GM8 Motor Output Shaft Coupling (Video)

Martin_H · 2011-12-01 11:31

I have a spare GM8 and it seems like a decent enough gearbox. But I am a little stumped how to couple something to the GM8's flat sided output shaft. Scribbler wheel hubs have a spline designed to couple with the GM8, but otherwise the shaft seems non-standard. What I would like to do is couple threaded rod from the hardware store to it and use the motor to turn the threaded rod. I could then use the threaded rod to translate the rotational motion into linear movement.

Anyone have ideas for coupling other things to these output shafts?

kwinn · 2011-12-01 11:48

A threaded spacer of a size to thread onto the shaft with a hole drilled through both for a locking pin?

Martin_H · 2011-12-01 12:07

kwinn wrote: »

A threaded spacer of a size to thread onto the shaft with a hole drilled through both for a locking pin?

That's not a bad idea, the shaft is kind of short, but I will take a look when I get home.

Franklin · 2011-12-01 15:05

This might help.

Solarbotics is the sole commercial outlet for the GM8, which is much like the GM2 gear motor but runs at nearly twice the speed. This 143:1 gear motor spins at 40RPM at 3V, drawing 400mA at stall generating 44.4 in*oz torque (free running at 50mA).
With a 7mm double-flat output shaft (avoid using the green double flat output - it's not meant to take rotational load), and a built-in clutch and built-in mounting screw holes, you can see why we like these motors!
These units have an overall size of 53.8mm (2.12") x 47.8mm (1.88") x 22.9mm (0.902") thick and weighs in at 31.5g or 1.13oz.
Do you need to mount something to the shaft of the motor? Although it doesn't work as well as on other motors because of the GM8's shorter output shaft, the GMW is a solid platform for a variety uses. Alternatively, you'll also find that a #2x1/4" SMS screw will provide a snug fit for your need.

erco · 2011-12-01 18:20

Don't have one in front of me, but isn't there a small steel shaft in the center of that round shaft with flats? If that's the case, it would preclude drilling a hole through it. Short of machining one yourself, Franklin's GMW looks like a winner.

Martin_H · 2011-12-01 18:47

erco wrote: »

Don't have one in front of me, but isn't there a small steel shaft in the center of that round shaft with flats? If that's the case, it would preclude drilling a hole through it.

There is a steel shaft, but it is in the green shaft which is on the back. The shaft in front is hollow so it can receive the GMW's retaining screw.

Franklin, thanks for the pointer to the GMW. I am not sure how I would connect that to the rod, but it is one possibility. I will think about it.

Phil Pilgrim (PhiPi) · 2011-12-01 21:55

I took this as a challenge and built a coupler. Here's a photo:

attachment.php?attachmentid=87345&d=1322805131

Here are the parts it's made from:

attachment.php?attachmentid=87346&d=1322805132

It includes a flex coupling made from 1/32" Delrin (the X-shaped part). This will keep any angular misalignment from tearing up the gears in the gearbox. (Radial misalignment has to be accommodated elsewhere -- probably with the motor mount). The attachment to the motor is accomplished with two pieces of Delrin: one to fit the flatted gearbox shaft, the other to accommodate a #2 sheet metal screw. The attachment to the 1/4-20 threaded rod is done via a castellated nut, which is pressed into a 1/16" piece of Delrin, and a jam nut and lockwasher on the other side, tightened on the shaft to keep it from coming loose.

Martin, I'd be happy to send you the parts shown in the second photo, if you like. Just email me your postal address, and they'll be on their way.

-Phil

Martin_H · 2011-12-02 03:00

Phil, thanks and that is quite generous. It also looks exactly like what I need. But I don’t have you email address, and you don't have private messages enabled. If you private message me with your email I will respond.

Phil Pilgrim (PhiPi) · 2011-12-02 08:32

Martin,

My email address is in my profile.

-Phil

Martin_H · 2011-12-19 16:35

So here's a picture of the linear actuator I have been toying with:

It is made from two metal printer rods and the slides from the printer. While it looks pretty good and I was hoping to give an update on this saying that it worked. But I can't as it binds on the rails at certain spots. But failure can be instructive because I think I know what I did wrong.

I stacked the end pieces and drilled them at the same time on the drill press, but I measured their spacing and made the sled separately. While the sled is nearly square and parallel with the rails, it is not close enough for the tolerances required, hence it binds towards one end.

What I will try next is building the sled on the rails. That way no measurement is required and it should be self aligned. Then I want to drive it with a threaded rod attached to the GM8. Ultimately this is related to my fiddling with stepper motors because I have a crazy idea of the building an XY table from scratch.

Martin_H · 2011-12-25 12:51

I rebuilt my linear actuator using some techniques I saw online. Basically I changed the shape to a rectangle to fully support the structure, so the ends and the rods stay parallel. As before I stacked the front and back parts and drilled them, but I used dowels, barrel nuts, and bolts to hold the rectangle together, so the entire structure can be disassembled. I built the sled on top of the rods which resulted in a perfect fit with a nice smooth ride with no binding. I installed the T nut, threaded rod, and Phil's GM8 to threaded rod coupler to move the sled back and forth. Here's a video:

[video=youtube_share;Yr8d61jDLJs]

It is a bit on the slow side because of all the gear reduction, but it certainly lets me test without a hassle of dealing with steppers. Now I need to install the end bearings, and figure out how to build a Y axis. The only thing I am sure of is I want to continue with the ability to bolt together and take apart the structure because that allows for tuning which is needed given the tight tolerances.

I've seen two different Y axis designs. One uses a fixed Y axis gantry which moves side to side over the X platform (like a plotter). In the other the Y axis rides on top of the X axis. The former seems simpler to build, but you need to work out the gantry height and position relative to the throw of the X axis which seems a bit tricky. The latter increases the load on the X axis motor, but has the advantage of not needing to work out the interrelations ahead of time. Not sure what's best here.

I also need to get over my fear of trying the stepper motors again. Those things intimidate me for some reason.

Martin_H · 2011-12-26 19:55

I bit the bullet and learned how to use a stepper motor I salvaged from a printer. I then attached the stepper motor to the slide and got good results. Here's a video:

[video=youtube_share;F1sF2BvbkNw]

I need to add limit switches and the X axis will be done, then it is on to the Y axis.

Martin_H · 2012-01-04 19:34

Here's the direction this thread is going:

Which makes this an example of something I DIY that saner people buy. The Y sled and Z axis combo was a large amount of effort. The Y axis seems to slide well enough, but the Z axis is not done so it is an, unknown.

So far the MDO, threaded rod and bearings are the only thing that I bought, and all rods, motors, and bushings are printer or scanner parts. So the cost has been low which is good as I am not sure how well this will work when I am done.

Martin_H · 2012-01-16 08:50

I'm shelving this thread because the project is no longer related to the GM-8 motor. I am also moving it to the builders forum because the project is morphing into a CNC machine. So here's the new URL:

http://forums.parallax.com/showthread.php?137289-Attempt-to-build-a-CNC-machine-out-of-old-printer-parts.&p=1066267#post1066267

Moderators let me know if I did something wrong in how I moved it from sub-forum to sub-forum.

GM8 Motor Output Shaft Coupling (Video)

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