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Lecture 12
HYDRAULIC ACTUATORS
Learning Objectives
Upon completion of this chapter, the student should be able to: 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. Analyze various lever systems using 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.
1.1 Introduction
Hydraulic systems are used to control and transmit power. A pump driven by a prime mover such as an electric motor creates a flow of fluid, in which the pressure, direction and rate of flow are controlled by valves. An actuator is used to convert the energy of fluid back into the mechanical power. The amount of output power developed depends upon the flow rate, the pressure drop across the actuator and its overall efficiency. Thus, hydraulic actuators are devices used to convert pressure energy of the fluid into mechanical energy. Depending on the type of actuation, hydraulic actuators are classified as follows:
- Linear actuator: For linear actuation (hydraulic cylinders).
- Rotary actuator: For rotary actuation (hydraulic motor).
- Semi-rotary actuator: For limited angle of actuation (semi-rotary actuator).
Hydraulic linear actuators, as their name implies, provide motion in a straight line. The total movement is a finite amount determined by the construction of the unit. They are usually referred to as cylinders, rams and jacks. All these items are synonymous in general use, although ram is sometimes intended to mean a single-acting cylinder and jack often refers to a cylinder used for lifting. The function of hydraulic cylinder is to convert hydraulic power into linear mechanical force or motion. Hydraulic cylinders extend and retract a piston rod to provide a push or pull force to drive the external load along a straight-line path. Continuous angular movement is achieved by rotary actuators, more generally known as a hydraulic motor. Semi-rotary actuators are capable of limited angular movements that can be several complete revolutions but 360o^ or less is more usual.
1.2 Types of Hydraulic Cylinders
Hydraulic cylinders are of the following types:
Single-acting cylinders.
Double-acting cylinders.
Telescopic cylinders.
Tandem cylinders.
1.2.1 Single-Acting Cylinders
A single-acting cylinder is simplest in design and is shown schematically in Fig.1.1. It consists of a piston inside a cylindrical housing called barrel. On one end of the piston there is a rod, which can reciprocate. At the opposite end, there is a port for the entrance and exit of oil. Single-acting cylinders produce force in one direction by hydraulic pressure acting on the piston. (Single-acting cylinders can exert a force in the extending direction only.) The return of the piston is not done hydraulically. In single-acting cylinders, retraction is done either by gravity or by a spring.
Figure 1.1 Single-acting cylinders
According to the type of return, single-acting cylinders are classified as follows:
Gravity-return single-acting cylinder.
Spring-return single-acting cylinder.
1.2.1.1 Gravity-Return Single-Acting Cylinder
Figure 1.2 Gravity-return single-acting cylinder: (a) Push type; (b) pull type
Load
(by spring)
Figure 1.4 Double-acting cylinder with a piston rod on one side
1.2.2.2 Double-Acting Cylinder with a Piston Rod on Both Sides
Figure 1.5 Double-acting cylinder with a piston rod on one side
A double-acting cylinder with a piston rod on both sides (Fig.1.5)is a cylinder with a rod extending from both ends. This cylinder can be used in an application where work can be done by both ends of the cylinder, thereby making the cylinder more productive. Double-rod cylinders can withstand higher side loads because they have an extra bearing, one on each rod, to withstand the loading.
1.2.3Telescopic Cylinder A telescopic cylinder (shown in Fig. 1.6) is used when a long stroke length and a short retracted length are required. 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 dump truck bed. Telescopic cylinders are available in both single-acting and double-acting models. They are more expensive than standard cylinders due to their more complex construction. They generally consist of a nest of tubes and operate on the displacement principle. The tubes are supported by bearing rings, the innermost (rear) set of which have grooves or channels to allow fluid flow. The front bearing assembly on each section includes seals and wiper rings. Stop rings limit the movement of each section, thus preventing separation. When the cylinder extends, all the sections move together until the outer section is prevented from further extension by its stop ring. The remaining sections continue out-stroking until the second outermost section reaches the limit of its stroke;this process continues until all sections are extended, the innermost one being the last of all.
Figure 1.6 Telescopic cylinder
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-lifting capacity decreases for each successive section.
