Patent Publication Number: US-2007103104-A1

Title: Power torque tool

Description:
FIELD OF THE INVENTION  
      This invention relates to a pulsed torque tool and to methods for measuring the torque generated in such a tool and for controlling operation of the tool to achieve a pre-determined torque.  
      The invention also relates to a method and apparatus for measuring the torque loss occurring along a torque transmission shaft; and to the determination of the torque applied to a load by such a shaft. This aspect of the invention has particular application to the measuring of torque loss in a power tool generating a pulsed torque drive and to the determination of the torque applied to a load by such a power tool.  
      The invention has particular, though not exclusive, application to powered tools for delivering a controlled torque without the operator having to measure or judge the torque exerted. Such tools may sometimes be referred to as powered torque wrenches.  
      Pulsed torque tools include two categories. One in which an impact generates a torque impulse: the other in which a pulse of controlled characteristics is generated, such as by a pressure pulse generated with the aid of a piston and cylinder mechanism. In both cases, a train of successive pulses is generated to produce increasing torque. Impact-type tools may be electrically or pneumatically driven. Pressure pulse-type tools may be hydraulically-driven (e.g. oil) or electrically driven.  
     BACKGROUND TO THE INVENTION  
      Power torque tools have been long used for applying a tightening torque to secure nuts to bolts, or similar operations, in manufacturing industry: automobile assembly is an example. They supply a succession of torque drive pulses. The pulses are generated at one end of an output shaft and are transmitted to an adapter at the other end configured to fit a nut or a bolt head. The pulses generated by power torque tools may be generally put in two classes in accord with the two categories of tool above mentioned.  
      The first class of pulses are short-duration impulses generated by impact power tools using a hammer and anvil type of mechanism in which a rotating hammer (dog) assembly percussively strikes an anvil coupled to the torque transmission shaft. This is an intermittent contact of hammer and anvil. A second class of pulses are longer duration impulses generated by pressure types of mechanism in which the shaft is continuously coupled to a piston and cylinder mechanism in which pressure pulses are generated to pulse the shaft. For convenience where a specific class of pulses is referred to herein the first and second classes of pulses may be referred to as impact pulses or impulses and pressure pulses or impulses, respectively.  
      As regards impact torque tools, reference may be made, for example, to U.S. Pat. Nos. 3,428,137 and 5,083,619. In such a tool a rotating motor, frequently pneumatically powered but it may be electrically powered, actuates with the aid of a cam a mechanism to drive hammer dogs in a linear axial direction and rotationally to engage anvil dogs whereby the rotational hammer motion transferred is to a rotary motion of anvil dogs as a step-wise pulsed motion. Usually there are two hammer impacts per rotation of the motor. The output shaft is driven by the step-wise pulsed motion of the anvil dogs. A clutch may be provided between the motor and the set of hammer dogs. Thus the anvil mechanism generates a train of torque impulses at the output shaft.  
      The delivery of torque to the shaft is not a simple relationship. It is very dependent on the nature of the load to which output torque is delivered. Tightening a nut up on a bolt or a bolt to a nut to a desired torque is a common example of a load and one much found in industrial assembly processes. In such industrial process it is often required that the same tightening procedure is repeated at frequent intervals and creates the need for a repetitive, reliable operation consistently achieving a required torque to which the nut or other part being tightened is driven. The torque is converted to other stresses by which the relevant parts or fixtures are secured.  
      It has been the practice to measure torque in the output shaft of an impact torque tool by means of a strain gauge assembly, the output of which is used to control the power to the motor. One problem with strain gauge sensors is that they are affixed to the output shaft. They are liable to become detached from the shaft due to the violent hammering and shaking of the shaft in an impact-type of operation. This is likely to be true of any sensor device that requires to be attached to the shaft. Another problem is the transmission of signals from the sensor device on the shaft to the processing electronics housed within the tool. The hammering and shaking of the shaft make the use of signal transmission by means such as slip rings unreliable. Yet another problem resides in the speed of response of the sensor device, or its related parameter bandwidth, bearing in mind that torque is generated as impulses in the shaft.  
      A more general problem which underlies the controlled operation of an impact torque tool is the lack of understanding to date of the torque impulsing and its interaction with the load which becomes stiffer as tightening progresses. If too high a degree of tightening is attempted, this may lead to damage, such as shearing of a bolt for example.  
     SUMMARY OF THE INVENTION  
      The present invention proposes in one of its aspects the employment of magnetic-based torque transducer technology having a transducer element that is formed integrally in the output shaft of an power torque tool. By this means the transducer element cannot become detached from the shaft. The element emanates a torque-dependent magnetic field which is detected by a magnetic field sensor arrangement which is not in contact with the shaft. The transducer element is of a kind described further below which has a fast response appropriate to sensing torque impulses.  
      In another aspect the invention proposes procedures by which the achievement of a given torque can be predicted or measured and used in controlling the operation of a powered power torque tool. The development of these procedures depends on an investigation, analysis and measurement of the characteristics of the train of torque impulses generated by the tool. This work has now been undertaken with the aid of the magnetic transducer technology mentioned in the preceding paragraph and is reported below.  
      The practice of the invention will be more particularly described below in relation to an impact torque power tool and the processing of impact type impulses generated thereby. It will be understood that the employment of the magnetic-based torque transducer technology is applicable to tools generating pressure pulses. Furthermore the pulse processing and measurement procedures taught below are generally applicable to both impact and pressure types of pulses.  
      In a further aspect the present invention proposes to make a torque measurement at two points of known spacing along a torque transmission shaft to which pulses of torque are applied. This provides a measure from which can be deduced a parameter representing the torque loss or the rate (per unit length) of torque loss along the shaft, and from which parameter the torque delivered to the load end of the shaft can be calculated. In the description given hereinafter the torque loss per unit length along the shaft is considered constant so that a linear extrapolation can be made. However, the teachings herein can be applied to other assumptions of the torque loss per unit length.  
      This last aspect of the invention will be described and discussed hereinafter with particular reference to its implementation in relation to power torque tools.  
      Aspects and features of the present invention for which protection is presently sought are set forth in the claims following this description.  
      The invention and its practice will now be further described with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows diagrammatically the main features of a power torque tool being used to tighten a load in the form of a nut and bolt engaging a fixture. Torque is applied to the head of the bolt to tighten it with respect to the nut.  
