Hydraulic and Pneumatic actuators
Introduction :
Definition : The actuator is a hydraulic element which converts oil energy into mechanical energy for performing the given application.
● In oil hydraulic system/Pneumatic system, the oil/air is allowed to flow through various elements and actuator is the last element in which oil enters and leaves from it to drain to the reservoir for recirculation.
● The actuator is the component which utilises the oil/air energy to produce output motion which may be linear or rotary.
● The movement of the actuator (linear/rotary) is used for performing the required application.
Clamping of wordpress using Actuator
● The linear actuator which produces push force using oil energy and it is used for clamping of work piece on the machine table.
● The heavy load can be lifted with the help of actuator movement. Hence it is an output member of hydraulic system.
● Actuators are the devices which converts fluid power into mechanical power.
Lifting of trolley/dumper
Need of the Actuators
(a) To produce push or pull force :
F = P.A
Where, F = Force in Newton
P = Pressure of oil
A = Area of piston
● When pressurised fluid enters from the rod side of piston, it produces pull force. The force produced is given by the formula :
F = P . (A - a)
Where, a = Area of piston rod
Push/pull force can be used for various applications like clamping, lifting, material handling, etc.
(b) To produce rotary motion (torque):
● It is useful for feed movement of machine table, winding/rewinding, etc.
Classification of Hydraulic/Pneumatic Actuator:
The chart shows classification of hydraulic/pneumatic actuators on the basis of:
(a) According to type of motion produced :
(i) Linear actuators: This type of actuator produce linear motion to and fro motion). It is known as hydraulic cylinder.
(ii) Rotary actuators: This type of actuator produces rotary motion (angular motion). It is known as hydraulic motors.
Linear actuators are further classified on the basis of:
(b) According to oil feed direction:
(i) Single acting cylinder: When oil is feed only from face side of the piston. It is called as single acting cylinder. Single acting cylinder has two types:
(a) Spring return single acting cylinder : When return movement of the actuator is performed by using spring force, it is called as spring return single acting cylinder.
(b) Gravity/External load return single acting cylinder : When single acting cylinder is mounted vertically, the return movement is carried out through the force of gravity.
(ii) Double acting cylinders: When cylinders are operated by feeding the oil to the cylinder in both directions i.e. face and rod side it is known as double acting cylinder.
They are further classified as:
(a) Double rod type : When piston rod is extended through piston on both side of the piston, it is called as double rod type double acting cylinder.
(b) Tandem type : When two cylinders are mounted in line (series) with common piston rod, it is called a tandem double acting cylinder.
(c) Ram type : When piston and piston rod of almost same size, the cylinder is known as ram cylinder.
(d) Telescopic : When number of cylinders are mounted in a short retracted envelope like a telescope lenses, it is known as telescopic cylinder.
Rotary Actuators:
Rotary actuators are classified as;
(a) As per direction of rotation :
(i) Uni-directional motor: When oil/air enters and rotates the motor in either direction (only one either clockwise or counter clockwise) it is known as uni-directional motor.
(ii) Bi-directional motor : When oil/air is feed alternately from inlets of motor to produce clockwise and counterwise motion, it is known as bi-directional motor.
(b) As per construction :
(i) Gear motors: When gear pair is used, it is known as gear motor.
(ii) Vane motors: When sliding vanes are used, it is known as vane motor.
(iii) Piston motor: When piston and cylinder are used,it is known as piston motor.
It has two types:
(A) Axial piston
(B) Radial piston
Other criterions for classification of actuators are :
(a) According to construction style :
(i) Tie rod cylinders : It consists of tie rods which are capable of taking impact loads.
(ii) Mill type cylinders : It is suitable for steel mills and metal processing industries where extra ruggedness is required.
(iii) One piece welded construction : The body is casted or the ends are welded to the tube. They are simple and inexpensive.
(iv) Threaded head cylinder: The ends are screwed to the tube by threading outside or inside.
