Motor Sizing and Power Requirements 1

[AdamDrover]

Hello.
New to this forum, but it seems to be just what I'm looking for. I am having a problem with a design and I'm hoping someone can help.

I have a horizontal powerscrew mechanism that needs to push a beam 55". I am using a higher than normal coefficient for friction just to ensure that the design will work. Coefficient is 0.6. The weight of the beam that needs to be pushed is 480 lbs. I have calculated that the friction will be 30 lbf, but now I need to solve the problem of how much force will it take to push this beam, along with how much power is required from a motor to push it. I have provided an FBD of the problem.

If someone could point me in the right direction, that would be great. Thank you in advance.

-Adam Drover

 

[alansimpson]

Some useful information in Lenze.com catalog if you download it. Other drives suppliers like sdp-si.com or misumi have good engineering guides.

 

[bvanhiel]

My advice: review your calculations, you're already underpredicting the force by 10X. Then review the difference between force and power. The worlds smallest motor could move your beam if Archimedes would lend his lever.

The right solution depends on more than the forces. It depends on how fast, how far and where you need to apply that force.

 

[imcjoek]

30 lbf seems awfully low, unless your slide has some kind of linear bearings in it.

Next step is to look at 'power screws' in a machine design book (or machinery handbook) so that you can relate linear force with torque to turn the screw.

Then you need to know what your desired motion profile looks like so that you have some speeds/accelerations to work with. Do not neglect the mass of the beam as you need to get that moving in the first place and the force to do so can be much higher than when acceleration=0.

 

[MintJulep]

480 x .6 is not equal to 30

There's this thing called inertia.

 

[dvd]

If your guiding is too sloppy there is a good chance for binding. With excessive misalignment you may damage the screw mechanism. Is this steel-on-steel? Some sort of hardness differential and lubrication provision is important. Linear bearings are always a good idea. Another point to raise is the buckling length of the screw. Most machine screw actuator manufacturers publish data. I happen to like Duff-Norton.

http://www.duffnorton.com/literature/mech/090-EngineeringGuide.pdf

See page 98

 

[tygerdawg]

Like alansimpson said. I've found that gearmotor catalogs downloaded for free from various manufacturers have a wealth of this type of information. My favorite was the English-language version of the SEW-Eurodrive engineering application guide downloaded from the German site. Don't even know if you can still get it or not. Many power transmission product companies have online calculators for certain mechanical configurations that may help. Dodge, Emerson PT, others.

I also recommend you websearch & find the "Smart Motion Cheat Sheet", it has all the dynamics equations summed up nicely.

Your path:
1. accurately determine your loads
2. accurately specify your required speeds and required accelerations of the various inertias up to the required speed. You must account for all inertias, translational and rotational.
3. specify your mechanics: lead screw, gearing, wheel types, bearings, etc. and determine how they impact output motion and/or torque requirements through friction or gearing.
4. plug all of this into the various equations to get various values (peak torque, running torque, horsepower)
5. use that to determine minimum motor size (and/or gearmotor if necessary). This is usually a bit of an iterative excercise.
6. from the catalog, pick the next higher motor size if you're confident of your estimates. If you're not conifident, then upsize it by 2X or 3X. Bigger is not necessarily a bad thing, but heavily oversized is wasted money. Gearmotor mfgs talk about "starts per hour" which is heat-related issue. Start your motor very frequently, and all those electrons will heat it up. If so, you'll need bigger thermal mass (upsized motor) to keep it from melting.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com