A high-density e-commerce fulfillment center reported that a Raymond 4750 narrow aisle reach truck's fork carriage juddered violently when lifting a 1.2-ton pallet above 5 meters. At full mast extension, the right fork consistently sat 15 millimeters higher than the left fork, preventing accurate pallet placement into the racking. The lift function also produced a rhythmic "clunk" as the mast ascended.
The technician recognized the symptoms of asymmetric mast chain tension. The 4750's mast uses two heavy-duty leaf chains-one routed over each side of the mast's crosshead-connected to a "Chain Equalizer Yoke" that distributes lift cylinder force equally between the two chains. He measured the chain tension on each side using a chain tension gauge: the left chain measured 380 kilograms-force, while the right chain measured only 180 kilograms-force. The 200-kilogram differential confirmed asymmetric tension.
He inspected the "Chain Equalizer Tensioner Pin"-the pivot pin that allows the equalizer yoke to self-center between the two chains. The pin was worn oval and deeply grooved, preventing the equalizer from pivoting freely to self-adjust chain tension. As the mast extended, the right chain's slack accumulated, and the left chain took all the load-causing the carriage to tilt (the 15-millimeter fork height difference) and the right chain to periodically snap back into engagement, producing the clunk. He also measured chain stretch using a 12-link gauge: both chains had stretched by 3.5 percent of their original pitch (spec allows 2 percent), indicating the chains needed replacement regardless of the equalizer issue.
The repair involved replacing both mast lift chains with new leaf chains, machining a new equalizer tensioner pin, and installing new bronze bushings in the equalizer yoke. The chains were lubricated with mast chain oil and tensioned to the specified equal tension of 400 kilograms-force on both sides using the tensioner adjustment nuts. The mast rails were inspected for scoring (clean, no damage) and the mast rollers were replaced as a preventive measure since they had been operating under the asymmetric load. After reassembly, the fork carriage lifted smoothly to full height with no judder, both forks sat level within 2 millimeters of each other, and the rhythmic clunking disappeared.
