GB2600571A

GB2600571A - Electromagnetic click wrenches

Info

Publication number: GB2600571A
Application number: GB2118721.6A
Authority: GB
Inventors: William Everitt Peter; James Collyer Timothy
Original assignee: Crane Electronics Ltd; Crane Electronics Inc
Current assignee: Crane Electronics Ltd; Crane Electronics Inc
Priority date: 2019-08-15
Filing date: 2019-08-15
Publication date: 2022-05-04
Anticipated expiration: 2039-08-15
Also published as: GB201911725D0; GB2586266B; GB2586266A; GB2600571B

Abstract

A torque wrench has a shaft 6 with a head 8 at one end and a handle 4 at the other end. The handle is coupled to the shaft to permit relative movement between the handle and the shaft, providing haptic feedback to an operator of the wrench, by a pivot 10, for example. A lock has a pole piece 14 in either the handle or the shaft, and an electromagnet 12 in the other of the handle or the shaft, the electromagnet is energised to attract the pole piece and engage the lock opposing the relative movement between the handle and shaft (fig.1a). A controller (fig.2,20) is programmed with a torque to be applied to each fastener and controls the supply of current to the electromagnet at a level corresponding to the pre-programmed torque for each fastener. The controller may also detect when the pole piece separates from the electromagnet (fig.1b) and then cease current supply de-energising the electromagnet; current level may be controlled by pulse width modulation.

Description

TITLE

Electromagnetic click wrenches

DESCRIPTION

Technical field

The invention relates to torque wrenches that are used to apply a measured torque to a nut and bolt or a similar threaded fastening. In particular, it relates to "click" wrenches, which provide haptic feedback to an operator when a desired level of torque is reached, in the form of a small relative movement between the handle and the shaft of the wrench.

Background

A torque wrench comprises a shaft, a head at a first end of the shaft for applying torque to a fastener, and a handle at a second end of the shaft, by which an operator grips the wrench to apply force to it. Electronic torque wrenches are now standard in manufacturing industry. Such a wrench electronically measures the torque of the joints being tightened, typically by using one or more strain gauges on the shaft of the wrench. It is possible to enter the set point for each joint using analogue or digital input means on the wrench itself Alternatively, an external computer can be used to pre-program a sequence of] oints to be tightened using the wrench, with the associated set point being stored for each joint. The wrench can then record the measured torque actually applied to each joint, which can be stored in the computer as evidence of whether the specified assembly program was correctly followed.

In click wrenches, a pivotal coupling is introduced somewhere between the handle and the head of the wrench to permit a small amount of relative movement. This specification describes a pivot located between the handle and the shaft but it may alternatively be between the shaft and the head or at other locations in the wrench and work in the same way. There may also be a wrench body fixed to the shaft such that the handle pivots with respect to the body, not the shaft itself During normal use of the wrench to apply torque to the fastener, a locking means rigidly secures the handle to the shaft and prevents pivotal movement between them. Upon the occurrence of a -2 -trigger condition, the lock can be released to permit a small amount of movement of the handle relative to the shaft, which provides haptic feedback that the operator may feel (and possibly also hear) in the form of a click. The axis of the pivot is parallel to the axis of the wrench head so that the movement of the handle occurs in the same plane as the movement of the shaft while applying torque.

The click indicates to the operator that they should stop applying torque to the wrench. The trigger condition may be that the wrench has reached the desired level of torque (the "set point"). Alternatively, it is known from patent GB 2506705 B for the wrench to measure the rate of change of torque applied by the operator and to trigger the click just before the set point is reached, in order to make allowance for the time the operator takes to react to the signal.

One known form of click wrench is described in patent GB 2524504 B. In that document, the locking between the handle and the shaft is magnetic. The force of a permanent magnet mounted on the handle attracts a pole plate mounted on the shaft to secure the handle rigidly to the shaft during normal use. An electromagnet is arranged in opposition to the permanent magnet so that, upon the occurrence of the trigger condition, the electromagnet can be actuated to oppose the attractive force between the permanent magnet and the pole piece. The force being exerted by the operator on the handle in order to apply torque to the wrench is then sufficient to overcome the net magnetic force so that the handle moves relative to the shaft and provides the desired haptic feedback. The patent also discloses that the locations of the magnet and the pole piece may be exchanged without affecting the principle of the invention.

There are various problems with the use of a permanent magnets for locking between the handle and the shaft. They can be relatively heavy and bulky components and become increasingly so for wrenches with a higher torque rating. It is difficult to source permanent magnets of consistent strength. The strength can also vary with age and with environmental conditions. The effectiveness of the lock depends on good contact between the magnet and the pole plate so that precautions need to be taken against the ingress of dirt during manufacture and use. That is made more difficult because a -3 -permanent magnet tends to attract metallic particles such as filings and other debris that are often present in a manufacturing environment. There may normally be an attractive force that helps to secure the magnet in its seat in the handle or shaft but, because the direction of the force is reversed when the electromagnetic field is applied, a strong adhesive or another anchoring means is still required. In order for an electromagnet to counteract the force of the permanent magnet effectively when the lock is released, the coil of the electromagnet may need to carry a current of several amps, which can be difficult to generate from a battery small enough to be located in the handle or body of a torque wrench. Although sustained for only a short time, this high current is a significant drain on the battery and it limits the number ofjoints that the wrench can be used to tighten before its battery must be recharged.