       FIG. 2  shows diagrammatically an experimental laboratory apparatus using a pendulum to deliver a torque impulse to a shaft;  
       FIG. 3  is a view of the pendulum apparatus of  FIG. 2  including a magnetic torque transducer;  
       FIG. 4  shows torque impulse responses derived from the transducer for a rigidly held shaft impulsed by different pendulum energies;  
       FIG. 5  shows torque impulse responses for a shaft that is stiffly but not rigidly held at different levels of stiffness and the same pendulum energy applied;  
       FIG. 6  is a set of diagrams A-F showing the nature of the impulsing as seen in  FIGS. 4 and 5 ;  
       FIGS. 7, 8  and  9  shows impulse responses over a longer time interval for an impact torque tool for a bolt that is relatively lose, very tight and hard tight respectively;  
       FIG. 10  shows superimposed trains of torque impulses to illustrate the pulse-to-pulse variation when a mechanical adapter is used on an impact power tool;  
       FIGS. 11   a - 11   c  are presentations in a three-dimensional graphical form of a data relating to a sequence of impulses, the presentations being of the same data from different perpsectives with respect to the axes of the graphs;  
       FIG. 12  is a presentation in a three-dimensional graphical form in the perspective of  FIG. 11   c  of data relating to a sequence of impulses having a different characteristic to that of  FIGS. 11   a - c;    
       FIG. 13  is a graph showing curves relating to a Signal Integration procedure;  
       FIG. 14  illustrates the shape of the curve showing the rise with successive impacts of torque in the output shaft of an impact torque tool, the measurements being performed on an oil-pressure chamber torque calibration unit;  
       FIG. 15  is a graph of the time interval between successive impacts over a train of impacts;  
       FIG. 16  illustrates parameters of a torque pulse train relevant to an Instantaneous Torque Calculation procedure;  
       FIGS. 17-19  are graphs relating to plots of various parameters measured and derived in the Instantaneous Torque Calculation procedure;  
       FIG. 20  is a graph showing the fit of a curve of  FIG. 14  to the curve plotted in  FIG. 19  to demonstrate a correlation between them;  
       FIG. 21  is a flow-type diagram illustrating the implementation of Instantaneous Torque Calculation or Signal Integration to a train of pulses;  
       FIG. 22   a  illustrates a representative sample of a train of pulses; and  
       FIG. 22   b  is a curve showing the resultant torque value at the load.  
       FIGS. 23   a  and  23   b  diagrammatically illustrate examples of impact and pressure pulses respectively generated by different types of power torque tool; and  
       FIG. 24  shows a transducer arrangement utilising two torque transducers in accord with the present invention.  
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       FIG. 1  shows diagrammatically elements of a power torque tool  10  to which the invention is applied. The tool may be of the impact pulse type or of the pressure pulse type but for the purposes of the description that now follows, the tool  10  is taken to be of the impact pulse type.  
      The impact torque tool  10  is illustrated as a hand-held implement having a housing  12  within which is an electrically or pneumatically powered motor  14 . The motor is coupled by an impact converter  16  to an output shaft  18  the distal end of which carries an adapter  20  engageable with the load to which torque is to be applied. In this example, the load is a bolt  22  which carries a nut  24  and which extends through an apertured fixture  26 . As shown the nut and bolt are being tightened on to the fixture  26 . The adapter  20  engages the head  28  of the bolt, being formed with an internal recess that matches the head  28 , e.g. an hexagonal head. The features of the tool  10  so far described are conventional and well-known to those in the art. As will emerge from subsequent discussion the impact converter, by which the rotation of the motor  14  is converted to a train of torque pulses in the shaft  16 , the transmission of those pulses to the bolt head  28  has been the subject of a new investigation yielding new information as to the manner in which the torque impulses are generated, transmitted and react with a tightening load, that is a load which progressively yields less as the tightening proceeds. In all the tests described below the shaft or bolt to which torque is applied is being stressed within its mechanical elastic limits to avoid permanent deformation of which shear or breakage is the extreme end-point.  
      A new feature of the tool is a magnetic transducer  30  by which the torque impulses in the shaft are detected and measured. The transducer comprises a torque-sensitive element  32  which is an integral region of shaft  18  which is assumed to be of ferromagnetic material. The region  32  is magnetised to have remnant or stored magnetisation so that it acts as a source of external magnetic field, the magnetisation being effected in such a way that the region  32  emanates a magnetic field or field component which is dependent on the torque. One form of magnetisation is circumferential (circular) magnetisation the employment of which in an integral region of a shaft is disclosed in WO99/56099. Another form of magnetisation usable in an integral region of a shaft is longitudinal magnetisation in which an annulus of stored magnetisation is formed about the axis of the shaft and the magnetisation is in the direction of the shaft. One kind of longitudinal magnetisation is that referred to as circumferential sensing and is disclosed in published PCT application WO01/13081, and another kind, referred to as profile shift magnetisation, is disclosed in published PCT application WO01/79801, incorporated herein by reference. The profile shift may be detected in respect of the radial or the axial profile. The documents just-mentioned describe the magnetic sensor arrangements appropriate to the field to be detected. The present invention has been developed, and the investigations reported below have been made, using a profile shift magnetisation kind of transducer element. The emanated torque-dependent magnetic field is detected by a non-contacting sensor arrangement  34  which is connected to a detector and control circuit  36  which in turn controls the operation of the motor  14 . The sensor arrangement may comprise more than one sensor device and further details of the nature of the emanated magnetic field and the placement of the one or more sensor devices is to be found for each form of magnetisation in WO99/56099, WO01/13081 and WO01/79801 above-mentioned.  
      The sensor device(s) employed in sensor arrangement  34  may be Hall effect or magnetoresistive devices. What has been preferably used is saturating core device(s) connected in a circuit such as disclosed in WO98/52063.  
      In operating the impact torque tool—which may also be referred to as impact torque wrench—it is of interest to measure and predict the build up of torque in the bolt  22 . The transducer built into the tool can only measure torque in the output shaft, though this torque will be affected by the tightness of the bolt. It is to be noted that the transfer of torque from the shaft  18  to bolt  22  depends on how well the adapter  20  seats on the bolt head  28  and the alignment between the axis of shaft  18  and the axis of bolt  22 , bearing in mind that the tool is hand-held and may be applied with some misalignment. It has been found that the efficiency of torque transfer between the tool and the bolt is not likely to exceed 30%. Additionally losses can occur between the bolt  27  and the part or fixture  26  to which it is being secured. In the case where the bolt is a snug fit within the aperture through which it extends and is very rusty and not greased the torque transmission losses from the bolt-head  28  to the bolt shaft itself may be more than 50%. In using a hand-held tool the torque delivered over a series of impulses can vary widely.  
      There are a number of different approaches to defining how a pre-determined torque is achieved in the load being tightened. The bolt previously discussed will be used as an example.  
