GB2195569A

GB2195569A - Power tools

Info

Publication number: GB2195569A
Application number: GB08623549A
Authority: GB
Inventors: Joshua Nee Adjah Okunor
Original assignee: Desoutter Ltd
Current assignee: Air Compressors and Tools Ltd
Priority date: 1986-10-01
Filing date: 1986-10-01
Publication date: 1988-04-13
Anticipated expiration: 2006-10-01
Also published as: DE3767437D1; GB8623549D0; GB2195569B; EP0266066A1; EP0266066B1

Description

1 GB2195569A 1

SPECIFICATION

Power tools This invention relates to power tools and in particular to power tools equipped with a de vice for cutting-off drive when the torque across the tool holder has reached a preset torque datum.

A power tool which has a device for cut ting-off drive is described in United Kingdom patent No. 2 096 51013. In this power tool, the device is in the form of a clutch opera tively connected between a motor and a tool holder. The clutch comprises torque sensitive clutch members which move axially relative to each other when a torque above a preset value is transmitted across the clutch. This relative movement gives rise to relative move ment between members of a Hall effect de vice which causes the motor to be cut-off.

The clutch disengages the tool holder from the motor. In order to vary the torque at which the clutch disengages the tool holder from the motor, a compression spring in the clutch can be adjusted by being tightened or loosened.

This power tool is disadvantageous in that it operates at full power regardless of the torque setting which gives rise to a significant 95 amount of inertia in the motor, gearing and the tool holder because full power is being supplied to the motor prior to cut-off. This gives rise to an indeterminate torque across the tool holder after cut-off, the value of this torque and rate at which it decays depending upon the friction between the tool and the workpiece, the inertia of the tool, tool holder, motor and gearing at cut-off.

The indeterminate torque gives rise to an undesirable degree of unpredictability in the operating characteristics of the power tool.

There has been proposed a clutchless power tool in which the torque across the tool holder is monitored, for example by measuring current supply in the case where the drive is an electric motor. In this power tool the power supply to the motor is cut-off when the torque reaches the preset level. This type of clutchless power tool also suffers 115 from inertia problems.

A previously proposed solution is to provide a power tool which has a variable free running speed, the cut-off being effected by means of a clutch or by current measurement as described above. Such power tools are disadvantageous because they cannot achieve high torques at low speeds.

It is an aim of the present invention to pro- vide a power tool which alleviates the afore- mentioned disadvantages so as to allow greater precision in cut-off torque adjustability while minimising undesirable torques after cut off.

According to the present invention there is 130 provided a power too[. comprising drive means for driving a tool holder, generator means for generating a signal indicative of torque across the drive means, means for switching substan- tially full power to the drive means when the torque is equal to or greater than a preset torque datum, and clutch means for cutting-off the drive to the tool holder in response to substantially full power being supplied to the drive means.

The torque datum may be the value of the torque across the drive means when the speed of the drive means is substantially equal to zero due to loading on the tool hol- der. Setting means may be provided for setting the value of the torque datum in dependence upon the free-running speed of the drive means.

Switch means may be operatively associated with the clutch means for switching off the power to the drive means when the clutch means cuts-off the drive to the tool holder. The switch means may be in the form of a Hall effect switch or a lever type switch.

The drive means may comprise an electric motor and the torque across the drive means may be measured in dependence upon current supplied to the electric motor.

The clutch means is preferably adjustable so that the torque at which it is operative for cutting-off the drive can be maintained greater than the torque datum.

A programmable microprocessor is preferably operative for determining the torque datum which corresponds to a given free-running speed by way of inspection of a pre-programmed set of corresponding torque data and free-running speed values. The programmable microprocessor may be operative for comparing the signal indicative of the torque across the drive means with the preset torque datum and for switching the drive means to full power when the torque is equal to or greater than the preset torque datum. The switch means may be incorporated into the microprocessor so that the microprocessor switches off the power supply to the motor immediately before the clutch means cuts-off drive to the tool holder.

Programming switches may be associated with the programmable microprocessor for setting an operating mode of the power tool, for example, for setting the tool. to be switched on/off in response to axial move- ment of the holder instead of by a manual switch. Additionally, the programming switches may provide means for switching the power tool to operate in forward or reverse functions.

The invention will now be further described by way of example, with reference to the accompanying drawings, in which:

Figure 1 illustrates variation of torque with time for a number of operating conditions of a power tool on a range of joints; 2 GB2195569A 2 Figure 2 shows the relationship between torque data and a range of freerunning speeds;, Figure 3 shows a schematic diagram of a power tool embodying the present invention; and Figure 4 illustrates variation of torque with time in relation to a preset torque datum Td for an embodiment of the present invention.