Pressure port
Three stages Extended position
Retracted position
Stage 3
Stage 2
Stage 1
Figure 1.8 Displacement cylinders
Example 1. A displacement-type cylinder has a rod of 65 mm diameter and is powered by a hand pump with a displacement of 5 mL per double stroke. The maximum operating pressure of the system is to be limited to 350 bar. (a) Draw a suitable circuit diagram showing the cylinder, pump and any additional valving required. (b) Calculate the number of double pumping strokes needed to extend the cylinder rod by 50 mm. (c) Calculate the maximum load that could be raised using this system.
Solution: (a) The circuit diagram is given in Fig. 1.9.
Figure 1. (b) The volume of rod displaced is equal to the volume of fluid entering the cylinder. Let the rod diameter be d , the distance rod extends be L , the displacement per double stroke of pump be V and the number of double pump strokes be S. Then Rod volume displaced = Fluid volume entering 2
4
^ d^ L V S
Load
Non-return valve to hold load in the position when the pump is not operated
Substituting values given in the problem and showing units for each value we get
652 4
mm^2 × 50 mm = 5 mL × S
× 50 mm^3 = 5 S mL
The units on both sides of the equation must be the same. Note: 1 mL = 1 × 10 ^3 L
1 L = 1 × 10 ^3 m^3
1 mL = 1 × 10 ^6 m^3
1 mm^3 = 1 × 10 ^9 m^3
Thus, for the dimensional equality
× 50 × 10 ^9 (m^3 )= 5× 10 ^6 (m )^3 × S
or
× 50 (m )^3 = 5× 10 (m )^3 3 × S
Therefore,
S =
2 3
=33.17 double strokes
(c) We have Maximum thrust = Pressure × Rod area Substituting the values given into the problem and showing units we get
Maximum thrust = 350 × 105 ×
× 10 ^622
N ×m m
= 35 ×
N = 116080 N = 116.080 kN
Example 1. A three-stage displacement-type telescopic cylinder is used to tilt the body of a lorry (Fig. 1.10). When the lorry is fully laden, the cylinder has to exert a force equivalent to 4000 kg at all points in its stroke. The outside diameters of the tubes forming the three stages are 60, 80 and 100 mm. If the pump powering the cylinder delivers 10 LPM, calculate the extend speed and pressure required for each stage of the cylinder when tilting a fully laden lorry.
Solution: First-stage First-stage diameter = 100 mm First-stage speed =Quantity flowing Area
3 3 2 2
10 10 m ( / 4) (0.1) min m
^ ^ ^ (^) (^)
= 1.27 m/min
1.3Standard Metric Cylinders Table 1.1gives preferred sizes for the cylinder bore and rod diameter of metric cylinders. Most cylinder manufacturers have based their standard range of metric cylinders on these recommendations, offering two rod sizes for each cylinder bore. A number of combinations have a piston rod to piston diameter ratio in the region of 0.7, which gives an annulus area of approximately one-half of the full bore area. This area ratio is of use in regenerative circuits to give similar values of speed and thrust on both the extension and retraction strokes. Table 1. gives the graphical symbols for various kinds of cylinders.
Table 1.1 Recommended cylinder bore and rod sizes Piston diameter (mm)
Piston rod diameter (mm)
Small 20 28 36 45 56 70 90 100 110 125 140 160 180 200 Large 28 36 45 56 70 90 100 110 125 140 160 180 200 220
Table 1.2 Graphical symbols of different linear actuators S. No.
Graphical Symbols Explanation
1. Single-acting cylinder with unspecified return
2. Single-acting cylinder with spring return
3. Double-acting cylinder – single piston rod
4. Double-acting cylinder^ – doublepiston rod
5. Telescopic cylinder–double acting
6. Telescopic^ cylinder–single acting 7. Double-acting cylinder– fixed cushion on one side 1. Double-acting cylinder–variable cushion on one side 9. Double-acting cylinder–variable cushion on both sides
1.4Cylinder Force, Velocity and Power The output force ( F ) and piston velocity ( v ) of double-acting cylinders are not the same for extension and retraction strokes.
Figure 1.11 Effective area during (a) extension strokes and (b)retraction strokes
During extension, entire piston area A p is subjected to fluid pressure
During retraction, annulus area A p− A r is subjected to fluid pressure
