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Old Sep 11, 2013, 04:19 PM
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United States, CA, Tehachapi
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Question
Prop shafts

Due to shaft angle some elect. motors end up sitting high in the hull and angled, I have seen a set up where the builder used two "U" joints with a short shaft between the motor and prop shaft. When I worked on full size motors & pumps etc. we had to maintain .004 axcial aliment between motor and shaft so has any one elese used the two "U" joint and shafts set up to lower motor in the hull, was thinking of trying this in a 1/20 scale PT boat.
Thanks, Keith
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Old Sep 11, 2013, 06:09 PM
Veni, Vidi, Feci
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Motor City
Joined Dec 2004
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TCR: Double u-joints is what ALLOWS you to run with significant misalignments. Does anyone hold 4-thou alignment between their trans and rear axle? Nope, they're connected with an intermediate shaft and double u-joints.
I do this all the time, including in my triple motor 1:20 PT:
http://matthewsmodelmarine.wordpress...e-expendables/

BTW, the much loved Dumas dogbone is essentially a short intermediate shaft with double u-joints. I will cut them in half, and jam them into a piece of tube to make a longer shaft. The PT, again:
http://www.rcgroups.com/forums/showt...9#post20897597

BTW #2: Don't try to do it with a single u-joint. This COULD allow motor and prop shaft centerlines to be angularly misaligned, but their centerlines have to intersect exactly, and in the middle of the u-joint. Double u-joints allows for offset and angular misalignment.
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Old Sep 11, 2013, 08:17 PM
Cheif Bottlewasher
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Canada, NS, Sydney Mines
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Something I haven't seen in the hobby world is a double cardan shaft....

They are out there..just not in a bolt on unit..
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Old Sep 11, 2013, 08:53 PM
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My choice, like Pat said, is Dumas. They just work!! Here is a Photo.
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Old Sep 11, 2013, 09:30 PM
Cheif Bottlewasher
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No argument there.
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Old Sep 11, 2013, 09:32 PM
Veni, Vidi, Feci
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Motor City
Joined Dec 2004
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Quote:
Originally Posted by more coffee View Post
Something I haven't seen in the hobby world is a double cardan shaft....

They are out there..just not in a bolt on unit..
???

All day long:
http://www.harbormodels.com/site08/m...propulsion.htm




Or just join two of these with as short or long a shaft as you like:

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Old Sep 11, 2013, 10:07 PM
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My problem with these is that they still transfer some load, as they DO NOT, have a true, "floating" center link. ANY, load transfer, ADDS wear.
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Old Sep 11, 2013, 10:14 PM
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United States, MA, Haverhill
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Dumas dogbone couplings

I'm just unlucky I guess. I have a Dumas dog bone that has one of the brass ends drilled approximatly .003 to 005 thousands off center. When I had it in my Dumas utility It vibrated very badly at high RPM. Had to take it out. Like Pat said, they are great if they are made right, if not, look out!!

DDWILLIE
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Old Sep 11, 2013, 10:57 PM
Cheif Bottlewasher
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Canada, NS, Sydney Mines
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Quote:
Originally Posted by patmat2350 View Post
???

All day long:
http://www.harbormodels.com/site08/m...propulsion.htm




Or just join two of these with as short or long a shaft as you like:

touché!!
got me there ..I have a dozen metal ones ,all under sized so I can resize them to what I need later.
My mind got up and left the building!!!
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Old Sep 11, 2013, 11:29 PM
Nickel Belter
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Northern Ontario
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Quote:
Originally Posted by DDWILLIE View Post
I'm just unlucky I guess. I have a Dumas dog bone that has one of the brass ends drilled approximatly .003 to 005 thousands off center. When I had it in my Dumas utility It vibrated very badly at high RPM. Had to take it out. Like Pat said, they are great if they are made right, if not, look out!!

DDWILLIE
None of mine have been drilled off-center, but some have been very roughly machined.

I feel like Dumas' quality control has slipped in recent years.
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Old Sep 12, 2013, 12:45 AM
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No argument there, but, still my preferred universal joint system!
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Old Sep 13, 2013, 09:09 PM
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The plastic stuff wears out down here in Fla. maybe I run my boats too much...
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Old Sep 13, 2013, 11:28 PM
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I have a Thompson Trawler with a 2 3/4'', 4bld prop, and it has the same Dumas universal joint parts, put in 25+ years, and how many hundreds of hours, of use. We estimate my LCM-6 based tug, has over 800+ hours on it, since built in 1989, and it still has the original Dumas parts in it. My boats are stored, and run, in temps from 40-100+ degrees, so no love there. Where they actual Dumas parts, or one of the many Chinese copies??
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Old Sep 14, 2013, 06:22 AM
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Raleigh NC
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I have "extended" dog-bones in my vesuvius project, strange shaft angles and narrow hull sections, allowed for the motors to be moved much farther forward and reduced the coupler angle from "not going to work" to "nice and quiet" . using vac-u-boat couplers, they have four pins on each end of the dog-bones.. round and smooth the inside machined edges of the brass coupler parts so they don't cut into the dog-bone pins.
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