Sizing a Machine to Pull Another Up Slopes

[MikePhua]

The Physics Behind Pulling on Inclines
When one machine is tasked with pulling another up a slope, the forces involved go far beyond simple towing. Gravity, friction, rolling resistance, and mechanical efficiency all play roles in determining whether the pulling machine can complete the task safely and effectively. The steeper the slope, the greater the gravitational component working against the pull. This is why a machine that easily tows a load on flat ground may struggle—or fail—on an incline.
The basic force required to pull a machine up a slope can be estimated using the following formula:
Required Force = Weight of Towed Machine × sin(θ) + Rolling Resistance
Where:

• θ is the slope angle in degrees
  
• sin(θ) represents the vertical component of the slope
  
• Rolling Resistance is typically 2–5% of the machine’s weight depending on terrain
  

For example, pulling a 40,000 lb dozer up a 20° slope requires overcoming roughly 13,680 lb of gravitational force, plus rolling resistance. That means the pulling machine must generate at least 15,000–18,000 lb of drawbar pull to move the load steadily uphill.
Drawbar Pull and Machine Capability
Drawbar pull is the horizontal force a machine can exert at its hitch point. It depends on engine torque, transmission gearing, traction, and ballast. Manufacturers like Caterpillar, Komatsu, and John Deere publish drawbar pull ratings for their machines, often measured at peak torque in first gear.
For example:

• A Caterpillar D8T dozer can produce up to 90,000 lb of drawbar pull
  
• A Komatsu WA500 wheel loader offers around 40,000 lb
  
• A John Deere 8430 tractor with duals may reach 25,000 lb
  

However, these figures assume ideal conditions—dry ground, good traction, and proper ballast. On wet clay or loose gravel, traction loss can reduce effective pull by 30% or more.
Traction and Surface Conditions
Traction is critical. Even if a machine has the theoretical power to pull a load, poor surface conditions can cause slippage. Rubber tires on wet grass or mud may spin without moving the load, while steel tracks on rocky terrain may grip better but risk damage.
To improve traction:

• Use dual tires or tracks for increased contact area
  
• Add ballast to increase ground pressure
  
• Use chains or cleats on tires in icy conditions
  
• Avoid sudden throttle changes that break traction
  

One operator in Alberta shared a story of using a Case 1150 dozer to pull a stuck excavator up a shale slope. Despite having enough power, the dozer spun out halfway up. After adding 2,000 lb of rear ballast and switching to a lower gear, the machine completed the pull without incident.
Safety Considerations and Load Control
Pulling uphill introduces serious safety risks. If the towed machine rolls backward or jackknifes, it can damage both units or injure operators. Always use controlled, low-speed pulls with communication between drivers. Use tow bars or rigid connections when possible to prevent slack shock.
Recommendations include:

• Use winches or snatch blocks for controlled ascent
  
• Avoid chain-only connections on steep grades
  
• Keep both machines aligned to prevent lateral drift
  
• Monitor engine temperature and transmission load
  

In one case, a contractor in Tennessee used a D6N dozer to pull a stuck dump truck up a 15% grade. The chain snapped under load, sending the truck backward into a culvert. After switching to a tow bar and adding a second machine for braking, the recovery succeeded.
Choosing the Right Machine for the Job
To select a pulling machine, consider:

• Weight of the towed machine
  
• Slope angle and surface type
  
• Required drawbar pull (based on formula)
  
• Available traction and ballast
  
• Operator experience and visibility
  

For slopes over 20°, tracked machines are preferred due to better grip and stability. For lighter loads on moderate slopes, articulated loaders or large tractors may suffice. Always verify manufacturer specs and consult experienced operators before attempting a pull.
Historical Context and Field Innovation
In the 1960s, logging crews in Oregon used cable skidders to pull stuck dozers up muddy slopes. These machines used winches and anchor trees to control ascent. Today, similar techniques are used in pipeline construction and mining, where terrain can be unpredictable.
One story from a coal mine in West Virginia involved a D9 pulling a broken-down drill rig up a 30° incline. The crew used a triple-block pulley system anchored to a haul truck, reducing the load on the dozer and allowing a slow, safe recovery.
Conclusion
Pulling a machine uphill is not just about horsepower—it’s about physics, traction, and control. By understanding the forces involved and selecting the right equipment, operators can perform uphill recoveries safely and efficiently. Whether using formulas, field experience, or a combination of both, the key is preparation and respect for the terrain.