GB2073062A

GB2073062A - Electromotively driven portable tool with a slip clutch

Info

Publication number: GB2073062A
Application number: GB8107723A
Authority: GB
Original assignee: Metabowerke GmbH and Co
Current assignee: Metabowerke GmbH and Co
Priority date: 1980-04-01
Filing date: 1981-03-11
Publication date: 1981-10-14
Also published as: GB2073062B; DE3012637C2; DE3012637A1

Abstract

An electromotively driven portable tool has a slip clutch between the motor armature (2) and a driven shaft (10). The speed of the motor armature (2) is reduced when the slip clutch slides through or on reaching a given current or when the driven shaft (10) stops. The reduction in speed of the motor armature can be achieved by arranging speed recorders in front of and behind the clutch and reducing the speed when the speed ratio differs from a predetermined value as a result of clutch slip. An electronic speed control may be provided and/or an overcurrent detector to reduce the motor armature speed in the case of excessive current consumption. A movement sensor may be provided between the clutch and the driven shaft and arranged to reduce the motor armature speed in the event of low speed or stoppage of the driven shaft. <IMAGE>

Description

SPECIFICATION Electromotively driven portable tool with a slip clutch This invention relates to an electromotively driven portable tool in which a slip clutch is provided between the motor armature and a driven shaft.

Such an electric tool is, for example, known from German Gebrauchsmuster No. 69 09 435.

The incorporated slip clutch reduces the accident risk for an operator, e.g. if the tool bites or catches.

In connection with gears and tools, the slip clutch offers the further advantage that in the case of suddenly occurring power peaks, e.g. when a saw blade tooth strikes a nail in a board, the clutch slides through, so that the considerable kinetic energy of the armature does not damage parts of the gear or tool.

When the clutch slides through, a certain wear occurs on the friction areas and is dependent on the force with which the friction discs are pressed against one another and on the rotary speed with which said discs slide on one another. In the case of a predetermined motor performance the service life of the slip clutch is dependent on the amount of use. If a higher motor performance is to be installed in the machine for the same amount of use and same life, the diameter of the friction discs must be increased for transferring the now larger torque, for the same speed. However, such an increase in the size of the friction discs leads to higher space requirements, which is particularly prejudicial in the case of portable tools.

The present invention aims to keep the wear of the slip clutch within acceptable limits in a portable tool of the aforementioned type, without increasing the constructional dimensions of the complete clutch.

According to the invention, there is provided an electromotively driven portable tool in which a slip clutch is provided between the motor armature and a driven shaft, wherein the speed of the motor armature is reduced when the slip clutch slides through, or on reaching a given motor current or when the driven shaft stops.

As the power consumed on the friction discs of the slip clutch on sliding through, which is converted into heat, is proportional to the torque and the speed difference between the driving and the driven disc, a reduction in the motor speed leads to reduced heat evolution and consequently to reduced wear.

It is possible to use as reference quantities for reducing the armature speed the speed difference between the two friction discs of the slip clutch or gear shafts located in front of and behind the slip clutch. However, it is also possible to perform a stationary measurement on the driven friction disc or on gear shafts located behind the slip clutch.

According to another feature of the invention the motor current can also be used as a reference quantity for reducing the armature speed.

When using the speed difference of the two friction discs or gear shafts located in front of and behind the slip clutch as the reference quantity it can be particularly advantageous to arrange a speed recorder in front of and behind the slip clutch. If the speed ratio differs from the normal value, the motor armature speed is preferably reduced by reducing the predetermined desired value given by an electronic speed control.

Advantageously the speed difference resulting from the sliding through of the slip clutch can be limited by reducing the motor armature speed. As in general the slip moment of the slip clutch is approximately constant, the power to be absorbed by the clutch is consequently limited to a still acceptable value.

Owing to the fact that when the slip clutch operates the main spindle is frequently blocked, i.e. through a jammed drill, the dropping below a certain minimum speed or the stopping of a main spindle can also be used as a reference quantity for reducing the motor speed. To this end a movement sensor is arranged between the slip clutch and the driven shaft and this enables the armature speed to be reduced on establishing a stoppage or an inadmissibly low main spindle speed. The movement sensor can be constructed as an electrical or mechanical sensor, for example as a centrifugal switch.

It is particularly advantageous to use the motor current as a reference quantity for reducing the armature speed, because then only limited extra constructional expenditure is required. For this purpose an overcurrent detector can be provided in the tool circuit, which reduces the armature speed when a current corresponding to the slip moment occurs. Advantageously the release current is fixed in such a way that the reduction of the armature speed takes place just before the slip clutch operates.

In the case of portable tools without an electronic speed control, for the operation of a slip clutch, the supply voltage can be reduced by phase lag by means of a controllable semiconductor to such an extent that the armature speed remains below a value which could cause serious wear to the clutch.

