# Find 1 RPM Motor for "Barn Door" Project!

• Cremer
In summary, the conversation is about a user seeking help with a project that involves building a device to track stars, known as a "barn Door." The user is having trouble finding a 1 RPM DC motor and is considering using a gearbox to achieve the desired speed. The conversation then discusses the potential issue of torque and the use of stepper motors and falling weight systems. The user is seeking clarification on how to calculate the number of steps needed for a stepper motor to achieve 1 RPM and shares a link to their project. Suggestions are given for using a straight rod instead of a curved one, and for using software to adjust the motor speed for different altitudes.
Cremer
Hello all.

This is my first post as I need some help.

I'm building a "barn Door" project which I'm sure you all know is a device to track the stars.

With the measurements I've built it to I need to drive the screw at 1 RPM.

I'm finding it very hard to get a 1 RPM DC motor either 230v or 12v.

Does anyone know of a source?

If I went the route of a gearbox of say 6000:1 what speed motor would I
need to attach to this gearbox to obtain 1 RPM.

Problem is I would like ample torque as well.

Hope you can help.

Cremer

Cremer said:
With the measurements I've built it to I need to drive the screw at 1 RPM.

I'm finding it very hard to get a 1 RPM DC motor either 230v or 12v.

Does anyone know of a source?

I would look at www.globalspec.com a search through the AC or DC motors. You'll never find a regular motor that runs at 1 rpm, you'll have to get a motor with a gearbox on it that reduces the shaft speed to your desired value.

Cremer said:
If I went the route of a gearbox of say 6000:1 what speed motor would I
need to attach to this gearbox to obtain 1 RPM.

A 6000rpm motor of course.

Cremer said:
Problem is I would like ample torque as well.

Torque is a more difficult issue, and will depend on the geometry and weight of the mechanism being moved.

Search on Gear Motors. You may also try Synchronous Gear Motors.

A better choice for this kind of application would be a stepper motor. You can turn them at pretty much any speed and they will be very accurate positionally. I would look around at suppliers to see if they have anything that will fit your specs.

Note that the problem of torque is largely self-solving due to the need for a large gear reduction for speed control, as long as the mount is going to be balanced.

It would seem like a stepper motor might tend to excite vibrations in the structure, not a good thing for an optical system.

Thanks guys and I'll look through those sites.

I did consider stepper motors but someone said that the steps to incite slight movement just like dr D has mentioned.

Dr.D said:
It would seem like a stepper motor might tend to excite vibrations in the structure, not a good thing for an optical system.
Stepper motors are used in highly accurate machining centers and the like. I have never heard of an induced vibration duer to one either. I am not quite sure how that would happen.

Telescope vibration, telescope motor vibration.

http://www.bbastrodesigns.com/vibration.html"

"Stepper motors snap from step to step when operating in fullstep or halfstep mode. This causes a jittery motion in the eyepiece that can be quite objectionable..."

Last edited by a moderator:
FredGarvin said:
Stepper motors are used in highly accurate machining centers and the like. I have never heard of an induced vibration duer to one either. I am not quite sure how that would happen.

Machining centers are usually very compact and stiff. Telescopes and other optical systems are often quite extensive (long dimensions) and necessarily not very stiff (too much weight) so that they cannot enjoy the stiffness of a machining center.

Phrak said:
"Stepper motors snap from step to step when operating in fullstep or halfstep mode. This causes a jittery motion in the eyepiece that can be quite objectionable..."
Not by the time it's gone through a 600:1 gearbox!

Have you considered just a falling weight - historically it's worked pretty well for smooth star tracking.

mgb_phys said:
Not by the time it's gone through a 600:1 gearbox!

Have you considered just a falling weight - historically it's worked pretty well for smooth star tracking.

The only thing the 600:1 gear box will add to the system is inertia. The frequency of the pulsations will go through the gear box unmodified, so that if it excites a structural resonance, you still have a problem.

The falling weight idea is a good one, provided that friction does not consume to much energy and does not develop stiction induced vibration.

So say I did use a stepper motor...

I have a 1.8 degree 200 stepper motor which means that one step = 0.06 degree.

How do I get it to turn exactly 1 RPM

360 degree divide by 0.06 degree = 6000 steps.

I think I need help

Last edited:
If you have a 200count stepper that bounces 1/2 step you are going to need a very tight gearbox for that frequency of movement to transmit through to the output shaft.

With a hanging weight you can always overcome friction (theres always more power available!) stiction could be a problem depending on what materials and machining services you have available.

OP - how big an instrument are you planning?

Cremer said:
So say I did use a stepper motor...
I have a 1.8 degree 200 stepper motor which means that one step = 0.06 degree.
360 degree divide by 0.06 degree = 6000 steps.
I think you have got a little confused.
A 200count stepper moves 1/200 of a circle on each step, ie 200steps for a full circle or 1.8deg/step.

You can't drive your telescope directly becuase on each step it would jump 1.8deg.
Normally you would have a large arc with gear teeth on the edge and a worm gear so that one complete rotation of the worm gear moved the arc 1 tooth ( perhaps 0.05deg) then it's easy to run the motor at some reasonable number of rpm to give exactly the correct rotation speed and the whole thing is fairly smooth

An easier arrangement is to use a threaded rod and a moving nut attached to your telescope

Description of a small system here http://home.att.net/~jsstars/slomo/slomo.html

Last edited by a moderator:
Don't use a curved rod - use a straight one and change the motor speed in software (trivial with a stepper)
For a camera you don't need sub-arc second tracking accuracy you can probably just have one speed for each 5-10deg altitude intervals.

In engineering the thing is also called a 'sine bar' because the change in the screw length is the sin of the angle.

You may also consider antibacklash gearing.

## 1. What is the RPM of the motor needed for the barn door project?

The RPM of the motor needed for the barn door project will depend on the weight and size of the barn door, as well as the type of mechanism used to open and close it. A general rule of thumb is to choose a motor with an RPM that is slightly higher than the desired speed of the door.

## 2. What type of motor is best for a barn door project?

The best type of motor for a barn door project will depend on the specific needs and preferences of the project. Some common options include AC or DC motors, gear motors, or servo motors. It is important to consider factors such as torque, speed, and power source when choosing the best motor for your project.

## 3. How do I calculate the necessary torque for a barn door motor?

To calculate the necessary torque for a barn door motor, you will need to know the weight and size of the door, as well as the type of mechanism used to open and close it. Once you have this information, you can use a torque calculator or consult with a motor expert to determine the appropriate torque for your project.

## 4. Can I use a motor from a different project for my barn door project?

It is possible to repurpose a motor from a different project for your barn door project, but it is important to make sure that the motor is suitable for the weight and size of your door, as well as the desired speed and torque. It is also important to consider the compatibility of the motor with the rest of your project components.

## 5. How do I properly install and wire the motor for my barn door project?

The installation and wiring process for a motor in a barn door project will vary depending on the type of motor and mechanism used. It is important to carefully follow the instructions provided by the manufacturer and consult with an expert if needed. Safety precautions should also be taken to ensure proper installation and avoid any hazards.

• Mechanical Engineering
Replies
19
Views
1K
• Mechanical Engineering
Replies
10
Views
597
• Mechanical Engineering
Replies
2
Views
1K
• Mechanical Engineering
Replies
24
Views
2K
• Mechanical Engineering
Replies
17
Views
3K
• Mechanical Engineering
Replies
15
Views
3K
• Mechanical Engineering
Replies
3
Views
1K
• Mechanical Engineering
Replies
4
Views
1K
• Mechanical Engineering
Replies
8
Views
2K
• Mechanical Engineering
Replies
2
Views
784