Hydraulic Motors
Problem
Oil motor stalls at bottom (no
unusual heat problems)
Oil motor stalls at top (no
unusual heat problems)
Low pressure (okay on down
stroke, sluggish on up.stroke —
high heat)
NOTE: Engine labors on
upstroke, idles back
at stall on the down
stroke.
Low pressure (both strokes -
high heat)
NOTE: Engine labors at stall
on both strokes.
Cause
1. Fluid pump piston seat unthreaded
2. Valve sticking or oil motor trip rod
shifter assembly separated
1. Valve sticking
2. Broken spring retainer (valve rod
assembly)
3. Broken spring or valve rod
4. Air in hydraulic motor
5. Air in fluid pump
1. Blown piston seal
2. Cracked piston
1. Blown center o-rings on spool valve
2. Bad hydraulic pump
Solution
1. If connecting rod is okay, remove cylinder
head plug and pop valve down. Replace
plug and start machine. If machine cycles
up and stops at bottom again, then problem
is piston seat on fluid pump. Check piston
seat. Repair or replace as necessary. If
piston seat is okay and problem does not
change, check oil motor.
2. Remove valve and check for scratches and
rough movement when sliding it up and
down. Replace valve and spool in this
condition. Check trip rod for possible
separation.and spool in this condition.
Check trip rod for possible separation.
1. Remove valve and check for scratches and
rough movement when sliding it up and
down. Replace valve and spool in this
condition.
2. Replace valve rod assembly.
3. Replace valve rod assembly.
4. Reset valve. Purge Air, generally
accomplished by low pressure cycling of
motor/pump assembly for 5–10 minutes.
Check for causes of air introduction:
• Loose fittings in tank.
• Loose fittings on hydraulic pump.
• Loose hose connections.
• Low oil in reservoir.
5. Stall at top can occur randomly when fluid
pump picks up air. Reset valve. Avoid air in
the fluid pump.
1. Before dismantling oil motor, start machine.
With pump cycling under pressure, touch
the hydraulic cylinder and the head to see if
cylinder or head gets hotter. This will help
determine if piston seal is blown or piston
nut is broken. If heat is on the head, check
the o-rings on spool valve.
2. Dismantle oil motor and check piston seals
cylinder bore and piston nut. Pay special
attention to piston nut. It can be cracked
and not show externally.
1. Before dismantling oil motor, start machine.
With pump cycling under pressure, touch
the head to see if the head becomes hotter.
This will help determine if center o-ring is
blown on spool valve. If hot, remove and
replace o-ring.
2. Replace hydraulic pump.
Troubleshooting
12
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