In Figure 1, four curves a to d show varia- tion of torque before and after a preset cut-off torque TP against time for a clutchless power tool (not shown), in the form of a screwdriver, which is driven by an electric motor (not shown) ' The curve a shows torque variations when the screwdriver is used to tighten a screw into a hard stop, the torque across the motor rises steeply as the screw is tightened. The torque across the electric motor is propor tional to the electric current supplied to the motor and so by measuring the current an indication of the torque can be derived. When the magnitude of the torque has reached a predetermined level T, the power to the elec- 90 tric motor is switched off. The screwdriver however continues to turn due to inertia of the rotating parts (ie, tool holder, gearing and electric armature). The torque across the elec tric motor continues to rise for a short time after the cut-off and then decays to zero as the speed of the screwdriver reaches zero.

Curves b, c, and d show different profiles of torque variation in the case where the screwd river is used to drive in screws into softer 100 materials.

Figure 2 illustrates how the torque varies with speed of the power tool for four different free-running speeds e to h. For example, in the case where full power is being supplied to the electric motor, the free-running speed of the screwdriver may be 1000 revolutions per minute (r.p.m). If a steadily increasing torque is applied across the electric motor, the speed of the screwdriver will steadily decrease until the electric motor stalls. In the case of e, the torque at which the electric motor stalls repre sents the maximum torque T,,, which the power tool is capable of supplying. If the ini tial free-running speed of the electric motor is 800 revolutions per minute then the maximum torque which can be supplied by the power tool is correspondingly lower (line f). Hence, by varying the free running speed of the elec tric motor, the maximum torque which can be supplied by the power tool can be varied ac cordingly. If it is desired to have a preset torque datum Td, then this may be achieved by setting the power tool to run at its maxi mum free-running speed of 1000 rpm and then cutting-off the power supply to the elec tric motor or disengaging the screwdriver and tool holder from the motor when the required torque datum Td is reached. However, in this case the speed of the screwdriver at cut-off is 850 rpm and so considerable inertia remains in the system.

In order to minimize the remaining inertia, the free-running speed may be set at 200 rpm (see line h of Figure 2) and so when the torque across the electric motor has reached the required torque datum Td, the speed of the screwdriver is substantially equal to zero and so very little inertia remains. Hence, if the screwdriver and tool holder is disengaged from the electric motor when this point is reached the inertia remaining in the motor and tool holder is much smaller.

The actual initial free-running speed may be set slightly higher than the theoretical value, say 220 rpm for the torque datum Td. This is due to resistive and frictional effects of gearing etc.

In practice, a certain amount of inertia will remain in the tool holder and screwdriver and so the absolute maximum torque is still slightly greater than the torque datum. This may be compensated for by setting a slightly lower torque datum than the desired maximum torque to be reached.

In Figure 3 there is shown a schematic diagram of a power tool embodying the present invention.

In Figure 3 an electric motor 2 is powered by a power amplifier 4 which is coupled to a microprocessor 6. The microprocessor 6 is operative for controlling the power supplied to the electric motor 2 by the power amplifier 4 in dependence upon the torque across the electric motor 2. An AD converter 8 receives a signal representative of the current i supplied to the electric motor and generates a digital signal indicative of the torque across the electric motor 2 and supplies this to the microprocessor 6.

A setting means which in this case is a speed select switch 10, sets the free running speed of the motor 2 and the datum torque Td (that is, the datum torque at which full power is to be switched on). The microprocessor 6 comprises a data memory 12 which may be in the form of a read only memory (ROM) which stores tables of torque data corresponding to different free-running speeds.

The microprocessor 6 is operative for comparing the digital signal representative of the torque across the electric motor 2 received from the AD converter 8 with the value of the preset torque datum T, which corresponds to the speed selected by the speed select switch 10.

When the torque across the electric motor 2 has reached the preset torque datum Td. the microprocessor 6 switches the power amplifier 4 to supply full power to the electric mo- tor 2. In response to full power being supplied to the electric motor 2, a mechanical clutch 14 responds immediately to the sudden increase in torque across the electric motor 2 so as to disengage the electric motor from a tool holder 15. Just before the tool holder 15 3 GB2195569A 3 is disengaged, a Hall effect switch (not shown), which is associated with the mechani cal clutch 14, generates a signal whic is fed to the microprocessor 6. On receipt of the signal, the microprocessor 6 switches off the power supply to the electric motor 2.

