TECHNOLOGICAL STUDIES. Intermediate 2. Mechanical Systems. Section 1. Outcome 1 Pneumatic Systems

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1 TECHNOLOGICAL STUDIES Intermediate 2 Mechanical Systems Section 1 Outcome 1 Pneumatic Systems 1

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3 OUTCOME 1 Outcome 1 Design, construct and evaluate pneumatic systems When the students have completed this unit of work they should be able to: Interpret pneumatic system and circuit diagrams. Describe the operation of pneumatic systems Pipe up/construct pneumatic systems. Have an appreciation of safety requirements when operating pneumatic systems. Perform calculations to determine cylinder pressure, piston force and area. Evaluate pneumatic systems. 3

4 INTRODUCTION Pneumatics is all about using compressed air to do work. Compressed air is simply the air we breathe forced into a small space. Pneumatic systems are more common than you would think and are used extensively in industry, for example to move objects off a conveyor or hold/clamp parts of an assembly together. In fact, pneumatics is used quite a lot every day. CAR TYRE BUS DOORS MS.Int 2.O1 fig 1 MOVING PARTS IN A PROCESS You will all have used compressed air when you have blown up a football or the tyre on a bicycle. To understand how pneumatics uses compressed air to do things we simply need to think of blowing up a balloon. It takes a lot of air to fully inflate the balloon and to stretch the balloon to its full size. If we inflate the balloon too much, the air will force the balloon to burst. If you release the balloon, the air will begin to escape and will propel the balloon around the room until all the air has escaped. All of this happens because you have squashed or compressed a lot of air inside the balloon and stored up energy. MS.Int 2.O1 fig 2 Most pneumatic systems rely on a constant supply of compressed air to make them work. This is provided by a compressor, which is an air pump driven by a motor or engine. The compressor sucks in air and stores it in a tank called a receiver. This compressed air is then supplied to the system through a series of pipes. 4

5 Health and Safety Pneumatic devices and systems are safe, clean and reliable but we must learn to use them properly to avoid accidents. When using pneumatic equipment, we must always follow these safety rules: 1. Never blow compressed air at anyone, not even yourself. 2. Never let compressed air come into contact with your skin, as this can be very dangerous. 3. Always wear safety goggles when you are connecting and operating circuits. 4. Check that all air lines are connected before turning on the main air supply. 5. Always turn off the main air supply before changing a circuit. 6. Keep your hands away from moving parts. 5

6 CYLINDERS Pneumatic equipment can be split up into two basic categories of cylinders and valves. Cylinders are the muscles of pneumatic systems as they are used to move, hold and lift objects. They can even be used to operate other pneumatic components. Cylinders are operated by compressed air and they covert the stored energy into linear motion. There are two types of cylinder that we will be using: a single acting cylinder and a double acting cylinder. Single Acting Cylinder A single acting cylinder requires only one air supply. If we supply compressed air to a single acting cylinder, the air pushes against a piston inside the cylinder and causes it to outstroke. When the piston has fully outstroked it is said to be positive. If we stop the supply of air then a spring inside the cylinder causes the piston to instroke to its starting position and the piston is said to be negative. The symbol for a single acting cylinder is shown below. MS.Int 2.O1 fig 3 Single acting cylinders are used in situations where their simple design and small size are an advantage. They produce small outstroke forces and their use is limited to operations that only require use of the outstroke. This is because the spring produces only enough force to instroke the piston. Double Acting Cylinder A double acting cylinder requires two air supplies, one to outstroke the piston and the other to instroke the piston. Compressed air is applied to one side of the piston then other to make it move positive and then negative. The symbol for a double acting cylinder is shown below. MS.Int 2.O1 fig 4 Double acting cylinders are used more often in pneumatic systems than single acting cylinders. They are able to produce greater forces and we can take advantage of both the outstroke and instroke forces to lift and move objects. 6

7 VALVES Valves provide the necessary control of the compressed air being supplied to the cylinder. They can turn the air on or off, change the direction that the air is flowing or even slow down the airflow. The most common type of valve is the 3/2 valve. 3/2 Valve A 3/2 valve is so called because it has 3 ports or connections and two states of operation. The ports are always numbered in the same way. Port 1 main air supply connection Port 2 output connection to other components Port 3 exhaust The valve has two states of operation. One state prevents air from being supplied to other components and the other allows the air to flow freely. State 1 Off/Unactuated State In this state, the main airflow through the valve is blocked and so air is unable to reach other components such as cylinders. However, any air within the cylinder is able to exhaust through the valve and this will allow the cylinder to return to its original position. Study the symbol below and ensure that you understand how the air flows through the valve. MS.Int 2.O1 fig State 2 On/Actuated State In this state, the main air supply is able to flow freely through the valve and supply components such as cylinders with air. Study the symbol below and ensure that you understand how the air flows through the valve. MS.Int 2.O1 fig The complete symbol for a 3/2 valve combines both states and is usually drawn in the off or unactuated state. The complete symbol is shown below. MS.Int 2.O1 fig 7 7

