Article by: Unnat Pinsopon, Department of Mechanical Engineering, Faculty of Engineering, King Mongkut’s Institute of Technology Ladkrabang.
Hydraulic power transmission is used in wide range of industrial applications from heavy machining to aviation flight control. The power is transmitted using pressurized hydraulic oil as media, and the power transmission within the system is shown in figure 1. Hydraulic pump receives mechanical power from a prime mover and produces the flow of hydraulic oil. The oil flows through pipes and valves, and flows into the actuator. The actuator finally delivers the power against the external load.
There are 2 types of hydraulic actuator: one that operates linearly (i.e. hydraulic cylinder), and one that operates angularly (i.e. hydraulic motor). Consider the operation of a hydraulic cylinder during its extension as shown in a simple diagram (figure 2). Pump produces the flow into the head end (HE) side of the cylinder, while the oil from the cylinder rod end (RE) flows back to tank. The speed of the cylinder piston (vcyl) depends on the flow rate of the hydraulic oil (QP) produced by the pump as shown in equation 1. AHE is the cross-sectional area of the cylinder piston at the HE side.
Figure 3 shows the free body diagram of cylinder piston under exerting forces (excluding friction). Equation 2 describes the force balance on the piston. PHE and PRE are oil pressures at cylinder HE and RE. ARE is the RE piston cross-sectional area and Fext is the external load force.
The value of cylinder RE pressure (PRE) usually is just large enough to overcome the pressure loss of the return line, however, is significantly much less than the cylinder HE pressure (PHE). Omitting the cylinder RE pressure, the cylinder HE pressure depends on external load force as shown in equation 3.
The pump outlet is connected to the cylinder HE during the cylinder extension (figure 2). The pressure of the oil at the pump outlet (PP) is therefore equal to the cylinder HE pressure plus the pressure loss between pump outlet and cylinder HE inlet. Neglecting the pressure loss, the oil pressure at pump outlet is roughly identical to the cylinder HE pressure, PP ≈ PHE.
The power delivered by the hydraulic pump is the product of oil pressure at pump outlet (PP) and oil flow rate (QP). Substituting the values of QP from equation 1, and PP, which is roughly equal to PHE, from equation 3, the pump delivered power can be computed as equation 4.
Equation 4 shows that when neglecting all the losses, the pump delivered power is roughly equal to the power delivered to load. Question such as “Could changing the size of the cylinder help saving the consuming power?” often arises in engineering conversations, and the equation 4 gives the answer. Upsizing the cylinder may reduce the value of the operating pressure according to equation 3.
However larger oil flow rate is needed for a bigger cylinder (equation 1), hence a bigger pump is needed. On the other hand, downsizing the cylinder leads to a smaller oil flow rate, hence a smaller pump. However, the system will operate at a higher-pressure level. No matter what size the cylinder is, the pump delivers the roughly same power to the load (equation 4). Changing the size of the cylinder or changing the size of the actuator does not alter the consuming power of the hydraulic system.