inginuity required

[Vass]

I'm building a turntable from wood that will operate outside, approx 3 feet in diameter. I have an old electric wheelchair motor that works on 12 volts and rotates approx 200 revs per minute. (no speed controller) I'd like to get this down to about 20 revs per min. I thought I could drop the turntable on one of the wheels and hope the friction would turn the edge but this does'nt appear to work Any ideas how I can get this to work with basic household/garage parts? (I've not got suitable chains and sprockets, belts and pullies in my garage) Come on uk.d-i-y here's a challenge for you. TIA

[Bob Minchin]

Why does your scheme not work then? To get the speed you want, a wheel of 3.6" diameter is needed. How about a childs push chair wheel or a wheel from a medium sized castor.

Bob

[The Natural Philosopher]

Drop the voltage to about a volt. With a capacitor to give full voltage 'start ' volts. Probably a very fat resistor of a few ohms, and a big electrolytic would do the job.

But really, you want a gearbox.

See if you can cannibalise an old belt drive turntable..

[The Medway Handyman]

Do you mean with the drive wheel vertical & the turntable horizontal?

How about using both horizontal so the wheel turns against the edge of the turntable?

[newshound]

More info required. Is the turntable just supported on an axle or will you use additional support at the periphery? What sort of load does the turntable carry, what sort of bearings will you use, is the load centred or does it put a moment on the axle. Once you have that lot, we can think about the torque required and see if it is feasible to have a frictional drive as you are suggesting, or whether you might need something like a chain or gear.

[Dave Liquorice]

Direct drive from the motor shaft to the edge of the turntable I take it?

Smooth timber against a smooth metal isn't going to have much friction. Try wrapping the shaft in self amalgamating tape to give a soft rubbery surface with lots of friction. You may well have to rig up some form of sprung pivot mount for the motor to keep the shaft pressed against the edge of the turn table unless your engineering of that is *very* good.

With the 10:1 reduction required the "shaft" dia will need to be about 3 1/2" that wouldn't be practical to build up with SA tape...

The simple and easiest solution is probably a belt drive of some sort that will take up the engineering tolerances for you. Pullies can be made from wood, a round block of suitable diameter with ply sides to create the grove for the belt. The belt can be an old fan/timing/washing machine drive belt.

[Clint Sharp]

In message , Vass writes

Does your turntable turn with little effort when not on the wheel? Wheelchair motors are fairly high torque so I can't imagine it doesn't have the power (unless you're loading it down with tons of weight), rather your mechanical design has a flaw (and that's not a criticism) that you've not spotted. Perhaps you need to sit the turntable on another two wheels or castors?

Find a scrap pushbike somewhere, judicious application of angle grinder or hacksaw will yield a nice selection of bits you can use.

[Vass]

I can use the shaft or keep the wheel on it. I'm having to fix the other end of the shaft or else the diff kicks in and I get no drive at all at one end Like the idea of putting the shaft against the edge of the turntable, that had not occurred to me. Everything horizontal, Table will carry approx 8 kilos. bearings for turntable? I have no idea, was going to use 4 small castors from a sofa. I could use bungies for the pressure against the table. will give this a go, maybe supply pics if this fails... Ta for help so far folks.

[newshound]

Well, four casters from a sofa aren't designed for high mileage. If it's just for intermittent use, OK, but if for continuous rotation for a year or more they might wear out. I assume you mean to use these to take the table weight? You might only need three, but you also need something to keep it centred. If you are using a belt drive then the belt tension will put a radial load on the table, as will pressure from a friction drive. Bungies are definitely a good idea for this. You might get away with metal on wood for your radial bearing, or you could use some more castors (at least two, using the belt or bungee tension to keep the table in contact with them, assuming they are "fixed" to something non-moving).

Personally, I would probably try to use rolling element bearings for low friction and long life. At low speed you can run them without lubricant even in wet conditions. Or, if you keep the rain off, grease will give even lower friction. You might adapt something like a washing machine drum bearing, a car wheel bearing assembly, or an old bicycle wheel. With this layout, a single bearing assembly (probably two bearings) will take both the radial and axial (vertical) load.

[Roger Mills]

In what way doesn't it work:

- is the speed wrong?

- does it stall the motor, or

- do you get wheel-spin?

[Andy Dingley]

Should do, so why not? Most likely you have insufficient force down onto the wheel, thus no traction. Either the disk must flex, the bearing must allow movement vertically, or you'll need to lift the drive wheel with some adjustable (and sprung) linkage.

[Vass]

its slipping but I dont have the load on the table yet.

[Vass]

its for 3 weeks

I'll pop down the local tip, see what they have on offer ta

[The Medway Handyman]

What ferzacerly is this for then? We are all intrigued?

[Vass]

we have a scarecrow festival soon, mine will be animated and kids can press a button to spin the crows :-)

[Vass]

here we are so far...

[The Medway Handyman]

How did you cut the circle & how 'circular' is it?

[Bob Minchin]

If you want the turntable speed to be one tenth of the shaft speed as you originally stated, then you need a wheel of 3.6 inches fitted to the motor shaft.

Bob

[Vass]

string and jigsaw, its pretty good speed appears usable with other shaft held so might just add something to the shaft to ensure good adhesion dont need it to be too quick, the scarecorws might fly off !