Pneumatic Problems Department of Technological Education Technological Studies
Pneumatic Problems Department of Technological Education Technological Studies Banff Academy
Department of Technological Education Forces in a single-acting cylinder n n When a single-acting cylinder outstrokes, it produces a force. 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 if we want a bigger force we can either use a larger piston or increase the air pressure. The instroke of a single-acting cylinder is controlled by a spring.
Department of Technological Education Pressure n n n Air pressure is measured in bars or in N/mm 2 (newtons per square millimetre). Whenever using pressure in calculations, the units will to be in N/mm 2. This sometimes means converting from bars to N/mm 2. This conversion is easy, as you simply divide the value in bars by 10.
Department of Technological Education Area n n The surface area of the piston is the area that the air pushes against to outstroke the piston. This area is circular. The area of a circle is calculated using the formula where r is the radius and d is the diameter of the circle.
Department of Technological Education Force n n The force produced when a single-acting cylinder outstrokes is calculated using the formula: n Force = Pressure Area where force is measured in newtons (N), pressure is measured in N/mm 2 and area is measured in mm 2.
Department of Technological Education Worked example n n n 1. A door requires a force of 400 N to slide it open. A single-acting cylinder supplied with a pressure of 5 bars controls the operation. Calculate the diameter of the piston required to produce this force. Use the formula: Using the scale, 5 bar = 0. 5 N/mm 2. A=F/P A = 400 / 0. 5 A = 800 mm 2 Diameter = 32 mm
Department of Technological Education Forces in a double-acting cylinder n n During the outstroke, the compressed air pushes against the surface area of the piston in the same way as in the single-acting cylinder. However, during the instroke the surface area is reduced because of the piston rod. This means that the compressed air does not have as big an area to push against and so it does not produce as big a force. The surface area, or effective area as it is known, can be worked out by calculating the area of the piston rod and subtracting it from the surface area of the piston. n Effective area = piston area – piston rod area
Department of Technological Education Worked example n n n 1. A double-acting cylinder is used to raise and lower a barrier in a car park. The air pressure is 0. 4 N/mm 2 and the piston has a diameter of 40 mm. The piston rod is 12 mm in diameter. What forces are produced when the piston outstrokes and instrokes? Force produced on the out-stroke: A 1 = d 12 4 A 1 = 402 4 A 1 = 1256 mm 2 F=Px. A F = 0. 4 x 1256 F = 502. 4 N Force produced on the in-stroke: Effective Area (A t) = A 1 – A 2 At = d 12 - d 22 4 4 2 At = 40 - 122 4 4 At = 1256 – 113. 04 At = 1142. 96 mm 2 F=Px. A F = 0. 4 x 1142. 96 F = 457. 18 N
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