I recently repaired an SP20 at the University of West Georgia on which the carriage had become stuck during the return travel. The photos above shows that top carriage bearings on the operator’s side had been removed during a previous repair attempt.
Typically what happens on SP series presses is that the trip spring inside the carriage side plate breaks causing the trip arm that shifts the carriage eccentric between print and trip to move freely.
A broken trip spring by itself doesn’t jam the carriage. In this case the retaining clip that hold the carriage eccentric arms linked to the trip arm stud had popped out. This caused the trip arm to flip over backward and hang down behind the print/trip pin that projects from the side of the bed. When this happens it impossible to repair the trip arm assembly without removing the side plate.
To remove the side plate, I had to also remove the tie rods, and the oscillating roller assembly. (The bottom carriage bearings were removed only for cleaning. Upon removing the side plate, I found that the brass Cylinder Eccentric (X-19423) remained attached to the crank shaft. A UGW Facilities worker assisting me, used a bearing extractor to remove it.
I then reassembled the mechanisms and new trip spring onto the side plate, I also reinstalled the carriage bearings, then remounted the side plate.
In consultation with Fritz beforehand, he suggested that the actual breaking of the trip spring was caused by another previous repair: the connecting bar between the trip/print operating handle and the trip/print pin that moves in and out of the side of the bed has a slot for a screw to hold it close against the bed press. The original low profile screw had been replaced with a hex head bolt, shown below, that having worked loose, made contact with the rear bottom carriage bearing. The operator forced the carriage past the bolt and this may have caused the trip spring to break.
After sharing my experience with Fritz, he sent me this Vandercook engineering change document for the SP series roller latch spring intended to eliminate disassembly of the carriage:
Fritz writes: “I interpret the change order as recognizing a design glitch where the loop of the end of the trip spring was almost a full circle and when the trip spring broke, they couldn’t get it off, thus a design change. We had new ones made for the -15 and -20 from the original blueprints and the spring company said “not a problem” though it sure looked like a problem to me.”