In this paper, the sealing characteristics of the piston ring of the piston in a bent axis type axial piston motor are clarified. In order to clarify the sealing mechanism of the piston ring, a test rig was developed and the behavior of the piston ring was visualized and measured. Next, the measured piston ring behavior was modeled and fluid structure interaction analysis was performed to analyze the leakage flow rate of the piston ring. The effect of the number and arrangement of piston rings on the leakage flow rate of the piston rings was also investigated. Comparison of experimental and analytical leakage flow rates indicated that the 2nd piston ring may have lower sealing performance than the 1st piston ring. The leakage flow rate from the piston ring gap was modeled by the flow between parallel plates and estimated the ratio of the leakage flow rate from that to the total leakage flow rate. The results showed that cross section profile of piston ring gap may affects the leakage flow rate.

Pilot-pressure supply circuit of construction machinery consumes 2-3% of engine's horsepower. A method of improving energy efficiency is studied by many researchers. Switched Inertance Hydraulic Converters (SIHC) which is one type of digital hydraulic circuit has been proposed as a solution to reduce throttling loss through control valve in a hydraulic circuit. A digital-hydraulic pressure reducing circuit is an example of SIHC which may show high energy efficiency because it does not use a throttling orifice. It can obtain output flowrate larger than supply flowrate. This paper aims to apply the digital-hydraulic pressure reducing circuit to pilot-pressure supply circuit of construction machinery. A control method of the pilot pressure to follow demanded pilot pressure is proposed. A simulation model is experimentally verified. The proposed control method is validated. Controllability and loss reduction effect are investigated by simulation based on transient pressure and flowrate data of an actual construction machinery in operation.