Applying full power to the electric motor 2 when the preset torque datum Td is reached has the advantage that the rotating parts of the tool do not have a significant amount of inertia because the parts are rotating slowly.

Hence, the torque/time graph is similar to that of curve V in Figure 1 regardless of the joint.

Consequently, for various types of joint, for example, hard or soft joints, the torque 80 achieved is substantially constant.

Figure 4 shows an example of torque against time for an embodiment of the inven tion where the torque datum has been set at T, and the maximum torque desired is Tmar When the torque datum Td is reached (at po int A of Figure 4), the screwdriver is supplied with full power and the torque increases until the clutch setting is reached (P of Figure 4 indicates the point at which the mechanical clutch 14 is set to disengage the tool holder from the electric motor 2) at which point the motor power is cut off, and disengage ment of the output drive occurs.

Embodiments of the present invention are advantageous in that the relatively low residual inertia of the tool holder 15 after disengagement enables a more precise determination of torque and provides for the possibility of ob- taining a high torque at a relatively low speed.

A touch switch pad 16 may be provided on the tool and is connected to the microprocessor 6 in order to provide an on/off switch for the power tool. An overload detecting means 18 is also associated with the microprocessor 6 in order electrically to disconnect the electric motor 2 from the tool holder and/or to disconnect the power supply from the power amplifier 4 in the case where there is an over- load due to over-heating or the like.

Four programming switches 20 may be coupled to the microprocessor 6 to enable the power tool to be programmed to operate in any one of up to sixteen modes. For example.

one switch may be operative for setting the microprocessor to be switched on/off in response to axial movement of the tool holder instead of by the touch switch pad 16. In addition, one of the programming switches may be operative for providing a slow start feature. Such a feature is useful in the case where the screwdriver is intended for delicate work where it may be desired to start the screwdriver slowly (thereby giving the opera- tor time to locate the screwdriver accurately in the screw head) and to increase speed gradually.

Claims

1. A power tool comprising drive means for 130 driving a tool holder, generator means for generating a signal indicative of torque across the drive means, means for switching substantially full power to the drive means when the torque is equal to or greater than a preset torque datum, and clutch means for cutting-off the drive to the tool holder in response to substantially full power being supplied to the drive means.

2. A power tool according to claim 1, wherein the torque datum is the value of the torque across the drive means when the speed of the drive means is substantially equal to zero due to loading on the tool holder.

3. A power tool according to claim 1 or claim 2, wherein switch means is operatively associated with the clutch means for switching off power supplied to the drive means when the clutch means cuts-off the drive to the tool holder.

4. A power tool according to claim 3, wherein the switch means is a Hall effect switch or a lever type switch. 90

5. A power tool according to any one of the preceeding claims, wherein the drive means comprises an electric motor, and the torque across the drive means is measured in dependence upon current supplied to the elec- tric motor.

6. A power tool according to any one of the preceeding claims, wherein the clutch means is adjustable so that the torque at which the clutch means is operative for cut- ting-off the drive can be maintained greater than the torque datum.

7. A power tool according to any one of the preceeding claims, wherein setting means is provided for setting the value of the torque datum in dependence upon the free-running speed of the drive means.

8. A power toot according to claim 7, wherein a programmable microprocessor is provided for determining the torque datum which corresponds to a given free-running speed.

9. A power tool according to claim 8, wherein the torque datum is determined by way of inspection of a pre-programmed set of torque datum and free-running values.

10. A power tool according to claim 8 or claim 9, wherein the programmable microprocessor is operative for comparing the signal indicative of the torque across the drive means with the preset torque datum and for switching the drive means to full power when the torque is equal to or greater than the preset torque datum.

11. A power tool according to any one of claims 8 to 10, wherein programming switches are associated with the programmable microprocessor, which switches are operative for setting an operating mode of the power tool.

12. A power tool according to claim 11, 4 GB2195569A 4 wherein the programming switches provide means for switching the power tool to operate in forward or reverse functions.

13. A power tool according to any one of claims 8 to 12, when dependent upon claims 3 and 7, wherein the switch means is incorporated into the programmable microprocessor so that the microprocessor is operative for switching off the power supply to the drive means immediately before the clutch means cuts-off drive to the tool holder.

14. A power tool substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

Published 1988 at The Patent Office, State House, 66/71 High Holborn, London WC 1 R 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Burgess & Son (Abingdon) Ltd. Con. 1/87.

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