Understanding Breakout Force and Digging Force in Excavator Design

[MikePhua]

Why Breakout Force and Digging Force Matter
In excavator engineering, breakout force and digging force are two critical metrics that define how effectively a machine can penetrate, lift, and manipulate soil or rock. These forces are not interchangeable—they originate from different hydraulic cylinders and serve distinct roles in excavation dynamics. Understanding the difference between them is essential for comparing machines, selecting the right equipment for a job, and interpreting manufacturer specifications.
Breakout force typically refers to the maximum force exerted by the bucket cylinder when curling the bucket inward, especially from a fully extended position. Digging force, on the other hand, is often split between the arm (stick) cylinder and the bucket cylinder, representing the combined effort during a digging stroke.
Terminology and Force Origins

• Bucket cylinder: Controls the curl and dump motion of the bucket; responsible for breakout force
  
• Arm (stick) cylinder: Controls the extension and retraction of the stick; contributes to digging force
  
• Breakout force: The peak force generated by the bucket cylinder when the bucket is curled inward against resistance
  
• Digging force: The total force applied during excavation, combining stick and bucket cylinder efforts
  

Breakout force is most relevant when dislodging compacted material or breaking through hard surfaces. Digging force is more representative of sustained trenching or loading operations.
Comparing Machines with Different Force Metrics
Manufacturers often report either breakout force or digging force, depending on regional standards or marketing preferences. For example, Liebherr may list breakout force with a specific stick length, while Hitachi might provide separate digging forces for the bucket and arm cylinders. This discrepancy can make direct comparisons difficult unless the forces are normalized or recalculated.
To estimate breakout force from digging force values:

• Identify the bucket cylinder’s bore diameter, rod diameter, and stroke length
  
• Determine hydraulic pressure at full extension
  
• Use the formula:
  Breakout Force = (Cylinder Area × Pressure) × Mechanical Advantage
  

Mechanical advantage depends on linkage geometry, which varies by model. Without precise geometry, estimates can be off by 10–20%.
Stick Length and Geometry Influence
Stick length plays a major role in force transmission. A longer stick increases reach but reduces leverage, lowering effective digging force. Conversely, a shorter stick improves force but limits reach. Manufacturers often publish force curves showing how digging force varies with stick length and bucket position.
In one comparison between a Hitachi EX1800-2 and a Liebherr 994, the Liebherr’s breakout force was listed with a specific stick length, while the Hitachi provided separate cylinder forces. Without knowing the linkage geometry and stick dimensions, direct comparison is speculative.
Real-World Implications and Operator Experience
Operators often describe breakout force as the “snap” or “bite” of the bucket when prying material loose. It’s the force that breaks through frozen ground, compacted clay, or fractured rock. Digging force, meanwhile, is the sustained power that keeps the bucket moving through soil.
One operator recalled using a machine with high breakout force but poor digging geometry—it could break through asphalt but struggled to trench efficiently. Another noted that a machine with balanced digging force across the stick and bucket cylinders felt smoother and more predictable during trenching.
Design Optimization and Force Balance
Modern excavators are designed to balance breakout and digging forces for optimal performance. Engineers adjust cylinder sizes, linkage geometry, and hydraulic pressures to achieve this balance. Overemphasis on breakout force can lead to jerky motion and increased wear, while underpowered digging force reduces productivity.
Design considerations:

• Cylinder bore and stroke sizing
  
• Linkage pivot placement
  
• Hydraulic pressure regulation
  
• Stick length options and modularity
  

Some manufacturers offer multiple stick lengths or heavy-duty linkage kits to tailor force profiles to specific applications.
Conclusion
Breakout force and digging force are foundational concepts in excavator performance, each rooted in distinct hydraulic and mechanical systems. While breakout force reflects the bucket’s ability to snap through resistance, digging force represents the sustained effort of both the stick and bucket working in tandem. Understanding these forces—and how they’re measured—enables smarter equipment selection, better jobsite planning, and more informed comparisons across brands and models. In excavation, force isn’t just about numbers—it’s about how those numbers translate into real-world productivity.