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Thread: Calculating the force required / Design of rack and pinion

  1. #1

    Calculating the force required / Design of rack and pinion

    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.

    Attached Images Attached Images

  2. #2
    Technical Fellow jboggs's Avatar
    Join Date
    Mar 2011
    Myrtle Beach, SC
    This is basically a hydraulic ram. I would suggest using an acme screw or ballscrew rather than a rack and pinion. You will get a much higher output force from a given input motor torque. And the higher the screw pitch the better. You cannot answer the question of how much torque you need until you can determine how much output force you need. Then it is a simple matter of choosing a gear reduction ratios to get the motor speed and torque you will require.

    The only way to measure the force required is to apply known forces in increments until you achieve the flow you need. You could use objects of a known weight and work out some kind of connection arrangement, direct or lever or pulley.

  3. #3
    Technical Fellow Kelly_Bramble's Avatar
    Join Date
    Feb 2011
    Bold Springs, GA
    To measure the force required to dispense the food you could remove the stepper motor and place static weights on the push ram incrementally to determine the axial force required to extrude the food.

    Once you have the axial ram force on the food you then need to back calculate the motor torque+gear ratio/diameter required to create that axial force.

    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  4. #4
    Hey Jboggs and Kelly Bramble,
    Thanks a ton for replying and helping me in this.

    I was able to place objects of known static weights and was able to find that, it requires 33-35 kg of force to dispense the food in the flow rate I want.

    Now, to create a gear mechanism to dispense the food, I was looking for types of threads for power screws. Found that, square threads are the most efficient to use for power screws, as it has the least friction in spite of being difficult to fabricate, . For square thread power screws, I was able to find an equation in Shigley's ME design book for the relation between torque and force. Attached the image of the equation.
    I dont know what a collar means, in a power screw. Please let me know why it is used and I also should use the same.

    Hoping that, I am not gonna use one in my design, I neglected the second part on the RHS of the equation.

    With that in place, for a load of 35 kgf, with screw's mean dia, lead, co-eff of friction and pitch to be 16mm, 4mm, 0.08 and 4mm respectively, around 4.5 kgcm torque is what I have arrived at to drive the lead screw.

    Now coming to the gears, what do you suggest? Spur gears or the worm ones? The constraint is, the product have to be cost efficient and should require very less maintenance.

    If it is spur gears, how about the one I attached in this reply? I am sorry it is a very rough sketch and please bear with me. Yeah, we will have guides with Internal threads both above and below the driven Spur gear for Screw Rod.


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