      1) Signal Integration  
      This method is based on measuring the torque delivered with each impact and integrating successive measurements to predict the achievement of a required torque within the shaft of the bolt. The method requires a calibration of the complete system of tool and load for which the bolt previously described will be used as an example. The measurement cycle begins from the point at which the bolt is just tightened to the part as by hand-tightening the bolt. At this point the torque in the bolt shaft is essentially zero.  
      This method assumes that the bolt-tightening under the action of the impact torque tool proceeds without interruption. It is applicable when the increase in torque with successive impacts on the bolt-head follows a defined curve that will be explained subsequently.  
      2) Instantaneous Torque Calculation  
      This method is also applied to the complete system of impact power tool and load. It relies on analysing the torque signal detected at each impact and calculating a torque dependent parameter for each impact. The succession of parameter are matched to a defined curve of torque v the number of the impact in a train of impulses.  
      The two procedures can be made available in a program operating in real-time and the program can include a decision function for selecting one or other procedure upon certain characteristics being detected as will be explained subsequently.  
      Both the above methods arise from work newly-done to investigate the impulse-type of action and the torque impulse arising out of it.  
       FIG. 2  diagrammatically illustrates the principle of a laboratory apparatus to investigate impact torque generation. A practical implementation is seen in  FIG. 3 . Referring to  FIG. 2 a  shaft  42  has one end  44  clamped against rotation in a fixture schematically shown as  43 . The fixture includes an aperture for receiving the bolt end  44  into which extends a clamping screw which can be adjusted from a setting allowing virtually free rotation of shaft  42  about its horizontal axis A-A, through degrees of resisted rotation, to no rotation. The other end  46  of the bolt is provided with a radially projecting peg  48  acting as an anvil. A pendulum  50  mounted to swing freely about horizontal axis B-B above axis A-A has a pendulum arm  52  carrying a weight  54 , the pendulum being dimensioned so that as it swings downwardly from a raised position the weight  54  strikes the peg  48  to generate a torque-energy impulse on shaft  42  from the momentum of the weight. The energy available to each torque impulse is determined by the initial height of the weight  54 .  FIG. 2  shows two initial positions  52  and  52 ′ of the pendulum arm and the weights  54 ,  54 ′ thereon, the height between which is h. Upon impacting the peg  48  it is deflected about axis A-A by an angle α shown exaggerated for clarity of illustration. The deflected peg position is  48 ′. The torque impulse in the shaft  42  due to the impact is measured by a magnetic-based transducer  60  comprising integral transducer region  62  of shaft  42  and sensor arrangement  64  in accordance with the transducer assembly  30  previously described. By way of example, the length L of the pendulum was 1.10 m, the weight  54  had a mass of 2 kg and the shaft  42  was of tensile steel of 15 mm diameter which is close to the diameter of the output shaft of the kind of specific input torque tool described below. The torque impulses detected by the transducer  60  are output as corresponding electrical signals and were monitored and displayed both as to duration and amplitude. The overall shape is also of significance.  
       FIG. 3  shows a practical laboratory apparatus working on the principle of  FIG. 2  and on which the results now to be reported were obtained. In  FIG. 3 , one end of the shaft  72  is secured in apertured block  74  in which adjustable bolts  76  are threadedly received in the block to act on shaft  72  and thereby control the degree of restraint against rotation. The other end portion of the shaft is rotatably mounted in support block  76  and the far end, projecting from block  76  carries the anvil  78  having a strike face  79 . The Figure also shows the lower end of the pendulum arm  80  carrying a hammer  82  about to strike face  79 . Weights  84  are secured to the pendulum. The magnetic field emanated by transducer region  86  in the shaft is detected by sensor arrangement  88 .  
      The first set of tests were performed with the shaft  72  held rigidly in block  74 . These are shown in the curves of  FIG. 4  in which the ordinate axis is the voltage output of the torque transducer  80  representing torque, and the abscissa axis is elapsed time in seconds, specifically gradations in milliseconds. The graph shows three curves  90 ,  92 ,  94  for different pendulum impulses for which initial height h of the hammer  82  above the strike position against face  79  was 17.4, 47 and 67 cm respectively. The torque pulse in each case is of a generally similar form—the torque rises, peaks, and then falls. The peak value rises with the increasing pendulum energy. No rotation of the shaft results. Curve  94  shows the end of the pulse swings below zero on the downslope as indicated at  94   a . This negative swing or rebound has a significance that will emerge later with reference to figures that have an extended time axis. It will be noted from  FIG. 4  that the pulses are generally symmetrical and have the same positive pulse duration.  
      Referring to  FIG. 5 , the graph shown is of the same kind as  FIG. 4  but the applied conditions are different. The pendulum strikes with the same energy, i.e. is raised to the same height in each case but the restraint on the shaft against rotation is varied. The bolts  76  have been released a little so that the shaft  72  is jammed but is capable of turning. It is again emphasised that the shaft  72  under test is operating within its elastic limits. The time axis in  FIG. 5  is also extended as compared to  FIG. 4 . Curves  100 ,  102 ,  104  and  106  are for increasing degrees of rotational restraint. All the four curves start to rise at essentially the same rate reflecting the same impact energy from the pendulum. The curve  100  pertaining to the lowest restraint against rotation rises to a peak  100   a  at which the shaft commences to turn at which time the applied torque drops rapidly to a lower value  100   b  and then tails off slowly, as the shaft turns and the impact energy is expended. There is no rebound. In this case the shaft  72  can be regarded as being pushed by the torque generated by the pendulum throughout the period the shaft turns. This is also true of the other curves.  
      Curve  102  requires more torque ( 102   a ) to commence rotation of the shaft. The torque then drops to a level  102   b  and then tails off in value during a period in which rotation continues, until the torque at  102   c  is no longer sufficient to maintain rotation at which time there is a sudden drop into a rebound phase  102   d  in which the torque reverses (becomes negative).  
      Curve  104  is for a still greater restraint against rotation. It reaches a higher peak  104   a  than that of curve  102 , descends rapidly to a value  104   b  from which it declines further to a zero value by which time rotation of the shaft has creased, and enters a rebound phase  104   d . The decline period shows the hint of a small rise at  104   e  after the peak has descended to  104   b . Curve  106  is a case where the shaft is now very light against rotation but is not hard tight to prevent any rotation. The peak value  106   a  reached is virtually the same as that of curve  104 . There is a descent quicker than curve  104  to a value  106   b  which is followed by a distinct rise  106   e  before the trailing decline into the rebound phase  106   d.    
