Connecting a threaded rod to a DC motor

[FlorianL]

Hello everyone,

What would be the 'best' way to connect a threaded rod to a DC motor? I am looking at efficiency, but also parts availability and cost.

The rod will rotate in a vertical position. The rod is a widely available one, 3/4" diameter and 10 TPI. Speed is not of the essence for my design although it would be nice. What I have right now is a tiny DC motor with what appears to be a worm gear at 10 TPI. That is not my only alternative however (in terms of motors) so please feel free to suggest something else if it improves in the area(s) of efficiency/reliability/availability/cost.

Any suggestions and ideas that you can provide me with would be most welcome. Many thanks!


Florian

 

[eromlignod]

Are you using this as a leadscrew? If so, I don't think regular unified threads are going to perform very well, especially if your motor is small. Turning a 3/4" allthread shaft in a nut with any vertical load is going to require some torque. You'd probably be better off with an acme screw and brass nut. You can get a 3/8-10 pair from McMonkey-Carr for about fifty bucks.

There are lots of ways to attach it. It depends on your motor shaft size and whether you need a misalignment coupling. Tell us more about the motor and the mechanism.

Don
Kansas City

 

[FlorianL]

Don, thank you for your reply. I will try to explain things a bit better so you and other people reading this can get a better idea of what I am trying to do.

You are right, I am trying to use it as a lead screw. The idea is to have a small platform attached to the nut and for the platform to be able to move up and down on command, carrying a small load - say, no more than 40-50 lb.

I am thinking of two ways of doing this. One is to rotate the whole shaft and to block the nut on the shaft, thus having the nut go up and down. OR, use a ball screw + nut assembly that rotates the nut while the shaft stays fixed in position. My guess is that the motor required to rotate the whole shaft is probably going to have to be more powerful than the one that only rotates the nut, however the first assembly is potentially safer to operate as it can be enclosed in some kind of tube.

Now, I've asked one of my friends (mechanical engineer) and after some preliminary calculations, the motor I'd need is a 35-40W if I will be using the ball screw + nut assembly. I haven't done any calculations yet for the motor needed when rotating the whole shaft.

The motor I have is this one:

http://www.mabuchi-motor.co.jp/en_US/cat_files/rs_385ph.pdf

I hope this makes it a bit clearer as to what my intentions are, please let me know if I can clarify matters further. Thanks again!


Florian
Toronto/Canada

 

[CoryPad]

I agree with Don, you should use an Acme thread.

I am not entirely sure of your configuration - do you want the motor shaft to be offset from the lead screw?

I think the most efficient design is an Acme threaded nut attached to the platform and an Acme threaded rod attached directly and coaxially to the motor shaft. Can you machine your motor shaft and lead screw to have splines or threads or other features?

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[FlorianL]

Thank you for your input Cory!

Unfortunately, I am not in a position to machine anything, that is why I am looking for a solution using off-the-shelf parts for now.

Also, I do have an ACME threaded rod and I know they are more efficient than your average threaded rod, however it is only 36" in length whereas I am looking for a 72" threaded rod for my project.

Regards,


Florian
Toronto/Canada

 

[handleman]

Based on the motor you linked to, you have way more important things to figure out than how to attach the motor to the shaft. Like how powerful to make the motor. The one you linked won't work. How are you going to guide the platform? Surely you don't plan to hang the thing from the motor shaft? Friction is going to eat your lunch.

-handleman, CSWP (The new, easy test)

 

[FlorianL]

Cory,

I seem to have missed one of your questions. Yes, most likely the motor shaft will probably have to be offset from the lead screw as I don't have the possibility to machine things right now. I wish I could connect the motor shaft to the threaded rod directly, but I don't think it will happen.

Regards,


Florian

 

[handleman]

I wish I could connect the motor shaft to the threaded rod directly, but I don't think it will happen.

If you try to do this, you will fail. Your motor is simply far too small. To generate 40lbf lift, you will require 0.16 N*m of torque, assuming a ball screw-like friction coefficient of 0.05. The larger motor on your linked page has a stall torque of 0.073 N*m. It quite simply won't turn!

