On Fri, Jun 17, 2022 at 12:25 AM John Dammeyer <jo...@autoartisans.com> wrote:
> Of course doesn't include the bolts nor the ball bearings but does give an > idea of rotational inertia. Monemts of inertia add together. If there is a wheel and a bolt, then compute the inertia of the wheel, then compute the ineria of the bolt when at it's radias from center then add the two to get the total. Typically you need to do this for all the parts, like the shaft and bearings. If the wheel is a complex shape like a disk with a wide rim the cut it up into simpler parts, find the inertia of each part then add the parts. I am in awe of anyone who can do this using American Costomary units (feet, pounds, and such) Even in the late 1970's when I took physics at UCLA the work was all in metric. Yes we did get some problems using feet or BTUs and such but the advice was to convert up front, work the problem then convert back. Seriously, the American unit of mass is "slugs", not pounds (ounds is an American unit of force, not mass) and if you don't keep this straight the answer is wrong. > -----Original Message----- > From: Chris Albertson [mailto:albertson.ch...@gmail.com] > Sent: June-16-22 10:29 PM > To: Enhanced Machine Controller (EMC) > Subject: Re: [Emc-users] Acceleration question. > > OK, here is the problem. > > 1) we can not know the distance from the mass to the center of the wheel. > Mass is always distributed, It all cannot be at the same radius. S We > define a concept called "moment of Inertia" that tell us how much a > rotating body resists changes in rotation speed. Your first step is to > compute the wheel's moment of inertia. If the wheel has a simple shape > then there are formulas you can use. If the wheel has a complex shape > then it is more work to find the moment. > This article explains what moment of inertia is and shows how to calculate > it for various shapes > <https://en.wikipedia.org/wiki/List_of_moments_of_inertia> https://en.wikipedia.org/wiki/List_of_moments_of_inertia > > 2) now that you know how much your wheel resists changes in rotational > speed you can compute how much torque is required to accelerate the wheel. > torque = (moment of inertia) x (rotational acceleration) > This is just like the better known "f = ma" that works for linear motion. > See here for more > <https://en.wikipedia.org/wiki/Angular_acceleration#Relation_to_torque> https://en.wikipedia.org/wiki/Angular_acceleration#Relation_to_torque > > 3) so now you know the torque required. You need to find a mother that has > this torque at the required RPM. Moros produce less toque at higher > speedSo you can not use the motor's rated torque at stall (zero RPM) you > must look at the torque vs. RPM graph > > The trick is to watch the units. Keep the rotation units in radians per > second and acceleration in read/second squared > > > > On Thu, Jun 16, 2022 at 7:50 PM John Dammeyer < <mailto: jo...@autoartisans.com> jo...@autoartisans.com> > wrote: > > > I'm not sure you are answering my question. > > > > Let me put it another way. Assume you know the distance of the mass from > > the center. And assume it's not possible to do any testing at this time. > > But you know the dimensions. > > > > Once the wheel is spinning it doesn't take a lot to keep it spinning. > > Friction and air resistance mostly unless it's also being asked to > > translate the spin back to some sort of work. > > > > Then let's say the need changes and now 2.5 seconds are required to bring > > it up to speed instead of 5 seconds. What size motor then? > > > > > > > > > -----Original Message----- > > > From: Chris Albertson [ <mailto:albertson.ch...@gmail.com> mailto: albertson.ch...@gmail.com] > > > Sent: June-16-22 6:56 PM > > > To: Enhanced Machine Controller (EMC) > > > Subject: Re: [Emc-users] Acceleration question. > > > > > > Knowing the mass of the wheel is not enough, you need to know how far the > > > mas is from the center of rotation. They call this "Moment of inertia" > > > There are ways of calculating this for simple wheel shapes like a plain > > > disk but for anything else you are best off if you just measure it. > > > > > > The simplest way you can find your answer with not much math is to wrap a > > > string around the wheel and attach a weight to the string and time how > > long > > > it takes for the weight to fall some distance. The weight will apply a > > > torque to the wheel equal to the weight times the radius of whatever you > > > wrapped the string around. > > > > > > Make the weight bigger until it works, then buy a motor that can supply > > > that torque, plus a bit more. > > > > > > if you really want to calculate the moment, perhaps because you have not > > > yet built the wheel then remember that the wheels moment is equal to the > > > some of the moments of the parts of the wheel (the parts add up) So > > divide > > > the wheel into (say) a rim, a thin disk and a hub find the moment of each > > > and then add them. > > > > > > But the string experiment is easier. > > > > > > On Thu, Jun 16, 2022 at 5:53 PM John Dammeyer < <mailto: jo...@autoartisans.com> jo...@autoartisans.com> > > > wrote: > > > > > > > OK. I realize this will be a dumb question but please bear with me > > > > especially since I've included the ability to accelerate in my > > Electronic > > > > Lead Screw project. > > > > > > > > A friend and I were discussing bringing a 300 pound flywheel up to > > speed. > > > > Vz=0 RPM, Vf=50 RPM. Reduction drive to the flywheel shaft is 32:1 so > > > > final speed of motor is 1600 RPM. > > > > > > > > Assume we're happy with 5 seconds to accelerate for Tz to Tf. Motor > > > > voltage is 12V. > > > > > > > > We have the mass, we have the velocity, we have the time and motor > > > > voltage. The question is what are the calculations to determine how > > much > > > > current the motor will require to create this acceleration? Assuming > > of > > > > course the motor is 100% efficient. > > > > > > > > We're getting all confused with F=ma and 1/2*a*t^2 etc. > > > > > > > > What size motor is actually needed to do this? > > > > > > > > Thanks. > > > > John > > > > > > > > "ELS! Nothing else works as well for your Lathe" > > > > Automation Artisans Inc. > > > > www dot autoartisans dot com > > > > > > > > > > > > _______________________________________________ > > > > Emc-users mailing list > > > > <mailto:Emc-users@lists.sourceforge.net> Emc-users@lists.sourceforge.net > > > > <https://lists.sourceforge.net/lists/listinfo/emc-users> https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > > > > > > > > > -- > > > > > > Chris Albertson > > > Redondo Beach, California > > > > > > _______________________________________________ > > > Emc-users mailing list > > > <mailto:Emc-users@lists.sourceforge.net> Emc-users@lists.sourceforge.net > > > <https://lists.sourceforge.net/lists/listinfo/emc-users> https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > > > > _______________________________________________ > > Emc-users mailing list > > <mailto:Emc-users@lists.sourceforge.net> Emc-users@lists.sourceforge.net > > <https://lists.sourceforge.net/lists/listinfo/emc-users> https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > -- > > Chris Albertson > Redondo Beach, California > > _______________________________________________ > Emc-users mailing list > <mailto:Emc-users@lists.sourceforge.net> Emc-users@lists.sourceforge.net > <https://lists.sourceforge.net/lists/listinfo/emc-users> https://lists.sourceforge.net/lists/listinfo/emc-users _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users -- Chris Albertson Redondo Beach, California _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