Summary of the invention

The invention provides a method of operating a torque wrench as defined in claim 1 The invention further provides a torque wrench as defined in claim 4.

Preferred but non-essential features of the invention are defined in the dependent claims.

It is recalled that the prior art used a permanent magnet to hold the pole piece until, on the occurrence of the trigger condition, a short pulse of current was applied to an electromagnet to provide an opposing force and enable separation between the pole piece and the magnet. The present invention does not use a permanent magnet and thereby avoids the problems associated with permanent magnets that were previously described. Instead, the present invention requires current to be supplied continuously to the electromagnet to engage the -lock" but it has surprisingly been found that this demands less energy than the prior art. At least in part, that is because the permanent magnet in the prior art needed to be strong enough to resist the force applied at the maximum rated torque value of the wrench (plus a margin of error). Therefore on every use of the wrench, the opposing electromagnet would similarly require the maximum level of current to be supplied. In contrast, the present invention only requires a current -4 -sufficient to hold the pole plate against the actual force at the set point of the particular fastener being tightened, which in most cases will be significantly less than the rated value of the wrench.

The drawings Figure la is a schematic plan view of a click wrench according to the invention before the set point is reached.

Figure lb is a schematic plan view of the click wrench of Figure 1 a after the set point has been reached.

Figure 2 is a schematic diagram of the control circuit of a click wrench according to the invention.

Figure 3a is an exploded view of an electromagnet and pole piece according to one embodiment of the present invention.

Figure 3b is a perspective view of a cross-section through the assembled electromagnet and the pole piece of Figure 3a.

Figure la schematically illustrates a click wrench comprising a shaft 6 with a handle 4 at one end and a head 8 at the other end for applying torque to a fastener. The shaft 6 incorporates an interface 7 with a display and various controls. The shaft may further incorporate one or more strain gauges 9 for use in measuring the torque applied by the wrench to the fastener. The size of the handle 4 in the drawing is exaggerated for clarity. An operator uses his/her fingers 2 to apply a force F to the handle 4 from one side, in order to generate torque at the head 8 and turn the fastener in a clockwise direction (as viewed in the drawing). The torque experienced at the head 8 is equal to the magnitude of the applied force F multiplied by the distance between the application point of the force F and the head 8. The handle 4 is coupled to the shaft 6 by a pivot 10 but is normally prevented from moving relative to the shaft by a locking means that comprises a magnet 12 that is attracted to a pole piece 14. As illustrated, the magnet 12 is attached to the handle and the pole piece 14 is attached to the shaft 6 but the locations could be exchanged. Instead of a simple pivot 10 as illustrated, the handle 4 and the shaft 6 can be coupled via a linkage that results in their relative movement about a virtual pivot point located inside or outside the handle, for example, near the head 8. -5 -

In order to be able to secure the handle 4 rigidly to the shaft 6 and prevent the handle 4 rotating about the pivot 10 prior to the occurrence of a trigger condition, the magnet 12 must be able to balance the torque that the force F exerts about the pivot 10. The force F increases as the fastener joint is tightened by the operator. In accordance with the present invention, the magnet 12 is an electromagnet, which must be energized with sufficient current to balance the applied force F until the trigger condition is reached. Upon occurrence of the trigger condition, the applied force F overcomes the strength of the electromagnet 12 and causes the handle 4 to undergo a small angular movement to about the pivot 10 to the position shown in Figure lb, in which the electromagnet 12 has separated from the pole piece 14 and the handle 4 rests at a slight angle from its original orientation. Abutment between elements (not shown) of the handle 4 and the shaft 6 prevents the relative movement between them continuing further. The angle through which the handle 4 moves is typically one to three degrees.

The electromagnet 12 must be able to create a force strong enough to secure the handle against a force F at least equivalent to that applied at the torque rating of the wrench. For example, a typical wrench may be rated at 75Nm and be 400mm long. If the operator applies a force F=200N at 350mm from the fastener axis, that equates to a torque of 70Nm on the fastener. If the electromagnet 12 were located opposite the application point of the force, so that both were equidistant from the pivot 10, the electromagnet would need to provide a balancing force of 200N to avoid separating from the pole piece 14. In fact, as shown in Figure 1, the electromagnet 12 can be located somewhat further from the pivot 10 than the application point of the force F in order to provide a mechanical advantage. By way of example, a small and lightweight electromagnet of dimensions 52mm x 28mm x 22mm can generate 400N of force using currents of the order of 50 to 150mA.

