Mast Drifting on Forklifts: Causes, Diagnosis, and Solutions

[MikePhua]

Mast drifting refers to the unwanted, gradual downward movement of a forklift’s mast when it is supposed to hold position. This issue can lead to safety hazards, such as loads dropping unexpectedly, and operational inefficiencies due to the need for constant correction by the operator.
Common Causes of Mast Drift

• Worn or Damaged Lift Cylinder Seals: Hydraulic lift cylinders raise and lower the mast. Aging or damaged seals inside the cylinders cause hydraulic fluid to bypass the piston, resulting in loss of holding pressure and causing the mast to drift down.
  
• Faulty Relief Valve or Relief Cartridge: The relief valve maintains system pressure by allowing excess hydraulic fluid to bypass. If this valve is stuck open, clogged, or damaged, it causes pressure loss, leading to mast drift.
  
• Cracked or Damaged Main Control Valve Body: Cracks in the valve body can cause internal leaks. Despite cleaning and inspection of the spool and cartridge, a crack can impair the control valve’s ability to maintain pressure and hold the mast steady.
  
• Pilot Controls Issues: Pilot-operated control valves manage hydraulic fluid flow. Malfunction or leaks can cause poor pressure regulation, although normal pilot controls suggest the problem lies elsewhere.
  
• Hydraulic Fluid Contamination: Dirty or contaminated hydraulic fluid can damage seals and valves, reducing system integrity.
  

Diagnostic Process

• Pressure Testing: Apply pressure tests (up to 3000 psi or system operating pressure) on lift cylinders to detect leaks internally or externally.
  
• Valve Inspection: Remove, clean, and examine the relief cartridge and spool valve for wear or debris, and check the valve body for cracks or other damage.
  
• Visual Hydraulic Leak Check: Look for fluid leaks around hoses, fittings, cylinders, and main valve bodies.
  
• Operational Observation: Monitor the mast during lifting and holding periods to identify drift rates and irregularities.
  

Solutions and Recommendations

• Cylinder Rebuild or Replacement: If seals or piston rods are worn or damaged, rebuild cylinders with genuine seal kits or replace them.
  
• Valve Maintenance or Replacement: Clean or replace stuck or faulty relief cartridges and valves. If the valve body is cracked, replacement is necessary.
  
• Hydraulic Fluid Maintenance: Flush and replace hydraulic fluid regularly to prevent contamination damage.
  
• System Maintenance: Check and maintain pilot controls and keep the hydraulic system clean.
  
• Safety Practices: When parking or leaving the forklift for extended periods, always lower the mast and support the forks on solid ground or blocks to prevent accidental descent.
  

Terminology Explained

• Mast Drift: The unintentional downward motion of a forklift’s mast.
  
• Lift Cylinder: Hydraulic cylinder responsible for raising and lowering the mast.
  
• Relief Valve/Cartridge: Hydraulic component that maintains pressure by bypassing fluid when pressure gets too high.
  
• Control Valve Body: The housing for internal control valves managing hydraulic fluid flow.
  
• Pilot Control: Secondary hydraulic control system directing main valve operations.
  

Small Story
A forklift user in Louisiana struggled with persistent mast drifting on a Taylor T-360-L forklift. Despite cylinder rebuilds and pressure testing, the problem persisted until a small crack was discovered in the main control valve body. After valve body replacement, the mast held pressure perfectly, restoring safety and confidence in operation. This story reveals the importance of considering structural damage within hydraulic controls, not just common seal failures, when diagnosing mast drift.
Summary List

• Mast drifting typically caused by hydraulic leaks or valve failure.
  
• Both lift cylinders and relief valves should be inspected.
  
• Cracked valve bodies require replacement.
  
• Regular fluid checks and maintenance prevent mast drift.
  
• Proper safety precautions reduce accident risk caused by drift.
  

By recognizing the signs of mast drifting, performing thorough diagnostics, and applying timely repairs, forklift operators and technicians can ensure safe, reliable mast operation and avoid costly downtime.