I'm too late, but a slack adjuster for a large truck S-cam air brake is a worm gear. The adjustment is done with a 9/16" hex head and rotates a hub with a splined bore. They are rugged and quite compact. Matching camshafts rarely wear the spines, instead they wear at the support bushings and sometimes the S-cam area. The camshafts get scrapped, so you can source a donor spline for cheap or free.
A friend once used one to rotate an engine overhaul stand. It worked very well.