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Author Topic: Curious vibration  (Read 3932 times)

hazmat

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Curious vibration
« on: June 16, 2010, 10:18:39 PM »

See the cad sketch, this is the power train layout of my tuglet. (and yes I'm hopeless at sketching too)
The problem is a funny vibration at 1/3, and 2/3 power.
It's almost harmonic and repeatable in water but NOT when running dry.
I thought it was the prop at first but I've changed that to a brass one and balanced it. Waste of money, same fault.
Someone thought it was the ESC?? Nope, changed that for a different make.
Anyone got any other  ideas?
couplers are no frills plastic with brass inserts.


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DARLEK1

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Re: Curious vibration
« Reply #1 on: June 16, 2010, 11:02:53 PM »

Is that the actual angle of your prop shaft?
 Paul...
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hazmat

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Re: Curious vibration
« Reply #2 on: June 16, 2010, 11:08:13 PM »

as drawn? no.
Working out the angle (bit of trigonometry) it comes to 15 degrees

Paul
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Umi_Ryuzuki

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Re: Curious vibration
« Reply #3 on: June 16, 2010, 11:21:20 PM »

Is that the actual angle of your prop shaft?
 Paul...

I think the actual angle is shallower,... "as dimensioned".
However that may be the actual order of connections, "as drawn".

Minimizing the connection angles is usually best, I can see where you might have
run into some issues. However, you might try aligining the shafts at the motor connection, and
see if that doesn't reduce some of the vibration that is being created.

Another thought, is that the set  screws at the shaft connection, from propeller to shaft, and
from motor to shaft, might have coincidentally aligned, creating a symmetrical offset of the shaft at
each end, when the set screws are cinched tight.  ... Just something to take a look at.... :)
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Bunkerbarge

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Re: Curious vibration
« Reply #4 on: June 16, 2010, 11:49:22 PM »

I think there is a slight misconception that UV joints will allow the transmission of power through any angle you care to think of.  They are only really designed to compensate for slight misalignements so going to the angles you describe is going to be a challenge.  Before everyone adds all sort of crazy angles thier own particular models operate at there will be some lucky people who have got away with it however the fact still remains that they should not be used for large angular changes.

If you think about how they operate, if the shaft was perfectly straight the pins in the UV joints would remain stationary in relation to the clevis bearing.  As the angle of the transmission increases the pins start to oscillate more and more in the clevis bearing until at large angles the pin is oscillating quite significantly and at the same frequency as the shaft speed!! %%.  Not surprisingly this leads to vibration and eventual failure of the clevis bearing.

When the model is in the water you are loading the propeller and so the transmission of the power puts the pins and bearings in a slightly different relationship to each other.  You are lucky that things run smoothly when the prop is out of the water as the pins find thier own natural location.  It wouldn't surprise me that if you oiled the pins and the clevis bearings that it runs smoothly in the water as well as you make the significant oscillation that bit easier for the bearings to cope with.

The only real long term reliable solution is to reduce the transmission angle and take the load off the pins but if this is not possible you might have to look at oiling the pins regularly to help the bearings and that is going to cause a bit of a mess.  Grease might do it as well but you will then need to take the units apart and that will soon lead to failure as well as you stress the clevis to dismantle them.

Depending on the power you might get away with a flexible joint such as a rubber tube type or a spring but both of those will fail significantly sooner with larger angles of operation as well.
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Peter Fitness

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Re: Curious vibration
« Reply #5 on: June 16, 2010, 11:57:05 PM »

Another thought, is that the set  screws at the shaft connection, from propeller to shaft, and
from motor to shaft, might have coincidentally aligned, creating a symmetrical offset of the shaft at
each end, when the set screws are cinched tight.  ... Just something to take a look at.... :)

I had a similar problem in an earlier model, and eventually discovered the cause to be aligned set screws. Offsetting them cured the vibration.

Peter.
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hazmat

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Re: Curious vibration
« Reply #6 on: June 17, 2010, 07:19:46 AM »

Thanks for all your input folks.

The shaft angle I can do nothing about but on inspection the fixing grub screws were all found to be on the same side. Nice easy thing to change so I'll try that. :-))
.
One of the things I don't like is high revving motors and the motor is geared 6:1 . I've always spun a large coarse prop at slow revs than a small one flat out. I figure that I can still shift enough water with this philosophy to make tugging round a pleasure and it keeps the radial stresses low. (Worked till now that is)
With an optical tacho (throwback to aeromodelling days), I don't exceed 8000 rpm (Slow to fast 500-8000 DRY) so I'm not in the 'shake it all about' revs that my mates run ( i.e. 13-19000 rpm).
Greasing the couplers I do as a matter of course.
Most of my boat innards are covered with a fine layer of 'spin off'. Better that than dry joints though. Anyway I figure it adds to the waterproofing. :embarrassed:

One thought was to use a double coupler at the motor end and shortening the shaft (must move quickly on this decision as the mastic is drying out holding the original shaft). The theory is that two clevis joints working on half the angle each will smooth things out.
Your thoughts appreciated on that one.

Paul  :}


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Marks Model Bits

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Re: Curious vibration
« Reply #7 on: June 17, 2010, 09:21:10 AM »

A universal joint (hardy spicer/cardan) with any mis-alignment, however small, speeds up and slows down as it rotates causing premature wear and vibration. To compensate for this two joints are used at 180 degrees opposed to each other, this tends to cancel out the differing speeds encountered as the shaft rotates, (look at any propshaft fitted to a rear wheel drive car or truck) As Bunkerbarge says fitting a rubber coupling will go some way to alleviate the problem, but it is better to get perfect alignment in the first place (not always possible though!!!)

