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Last time, we learned how a pneumatic actuator was connected to a depositor’s positive displacement piston pump so that it could extract jelly filling from a hopper, and deposit it through a nozzle onto a passing pastry. The pneumatic actuator imparted linear motion to the pump during this process. Since the pistons in the actuator and pump both move in a straight line, it was very easy and straightforward to connect the actuator to the pump. For the depositing process to work, we must have an additional actuator to rotate the diverter valve as the pump operates. The valve changes the flow path of the jelly between the hopper and the nozzle. More specifically, the valve must rotate clockwise to create a flow path between the hopper and the pump while the pump extracts jelly from the hopper.
The Diverter Valve Rotated Clockwise
When the pump is full of jelly, the diverter valve must rotate counter-clockwise to create a flow path between the pump and the nozzle. This path allows the pump to empty its contents trough the nozzle.
The Diverter Valve Rotated Counter-Clockwise
Although the diverter valve’s motion is rotary, it can be operated with the linear motion of a pneumatic actuator. To convert the linear motion of the actuator to the rotary motion needed to operate the valve, we can employ a device known to engineers as a Scotch Yoke.
The Depositor’s Scotch Yoke
In the Scotch Yoke, the pneumatic actuator’s piston rod is connected to a slider. As the piston moves back and forth in the pneumatic actuator, the slider is free to move back and forth along a fixed guide rod. A pin is located on the slider. The pin loosely engages a slot in the yoke mechanism. As the slider moves, the pin can move freely in the slot. The yoke mechanism is rigidly attached to the rotating diverter valve shaft. Next time, we’ll look at the rotary motion of the Scotch Yoke as the pneumatic actuator piston moves to the right and then to the left during the jelly depositing process. Copyright 2018 – Philip J. O’Keefe, PE Engineering Expert Witness Blog ____________________________________ |
