Starting Torque
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MECH1300 Pneumatic Components Topics Pneumatic Cylinders Pneumatic Motors Other Pneumatic Actuators Pneumatic Directional Control Valves Pneumatic Flow Control Valves Air Preparation Air Distribution MECH1300 Air Tools MECH1300 Pneumatic Cylinders Same basic construction and configuration as hydraulic cylinders Since they operate at much lower pressures, they are of much lighter construction Quicker acting, less precise Still use Pascal’s law: πΉπΈ = π β π΄π πΈπ₯π‘πππ πΉππππ πΉπ = π β π΄π − π΄π π ππ‘ππππ‘ πΉππππ Pressure is in psig Gauge pressure is the pressure above atmospheric pressure MECH1300 Pneumatic Cylinders EX: A double acting pneumatic cylinder with a 3 in bore and a 1.5 in rod is to be used in a system with a supply pressure of 100psig. What is the force output of the cylinder on the extend and retract strokes? Calculate the piston area: π β π·π 2 3.142 β 3 ππ 2 π΄π = = = 7.06ππ2 4 4 Calculate the rod area: π β π·π 2 3.142 β 1.5 ππ 2 π΄π = = = 1.77ππ2 4 4 Calculate the extension force: πππ πΉπΈ = π β π΄π = 100 2 β 7.677ππ2 = 706πππ ππ Calculate the retraction force: πΉπ = π β π΄π − π΄π = 100 πππ β 7.06 − 1.77 ππ2 = 529 πππ 2 ππ MECH1300 Pneumatic Cylinders To calculate the air consumption, you must calculate the volume of air required to extend and retract the cylinder. ππΈ = π΄π ∗ π ππ = π΄π − π΄π ∗ π S is stroke length Slide – a cylinder with guide rods that run parallel to the piston rod. MECH1300 Pneumatic Cylinders EX: A double-acting pneumatic cylinder with a 3 in bore, 1.5 in rod and a 5 in stroke length must cycle (extend and retract) 40 times per minute. The supply pressure is 100 psig. What is the air consumption of the cylinder in scfm? Calculate the piston area: π β π·π 2 3.142 β 3 ππ 2 π΄π = = = 7.06ππ2 4 4 Calculate the rod area: π β π·π 2 3.142 β 1.5 ππ 2 π΄π = = = 1.77ππ2 4 4 Calculate the total volume per cycle: ππΈ = π΄π ∗ π = 7.06 ∗ 5 = 35.34ππ3 ππ = π΄π − π΄π ∗ π = 7.06 − 1.77 ∗ 5 = 26.49ππ3 ππ‘ππ‘ππ = ππΈ + ππ = 35.34 + 26.49 = 61.83 Convert to ππ‘ 3 : 3 1ππ‘ 61.83ππ3 ∗ = 0.0357ππ‘ 3 3 1728ππ Calculate the equivalent uncompressed volume since it is required at 100 psig: π2 ∗ π2 0.0357 ∗ (100 + 14.7) π1 = = = 0.2792ππ‘ 3 π1 (0 + 14.7) MECH1300 Pneumatic Motors Similar in function and construction to hydraulic motors Used where low to moderate torque is required Can operate at speeds in excess of 10,000 rpm Hydraulics motors are limited to 5,000 rpm Advantages: Can stall under full load for indefinite periods of time Can be use were electric motors would be considered a fire hazard Lighter and less expensive that electric motors MECH1300 Pneumatic Motors Running Torque - The torque generated when the motor is operating at a particular speed Starting Torque – The torque generated when a motor is started from a dead stop Stall Torque – The torque required to stop a rotating motor at a particular supply pressure. It is always higher than the starting torque, because it must overcome the momentum of the motor. Speed depends on load and supply pressure If the load is increased, torque will increase, speed will decrease If supply pressure is increased then speed increases, if supply pressure decreases then speed decreases MECH1300 Pneumatic Motors Free Speed- The speed under no load at a particular supply pressure Design Speed – The rotational speed at the maximum power output. Graphic symbols are the same as hydraulic motors only the arrows are not shaded. MECH1300 Other Pneumatic Actuators Grippers – used to pick up and manipulate parts Escapements – an actuator that allows parts to be released one at a time MECH1300 Pneumatic Directional Control Valves Similar to hydraulics Two-way, three-way, four-way – the number of ways means the number of ports Two-way – Pressure port is connected to a compressed air source and the outlet is connected to the actuator. Three Way – Pressure port is connect to a compressed air source, outlet is connected to the actuator and the exhaust port is open to the atmosphere. MECH1300 Pneumatic Directional Control Valves Four-Way – Exhaust ports are combined. Most commonly used to control double acting cylinders Hydraulics use a Detent – a mechanism that holds a valve securely in its position. Pneumatics low pressures and tight seals allow valves to maintain position using friction. MECH1300 Pneumatic Directional Control Valves Some Basic Pneumatic DCV actuation types: MECH1300 Pneumatic Directional Control Valves Check Valves – allow flow