(b) According to special applications :
(i) Plunger/ram cylinders
(ii) Telescopic cylinders
(iii) Cable cylinders
(iv) Diaphragm cylinders
(v) Bellows cylinders
(vi) Tandem cylinder
(vii) Duplex cylinder
(viii) Rotary cylinders, etc.
Linear Actuators (Hydraulic/Pneumatic Cylinders) :
Definition : Hydraulic/Pneumatic cylinder is a device which converts fluid power into linear mechanical force or motion to perform the task.
● It is most useful and effective in conversion of oil /air energy to produce output force in a linear direction which may be horizontal, vertical or at an angle.
● It is suitable for performing work such as pulling or pushing in machine tools, earth moving equipments, cranes, construction equipments and space application.
● In oil hydraulic system, they are used at high pressure up to 700 bars and produce large forces and precise movement.
● In pneumatics, actuators are designed to handle and operate at low pressure up to 10 Bar. Their strength is less as compared to hydraulic actuators.
Single Acting Cylinders:
Definition : When pressurised oil I compressed air acts only from face side of the piston and return stroke is completed by spring/external load, it is known as single acting cylinder.
Construction :
It consists of:
(a) Cylinder bore (tube) : It is hollow cylinder seamless tube chrome plated for corrosion and scratch resistance.
(b) End covers : The end covers are joined with the tube by screw fasteners, or welded.
(c) Piston : The piston is a sliding element closely fitted with the help of O-rings.
(d) Piston rod : Piston rod is attached to the piston at the centre and transmits movement of piston.
(e) Compression spring : It is provided to exert force during retracting of piston.
(f) Bush : It helps for smooth movement of the piston rod in cylinder bore.
Working :
● The pressurised oil/compressed air enters through inlet of the cylinder and exerts a force on the face of the piston.
● When sufficient oil force is developed to overcome spring force, it pushes the piston in the extended direction and completes the forward movement.
● When the oil supply is stopped / out of, the oil pressure drops at the face side and spring forces the oil through inlet (at this position -outlet) and brings the cylinder to the normal position.
● When it is mounted vertically with external load, there is no need to provide spring, the cylinder retract due to force of gravity. E.g. lifting of vehicle in automobile service station.
Applications :
1. It is used when push force is required e.g. clamping of work piece.
2. It is used when work is to be completed in forward direction only.
Advantages :
1. Simple in construction
2. Quick return due to spring action
Limitations :
1. It is applicable for handling load in one direction only.
2. Spring selection is critical to handle load and speed of actuator.
3. It cannot be used for longer stroke lengths due to problem related with spring.
Double Acting Cylinder
Definition: When pressurised oil compressed air acts on both side of piston ie. on face side for forward movement and piston rod side for return movement. It is known as double acting cylinder.
Construction :
It consists of:
(a) Cylinder body : It is seamless tube of Al alloy or steel with chrome plated to provide better corrosion and scratch resistance, stainless steel, cast iron also used for body.
(b) Piston and piston rod: The pistons are made of aluminium alloy castings. They are fitted precisely in the cylinder bore with the help of piston seals. Piston rod is attached to the piston and made of steel bar.
(c) End covers: The end covers are provided at the front end as well as at rear end. They can be joined with the help of screwed fasteners, welded or tie rod.
(d) Piston seals: It is provided for leak proof assembly between cylinder bore and piston.
(e) Bushing: Bush are provided for smooth movement of piston rod and for reducing leakage of oil. They are made of brass, bronze, Babbitt metal or split nylon and press fitted to the cap end.
Working:
● When pressurised oil/compressed air enters through inlet A from face side of piston, it pushes the piston to the right hand side to complete forward stroke while the oil on the rod side is delivered through the outlet to the tank through the outlet B.
● When pressurised oil/compressed air enters through inlet B from piston rod side. It moves the piston to the left hand side and completes the return stroke (retraction). The oil from the face side is drained out through outlet A to the tank.
● It gives to and fro (reciprocation) movement of piston.
