Why Did the Hydraulic Pump Housing Crack on a P&H 18 Crane

[MikePhua]

Quick answer
The pump housing likely cracked due to excessive pressure buildup caused by a missing or misconfigured relief valve, deadheading of a fixed-displacement pump, or incorrect hose routing that blocked flow. Even new pumps can fail catastrophically if system pressure exceeds design limits.
P&H 18 crane background and hydraulic configuration
The P&H 18 is a mid-sized hydraulic crane produced by P&H Harnischfeger, a company with deep roots in American lifting equipment dating back to the early 20th century. These cranes typically use fixed-displacement gear or vane pumps to power swing, steering, and outrigger functions. The hydraulic system includes:

• A fixed-displacement pump driven by the engine
  
• Directional control valves for swing and outrigger extension
  
• Pressure relief valves to protect components
  
• Return lines and tank circuits
  

Fixed-displacement pumps deliver constant flow regardless of system demand. If flow is blocked—known as “deadheading”—pressure spikes rapidly, often exceeding 3,000 PSI, which can crack housings or rupture seals.
Failure scenario and contributing factors
In this case, the operator installed a new swing and outrigger pump, started the crane, and successfully operated the boom swing. However, when extending the outriggers, the pump housing cracked. All hoses were reportedly connected correctly, and none had been replaced.
Possible causes include:

• Missing or misadjusted relief valve: Without a properly set relief valve between the pump and control valves, pressure has nowhere to go when flow is blocked.
  
• Deadheaded pump: If the control valve was closed or a port was plugged, the pump would build pressure until failure.
  
• Incorrect hose routing: A return line connected to a pressure port or a blocked outlet can trap flow.
  
• Overcompensation from prior leaks: If the old pump was leaking, someone may have increased relief pressure to compensate, leaving the new pump vulnerable.
  

One technician noted that extending outriggers typically requires low pressure, as the beam slides outward with minimal resistance. This suggests the failure was not due to load demand but to hydraulic misrouting or valve malfunction.
Understanding relief valve function and placement
A relief valve protects the pump by diverting excess pressure back to the tank. It must be placed between the pump outlet and the control valves. If installed downstream or omitted entirely, the pump is exposed to full system pressure.
Recommendations:

• Install a relief valve rated for the pump’s maximum pressure, typically 2,500–3,000 PSI
  
• Verify valve orientation and flow direction
  
• Check for plugged ports or incorrect fittings
  
• Inspect control valve spools for sticking or misalignment
  

One mechanic recalled a similar failure on a loader where a relief valve had been removed during a rebuild and never replaced. The new pump lasted less than five minutes.
Preventive measures and repair strategy
To prevent future damage:

• Always verify relief valve presence and setting before startup
  
• Use pressure gauges during initial testing
  
• Avoid deadheading fixed-displacement pumps
  
• Label hoses and ports during disassembly to prevent misrouting
  

If the pump housing is cracked, replacement is the only viable option. Welding cast aluminum or iron pump bodies is unreliable and often leads to secondary failures.
Conclusion
The cracked pump housing on the P&H 18 crane was likely caused by excessive pressure due to a missing relief valve or blocked hydraulic flow. Fixed-displacement pumps require careful system design to avoid deadheading. Proper valve placement, hose routing, and startup testing are essential to protect new components and ensure safe operation.