8 ACTUATORS There are a number of different ways that we can change the state of the 3/2 valve. The most common way is by using a push button. A push button is useful for quick and easy starting of a sequence or as a safety circuit breaker. By pressing the button, the valve changes to the actuated state and allows main air to flow through to other components. If we release the button, a spring inside returns the valve to its off state. The symbol for a push button, spring return 3/2 valve is shown below. MS.Int 2.O1 fig 8 Below is a list of the most common types of actuators. They are always drawn onto the standard symbol for the 3/2 valve. PLUNGER ROLLER TRIP PUSH BUTTON SOLENOID LEVER PILOT AIR ROLLER DIAPHRAGM SPRING MS.Int 2.O1 fig 9 Not all of these actuators are intended to be used by a human operator. For example, an outstroking piston from a single or double acting cylinder actuates the roller trip. The solenoid uses an electrical pulse to change the state of the valve and this allows us to use combined electrical and pneumatic systems. 8

9 SIMPLE CIRCUITS A bottling plant uses a single acting cylinder to press lids onto bottles. The cylinder is controlled by a push button, spring return 3/2 valve. An operator presses the button when the bottle is in the correct position. MS.Int 2.O1 fig 10 The pneumatic circuit is shown below. MS.Int 2.O1 fig 11 When the button is pressed, the valve changes state and supplies air to the single acting cylinder. This causes the piston to outstroke with enough force to press the lid onto the bottle. When the button is released, the valve returns to its original state and the piston is able to instroke ready for the process to begin again. 9

10 Assignment 1 1. Build the circuit shown above for the bottling plant. a. Press the button on the valve and keep it pressed. Explain what happens. b. Release the button and explain what happens. c. Using the correct terminology, explain how the circuit operates to seal the bottles with the lids. 10

11 In a section of a recycling plant, metal cans are dipped in a chemical solution to remove traces of dirt, oil and paint. A double acting cylinder is used to raise and lower a basket containing the cans. MS.Int 2.O1 fig 12 The pneumatic circuit is shown below. Valve A Valve B MS.Int 2.O1 fig 13 When the operator presses the button on valve A, air is supplied to the double acting cylinder which outstrokes and lowers the basket into the chemical solution. When the button on valve B is pressed, air is supplied to the other side of the piston which instrokes and raises the basket. Any air in the cylinder at this time is able to exhaust through port 3 of valve A. 11

12 Assignment 2 1. Build the circuit shown above for raising and lowering the basket. a. Press the button on valve A to outstroke the cylinder. Does the piston instroke when you release the button? b. Press the button on valve B. What happens to the cylinder? c. What happens when you press both buttons at the same time? d. Leave both valves unactuated and try moving the piston. Why is it so easy to move? e. Using the correct terminology, explain how the circuit operates to raise and lower the basket. 12

13 5/2 VALVES The problem with controlling a double acting cylinder with two 3/2 valves is that when neither valve is actuated, no air is being supplied to the cylinder to hold the piston in place. Any air within the cylinder is able to exhaust through port 3 on each valve. This means that any force or effort placed on the piston will make it move easily. A further disadvantage is that the 3/2 valve needs to be actuated until the double acting cylinder has fully outstroked or instroked. Releasing the valve before the stroke is complete will result in the piston stopping short of its final position. We have greater control over a double acting cylinder if we control its outstroke and instroke using a 5/2 valve. This valve has 5 ports and two states of operation. The ports are always numbered in the same way. Port 1 main air supply connection Port 2 output connection to cylinder Port 3 exhaust Port 4 output connection to cylinder Port 5 exhaust The valve has two states of operation. One state supplies air to outstroke a double acting cylinder and the other state will cause it to instroke. State 1 - Instroke In this state, the main air flows through the valve from port 1 to port 2. Any air within the cylinder is able to exhaust through the valve from port 4 to port 5. In this state, a 5/2 valve will cause a double acting cylinder to instroke or hold the piston in the negative position as air is always being supplied to the cylinder. Study the symbol below and ensure that you understand how the air flows through the valve. AIR FROM CYLINDER 4 AIR TO CYLINDER 2 MS.Int 2.O1 fig

14 State 2 Outstroke In this state, the main air flows through the valve from port 1 to port 4. Any air on the other side of the piston is able to exhaust through the valve from port 2 to port 3. In this state, a 5/2 valve will cause a double acting cylinder to outstroke and hold it in the positive position. Study the symbol below and ensure that you understand how the air flows through the valve. AIR TO CYLINDER 4 2 AIR FROM CYLINDER MS.Int 2.O1 fig The complete symbol for a 5/2 valve is shown. The cylinder, main air and exhaust lines are only ever drawn on one half of the symbol depending on which part is active in the circuit. It is usual, however to draw the symbol in such a way as it would hold a double acting cylinder in the negative position. MS.Int 2.O1 fig 16 5/2 valves can be operated or actuated in the same way as 3/2 valves however, the most common way of actuating a 5/2 valve is pilot air. A pilot air 5/2 valve will change state when a brief air signal acts at either end of the valve. This signal is most often supplied from a 3/2 valve. In the example shown below, the button on Valve A only needs to be pressed momentarily in order to change the state of the 5/2 valve. The 5/2 valve will supply the double acting cylinder with air in order for it to outstroke. 14

15 Valve A MS.Int 2.O1 fig 17 Notice that the pilot air lines to the 5/2 valve are drawn as broken or dashed lines to distinguish them from the other air lines in the circuit. 15

16 Assignment 3 1. A hopper door is connected to the end of a double acting cylinder. Sand is fed into the hopper from above and when the hopper is full, the door of the hopper must open to allow the sand to fall through onto a wagon. The hopper door then closes and the process is able to begin again. Valve A Valve B MS.Int 2.O1 fig 18 a. Build and test the circuit shown above for opening and closing the hopper door. b. Does pressing Valve A or Valve B close the hopper door? c. Using the correct terminology, explain how the circuit operates. 16

