Excavator Breakout Force and Its Impact on Performance and Design

[MikePhua]

What Breakout Force Really Measures
Breakout force is a critical specification in excavator design, representing the maximum force exerted at the tip of the bucket during curling motion. It determines how effectively an excavator can pry, fracture, or dislodge compacted soil, rock, roots, or debris. This force is generated primarily by the bucket cylinder and is influenced by hydraulic pressure, cylinder size, linkage geometry, and the distance between pivot points.
Terminology annotation:

• Breakout Force: The peak force applied at the bucket’s cutting edge during curl, measured in kilonewtons (kN) or pounds-force (lbf).
  
• Bucket Curl: The rotational movement of the bucket toward the cab, driven by the bucket cylinder.
  
• Linkage Geometry: The arrangement and length of arms and pins that transmit hydraulic force to the bucket.
  
• SAE J1179 / ISO 6015: International standards used to calculate breakout force under controlled conditions.
  

Most manufacturers publish breakout force values based on a standard GP bucket directly pinned to the stick, without couplers or specialized attachments. Adding a quick coupler or taller bucket reduces breakout force due to increased distance from the pivot point, weakening mechanical leverage.
Breakout Force vs Digging Force
Breakout force is often confused with digging force, also known as crowd force. While breakout force is generated by the bucket cylinder curling the bucket inward, digging force comes from the stick cylinder pushing the bucket through material.
Comparison:

• Breakout Force
  • Generated by bucket cylinder
    
  • Ideal for prying, breaking, and lifting compacted material
    
  • Affected by bucket height and coupler configuration
    
• Digging Force
  

• Generated by stick cylinder
  
• Ideal for trenching and penetrating loose soil
  
• Affected by arm length and cylinder angle
  

For example, the Bobcat E35 mini excavator delivers a breakout force of 29.1 kN and a digging force of 22.5 kN. The Kubota KX040-4 offers 3,315 kgf breakout force, making it more effective for stump removal and hard ground penetration.
Factors That Influence Breakout Force
Several design and operational factors affect breakout force:

• Hydraulic Pressure
  • Higher system pressure increases cylinder output
    
  • Relief valves limit maximum pressure to protect components
    
• Cylinder Size
  • Larger diameter cylinders produce more force
    
  • Stroke length affects speed and control
    
• Bucket Design
  • Squatter buckets improve leverage and increase force
    
  • Taller buckets reduce force due to longer moment arm
    
• Coupler Height
  

• Quick couplers add distance between pivot and cutting edge
  
• Reducing coupler height improves breakout efficiency
  

Recommendations:

• Use direct pin-on buckets for maximum breakout force
  
• Choose squat-style buckets for prying applications
  
• Avoid oversized attachments that reduce leverage
  
• Maintain hydraulic system pressure within manufacturer specs
  

In one forestry application in Oregon, switching from a tall rock bucket to a squat GP bucket increased stump removal efficiency by 25%, reducing fuel consumption and cycle time.
Breakout Force and Machine Size Selection
Choosing the right excavator size depends heavily on breakout force requirements. Smaller machines may struggle with tasks like stump removal or rock trenching, even if they fit the jobsite better.
Typical breakout force ranges:

• 2-ton mini excavator: ~15–20 kN
  
• 5-ton compact excavator: ~25–35 kN
  
• 20-ton full-size excavator: ~120–150 kN
  
• 35-ton production excavator: ~180–220 kN
  

For heavy-duty tasks like removing tree stumps or breaking shot rock, a machine with at least 100 kN breakout force is recommended. In agricultural land clearing, operators often upgrade from 6-ton to 12-ton machines to avoid overloading smaller frames and hydraulic systems.
Protecting the Machine During High-Force Operations
While breakout force is a measure of capability, it also defines the stress limits of the machine. Overuse or abuse—such as prying with the stick fully extended or forcing the bucket against immovable objects—can lead to frame twisting, cylinder damage, or premature wear.
Preventative measures:

• Avoid exceeding relief valve settings
  
• Use breakout force within rated duty cycles
  
• Monitor hydraulic temperatures during continuous digging
  
• Train operators to recognize resistance and adjust technique
  
• Inspect pins, bushings, and linkage for signs of stress
  

In one case from New Zealand, an operator damaged the stick cylinder on a 6-ton excavator while attempting to remove a buried concrete footing. The machine’s breakout force was insufficient, and the relief valve had been tampered with. After repair, the company implemented a breakout force chart for each machine and trained staff accordingly.
Conclusion
Breakout force is more than a number—it’s a reflection of an excavator’s ability to perform demanding tasks safely and efficiently. Understanding how it’s generated, how it compares to digging force, and how it’s affected by attachments and geometry allows operators and fleet managers to choose the right machine for the job. Whether trenching in soft soil or prying out stubborn stumps, breakout force defines the edge of performance—and the boundary of mechanical respect.