A 5-ton pneumatic forklift exhibited terrifying "free-steer." The operator would turn the wheel 90 degrees, and the rear wheels wouldn't move. Then, suddenly, the wheels would snap to full lock. The shop replaced the hydraulic steering cylinders, but the ghost remained.
We disassembled the orbital steering valve (char-lynn motor). Inside, the metering spool and sleeve have a 5-micron (0.005mm) tolerance, lubricated exclusively by the hydraulic oil. An oil analysis revealed a NAS 12 contamination level (equivalent to ISO 4406 23/21/18). The oil contained over 250,000 particles per milliliter larger than 6 microns.
These hard particles, ingested past a breached 10-micron return filter, acted as a lapping compound. They wore the cross-center spool bore by 15 microns. When the operator turns the wheel, the spool must direct 8 L/min of pilot flow to the gerotor. Because of the 15-micron clearance, 6 L/min of that flow bypassed the spool directly to the tank (internal leakage). The gerotor didn't receive enough flow to overcome the 20 Bar pilot spring, causing the steering wheel to "slip" 90 degrees before pressure built up. The shock load then surged the pressure past the worn spool, snapping the wheels to lock. Replacing the $1,200 orbital valve was only half the fix; the 100-gallon hydraulic tank had to be flushed and filled with NAS 9 clean oil, or the new valve would wear out in 200 hours.