17 2. The door in a secured entry system is controlled by pneumatics. A simplified circuit diagram is shown below with some of the piping missing. C SLIDER TRACK A CRANK CYLINDER B MS.Int 2.O1 fig 19 a. Name each of the components A, B and C used in the pneumatic circuit shown above. b. Complete the missing piping in the circuit diagram. c. Using appropriate terminology, describe how the systems works from when Valve A is actuated. 17

18 3. A barrier to a company car park is controlled by a security guard. The guard carefully checks all cars entering the premises. The barrier is to be raised and lowered by a double acting cylinder. MS.Int 2.O1 fig 20 a. Design a pneumatic system that will solve this problem using equipment that you have available. b. Pipe up and test your solution. c. Produce a list of all the components used in your solution using their correct names. d. Using the correct terminology, describe how your solution raises and lowers the barrier. e. Why is a double acting cylinder used instead of a single acting cylinder in these circumstances? 18

19 FLOW CONTROL VALVES You will have noticed in all the circuits you have built so far, that the cylinders operate very quickly. Sometimes this is exactly what we want but very often it would be dangerous or impractical for the cylinder to work this fast. For example, think back to the exercise where a basket was lowered into a chemical solution. If this happened too fast then the chemical would splash out of the container. In this case, we would want the cylinder to raise and lower the basket slowly. We can slow down the airflow to a cylinder by using flow control valves. The most common type is the unidirectional restrictor. This type of valve slows down the airflow in one direction only. The symbol is shown below. MS.Int 2.O1 fig 21 When air flows into the restrictor in the direction shown in figure 22, some of the air takes the bypass route and forces the ball to seal the path. The air is then forced to pass through the restriction and the flow is slowed down. We can alter the flow by adjusting the small screw on the top of the restrictor. RESTRICTION AIR FLOW MS.Int 2.O1 fig 22 BYPASS When air flows in the opposite direction into the restrictor, most of the air takes the bypass route and forces the ball to open up the path. This allows the air to flow freely and unrestricted. AIR FLOW MS.Int 2.O1 fig 23 If we want to restrict the stroke of a cylinder, it is normal practice to restrict the exhaust air coming from the cylinder rather than the main air being supplied to the cylinder. This provides a much smoother motion in the piston. 19

20 Remember our example with the hopper door. It would be much more practical for the door to open slowly, as this would allow the sand to fall more gently into the wagon below. We would still want the door to close quickly so that the process could begin again as soon as possible. Study the new circuit diagram below. Valve A Valve B MS.Int 2.O1 fig 24 To open the door of the hopper slowly, we need to control the speed of the instroke. When Valve B is pressed, the pilot air 5/2 valve changes state and supplies the air to instroke the double acting cylinder. However, air is trapped behind the piston and this must escape or exhaust in order for the piston to move. The restrictor is connected in such a way that the exhausting air must pass through the restriction. This slows down the exhaust air and so slows down the speed that the cylinder can instroke. 20

21 Assignment 4 1. Build and test the pneumatic circuit shown above. a. Using appropriate terminology, explain how the circuit works. b. Explain why the exhaust air has been restricted in order to slow down the instroke. 2. For safety reasons, the entrance door to a storeroom in a warehouse must open and close slowly. A double acting cylinder is used to slide the door. A simplified circuit diagram is shown below. MS.Int 2.O1 fig 25 MS.Int 2.O1 fig 26 Valve A Valve B a. Build and test the pneumatic circuit shown above. b. Explain why two restrictors are needed in this circuit. c. Where would you place the push button, spring return 3/2 valves in relation to the warehouse door? 21

22 Sometimes we might want to control the instroke and outstroke of a single acting cylinder. We can control the instroke in the same way as a double acting cylinder, by restricting the exhaust air. However, when a single acting cylinder outstrokes the piston pushes against a spring and there is no exhaust air to control. In this case, we use two back to back restrictors. The spring prevents the jerky action of the piston that occurs when this arrangement is used with a double acting cylinder. MS.Int 2.O1 fig 27 A B A simpler device for controlling the speed of the outstroke and instroke of a single acting cylinder is a bi-directional restrictor. This restrictor has just one route for the air and so the same rate of flow is achieved in both directions. A small screw on the top of the restrictor adjusts the flow rate Although this device is more economical than using two unidirectional restrictors, it means that the instroke and outstroke speeds will be the same. If used with a double acting cylinder then a jerky motion is produced rather than a smooth controlled one. The symbol for a bi-directional restrictor is included in figure 28 below. MS.Int 2.O1 fig 28 22

23 Assignment 5 1 a. In figure 27, which unidirectional restrictor controls the speed of the outstroke? b. What is the purpose of the other restrictor in this circuit? 2. Describe some of the disadvantages of using a bi-directional restrictor to control the speed of a double acting cylinder. 3. Part of a production line requires the company logo to be stamped on the packaged goods. The content of the packages is fragile and so care must be taken not to damage the boxes. It has been decided that a single acting cylinder is used but it should outstroke slowly to avoid damaging the goods. STAMP BOX BOX BOX BOX MS.Int 2.O1 fig 29 a. Design a pneumatic system that will perform this task. b. Build and test your solution. c. Name each of the components used. d. Using the correct terminology, describe how the circuit operates. 4. As part of a drilling operation, the work piece requires to be held in a vice. The vice is to be operated pneumatically by a double acting cylinder. To allow the operator time to position the work piece, the vice must close slowly. MS.Int 2.O1 fig 30 a. Design a pneumatic system that will perform this task. b. Build and test your solution. c. Name each of the components used. d. Using the correct terminology, describe how the circuit operates. e. The weight of the vice means that a large force is required to open and close it. It is suggested that two cylinders operating at the same time would be better. Using a T-piece, alter your circuit so that two double acting cylinders operate at the same time. 23