      There are some time relationships which should be noted in  FIGS. 4 and 5 . It has already been mentioned that the positive pulse torque period for the three pulses is virtually the same at about 5 mS. In  FIG. 5  the restraint on the shaft  72  for curve  106  is closely approaching the total restraint applicable to the curves of  FIG. 4 . If the descent from the peak  106   a  is extended as shown by dashed line  106   f  it intercepts the zero torque axis close to 5 mS as with the curves of  FIG. 4 .  
      In  FIG. 5 , it is also the case that where the curves show a rebound phase, i.e.  102 ,  104  and  106 , they all cross the zero torque axis at virtually the same time, 8 mS. Thus the positive pulse portion driving the shaft  72  to rotation has the same total length in each of these cases.  
      One factor that is not directly seen from the curve of  FIG. 5  is the speed at which the shaft rotates and strike face  79  moves relative to the hammer  82  in ( FIG. 3 ). Also in tightening up a bolt with an impact torque tool such as outlined above, the torque required for rotation increases as the bolt rotates and the delivery of the input energy may be different from the pendulum case. Nonetheless, the pendulum apparatus experiments provide valuable guidance as to further investigations to be undertaken with an impact torque tool itself. There is an indication in the curves of  FIG. 5  that as the torque impulse trails away there is another effect that needs to be taken into account.  
      Referring to  FIGS. 4 and 5 , it is not surprising that where the shaft is held rigid against rotation, the peak torque impulse in the shaft increases with increasing pendulum energy and is followed by a significant rebound as the shaft relaxes. The pulses of  FIG. 4  appear to indicate effectively a single strike of the anvil by the pendulum.  FIG. 5  indicates something more complex in the pulse structure. Also  FIG. 5  illustrates a situation which appears analogous to static friction (striction) and dynamic friction. There is a limiting friction required to be overcome before the shaft rotates, whereafter rotation continues at a lesser torque value until the torque reduces to a level at which rotation cannot be maintained. This is exemplified in the curves of  FIG. 5 . If rotation can be maintained for a period as in curve  100 , it appears that all the impact energy is dissipated without a rebound pulse.  
      The following discussion is put forward as a theoretical explanation of a hammer action in an impulse torque tool based on the results seen in  FIGS. 4 and 5 . Reference will be made to the diagrams of  FIG. 6  which illustrates six conditions A-F of a hammer striking the anvil in an impact torque tool tightening a bolt head  28  as in  FIG. 1 . Each shows the torque amplitude (ordinate) as a function of time (abscissa). Diagram A shows the bolt starting in a relatively loose state—e.g. hand tight. There is an first positive torque impulse  110  followed by a lesser amplitude negative rebound or recoil  112 . In response to the positive impulse head  28  initially flies ahead of the hammer action but as the diagram shows there is a second impulse shown as a secondary peak  114  followed by a secondary rebound  116 . In diagram A, the secondary impact is distinct from the primary impact. As the bolt tightens it requires more torque to turn it. The bolt both rotates less and for a shorter period so that the time between the primary and secondary impacts shorten. This is indicated by arrow  118  showing the secondary impulse advancing in time towards the primary impulse. This is the situation shown in diagram B. As the tightening continues, the secondary pulse moves still nearer the primary as in diagram C until as illustrated in diagram D, the secondary positive peak  110  overlaps the negative primary pulse rebound  112 . This substantially flattens the negative swing and may cancel it altogether. As the bolt turns less and less on each impact the positive part of the secondary pulse occurs within positive part of the primary pulse moving steadily up the trailing edge. It is at this stage that the conditions applying to  FIGS. 4 and 5  arise. The bolt head which has been flying ahead now enters the push mode above-mentioned. Diagram E shows the secondary pulse slightly lifting the trailing edge at  114   a  and as is seen at  104   e  in  FIG. 5  and more so at  106   e . Finally the bolt ceases to turn further and effectively the secondary pulse disappears or may be regarded as coincident with the primary pulse. There is a single impact which causes a torque pulse like that exerted in  FIG. 4 . The peak torque exerted is the same as in the earlier diagrams. This is considered to be consistent with curve peaks  104   a  and  106   a ′ in  FIG. 5 . It is, of course, to be remembered that the graph of  FIG. 5  relates to the pendulum experiment, whereas the explanation given with reference to the diagrams of  FIG. 6  assumes a rapidly and repetitively driven impact converter in a impact torque tool.  
      The theoretical nature of  FIG. 6  was explored by some practical tests.  
      Using the tool equipped with a transducer  30  as shown in  FIG. 1 , investigations were made on the torque impulses generated in the output shaft  18  of the tool itself when driving a bolt load as illustrated. The tool used was a “CP733” pneumatically powered impact torque wrench available from Chicago Pneumatic Tool Company of Rock Hill, S.C. The results of these investigations are shown in  FIGS. 7-9 . Each figure is a graph of the transducer output representing torque as a function of time for a single impact in the converter  16  of  FIG. 1 .  FIGS. 7-9  relate to different conditions of bolt tightness, loose (hand-tight), tight but still capable of a little rotation and hard tight respectively. The same reference numerals are employed as in  FIG. 6  for the pulse portions like to those of  FIG. 6 .  
       FIG. 7  shows the first impact impulse  110  which is transmitted to the load (the bolt) which being relatively loose flies ahead. The output shaft  18  of the tool also initially flies ahead of the hammer mechanism in the tool going through a rebound  112  from which the torque rises positively as the shaft shows whereupon there is a second impulse  114  from the hammer mechanism which is followed by its own secondary rebound  116 .  
       FIG. 8  relating to driving a very tight bolt, shows the situation as in Diagram E of  FIG. 6  (and curves  104  and  106  in  FIG. 5 ) in which the torque impulse reaches a value sufficient to slightly turn the bolt and the output shaft at which point the torque drops. By this stage, the secondary impulse has sufficiently advanced in time for the portion  114  to appear as a small peak  114   a  on the trailing part of the primary impulse.  
       FIG. 9  shows the impulse waveform when the bolt being driven is hard tight. There is a peak value torque pulse  110  which does not move the bolt followed by a rebound  112 . This is consistent with  FIG. 1 .  
      Attention is now turned to the more practical use of the investigations reported above in determining when a impact torque wrench will achieve a given torque on the load. The tool used was the above-mentioned CP733 together with a standard oil-pressure chamber torque calibration unit. This unit comprises a nut and bolt which are tightened up on an oil-filled chamber the pressure in which is measured as representing the applied torque. The bolt head is drive by the tool with the aid of an adapter as shown in  FIG. 1 . The CP733 tool was supplied with 6 Bar of air-pressure and was operated in its highest tool-force setting ( 4 ) in the forward mode. What has been investigated is the build up of torque during successive impacts in the tool and how the insight gained leads to practical measures that can be used in a predictive fashion to control operation of the tool.  
