Posts Tagged ‘Scotch Yoke’

The Depositor’s Scotch Yoke Motion

Monday, July 16th, 2018

    In my previous article, I introduced a mechanism known to engineers as a Scotch Yoke.   It converts linear motion of a pneumatic actuator into rotary motion.   With regard to the jelly filling depositor on a pastry production line, the Scotch Yoke converts the pneumatic actuator’s linear motion into the rotary motion needed to operate the depositor’s diverter valve.   Now, let’s follow the Scotch Yoke’s motion in this application.

    When the pneumatic actuator’s piston is all the way to the left, the Scotch Yoke’s slider is all the way to the left on the guide rod.   The slider pin is at the top of the slot in the yoke mechanism.   The diverter valve is positioned to create a path for the jelly so it can be emptied from the pump through the nozzle.

 The Depositor’s Scotch Yoke

The Depositor’s Scotch Yoke Slider Is Full Left

   

    As compressed air is introduced to the left side of the actuator’s piston, the piston moves to the right, and the slider also moves to the right.   As this happens, the slider pin begins to move in the yoke mechanism’s slot, and the diverter valve shaft begins to rotate clockwise.

The Depositor’s Scotch Yoke Clockwise Rotation

The Depositor’s Scotch Yoke Clockwise Rotation

   

    After the piston moves all the way to the right, and the diverter valve shaft stops its clockwise rotation.   The diverter valve is positioned to create a path between the pump and hopper so the pump can suck in jelly from the hopper.   When the pump is full of jelly, compressed air is introduced to the right side of the piston.

 The Depositor’s Scotch Yoke Slider Is Full Right

The Depositor’s Scotch Yoke Slider Is Full Right

   

    As compressed air is introduced to the right side of the actuator’s piston, the piston moves to the left, and the Scotch Yoke’s slider also moves to the left.   As this happens, the slider pin begins to move in the yoke mechanism’s slot, and the diverter valve shaft begins to rotate counterclockwise.

 The Depositor’s Scotch Yoke Counterclockwise Rotation

The Depositor’s Scotch Yoke Counterclockwise Rotation

   

    After the piston moves all the way to the left, the diverter shaft stops its counterclockwise rotation.   The diverter valve is once again positioned to create a path so the jelly can flow from the pump through the nozzle.   After all the jelly is emptied from the pump, compressed air is introduced to the left side of the piston to repeat the previously described motion.

    But what selectively admits compressed air to either the right or left of the pneumatic actuator’s piston?   Next time, we’ll find out when we discuss a device called a solenoid valve.

Copyright 2018 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog

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The Depositor’s Scotch Yoke

Monday, July 9th, 2018

    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

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

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

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

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