24 AND CONTROL We can create basic control circuits using pneumatic equipment relatively easily. On many production lines, operators become careless because they are so familiar with the machine that they constantly use. One way to avoid accidents happening is to fit the machine with a guard system, where the machine will only operate if the guard is in position and the machine is switched on by the operator. SLIDING DOOR START/STOP VALVE VALVE POSITION MS.Int 2.O1 fig 31 This type of control can be achieved if we connect two 3/2 valves together in series. Valve A Valve B MS.Int 2.O1 fig 32 The single acting cylinder in this circuit will only outstroke if both 3/2 valves are actuated at the same time. This is because Valve B is supplied with air coming from Valve A. Pressing the button on Valve A supplies air to Valve B and if we press the button on Valve B at the same time, this allows air to flow into the single acting cylinder and cause it to outstroke. 24

25 We can summarise the behaviour of this type of circuit in a truth table. VALVE A OFF ON OFF ON MS.Int 2.O1.fig 33 VALVE B OFF OFF ON ON CYLINDER INSTROKE INSTROKE INSTROKE OUTSTROKE 25

26 Assignment 6 1. A machine in a factory is very difficult to guard without affecting how the machine carries out its task. The manager is very concerned that the operator may have an accident as a result. In order to prevent this, the manager wants to redesign the start/stop system of the machine so that the operator needs to press two buttons before the single acting cylinder outstrokes. MS.Int 2.O1 fig 34 a. Design a system that will only work if the operator is pressing two buttons at the same time. b. Build and test your system to check that it operates correctly. c. Draw a truth table of your results. d. Using appropriate terminology, describe how the system operates. e. The manager thinks that the single acting cylinder operates too quickly. Alter your design so that the cylinder outstrokes more slowly. 2. A single acting cylinder is used to bend metal brackets using a former attached to the end of the piston rod. In order to operate safely, the system must satisfy the following conditions: The cylinder must only outstroke if the guard is down. The process must only start if the operator presses both start buttons. The process can only begin when the metal is in the correct position and actuates a 3/2 valve. a. Design a pneumatic circuit to carry out this operation. b. Build and test your solution and ensure that it operates properly. c. Using appropriate terminology, explain how your circuit controls the action of the single acting cylinder. 26

27 OR CONTROL In many industrial applications, pneumatic systems are controlled by either of two operators. They may be working together on the same process or controlling a process from two different positions. Quality control checks on production lines usually involve two or more people. Damaged or unsatisfactory goods are rejected if one or other person detects a fault and the goods are removed from the line by a single acting cylinder. MS.Int 2.O1 fig 35 This type of control requires two 3/2 valves to be connected together in parallel and the use of an additional component called a shuttle valve. Shuttle Valve MS.Int 2.O1 fig 36 Valve A Valve B 27

28 A shuttle valve is a very simple device but it is essential to how an OR control circuit works. If the button on Valve A is pressed, the ball in the shuttle valve is blown across towards Valve B and air is directed towards the single acting cylinder. This will outstroke the piston. If the button on Valve B is pressed, the ball is blown across towards Valve A and air is directed towards the cylinder. This also causes the piston to outstroke. We can summarise the behaviour of this circuit in a truth table. VALVE A OFF ON OFF ON VALVE B OFF OFF ON ON CYLINDER INSTROKE OUTSTROKE OUTSTROKE OUTSTROKE MS.Int 2.O1 fig 37 28

29 Assignment 7 1. A window in a large factory is to be opened and closed by a single acting cylinder. The cylinder is to be controlled by lever operated 3/2 valves, one placed at either end of the factory floor as this provides the workers with nearby control. If either of the two 3/2 valves is actuated, the cylinder should outstroke and open the window. MS.Int 2.O1 fig 38 a. Design a system that will work if a person at either end of the factory activates a lever. b. Build and test your system to check that it operates correctly. c. Draw a truth table of your results. d. Using appropriate terminology, describe how the system operates. e. Someone suggests changing the shuttle valve to a T-piece. Explain why this is not a good idea. 2. A pneumatic circuit has been devised for use in operating a sliding door to a refrigerated store in a supermarket. For safety reasons, it must be possible for the door to be opened or closed from both inside and outside the room. Additionally, the door should open and close slowly. MS.Int 2.O1 fig 39 29

30 A layout of the components to be used in the system is shown in figure 40. CLOSE (OUTSIDE) OPEN (OUTSIDE) MS.Int 2.O1 fig 40 CLOSE (INSIDE) OPEN (INSIDE) a. Complete the circuit so that it operates properly. b. Using appropriate terminology, explain how the circuit operates. c. It is decided after a while that the door should open as quickly as possible but continue to close slowly. Adjust your circuit to suit these new conditions. 30

