I) Clamping the work piece II) Drilling the work piece. III) Unclamping the work piece. 10

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1 Seventh Semester B.E. III IA Test, 2014 USN 1 P E M E PES INSTITUTE OF TECHNOLOGY (Bangalore South Campus) (Hosur Road, 1KM before Electronic City, Bangalore ) Department of Mechanical Engineering Date: Max Marks: 50 Subject & Code: Hydraulics & pneumatics (10ME76) Time: 90 Min. Name of faculty: JagadeeshBagali Note: 1.Answer ANY FOUR FULL Questions from PART- A. 2. PART- B is Compulsory. PART-A 1. Design and explain the hydraulic power circuit, for the sequencing of the following operations in a drilling M/c. I) Clamping the work piece II) Drilling the work piece. III) Unclamping the work piece. 10 Clamp and work circuits use at least two cylinders, hydraulic. One cylinder advances first and clamps the work piece in a fixture. Then the other cylinder(s) moves in with tools for cutting, drilling, pressing, forming, or other operations on the work piece.sequence valves provide the most positive means for delaying advance of the work cylinder until the clamp has reached nearly full clamping force. The circuit should be arranged so both cylinders are controlled from a single 4-way valve by the operator. Starting of the work cylinder should be initiated by the rise of clamping pressure and not require extra attention by the operator. Sequence Valves A sequence valve is different from a relief valve in that the spring chamber is vented directly to atmosphere instead of to the outlet port on the valve. This allows full pressure to appear on both cylinders when they stall, instead of the pressure having to divide between them.

2 Figure 1. Simple Sequencing of Small Cylinders Figure 1. This illustrates the basic principle of sequencing. It is a good working circuit if an air sequence valve can be purchased. Sequencing is in the forward direction of both cylinders - they retract randomly. When the operator shifts and holds the manual or foot valve, 1, the single-acting clamp cylinder immediately extends and stalls at its forward position. This allows air pressure to build up to the opening pressure set on the sequence valve, and air can flow to the work cylinder. The work cylinder then advances to perform its work. When the operator releases Valve 1, both cylinders retract.

3 2. a) Discuss in detail the application of hydraulic accumulators for protection against shocks. 05 Accumulator as a hydraulic shock absorber: One of the important applications of accumulators is the elimination of hydraulic shock. The schematic diagram is shown in Fig. Hydraulic shock is caused by the sudden stoppage or declaration of a hydraulic fluid flowing at relatively high velocity in a pipe line. Rapidly closing a valve creates a compression wave. This compression wave travels at the speed of sound upstream to the end of the pipe and back again to the closed valve, which causes an increase in pressure. The resulting rapid pressure pulsations or high-pressure surges may cause damage to the hydraulic system components. If an accumulation is installed near the rapidly closing valve, the pressure pulsations or high-pressure surges are suppressed. b) Explain the construction and operation of bladder-type accumulators. 05 (c) Bladder accumulator: It functions in the same way as the other two accumulators. Schematic diagram of bladder accumulator is shown in Fig Here the gas and the hydraulic fluid are separated by a synthetic rubber bladder. The bladder is filled with nitrogen until the designed pre-charge pressure is achieved. The hydraulic fluid is then pumped into the accumulator, thereby compressing the gas and increasing the pressure in the accumulator. The port cover is a small piece of metal that protects the bladder from damage as it expands and contacts the fluid port.. 3. a) Mention the merits and demerits of pneumatics over hydraulics. 05 MERITS Simplicity of design and control Machines are easily designed using standard cylinders and other components, and operate via simple on-off control.

