need to build drive shafts 4mm each end..what do you suggest? - RC Groups
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Feb 29, 2012, 05:04 PM
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need to build drive shafts 4mm each end..what do you suggest?

I am working on a scratch build on the forum, and I am wanting to build some drive shafts for my motors.

Right now I am not sure of the length, I could go as short as 2 inches on the drive shafts, or as long as 10.. but there is no reason to be that long unless its an advantage... I am guessing I will end up around 4 inches long.
What would you suggest for the end fittings ?

I need to fit 4mm shafts on each end. I can solder fairly well, but I am not sure if there is something out there that will use a joint or allow some flexing . I admit I am pretty oblivious to all the possible ways to do this. I am better at hull or airframe construction than I am at drive trains

Maybe you know of a site with fittings you have had good experience with..I have been looking for some sort of universal joints, but finding mostly speed boat gear so far..

Oh I forgot to add my drive shafts will not be exiting thru the hull, since I am using some older nozzle type drives. The shaft will be direct without any stuffing box setup.

Thanks for any ideas.
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Feb 29, 2012, 06:02 PM
Mmm, tugs...
patmat2350's Avatar
You can find some ideas here:

I assume you have a 4mm shaft sticking out of a drive unit (jet?)... and a 4mm stub on your motor.

NEVER try to direct couple with a solid shaft, always build in flex (dogbones, u-joints)... and use two joints (a dogbone with a ball on each end, or TWO u-joints)... there are good geometrical reasons for this.

If you have access to a lathe, it's easy to open up the Dumas 1/8" brass cups to 4mm... best if you ream with a 4mm reamer. And the nylon dogbone itself can be cut in half, pressed into 7/32" OD (iirc) aluminum tube, and cross drilled and pinned (I use #1 brass screws and nuts from the model train store). This allows for any length connecting drive shaft... mine are about 6" long in my PT boat.

Mar 01, 2012, 08:27 AM
GILL RC's Avatar
Pat is right you need a coupling.
Try looking at somme of the R/C car stuff. Some use 4mm cups with set screws. The dog bone can be shortened or lengthened as Pat says.
OR you can use a regular coupling. I have used these in lengths from 2 inch up to 12 inch.
The only time I use a hard coupling is on some of my jet drives that have a motor mount built in.
Mar 01, 2012, 12:46 PM
Registered User
I had been wondering if any of the car parts and dog bones would work? The local hobby shop has tons of that kind of stuff, and barely ANY boat parts. I am seeing what I need to do now....

The boat units look great, I will try to do it on the cheap if I can.. but its good to see there are some perfect parts I can fall back to if i don't see a financial shortcut hahahahah

now I see what I need to do ....

thanks guys
Mar 01, 2012, 08:48 PM
xanin's Avatar
Gill, I've never seen those before with the swappable inserts. Where could I order something like that?
Mar 01, 2012, 10:34 PM
Registered User
Rob_P's Avatar
Try Cornwall Model Boats
Mar 01, 2012, 11:27 PM
Mopar Musclecar fanatic
xx29l9's Avatar
Originally Posted by Rob_P
Are these strong enough to hold up? Or are these the ones people have had explode apart due to the plastic?
Mar 02, 2012, 01:29 AM
RELAX. You'll live longer
785boats's Avatar
I run those in most of my scale boats (as do many others) & they hold up pretty well.
I have blown one apart at 32,000 rpm in a little fast boat, but i half expected that to happen when I did it.
For security of mind I put some heat shrink around the coupling in the faster scale boats. It quietens them down a bit too.
In the attached pic you are looking at 2 x Johnson 785 motors that I run on 4s lipo in a 44" Higgins P.T. Boat. The couplers have the heat shrink on them just for safe measure.
All the best
Mar 02, 2012, 05:58 AM
woodybob's Avatar
Harbor Models have the Kardan Style Motor Couplers.
Mar 02, 2012, 09:15 AM
GILL RC's Avatar
Polk used to sell them, Harbor models, and Cornwall models.
I like them. Strong, well balanced, will fit any thing and easy to use. The picture is the one I used in our catalog.
We had 2mm up to 6mm bore and 10-32 (3/16) and 4mm threaded.
Mar 02, 2012, 06:49 PM
Registered User
If I hook a nice Kardan style coupler directly between the motor and my drive units.. would that be enough? or am I going to need a longer shaft with couplers on both ends?

I did find some drive shafts in the car dept. of the hobby shop that could be made to work.. but they needed an extra part ordered to make it work correctly, I think to build two driveshafts that way was going to be about 25 dollars. They wern't as nice as the Kardan ones, but they did telescope.. making the length not an issue.

here is what I am using for drivetrain, I have been told my drive units are not real sturdy, and lack replacement parts on the (Anyone build this online tug) thread, but I am gonna give it a shot...
Mar 02, 2012, 06:55 PM
Registered User
Pat, do you get two dog bones, and two of the cups on the Dumas kit? so that you can make one complete drives haft with two universal ends? I just wondered as the picture doesn't show a second dogbone.
Mar 02, 2012, 06:59 PM
Mmm, tugs...
patmat2350's Avatar
Various combos are available
Mar 02, 2012, 08:47 PM
Registered User
found and ordered !

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