Torque is transmitted by a specially alloyed dry ferromagnetic powder, it apparent viscosity may be changed by modulation of the field coil current.
The couplings can sustain continuous slip (under maximum heat ratings) at a precise and stable torque value, which is determined by excitation level. Slip between input and output members is not necessary in order to transmit torque, and provided load torque does not exceed the torque for which a unit is excited to transmit, locked-in synchronous operation will take place.
Conversely, if load torque exceeds the energised torque level, slip will occur in an absolutely smooth manner at the predetermined torque value. For all practical purposes, static and dynamic coefficients of friction are sensibly equal; also, output torque is independent of speed, or slip speed.
Powder performance is unaffected by temperature rise on the working sur- faces, and couplings will, have at all times, a torque directly proportional to current. It should be noted that use of a dry powder rather than a fluid suspended powder medium promotes consistency and accuracy of torque control.
Briefly, the coupling comprises two concentric members, the body or input member containing the field coil, and within it, and separated by a small annular gap, an inner rotor, or output member. The annular gap contains a ferromagnetic powder, which is activated when the coil is energised.
The resultant flux which is generated, passed through the powder, causing it to align with the flux path, and thereby creates a driving bond between input and output members, the strength of which is only determined by the value of DC current applied to the coil.
The torque transmitted by powder couplings is proportional to field current, and is infinitely variable from maximum design rating down to practically zero in all models.
Torque/current characteristic curves can vary by 5% dependent on whether current is increasing or decreasing. This is due to magnetic hysteresis. For all practical purposes torque is independent of speed, either with or without the presence of slip, and can be maintained within an accuracy of 5 % for speeds within the recommended operating range 50 to 3000 RPM.
De-energised drag torque due to residual iron circuit magnetism, bearing and seal friction, is less than 1% of design torque rating for any coupling.
Response time to torque is determined by field coil inductance to resistance ratio plus a magnetic delay due to eddy current loss.
Note: to ensure satisfactory operation, all couplings and brakes must be mounted in horizontal use.