in one direction only (like a LED in electronics). Can flow from X to Y but not from Y to X Shuttle Valves – allow flow from either of two inlets (p1 or P2) to be sent to a common outlet (sometimes call an OR valve) AND Valves – requires pressure at both P1 and P2 simultaneously for flow to be sent to the outlet. (used for safety applications) MECH1300 Pneumatic Directional Control Valves Air Logic Valves – AND & OR valves. They can be used in areas where electrical control presents a fire hazard. Quick Exhaust Valve – used to increase speed by speeding up the exhaust flow MECH1300 Pneumatic Directional Control Valves Common flow coefficient: A parameter that relates the pressure drop across a valve and the flow rate through it πΆπ = π 22.48 π βπβπππ’π‘ πΆπ = π 68.7 π βπβπππ’π‘ π = air temperature (β°R,K) βπ= pressure drop (psi, kPa) πΆπ is a sizing tool. A value with a large πΆπ will have a smaller pressure drop across the valve for a particular flow rate. Critical pressure ratio - ππππ ππΌπ Decreasing the ratio results in no further increase in flow and the valve becomes choked MECH1300 Pneumatic Directional Control Valves EX: An air-powered machine consumes 25 scfm at 90 psig. If the directional control valve that will control this machine can have a pressure drop of no more than 5 psi, what is the minimum πΆπ ? Assume the temperature of the air is 80β°F. Convert to absolute values: ππππ = 90 + 14.7 = 104.7 ππ ππ T=80+460=540°π Calculate: π π 25 540 πΆπ = = = 1.13 22.48 βπ β πππ’π‘ 22.48 5 ∗ (104.7) Choosing a value of 1.13 or higher ensures the pressure drop will not be more than 5 psi MECH1300 Pneumatic Directional Control Valves EX: An air-powered machine consumes 750 l/min at 650kPa gauge. If the directional control valve that will control this machine can have a pressure drop of no more than 40 kPa, what is the minimum πΆπ ? Assume the temperature of the air is 30β°C. Convert to absolute values: ππππ = 650 + 101 = 751 πππ πππ T=30+273=303 K Calculate: π π 750 303 πΆπ = = = 1.09 68.7 βπ β πππ’π‘ 68.7 40 ∗ (751) Choosing a value of 1.09 or higher ensures the pressure drop will not be more than 40 kPa MECH1300 Pneumatic Directional Control Valves EX: A valve with a πΆπ of 1.25 is used in a system with a line pressure of 100 psig. The pressure drop across the valve is 10 psi. What is the flow through this valve? Assume the temperature of the air is 80β. Convert to absolute values: Outlet pressure will be 100 - 10=90 ππππ = 14.7 + 90 = 104.7ππ ππ T=80+460=540 °π Calculate: π = 22.48 ∗ πΆπ βπ β πππ’π‘ 10 ∗ 104.7 = 22.48 ∗ 1.25 = 39.1 π πππ π 540 Larger pressure drop will have a higher flow rate MECH1300 Pneumatic Flow Control Valves Valves with a return check for free flow in the reverse direction. Flow is controlled from A to B, but B to A is unrestricted Connected in a meter-in or meter-out configuration Meter-in: Flow rate into the actuator is being controlled Meter-Out: Flow rate out of the actuator is being controlled MECH1300 Air Preparation Pressure Relief Valves – Spring loaded valves that open when a preset maximum pressure is reached to prevent further rise in pressure Pressure regulators – Used to supply pressure to an appropriate level for a particular machine Filters – Filters that remove contaminates from the air that could cause damage to components Lubricators – a fined oil mist that is sprayed into the airflow that will be carried downstream and precipitate out ono the components. Combined Conditioning Units – A filter, regulator and lubricator combined into one unit (called a FRL or trio) MECH1300 Air Preparation Water removal – in pneumatics water exists as water vapor and as liquid water. When it is in liquid form it becomes problematic and can cause corrosion and reduction of effectiveness Relative humidity – the percentage of water in the as compared to the maximum amount it could hold Dew Point – The temperature at which the air becomes saturated with water Air dryers reduce the water in the air Refrigeration dryers (chillers) – uses a refrigerant that is compressed and heated then expanded and cooled Absorption dryers use a gel material that absorbs water as the air is passed through it. They are used to drop the dew point. MECH1300 Air Distribution Pneumatics use a centrally located compressor and receiver tank that supplies multiple machines. It’s distributed throughout the facility through a distribution header Lines are dropped from the conduits to supply each machine Usually uses schedule 40 steel pipe Just like the setup in our lab
