In this experimental study, a Direct Driven Hydraulics (DDH) system of the closed circuit type was utilized for cyclic vertical actuation in heavy load material handling. The actuator was controlled by a speed-controlled fixed displacement pump. The high energy saving potential of this system has been demonstrated in previous studies by the authors, but the dynamic characteristics of the ramped and P-controlled base system were considered unsatisfactory. Therefore, the system was implemented with an open-loop S-curve control that utilized a pre-calculated RPM (revolutions per minute) profile for the electric motor in order to realize a smooth actuator and load transition as a function of time. The results indicate that S-curve control is exceptionally well suited for producing a controlled lifting–lowering rapid motion with a heavy load, while still keeping the actuator chamber pressures within acceptable limits. In comparison, the motion produced by P-control was characterized by large unwanted pressure peaks together with velocity fluctuations and vibrations at the end of the stroke. Using a combination of S-curve control and hydraulic load compensation, a mass of 1325 kg could be moved 0.26 m in less than 0.5 s. The load compensation reduced the energy consumption by 64%, which would allow downsizing the electric motor and enable cost-efficient DDH implementation.

In a hydraulic cylinder, forces are generated by high-pressure oil. To contain the pressurized oil inside the cylinder, tight sealing is needed. The sealing tightness causes friction that directly decreases the performance of a cylinder. Additionally, at very low operating speeds, the sealing friction can cause stick–slip phenomenon and worsen positioning accuracy. In this paper, friction forces of hydraulic rod seals made of polyurethane and bronze-filled PTFE were measured on three different rod coatings. Measurements were planned according to the standard ISO 7986 and were conducted in pressure points of 63, 110, and 160 bar in constant sliding speeds of 50, 150, 300, and 450 mm/s. The results show that the sealing choice has the biggest impact on the sealing friction: the tested polyurethane U-cup seal induces friction forces 1.8–8.5 times higher than the PTFE-bronze seal depending on the operating conditions. On average, the polyurethane seal induces forces 4.1 times the forces of the PTFE-bronze seal. For the same seal, changing the rod coating can change friction values up to 43% for the polyurethane and up to 27% for the PTFE-bronze seal. Therefore it is clear that sealing type and rod coating have a big impact on cylinder friction.