Inertia of a electrically driven cart

[SpecDor]

When sizing motor/gearbox/brake combinations, inertial calculations ar often used. I have to select a gearbox with brakemotor to drive a 10 ton train car back an forth through a factory. I would like to know is there a simple way to estimate the HP. Do I have to calculate the inertia of every moving part to calculate the stopping distance.

 

[Barry1961]

Just calculate the axle and wheel inertia and the rest as pure linear mass. The moving parts of the gearbox will not be significant.

Depending on how fast you are planning on going the wheel and axle inertia could probably be just fudged in.

I would start with a required stop distance or rate of acceleration and calculate horsepower from there. As just a wild guess at what you are doing, 24 in-sec top speed, .05 cf bearings, .05 g accel., 15 inch stop distance, level ground and no speed bumps.

20,000 lb * .05 cf = 1000 lb force to move
20,000 lb * .05 g(19 in-sec^2) = 1000 lb force to accel
(2,000 lb total force * 120)/33,000 fpm = 7.3 hp
7.3 hp * 1.4 efficiency = 10.2 hp
10.2 * fudge = 15 hp

Hopefully your wheel co-efficient of friction will be lower and your slow down distance longer.

One thing to watch when running heavy cars is operators bumping the car into a hard stop at end of line. This will usually cause the input pinion to snap. If you need to put in a torque limiter to prevent this double check the calculations of the torque limit mfg or it may be grossly oversized since they are mostly rated for much higher duty cycles.

The slow down distance in inches would be, .5 * rate in-sec^2 * ( time^2)
To convert from in-sec^2 to g(gravity) would be, rate * 386
To find rate in in-sec^2, from time and distance, (2 * dist”)/ time^2

Barry1961

 

[tygerdawg]

The LazyMan's method: go to the SEW Eurodrive US website and set yourself up to use their PT Pilot software application. If I recall correctly, one of the models they use for sizing their gearmotors is just what you are looking for. After you plug in all of the values (masses, sizes, speeds, accelerations, etc.), it will spit out a list of the correctly sized gearmotors (hp and output rpm) to do the application in ascending order of cost. Similar applications can probably be found at the websites of other gearmotor manufacturers like Dodge, Falk, Nord, etc. I just like SEW because them German guys are very rigorous in their math and methods. For example, SEW will try to account mathematically for all masses and friction loads for this application and produce something close the exact solution. The Dodge software just oversizes it. This seems to be the essence of the difference in the respective approaches of Euro versus US methods: Finesse versus BiggerHammer. Different, but they both work pretty well.

If you want to learn how to do the calculations manually, you can keep digging in the SEW website for the "electronic documentation" link. This will take you to their German English-language website where you can download PDF's of their "practical engineering" manuals. Has examples and instruction so you can learn the stuff we all should have learned in sophomore dynamics.

TygerDawg

 

[MintJulep]

As a very rough estimate, for a subway car each axle/gearbox/motor combination adds the equivalent of about a ton of linear momentum. However, your 10-ton car is very light compared to a typical subway, so your drive gear will likely also be much lighter.

As you obviously know, to really work it out properly you need to calculate the actual rotating inertia. Everything is either a disk or a cyliner, so its really pretty easy.