Need to know screw size for the front of a tower pro micro Servo motor

Hi,

I'd like to know what the screw is that goes on the front of the tower pro micro servo motor. This is the screw that holds the little propeller blades in place. I want to buy a standoff/spacer for this, but don't know the correct size.

If no standoffs exist in that size, does anyone have any ideas for having the item to be rotated about a centimetre away from the motor shaft? What I've tried so far is to have the screw head at the centre of the item to be rotated glued to the end of the shaft or the propeller. I've tried carpenters' glue, hot glue, gorilla glue, silicone adhesive, epoxy. No kind of glue holds because the pressure against it is twisting pressure, not pulling. The glues seem to work better against pulling.

Thank you.

You have an error in your title - which describes the device as a step motor. That will discourage people who may know about servos.

Sorry, I don't know the screw size. Have you enquired on model flying websites?

...R

Oops..you're right. I don't think I can fix it, though.

Not quite sure what you mean - by propeller you mean the servo horn? (google)

What is the 'front' of a (hobby) servo? Maybe there is an understanding on this, but I'm not aware of it - my intuition says it's not the side with the servo horn on it, that is the top - guess, I could be about to learn something :slight_smile:

So you mean the plastic/wood (sharper and more course than an equivalent metal thread) screw that screws through the horn and into the motor shaft ?

Have you noticed the little splines on the shaft (little zigzags going around it) ? You'll also note that there is a matching set on the horn, this is to provide a degree of perpendicularity for the forces to be transmitted from the motor to the load. Technically, if you're interested, it moves the shear load away from the gap between the two parts to be effectively inside the parts.

Using glue as you suggest, only leaves a small ring shaped face for the load to be applied, not as pressure, but as a shear load. As you've figured out, you'd need some pretty special adhesive to achieve the same torque specifications :wink:

Maybe have a noodle around here:

https://www.servocity.com/html/servo_shafts___couplers.html

Still, not sure I understood your query... ?

Yes, I mean the servo horn. Thank you.

So I have a wheel that I want to turn to a particular angle with the servo. The wheel cannot be right up against the servo but needs to be a little distance from it -- about a centimetre. So basically, if that little horn ring where a centimetre deep/long, I'd be golden. But it's not. So I would like to extend it.

My glue-based attempts left the little propeller attached and glued the propeller to try to get more area to press against. Not good enough.

So my thought was that if I could find a spacer/standoff in the size of the screw used to hold the propeller on the horn, then I could use that to attach my wheel (I can make the attachment at the wheel side any size I want). But to find a standoff/spacer that size, I need to know what that size is. So I'm hoping someone knows what that size is. I've checked data sheets and found nothing.

Sorry for my repeatedly wrong terminology. This is my first project and I've been surprised to discover that figuring out the mechanics has been much harder than figuring out the wiring and programming.

Why not build a stand off from the horn?

It's already got holes in it... Especially easy if it is the symmetrical type (that looks like a (useless) propeller).

Essentially I have no room in front of the horn for any real structure. It's horn, wall, wheel. I just need something to go through a small hole in the wall. I've tried and hopefully this will work, to build a small disk around the horn with the propeller (screwed into the horn) embedded in the disk. The disk is made of epoxy which is drying to hard as rock, hopefully. Also embedded in the disk at the centre of the propeller is the flat head of a screw. Said screw can go through the wall and into my wheel, and hopefully this will work.

I'm surprised that having some sort of horn extension rod isn't something that would come up a lot and be easily available, but oh well. Hopefully this will work.

Too many possible interpretations of your explanation for me to think about ::slight_smile:

(picture is with a thousand words)

Good luck though.

Here's a picture. I think the epoxy disk will hold, but we'll see. If not, time for plan B (well, more like G), I guess.

Imgur

While that picture might be worth a 1000 words, 800+ of them aren't relevant to the topic, and the 200 remaining are more poetic than technical.

:slightly_smiling_face:

so... you don't have control over that structure?

Like just, get handy ??

