Possibility of turning a threaded rod with spur gears using a motor

  • Thread starter AdamQuinn
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  • #1
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Summary:

Need help with gearing of barn door drive, for astrophotography.

Main Question or Discussion Point

So I'm a college student currently doing A-Level Physics, I am attempting to build a type 4 double arm barn door tracker and although it has been a complicated project, I believe I've got all the planning done, however, I have a 2Rpm motor, a PWM motor controller, 2 9v batteries, an M8 threaded rod. I want to know how I can use the motors turning action to turn the threaded rod. I was thinking of using cogs/spur gears but in my head I need something that has a threaded inside (like the inside of a nut) to allow the friction needed to turn the rod. Please could someone advise me on what I would need and if the electrical circuit (motor, pwm controller and 2 9V batteries) is viable.
Cheers
Adam
 

Answers and Replies

  • #2
berkeman
Mentor
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6,365
Welcome to the PF. :smile:

Do you want the threaded rod to be linearly stationary with respect to the motor (use a shaft coupler), or do you want the threaded rod to move through/past the motor with its linear movement?

If you want the rod to move linearly with respect to the motor, I think I've seen motors with embedded nuts (but I'm not finding them online at the moment), or you can use an offset-driven-nut like this one:

https://1ugn2l2m3z2y31d4y73c1m5w-wpengine.netdna-ssl.com/wp-content/uploads/2014/11/Driven-Nut.jpg

1576014242820.png
 
  • #3
  • #4
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I want to build exactly that. I find this the easiest to understand https://www.google.com/url?sa=i&source=images&cd=&cad=rja&uact=8&ved=2ahUKEwim1vH_4K3mAhVyDWMBHfq_B6kQjRx6BAgBEAQ&url=http://www.astronomyboy.com/barndoor/gagnon.shtml&psig=AOvVaw1-Fh6B-DSnw2AA3xb5RC-e&ust=1576159193381100 .

This is the idea I had in mind

barn door.jpg


In terms of the turning of the camera board, I think that it is a good idea to have one on top of the other, where would I mount the motor though? And also how will avoid hypotenuse error? I've got two options, using a curved rod (I'd have to bend it myself) or a corrective cam. In either case where would I mount the motor? Would it be possible to mount it on the base board (underside)?

Cheers
barn door.jpg
 
  • #5
9
0
Welcome to the PF. :smile:

Do you want the threaded rod to be linearly stationary with respect to the motor (use a shaft coupler), or do you want the threaded rod to move through/past the motor with its linear movement?

If you want the rod to move linearly with respect to the motor, I think I've seen motors with embedded nuts (but I'm not finding them online at the moment), or you can use an offset-driven-nut like this one:

https://1ugn2l2m3z2y31d4y73c1m5w-wpengine.netdna-ssl.com/wp-content/uploads/2014/11/Driven-Nut.jpg

View attachment 253936
I want the rod and motor to be in a fixed position whilst the camera board moves upwards. How will a coupler work with a threaded rod? (I haven't received it yet but I presume it would need the same threading as the rod for it to work)
 
  • #6
1,616
618
How will a coupler work with a threaded rod? (I haven't received it yet but I presume it would need the same threading as the rod for it to work)
Are you saying, the motor shaft is threaded? And asking, how to keep the threaded coupling (motor shaft to the barndoor threaded rod) from unscrewing when the device is operating?

I can think of several ideas for that: use blue loctite thread locker; or assemble the connection and then drill it radially, insert a pin or cotter pin in the radial hole.

Get a long bolt, or piece of allthread and a few nuts that fit - you can find "coupling nuts" which are long nuts that will let you thread two bolts together; then play around with holding one nut fixed while turning the rod. I think this exercise will help you visualize what the barndoor hardware has to do.

I have not built a barndoor myself but I have been thinking about it on and off. Good luck, and please keep us informed as you make progress.
 
  • #7
Spinnor
Gold Member
2,102
316
If you have a computerized motor controller you could program it so as to eliminate the large rotation angle error with such a mount.

You definitely do not want to bend the rod, it wants to be straight as seen on other such mounts.

