Abstract: The compound digging force concept was proposed based on the digging process in actual operation conditions of the composite motion of three hydraulic cylinders. According to the main function of boom， arm and bucket， the restricted conditions of the compound digging force were determined. The value range of orientation angle was up to the resistance angle and the difference angle. The calculating model of compound digging force was established based on a compound digging force hypothesis， which is changing gradually with the orientation angle. Taking a 36 t backhoe hydraulic excavator as an example， the validity of the hypothesis was proved. Taking the actual digging trajectory and the digging resistance obtained by testing as the reference， the calculating results of the existing theoretical digging force model were compared with that of the actual one. The comparative results indicate that the model can exactly estimate the real compound digging ability of the excavator. In this way， it can provide evaluation criterion and calculation basis to optimal design and strength analysis of the excavator working device.

Key words: hydraulic excavator, theoretical digging force, compound digging force, resistance characteristics, orientation angle, digging ability