I am building a structure with some moving parts. One of the plastic gears that fits onto a metal rod is not firmly secured and is slipping. In other words the rod (shaft) is rotating, but the gear stands still instead of turning with the shaft.
Any suggestions on how to fix this problem?
Thanks.
Rich
There are a few methods to try. If the gear can easily be slid to to sid of the shaft, you could knurl the shaft with serrated jaw pliers, before repositioning add a drop of Loctite red and the position the gear as quickly as you can. Clean up any excess that shows. Epoxy is another way to secure the gear.
It is exactly what I would do. I am a big fan of epoxy for tough jobs, but you have to give the material some tooth to adhere to, and that means removing the gear and doing something to score the shaft. This doesn’t mean goughing it deeply, but just rough it up visibly with sand paper or a small file. A good approach would be to do a cross-hatching pattern by turning the shaft lathe-like while you run the file diagonally one direction, and then stand on the other side of the shaft as it continues to turn and run the file the same way relative to you. It would ideally produce something like a light knurling, and that would really help the epoxy.
I would also cause some grooves in the hole of the gear if it could be done relatively easily and safely…you don’t want to actually compromise the gear since so many of them have failed in the hobby these years. But a roughened inner lining to the hole would help the epoxy to grip the gear and ensure you get the action out of the shaft and gear combo that the motor means to impart to it.
Crandell
Another method to do this (called “upsetting the shaft”) is to remove the gear, then roll the shaft between two fairly coarse mill files. This has the effect of slightly increasing the shaft’s diameter, allowing for a tighter fit of the gear. Add a drop of Loctite or apply a drop of ca after the gear has been pressed into place.
Wayne
Thanks, guys, this all makes good sense.
What I am considering, based upon this advice, is to rough up the shaft with sandpaper or a metal file, use Loctite right over the spot on the shaft where the gear will go, and then epoxy on each side of the in place gear to really firm it up.
Any thoughts or comments on that procedure?
Rich
Wayne,
This is an interesting comment about “upsetting the shaft”. Obviously, rolling the shaft between two fairly coarse mill files is not going to increase the diameter of the shaft. But, what does it do to allow for a tighter fit of the gear?
Rich
I would use one or the other, not both. Personally, based on my experience using epoxy as a go-to adhesive/fixer, it gets the nod. You would need only a very tiny amount, maybe 8 cubic mm.
It won’t be an easy procedure. Because it is fluid, you would have to dam one side of the gear with tape tight to the shaft and gear’s one side, then clamp the shaft vertically in a vise or heavy vise-grip so that the dammed face is lowermost. Mix your tiny dollop of two part epoxy well, and then let it drip slowly into the area of the shaft so that it can run into the tiny orifices.
Come to think of it, it might be better to try a gel CA or Loctite first, let it cure thoroughly, and then give the gear a firm twist to see if it will withstand the torque. The epoxy solution would be quite involved, but if it penetrated the hole well, it would be like a diamond.
Crandell
Crandell,
Thanks for that good advice.
Here is what I have done so far. I used a metal file to rough up the metal rod then applied Loctite only on the part of the shaft that the gear rests on, but not on the sides of the gear. So, only the center hole of the gear is sitting on the shaft where the adhesive has been applied. So far, so good. It seems to be holding firm, but I will wait 24 hours to apply any torque with my fingers. Then, I will apply epoxy on both sides of the gear and onto the rod on either side of it. I have actually done this before on high stand ground throws and it works quite nice. Of course, in that instance, we are talking about manual torque, not a 6 volt motor torque. But, I am optimistic that this will work. If it doesn’t, then I may not have a motorized structure.
Rich
But it DOES increase the diameter of the rod and gives it some tooth. You are not FILING material off the rod. You are ROLLING the rod on the file. This impresses the tooth pattern of the file on to the rod. Some of the rod gets dimpled, and alternate sections raise up (just a hair) increasing the effective diameter of the rod a few thousandths, giving the hub (and loctite) something to grab on to.
I usually roll the rod between the file and a hard surface, using the small edge of the file since our hubs are so narrow.
Phi
[quote user=“THE.RR”]
richhotrain:
doctorwayne:
bogp40:
There are a few methods to try. If the gear can easily be slid to to sid of the shaft, you could knurl the shaft with serrated jaw pliers, before repositioning add a drop of Loctite red and the position the gear as quickly as you can. Clean up any excess that shows. Epoxy is another way to secure the gear.
Another method to do this (called “upsetting the shaft”) is to remove the gear, then roll the shaft between two fairly coarse mill files. This has the effect of slightly increasing the shaft’s diameter, allowing for a tighter fit of the gear. Add a drop of Loctite or apply a drop of ca after the gear has been pressed into place.
Wayne
Wayne,
This is an interesting comment about “upsetting the shaft”. Obviously, rolling the shaft between two fairly coarse mill files is not going to increase the diameter of the shaft. But, what does it do to allow for a tighter fit of the gear?
