36 suitable for this purpose and integrated into the cut-out device 26. The acceleration sensor 36 illustrated in Fig. 2 is a so-called crash sensor, as used, among other things, for detecting decelerations in motor vehicles caused by accidents. The acceleration sensor 36 comprises a piezoelectric flectional bar 40, which is clamped in a holder 38 on one side, is formed by a biomorph plate and bends owing to its inertia when there is an acceleration pulse in the direction of arrow P. as illustrated by broken lines in Fig. 3. The deformation of the piezoelectric bar 40 leads to a loading displacement at the clamped end of the bar 40, which is converted into a voltage signal at the output 44 of the sensor 36 via a loading amplifier 42.

As best illustrated in Fig. 3, the axial acceleration pulses a generated during the overlatching process in commercially available cut-out screwdrivers 2 of the applicant have a strength of between 100 and 250 g (1 g gravitational acceleration), corresponding to a voltage signal of between 1.06 V and 2.65 V generated at the output 44 of the sensor 36.

This voltage signal generated at the output 44 of the sensor 36 is delivered to the control electronics 10 of the electric motor 8, which comprise a microcontroller (not illustrated), which compares the strength of the voltage signal with a predetermined threshold value, for example 0.5 V, and if the threshold value is exceeded causes immediate cutting out or slowing down of the electric motor B. Although the cut-out device 26 with the acceleration sensor 36 is illustrated in the cut-out screwdriver 2 illustrated in Fig. 1 together with the control electronics 10 in the rear part of the housing 4, the sensor 36 can basically be mounted at any point of the cut- out screwdriver 2 and, moreover, also at any orientation, as the radial and the axial acceleration pulses are discernible everywhere and are superimposed on one another.

Claims

Patent claims 1. Cut-out screwdriver with a drive motor, an assembly tool

receptacle driven by the drive motor, an overlatching clutch arranged in a drive train between the drive motor and the assembly tool receptacle and also a cut-out device for cutting out the drive motor when the overlatching clutch disengages as a result of reaching a pre-set torque in the drive train, characterized in that the cut-out device (26) comprises a sensor (36) which responds to an acceleration pulse generated when the overlatching clutch (22) disengages.
2. Cut-out screwdriver according to claim 1, characterized in that the sensor is an active sensor (36) which on response generates a voltage signal.
3. Cut-out screwdriver according to claim 2, characterized in that the sensor (36) is a piezoelectric sensor.
4. Cut-out screwdriver according to one of the preceding claims, characterized in that the sensor (36) responds to an axial acceleration pulse.
5. Cut-out screwdriver according to one of the preceding claims, characterized in that the sensor (36) responds to a radial acceleration pulse.
6. Cut-out screwdriver according to one of the preceding claims, characterized in that the sensor (36) responds to an acceleration pulse with radial and axial acceleration components.
7. Cut-out screwdriver according to one of the preceding claims, characterized in that the sensor (36) responds to an acceleration pulse of more than 50 g and preferably of more than 100 g.
8. Cut-out screwdriver according to one of the preceding claims, characterized in that the cut-out device (26) comprises a microcontroller for evaluating the voltage signal generated by the sensor (36).
9. Cut-out screwdriver according to claim 8, characterized in that the microcontroller compares the voltage signal with a predetermined threshold value and if the threshold value is exceeded triggers immediate cut-out or slowing down of the drive motor (8).
10. A cut-out screwdriver substantially as herein described with reference to the accompanying drawings.