An IC forklift would lift heavy loads very slowly and with a shudder, but lowering was smooth and normal. The shop replaced the lift cylinder piston seals, thinking internal bypass was causing the jitter. No change. They then replaced the lift spool in the control valve. Still no change.
We had to rethink the hydraulics. If the cylinder seals and spool are good, but lift is slow and lowering is normal, there must be a one?way restriction in the line feeding the cylinder. We disconnected the supply hose at the bottom of the lift cylinder and ran a jumper hose directly back to tank. The forklift lifted quickly and smoothly. The problem was that 6?foot high?pressure hose.
We took the hose to the bench and blew compressed air through it. Air barely hissed out. Then we skived off the outer cover. The inner tube of synthetic rubber had delaminated from the wire braid-inner delamination. When the control valve sent oil to the cylinder, pump pressure pushed the loose inner tube against the wire braid, acting like a flapper valve and choking the flow. When lowering, flow reversed, peeling the inner tube away from the braid, and the path opened up. We replaced that $50 hose, and the jitter was gone. Whenever a circuit shows a one?way flow restriction, suspect inner delamination of a hose before you tear the valve apart.
