I would like to make my own rotating spits for the grill. My original plan was to use my 12VDC 6W vent motor for it but as it rotates at 2000rpm (minimum is 1‚200rpm) I would need a few reduction gears to take it to wanted 5-6rpm rotation. Also I need the motor to be strong enough to rotate cca. 1.5kg (2 kilograms at most) load (preferably divided on to 3 spits with each getting 0.5-0.6kg load lets say) . Does anyone have an idea how much should my motor have to do that and any suggestions on a motor that would require minimum hardware to accomplish this?
Gear reduction sounds like the right solution.
This puts the normal operating speed of the motor into the right range for your application.
It also greatly increases the torque so that the spit is able to turn unbalanced foods.
Without a gearbox you would need a huge motor as torque depends on the motor volume. 200:1 reduction gearing sounds about right.
Also without a gearbox you'd need a servo system to rotate the motor that slowly accuracy and stably.
Gearmotor is the answer, perhaps a wormdrive is a good option, large ratio in one step and relatively quiet.
Guess what, many commercial spit drives do exactly this!
How much power do you think my motor should have for a load of max. 2kg spread across 3 spits that would be lets say 0.5m long each? (like Ive said my I would settle for 5 or 6 rpm).
Not that much, 6rpm is 0.6 rad/s so even 5W will give 5/0.6 = 8Nm of torque (assuming no losses and that the gears are built to take it).
2kg 20cm off-axis will need a torque of 4Nm, and its likely the off-axis distance is way less than that, so 1Nm is
likely to be plenty.
Assuming 200:1 reduction gear with 30% efficiency that demands a motor of 200*6 = 1200rpm and 16mNm
of torque.
The basic relationships to use are:
power = torque x speed (units watts, Nm and rad/s respectively)
mechanical power = efficiency * electrical power
Note most motors are around 80% efficient, most cheap gear systems 50 to 70%
Another approach would be to look at bbq spit motors on line and see how much power they draw.
wildbill:
Another approach would be to look at bbq spit motors on line and see how much power they draw.
I once bought such a device, thinking I would build something out of it. Never did. Motor was powered by two "D" cells. Would run for hours.
Paul
So basically if I order the 12vdc 10rpm with no load one from this link:
I should be fine?
Well I can't follow the specifications on that page - what is the output torque for that version?
There are 2 I could use, both 12V versions:
- first one is 10rpm without load, 7rpm with load and max torque is 12kgcm
- second one is 6rpm without load, 4rpm with load and max torque is 21kgcm
You know its possible to measure torque? Then you can be sure of the max torque needed.
Do you think 21kgcm (That's around 2Nm) would be enough for what Im trying to do?
What is the torque required? If its less than 2Nm then yes. If the only appreacable torque is due to off-centre mass, then estimate the max mass (kg) and max eccentricity (m) and multiply them together and then by 9.8 for gravitation. So 3kg that's 0.1m off-centre will need 2.94Nm, ignoring friction.
And you can measure torque as I've said as a double check.
Maybe get a wipermotor, they are cheap and have a lot of torque
So if that's correct lets say my 1.5kg-2kg load is equally placed on a 0.5m long spit which is on one end has a motor and on other end its just something to hold it while it rotates, so the center of mass would approximately be on the half of the spit, I would need 2kg0.25m9.81 that equals to 4.9Nm torque? is that too much or just right?
No, not unless this is a large rotating wheel 1m across with the spits sticking out from the hub.
I made a quick 
sketch on how I imagined the whole thing and attached it on this post. I would think that cca. 20kgcm torque would be enough for this?
tuadru's image...
20kgcm seems kind of small for that arrangement. If it is always loaded symmetrical and the pivots are not completely terrible then it should work.
Ive found another motor in the picture. Its specs are:
Voltage
Workable range: 12-30V
Rated: 24V
No load
Speed:8rpm
Current:<=80mA
Load
Speed:6rpm
Current:<= 0.6A
Torque: 30kgcm
Stall
Torque: 25kgcm
Current: 1.8A
Reducer
Ratio: 1000:1
Link:
I think this should do the job?
You have the luxury in this project that you don't have an MBA looking over your shoulder urging you to minimize the cost of the motor.
You may find that your mechanicals are less than perfect and add more friction than you expected.
You will probably decide at some point that you want to cook something asymetric and larger than you originally anticipated.
So over-engineer it a bit. Take your original torque estimate and double it (or more).
Also, remember that this motor will probably be running in hot sun near a hot BBQ. Not ideal conditions for the poor thing. Maybe add a bit more headroom for this too. After all, you're only building one and it'll be more expense and a redesign if you undersize the motor. Don't ask me how I know 