31 3. A single acting cylinder is used to punch a hole in thin aluminium sheet. The sheet forms part of a household accessory that the company manufactures. The process can be controlled locally by an operator who must press two buttons at the same time to outstroke the cylinder. The alternative is to control the process from the control area directly above the production line. Both arrangements help to prevent accidents during this potentially dangerous process. The pneumatic circuit to carry out this process is shown. MS.Int 2.O1 fig Valve A Valve B Valve C a. Build and test the pneumatic circuit shown above. b. Draw a truth table of your results. c. Using appropriate terminology, describe how the system operates. d. When the process is controlled from the control area above the production line, it is possible that someone working near the cylinder could still be injured, as the machine is not guarded. Redesign the circuit to ensure that the single acting cylinder will only outstroke if a guard is in position. e. Explain the changes that you have made and how they improve the overall performance of the circuit. 31

32 TIME DELAY CIRCUITS Sometimes we require a time delay or pause in a circuit before something else happens. Time delays are easily achieved with the use of two additional components inserted into the system at the point where the delay is required. The components required are a unidirectional restrictor (which we have already used) and a reservoir. The arrangement is shown below. Air MS.Int 2.O1 fig 41 A reservoir is simply an empty container. When placed in a circuit it takes some time to fill up with air and restore the working pressure of the circuit. The time it takes to fill creates the pause or delay. We can change the length of the delay in two ways. The first is to use a reservoir of a different size as large reservoirs take longer to fill. The second way is to adjust the flow of air into the reservoir using a unidirectional restrictor. We can slow down the airflow using the restrictor so that the air takes a much longer time to fill the reservoir. Time delay circuits can be useful in clamping or pressing applications where pressure is applied by a cylinder for a set time. Such a circuit can be used to make plastic containers. This type of pressing system makes use of a double acting cylinder to press a hot plastic sheet into a mould. The plastic needs to be held in position for several seconds to allow it to cool. The circuit for this system is shown in figure

33 1 2 HOT PLASTIC SHEET MS.Int 2.O1 fig 42 When the push button, spring return 3/2 valve is pressed, the 5/2 valve changes state and the cylinder containing the former outstrokes. When fully outstroked, the former forces the plastic sheet into the mould and at the same time trips the roller. Although this 3/2 valve has been actuated, the instroke is delayed because of the time delay. The airflow to the reservoir is slowed and it takes several seconds for the reservoir to fill. This allows the plastic container to cool before it is released when the piston finally instrokes. 33

34 Assignment 8 1. Build and test the pneumatic circuit shown in figure 42. a. Adjust the circuit to achieve a 2 second delay. Explain how to do this. b. Explain the difference between piston speed control and a time delay. c. Describe how the time delay could be increased to 10 seconds. 2. A machine is designed to test for wear and tear on drawer guides in a kitchen unit. A double acting cylinder is used to constantly open and close the drawer and the runners are checked regularly. PISTON MOVEMENT KITCHEN UNIT B DRAWER DRAWER GUIDES X A C MS.Int 2.O1 fig 43 a. Build and test the pneumatic circuit shown in figure 43. b. Name the components labelled A, B and C. c. Describe how the circuit operates to complete one cycle of operation from the position shown. d. To allow the system to be stopped and checked, a 3/2 lever operated spring return valve is inserted at point X in the circuit. Adjust the circuit to contain this valve. Explain why this is an improvement to the original design. 34

35 3. A double acting cylinder is used to mark out even sections of fudge on a food production line. It is important that the process runs continuously and that each fudge bar is the same size. Pneumatics is used in many food production lines because it is clean, safe and reliable to use. The circuit diagram is shown below. MS.Int 2.O1 fig a. Build and test the circuit shown above. b. Adjust the restrictors so that the delay on each side is the same or as close as you can make it. c. This circuit is designed to run constantly but sometimes the machinery needs to be cleaned. The only way to shut the process down is to shut down the main air supply. Insert a 3/2 valve in the circuit that will allow the process to be stopped at any time. Explain your choice of actuator for this valve. d. Are there any other problems with this system that you think may affect the size of the fudge bars? e. Why is pneumatics a good choice for this type of production line? 35

36 AIR BLEED CIRCUITS When designing pneumatic circuits it is often necessary to detect if something is in place before the operation of the circuit continues, for example that a work piece to be drilled is in the correct position. It is possible to use some actuators on 3/2 and 5/2 valves (such as a plunger or roller trip), to detect this but sometimes they interfere with the set up of the system or are not sensitive enough. To overcome these problems, we use what is known as an air bleed circuit. The diagram below shows the junction between two conveyor systems in a factory. Boxes are transferred from one conveyor to another by the action of a single acting cylinder. The boxes are fairly light and an air bleed is used to detect when a box is in the correct position ready for transfer. AIR BLEED SINGLE ACTING CYLINDER CONVEYOR BELTS MS.Int 2.O1 fig 44 The circuit used to achieve this is shown below. MS.Int 2.O1 fig 45 Main air in the circuit is connected to a unidirectional restrictor in such a way as to slow down the airflow. Air passes through the restrictor to a T piece, one end of which has an unconnected pipe that allows the air to escape or bleed out into the room. When the boxes block the air bleed, the pressure in the pipe increases and this air provides a pilot air signal to the diaphragm valve. This actuates the 3/2 valve which in turn outstrokes the cylinder and the box is moved from one belt to the other. The single acting cylinder instrokes automatically ready for the process to begin again. 36

