Actuators in hydraulic system

HYDRAULIC ACTUATORS

1. actuators

2. Actuators Hydraulic systems are used to control and transmit power. A pump driven by prime mover such as electric motor creates a flow of fluid in which the pressure, direction and flow rate are controlled by valves An actuator is used to convert the energy of fluid back into mechanical power. The amount of output power developed depends upon the flow rate, pressure drop across the actuator and its overall efficiency Thus they are devices which used to convert pressure energy of the fluid into mechanical energy.

3. types Depending on the type of actuation, hydraulic actuators are classified as, Linear actuator: for linear actuation (hydraulic cylinder) Rotary actuator: for rotary actuation(hydraulic motor) Semi-rotary actuator: for limited angle of actuation (semi-rotary actuators)

4. Hydraulic linear actuators As their name implies, it provides motion in a straight line. The total movement is a finite amount determined by the construction of the unit They are referred to as cylinders, rams and jacks The function of a hydraulic cylinder is to convert the hydraulic power into linear mechanical force or motion Hydraulic cylinder extend and retract a piston rod to provide a pull or push force to drive the external load along a straight path

5. Types of hydraulic cylinders Hydraulic cylinder are of following types: Single-acting cylinders Double-acting cylinders Telescopic cylinders Tandem cylinders

6. Single acting cylinder

7. Single acting cylinder It consist of piston inside a cylindrical housing called as barrel On one end is attached a rod, which can reciprocate At the opposite end, there is a port for the entrance and exit of the oil They produce force only in one direction by hydraulic pressure acting on the piston The return of the spring is not done hydraulically, it is either done by gravity or spring.

8. double acting cylinder There are two types of double acting cylinder: Double acting cylinders with piston rod on one side Double acting cylinders with piston rod on both side

9. Double acting cylinder

10. Double acting cylinder DOUBLE ACTING CYLINDER WITH PISTON ROD ON ONE SIDE To extend the cylinder, the pump flow is sent to the blank-end port as shown in fig. The fluid from the rod end port returns to the reservoir Now to retract the cylinder, the pump flow is sent to the rod end port and the fluid from the blank end port returns to the tank as shown in another fig.

11. Double acting cylinder DOUBLE ACTING CYLINDER WITH A PISTON ROD ON BOTH SIDES

12. Double acting cylinder A double acting cylinder with piston on both sides is a cylinder with a rod extending from both the ends The application involves in a process where work can be done by both the ends of the cylinder, thereby making the cylinder more productive Double rod cylinder can withstand higher side loads because they have an extra bearing on each rod to withstand the loading.

13. Telescopic cylinder A telescopic cylinder is needed when a long stroke length and a short retracted length are needed. The telescopic cylinder extends in stages, each stage consisting of a sleeve that fits inside the previous stage One application for this type of cylinder is raising a dumping truck bed. Telescopic cylinders are available both in single acting and double acting models.

14. Telescopic cylinder

15. Telescopic cylinder Construction: They generally consist a nest of tubes and operate on the displacement principle The tubes are supported by bearing rings, the innermost set of which have grooves or channels to allow the fluid flow The front bearing assembly on each section includes seals and wiper rings Stop rings limit the movement of each section, thus preventing the separation For a given input flow rate, the speed of operation increases in steps as each successive section reaches the end of its stroke. Similarly, for a specific pressure, the load shifting capacity decreases for each of the successive section

16. Tandem cylinder

17. Tandem cylinder A tandem cylinder is used in application where a large amount of force is required to be applied from a small diameter cylinder Pressure is applied to both the pistons, resulting in a increased force because of a large area. The only drawback is that this cylinders must be longer than a standard cylinder to achieve an equal speed because the flow must go to both the pistons simultaneously.

18. Graphical symbols

19. Graphical symbols

20. Cushioning of cylinders

21. Cushioning of cylinders For prevention of shock due to stopping loads at the end of piston stroke, cushion devices are used. Cushion may be applied at either end or both the ends They operate on the principle that as the cylinder piston approaches the end of the stroke, an exhaust fluid is forced to go through an adjustable needle valve that is set to control the escaping fluid at the given rate. This allows the deceleration characteristic to be adjusted for different loads When the cylinder piston is actuated, the fluid enters the cylinder port and flows through a little check valve so that the entire piston area can be utilized to produce forces and motion.

22. Operation of cylinder cushioning

23. Operation of cylinder cushioning

24. Cushioning pressure During deceleration, extremely high pressure may be developed within a cylinder cushion. The action of the cushioning device is to set up a back pressure to decelerate the load.

25. Maximum speeds in cushioned cylinders The maximum speed of the cushion rod is limited by the rate of fluid flow into and out of the cylinder and the ability of the cylinder to withstand the impact forces that occurs when the piston motion is arrested by the cylinder end plate. For uncushioned cylinder: 8 m/min For cushioned cylinder : 12 m/min For high speed or externally cushioned cylinders: 30 m/min

26. Acceleration & Deceleration of cylinder loads Cylinders are subject to acceleration and deceleration during their operation. Cylinders are decelerated to provide cushioning and cylinders are accelerated to reduce cycle time of the operation. Acceleration Equation:

27. questions Explain the classification of hydraulic actuators. Explain various types of hydraulic cylinders. Describe the construction and working of double-acting cylinders. Derive an expression for force, velocity and power for hydraulic cylinders. Explain the importance of cylinder cushioning. Explain various types of cylinder mountings used in fluid power. Evaluate the performance of hydraulic systems using cylinders.