Author Topic: force, torque and motor selection basics  (Read 1570 times)

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Offline JohnBotTopic starter

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force, torque and motor selection basics
« on: October 16, 2011, 10:21:09 AM »
Hi all,

So, I'm attempting to build an egg launching sled. This will be roughly 2.0 to 2.5 feet long, and be powered by surgical tubing. Ultimately, I would like to increase the range to around 100 feet. When the first phase is built and working (i.e. egg will launch), I will add automation mechanics: lead screw and carriage to re-set the launching sled, and a servo controlled release mechanism to fire. I think I have a pretty good design, but I'm very new to this, and need some basic context on the force required, if anyone knows and would be kind enough to offer some advice.

These are my questions:
1. How much force would I need to generate to launch an egg around 100' via the tubing? And is a 2' rig long enough?
2. If I succeed in building it, how do I size the motor and lead screw? I've spent a lot of time researching r/c hobby motors and typical d/c and servo motors/controllers, but I'm just not sure. Can anyone provide just a little context- for example, do I need a small, d/c gear motor or maybe a modified servo for the lead screw, or are we talking something similar to the d/c motors that drive scooters?

Any help would be much appreciated. I included a very basic image below of what I'm looking to build. But this is just for reference, not really to scale and no mechanical details included.   


Offline waltr

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Re: force, torque and motor selection basics
« Reply #1 on: October 16, 2011, 07:06:25 PM »
Steps in calculations:

1- calculate the initial velocity and launch angle required to send a egg 100 feet (trajectory calculations from a Physics text).
2- from the 'initial velocity' now calculate the acceleration required in a distance of two feet.
3- from the acceleration and mass of the egg calculate the force (hint: F = ma from any Physic text book).

4- You'll need to experiment with the surgical tubing to find out the force it exerts with stretch.
5-  The force for the surgical tubing is greatest fully stretched and less half-stretched so the Force for accelerating (step 3) is the average force so the surgical tubing force will be over twice this fully-stretched. This is the Force required by the lead-screw motor to fully pull it back.

6- Now calculate the Torque required to turn the lead-screw (Thread pitch and friction come into play here).
7- Determine how fast you want the lead screw to turn (re-load time) and pick a Gear motor to do the job. I'd not use a servo.


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