In the case of portable tools with an electronic speed control the signal obtained from the reference quantity can be directly used in the electronic component in order to pass from normal operation to a reduced desired armature speed or a predetermined maximum speed difference between the two friction discs.

The invention will now be further described, by way of example, with reference to the drawings, in which: Fig. 1 shows a part section through a portable electric drill according to the invention; Fig. 2 shows diagrammatically the gearing employed in the portable tool shown in Fig. 1; Fig. 3 is a circuit diagram of a speed reduction control with the measurement of two speeds; Fig. 4 is a circuit diagram corresponding to Fig.

3 with only a stationary monitoring of the driven shaft; and Fig. 5 is a circuit diagram corresponding to Figs.

3 and 4 with the use of an overcurrent detector for controlling the speed reduction.

In the case of the portable tool partly shown in Figs. 1 and 2, a motor armature 2 is rotatably mounted in a multipart casing 1 and its pinion shaft 3 meshes with a gear wheel 4 of an intermediate shaft 5 of a transmission gear 6. The intermediate shaft 5 of the transmission gear 6 has two gear wheels 7, 8 by means of which, as required, a driven shaft 10 can be driven at two different speeds by means of a ratchet wheel 9.

The ratchet wheel 9 can be operated by means of a transmission reversing device 1 A tool mounting, e.g. in the form of a drill chuck 12, is provided on the driven shaft 10.

A slip clutch 13 is arranged in the gear wheel 4, so that, on reaching a predetermined torque, the latter can slide on the intermediate shaft 5.

During operation, a speed recorder 14 or 15 is arranged in front of and behind the slip clutch 13.

As shown in Fig. 2, the teeth of the pinion of the pinion shaft 3 and of the gear wheel 7 serve as pulse generators for the speed recorders 14 and 15. The speed recorders 14 and 15 can be constructed as tacho-generators. However, it is also possible to use inductive and capacitive measuring methods for the indirect speed determination of the motor armature 2 and the intermediate shaft 5 or driven shaft 10.

Fig. 3 shows diagrammatically the arrangement of Fig. 2. The two speed recorders 14, 1 5 are coupled with an electronic speed control 16, so that the speed of the motor armature 2 is reduced in known manner when the slip clutch 13 slides through, i.e. when there is a variation from the normal frequency response ratio of the frequencies from speed recorders 14, 1 5. Speed is reduced to such an extent that there is no drop below a certain predetermined speed difference at the slip clutch 13, so that the energy converted there into frictional heat does not cause clutch damage. An operator therefore has enough time to eliminate the overloading or blocking of the driven shaft 10 or to stop the drive manually, if this is not possible in a short time. The speed difference on the slip clutch 13 can also be maintained in the case of a complete blocking of the driven shaft 10.

As the driven shaft 10 is often blocked in the case of overloading, as shown in Fig. 4 such a blocking action can be determined by a movement sensor 1 5' and as a result the desired value predetermined by the electronic speed control 16 can be adjusted from a normal value to a reduced value.

In the embodiment of Fig. 5, an overcurrent detector 17 is arranged in the motor circuit, so that on exceeding a predetermined current value' the desired speed predetermined by the electronic speed control 1 6 is reduced and consequently the motor speed is reduced in accordance with the power absorption capacity of the slip clutch 13.

The circuit diagrams of Figs. 3 to 5 diagrammatically show the power supply 18 and an operating switch 19.

Claims

1. An electromotively driven portable tool in which a slip clutch is provided between the motor armature and a driven shaft, wherein the speed of the motor armature is reduced when the slip clutch slides through, or on reaching a given motor current or when the driven shaft stops.

2. A portable tool according to claim 1, wherein a speed recorder is provided both in front and behind the slip clutch and the speed of the motor armature is reduced when the speed ratio differs from a predetermined value.

3. A portable tool according to claim 2, with an electronic speed control and at least one tachogenerator, wherein the speed recorders are constituted by two tacho-generntors which reduce the motor armature speed via an electronic speed control in the case of a variation from the desired speed ratio.

4. A portable tool according to claim 2 or claim 3, wherein the teeth of two gear wheels arranged in front of and/or behind the slip clutch cooperate with the associated speed recorders.

5. A portable tool according to claim 1, with an electronic speed control, wherein an overcurrent detector is provided to reduce the motor armature speed in the case of excessive current consumption.

6. A portable tool according to claim 1, wherein a movement sensor is provided between the slip clutch and the driven shaft and serves to reduce the motor armature speed when a low speed or stoppage is established.

7. A portable tool according to any preceding claim, wherein the speed difference caused by sliding through of the slip clutch is limited by reducing the motor armature speed.

8. A portable tool according to any preceding claim, wherein for reducing the speed the electric motor supply voltage can be reduced by means of a thyristor or triac.

9. A portable tool substantially as described herein with reference to the drawings.