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 Help! Inertia / Forces Post Reply Forum
 Posted by: newpunkrkr ® 02/21/2006, 09:44:46 Author Profile eMail author Edit I posted this before but didn't any help. I'm not a student doing homework problems. I'm production manager for a steel brokerage and am building some machinery. Any help or advice would be greatly apprecaited. Thanks in advance!I've got an aluminum rectangular mass with two steel pins that I need to rotate at a high speed. It is 26" x 4.5". The aluminum weighs 46lbs, and the pins are 8.5lbs each. The pins are centered 11.25" left and right off center. I need to spin it 90 degrees in .450 sec (start and stop), via a 1 7/16" shaft mounted below in center. I'm having trouble calculating the moment of inertia and the forces/torque required to preform that operation. I looked at the calculators on this page but they are all for cylinders, so I'm unsure on the conversion to my application. Any help or suggestions to get me in the right direction would be appreciated. Build it, Break it, and then Build it Better

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 Re: Help! Inertia / Forces Re: Help! Inertia / Forces -- newpunkrkr Post Reply Top of thread Forum
 Posted by: zekeman ® 02/21/2006, 11:43:41 Author Profile eMail author Edit The moment of inertia of the rectangular mass is: W/12(d^2+b^2) and the two pins are approximately 2*w1*r^2 where W= 46 w1= 8.5 b=26 d= 4.5 r=11.25 I get I=2669+2151=4820 lb in^2 If you want ro rotate this mass in .45 sec, the most efficient way is to accelerate it at a constant rate for 1/2 the time and decelerate it for the remainder bringing the mass to rest in .45 sec. The average angular vbelocity would be (p1/2)/t rad/sec and the peak is double or pi/t=6.98 rad/sec Since you accomplish this in .45/2 seconds the acceleration is 6.98/.225=31 rad/sec^2 From basic mechanics the minimum torque required excluding the driver would be: I*30.2=4820*30.2/g=4820*31/32.2=4645 inch pounds. To be more accurate you would have to include the moment of inertia of the driver and the pins about the1r own centerline. How do you plan to accomplish this?

 Re: Help! Inertia / Forces Re: Re: Help! Inertia / Forces -- zekeman Post Reply Top of thread Forum
 Posted by: newpunkrkr ® 02/21/2006, 12:41:41 Author Profile eMail author Edit My calculations matched yours. And I used: 2*(Icom + M h^2), to adjust the Inertia for the pins adjusted on center. Does that make sense. also, for "I*30.2=4820*30.2/g=4820*31/32.2=4645 inch pounds" - where did the 32.2 come from?Thank you for your help Build it, Break it, and then Build it Better

 Re: Help! Inertia / Forces Re: Re: Help! Inertia / Forces -- newpunkrkr Post Reply Top of thread Forum
 Posted by: newpunkrkr ® 02/21/2006, 12:46:54 Author Profile eMail author Edit I retract that question... I realised it's gravity. I didn't see the g in there. Thanks. Build it, Break it, and then Build it Better

 Re: Help! Inertia / Forces Re: Re: Help! Inertia / Forces -- zekeman Post Reply Top of thread Forum
 Posted by: newpunkrkr ® 02/21/2006, 11:54:30 Author Profile eMail author Edit The idea is to spin it with a Servo via Belt Drive. We require +/- .001 tolerance in the stop locations around the circumfrence, so chain and direct drive gears seem out of the question due to play and wear issues. Any thoughts? Build it, Break it, and then Build it Better

 Re: Help! Inertia / Forces Re: Re: Help! Inertia / Forces -- newpunkrkr Post Reply Top of thread Forum
 Posted by: zekeman ® 02/21/2006, 12:57:38 Author Profile eMail author Edit I think you had better rethink that .001" tolerance at the pin locations. A belt drive will never do it, since it will be encumbered with slack and wear problems, etc. If you really need that tolerance then other very expensive solutions are available. If you have the bucks you can get a solution.

 Re: Help! Inertia / Forces Re: Re: Help! Inertia / Forces -- zekeman Post Reply Top of thread Forum
 Posted by: zekeman ® 02/21/2006, 11:48:08 Author Profile eMail author Edit Correction The result is ok but I erroneously used 30.2 instead of 31 for the acceleration in some of the steps.

 Re: Help! Inertia / Forces Re: Help! Inertia / Forces -- newpunkrkr Post Reply Top of thread Forum
 Posted by: swearingen ® 02/21/2006, 10:05:50 Author Profile eMail author Edit Can you provide a dimensioned picture of the item? I'll be able to give you an answer, but I'm having trouble visualizing what this thing looks like.

 Re: Help! Inertia / Forces Re: Re: Help! Inertia / Forces -- swearingen Post Reply Top of thread Forum
Posted by: newpunkrkr ®

02/21/2006, 10:40:09

Author Profile eMail author Edit

Thanks - attached is a picture.

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