It is fun to build stuff and quite educational but in the end what you want to do is take great pictures,

1576104356246.png


$189 and if you lose interest you can probably sell it for a fair amount of money.

https://www.telescope.com/catalog/product.jsp?productId=27154&gclid=CjwKCAiAxMLvBRBNEiwAKhr-nEoj7Yev_gFtjLmjbAbE51buZcm2o8GKx-wVEfkvx3Eyzqj0Qh5apxoCWbIQAvD_BwE

https://www.google.com/search?q=astrophotography+camera+tracking+mount&safe=off&sxsrf=ACYBGNQ42GilboyV7YaF6eiIiKGHStO6pQ:1576106013231&source=lnms&tbm=isch&sa=X&ved=2ahUKEwiE56-43K7mAhUKc98KHTEECIIQ_AUoA3oECAwQBQ&biw=931&bih=604


Good luck and clear skies to you.
 

Attachments

Last edited:
  • #8
9
0
Are you saying, the motor shaft is threaded? And asking, how to keep the threaded coupling (motor shaft to the barndoor threaded rod) from unscrewing when the device is operating?
The motor shaft is circular with a slice taken out of it (sort of like a semi-circle). What I'm saying is how would I go about moving a single plate?

I can think of several ideas for that: use blue loctite thread locker; or assemble the connection and then drill it radially, insert a pin or cotter pin in the radial hole.

Get a long bolt, or piece of allthread and a few nuts that fit - you can find "coupling nuts" which are long nuts that will let you thread two bolts together; then play around with holding one nut fixed while turning the rod. I think this exercise will help you visualize what the barndoor hardware has to do.
Moving onto the practical you advised, if I fix the nut with my hand then the rod will move up/down. That is what I want to do with an entire board. Would I be able to glue a nut onto the board so that it moves up and down with the nut? Would that be a feesable idea?

Cheers
 
  • #9
9
0
If you have a computerized motor controller you could program it so as to eliminate the large rotation angle error with such a mount.

You definitely do not want to bend the rod, it wants to be straight as seen on other such mounts.

It is fun to build stuff and quite educational but in the end what you want to do is take great pictures,

View attachment 254003

$189 and if you lose interest you can probably sell it for a fair amount of money.

https://www.telescope.com/catalog/product.jsp?productId=27154&gclid=CjwKCAiAxMLvBRBNEiwAKhr-nEoj7Yev_gFtjLmjbAbE51buZcm2o8GKx-wVEfkvx3Eyzqj0Qh5apxoCWbIQAvD_BwE

https://www.google.com/search?q=astrophotography+camera+tracking+mount&safe=off&sxsrf=ACYBGNQ42GilboyV7YaF6eiIiKGHStO6pQ:1576106013231&source=lnms&tbm=isch&sa=X&ved=2ahUKEwiE56-43K7mAhUKc98KHTEECIIQ_AUoA3oECAwQBQ&biw=931&bih=604


Good luck and clear skies to you.
The only reason I mentioned bending is the rod as I looking into the arduino device however I am not the most able programmer. I did recieve the threaded rod today and it is safe to say that there is no way I'll be able to bend the rod. I believe that this https://cdn.instructables.com/FQS/2TT6/HOHYJG7X/FQS2TT6HOHYJG7X.LARGE.jpg?auto=webp&width=1024&height=1024&fit=bounds would be the easiest method of correcting the tangent error. I have only got programming experience in small basic and visual basic; I wouldn't know where to start on implementing a program onto a physics PCB with the motor etc.

Cheers
 
  • #10
1,616
618
The motor shaft is circular with a slice taken out of it (sort of like a semi-circle). What I'm saying is how would I go about moving a single plate?
The motor shaft can fit into a circular hole in whatever you want to turn; then drill & tap a hole perpendicular to the shaft axis and screw in a setscrew to bear against the flat spot (where the semicircular slice is). If the slice takes off half the shaft, you can file the mating end of your threaded rod to match and then use a coupling to hold the two together:

6412k140l.gif


Would I be able to glue a nut onto the board so that it moves up and down with the nut? Would that be a feesable idea?
Yes, That's the idea.

