080923-1855 EST
Besoeker:
In fact we want the large voltage spike because we want the clutch to dropout as fast as possible. A reversed bias diode across the coil would have a voltage of about 1.5 V and a very long dropout time, relatively speaking.
On one machine we changed to an air clutch to get a better dropout time, but this had pullin time problems. This particular application had bearings with a very unstable build characteristic, and dropout time was more important than the pullin time.
The nut and drag torque plots on my web site were for a front axle with aluminum carrier and Timken bearings. Generally I find the build characteristics of drag torque are better with Timken bearings than other brands, and after build the drag torque remains more stable. Assuming proper lubrication long term drag torque drops off.
The Potter and Brumfield KUP relay has a relatively fast dropout as compared to a machine tool relay with larger contacts, and being a plugin it is easily changed. It is not too expensive. What happens in switching DC is that metal transfers from one contact to the other. This creates a pit in one and a mating conical mound on the other. True there are ways to reduce this transfer, but that is a more complex circuit and less for the electrician to do.
The electronics had relatively few failures, so changing relays every 4 months or so was a useful activity for the electricians.
There are a lot of variations in collapsible spacers and bearings, and generally on most parts we could build within +/-2 #-in at a mean of 22 #-in. long term average, meaning 10 seconds. The specifications for many years have been like 15 to 30 #-in for the adjusted average value at the time of assembly. The drag torque variation at 30 RPM with an 0.3 second averaging time might have a peak to peak excursion of 3 #-in after adjustment of the part and a preceding several second runin of the bearings.
.
