External force acting on a hydraulic excavator bucket is calculated precisely. An external force measurement system for a work machine includes a hydraulic cylinder for driving a front unit of the work machine and a plurality of strain gauges for sensing strain amounts of a cylinder rod of the hydraulic cylinder. The plurality of strain gauges are formed of at least two sets of strain gauges. The at least two sets of strain gauges are arranged to oppose each other as viewed in an axial direction of the cylinder rod. The external force measurement system includes a load calculation section that is configured to calculate a load acting on the hydraulic cylinder based on difference between the strain amounts of the strain gauges arranged to oppose each other in each set.

A load cell (1) has a deformable body with upper and lower contact surfaces (3, 4), through which a force is introduced. Support points (5) of the contact surfaces define a line of action (6) of the force. At least one column-shaped portion (7) of the deformable body has a central longitudinal axis (8) and a circumferential surface whose generating line runs parallel to the central longitudinal axis (8). A first determining means (9) installed on the column-shaped portion converts a mechanical deformation into an electronic signal, and a second determining means (10) installed on the column-shaped portion converts a deviation of the central longitudinal axis (8) from the line of action (6) into a representative signal. Each determining means has at least one strain gauge.

A balance device comprises first, second and third components. The first, second and third components include substantially cylindrical first, second and third central portions, respectively, and which are coaxial with one another. The balance device further comprises a first set of connectors for coupling the first component to the third component and a second set of connectors for coupling the second component to the third component. The connectors accommodate axial movement in response to relative axial forces between the first and second components and minimise rotational movement about the axial direction in response to relative axial torque between the first and second components.