Hydraulic Pump Screeching on a Custom Crane Truck

[MikePhua]

Hydraulic systems are the lifeblood of cranes, boom trucks, and lifting equipment. When a hydraulic pump begins to screech loudly, especially after replacement, it signals a serious issue that must be addressed before the machine can be operated safely. A custom‑built 13‑ton crane truck powered by dual Chevrolet V‑8 engines provides a unique case study in diagnosing pump noise, cavitation, and flow restriction. The machine’s rear engine drives a Haldex hydraulic pump that supplies pressure to the boom, winch, outriggers, and turntable. When the pump began screeching under load—and continued to do so even after replacement—it became clear that the problem lay deeper within the hydraulic system.
This article explores the likely causes of pump screeching, the behavior of cavitation, the risks of improper plumbing, and the diagnostic steps needed to restore proper hydraulic function.

Background of the Crane System
The crane in question is not a factory‑built unit but a hand‑constructed boom truck assembled from the ground up. Custom machines like this often combine components from multiple manufacturers, making troubleshooting more complex. The rear Chevrolet 350 V‑8 engine powers the hydraulic pump through a transaxle connection, while the front engine drives the truck itself.
Terminology notes:

• Cavitation: The formation and collapse of vapor bubbles inside a pump due to insufficient inlet pressure.
  
• Bypass relief: A safety valve that opens when system pressure exceeds a set limit.
  
• Suction line: The low‑pressure hose or pipe feeding oil from the reservoir to the pump.
  
• Foaming: Air entrainment in hydraulic oil, often caused by suction leaks.
  

Because the crane is custom‑built, the hydraulic layout may not follow standard OEM design principles, increasing the likelihood of plumbing‑related issues.

Symptoms of the Problem
The operator reported several key symptoms:

• Loud screeching from the hydraulic pump
  
• Screeching increases as the system warms up
  
• Noise becomes louder when operating the winch
  
• Hydraulic oil foams lightly in the reservoir
  
• Replacing the pump did not solve the issue
  
• Replacing the suction hose with steel pipe did not help
  
• Controls still function, but noise worsens under load
  

These symptoms strongly indicate cavitation or severe flow restriction on the suction side of the pump.

Why Cavitation Causes Screeching
Cavitation occurs when the pump is starved of oil. Instead of drawing a solid column of fluid, the pump ingests air bubbles or vapor pockets. When these bubbles collapse under pressure, they create a screeching or grinding noise.
Common causes include:

• A suction line leak
  
• A collapsed or internally damaged hose
  
• A clogged suction strainer
  
• A suction line that is too small
  
• A pump mounted above the reservoir
  
• A restriction caused by improper fittings
  

The presence of foamy oil in the tank is a classic sign of air being drawn into the system.

The Risk of Using Steel or Galvanized Pipe
One notable detail is the replacement of the suction hose with steel pipe. While steel pipe is sometimes used in industrial hydraulic systems, it must be seamless, properly sized, and free of galvanizing.
Potential issues include:

• Galvanized pipe contamination: Zinc reacts with hydraulic oil, shedding flakes that clog valves and pumps.
  
• Rigid pipe misalignment: Even slight misalignment can create micro‑gaps that pull air into the pump.
  
• Improper fittings: Tapered pipe threads can restrict flow or introduce air leaks.
  
• Insufficient diameter: Suction lines must be larger than pressure lines to prevent starvation.
  

Replacing a flexible suction hose with rigid pipe often introduces more problems than it solves.

Downstream Restrictions and Valve Issues
If the suction side is not the culprit, the next likely cause is a restriction downstream of the pump. A blocked line, stuck valve spool, or malfunctioning relief valve can force the pump into bypass, generating heat and noise.
Indicators of downstream restriction include:

• Screeching increases when operating specific functions
  
• Cylinders move slower than normal
  
• Pump noise changes depending on which control is used
  
• Relief valve chatter or vibration
  

In this case, the winch produces the loudest noise, suggesting that the winch circuit may have a blockage or stuck valve.

Foaming Oil as a Diagnostic Clue
Foamy hydraulic oil almost always indicates air ingestion. Air can enter the system through:

• Loose clamps
  
• Cracked suction hoses
  
• Poorly sealed pipe threads
  
• Worn pump shaft seals
  
• Improper reservoir venting
  

Air in the oil reduces lubrication, accelerates pump wear, and causes erratic control behavior.

Field Experiences and Operator Stories
Mechanics who have worked on custom cranes often recall similar issues. One technician described a boom truck that screeched loudly whenever the outriggers were deployed. The cause turned out to be a tiny pinhole in the suction hose hidden beneath a clamp. Another operator remembered a crane that foamed its oil every morning; the culprit was a galvanized pipe nipple that had shed zinc flakes into the suction line, clogging the pump inlet.
These stories highlight the importance of proper suction plumbing and contamination control.

Company Background: Haldex Hydraulic Pumps
Haldex, originally known for brake systems and industrial components, also produced hydraulic pumps widely used in mobile equipment. Their gear pumps are reliable when supplied with clean oil and proper inlet conditions. However, like all gear pumps, they are extremely sensitive to suction starvation and air ingestion.

Practical Recommendations

• Replace all suction‑side steel or galvanized pipe with a proper hydraulic suction hose.
  
• Inspect the suction line for pinholes, cracks, or loose fittings.
  
• Ensure the suction line diameter meets pump specifications.
  
• Check for an internal suction strainer inside the reservoir.
  
• Inspect the winch circuit for blockages or stuck valves.
  
• Test system pressure with gauges to identify abnormal relief activity.
  
• Verify that all valve spools return to center properly.
  
• Check for contamination in the reservoir and flush if necessary.
  

Conclusion
A screeching hydraulic pump on a custom crane truck is almost always caused by cavitation, suction leaks, or downstream restrictions. Replacing the pump does not solve the problem if the underlying cause remains. By focusing on the suction line, checking for air leaks, removing galvanized components, and inspecting the winch circuit, operators can restore proper hydraulic function and eliminate damaging pump noise. With careful troubleshooting and proper plumbing practices, even a hand‑built crane can operate smoothly and reliably.