How to calculate the force required?

[Govardhanan]

Hello everyone. This is Govardhanan. I am still a student, when mechanics is considered, so please tolerate my simple questions and pls do help me out.

I am trying to dispense some food from the container using a rack and a pinion, but was not successful in that, as the stepper motor I use doesnt give enough torque. So I need to know the torque I am supposed to generate to dispense the food. But I am not sure how to measure the force required to dispense the food. Please help me out in designing this. Thanks a ton in advance.🙂

I have attached the rough explanation drawing and an image of what I have done already.

[G B Reid MIMechE, SIMarEST]

Hi Govardhanan!

You will likely have problems with this design due to the foodstuffs not flowing sufficiently. 

If the foodstuff is too viscous, it will not exit through the holes at the bottom...in fact in some instances, the container will fail before it does.  If you look at dispensers available commercially, you will find that the majority have a cone at the bottom to facilitate exiting of the material. 

Whilst you acknowledge that you are a student of mechanics, I am sure you will be familiar with the phrase "every action has an equal and opposite reaction." (Isaac Newton) .

Basically, in this situation, all of the force from the piston is reflected back into the piston with nothing facilitating flow through the exit holes.  If this exit was altered to a cone of - say - 60 degrees (radially), 86.6% of the force would push down and 50% of the force would be deflected to "feeding" the material through towards the target exit. [Yes that does exceed 100%, but that's trigonometric mechanical advantage]

Note: I have not sketched this out and checked as have no paper at the moment, so it's a "finger in the air" calc. but at very least the principle is right if the figures are wrong...

Good luck!

Bruce

[Mucugia]

Hi Govardhanan,

Allow me to answer equally simply.

1. I can see the setup has two main phases; the motor, rack and pinion phase, and the piston food in a cylinder phase.

2. To size the motor you need to determine the load required to desirably dispense the food.  This can be achieved through determining the required load at the piston.  The exercise may be approached in varied ways including; using physical loads on piston.

3. Upon determining the required load range, work it backward to size the motor.  Remember energy input at motor rack phase = energy output at piston * efficiencies(may start with 80%).

 

Best of lack

[Govardhanan]

Thank you both of you. With all the inputs, I was able to calculate the force required. I changed the dispensing mechanism from rack and pinion to Power screw and all works fine now. Thanks a ton for your time and help.