IainB
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| posted on 8/12/05 at 05:23 PM |
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Arrgghhh!, in need of help with DC motors/gearboxes for uni project!
The project involves designing a golf cart type buggy for off road use, has to be electric so no rover V8s 
We are having a DC motor (with wormwheel type gearbox) powering each wheel with independent skid steering. Im trying to calculate torque per wheel
required for buggy to climb 40 degree slope at const speed (assuming no friction just now). Target speed is 7m/s (420m/minute)
Mass of buggy = 360k / 90kg per wheel
(assume 50:50 weight distribution)
wheel radius = 0.225m
Slope = 40 degrees.
Does any one have knowledge of large DC motors? I calculate a torque of just under 130Nm needed per wheel.
Similar types of motor i have found seem to rev to around 3000rpm.
With vehicle speed set at 7m/s, wheel rpm is just under 300rpm.
Using a gearbox ratio of 10:1 this would mean a motor torque of 13Nm is required.
Is this impossible? Not feasible?
Also is the stall torque of a DC motor larger than its continuous rated torque? (need to calculate starting from standstill on slope)
If ur lot's knowledge of this is anything near that of cars we should have this sorted in no time!!
Will be eternitily grateful if any one can help here, large crate of Tennants waiting if ur in Glasgow!!
Thanks,
Iain
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MikeR
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| posted on 8/12/05 at 05:30 PM |
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instead of using worm gears look at what NASA did for the luna rover. They designed a 'new' type of gearing that was very effective.
Made a copy in lego when i was a kid and it worked quite well!
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Deckman001
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| posted on 8/12/05 at 05:33 PM |
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A link would be nice ???? 
Jason
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MikeR
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| posted on 8/12/05 at 06:35 PM |
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don't have one, sorry, but a search should find something......
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chriscook
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| posted on 8/12/05 at 06:50 PM |
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Did some research on DC motors for a solar powered vehicle we built at uni. If you look up Maxon Motors their catalogue has all the calculations you
need for motors.
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bigandy
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| posted on 8/12/05 at 07:18 PM |
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I've just bought four servo motors (DC brushed) for a CNC project that I am hoping to start designing.
The continous rated current of these is only 2Amps (at 60V), with the continuous torque being 28.5N/cm. However the startup/Stall current is given as
19.2A (at 60V) with the torque given as 200N/cm. However, there is a note in the datasheet to say that the Stall current can only be applied for
limited periods of time.
These figures are taken direct from the data sheet, so in answer, yes the stall torque is greater than continuous rated torque, but it can only be
used intermittently.
Hope that helps!
Cheers
Andy
Dammit! Too many decisions....
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IainB
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| posted on 8/12/05 at 07:19 PM |
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Cheers guys, The Maxon Motors site is great, Thank you!
A 250W motor has a rated "Stall Torque" (From 0 RPM i guess) of 13.9Nm. Is this the same as continuous torque though?
Cant seem to find much on the luna rover gears.
Keep it comin, Ta!
http://s43.photobucket.com/albums/e365/IainB1986/
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IainB
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| posted on 8/12/05 at 07:32 PM |
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Ahh cheers Andy, that explains some of the data i have found about motors 
I am going to re calculate from scratch anyway as i think i have been a moron somewhere along the way...
I hate electrics. Couldnt even get mates indicators workin on his Tiger Cat! (he did tell me a week later that the stalk switches were broken!
grr)
Anyway, cheers for all the help. Have to get this finished by tomorrow so i can play with spanners, bodyfiller and, oh wait, more (Italian) electrics
on sat, god help me
Iain
http://s43.photobucket.com/albums/e365/IainB1986/
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smart51
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| posted on 8/12/05 at 07:41 PM |
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Stall torque is the torque that the motor can provide when the rotor is stationary or stalled. for a DC motor this is the peak torque.
Your 13Nm 300W motor will have a gearbox on it and the output of the gearbox will be 13 Nm.
A motor that has a direct output torque of 13Nm and a rated speed of 3000 RPM will have a rated output of 4kW or so. If that motor and its'
drive circuit are 75% efficent then you will need to put 5kW of electricity into it to get full power. You won't get that out of a car battery,
at least not for long.
If you have one of these connected to each wheel then you can expect a top speed towards 60MPH in a buggy. Perhaps your maths is a bit out.
[Edited on 8-12-2005 by smart51]
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omega 24 v6
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| posted on 8/12/05 at 07:41 PM |
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Don't know to much about the motors but thought about the 40 degree slope and then wondered if your thinking of 90kg per wheel was accurate.
Surely the rear wheels will bear more of the weight than the front ( while it is on a slope that is)
[Edited on 8/12/05 by omega 24 v6]
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steve_gus
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| posted on 8/12/05 at 07:44 PM |
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http://www.deutsches-museum.de/ausstell/dauer/masch/e_masch3.htm
and
http://www.engineeringtalk.com/news/vct/vct117.html
they are called harmonic drives
lunar its spelt, NOT luna
atb
steve
[Edited on 8/12/05 by steve_gus]
http://www.locostbuilder.co.uk
Just knock off the 's'!
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MikeR
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| posted on 8/12/05 at 10:42 PM |
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one r, i dropped an r ............ tish, some people are just soooooo sensitive :p
i mean what difference does an r make?
Just read the links and the harmonic drive is what i'm talking about.
In lego you take a toothed wheel with a input shaft on it, put a chain around it that is a couple of links too big. then take another input shaft,
attach a lever to it which then has something that fits in the gap between the toothed wheel and chain. Rotate the second input shaft and the first
input shaft (ok, ok, output shaft) moves slowly.
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steve_gus
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| posted on 9/12/05 at 12:23 AM |
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my last post almost looks as if I knew something about the subject...... thats the effect of adding an 'r'
and google
atb
steve
http://www.locostbuilder.co.uk
Just knock off the 's'!
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IainB
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| posted on 14/12/05 at 09:51 PM |
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Cheers guys, got the report in yesterday and its all lookin good
the team now think im a member of some kind of online cult the way everyone chipped in and helped out!!
thanks again,
Iain
http://s43.photobucket.com/albums/e365/IainB1986/
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MikeR
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| posted on 14/12/05 at 10:44 PM |
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errrm, you are part of an online cult!
now i hope we get credit for your work, if not we'll have a quiet word in teachers ear :p
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Simon
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| posted on 15/12/05 at 12:02 AM |
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Iain,
Try contacting a gent on here call Bob C - check his website, I could be wrong but think he makes his living from this sort of thing.
ATB
Simon
PS http://freespace.virgin.net/bob.carter/
[Edited on 15/12/05 by Simon]
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DarrenW
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| posted on 15/12/05 at 10:25 AM |
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I bet DeWalt cordless drill motors would do the job very nicely. The factory in Spennymoor, Co. Durham still has a design centre their. It could be
worth dropping them a line. When i worked their they were always helping people like yourself. One of the young lads there built a robotWars robot
(Firestorm) using them. They are not the commercially available Chinesemadebythemillion motors, they are a special development made by DeWalt
themselves.
One of the Engineering Managers is called Vince Carr. Dont know if he is guru with motors but al least you will get letter passed reception. iirc
address isBlack & Decker / DeWalt Power tools Ltd, Green Lane Ind Est, Spennymoor, Co. Durham. Do a search to get post code.
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