The only reason I mentioned bending is the rod
I have seen lots of barndoor designs on the web using a bent threaded rod. The radius of the bend is fairly large; I think the builders slowly and gently make the bend by pulling the rod around an appropriately sized cylinder. The ends of the rod can then be cut off, leaving the radiused portion for use. The radius has to be large enough to get the desired motion of the door, and at the same time not binding the nut.

Barndoor designs are cheap home made devices for relatively short exposures, like up to 10 or 20 minutes; the motor-drive equatorial mounts mentioned above are in another league.
 
  • #11
9
0
The motor shaft can fit into a circular hole in whatever you want to turn; then drill & tap a hole perpendicular to the shaft axis and screw in a setscrew to bear against the flat spot (where the semicircular slice is). If the slice takes off half the shaft, you can file the mating end of your threaded rod to match and then use a coupling to hold the two together:

View attachment 254055
So... Building of my barn door tracker is mostly complete, wood assembled, motor/pwm circuit working, camera mount mounted. However the motor/threaded rod issue persists. The issue I am having is that I am having to mount the motor at the bottom of the threaded rod (essentially hanging). I have a M8 coupling nut which fits the threaded rod however the motor shaft is probably M5/6 size. I have tried soldering the shaft to the coupling nut, I've tried using a paper clip to widen the shaft, I've even tried using tape just to increase the diameter of the shaft to the internal diameter of the coupling nut; all to no prevail.

IMG_5246.JPG


Would it be easier to use an M6 threaded rod instead?
How do I make the motor shaft turn the rod not the rod turn the motor?
How do I support the motor housing as it is hanging on its own?
Best solutions for the thread/shaft joining?

Cheers
Adam
 
  • #12
Spinnor
Gold Member
2,102
316
So... Building of my barn door tracker is mostly complete, wood assembled, motor/pwm circuit working, camera mount mounted. However the motor/threaded rod issue persists. The issue I am having is that I am having to mount the motor at the bottom of the threaded rod (essentially hanging). I have a M8 coupling nut which fits the threaded rod however the motor shaft is probably M5/6 size. I have tried soldering the shaft to the coupling nut, I've tried using a paper clip to widen the shaft, I've even tried using tape just to increase the diameter of the shaft to the internal diameter of the coupling nut; all to no prevail.

View attachment 254371

Would it be easier to use an M6 threaded rod instead?
How do I make the motor shaft turn the rod not the rod turn the motor?
How do I support the motor housing as it is hanging on its own?
Best solutions for the thread/shaft joining?

Cheers
Adam

I suggested using the proper size flexible rubber tubing with hose clamps. What are the downsides to that fix? An improvement on that method, wrap electrical tape around the motor shaft to build up the shaft diameter so that it is close the the diameter of the threaded rod, then use some flexible tubing available at any hardware store or in my garage.
 
  • #13
Tom.G
Science Advisor
2,937
1,691
Her is a labor intensive approach that doesn't always work, but sometimes does.

1) Put 3 wood toothpicks in the coupling nut, equally spaced around it
2) Insert the motor shaft. The toothpicks act as shims and center the shaft in the nut
3) Drill a small hole completely thru the coupling nut and the motor shaft
4) Insert a tight fitting metal pin in the hole. (a 'spring pin', or 'roll pin' if you have a good hardware store near you)

Of course any hardwood scraps can be used for the toothpicks, they should all be the same diameter or thickness to center the motor shaft.

Cheers,
Tom
 
Last edited:
  • #14
9
0
I suggested using the proper size flexible rubber tubing with hose clamps. What are the downsides to that fix? An improvement on that method, wrap electrical tape around the motor shaft to build up the shaft diameter so that it is close the the diameter of the threaded rod, then use some flexible tubing available at any hardware store or in my garage.
There's nothing wrong with the idea, I just thought there may have been a more thorough solution. In terms of the electrical tape, I used that yesterday and the coupling nut shredded the tape. If I used tubing, I presume that I need to have the clamps very tight to the shaft and rod? Would this last a long time? And how to I make sure that the motor shaft turns the rod instead of the motor spinning and getting tangled in the wire.
 
  • #15
9
0
Her is a labor intensive approach that doesn't always work, but sometimes does.

