The hollow shaft of rotational speed measuring devices in the A4 design (such as hollow-shaft encoders) is pushed directly onto the drive shaft and frictionally locked to it. The reactive torque of the housing (breakaway torque of the ball bearings, friction of bearings and seals, mass inertia) is then taken up by a torque arm. The swivel joints of torque arms in size 6 are conform to DIN ISO 12240-4, size series K, and are implemented as ball joints. This makes it possible for them to compensate the mounting tolerances of the drive shaft in an axial direction (e. g. as a result of thermal expansion of the shaft) and radial direction (e. g. as a result of runouts). Torque arms are available for a wide variety of applications, in standard, insulated and stainless steel versions. Definite length on request.