Rich
But it DOES increase the diameter of the rod and gives it some tooth. You are not FILING material off the rod. You are ROLLING the rod on the file. This impresses the tooth pattern of the file on to the rod. Some of the rod gets dimpled, and alternate sections raise up (just a hair) increasing the effective diameter of the rod a few thousandths, giving the hub (and loctite) something to grab on to.</
Thanks, Phil: you’ve pretty-well explained the principle behind “upsetting”. The material displaced from the minor grooves created by the process end up as small ridges, slightly increasing the overall diameter. Filing the shaft, on the other hand, may result in not only a smaller shaft diameter, but the chance that the gear’s centre will not be concentric with the shaft’s original diameter. If you were able to then somehow fix the gear onto the shaft, you could end up with variable gear mesh as the shaft rotates - not a good thing. [swg]
The pressure required to perform this operation isn’t especially great, and it works well on fairly hard material, such as steel music wire, the same material NWSL uses for their axle stock. Setting the shaft atop one file, then placing a second file atop the shaft and pushing along the length of the first file while applying moderate downward pressure usually does the trick in a single short pass.
Wayne
Most of the suggestions have involved an adhesive or “roughing” he shaft area where the gear sits. There is another possibility: Remove the gear, and using a cutting disk, cut a very shallow grove in the shaft at the point where the gear is to be located. Then using a triangular file, cut a very shallow notch in the hole in the gear.
Place the gear on the shaft, and align the grove and notch. Then drive a metal rod of appropriate size into the “hole” created between the grove in the rod and the notch in the gear. What this does is create a “spline” which will bind the gear in position, and prevent it from rotating about the shaft.
This “fix” is entirely mechanical, and does not depend on the adhesion of glue, epoxy, or loc-tite to two dis-similar materials.
That’s also a good suggestion, to use a key. A cut-off disk might help to score the key-hole along the shaft, but it will be very tricky to keep the disk wanting to say aligned along the axis of the shaft. A machining tool would be better.
The upsetting process sounds good to me, although my only worry would be the integrity of the gear when trying to press it onto the slightly enlarged, and now quite rough, shaft. I’m thinking about split gear, the great dread. [:S]
I guess there is always NWSL gears if none of this helps.
Crandell
Keep in mind that the plastic gear is being pressed onto a metal shaft whose diameter has been increased very slightly, but not over the entire circumference. The ridges which have been raised on the shaft will cut the walls of the gear’s bore only where they make contact, acting in effect like miniature keys. Upsetting (a procedure recommended by NWSL, incidentally) probably works best where the shaft material is harder than that of the gear. If the gear material were similar to, or harder than, the shaft, much of the upset material would likely be removed during the pressing process, negating the effect.
As an experiment, I upset the end of a piece of 3/32" (.0938") steel axle stock. A single pass of about 3"-4" between the two files resulted in a new diameter of about .096" (+/-), but, of course, the upset area was no longer smooth. Unfortunately, I didn’t have a suitable sloppy gear available on which to test the results. [swg]
A key and keyway is an excellent solution, but one best executed with accuracy beyond the capabilities of the modelling tools which most of us have on hand.
Wayne
SUCCESS !!!
Here is what I did. I lightly roughed up the metal rod surface with a metal file, then applied a coat of CA adhesive and inserted the gear. After letting the glue dry for several hours, I used the pointed end of an Exacto Knife blade to apply JB Weld epoxy on either side of the gear where it touches the metal rod. I let the epoxy set and cure overnight, then tested it today with the motor torque, and it works !
Thanks guys for all of your help and advice.
Rich
Excellent! And you have your loco back in service, or will have shortly. Very good news.
Crandell
Congrats Rich. The use of CA is fine for the application of the machine shop/ structure.
I would only like to add, that CA is a brittle joint. If this was to be used on holding a gear/ part that would see torque, shock and other stress, as an axle shaft for a loco, Epoxy or Loctite is a better choice.
I use a product that contains cyanoacrylate ester, so I refer to it as CA glue, rightly or wrongly.
Also, Loctite is a brand name for a variety of products, but I thought that the principal ingredient in Loctite adhesives was cyanoacrylate.
Which Loctite product are you recommending? Does it contain something other than cyanoacrylate ?
Thanks.
Rich
I think that regardless of whether you use Loctite or ca, neither is acting primarily as an adhesive but rather as a means of increasing the “fit” between parts - applied after the gear is in place, it’s drawn into the minute and irregular gaps between the shaft and the gear’s bore, creating a rough equivalent of an interference fit.
Wayne
I agree, Wayne, it may not be very strongly adhered to the metal or the slippery nylon (?) gear, but its actual structure filling the gap acts more like a whole bunch of those keys we were talking about, or maybe more like a weird-toothed inner gear in its own right that forces both the axle and gear to rotate fully synchronously.
Crandell