1) Put 3 wood toothpicks in the coupling nut, equally spaced around it
2) Insert the motor shaft. The toothpicks act as shims and center the shaft in the nut
3) Drill a small hole completely thru the coupling nut and the motor shaft
4) Insert a tight fitting metal pin in the hole. (a 'spring pin', or 'roll pin' if you have a good hardware store near you)

Of course any hardwood scraps can be used for the toothpicks, they should all be the same diameter or thickness to center the motor shaft.

Cheers,
Tom
This sounds like what I was trying to achieve yesterday. I tried drilling the coupling nut, I got about 2mm in and the drill bit snapped in two. Instead of using wood toothpicks, I used tin solder, it did work temporarily though.
 
  • #16
Baluncore
Science Advisor
2019 Award
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And how to I make sure that the motor shaft turns the rod instead of the motor spinning and getting tangled in the wire.
The motor must be attached = two screws to the mounting, which will stop it tangling wires. There needs to be a hinge on the motor mount. See the motor mount pictured in post #3.
 
  • #17
9
0
The motor must be attached = two screws to the mounting, which will stop it tangling wires. There needs to be a hinge on the motor mount. See the motor mount pictured in post #3.
Would something like this work? Would it be feesible to use it as the joining to the rest of the tracker?
FC4PKNEHOHYJG7D.LARGE.jpg
 
  • #18
Spinnor
Gold Member
2,102
316
There's nothing wrong with the idea, I just thought there may have been a more thorough solution. In terms of the electrical tape, I used that yesterday and the coupling nut shredded the tape. If I used tubing, I presume that I need to have the clamps very tight to the shaft and rod? Would this last a long time? And how to I make sure that the motor shaft turns the rod instead of the motor spinning and getting tangled in the wire.
Because the threaded rod has such a large mechanical advantage there should not be a large torque required to turn the threaded shaft unless something is binding? Does the threaded rod turn freely without the motor attached, if so the flexible tubing should work? Flexible couplings are used in power transmission all the time, they can be very strong.
 
  • #19
Baluncore
Science Advisor
2019 Award
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Would something like this work? Would it be feesible to use it as the joining to the rest of the tracker?
If the nut is in the fixed lower door and the motor can slide along the two fixed rods, then the far end of the screw cannot be fixed, so it presses on the upper door, while the carved notch there provides some additional correction.

You need to sketch a section so we can see where the nut is, and how it will work.
 
  • #20
jrmichler
Science Advisor
976
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Some general rules for connecting gearmotors to shafts:

1) If the shaft has bearings on both ends and the motor is rigidly mounted, you need a flexible coupling.

2) If the shaft has bearings on both ends and the motor is rigidly connected to the shaft, the motor mount should be flexible. One way to do this is to restrain the motor from spinning with a torque link, plus one shaft bearing needs to restrain the shaft in the axial direction.

3) If the motor is rigidly supported and rigidly coupled to the shaft, support only the far end of the shaft with a bearing.

Both the shaft and motor have six degrees of freedom, three translational and three rotational. Both the motor and shaft need to be constrained in exactly all six degrees of freedom. Too many constraints can result in high internal forces. An example of overconstraint is a rigid shaft with three bearings.

For example, in #2 above, two bearings restrain the shaft against three degrees of freedom in translation and two degrees of freedom in rotation, and the motor restrains the shaft in rotation about the shaft axis. The rigid coupling then restrains the motor against three degrees of freedom in translation and two in rotation, while the motor torque link restrains the motor against the third degree of freedom in rotation. The shaft rotation degree of freedom is controlled by the motor when the motor is running, and by friction when the motor is off.

BTW, I have successfully used #2 for large gearmotors weighing several hundred pounds.
 
  • #21
789
268
I've used an MFA/Como Drills 'universal joint' adaptor. Inserts come with a motor shaft hole and grub-screw. I chose one to match the gear-motor. Other, I carefully hand-tapped to M6 per the actuator thread. It got the grub-screw, a lock-washer and a locking-nut...

For another task, given the very short, lefty-threaded stub axles on the windscreen-wiper motors I'd available, I part-threaded the short neck of stub axle to M10, fitted a chunky nut, locked it with the motor's M8 lefty nut, then used half of a deep 19mm =M10 box-spanner to pass power to road-wheel flange. But yet it moves !!
 

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