This may help you:

http://www.racointernational.com/screw_formula.htm

-handleman, CSWP (The new, easy test)

 

[eromlignod]

If I were you, I think I'd use a ballscrew & nut and a gearmotor, rather than direct drive. You could probably get by with a fairly small gearmotor as long as you're not worried about speed. With such a long, small-diameter screw you are going to have terrible vibration problems at even moderate speeds anyway.

You will have to prevent the load/nut from turning somehow. Perhaps a stationary rod parallel to the screw that passes through a hole or slot in the stage.

The ballscrew will probably come with machined journal ends. Both ends will have to terminate in a mounted bearing. You could attach a timing pulley or a sprocket to the drive end and another pulley to the motor shaft and drive it that way. This doesn't mean much machining, but you're at least going to have to use a drill to mount all this stuff down to something.

Don
Kansas City

 

[ScottyUK]

Not sure where you are located but if your manufacturing industry is in a state like ours in the UK, I would look for a machinery breakers and see if I could acquire a leadscrew and nut from an old lathe.

The gear motor is good advice.


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If we learn from our mistakes I'm getting a great education!

 

[DesignerGuy16]

If your cheap and dirty doens't work out look into a full up drive screw actuator. That would be a lot easier to mount and setup than trying to rig something cheap and dirty (since it's more likely you'd have to custom make an adapter to mount the screw to the motor shaft).

Think something like:

http://tolomatic.com/products/item_detail.cfm?tree_id=72

I'm sure you can find cheaper out there, just linked the first one I could find easily.

James Spisich
Design Engineer, CSWP

 

[FlorianL]

First of all, I would like to thank all of you for trying to help me and for sharing your thoughts and advice on my project. You people are awesome!

I have decided to go 'cheap and dirty' as James mentioned above for now as the parts I have found so far are pretty expensive.

The shaft will be 72" long and 3/4" in diameter. Hopefully I can find one that is one piece or I will connect 2 x 36" rods with a connection nut. I bought two ball bearings that will go at the ends of the shaft, allowing it to rotate vertically and at the same time they can be mounted on supports which will allow the system to reduce vibrations.

The motor will NOT be connected directly to the shaft, it will be placed to the left or right of the shaft and will connect to the shaft using a gear or a belt. This of course implies I will have to connect a gear of some sort on the shaft.

That's it so far, please give me your thoughts on how I can make the best of this system, mainly what do I need to do in order to minimize vibrations, size the motor and improve the speed/reliability of the system.

If you have references [books, articles, web pages] where I can educate myself on the subject, it would be much appreciated.

Regards,


Florian
Toronto/Canada

 

[handleman]

The shaft will be 72" long and 3/4" in diameter. Hopefully I can find one that is one piece or I will connect 2 x 36" rods with a connection nut.

What kind of stroke are you wanting? Won't a "connection nut" in the middle of your shaft interfere with the stroke? How will you keep the shafts from coming apart during running?

Small motors like the one you've linked to usually have a plastic (or maybe brass) gear press-fit onto the shaft, perhaps fixed with a little glue. If you use a belt to drive the screw, be sure it is a toothed belt. The tension required to keep a smooth belt from slipping would ruin your little motor shaft/bearings.

A gearmotor will have a somewhat more substantial shaft, probably with a keyway.

As for books/magazines/articles, I would recommend a good college-level calculus-based physics book.

-handleman, CSWP (The new, easy test)

 

[FlorianL]

Hi handleman,

Thank you for your reply. I only need the nut to be able to move on the upper part of my shaft (30" or so). So theoretically, I wouldn't need a fully threaded rod, but I have been unable to find a half threaded rod . I am going to keep the shafts from coming apart by using a metal bonding adhesive inside the connection nut.

Regarding the motor, I was planning to size the motor once I have all the data I need. I am not sure for example how fast I should rotate the whole shaft in order to minimize vibrations and friction/damage to the whole assembly.

I don't have any experience in mechanical systems design and that's why I am asking for some reference material.

Regards,


Florian
Toronto/Canada

 

[hydtools]

As for speed, you need to decide how fast you want to move the platform. This will then determine how fast you need to turn the screw since you know the thread pitch. If you use a 1:1 toothed belt sprocket drive, your motor speed will then be the same as the screw rpm. If the screw speed needs to be less than the motor speed you can decide on an appropriate speed reducing ratio for the sprockets.

