go_away

### Author Topic: Rotating saucer or plate  (Read 1912 times)

0 Members and 1 Guest are viewing this topic.

#### Narkaleptic

• Beginner
• Posts: 1
##### Rotating saucer or plate
« on: July 09, 2008, 08:59:53 PM »
Think your lazy susan spice rack.  Something that can support unbalanced weight on top of the plate around the perimeter of the base (ball bearings or wheels) with a motor perpendicular to the axis with a couple of bevel gears.  I want to accurately select the angle, possibly a cheap encoder?

How would I go about designing something like that?  What should I use for material?  What should I use for a motor and gears?  I am not sure how to determine the required torque.  Any advice would be greatly appreciated.

The plate would be between 12" and 18" and holding between 5 and 10 pounds.

My budget for that piece is about \$100 US.

#### Cotowar

• Full Member
• Posts: 50
• Ubuntu for Humans
##### Re: Rotating saucer or plate
« Reply #1 on: July 10, 2008, 12:44:38 AM »
Okay, it took me a second to understand what you were talking about, but its 2:35 Am here and I've been up since 7 yesterday morning. Don't ask me why, its been a long day.

Here is my answer to you. I do not have a pencil or paper handy, nor do I remember the formulas from school, but I think you can find out how much torque is required to rotate the platter by assuming the weight is all along the axis of rotation. Then you can calculate the torque supplied through the gear ratio you have selected. Pick a motor with an applied torque value just larger than the required value, leaving room for error, so say ~15% - 20% above the required value.

As my father has spent the last 20 years working with gears, and I have grown up my entire life around them, I can tell you this. Do NOT use a bevel gear. Use Helix gears, they are smoother, and loads more efficient. Also, you have the beautiful feature of being able to just slam the motor to a stop and not break the teeth off. They will be amazingly quiet, and are really the best bet.

My calculations could be incorrect, you might want to check this, and use some logic before you take that piece of advice, but do listen to me about the gear type. bevels are a pain in the ass.

Check McMaster.com for motors, gears, and everything else you will need. I would suggest zinc coated steel for the gears, but thats type I'm most familiar with. Excellent corrosion resistance, good wear resistance, and you'll get 50k cycles easy from them.
--Cotowar--

#### Soeren

• Supreme Robot
• Posts: 4,672
• Mind Reading: 0.0
##### Re: Rotating saucer or plate
« Reply #2 on: July 10, 2008, 07:26:25 PM »
Hi,

Better use a worm gear. That way you can use a small (and cheap) motor, since the gear ratio will be high (1:n, where n is the number of teeth around the perimeter of your plate - the torque of the motor will be multiplied by n).
Worm gears has the added benefit of "locking" when the motor is not rotating.

two plates with a circular groove near the perimeter with steel or glass "balls" will make the plate go round with little friction and good support for unbalanced loads. Top half should contain the teeth driven by the "worm" mounted on the motor axle.

An angular decoder can be built into it or along the rim - number of bits depends on the resolution needed of course.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

#### Cotowar

• Full Member
• Posts: 50
• Ubuntu for Humans
##### Re: Rotating saucer or plate
« Reply #3 on: July 10, 2008, 09:53:40 PM »
worm gear works too, but is less efficient. Never really used them much, mainly just bevels, helixes, and standards. Locking is an added bonus, but at the same time, if your motor locks when it shuts off(some do), you wont have to worry about that.
--Cotowar--

• Supreme Robot
• Posts: 11,680
##### Re: Rotating saucer or plate
« Reply #4 on: July 19, 2008, 11:41:28 AM »
Considering the items going on this table could vary a lot, you should calculate torque for worst case.

torque = force * radius = mass * acceleration * radius = mass * radius^2 * rotational_acceleration

You will have to figure out your mass, radius is the distance the center of mass is away from the center of rotation, and make up your own desired acceleration.