Troubleshooting Challenge: Engine block foundry’s motor circuit not rotating properly

A sand core forming machine at a cast iron foundry was experiencing a problem with its mold rotation motor circuit. Personnel could rotate a mold full of sand clockwise, but when a cycle required rotating an empty mold back to the home position, it would not. This was because the line pressure could reach 2700 psi for clockwise rotation but only 300 psi for counter-clockwise.

Unfortunately, the workers chose to replace the pump with a new one because they couldn’t get pressure to exceed 300 psi in one direction. If the pump could develop 2700 psi rotating the mold in one direction, it certainly should be able to reach the same pressure when rotating the mold in the opposite direction.

Workers then replaced a pilot-operated proportional valve with a new one. It didn’t fix the pressure problem at the motor either. When they considered replacing the hydraulic motor, the foundry supervisor decided to call in outside help.

When I arrived, personnel told me about the lack of pressure when trying to rotate the mold back to the home position and that another problem developed after replacing the proportional valve: when they rotated the mold to the fill position, it completed its cycle, but at a slower rate. Also, pressure still would not exceed 300 psi when trying to return the mold. After inspecting the system and reviewing the circuit drawing, I determined they also had an overheating problem.

The hydraulic system had several other functions that ran off of another pressure-compensated, loadsensing pump. Two of the functions used pilot-operated directional valves that featured a separate pilot pressure pump circuit set at 250 psi. This also provided pilot pressure to the circuit they were troubleshooting. The circuit for the problem motor is shown, including the pilot pressure circuit.

Any ideas as to what the problem was?

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Solution to September’s processing line problem

When a hydraulic cylinder lunges, it could indicate air has entered it somehow. If cycling it several times resolves the lunging problem, then more than likely it was an air problem.

The check valve located in the return line at the filter was designed to prevent the weight of the oil from drawing a vacuum in the cylinder lines. One foot of oil weighs about 0.4 psi. If you multiply this by 60 ft of head, it would cause a pressure of 24 psi at the check. They used a check with a 50 psi opening spring. If this check valve was not present, the oil in the line would pull a vacuum of approximately 30 in. of mercury at the cylinder ports.

They found the check at the filter had a piece of O-ring holding the check open. This caused a high vacuum on both sides of the cylinder. Rod seals are “V” or “U” shaped with the open end facing the pressurized area. The cap end vacuum caused the cylinder to drift in while the rod side slowly pulled in air through the rod seal. Most rod seals collapse when a vacuum is pulled on the open side of the seal.

Removing the piece of O-ring solved the problem.

Mobile Equipment Training Challenges

Individuals that work on mobile equipment often require hydraulic and electrical knowledge beyond that necessary for maintaining equipment in fixed factory settings.

Industries such as energy, mining, construction, rail, waste management, agriculture, and oil and gas are loaded with mobile equipment. If you are faced with finding professional training for your fleet or unique mobile equipment, CFC Solar may be able to help. CFC-Solar is one of the most respected names in the mobile industrial training market with a history of developing training programs that teach individuals how to maintain today’s technically advanced mobile equipment. In addition to product specific training, CFC-Solar has standard training classes developed for the mobile industry that include:

  • Level 1 Mobile Hydraulics – In Depth Fundamentals
  • Level 2 Mobile Hydraulics – Advanced Maintenance
  • Level 3 Mobile Hydraulics – Design and Sizing
  • Troubleshooting Mobile Systems using Schematics
  • Hydrostatic Closed Loop Systems
  • Level 1 Mobile Electrical – Fundamentals
  • Level 2 Mobile Electrical – Multiplex Systems

CFC-Solar is one of the primary training firms that provide Fluid Power Society Mobile Hydraulic Certification. For more information, contact [email protected] or visit

Robert J. Sheaf Jr., is the founder of Certified Fluid Consultants (CFC) and President of CFC-Solar Inc. CFC-Solar provides technical training, consulting, and field services to any industry using fluid power technology. Visit for more information