Knowing the load and its speed will let you calculate power demand of moving the load.

Ted

 

[eromlignod]

Vibration is going to be a real problem unless you run at slow speeds. Anything fast and it's going to vibrate like a bass fiddle.

As a young engineer, I once designed a machine that was very similar. It had a 3/4" ball screw that was about six or seven feet long with a carriage that moved along it. At even moderate speeds, the resonant vibration was terrible...the screw would vibrate violently and the machine would walk around the floor like an old washing machine.

What was interesting is that as the carriage moved along, the natural frequency of both exposed ends of the screw would change, just like moving your finger along a guitar string. I made a graph of the critical speed of each end of the screw in relation to its distance from the carriage. It turned out like two mirrored exponential curves that crossed in the center of the graph. So I programmed the motor to ramp quickly up to the "crossing" speed and plow right through it. That way both ends crossed through their critical speeds at the same time (at the center of travel) and only spent a fraction of a second in the critical region, so the shaft didn't really have enough time to produce much resonance. Otherwise you had to sneak in well under both curves at a low speed. That was in 1989 and I believe that machine is still in service.

If you don't need the shaft to be entirely threaded, you might consider adding an intermediate bearing at the middle (instead of a coupling) and secure it. That way your spinning members have a much stouter diameter-to-length ratio.

Don
Kansas City

 

[DesignerGuy16]

Or configure it so it's only threaded for the lifting distance. Shouldn't be hard if you fix the nut to the platform and turn the rod from below. I don't know what kind of weight and size this platform will be, but have you considered a seperate guide rod to stabilize the platform?

We have a setup on one of our machines where I work where it has a thickness adjust for a tenderizer. There's a small motor mounted to a frame that has 2 threaded shafts passing through it with fixed "elevator" nuts on each. The motor is chained to both shafts in a setup that will raise turn both rods with the same speed (this is very important or you'll likely get binding).

James Spisich
Design Engineer, CSWP

 

[hydtools]

Can you mount the motor on a pedestal and use just a short shaft to the platform? A quick and dirty pedestal would be a pipe screwed into a flange bolted to the floor.

Ted

 

[FlorianL]

Don, thank you very much for the story, at least now I know what to expect! Unfortunately, I don't have the tools to measure vibrations so I will have to rely on good planning to minimize the number of problems I am faced with, however I will keep your advice in the back of my mind. The idea of a middle bearing is very interesting and intriguing at the same time.

The way I've designed things so far is that the shaft is sitting nicely in a 4 bolt flange bearing fastened to a pedestal. Since I am planning to also fasten the motor to the pedestal, I am somewhat intrigued as to how I will be able to rotate the upper shaft with a bearing in the middle. I believe I might have not understood you completely there.

James, thank you for the suggestion - I have considered a guide rail to stabilize the platform. However, I would like to try and see how I am doing without one first, and only use one if necessary. You are right - the nut will be fixed to the platform although I am afraid I didn't quite understand what you meant when you said that I can configure the rod to be threaded only for the lifting distance.

Ted, that is exactly what I am planning to do, mount the motor to a pedestal. Except in my design I am using a longer shaft to connect to the lifting rod.

I am aware that I can minimize vibrations with a shorter length to diameter ratio. However, this whole thing will be built in my room for now and as such, I am considering potential safety issues. I just thought that fastening the motor and gears to a pedestal on the floor where I could potentially encompass everything in a box maybe would be safer than suspending the assembly 3ft above the ground even though I realize that mechanically there are drawbacks to my decision. Am I wrong/right about potential safety issues? Thank you.

 

[eromlignod]

I guess I shouldn't have said "instead of a coupling". What I meant is to run the end of the screw through the center bearing, then attach your coupling, just below it, to the plain shaft. That way your screw is fixed at both ends but shorter, which should greatly reduce the vibration problem.

Actually, you could circumvent the resonance entirely by using a rotating nut setup instead. The screw would be fixed to the carriage and non-rotating. The nut would rotate in a stationary bearing and convey the carriage and the screw up and down. You would have to have a long enough nut to fit into the bearing and yet protrude enough to attach a pulley/sprocket/gear to its OD to drive with the motor. Good luck.

Don
Kansas City