4 Reliability Pneumatic systems generally have long operating lives and require little maintenance. Because gas is compressible. Equipment is less subject to shock damage. Gas absorbs excessive force, whereas fluid in hydraulics directly transfers force. Compressed gas can be stored, so machines still run for a while if electrical power is lost. Safety There is a very low chance of fire compared to hydraulic oil. Machines are usually overload safe. DEMERITS When precision matters PS lags behind as compared to HS since air is compressible it is not possible to precisely control actuators velocities or control position. PS are mostly used for low power systems. Since air contains 20% of oxygen, hence not a good lubricant, therefore a fine mist of oil is injected into air which reduces wear and tear of closely fitting moving parts. Another major problem in PS working medium is moisture in air which again increases wear and tear therefore air dryers are used. Since PS exhaust directly into the atmosphere preventive steps should be taken to see that the exhaust gases should not harm the environment. b) Give complete classification of pneumatic actuators. 05 Pneumatic Actuators An output device used for the conversion of supply energy into useful work Pneumatic actuators convert the air pressure into linear or rotary motion depending upon their design. Classified into two types Linear motion actuators SINGLE Acting CYLINDER Double Acting Pneumatic Cylinder Rod less cylinders These are the cylinders without any rod extending from them, it has a barrel with a rod less piston. Used when there is no enough space for the rod extension or when the stroke length required is to high. Rotary motion Rotary Cylinder Type Actuator Rotary Cylinders Pneumatic devices that produce rotary motion 360 degrees Vane type Semi rotary actuator Rotation is limited to less than 360 deg usually 270 deg Rack and Pinion actuator Air motors are pneumatic actuators that are designed for continues rotation vane and turbine type air motors are widely used.ex:vane type air motor, piston type air motor.

5 4. Explain with neat sketch, end cushioning in double acting cylinder. 10 Cylinder Cushions For the prevention of shock due to stopping loads at the end of the piston stroke, cushion devices are used. Cushions may be applied at either end or both ends. They operate on the principle that as the cylinder piston approaches the end of stroke, an exhaust fluid is forced to go through an adjustable needle valve that is set to control the escaping fluid at a given rate. This allows the deceleration characteristics to be adjusted for different loads. When the cylinder piston is actuated, the fluid enters the cylinder port and flows through the little check valve so that the entire piston area can be utilized to produce force and motion. A typical cushioning arrangement is shown in Fig During deceleration, extremely high pressure may develop within a cylinder cushion. The action of the cushioning device is to set up a back-pressure to decelerate the load. Ideally, the back-pressure is constant over the entire cushioning length to give a progressive load deceleration. In practice, cushion pressure is the highest at the moment when the piston rod enters the cushion(fig ).Some manufacturers have improved the performance of their cushioning devices by using a tapered or a stepped cushion spear. Wherever high inertia loads are encountered, the cylinder internal cushions may be inadequate but it is possible for the load to be retarded by switching in external flow controls. Deceleration can then take place over a greater part of the actuator stroke. 5. What do you mean by quick exhaust valve? Explain its working principle with neat sketch. 10

6 Quick Exhaust Valve In many applications especially with single acting cylinders, it is a common practice to increase the piston speed during retraction of the cylinder to save the cycle time. This is carried out by incorporating a Quick exhaust valve. The Quick exhaust valve has essentially three ports Supply port 1, is connected to the out put of the final control element (Directional control valve). The Output port, 2 of this valve is directly fitted on to the working port of cylinder. The exhaust port, 3 is left open to the atmosphere. Forward Motion During forward movement of piston, compressed air is directly admitted behind the piston through ports 1 and 2 Port 3 is closed due to the supply pressure acting on the diaphragm. Port 3 is usually provided with a silencer to minimize the noise due to exhaust. Return Motion During return movement of piston, exhaust air from cylinder is directly exhausted to atmosphere through opening 3 (usually larger and fitted with silencer).port 2 is sealed by the diaphragm. Thus exhaust air is not required to pass through long and narrow passages in the working line and final control valve Explain with a pneumatic Circuit, the control of extension of a double acting cylinder using OR and AND logicgates.

