Torque is the rotational force equivalent in mechanical systems, describing the turning effect produced by an engine, hydraulic motor, or hydraulic cylinder acting through a lever arm. In construction equipment, torque is expressed in newton-meters (Nm) for smaller components such as wheel motors and fasteners, or kilonewton-meters (kNm) for larger assemblies such as swing drives and final drives. It is one of the primary specifications used to compare the mechanical capability of excavators, loaders, cranes, and drilling rigs.
Physical Principles
Torque is calculated as force multiplied by the length of the lever arm over which it acts (T = F × r). In a hydraulic system, torque output from a motor is a function of system pressure (bar or psi) and motor displacement (cm³/rev). Increasing displacement or pressure increases torque, while flow rate (L/min) determines rotational speed, not torque. Power (kW) is the product of torque and angular velocity, meaning two machines with identical power ratings can have different torque and speed characteristics depending on gearing and hydraulic circuit design.
This distinction matters in practice: torque determines how much resistance a component can overcome—such as breaking into compacted soil or rotating a loaded upper structure—while rotational speed determines how quickly that movement occurs. A high-torque, low-speed setup favors force transmission; a low-torque, high-speed setup favors cycle time.
Applications in Construction Equipment
Several torque values appear on equipment spec sheets, each describing a different subsystem:
Engine torque describes the output of the diesel or electric drive motor, typically specified at a given rpm (e.g., peak torque at 1,400–1,600 rpm for many mid-size diesel engines). This figure affects how an excavator or loader responds under load without stalling or requiring a downshift.
Swing torque describes the rotational force available at the upper structure's slewing ring, generated by one or more hydraulic swing motors acting through a pinion and ring gear. It affects how effectively the upper structure can rotate while loaded, particularly on slopes or when swinging into a bank.
Travel and drive motor torque describes the force delivered to the final drives or wheel motors, affecting gradeability and traction, independent of travel speed.
Hydraulic cylinder torque-equivalent force is usually expressed directly as breakout or tearout force (kN) rather than torque, since cylinders produce linear rather than rotational force; torque becomes relevant again once that force is transmitted through a linkage.
Relationship to Other Performance Metrics
Torque is frequently cited alongside breakout force and swing force in excavator spec sheets. These values are related but not interchangeable: breakout force is a linear force (kN) generated by the bucket cylinder through the linkage geometry, while swing torque (kNm) describes the rotational force at the slewing ring. Comparing machines solely by engine torque or swing torque without accounting for gear ratios, hydraulic pressure settings, and linkage geometry can produce misleading conclusions about actual digging or lifting performance.
Coverage in This Publication
Swing torque and its relationship to breakout force is examined in detail in Swing Torque and Breakout Force: What They Mean for Your Excavator's Performance, which distinguishes the two metrics and their effect on digging and slewing performance. Torque ratings as part of a manufacturer's model range are discussed in Doosan DX-Series Crawler Excavators: Performance, Efficiency and Operator Control. The role of torque in hydrostatic drivetrains, including motor sizing and torque requirements by machine class, is covered in Hydrostatic Transmission in Tracked Excavators and Dozers: How It Works and When It Pays. The mechanical transmission of swing torque through the slewing ring and gearing is detailed in Swing Bearing Design in Excavators: Components, Function, and Maintenance.
