Graupner versus Raboesch U-Joints

[Kmot]

I just got a set of Raboesch u-joints to replace the Graupner u-joints that failed in my PT boat. The difference is significant. I thought others might like to know.

Graupner
"U" OD:.......................10mm (.393)
"U" Thickness:...............2mm (.075)
Pin OD:........................2mm (.075)
"U" Length:.................21mm (.830)

Raboesch
"U" OD:......................13mm (.504)
"U" Thickness:..............3mm (.100)
Pin OD:.......................3mm (.100)
"U" Length:.................25mm (.980)

[HorribleHarry]

Good post Tom, I have been a champion of the Raboesch since I got mine from Al at Ships n Things. They are slated to go into my ironclads, but at the low speeds I will be running them at, I may switch to dumas, and save these for my "bigger" stuff.
Thank you for being the village scientist!

(I'm going to start calling you "professor"!)
Cap'n Horrible

[jeepers1940]

Kmot,
It appears that the Raboesch units are dimensionally larger overall than the Graupner units. Like Harry, I appreciate your doing the research for the rest of us. Question though - Are the units made of the same material, or does that differ also?
Bill

[Kmot]

Bill, the "U" units are both made of plastic. That's all I can tell. Whether they are carbon, or glass fiber reinforced I do not know. The Raboesch insert is brass. The Graupner insert is chrome plated brass.

[green-boat]

I have found out that the Dog Bone couplers from Vac-U-Boat are of the 4 pin variety where the Dumas ones have 2 pins per end. The 4 pin ones are stronger, quieter and cheaper.

[u163665]

Those Raboesch couplers are fantastic! They are in my Dumas Donzi and have not failed me yet. My Graupner couplers installed on my Dumas Prime Time have failed a couple of times.

[patmat2350]

The Raboeschs are definitely bigger, which is good IF you need the capacity. But I also find them to be stiffer, and maybe not a great choice for smaller models.
Both have glass-filled nylon bodies.
R. has swappable splined brass inserts with different bores available, while G. requires swapping the entire end (brass insert is molded in).

Kmot, you won't berate me for mentioning that I covered these guys and more in the Jan. 2007 Ships in Scale, I hope?

Pat Matthews

[Kmot]

LOL!!! Well Pat, you know the more you get hit in the face, the less it hurts. :p

[Kmot]

The Raboesch won't work in my PT boat:

http://www.rcgroups.com/forums/showpost.php?p=6941735&postcount=75

[Ghost 2501]

what I worry about rergarding those roebush units is the splines look delecate and could strip easily if a large amount of power is fed through them. Robbe's najade has a dual cog and collar joint one cog goes on the motor one goes on the propsaft and a collar goes between the two gogs, the teeth of the "cogs" acting like the splines on the roebush units. what it cant do is go through steep angles like a UJ but it does absorb shocks transmitted up the shaft

[Ghost 2501]

Tom post a pic of your pt boat transmission

[patmat2350]

Ghost, those splines are stronger than you might think, especially since the bodies aren't plain plastic, but chock full of glass fiber reinforcement.

A number of 1:1 scale autos these days are using carbon fiber drive (prop) shafts. The most popular technology (used on Aston Martin, Mitsubishi, Nissan Z, and Mazda RX8) uses a carbon fiber tube with a bit of glass fiber on the inside for "crush". A splined metal end is then jammed into the tube. No other joining material needed, just a dry pressed spline... high performance cars have been sold for years this way.

Pat M

[pusar]

In my experience the Raboesch splines have no problem taking the punishment. My Dauntless (about 20lbs. and AstroFlight 25's) can take from standing to full throttle with no problems time and time again. The splines fit so tightly that it is almost like they are glued in. You need a pair of pliers to remove them. There is no slip at all. The splines begin to wear if there is slip. No chance of that here.

My Dauntless is also using the Raboesch Props and shafts. In my experience their products are first class.

Marty

[Kmot]

Tom post a pic of your pt boat transmission
Mark, my PT boat does not have a transmission. Just a motor shaft and a prop shaft.

The issue with my PT boat that makes the Raboesch, or any single u-joint for that matter unusable, is that the motor to propshaft is not on the same plane. If it were just an 'angle' issue the single u-joint would work fine. That is what u-joints do. But the problem is the motor to propshaft in my boat are on different levels. So attaching a single u-joint instantly creates a bind in it.

The proper solution is to rip out the motor mounting structure in the boat and build back in a new motor mounting structure that is aligned correctly. I am trying to take the easy way out by only installing a double u-joint. And it works. ;)

[pkboo]

LOL!!! Well Pat, you know the more you get hit in the face, the less it hurts. :p

I believe that´s Pavlov in reverse :) Kmot. Now if Pat goes with Pavlov he´ll ask you first the next time before he wants to writes an article! This is so much fun :D Eugène

[Ghost 2501]

kmot you can get adaptors to double up on the roebush units, My glynn guest warship used a double UJ. growls like hell but it worked

[Shaun Hendricks]

I just finished balancing and testing one of the Graupner U-Joints for my Highwind conversion. Balancing was pretty straightforward with my prop balancer and it made a huge difference in the noise and vibration of the running shaft. I also noticed a little bit of friction heat at the joint after running a few minutes. I know Graupner says they use 'self lubricating' plastic (the stuff kind of acts like Delrin) but I oiled it with some paraffin turbine oil and ran the joint again. Almost no heat the second time around. It's entirely possible that Kmot's problem may have started with a heating at the joints and worsened from there.