7 Billiards balls are distributed from a gravity magazine via distributor shafts by two packing stations for individual packing. The signal for the return stroke must be capable of being given by the machine operator by means of either a manual push button or a foot-operated valve. The advance stroke of the piston is triggered by the piston rod when the rearmost end of position is reached. The piston must execute a return stroke only when balls are present in the gravity magazine a) write short notes on: I) Time dependent control and pressure dependent control. PRESSURE DEPENDENT CONTROL Pressure Dependent Valves The following Pressure Dependent Controls are often used in Pneumatic applications Pressure Sequence Valve Pressure Relief Valve Pressure Regulator Pressure Sequence Valves Pressure Sequence valve is essentially a switch on or off valve. Sequence Valve generates a pneumatic signal if the sensing pressure [signal input] is more than the desired set pressure. This generated out put signal is used to control the movement of cylinder by using it as a set signal or reset signal to the final control valve to obtain forward or return motion respectively. Sensing pressure signal is introduced at port 12 Manual adjustment of pressure setting is done with the help of a cap screw/knobwhich is spring loaded. Clock wise rotation of knob results setting for higher pressuresetting and anticlockwise rotation of knob results in lower pressure setting. The right section is basically a 3/2 directional control valve [NC] - pilot operatedusing pressure signal derived from left section. Figure Pneumatic Timers are used to create time delay of signals in pilot operated circuits. Available as Normally Closed Timers and Normally Open Timers. Usually Pneumatic timers are On Delay Timers Delay of signals is very commonly experienced in applications such as Bonding of two pieces. Normally Open Pneumatic Timer are also used in signal elimination Normally Open Pneumatic Timers are used as safety device in Two Hand Blocks

8 TIMEDEPENDENT CONTROL Pneumatic Timers are used to create time delay of signals in pilot operated circuits. Available as Normally Closed Timers and Normally Open Timers. Usually Pneumatic timers are On Delay Timers Delay of signals is very commonly experienced in applications such as Bonding of two pieces. Normally Open Pneumatic Timer are also used in signal elimination Normally Open Pneumatic Timers are used as safety device in Two Hand Blocks. A Pneumatic Timer is a combination valve which consists of three parts 1. 3/2 way pilot operated directional control valve [NC or NO], 2. A one way flow control valve and 3. An accumulator Signal input is supplied at port 1 and delayed signal out put is taken at 2. A signal source is connected at port 1 TIME DELAY VALVE A signal is restricted to slow the rate of pressure build up on a pressure switch (3/2 differential pressure operated valve). When the pressure switch operates a strong un-restricted output is given A reservoir provides capacitance to allow less fine and sensitive settings on the flow regulator making it easy to adjust. II) Meter-in and Meter-out circuits. Supply Air Throttling(Meter-incircuit) Supply air entering the cylinder through either of the working ports, undergoesthrottling as the non return valve is closed in the direction of flow. During exhaust, the compressed air leaving the cylinder is by passed through the non return valve and escapes freely as it does not undergo throttling. Supply air throttling is used for single acting cylinder and small volume cylinder Exhaust Air Throttling(Meter-out circuit) Supply air flows freely to the cylinder through the bypass passage of the non return valve. The supply air does not under go any throttling Exhaust air leaving the cylinder has to under go throttling as the non return valve is closed in the return direction The piston is loaded between two cushions of air Exhaust throttling should always be used for double acting cylinder Not suitable for small volume cylinders and cylinders with short strokes as

9 effective pressure cannot build up sufficiently. 10 PART-B 8. Explain the principle of cascade method with a suitable sequence example with step displacement And step time diagrams. 20 In order to develop control circuitry for multi cylinder applications, as done before in classic method, it is necessary to draw the motion diagram to understand the sequence of actuation of various signal input switcheslimit switches and sensors. Motion diagram represents status of cylinder position -whether extended or retracted in a particular step. CASCADE METHOD A Bi-stable memory valve or reversing valve can be used to eliminate signal conflicts. Signal conflict is avoided by allowing the signal to be effective only at times when they are needed. Step 1: Write the statement of the problem: First cylinder A extends and brings under stamping station where cylinder B is located. Cylinder B then extends and stamps the job. Cylinder A can return back only cylinder B has retracted fully. Step 2: Draw the positional layout.