37 Assignment 9 1. Build and test the pneumatic circuit shown in figure 45. a. Explain what happens when the air bleed is blocked. b. Why is an air bleed more useful for sensing when an object is in place than a plunger operated 3/2 valve? 2. A paper mill uses a large guillotine controlled by a double acting cylinder to cut the reams of paper to size. An air bleed is used to detect when the paper is in position ready to be cut. MS.Int 2.O1 fig 46 The pneumatic circuit to solve this problem is shown below. MS.Int 2.O1 fig 47 a. Build and test the circuit shown above. b. What is the purpose of the roller trip, spring return 3/2 valve? What would you expect to activate this valve? c. It is discovered that the roller trip isn t always actuated and needs to be reset manually by an operator. It is very dangerous for the workers to be so close to the guillotine blade and so it is decided to change the roller trip to an air bleed. Redesign the circuit to take account of this. 37

38 3. A double acting cylinder is used to form hot acrylic sheet into the shape of soap dishes. The process is started by an operator pressing a button which causes the double acting cylinder to outstroke. It is important that the plastic is pushed fully into the mould before the cylinder instrokes. To ensure this, two air bleeds are used to sense that the soap dish has been fully formed. MS.Int 2.O1 fig 47a a. Design a pneumatic circuit that will carry out this operation. b. Build and test your solution to ensure that it operates correctly. c. Using the correct terminology, describe how your circuit operates. d. Explain why air bleeds are a good choice for detecting that the dish has been formed correctly. Can you suggest another way of achieving this? e. A further improvement to the system is that it should only operate when the acrylic sheet is in the correct position, otherwise some of the dishes are not formed properly. Redesign the circuit taking this into account. 38

39 4. An automatic door is designed so that, when the door is opened there is a time delay before it automatically closes again. An air bleed is used to detect when a person is approaching the door as the piping can be easily hidden under the entrance mat. The pneumatic circuit is shown below. MS.Int 2.O1 fig 47b a. Build and test this circuit and ensure that it operates properly. b. Study the circuit diagram carefully and explain what is controlling the instroke of the cylinder. c. Using appropriate terminology, explain how the circuit operates. d. Alter the circuit so that the door closes slowly. e. Describe any problems that may be encountered if this system was to be used at the entrance of, say, a hospital. 39

40 SEMI-AUTOMATIC CIRCUITS In a semi-automatic circuit, either the instroke or outstroke of the piston is automatic. The most commonly used semi-automatic circuits involve an operator starting the circuit and when the piston is fully outstroked, it actuates a valve that controls the instroke. We have come across examples of semi-automatic circuits in the course already. On some production lines, drilling operations are started by an operator who is able to ensure that the component to be machined is in the correct position. When the hole has been drilled to the correct depth, a roller trip is activated which automatically instrokes the cylinder. DEPTH STOP ROLLER TRIP DRILL WORK PIECE MS.Int 2.O1 fig 48 The pneumatic circuit that controls this operation is shown below. Valve A MS.Int 2.O1 fig 49 40

41 The operator starts the operation by pressing the push button. This changes the state of the 5/2 valve and the cylinder outstrokes. The cylinder pushes the drill into the work piece and when the correct depth has been reached, the roller trip is activated. This changes the state of the 5/2 valve and the cylinder instrokes. The process is ready to begin again. 41

42 Assignment Build and test the pneumatic circuit shown in figure 49. a. Why is this type of circuit described as semi-automatic? b. The drill is inserted into the work piece too quickly for the hole to be drilled properly. Alter the circuit so that the outstroke is slower. 2. A guard on a plastic forming machine is held in position by a double acting cylinder. The guard is raised to allow the worker to remove the component and insert a blank. The guard is raised on the push of a button but lowers automatically. GUIDES TRAVEL OF DOOR MS.Int 2.O1 fig 50 a. Design a semi-automatic circuit that will automatically lower the guard by outstroking the piston. b. The operator complains that the guard raises and returns too quickly and he doesn t have enough time to remove the component. Someone suggests slowing down the outstroke while another designer thinks that a time delay circuit would be better. How would you improve the circuit? Explain your decision. c. Rebuild the improved circuit and test it. d. Using appropriate terminology, explain how the circuit operates. 42

43 3. Quality control on a production line is very important to ensure that all products are made to the correct standard. A quality control officer can randomly select goods on the line to be checked. He does this be pressing a button which activates a double acting cylinder which pushes the object off the line. However, sometimes the goods can become jammed and the piston should automatically instroke. A circuit is shown below. VALVE B MS.Int 2.O1 fig 50a VALVE A a. Build and test the circuit shown above. b. Explain the purpose of Valves A and B. c. Why is this described as a semi-automatic circuit? 43

44 AUTOMATIC CIRCUITS Automatic circuits can perform tasks over and over again without the need of input from an operator. These circuits make use of actuators such as a roller trip and plunger to sense the position of the piston. We have come across an automatic circuit already in the course. Automatic control circuits are used in applications that require reciprocating motion of the piston. For example, a production line requires the finished products to be directed towards two loading bays ready for shipping out to customers. A double acting cylinder controls the movement of a chute so that every second container is sent to each bay. MS.Int 2.O1 fig 51 The pneumatic circuit that controls this operation is shown below. Notice that roller trip, spring return 3/2 valves are used to detect when the piston is positive and negative. Valve A Valve B Y X MS.Int 2.O1 fig 52 As the piston instrokes, it trips Valve A and the 5/2 valve changes state and the piston is sent positive. When it is fully outstroked, it trips Valve B and the 5/2 valve returns to its original position allowing the piston to instroke. The process begins all over again and continues to operate. 44