Advantages :
1. The force is thus applied in both directions
2. The structure of this hydraulic cylinder is very simple
Limitations :
1. Costly compared to S.A Cylinder
2. Movement depends upon two inlets/outlets
Applications :
1. When work is to be completed in both the directions. It can be used for push and pull forces.
2. It is used in machine tools like shaper machine,table movement of milling and grinding machine, hydraulic press machines, etc.
3. It is used in material handling equipments, construction equipments, etc.
Cushioning of Double Acting Cylinders:
● Cushioning of cylinders means gradual deceleration of the piston in the end of its stroke. It means slowing down the speed of the piston in the end of cylinder body.
● It is very helpful to reduce shock or impact of load on the cylinder end covers especially when a heavy load is connected to the rod or the cylinder and is working at very high speed.
● The cushion assembly is around 25 mm long for a standard cylinder. It consists of a small passage to allow entrapped oil to the port with a cushion needle with a check valve to allow free flow of oil during reverse flow of oil during reverse start of piston travel.
● The end of the cushion nose is tapered in order to enter more easily into the cushion chamber.
● The fluid is normally leaves through the outlet port directly, but when cushioning boss enters the recess,the fluid around the piston is trapped.
● The only way the fluid can escape is through the secondary path which is restricted by needle valve.
● The needle valve is adjusted so that the piston is slowed up over the last part of its stroke by a pressure build up in the fluid escaping past the needle valve.
Double Rod Double Acting Cylinder:
Definition : In double acting cylinder, when piston rod is provided on both side of the piston, it is called as double piston rod double acting cylinder.
Construction :
● It consists of cylinder body with end covers; the piston is fitted with piston seals.
● The piston rod is extended and provided on both side of the piston. Hence area on both side of the piston is same (which is different in previous double acting cylinder).
Working :
● When pressurised oil/compressed air enters through inlet port A, it pushes the piston to complete forward stroke by draining the oil from the other port B.
● When pressurised oil/compressed air enters through inlet port B, it moves the piston to complete return stroke by draining the oil from the other port A.
● As the area on both side of piston is same, it produces equal forces in both directions.
● The area on both side is,
a = π /4 ( D - d )2
D = Diameter of cylinder bore
d = Diameter of piston rod
● The maximum force of this cylinder for a given cylinder size is smaller than the single rod type.
Advantages :
1. The force is equal in both directions
2. The velocity is same in both strokes.
Limitations :
1. Costly compared to conventional D.A Cylinder
2. Piston rod area decreases the oil flow area in cylinder
Applications:
1. It is used when equal force and speed is needed in both the directions.
2. It is useful in robotic mechanisms where the rod is clamped at both ends and the body moves.
3. It is used in table movement of grinding machine.
Tandem Cylinder :
Definition: When two double acting cylinders are mounted in line with the pistons connected to a common piston rod, it is known as tandem cylinder.
Construction:
● It consists of two double acting cylinders joined with the common partition.
● The pistons are connected to a common piston rod. The four ports are provided which includes two ports of each double acting cylinder.
● It has more linear dimension compared to simple double acting cylinder. Hence occupy more linear space.
● Modification of tandem cylinder is the duplex cylinder.
Working :
● When pressurised oil/compressed air enters in the cylinder 1, it produces force F1 and when enters in the cylinder 2 at the same time produces F2.
● Hence at the piston rod, we can get double force (F1 +F2) during movement of pistons.
● Similarly for the return movement of the piston, we can get double force.
Beneficial Property:
● It will produce double force of two cylinders during movement.
● Less axial dimensions due to common wall between two cylinders.
● They are designed for applications where high force must be generated within a narrow/less radial space where axial length is easily available.
Applications :
1. They are widely used in various applications which have two or more independent systems like power operated flight control system in naval aircraft.
2. Work holder devices and lathe equipments.
3. Large press cylinders.
Telescopic Cylinder
Definition : When number of cylinders are arranged in a compact envelope similar to telescope lenses, it is known as telescopic cylinder.