      In using the tool and in some of the graphs referred to below, account needs to be taken of the effect of the mechanical adapter ( 20  in  FIG. 1 ).  FIG. 10  shows superimposed curves representing a series of torque impulses such as  120 . Each pulse is two superimposed pulses, the one  122  on the right relating to driving a load without an adapter: the other 124 on the left is driving a bolt head through an adapter, the tool being secured in a jig to eliminate hand-held variation. Nonetheless it is seen that whereas the pulses on the right are of essentially constant peak positive amplitude, those on the left show a significant range of positive peak amplitude and tend to vary in a cyclic manner. These variations may be due to the lose mechanical fittings between the output shaft of the tool, the adapter and the bolt head; and varying recoil forces from impact to impact.  
      Reverting to the explanation of the nature of torque impulses with reference to  FIG. 6  and the investigative support for the explanation given with reference to FIGS.  7  to  9 , the results of a whole series of sequential impacts is seen in  FIGS. 11   a ,  11   b  and  11   c  which show the same data presented in three-dimensional graph from but from different perspectives. These figures relate to data from the magnetic torque transducer in the tool.  FIG. 11   a  shows the results of a sequence of torque impulses S 1  to S 37  in a Z-direction out of the plane of the paper. Thus the last event is at the foreground. Each pulse extends in time along the X-axis but to the left with a zero time point at the right. The Y-axis shows each torque pulse as measured as a voltage (V) from the magnetic transducer output.  FIG. 11   b  shows  FIG. 11   a  “looking from the rear” with the first event in the foreground and the time axis running to the right. Fib.  11   b  shows the early pulses have a primary impulse (with rebound) and the distinct secondary impulse (darker) indicative of the bolt head flying ahead as previously discussed. The peak amplitude of the primary pulse is restricted. However as the bolt head tightens the primary impulse rebound disappears. This is at about impact S 10  and essentially all the torque impulse energy is dissipated in turning the bolt head and any ancillary losses. As the bolt tightens further the peak positive amplitude of the primary pulse is increasing and the rebound portion of it reappears. At this stage the progress is better seen in  FIG. 11   a  which shows the peak achieving a maximum value as the bolt approaches the hard tight state. However, it will be seen from  FIG. 11   a  that at the very last impacts the peak amplitude drops which may be due to the head in fact turning a little more.  
      Referring now to the presentation of  FIG. 11   c , this shows that sequence of impulses S 1 - 37  looked at “end-on” and looking toward time zero. The events are on the X-axis, time is in the Z-axis with time zero being in the background and the Y-axis is again the signal amplitude. What can be seen is that the output signal is generally increasing through the sequence of inputs save for the sudden drop at the end already noted. Time t is in units of 40 μS.  
      The data presented in  FIGS. 11   a - 11   c  is used in a manner that will be described below particularly relying on the increase in the peak amplitude over the series of inputs. However, before describing this further attention is drawn to  FIG. 12  which is presented in the same manner as  FIG. 11  and shows a sequence of impulses which is at a substantially constant peak pulse amplitude. Nonetheless torque on the bolt-head is increasing during the sequence. If such a case is detected, operation of the tool is predicted or measured in a different manner as will also be described.  
      The procedures for deriving control signals or commands for the operation of the tool will now be described. They fall under the two heads earlier mentioned, namely “Signal Integration” and “Instantaneous Torque Calculation”.  
      It has been found that whether bolt tightening proceeds according to  FIGS. 11   a - c  or to  FIG. 12  depends on the condition of the bolt in the fixture ( FIG. 1 ). Where the bolt is well greased so that tightening proceeds smoothly, the  FIG. 11   a - c  characteristic is more likely to apply. Where the bolt is for example rusty and binds, the pulse characteristic of  FIG. 12  is more likely.  
      The Instantaneous Torque Calculation involves manipulating the data of the recorded torque impulses to best fit a curve of a form described below with reference to  FIG. 13 . However, this curve fitting technique may not apply in cases, such as that of  FIG. 12 , in which the positive peak of each successive torque impulse remains essentially constant. The Signal Integration procedure can be used for such a case and will be explained first before going on to the Instantaneous Torque Calculation procedure.  
      1) Signal Integration  
      First of all it will be recalled from  FIG. 6  that it is postulated that a impact pulse may comprise a primary pulse and a secondary pulse as shown in  FIG. 6A  and that as the bolt tightens, the secondary pulse advances in time with respect to the primary pulse until they merge. Once the bolt tightens significantly (no longer flies ahead), a torque pushing mode is entered as illustrated in  FIG. 5 . The period of the positive portion of the primary pulse remains much the same.  FIG. 12  illustrates circumstances in which the pulse positive peak value is near constant.  
       FIG. 13  shows a graph in which curve  130  shows an integration or summation of the positive peak torque pulse value (ordinate). It proceeds in a step-wise fashion per impact. As will appear below with reference to  FIG. 15  the impact rate remains relatively constant over a train of impulses at  17 - 20  impacts/second for the tool investigated so the step-wise curve  130  may be expressed in terms of time as is the case in  FIG. 13 . The integrated value on the ordinate axis is calibrated to relate to torque values so that, the tool can be controlled for a pre-determined number of impacts or for a pre-determined time for a desired torque to be achieved. In processing the positive pulse portions it is desirable to set a threshold which the pulse must exceed to be recognised.  
      Another possibility is to integrate each pulse over its positive portion as is done in the Instantaneous Torque Calculation procedure described below. This is effectively the area under the positive pulse curve—see  FIG. 16 . The individual pulse integrals or areas are then themselves integrated summed.  
      It has been found that the rate of rise (slope) of curve  130  in  FIG. 13  is dependent on the air pressure in the tool. The higher the pressure the greater the rate of rise but the curve remains generally semi-logarithmic as shown.  
      By way of comparison  FIG. 13  also shows the signal integration applied to the absolute value of the negative (rebound) portions of the impulses. This is curve  132 . It still rises with the increasing number of impacts but the curve generated is not as regular as using the positive pulse portions. It is found generally that the rebound pulse portions tend to be more erratic from impact-to-impact. The time values on the abscissa are in increments of 40 μs.  
      2) Instantaneous Torque Calculation  
      The starting point for the procedure to be described is the curve of  FIG. 14 .  FIG. 14  shows a typical curve  140  of the torque exerted as a function of the number of impacts. It is seen that the torque rises in a non-linear fashion, rising relatively rapidly for an initial number of impacts but the torque increase per impact is showing all the time. Eventually the curve would become asymptotic towards a maximum torque value. Practicability requires that the tool should be operated within this maximum torque rating such that a desired torque is reached reasonably quickly. The shape of the curve of  FIG. 14  is generally applicable as a model or template. It can be stored as an algorithm defining a semi-logarithmic relationship of torque to the number of impacts. What will be described below is how actual torque pulse measurements can be fitted with the curve to provide a prediction or measure of the number of impacts or the time required to obtain a desired torque.  