45 Assignment Build and test the pneumatic circuit shown in figure 52. a. Sometimes the containers get trapped or jammed in the system and it needs to be switched off. How do you get the circuit to stop? Is this an acceptable way of controlling the circuit? b. Someone suggests placing a lever operated 3/2 valve in the circuit at position X but another worker disagrees and believes it should be point Y. Try both positions and explain how these changes affect the operation of the circuit. c. Why must a lever operated 3/2 valve be used instead of a push button? 2. A polishing machine requires reciprocating motion of a double acting cylinder. So that the polished work piece can be removed, a valve is required to stop the operation of the complete circuit. Experience of this type of process shows that if the piston operates too quickly, the brush becomes hot and sticks to the surface of the material. MS.Int 2.O1 fig 53 a. Design a pneumatic circuit that will carry out this operation. b. Build and test your solution and ensure that it fully solves the problem. c. Using appropriate terminology, explain how you circuit works. d. Why do we describe this type of circuit as automatic? 45

46 SEQUENTIAL CIRCUITS Many pneumatic systems and machines are designed to perform a range of tasks in a certain order or sequence. This usually involves the use of two or more cylinders working together to complete the task. For example, a company has automated its production line that involves metal blocks being placed in a furnace for heat treatment. One cylinder is used to open the furnace door and another pushes the metal blocks into the furnace. A MS.Int 2.O1 fig 54 The sequence of operations for this process is: a. An operator pushes a button to start the process. b. The furnace door is opened. c. The block is pushed into the furnace and the piston retracts. d. The furnace door is closed. e. The sequence stops. For this system to work successfully, we need to fully understand the order and movement of cylinders A and B. Stage 1 Cylinder A instrokes to raise the furnace door. Stage 2 Cylinder B outstrokes and pushes the metal block into the furnace. Stage 3 Cylinder B instrokes. Stage 4 Cylinder A outstrokes and closes the furnace door. 46

47 The pneumatic circuit that carries out this operation is shown below. CYLINDER A VALVE G VALVE H VALVE B VALVE C VALVE A CYLINDER B VALVE F VALVE I VALVE E VALVE D MS.Int 2.O1 fig 55 47

48 The system begins by actuating Valve A. This changes the state of Valve B and causes Cylinder A to instroke, raising the door. When fully instroked or negative, the piston trips Valve C and this sends a signal to Valve D. This 5/2 valve changes state and sends Cylinder B positive. When fully outstroked, the piston trips Valve E and the cylinder instrokes. When negative, Valve F is actuated and causes Cylinder A to outstroke and stay in the positive position. The system stops and waits for a signal from Valve A. We can summarise the sequence of this circuit as follows: Start, A-, B+, B-, A+, Stop 48

49 Assignment Build and test the pneumatic circuit in figure 55. a. Name the components labelled Valve D, Valve F and Valve H. b. If Valve H was removed from the circuit, explain the effect this would have on the operation of the furnace door. c. Using appropriate terminology, explain how the circuit operates from when Valve A is pressed. d. A short delay is required before Cylinder B goes positive. Alter your circuit to take this into account. 2. A company has automated a small section of its production line. The system involves a component being pushed into position by a cylinder, drilled and then released ready to begin again. The system involves two cylinders working together in sequence: A+, B+, A-, B-. The system is automatic and should continue to work through the sequence. However, a start/stop button should be incorporated that would allow the system to be shut down in the event of a jam or the drill bit breaking. CYLINDER B D RILL CYLINDER A MS.Int 2.O1 fig 56 GUIDES a. Design a pneumatic system that would carry out this sequence of operations. b. Build and test your solution. c. Using appropriate terminology, describe how it completes one sequence. d. Explain your choice of actuators that sense the fully instroked and outstroked positions of the two pistons. 49

50 3. Engraving is an operation that requires great skill and is very time consuming. In an attempt to speed up the process, two double acting cylinders are needed to work together in sequence to position the component and hold it in place. Cylinder A pushes the component to be engraved into position. The clamping process is achieved by connecting a parallel linkage system to the end of cylinder B. When the cylinder instrokes, the linkage closes around the component to hold it steady. The complete sequence required is A+, B-, A-, B+. a. Design a pneumatic circuit to carry out this sequence. b. Build and test the circuit and ensure that it operates correctly. c. It is felt that the clamping operation works too quickly and sometimes the component is moved out of line. Alter your circuit so that cylinder B instrokes more slowly. d. Not enough time is allowed for the engraving operation to take place before the component is released. A short delay is needed in the circuit. Using the correct components, position the delay where you think it would be most effective. Explain your reasons for inserting it there. 50

