The modern day hydraulic system has a very close resemblance to the human body. The heart does the work of a pump by pumping blood into all the parts of the body. The brain is the central unit controlling eyes and ears (sensors), and human muscles which work as output devices that perform various tasks. These output devices in hydraulic systems are called actuators.
What is a hydraulic actuator?
A hydraulic actuator is a completely mechanical component and has no sense of the amount of power that needs to be delivered and at what speed. In order to perform the required set of operation, servo controlled feedback loops are used to command controlling valves (pressure control valve, flow control valve and direction control valve) to control the amount of hydraulic energy going into the actuator. Hydraulic actuators blindly convert that hydraulic energy into mechanical energy (a function of force and velocity or torque and rotational speed) without the need to think.
Hydraulic actuators can be classified into two broad categories: Linear and rotary, and provide different types of force.
Different types of force
Linear hydraulic actuators provide force in a linear manner, whereas rotary actuators provide torque and rotational speed. The major difference between a linear and rotary actuator is that a linear actuator provides different forward and backward speeds (due to the area occupied by the piston rod), whereas a rotary actuator will always provide the same rotational speed in either direction.
Rotary actuators require sliding seals which can cause more leakage problems than the piston and cylinder type. To prevent that, for most rotary applications, linear motion is developed by linear actuator and is converted into rotary motion by rack and pinion. So linear hydraulic actuators provide the maximum force to weight ratio and are mainly used for mobile application (cranes, loading & unloading trucks, special purpose mine drilling machines etc.). They are also used for applications demanding very high force/pressure, e.g. punching machine, as linear actuators can either be single acting or double acting.
A single acting cylinder is actuated by hydraulic force during forward motion and returns to the original position by spring. Double acting cylinder has two ports on the opposite ends and fluid can enter from either side depending upon DCV. Hence double acting cylinder can provide force in both directions which is a very useful option.
Selecting the right hydraulic actuator
The selection of the hydraulic actuator will depend upon the application for which is it required. Maximum force developed using a hydraulic cylinder depends on the maximum pressure the pump can generate and area of the actuator. Since max pressure generated by pump is limited by the safe working pressure the system (pipes and valves) can handle without failure, the area of a cylinder plays a major role in the selection process.
To determine the right size of actuator for any application you need to:-
- Find the force requirement. If it is for lifting purpose, the force must match the weight of the unit. If the application demands shifting of a load on the ground, the hydraulic force will just have to cover the frictional force (which will be much less).
- Figure out the force location. If the force is eccentric, consider a piston rod of higher diameter to avoid buckling.
- Calculate the bore diameter of the cylinder depending on the maximum force requirement and maximum pressure the system is capable of developing. Since a piston rod is only on one side the area will be different as one side is completely open to the fluid whereas the other side is connected to the piston rod. Hence the area available will be less. Empirically, the diameter of piston rod should be half of the internal diameter of the hydraulic cylinder hence the area available on the piston rod side is 75% of the open sided area. Volume is directly proportional to area as stroke length is constant and hence, for the same flow rate provided from the pump, the differential volume changes the speed. To keep the speed the same in either direction, double rod cylinders are used, which have a cylinder rod on either side of the piston. The force generated by a single rod double acting cylinder is also different as force is directly proportional to area.
Speed= Volume of cylinder/flow rate
To avoid the impact of the piston on the cylinder end-cap and improve the life of the hydraulic cylinder seals, quick deceleration is required at the end of the stroke. Ideal cushioning is when the velocity of the piston hits zero as it reaches at the end of the cylinder without impact. As the piston reaches the end, some of the fluid is trapped and is passed through a throttle valve which provides an additional resistance and hence deceleration. The profile of a throttle valve can be adjusted using a screw to control deceleration.