      For comparison,  FIG. 14  also shows a second curve  142  which is of the same general shape as curve  140  but arising from a poor air source. The curve  140  pertains to a high, stable air-pressure (e.g. well buffered) at a steady 6 Bar, while curve  142  pertains to a lower, unstable air-pressure (e.g. poorly buffered) at 5 Bar. The maximum torque attainable on curve  142  is less than that on curve  140 . The impacts (events) were at a rate of 17.20 per second.  
      The torque expressed in  FIG. 14  was measured via a use of an oil-filled chamber as mentioned above. For the present purposes the curve  140  can be regarded as at model or template defining the build-up of torque with the numbers of impacts (uninterrupted sequence), though it is subject to scaling.  
       FIG. 15  is a graph of the impact rate, that is the impact interval through the sequence of impacts plotted as curve  144 . As can be seen the interval generally increases over the sequence but not greatly. This is referred to below.  
      What has been found to be a very important parameter relates to the shape and duration of the individual pulses as seen in  FIG. 16 . The figure diagrammatically shows a sequence of three pulses  150 ,  151 ,  152 , diagrammatically exemplified as being of the same shape and having equal characteristic parameters. Each pulse has the form discussed previously (see  FIG. 6 ) with an initial positive portion  154  followed by a rebound portion  156  of opposite polarity. The interval between impacts (as used in  FIG. 15 ) is indicated as t e , and the duration of the positive portion of a pulse as t p . The total pulse duration is denoted t t . For each pulse further characteristics can be derived from the variation of pulse amplitude with time. These are 
          PA p : the area of the positive pulse as indicated for pulses  151  and  152  and which in an integral of the pulse curve with time.     PA n : the area of the negative or rebound portion of the curve as indicated for pulse  152 .        

      What has been found to be of particular interest is a factor which is obtained by multiplying each positive pulse area by its duration, namely PA p ×t p . It is to be noted that, in cases where a distinct secondary pulse occurs ( FIG. 6A ), this multiplication applies to the primary pulse. The secondary pulse can be distinguished in processing a train having both by a gating process based on the a priori knowledge that the interval t e  from one primary pulse to the next is much longer than that between a primary pulse and associated secondary pulse.  
      Investigations have shown that it is advantageous to rely on the positive torque pulse portions only. The inclusion of negative (rebound) pulse portions does not lead to a clear correlation with the template curve  140  of  FIG. 14 . As already noted, the rebound pulses tend to be far more variable than the positive torque pulses. Data obtained for a sequence of impulses will now be given by way of the graphs of  FIGS. 17-19 . It will be apparent in all the curves shown (which are in fact event-by-event plots) that there is considerable pulse-to-pulse variation. The impulses are generated in a sequence generally of the form of  FIGS. 11   a - 11   c.    
       FIG. 17  shows curves  160  and  162  both plotted as a function of the impact number of a train of impacts increasing to the right. Curve  160  shows the positive pulse width t p  per impact (left-hand ordinate axis). Curve  162  shows the positive pulse area per impact (arbitrary scale on right-hand ordinate axis). The pulse width increases relatively rapidly at first (this may be associated with a distinct secondary pulse). The pulse width then increases at a lesser rate where the pulse forms and durations are nearer to being seen as in  FIG. 5 .  
      On the other hand the positive pulse area PA p  per impact increases little initially and then far more rapidly, though its pulse-to-pulse variations are greater than those of the pulse width and become out-of-phase with them as is clearly seen on the right of the graph.  
      It is to be noted in  FIGS. 17-19  that in comparison with the control exercisable in the pendulum laboratory apparatus, the measurements now presented are in a rapidly rotating machine. One factor in the machine is that there is not only an impulse delivered by a striking hammer but there is a reaction on the hammer introducing a bounce into its travel and timing.  
       FIG. 18  again shows curve  162  this time plotted in conjunction with a curve  164  which is the interval between successive pulses (t e  in  FIG. 16 ). Of more interest and considerable importance are the curves plotted in  FIG. 19 . Curve  170  (heavier line) is a combination of curves  162  ( FIGS. 17 and 18 ) and  160  ( FIG. 17 ), namely a value given by pulse area multiplied by time, the value being expressed in units shown on the left-hand ordinate axis. Also plotted for comparison is curve  172  of the positive pulse peak signal amplitude (i.e. no integration of the pulse)—see right-hand ordinate scale. No correlation can be seen between curve  172  and the curve  140 . However, the shape of curve  170  does show such a correlation as is seen in  FIG. 20  in which curve  170  is replotted (squares) against the template curve  140  (triangles) of  FIG. 14 . The correlation between the two is evident. The plot of curve  170  has been subject to some filtering during signal processing but curve  140  is not derived by a best-fit procedure. It has been found that the use of the real-time calculated points of curve  170  are sufficient to control a torque-impact tool within the limits required in normal industrial use.  
      The techniques and procedures described above for processing the impulse torque signals can be implemented in computer programs. Curve fitting procedures and algorithms for defining curves are well-known. The curve such as  140  for a given tool operating under specified conditions can be generated from a general algorithm defining the curve adjusted to specific parameters of the tool in question. Whatever procedures are used, the program(s) can be stored in firmware and performed by a microprocessor or microcontoller with appropriate memory capacity. The facilities provided can also include the ability to learn and store the control data required for a particular task. Thus it is contemplated that all the electronics be mounted with the tool as indicated at  36  in  FIG. 1 . The electronic circuit will then issue the required commands to control operation of the motor  14 .  
      The foregoing specific description has been given in relation to impact torque tools in which the successive impacts give rise to torque pulses or impulses as has been described. The magnetic transducer technology can also be applied to another type of pulse torque tool which does not rely on impacts to generate pulses but includes means for generating controlled pulses in a train. The Signal Integration procedure can be applied to such pulses and the Instantaneous Torque Calculation adapted to such pulses. One such other type of pulse torque tool uses a piston and cylinder mechanism which is continuously coupled to the output shaft. Pressure pulses are generated in the piston and cylinder mechanism and are transmitted to the shaft.  
