bar being held in a fishplate structure 40 at part 31 the fishplate structure 40 and thus part 31 and the tie bar 39 are angularly adjustable with respect to part 30 about the axis 32 by means of setscrew 41 by virtue of this arrangement the clearance s at the point of application of force 38 between the end 37 of the force detector 34 and the tie bar 39 can be adjusted to a value which corresponds to a desired dead pivot angle between parts 30 and 31 upon transition from the force free position of the puller head 14 into the force transmitting position thereof the necessary sensitivity of the clearance adjustment is warranted by the rather great distance x of the point of application of force 38 from the axis 32 the measuring body not shown furnished with wire strain gauges likewise not shown and positioned between the ends 37 and 33 of the force detector 34 in operation is subjected to bending stress by the tie bar 39 the extension of the wire strain gauges thus caused is converted into an electrical voltage which is proportional to the pull exerted by way of the tie bar 39 and applied by cable 42 to a control means.