[Kmot]

kmot you can get adaptors to double up on the roebush units, My glynn guest warship used a double UJ. growls like hell but it worked
Yes I know. I posted that in the other thread. They would be too long.

I also noticed a little bit of friction heat at the joint after running a few minutes. It's entirely possible that Kmot's problem may have started with a heating at the joints and worsened from there.

I actually measured the temperature of the Raboesch joints when I was running them and they were quite hot after a few seconds, over 120°F. After I changed over to the Graupner u-joints, I lubed them while running and while my Watts Up meter was connected. The amp draw dropped an average of .25A after spraying some Prolong lube on the knuckles and the temp dropped from 90°F to 70°F. :cool:

[ernest2]

i have also experience heating, or shall i say overheating in my graupner joint (single), is this normal?
also in your opinion and experience, which is better, the u-joints or the dogbones type?

[Kmot]

In my experience, limited as it is, the dogbones and the u-joints are equal. It just depends on whether or not you can use them in the application. In my application, the dogbones worked but were very noisy. The u-joints are far quieter. If the motor to shaft were aligned properly, there might be no difference.

However, I have tons of experience with dogbones and u-joints in r/c cars and trucks. That tells me that u-joints are far superior. The dog bone pins start to wear the u-cup slot and before too long their is a lot of play in there. And eventually the dogbone will have worn so far it will pop out. That does not mean that u-joints do not wear out as well. They do. But much, much slower. Eventually the pins in the coupler will elongate the holes in the u-cup. But in an r/c truck their is constant shock loading of the driveline.

At high rpm, the u-joints again are quieter in a r/c truck. Dogbones tend to 'rattle' as everything spins up really fast.

[Shaun Hendricks]

I'm surprised someone hasn't adapted the CVD style joint that is currently popular in performance RC cars to boats...

They are very quiet, extremely tough, easy to keep lubed and super slick to work with.

[patmat2350]

I wonder... The Dumas dogbone is essentially the same geometry as those joints- a ball and socket with drive pins. Problem is that the car-style cups are always integral with bespoke ends- theaded to fit hubs or whatever... and the joints are pretty long shafts (half shafts), which might be inconvenient to deal with in the confines of many boats.

Another question- what's the long term durability like, with metal-on-metal? Car wheels turn, what, a few hundred RPM? Boat prop shafts spin 5-10,000 RPM.

One item I like- the sliding prop shafts with u-joint (Cardan) ends. I use these for the center drive shafts to my DUKW's axles:
http://www.geocities.com/y2patmat/dukw/ see "Driveline" section.
But I had to modify the ends to fit my shafts...

Pat M

[rlboats2003]

Thanks for putting one of the SIS pages in about joints, I must have missed that whole article, need to go back to the issue and take a look. By the way the last couple of weeks you have been dropping some great pearls of modeling wisdom throught out different threads. Thanks for those, too.

Happy Modeling,
Rich

[Shaun Hendricks]

A Micro (1:18th scale) car with a 2.25" diameter wheel travelling 45 mph turns about 6800 rpms on those wheels and has far more pressure on the drive line than any boat would (direct friction contact with a surface rather than the mush of a liquid). I don't think a boat could ever generate the same torque forces from a prop that a car does from standing stop to full throttle, not to mention the abuse from braking, sliding, bouncing, rolling, slamming, skidding and the like. A boat would come close when its prop was stuck or bound though.

My Mini-T has CVD's on it and they are titanium and in theory could be made to any length you wanted, probably down to ¼" (enough to clear an angle of less than 40° for the application, I don't think they'd work well beyond that total bend radius).

Slot with set screw would work fine on each end and the 'head' of the joint could be made as part of the lead slotted drive fitting. Then pin in the bone, slip some latex tubing with grease in it over the whole joint and you're done. Little to no wear, silent, and long, worry free running.

Anyone with a decent machining lathe could make one up. Too bad I don't have one! LOL! :D

[Kmot]

Not all CVD's for cars come with the threaded axle on the end. The ones made for the center of the E-Maxx are cups and would work in a boat if you have the length to use them. I have investigated this, and have another option I mentioned once before in another thread. The sliding driveshafts that Traxxas uses. These could easily be cut down to the lenght needed.

[Shaun Hendricks]

Here's a quick mock up of what it could look like (just using Paint, sorry don't have photoshop on this PC to make it nicer looking!)

It should work though.