      The foregoing description has discussed the effect of the nature of the load on the pulse generation. Another factor which is also of relevance is the weight (mass) of the output shaft of the tool and the adapter connected to it. Investigation has been made of the loss of torque in transmission of torque pulses along a shaft and this is further discussed below under the heading “Torque Loss Measurement”. A torque pulse applied to the input end of the shaft has the affect of winding (angularly rotating) the input end which winding has to be transmitted along the shaft if torque is to be achieved at the far, load end. The subsequent description discusses torque loss along the shaft and the effect of the form of the torque pulses on the efficiency of transmission. The mass of the shaft and adapter has been found to be a factor possibly due to the local inertia of the shaft and adapter which the propagating torque pulse has to overcome.  
      There has been described above how the cumulative effect of a torque pulse, and particularly the pulse area×pulse time product, can be used to determine when a predetermined torque is reached at the load under what has been called the Instantaneous Torque Calculation procedure and with particular reference to  FIG. 20 . Alternatively when a pulse train becomes a series of near constant amplitude pulse a Signal Integration procedure can be employed as particularly described with reference to  FIG. 13  and  FIG. 15 .  
       FIG. 21  shows a flow diagram for a procedure for deciding which of the two signal processing applications is to be applied and the manner of so doing.  FIGS. 22   a  and  22   b  exemplifies show the decision procedure is performed.  
      Referring to  FIG. 21  it illustrates the decision making process  200  applied to a train of pulses detected by the transducer  30  of  FIG. 1 . The train of pulses is shown in  FIG. 22   a  which shows a representative sample of pulses. As will be clear from the pulse trains given earlier, the actual number of pulses is large in achieving a desired torque.  
      Each fresh pulse acquired at step  202  has its amplitude entered in a memory store or register at step  204 . The pulse amplitude is then compared at step  206  with the preceding pulse amplitude held in a comparator register  218  to decide whether it is part of a rising curve of pulse amplitude or is to be considered a part of a curve of substantially constant amplitude. Because of pulse-to-pulse variations the decision is not necessarily made on the basis of just two next following pulses but by assessing the amplitude of the newly acquired pulse relative to an amplitude value derived from more than one immediately preceding pulse to judge the trend in the pulse amplitude curve.  
      If the decision at step  204  is that the new pulse is of greater amplitude according to a predetermined criterion, it is processed according to the above Instantaneous Torque Calculation procedure at step  208  and the resultant torque value is stored at step  210 . On the other hand if the decision at step  204  is that the new pulse is not of greater amplitude, that is the pulse is one of a series of essentially constant amplitude pulses, it is processed according to the above signal Integration procedure at step  212  adding another increment to the output torque value stored at step  210 . It may be that step  206  only provides a decision or a change of decision after a given number of pulses in which action under steps  208  and  212  is then applied to a number of pulses preceding and including the new one using the values stored at step  204 .  
      The process shown in  FIG. 21  allows processing of a pulse train according to each of step  208  and step  212  at different stages in the pulse train.  FIG. 22   a  shows a series of pulses which up to pulse N are subjected to Instantaneous Torque Calculation as shown by the semi-logarithmic (exponential) form of the initial portion  220  of the curve of  FIG. 22   b  which represents the value stored at step  208 . Thereafter, the decision is to proceed by Signal Integration leading to the substantially linear portion  222  of the curve of  FIG. 22 .  
      Reverting to  FIG. 21 , the torque value stored at step  210  is compared at step  214  with a predetermined or pre-set torque Ts. If the pre-set torque has been reached a command  216  is issued for stop the power torque tool or at least the transmission of generated pulses to the load. If the torque is less than the desired pre-set value the torque pulsing of the load continues, with the comparison register set to the value stored in register  204  or a value derived from it and a number of preceding pulses.  
      The description thus far has assumed the power torque tool is of the impact type. However, where the context clearly refers to impact torque impulses, the description of pulse processing procedures given above, including with reference to  FIGS. 21-22   b , applies also to the pressure type of impulses referred to earlier.  
      The teachings of the invention as regards torque loss measurement are applicable to torque pulses, however generated and however measured. The description given below will be in the context of impact or pressure pulses generated in a power torque tool and measured by use of the magnetic based technology described above.  
      It will be recalled that  FIG. 1  diagrammatically shows a power torque tool in which a single magnetic-based torque transducer is employed. The torque tool  10 —also referred to as a torque wrench—is illustrated in  FIG. 1  as a hand-held implement having a housing  12  within which is an electrically or pneumatically powered motor  14 . Pneumatic power is more usual. The motor is coupled by converter  16  to an output shaft  18  the distal end of which carries an adapter  20  engageable with the load to which torque is to be applied. In this example, the load is a bolt  22  which carries a nut  24  and which extends through an apertured fixture  26 . As shown the nut and bolt are being tightened on to the fixture  26 . The adapter  20  engages the head  28  of the bolt, being formed with an internal recess that matches the head  28 , e.g. an hexagonal head. The features of the tool  10  so far described are conventional and well-known to those in the art. By means of the converter  16  the rotation of the motor  14  is converted to a train of torque pulses in the shaft  18  and those pulses are transmitted to the bolt head  28 . The converter may be an impact type of mechanism generating a train of impact pulses or a pressure type of mechanism generating a train of pressure pulses as has been outlined above.  
      As has been described, a new feature of the tool of  FIG. 1  is the employment of a magnetic transducer  30  by which the torque impulses in the shaft are detected and measured. The transducer comprises a torque-sensitive element  32  which is an integral region of shaft  18  which is assumed to be of ferromagnetic material. The region  32  is magnetised to have remnant or stored magnetisation so that it acts as a source of external magnetic field, the magnetisation being effected in such a way that the region  32  emanates a magnetic field or field component which is dependent on the torque. The forms of magnetisation that may be used are set out above. As previously indicated the present invention has been developed using a profile shift magnetisation kind of transducer element. The emanated torque-dependent magnetic field is detected by a non-contacting sensor arrangement  34  which is connected to a detector and control circuit  36  which in turn controls the operation of the motor  14 . The sensor arrangement may comprise more than one sensor device, preferably saturating core device(s) connected in a circuit such as disclosed in WO98/52063. Sources of further information on sensor devices are given above.  
      It has been found that using an impact torque tool as an example of the tool illustrated in  FIG. 1  that the impulses measured with the aid of transducer  30  are irregular in both pulse spacing and in amplitude which may be due to reaction or bounce of the hammer with respect to the anvil sometimes leading to a double impact. It is difficult to predict the moment at which a desired torque is achieved at bolt-head  28 .  FIG. 23   a  diagrammatically illustrates the sharp spiky nature of impact pulses and their irregularity in time and amplitude.  
      If the tool illustrated in  FIG. 1  is of the pressure mechanism type producing pressure pulses, it has been found using the transducer  30  that the train of pulses generated is more regular and the individual impulses are of longer duration than impact pulses (it is noted here that in the art impact power tools have been simply referred to as such, while what is referred to herein as pressure power tools have often been referred to as impulse torque tools). For comparison  FIG. 23   b  diagrammatically illustrates the generation of the smoother more regular pressure pulses.  