Well, the structure has to serve a particular purpose and the things around it have necessary functions, too, and need to be in particular places relative to one another. So my control is constrained control. I'm hoping the epoxy disk will be handy enough. It's certainly rock hard. I have to wait a few more hours for it to be fully cured before testing it.

Quick Edit:Modify - lets you edit the title. I fixed a few for you.

Epoxy - can bake at low temp for a couple hours, 125F to 150F, to cure it faster.
I do that with some thermal epoxy we encapsulate boards in - otherwise the cure time is weeks.
Toaster oven in the garage this time of year, out on the back porch otherwise.

Thank you for fixing.

So the epoxy cured (it was a fast cure kind - 5 minutes working time and 8 hours cure, though I doubled that just to be safe). The epoxy disk held. But (of course there's a but!), the screw turned inside the wheel instead of the wheel turning. Basically, it turned as designed twice and then the screw was too loose, so when the motor rotated, the wheel didn't turn.

I've made a new wheel so the screw threads will be tighter (the last version had been through lots of screwing and unscrewing by this point) and sealed the threads with gorilla glue instead of epoxy this time. My internet research suggested gorilla glue is best for shear force.

Let's hope this is the final iteration.

CrossRoads:
Epoxy - can bake at low temp for a couple hours, 125F to 150F, to cure it faster.
I do that with some thermal epoxy we encapsulate boards in - otherwise the cure time is weeks.
Toaster oven in the garage this time of year, out on the back porch otherwise.

Not to hijack the thread... but...

CR... do you use any specific epoxy for this or just standard 2 part epoxy? I have some RC boat parts that will be load bearing (boom vang and others) that will get wrapped tightly in twine and epoxied for reinforcement. last season, the epoxy never cured right (mismix on the 2 part and length of time), but other parts of the setup will be sensitive to heat (wood and plastic).

Here's what I'd try to extend the servo shaft:

  1. First - take the servo apart; basically just the top off, then carefully remove the output gear - the design of the servo will influence this step, so be careful. If it doesn't look like it can be done, then you might need another method.

  2. Once you have the output shaft gear removed, get a piece of brass tubing slightly larger than the output shaft, from a hobby and crafts store (or similar); a snug fit on the shaft would be best. Cut a piece of the tubing off (using a tubing cutter), as long as you need for your axle.

  3. Fit the tube over the output shaft on the gear, then make a mark on the tubing about halfway between the end of the shaft and the end of the tubing.

  4. Remove the tube, then fit a piece of doweling inside the tube (you can get this at the hobby shop as well) - the dowel should be a snug, but not tight fit.

  5. Now - using your mark as a guide, take a small nail and use a hammer to "indent" the mark - don't hammer through it - you're just making a guide detent.

  6. Then - use a small drill (a hobby drill press would be best, but it can be done with a dremel tool and a small drill bit - provided you put the piece in a vice, and hold the drill perfectly vertical) to drill through the tube, dowel, and other side of the tube - the detent you made in step 5. Clean up any burrs from the drilling, and remove the dowel from the tube.

  7. Fit the tube over the end of the servo gear output shaft. Using the holes as a guide, drill through the shaft (slowly and carefully - don't rush it). The plastic should be soft enough to make this fairly easy.

  8. Remove the tubing from the shaft, then reassemble the servo with the now drilled gear output shaft.

  9. Replace the tubing over the output shaft, aligning the holds. Install a small pin (a small brad nail would work OK) through the hole.

There's the shaft extension. The tubing can serve as the axle; the brad keeps the tubing secured and rotating with the servo output shaft.

If you wanted a solid axle, shorten the tube (so only a cm or so extends away from the servo), and get a piece of brass rod (hobby store again) that will slip-fit inside the tubing. With the tubing off the servo, slip the brass rod inside the tubing slightly, then use a small torch to flow solder in the gap between the rod and the tubing (capillary action will draw the solder in). Let it cool, then re-attach the tube to the servo output shaft with the pin.