51 INTERFACING AND ELECTRONIC CONTROL Electronic Control Although pneumatic circuits have many advantages, they can become complicated and expensive when lots of components are needed. They can also be difficult to set up and control. One possible way of overcoming these problems is to use electronics or a computer interface to control the operation of pneumatic circuits. The advantage of this is that electronic signals can be transmitted over much greater distances than pneumatic signals. However, the main advantage is that electronic signals respond faster than pneumatic signals and use less energy. To control a pneumatic system electronically, we require the use of a solenoid operated 3/2 valve. This valve is actuated when an electric current energises the coil of the solenoid. If we can control the current flowing to the coil, then we can control the operation of the entire circuit. This can be achieved easily by connecting switches or sensors in series with the solenoid. For example, a ten-pin bowling complex uses double acting cylinders to set up the skittles once they have been knocked down. The cylinder is controlled by an electric switch arrangement which energises solenoid operated 3/2 valves. MS.Int 2.O1 fig 57 The circuit diagram is shown below. A 12V dc 12V dc B C MS.Int 2.O1 fig 58 51

52 Assignment Build and test the electrical and pneumatic circuit shown in figure 58. a. What advantages are there of controlling pneumatic circuits electronically? b. If only pneumatic components were available, redesign the circuit so that it would carry out the same operation. 2. A waste disposal system makes use of a hopper and container. The container moves forward under the hopper, which tips up to empty its contents. Limit switches are used to detect if the container is in place and if the hopper has been tipped up properly. The system needs a start/stop button that will allow overall control of the circuit. a. Design an electronic and pneumatic system to fully solve this problem. b. Using appropriate terminology, explain how the circuit operates. c. Give examples of other sensors that could be used to detect if the container and hopper are in the correct place. d. It is discovered that once the hopper has tipped right over, the double acting cylinder begins to instroke before all the contents have emptied out. Insert a time delay into the circuit that will overcome this. 52

53 Reprogrammable Interfacing Most industrial pneumatic systems include a number of cylinders working together in sequence. Certain sequences of operation are difficult to control using the equipment we have come across so far. The circuits also become expensive to build and are difficult to set up. However, we can overcome these problems by using a computer interface to control complex tasks. For example, a post office sorting system is used to separate three different sizes of packages. The packages are carried on a conveyor belt to the first sort. At this point, a single acting cylinder removes the largest packages, which are detected by use of a switch. At the second stage, the medium packages are separated from the smaller size ones, which continue to the end of the conveyor. As the content of the packages is unknown, they should be removed carefully from the conveyor. For safety reasons, the system should not operate unless the guard around the conveyor is in position. MS.Int 2.O1 fig 60 53

54 The operation of this circuit can be summarised in a flow chart. START IS GUARD DOWN? NO YES IS PACKAGE LARGE? NO YES EJECT PACKAGE IS PACKAGE MEDIUM? NO YES EJECT PACKAGE MS.Int 2.O1 fig 61 54

55 Assignment 14 1a. Write a program that will control the operation of this circuit. b. Build the necessary pneumatic and electronic system and connect it to your interface. Test that your circuit operates properly. c. It is suggested that there should be a start/stop button to halt the system in case of a jam. Make the necessary adjustments to your program and circuit to accommodate this. 55

56 2. A system is designed to control a chemical process. A cylinder moves a container over the tank containing the chemical solution. Another cylinder lowers the container into the tank. The chemical reaction takes several seconds to complete. The cylinder lifts the container and it returns to its original position to be unloaded. CYLINDER A CYLINDER B TANK MS.Int 2.O1 fig 62 a. Write a program that will control this operation. b. Build the necessary pneumatic and electronic system and connect it to your interface. Test that the circuit operates properly. c. Investigate the reasons why this circuit would be very difficult to control using pneumatic equipment only. 56

57 3. Packets of sweets in a factory are boxed automatically. The packets pass along a conveyor belt and drop into the boxes. Each box should contain 12 packets only. When full, the box is pushed onto another conveyor by a double acting cylinder and taken away to be labelled and dispatched. A single acting cylinder pushes an empty box into position ready for the process to begin again. a. Draw a flow chart that clearly shows this operation. b. From your flow chart write a computer program that will control the counting sequence and the operation of the cylinders. c. Build and test your solution and ensure that it works properly. d. Describe what would happen if no empty box was in place to collect the sweets. How could you overcome this problem? e. Describe any necessary safety precautions that would need to be in place for this system to work safely. Have you included these in your design? 57

58 FORCES IN A SINGLE ACTING CYLINDER When a single acting cylinder outstrokes it produces a force. We can use this force to lift objects or push them around. The size of the force produced by the cylinder as it outstrokes depends on two things - the air pressure supplied to the cylinder and the surface area of the piston. This means that we can increase the force by either increasing the air pressure or increasing the size of the piston. However, systems and components are designed to operate at specific pressure levels and we should never increase the pressure beyond these levels. The force produced by the cylinder as it instrokes is not affected by either of these things. The instroking force is controlled by the return spring. Pressure Air pressure is measured in bar or in N/mm 2. We can measure the pressure in a pneumatic system using a pressure gauge. A gauge will always be connected to the compressor but other gauges may be distributed throughout large systems to monitor the pressure. This helps to detect leaks, as the pressure in the system would begin to fall if air was escaping from the pipes. Whenever we use pressure in calculations we require the units to be in N/mm 2. This often means converting from bar to N/mm 2. This conversion is easy as you simply divide the value in bar by 10. For example, if the pressure supplied to a system is 5 bar, we can find the equivalent value in N/mm 2 by simply dividing 5 by 10. Therefore, the value would be 0.5 N/mm 2. The chart below provides a quick reference. bar N/mm MS.Int 2.O1 fig 63 Area The surface area of the piston is the area that the air pushes against to outstroke the piston. This area is circular or round. MS.Int 2.O1 fig 64 58