Construction :
● It consists of number of cylinders, with decreasing in size. The main cylinder accommodates the number of cylinders arranged one inside the other.
● The inlet is provided for main cylinder to enter the pressurised oil. The corresponding inlets are also provided to the each cylinder.
● The cylinders are arranged in a short envelope hence space required is less.
Working :
● When pressurised oil enters in the main cylinder 1 it will push the cylinder 2 in the upward direction when sufficient oil pressure is generated.
● Now the cylinder 2 is completely lifted and oil will enter into the inlet of cylinder 2.
● The oil will flow in the cylinder 2 and push the piston in upward direction. It will raise the piston for stroke length of cylinder 2.
● In this manner, the total length of stroke through which the load is lifted is (L1 + L2).
Applications:
1. Is suitable where less space is available for mounting of actuator.
2. Used in equipments like tilting of truck dump bodies, fork lift trucks, hydraulic cranes.
3. Tipping platform, lifting platforms commercial vehicle applications.
Ram Cylinder (Plunger Cylinder)
Definition: When piston is larger and almost equal to the piston rod, it is known as ram cylinder.
Construction:
● It consists of cylinder in which ram or plunger is mounted. The plunger is a large size piston with d = l , (d → diameter of piston and l → length of piston rod). Hence piston and piston rod are same as a single element.
● The platform is provided at the top of the ram for placing the heavy loads conveniently.
● The single inlet is provided in the cylinder for oil feed. The ram is always mounted vertically. It is single acting type.
Working :
● When pressurised oil enters through inlet, it will lifts the ram in the upward direction. The load is lifted to the required height.
● When oil supply is cut off to lower the load, the ram comes downward due to gravity force and oil is drained to the tank through the outlet port.
Applications:
1. Commonly used as a lift in automobile service stations.
2. Material handling equipments.
Specifications hydraulic Cylinders:
(a) Pressure rating in Pascal.
(b) Length of stroke in meters.
(c) Diameter of the cylinder bore.
(d) Diameter of piston rod.
Cylinder Mountings
● The hydraulic cylinders are used for variety of applications and it is required to mount or install the cylinder conveniently considering function and maintenance activity.
● Mountings may be necessary at one end or both ends and it may not be same at both ends.
● Mounting styles affects the design life and frequency of maintenance of the hydraulic installation.
Types of Cylinder Mountings
(a) Centre line mounting
(b) Foot mounting
(c) Pivot mounting
(i) Clevis mounting
(ii) Trunnion mounting
(a) Centre line mounting:
● The best way to support a cylinder is along its centre line. It uses rectangular flange or square flange connected to cap end or head end.
● Centre fine mounting in which lugs are attached to the sides of both ends of the cylinder centre line. The flanges are attached at both the ends.
● Tie rod mountings are also used but they are less stronger than the flange mountings.
(b) Foot mounting :
● Foot mounting consists of mounting the cylinder with the help of side end lugs or side covers.
● These mountings are used where cylinders are to be mounted on to surfaces parallel to the axis of cylinder.
(c) Pivot mountings:
● In pivot mounting the cylinder can be able to rotate freely while it reciprocates.
● The pivot centre line normally intersects the cylinder centre line.
● There are two methods of pivot mounting:
(a) Clevis mounting
(b) Trunnion mounting
● This mounting gives rotational freedom in one plane. If freedom is required in all planes, universal joints should be used.
(i) Clevis mounting:
● The clevis is a part of cap end and can be either fixed or universal
● Clevis mounting in which the cap end is pivoted so that it can rotate the cylinder end during to and fro movement of the cylinder.
● While the rod end is attached to the links with the help of pin joint. One link is pivoted and other link can slide to apply the force.
(ii) Trunnion mounting:
Trunnion mounting has three types:
(a) Trunnion mounting at head end.
(b) Trunnion mounting at cap end.
(c) Trunnion mounting at intermediate.
● Trunnion is a type of rotary slider which allows rotary movement.
