DESCRIPTION 
     1. Field of Invention 
     The invention relates to torque transducing attachments for air driven impact tools used extensively in assembly operations to fasten nuts etc. 
     2. Background of Invention 
     In an article in Eureka 1983 a torque transducing system made by Adams Limited is described. This uses a pair of adjacent gratings through which varying amounts of light are transmitted depending on torque level using six light sources and six photo detectors. 
     In endeavouring to provide a torque transducing system for use on air driven impact tools used frequently by operators to tighten nuts, one faces severe problems not encountered where one basically merely checks that a nut has been properly fastened. There is considerable vibration. In addition there are extremely high acceleration and deceleration forces in the use of air-driven impact tools which necessitate that the torque occurs only briefly and then drops rapidly so that the measurement at the moment of impact has to be correct if the attachment is to operate dependably. 
     It is amongst the object of the invention to overcome these difficulties to provide reliable and accurate torque measurement for use by factory operatives in assembly operations. 
     SUMMARY OF INVENTION 
     In a first aspect, the invention utilises the momentary angular off-set of a pair of apertured integral bushes to vary light transmission through the apertures from a stabilised LED source. 
     The LED source is compact and rugged, has a low current consumption and yet provides enough radiation to provide a meaningful signal from the bush apertures if it is stabilised. The integral bush construction, preferably from a light material, permits the aperture arrangement to withstand the extremely high G-forces without material distortion. 
     In a second aspect of the invention there is provided a torque transducing system including a housing, bearings in the housing, a rotatable shaft rotatably supported by the bearings, said shaft having a torque receiving input end and an output end for supplying torque to a workpiece, a first annularly arranged light transmitting means mounted to be in substantially fixed angular relationship to the input end, a second annularly arranged light transmitting means mounted to be in substantially fixed relationship to the output end, respective angular movement of the first and second light-transmitting means leading to a variation in the total amount of light transmittable, a low power light source for illuminating the first and second light transmitting means at a peripheral position thereof through a fibre-optic bundle, first means for sensing the amount of light transmitted at that position, and a second means for sensing the amount of light emitted by the source through a first branch of the fibre optic bundle, and a control means responsive to the second light sensing means for maintaining the amount of light emitted from the low power source substantially constant. 
     Other features of the invention permitting bi-directional torque detection and the avoidance of dark current effects are set out in the claims. A fibre-optic arrangement permits the various features to be provided simply and compactly. 
    
    
     DRAWINGS 
     FIG. 1 shows a side view partly in section of an attachment according to the invention secured to an air-driven impact tool; 
     FIG. 2 shows a section through a measuring head of the attachment of FIG. 1; 
     FIG. 3 shows a view from below of the attachment of FIG. 2; 
     FIG. 4 shows enlarged a detail of the measuring head of FIG. 2; 
     FIG. 5 shows enlarged, other details of the measuring head of FIG. 2; 
     FIGS. 6 and 7 show partial sections through bushes of the attachment of FIG. 1; 
     FIGS. 8 and 9 shows plan views of the bushes of FIGS. 6 and 7; FIG. 9 shows the circumferential detail enlarged; 
     FIG. 10 shows the electrical and optical circuitry for the attachment of FIG. 1. 
    
    
     DESCRIPTION WITH REFERENCE TO DRAWINGS 
     An attachment 2 (FIG. 1) is secured by clamping collar 4 and screws 6 to air driven impact tool 8 having an output member 10. 
     The attachment has a housing 12 mounting a measuring head 14 and a pair of spaced bearings 16 supporting directly and indirectly through collar 18 a shaft 20. 
     The shaft 20 has a necked portion 22 capable of transforming under torque. Integral titanium bushes 24 and 26 are mounted by pins 28 adhesive etc non-rotatably and impact resistant on either side of the necked portion 22. A sleeve 31 is force-fitted and glued onto the bushes 24 and 26 to hold pins 28 captive. The bushes 24 and 26 (see FIGS. 6 and 7) have flanges 30 with outwardly radiating slots 29 (see FIGS. 8 and 9). The shaft 20 is adapted at its front end to engage a workpiece (not shown). The bush 24 has a journal portion 33 ensuring optimum mutual alignment of the bushes 24 and 26. 
     The measuring head 14 (FIGS. 2 and 3) has a body with recesses for mounting a printed circuit board 32 and associated fibre-optic components for straddling the flanges 30 at the measuring area 34. The fibre-optic components include an LED at 36; a feed-back loop to a photo sensor 40; an irradiating head 42 at the area 34; a light receiving head 44 at the area 34; and a photo-sensor 46. Fibre optic bundles interconnect the aforementioned components including bundle 48 between LED 36 and head 42; bundle 50 between head 44 and sensor 46; bundle 52 for the feed back loop between LED 36 and sensor 40; and by-pass bundle 54. 
     An end view of the bundles at the radiating heads 42 and 44 is shown in FIG. 4. Four rows of fibres are clamped at 60 between mounts 62 and 64. The rows extend arcuately conforming to the arc formed by the slots 29. 
     Optionally internally reflecting light guides of similar section may be interposed between the bush flanges and the heads 42 and 44. The necessary intermingling of light from the different fibres can also be achieved by spacing heads 42 and 44 approximately 5 mm without confining the light with guides, the irradiating head 42 being spaced by at least 1 mm from the bush flanges. It may be up to 10 mm away. 
     The LED 36 illuminates a fibre array as shown in FIG. 5 providing a large central area 66 for the fibres leading to the head 42, a small outer area 68 for the fibres of the feed back loop 38 and a small intermediate-area 70 for the fibres of the bundle 54. The fibres of bundle 54 join the fibres from the head 44 in a similar fashion at the photo-sensor 46. 
     FIG. 10 schematically illustrates light paths through optic fibres in dashed lines. The feed back control of LED 36 stabilises its output. The torque-dependant signal (kept out of the dark current area by by-pass 54) is supplied to an amplifier 72 which in turn controls an amplification circuit with a capacitive feed back network 74 with a capacitor designed to give a frequency response selected by trial for a given application to provide an output substantially corresponding to the torque &#34;retained&#34; by a nut secured by the rod independant of the torque applied. This means that the output will be proportional to the torque required to turn the nut itself, even though temporarily a much higher impact torque may have been applied by the tool to the nut. An appropriate capacitive feed back network can provide such a proportional output both where the nut tightens slowly in a soft joint or where it tightens quickly in a hard joint. 
     The system need only consume a small amount of current and is sufficiently strong to stand up to the battering occurring in an impact to a environment yet provides a prolonged period in which a dependable read-out can be obtained.
A torque transducing attachment (2) can be fastened to a air-driven impact tool (8) at its rear end. A shaft (20) in bearings (16) in a housing (12) has a torque transformable part (22) with integral bushes (24, 26) with apertures (29) on either side. The relative displacement under torque of the apertures (29) varies the amount of light transmitted from a stabilized LED source (36) to a photo detector (46) and the resulting signal is passed through frequency responsive network.