Troubleshooting the Differential on a Simon Aerial Lift

[MikePhua]

The Simon Lift Legacy
Simon Aerials, originally a UK-based manufacturer, gained prominence in the 1970s and 1980s for producing reliable boom lifts and truck-mounted aerial platforms. Their equipment was widely used in utility work, building maintenance, and industrial access. Though the brand eventually merged into larger entities like Terex and ceased standalone production, many Simon lifts remain in service today, especially in municipal fleets and small rental yards.
One of the enduring models is the Simon S1932 and its variants, often mounted on Ford or GM chassis. These lifts feature hydraulic outriggers, telescoping booms, and a drivetrain integrated with the host vehicle’s differential system. When the lift is mounted on a truck chassis, the differential becomes a critical component—not just for mobility, but for hydraulic power delivery and stability.
Understanding the Differential’s Role
In automotive and industrial terms, a differential is a gear assembly that allows drive wheels to rotate at different speeds, especially during turns. In the context of a Simon lift mounted on a truck, the differential also serves as the final gear reduction point for torque delivery from the transmission to the rear axle.
Key components include:

• Ring gear and pinion
  
• Carrier bearings
  
• Spider gears (for open differentials)
  
• Locking mechanism (if equipped)
  
• Axle shafts and seals
  

When the lift is stationary and the truck is used only as a platform, the differential may seem irrelevant. But in reality, it supports the hydraulic pump (often PTO-driven), stabilizes the rear axle under load, and ensures proper weight distribution.
Common Differential Issues in Lift Applications
Operators of older Simon lifts often encounter differential problems due to age, wear, or improper maintenance. Typical symptoms include:

• Grinding or whining noises during movement
  
• Fluid leaks from the axle housing
  
• Difficulty engaging drive or reverse
  
• Vibration under load
  
• Uneven tire wear
  

In one case from a utility crew in Michigan, a Simon lift mounted on a Ford F-450 began making loud clunking noises during transit. Inspection revealed that the carrier bearings had failed, causing the ring gear to misalign. The repair involved replacing the entire differential assembly and re-shimming the gear set.
Identifying the Correct Differential Model
Simon lifts were mounted on various truck platforms, including Ford, Chevrolet, and Dodge. Identifying the correct differential requires:

• Locating the axle tag or stamping (usually on the differential cover or housing)
  
• Measuring the ring gear diameter
  
• Counting the gear teeth for ratio calculation
  
• Checking spline count on the axle shafts
  

For example, a Ford Dana 60 rear axle typically has a 10.5-inch ring gear and 35-spline axles. GM trucks may use Eaton or AAM axles with different configurations. Matching the correct parts is essential for rebuilds or replacements.
Repair and Replacement Strategies
When a differential fails, operators have several options:

• Rebuild the existing unit with new bearings, seals, and gear sets
  
• Swap in a remanufactured differential from a reputable supplier
  
• Upgrade to a limited-slip or locking differential for better traction
  

Rebuilding costs range from $800 to $1,500 depending on labor and parts. Remanufactured units may cost $1,200 to $2,000 but save time. Upgrades like Eaton Truetrac or Detroit Locker can improve performance in off-road or uneven terrain, especially if the lift is used in rural or construction zones.
Preventive Maintenance for Long-Term Reliability
To extend the life of the differential in a Simon lift application, follow these maintenance tips:

• Change gear oil every 30,000 to 50,000 miles or annually
  
• Use synthetic gear oil rated for high-load applications (e.g., 75W-140)
  
• Inspect seals and breather vents for leaks or blockages
  
• Check backlash and gear wear during routine service
  
• Monitor for unusual noises or vibrations during operation
  

A fleet manager in Alberta shared how regular oil sampling helped detect early metal wear in their lift trucks. By catching the issue before catastrophic failure, they avoided costly downtime and extended axle life by over 40%.
Hydraulic Integration and PTO Considerations
Many Simon lifts rely on a power take-off (PTO) system connected to the transmission or transfer case. The differential indirectly supports this setup by stabilizing the rear axle and absorbing torque fluctuations. If the PTO is misaligned or overloaded, it can transmit stress to the differential housing.
Operators should:

• Ensure PTO engagement is smooth and free of chatter
  
• Use hydraulic fluid with anti-foaming additives
  
• Inspect PTO shafts and u-joints regularly
  
• Avoid sudden directional changes while the lift is extended
  

In one incident in Georgia, a Simon lift’s PTO pump seized due to contaminated fluid. The resulting torque spike cracked the differential housing, requiring a full rear axle replacement.
Modernization and Retrofit Possibilities
While Simon lifts are no longer in production, many owners retrofit their units with modern components. This includes:

• Installing newer axles with disc brakes and ABS sensors
  
• Upgrading hydraulic pumps to variable displacement models
  
• Replacing analog controls with digital load monitoring systems
  

Some municipalities have converted their Simon lifts to hybrid platforms, using electric motors to drive the hydraulic system while retaining the original boom structure. These retrofits improve efficiency and reduce emissions, especially in urban environments.
Conclusion
The differential in a Simon aerial lift may seem like a background component, but it plays a vital role in mobility, hydraulic power, and structural integrity. Understanding its function, identifying the correct model, and maintaining it properly ensures safe and reliable operation. As these legacy machines continue to serve in the field, attention to drivetrain health becomes not just a technical necessity—but a tribute to the enduring engineering of a bygone era.