      Investigation has now shown that in transmission along the output shaft  18  of a power torque tool, the energy of impact pulses is absorbed and dissipated far more rapidly than is that of pressure pulses. The mechanism of torque transmission along a shaft to a load whose characteristics vary as tightening proceeds is not easy to define and analyse.  
      The following is put as a consideration of factors involving the transmission of a torque pulse applied at the input end of the output shaft  18  to the far, load end of the shaft.  
      Consider first a continuous torque being applied—which may be thought of as analogous to D.C. energisation of an electrical transmission system. The shaft is wound about its axis by applied torque so that the shaft itself both absorbs energy and stores it in the resilience or elasticity of its material. This winding action is propagated along the shaft and with the continued torque applied at the input end, torque is eventually delivered at the load end. The loss along the shaft is a linear function of distance along the shaft.  
      Turning now to pulsed torque, to use the electrical analogy, this may be considered a case of A.C. pulse propagation, though substantially a unipolar A.C. case. A pulse of torque applied to the input end of shaft  18  but as the shaft winds under the applied torque, the torque ceases in the pulse interval so that there is no continuing torque to ensure further winding propagating along the shaft. Stored energy may cause relaxation of the shaft. The investigations made to date indicate that the short pulses of  FIG. 23   a  are less likely to cause an effective torque pulse to propagate along the shaft due to losses and elastic rebound. Whatever, the reason the short impact pulses tend to dissipate relatively rapidly.  
      In contrast the longer duration pulses of  FIG. 23   b  have been found to be more effective in propagation along the shaft and developing torque at the load end. It has been found that the area under the pulse is important to the torque developed at the load. It is surmised that a higher mark/space ratio is advantageous—that is pulse duration/pulse spacing.  
      The pulses illustrated in  FIGS. 23   a  and  23   b  are much simplified. It is known that the impact pulses of  FIG. 23   a  are more complex in reality and that their shape will change with the state of tightness of the load. Also of potential relevance to this is to what extent the impact pulse generator “sees” the load which becomes more remote, the longer the transmission shaft.  
      Another factor that has been found to be relevant is the weight or mass of the shaft which is to transmit the shaft together with the mass of the adapter coupled to the end of it. Current investigation suggests the lower the mass, the greater the efficiency of torque propagation. A mass-related parameter that may relate to the finding is that the progressive winding of the shaft, and eventually the shaft plus adapter at the far end, also entails overcoming the local inertia of the shaft.  
      Whatever the underlying theory of the transmission of torque pulses along a shaft, there remains a general need to be able to investigate the pulses transmitted and to be able to obtain some measure of the losses entailed in transmission.  
       FIG. 24  illustrates an embodiment of the transducer arrangement of the present invention. It shows a torque pulse converter  16  coupled to an output shaft  18  terminating in an adapter  20  engaging a bolt head  28  as in  FIG. 1 . In  FIG. 24  two transducers  30   a  and  30   b  are utilised each of the same kind as transducer  30  in  FIG. 1 . In  FIG. 24  the signals from respective sensor arrangements  34   a  and  34   b  are processed by signal processing unit  38  which may be realised in hardware and/or software. The respective transducer regions  32   a  and  34   a  are spaced apart along the shaft by a distance s. If the torque as measured at sensor arrangement  34   a  is Ta and that as measured at sensor arrangement ( 34   b ) is Tb, any torque loss T L  in transmission of a torque pulse along the shaft between sensors  34   a  and  34   b  is given by: 
   T   L   =Ta−Tb    
 and the rate of loss R L  expressed as loss or dissipation per unit distance along the shaft is 
   R   L =( Ta−Tb )/ s.    
      It is assumed that over the spacing s, the rate of loss R L  can be taken as a constant per unit length. In the first example given below the loss or dissipation per unit length is taken to be constant along the length of the shaft. If this does not apply s should be a sufficiently short increment of distance that the value of R L  can be used in calculating the torque loss over the length of the shaft to the load. In a test power tool according to the embodiment of  FIG. 24 , the spacing s was 15 mm.  
      The rate of loss R L  expressed as loss or dissipation per unit distance along the shaft is 
 
 R   L =( Ta−Tb )/ s  
 
      If the dissipation is constant then at a distance l to the load from sensor arrangement  34   a  total loss T T  is given by 
 
 T   T   =l .( Ta−Tb )/ s  
 
 and the torque delivered, Tr, from the shaft is given by 
 
 Tr=Ta−l ( Ta−Tb )/ s.  
 
      This expression is likely to be less true the shorter the pulses become, as with impact pulses. It will be understood that the same arrangement of two spaced transducers can be employed even if the dissipation is not a constant absolute value. For example if the dissipation is akin to an attenuation expressed as a fractional or percentage loss per unit distance, the dissipation of loss factor, D, can be determined as 
 
 D =( Ta−Tb )/( Ta.s ). 
 
      In this case the decline in torque delivered is exponential with distance and the torque delivered can be expressed as 
 
 Tr=Ta.e   −lD . 
 
      This expression of the torque loss is a form familiar from the transmission of A.C. electrical signals to return to the electrical analogy given above. It may be that the expression to be used is somewhere between the A.C. and D.C. cases. It may thus be important to know the form of torque pulses being transmitted at any moment. A magnetic torque transducer of the kind referred to herein enables the pulse train and its waveform to be analysed. Such facilities and functions can be provided in unit  38 .  
      By the adoption of a predictive technique of when a required torque is reached at the load based on a measure of torque at a point preceding the load such as torque Ta, the unit  38  can be employed to deliver control signals to the motor  14  ( FIG. 1 ) which are better related to actual load conditions.  
      It will be also understood that the predictive technique applied downstream of the torque measure to the load end of the shaft can also be applied to the actual torque delivered to the converter end of the shaft.  
      It will be understood that to determine torque loss and to make predictive calculations from it the measurement of torque at two spaced points along the shaft can be done by transducers other than those specifically referred to above, both magnetic and otherwise. The concept of measuring torque loss along a shaft, particularly for pulsed torque, by torque measurement at two spaced points is considered novel. However, as already mentioned magnetic-based transducers can provide signals which convey a waveform representing the instantaneous value of the torque and which can be analysed for pulse period, mark/space ratio and area under the pulse.  
      Although the measurement of torque loss has been described in relation to its application to power torque tools, it is considered that the teachings herein are of wider utility in measuring torque transmission by a shaft, particularly where the applied torque is of a pulsed nature and/or the load as such as to require increasing torque to drive the load.