10 Step3: Represent the control task using notational form Cylinder Aadvancing step is designated as A+ Cylinder Aretracting step is designated as A- Cylinder B advancing step is designated as B+ Cylinder B retracting step is designated as B- Given sequence for clamping and stamping is A+B+B-A- Step 4 Draw the Displacement step diagram Step 5 Draw the Displacement time diagram

11 Step 6: Analyse and Draw Pneumatic circuit. Step 6.1 Analyse input and output signals. Input Signals Cylinder A Limit switch at home position ao Limit switch at home position a1 Cylinder B - Limit switch at home position bo Limit switch at home position b1 Output Signal Forward motion of cylinder A ( A+) Return motion of cylinder A (A-) Forward motion of cylinder B ( B+) Return motion of cylinder B( B+) Step 6.2 Using the displacement time/step diagram link input signal and output signal. (Figure 1.31) Usually start signal is also required along with a0 signal for obtaining A+ motion. 1. A+ action generates sensor signal a1, which is used for B+ motion 2. B+ action generates sensor signal b1, which is used group changing. 3. B- action generates sensor signal b0, which is used for A- motion 4. A- action generates sensor signal a0, which is used for group changing Above information (given in 6.2) is shown below graphically Step 7 Draw the power circuit (Figure 1.32) i) Divide the given circuits into groups. Grouping should be done such that there is no signal conflict. Do not put A+ and A- in the same group. Similarly B+ and B- should not be put in the same group. In other word A+ and A- should belong to different group to avoid signal conflict. In our example of A+ B+ B- A- we can group as ii) Choose the number of group changing valve = no of groups -1 In our example, we have 2 groups so we need one group changing valve Connect the group changing valve as follows. From the figure it is clear that when the control signals I and II are applied to group changing valve, the air (power) supply changes from Group 1(G1) to Group 2 (G2)

12 iii) Arrange the limit switch and start button as given below (Figure 1.33) iv) Draw the power circuit (Figure )

13 Step 9 Analysis of pneumatic circuit 1. Assume that air is available in the line G2 to start with. (Say from last operation) 2. When the start button is pressed, Air supply from Group G2 is directed to line 2 through actuated limit switch a0. Now the air available in line 2, actuates the Group changing valve (GCV) to switch over to position I. This switching of the GCV causes air supply to change from G2 to G1. 3. Now the air is available in line G1. The air supply from group G1 is directed to port 14 of the valve 1.1. As there is no possibility of signal conflict here, valve 1.1 switches over causing the A+ action. 4. Sensor a1 is actuated as the result of A+ action, allowing the air supply from the Group G1 to reach to line 1 through a1. Now the air available reaches port 14 of valve 2.1. As there is no possibility of signal conflict here, valve 2.1 switches over, causing B+ action automatically. 5. Sensor b1 is actuated as result of B+ action, allowing the air supply in line 3. Air from line 3 allows the air to reach port 12 of Group changing valve (also called reversing valve). As a result, the Group changing valve switches over, causing the group supply to change from G1 to G2. 6. Now the air is available in G2. Air from G2 acts on port 12 of the Valve 2.1. As there is no possibility of signal conflict here, valve 2.1 switches over, causing B- action automatically. 7. Sensor is actuated as the result of B- action. Now the air is available in line 4.Air from line 4 reach port 12 of the valve 1.1, As there is no possibility of signal conflict here, valve 2.1 switches over, causing A- action automatically. The cascade system provides a straightforward method of designing any sequential circuit. Following are the important points to note: a) Present the system must be set to the last group for start-up b) Pressure drop- Because the air supply is cascaded, a large circuit can suffer from more pressure drop. c) Cost Costly due to additional reversing valves and other hardware.

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