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Last time we learned that a fruit jelly depositor in a food manufacturing plant is an example of a positive displacement pump at work. Today we’ll see how pieces of equipment on the depositor, known as a pneumatic actuators, work. Pneumatic actuators do not come in contact with the jelly flowing through the depositor. In other words, no jelly flows through the actuators. The jelly only flows through the transfer valve and positive displacement pump as we saw last time. The pump and valve can’t move by themselves. So, they need some device to set them in motion. That’s where the pneumatic actuators come into play. They impart movement to the pump and transfer valve to get the jelly flowing from the hopper and down through the nozzle and onto the pastry. A pneumatic actuator is a device that operates using compressed air. Compressed air, from an external air compressor, enters into a tube in the actuator known as a cylinder. Inside the cylinder is a piston that can move along the length of the tube. Attached to the piston is a piston rod which extends to the outside of the cylinder. When compressed air is introduced into the cylinder on the left side of the piston, it forces the piston and piston rod to move towards the right side of the cylinder. But, air must be vented out to atmosphere from the right side of the piston for this movement to occur. If no venting took place, trapped air to the right of the piston will get squeezed between the piston and the right end of the cylinder. When the air gets squeezed, it becomes pressurized. The pressure will impede the movement of the piston. Likewise, when compressed air is introduced into the cylinder on the right side of the piston, it forces the piston and piston rod to move towards the left side of the cylinder. The Depositor’s Pneumatic Actuator
So, depending on which end compressed air is admitted to the pneumatic actuator’s cylinder, the piston rod will move to the left or the right. In engineering terms, the actuator imparts linear motion to machines. In other words, the piston rod moves back and forth in a straight line. Next time, we’ll see how the pneumatic actuator is connected to the depositor’s pump to impart the linear motion that draws jelly from the supply hopper and sends it streaming out of the nozzle onto a passing pastry. Copyright 2018 – Philip J. O’Keefe, PE Engineering Expert Witness Blog ____________________________________ |
