Hydraulics are also more efficient in applications that involve clamping. No additional energy needs to be consumed to hold a hydraulic actuator motionless, given a properly selected valve. This is in contrast to attempting to hold precise position with an electric motor against a force or weight. To avoid wasted electrical energy due to the electrical fields of the motor actively working against the mass, a complex clutching or braking mechanism may be needed to hold the weight.
Of course, hydraulic systems also lose power due to fluid friction in the pumps, valves, and piping, and the design of hydraulic systems should be optimized accordingly.
Improving efficiency of hydraulic systems
Proper hydraulic system design starts with selecting and sizing the components correctly. Pumps and accumulators that are too large can waste power, and if they're too small, cylinders and valves may not have the power or the ability to react fast enough to meet the requirements of the machine. For actuators moving moderate to heavy masses, acceleration, velocity, and deceleration are limited by available force - not by oil flow. Because a cylinder bore determines the force it can produce, if the bore is too small, the cylinder may not be able to attain the speeds or cycle times required by the application.
Hence, the proper sizing of components starts with understanding the natural frequency of the system. A control system's natural frequency is the rate at which it would naturally tend to oscillate if given a stimulus. For proper controllability, a system's natural frequency should be at least four times the rate that it will be moved normally. With that in mind, the hydraulic cylinder should be sized such that:
Aavg = (f ื 4)2 ื π2 ื LS ื WL / (g ื β)
Where, Aavg is the average area of the piston, f is the cycle frequency, LS is the stroke length, WL is the weight of the load, g is the acceleration due to gravity (32ft./sec2), and β is the bulk modulus (incompressibility constant) of oil (~200,000psi).
Note that if there is distance between the valve and cylinder, and/or if hose is used instead of pipe to carry oil, then the equation above tends to underestimate the correct cylinder bore.
Proper sizing of the accumulator is more a function of volume. Generally accepted design requirements are that the gas volume in the bladder should be large enough to prevent the pressure from dropping below a desired minimum, and the accumulator should never be allowed to run out of oil. To calculate the volume needed, start with the following equation:
P1 ื V1 = P2 ื (V1 +ΔV)γ<
Where P1 is the supply pressure, V1 is the gas volume at steady state, P2 is the minimum pressure, ΔV is the maximum change in gas volume plus a small safety factor, and γ is the ratio of specific heat, which is about 1.4 for diatomic gas.
Then,
V1 = ΔV / (γรP1 / P2 1) and the total accumulator volume required is:
V2 = V1 + ΔV
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