DE3011164C2 - Protective device for electric machine tools - Google Patents

Description

50

The invention relates to a protective device for hand-held electric machine tools, in particular Do-it-yourself machines, according to the preamble of claim 1, to when the machine is overloaded «Urn standstill (blocking) to cause an automatic shutdown atu.

In addition to the safety devices against contact with live parts, the protective devices of modern machine tools usually also include an electrical overload protection switch in order to exclude thermal overloading of the motor ^ The various tools and attachments must also meet the relevant safety regulations bring that is Enlarges mainly by _(that laymen often work with the machines, which is not generally conscious to what extent injuries can result from improper operation of the machines.

One of the specific dangers that are particularly common when working with machine tools can often occur is the jamming of the tool in the workpiece. Examples of this phenomenon are drilling in thin sheet metal - usually when the drill is just piercing the sheet metal - circular saw cuts and cutting (abrasive cutting). Even a slight tilting of the machine or workpiece the tool is jammed and the machine blocked during the last-mentioned work. the In this case, the safest measure is to switch off the machine, unfortunately this precaution will be implemented but too seldom noticed. If you try to withdraw the switched on machine or To resolve the cause of the deadlock, the sudden restart of the machine can be very uncomfortable Have consequences.

This is especially true in cases where pieces of wood jam the circular saw blade when it is restarted be thrown away, or if the grinding wheels tilt, where the wheel often breaks arises and the parts of the grinding wheel can loosen.

In machine tools, higher power is common a safety slip clutch is provided in the event that the tool is blocked a certain torque so that the motor continues to run, but the tool stops remain. However, this safety slip clutch can in no way mitigate the dangers outlined, because it does here, if the cause of the jamming of the tool is no longer present, an abrupt restart occurs.

An immediate shutdown of the machine when it comes to a standstill (blockage) - what from CH-PS 3 40 889 is already known - but there is no remedy for the problem. Just at Machine tools are conceivable operating states where the full drive torque is available got to. it would also lead to disruptive work stoppages in many cases, if every - too Brief - blocking of the machine would result in a shutdown.

Thus the task arises of an automatic shutdown of the machine within a period of time after the machine has come to a standstill, which is below the reaction time of the worker so that there is no attempt to withdraw the machine with the motor switched on. on the other hand the machine still has to deliver its full moment fractions of a second after the blockage in order to achieve the To be able to fully exploit performance limits.

This object is achieved according to the invention in the protective device according to claim 1 by its characteristic features solved.

This can change the time until the overload protection switch (7) responds that the resistance (6) is changeable, the time constant can thus correspond to the stresses. adapted to speak.

The selected circuit that the circuit breaker (7) iri includes the function of the protective device when the machine is blocked, has opposite the solution known from CH-PS 3 40 889, directly a disconnector in the main control circuit

of the machine that causes the safety shutdown has two advantages: It avoids a to arrange additional control element on the machine. The tripping of the overload protection switch (7) in continuous operation or by the locking device according to the present invention leads to the same result. The machine can be operated as usual - even with the extended protective device - take place. The decisive advantage, however, is to be seen in the fact that the overload protection switch (7) after its response for some time due to its thermal overload and that of cooling down required period of time cannot be switched on again immediately, in which an additional safety element is established. While the circuit breaker cools down, there is an opportunity to identify the cause the blockage of the machine can be removed safely.

F i g. 1 and 2 show different exemplary embodiments for mechanical devices for operating the switch (5) on the basis of schematic longitudinal sections by a hand drill. In the case of the in FIG. 1 is one on the Drive shaft (1) of the drive motor? igeorünete Device in which the switch (5) is connected to a sleeve (3) which can be displaced on the drive shaft (1) is. This sleeve is clamped between a compression spring (2) and tiltable centrifugal weights (4). at rapid rotation of the drive shaft (1), the flyweights (4) are pushed outwards. This will Sleeve (3) moved against the pressure of spring (2), switch (5) is open. When the However, the drive shaft (1) cannot compensate for the force of the compression spring (2) by the flyweights (4) the sleeve (3) is moved against the flyweights and closes the switch (5).

Such constructions are generally known as "centrifugal switches". With the one described here Application, a circuit is closed over the resistor (6), which together with the over The current flowing in the motor circuit of the machine causes the overflow protection switch (7) to respond.

In the device shown in Fig. 1, the switch (5) is also when the machine is switched off closed, which means that when the machine is switched on during run-up via the resistor (6) a stream would hire until a drive shaft speed (t) is reached at which the flyweights (4) the sleeve (3) can move against the spring force of the compression spring (2) so that the switch (5) opens will.

This disadvantage can be avoided if the resistor (6) has a negative temperature coefficient is carried out, so that it initially has a very high resistance value, but over time the

ίο load decreases.

In the case of machine tools with a safety slip clutch, however, a device can be implemented in which there are no disadvantages when the machine is switched on.

Fig.2 shows such an arrangement in which the Switch (5) via a sleeve (8) is actuated by the relative movement of the two halves (10 and 11) of the Safety slip clutch can be moved in the longitudinal direction. This is done through a thread (9) on the outer circumference of the output-side half (11) of the Slip clutch. The inside of the sleeve (8) has a corresponding thread (<*> f see. The socket is used in normal operation (if Deide parts of the Rotate the slip clutch together) by the spring

(12) held in its end position. When reading the Slipping clutch, however, if the output-side half (1 <) «because the load torque is exceeded stops, is done by further turning the drive-side half (10) of the slip clutch a friction surface arranged on the outer circumference of the coupling half (10) entrains the sleeve (8th). This sleeve rotates with respect to the stationary coupling half (11) and is accordingly the pitch of the thread (9) shifted in the axial direction, the switch (5) actuated and the Circuit is closed via the resistor (6).

In a similar way, the function of the protection device can also be performed using electronic Components are realized. This can be done, for example, by light barriers that the Observe the rotation of the drive shaft (1). Pr the resistor (6) are also electronic equivalent circuits possible that a time delay or a change in the electrical resistance cause.

For this purpose 2 sheets of drawings

Claims (8)

Patent claims: 1. Protective device for hand-held electrical machine tools, in particular do-it-yourself machines, with a drive motor, with a self-actuating overload circuit breaker and with one of the rotary movement of the drive shaft of the drive motor operated switch, which the The drive motor switches off when it comes to a standstill under overload, characterized in that that the switch (5) and a resistor (6) in a circuit parallel to the Main circuit of the machine are arranged, and that this additional circuit together with the main circuit loads the overload protection switch (7) when the machine is blocked. 2. Protection device according to claim 1, characterized in that the resistor (6) is variable is 3. Protection device according to claim 1 or 2, characterized in that the resistor (6) has a negative temperature coefficient. 4. Protection device according to claim 1 or 2, characterized in that the resistor (6) can be replaced by an electrical circuit known per se, which initially becomes high over time but represents sharply decreasing resistance 5. Protection device according to claim 4, characterized in that the change over time of the Resistance of the circuit is made adjustable. 6. Protection device according to claim 1, characterized in that the Scnalter (6) with one on the output shaft (1) \ sliding sleeve (3) is connected and that this uluffe between a Compression spring (2) and tiltable centrifugal weights (4) is clamped. 7. Protection device according to claim 1, characterized in that the switch (5) has a sleeve (8) is connected, which is displaceable in the longitudinal direction via a thread (13) against the force of the spring element (12) is mounted on the outer diameter of a safety slip clutch (10, 11). 8. Protection device according to claim 1, characterized in that one of the rotational movement of the Drive shaft (1) controlled electronic circuit for actuating the switch (5) is provided.