Figure 2 is a schematic diagram of the control circuit of a torque wrench according to the invention. A controller 20, indicated generally by dot-dash lines, comprises a processor 22 and may also comprise associated memory and input/output devices (not shown) that permit the controller 20 to be programmed with the set points of one or -6 -more fasteners, and to record and export measurements of the torque applied by the wrench. An adjustable current source 24 supplies current to a coil 26 of the electromagnet to control the strength of the electromagnet in accordance with a signal received from the processor 22. It is preferred to control the level of current using a pulse width modulation (PWM) technique but it is equally possible to control the current's absolute magnitude. A feedback signal 27 representing the actual current passing through the coil 26 is provided to the processor 22 via an analogue-to-digital converter (ADC) 28. Its use will be described below. As previously indicated, the torque wrench may optionally be provided with one or more strain gauges 9. If present, their output is also provided to the processor 22 via a signal filtering and amplification function 29 and the ADC 28, for use by the processor 22 in measuring the torque applied by the wrench.

Figure 3a shows an exploded view of one example of an electromagnet 12 and pole piece 14 suitable for use in a click wrench according to the present invention, while Figure 3b is a cross-section through the same electromagnet 12 following assembly, with the pole piece 14 adjacent to it. The electromagnet 12 comprises a coil 30 wrapped around an annular former 32. A body 34 is formed of soft iron or an alternative magnetically conductive material. It has an annular recess 36 for receiving the former 32 and the coil 30 such that the centre of the body 34 provides a core 38 for the coil 30. As shown, the body 34 may be narrowed so that the coil 30 protrudes beyond the sides of the body. This reduces the amount of material required for the body 34 and consequently its weight, while providing access for electrical connections (not shown) to the coil 30.

In use, the torque wrench is pre-programmed with the torque to be applied to each fastener at the set point. The programming may be done through manual input via the controls on the wrench itself or through a separate computer with which the wrench is in communication during or prior to use. An operator engages the wrench with a fastener and turns it to apply torque to the fastener. The torque increases as the joint is tightened until the occurrence of the trigger condition. Then the applied force F overcomes the strength of the electromagnet 12 and permits the small movement of the -7 -handle 4 relative to the shaft 6, which the operator senses as a click, thereby providing haptic feedback to indicate that the set point of the fastener has been reached and no more torque needs to be applied.

In a method of operating the wrench, which does not depend on the use of strain gauges 9, the controller 20 controls the current supply to the electromagnet 12 such that the strength of the electromagnet will exactly balance the applied force F at the set point of the fastener being tightened. The relationship between the current and the torque set point may be calculated according to a formula or may be obtained from a look-up table. Preferably the look-up table has been populated during initial testing and calibration of the torque wrench. (Of course, the wrench may subsequently be recalibrated and the look-up table updated accordingly.) As long as the increasing torque has not yet reached the set point, the strength of the electromagnet 12 is sufficient to resist the applied force F so the electromagnet 12 is held firmly against the pole piece 14 and the handle 4 and the shaft 6 are locked together to behave as a rigid assembly. As soon as the torque exceeds the set point, the applied force F is sufficient to overcome the strength of the electromagnet 12, which separates from the pole piece 14 to provide haptic feedback to the operator.

The separation of the pole piece 14 from the electromagnet 12 changes the flux path of the magnet and the inductance of the electromagnet system, which can be sensed via the pick-up 27 from the coil 26. The controller 20 can then switch off the current supply to the electromagnet 12, which serves no further purpose. The controller can also log the fact that the set point was reached, as evidence that the fastener was fully tightened.

The trigger condition may coincide with the set point being reached or may be determined to occur just before the set point is reached, based on a predictive algorithm, in order to make allowance for the time the operator takes to react to the haptic feedback signal. -8 -

Claims

CLAIMS1. A method of providing haptic feedback to an operator of a torque wrench, wherein the torque wrench comprises: a shaft having a first end and a second end; a head at the first end of the shaft for applying torque to a fastener; a handle at the second end of the shaft, the handle being coupled to the shaft in a manner that permits relative movement between the handle and the shaft to provide said haptic feedback; a pole piece in one of the handle and the shaft; and an electromagnet in the other of the handle and the shaft; and wherein the method comprises: when the operator uses the wrench to apply torque to the fastener, supplying current to energize the electromagnet to attract the pole piece and thereby oppose said relative movement between the handle and the shaft, the current being supplied at a current level that is predetermined for each fastener.
A method according to claim 1, further comprising: detecting when the pole piece separates from the electromagnet; and ceasing the supply of current to de-energize the electromagnet.
3. A method according to claim 1 or claim 2, wherein the current level is controlled by pulse width modulation of the current A torque wrench comprising: a shaft having a first end and a second end; a head at the first end of the shaft for applying torque to a fastener, a handle at the second end of the shaft, the handle being coupled to the shaft in a manner that permits relative movement between the handle and the shaft to provide haptic feedback to an operator of the wrench; and a lock that can selectively be engaged to oppose said relative movement between the handle to the shaft, the lock comprising a pole piece in one of the handle and the shaft and an electromagnet in the other of the handle and the shaft, such that energizing the electromagnet to attract the pole piece engages the lock, and a controller capable of being programmed with a torque to be applied to each fastener and of controlling a supply of current to the electromagnet at a current level corresponding to the pre-programmed torque for each fastener.5. A torque wrench according to claim 4, wherein the controller is further capable of detecting when the pole piece separates from the electromagnet and then ceasing the supply of current to de-energize the electromagnet.