A high-reach, triple-mast reach truck was experiencing a violent and dangerous symptom. When the operator raised a heavy pallet to the top rack, the lift chain on the right side would jump off the drive sheave roller at the top of the mast, dropping the load a few inches before the chain jammed. The shop had replaced the chain, assuming it was stretched, and replaced the roller, assuming it was worn. The chain kept jumping.
We examined the mast while the forks were resting on the ground. The chain tension was correct, and the sheave spun freely. We looked closely at the bottom anchor point where the chain attaches to the inner carriage. The chain is held by a heavy steel yoke that pivots on a thick anchor pin.
We put a wrench on the anchor pin bolt and found it was loose. We pulled the pin out and found the real problem. The steel anchor pin had been spinning in its bore. Over thousands of hours, the micro-movements of the carriage under load had caused the pin to wear the softer steel of the carriage mount, elongating the perfectly round pin hole into an oval shape.
When the mast was empty, the chain tracked fine. But under a 4,000-pound load, the anchor pin shifted backward in the elongated hole. This shift effectively lengthened the chain on that side, destroying the tension synchronization between the left and right chains. The right side became loose enough to skip a tooth on the top sheave under the shock load of a sudden stop. We welded the elongated hole shut, re-drilled it to factory spec, and installed a new pin with Loctite. The chain never jumped again.
