Using Inventor for Arm torque calcs

[Greg Needel]

Originally Posted by AdamHeard

Rather than estimation and integrating the weight of an arm + whatever it is lifting to calculate what kind of gearing/motor to use... is there a feature in inventor that can give you those numbers? It'd be nice to simply put whatever object that needs to be lifted (with accurate weight) at the end of the arm and see the actual numbers.

As continent as it may seem you will never beat the speed of just a simple paper and pencil, or you can use a spreadsheet/ mathcad document. But there are just too many issues setting up complex dynamic simulations to
make it worth the effort for something like this.

Originally Posted by Molten

Who needs such analysis for this? Just make it good and strong and you will be fine. Besides, My philosophy is that if it doesn't look like it might break then it won't. Also, even with stress analysis and exact figures for the computer will still not help anyone out. There is always the fluke accident and though I will agree that stress analysis is good for long-term projects like NASA, it is not that useful for a six-week project like FIRST. Unless you are already an expert in which case go ahead.

I don't agree with this at all. You need to be able to somewhat optimize your design otherwise you are going to end up with an arm that is to heavy and/or a gearing that won't be able to lift it. In many cases it is true that you can overlook doing a complete analysis but teams should always do some form of calculations to get a ball park of what will work. On another note, FIRST robot design is about teaching kids engineering methods, and just saying " make it good and strong" doesn't teach a student anything.



Now here is how simple it is to calculate the torque required on an arm. 1) draw a simple Free Body Diagram (look below)



The Force of the arm = How much your arm weighs total (lbs)

Force of the Ball = How much the ball weighs (lbs)

Arm Length (Al) = The distance from the arm pivot (A in this example) to the center of gravity of the arm. this can be a best guess. If your gripper is at the end then your center of gravity should be somewhere between the middle of your arm and the gripper.

Ball Length (BL) = The distance from the arm pivot (A in this example) to the center of gravity of the ball. (In some cases this can be beyond the end of your arm)

Plug and Chug....Thats really how simple it is. Your answer will be in torque units (ft-lbs, in-lbs, N-m, etc) depending on what units you use, convert as needed. Now remember this is the MINIMUM torque required to get your arm moving and you will need to add a factor of safety to this to make it usable, but ultimately this will give you a good ball park number to start from.