Determining necessary motor power to move a cart along tracks.

[SCTrojan09]

I'm looking into developing the drivetrain for a cart which will be battery powered and travel at low speeds (around 0.33 m/s) along metal guide rails using v-groove wheels and weighing in around 4,000 kg. I'm trying to determine the necessary electric motor size to power the cart but my calcs just seem low. Am I missing something?

It looks like to accelerate at 0.1m/s^2 requires 12Nm of torque using a 60mm diameter wheel. (F=ma for a force of 400N times the 0.03m radius) This appears to work out to 3 hp when using an 1800 rpm electric motor. The rolling resistance appears to be 196N using a rolling resistance coefficient of 0.005. That works out to 0.09 hp using the 0.33 m/s. Summing that up comes to 3.1 hp, which even with a transmission efficiency of 80% is still only 4hp. Can I really get away with a 4 hp motor? I realize I will add on an additional fudge factor but it still seems low.

 

[hydtools]

Compare with existing equipment.
For example:

http://www.saferack.com/capstans-and-railcar-movers.cfm

Ted

 

[drawoh]

SCTrojan09,

I am getting your numbers. It will take 3HP to get your cart up a 10[°] grade. You might want to work out the power needed to get your cart over a rock or a bolt sitting on your tracks. Your cart is moving very slow.

I visited a marine museum where they had a propeller on display. It was from a fishing boat. The propeller was about my size, around six feet. The boat's motor was capable of around 17HP. My uncle had a 200HP motor boat with a propeller I recall as being around 10" diameter, so I was a little shocked. I worked it out, and it turns out that at a believable fishing boat speed of 10mph, you have over 600lb thrust.

JHG

 

[MikeHalloran]

Your stated wheel diameter of 60mm just seems rather small, given the load.

Mike Halloran
Pembroke Pines, FL, USA

 

[SCTrojan09]

Thanks for the replies, I will take a closer look at the effect of debris on the tracks and such. Also, 60mm seems a little small to me also, but I only just started looking today and those were the largest v-groove wheels I had found at the time. If you have any suggestions as to where I might look for larger v-groove drive wheels I would appreciate it.

 

[MikeHalloran]

V-groove wheels are available from caster manufacturers, and are typically much larger than 60mm diameter.

I'm no fan of metal/metal friction drive, either. If you can guarantee a dry rail, maybe it will work.





Mike Halloran
Pembroke Pines, FL, USA

 

[hydtools]

If you use two v-rails, make sure there is lateral play in the wheels on one side. Otherwise, I assume one rail is flat to accommodate rail gage variation and only one v-rail actually guides the car.

Ted

 

[tygerdawg]

I didn't see some essential calculation elements, perhaps you already did it:
1. acceleration time to achieve steady state speed
2. did you account for all of your rotational masses in the acceleration & torque calculation?

It really doesn't require much torque to keep a moving mass in motion. You must supply the peak torque to get it moving though. Peak torque is a function of translational mass, rotational mass, friction torque, efficiency loss, and on and on and on. Then the faster you want to get it moving, the higher the power required to do so.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[MintJulep]

Well, five guys can get a subway car moving, so 4 HP should be plenty for one little cart.

What's wrapped up in the "rolling resistance coefficient"?

Do you have bearing resistance in the mix?

I agree with others that 60 mm diameter is too small. In fact, why use V-groove wheels? Something like

http://www.grainger.com/Grainger/HAMILTON-Single-Flanged-Cast-Iron-Track-Wheel-5VK10?Pid=search

would seem to be a better choice.

 

[Bobber1]

The rotating restaurant at the top of the Space Needle in Seattle is powered by 1/4 horsepower electric motors. But I'm not sure how many. And it doesn't have to accelerate except when they turn it off for maintanance. And it only goes one revolution per hour.

The bathrooms are in the stationary central core, and if you spend more than a couple of minutes in there, you will never find your way back to your table. More than once I returned to "my" table and sat down with a stranger.