Mark.
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hazmat

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Re: Curious vibration
« Reply #8 on: June 17, 2010, 09:48:36 AM »

Right, understand that but as alignment laterally is ok, I'm trying to lessen the impact of 15 degree(ish) angle on the couplers (I think).
So, dual couplers, won't that half the angle in each coupler??
If it does, will that reduce the vibration or do I have a tuning fork situation where everything is just vibrating at specific frequencies.

Paul
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Drkomen86

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Re: Curious vibration
« Reply #9 on: June 17, 2010, 10:27:28 AM »

personaly I would advise trying one of these.

http://cgi.ebay.co.uk/RC-Boat-Aluminium-4mm-6-35mm-21mm-U-Joint-/110547205182?cmd=ViewItem&pt=UK_ToysGames_RadioControlled_JN&hash=item19bd207c3e

there about the same size so can be a direct swap and in my experiance they can handle more of an angle without vibrations, just an idea.


Chris
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hazmat

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Re: Curious vibration
« Reply #10 on: June 17, 2010, 11:10:24 PM »

Fascinating little Universal joint Chris.
Apart from Flea-Bay, any idea where you can get them from as the one shown has expired (I think that is the correct term)?

Paul
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tobyker

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Re: Curious vibration
« Reply #11 on: June 18, 2010, 12:38:25 AM »

can you not run a shaft from the propellor and one from the motor parallel to each other and link them with pulleys and a belt?
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RaaArtyGunner

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Re: Curious vibration
« Reply #12 on: June 18, 2010, 12:59:35 AM »

can you not run a shaft from the propellor and one from the motor parallel to each other and link them with pulleys and a belt?

A bit of lateral thinking outside the Box. :-))
Hazmat, Not a bad idea,  O0 Will it work? {:-{
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HS93 (RIP)

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John W E

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Re: Curious vibration
« Reply #14 on: June 18, 2010, 07:18:31 AM »

Fascinating little Universal joint Chris.
Apart from Flea-Bay, any idea where you can get them from as the one shown has expired (I think that is the correct term)?

Paul

hi there Paul, sorry but I just bought that Universal joint for use in a glow plug model  :-)) good little coupling I think.   I have used them before but mainly on I.C. powered models

aye
john e
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HS93 (RIP)

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Re: Curious vibration
« Reply #15 on: June 18, 2010, 07:22:13 AM »

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Umi_Ryuzuki

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Re: Curious vibration
« Reply #16 on: June 18, 2010, 07:24:44 AM »


Quote from: Tobyker
can you not run a shaft from the propellor and one from the motor parallel to each other and link them with pulleys and a belt?


A bit of lateral thinking outside the Box. :-))
Hazmat, Not a bad idea,  O0 Will it work? {:-{



Someone started something like that here, I don't recall if the boat was ever completed as proposed.

http://www.rcgroups.com/forums/showpost.php?p=12345822&postcount=6224
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Peter Fitness

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Re: Curious vibration
« Reply #17 on: June 18, 2010, 07:55:31 AM »

can you not run a shaft from the propellor and one from the motor parallel to each other and link them with pulleys and a belt?

Do you mean something like this? This is the setup I use in my pusher tug, although I have replaced the single belt pulleys with double belt ones since this photo was taken.

Peter.
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Drkomen86

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Re: Curious vibration
« Reply #18 on: June 18, 2010, 08:26:29 AM »

theres lots of thos ball joints on ebay and I have bought them from my local model shop before, I thought they were quite common
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Martin [Admin]

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Re: Curious vibration
« Reply #19 on: June 18, 2010, 08:52:42 AM »

Do you mean something like this? This is the setup I use in my pusher tug, although I have replaced the single belt pulleys with double belt ones since this photo was taken.

Peter.

Nice to see good 'earthing' straps Peter!
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RaaArtyGunner

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Re: Curious vibration
« Reply #20 on: June 18, 2010, 01:45:33 PM »

Nice to see good 'earthing' straps Peter!

Peter,
A picture is worth a thousand words,  :-)) digressing from the thread, %)  had not considered "Earthing".
Would I be right in saying that earthing to shaft (ie water) will reduce interference/static etc, than the normal closed loop circuit. O0
Or doesn't it make any difference.
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HS93 (RIP)

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Re: Curious vibration
« Reply #21 on: June 18, 2010, 02:09:09 PM »

a couple of pictures of different dive setups made recently

Peter
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Peter Fitness

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Re: Curious vibration
« Reply #22 on: June 18, 2010, 11:22:06 PM »


Would I be right in saying that earthing to shaft (ie water) will reduce interference/static etc, than the normal closed loop circuit. O0
Or doesn't it make any difference.


Yes, that's the theory. I was having trouble with glitches on that boat for some reason, and the earth straps appeared to cure the problem. It's not necessary with 2.4Ghz equipment though, interference doesn't seem to bother it.

Some very nice, neat engineering there Peter (HS93).

Peter.
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hazmat

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Re: Curious vibration
« Reply #23 on: June 18, 2010, 11:43:00 PM »

MFA Gear boxes i've played with before with good results but have always had loads of room which I'm a bit pushed for in this baby.
Still, I can always move a bulkhead if all else fails. I have found in my junk box a very old double wound spring U.J. so I'll give that a go as everything is now in pieces.  <:(
Earth strapping used to work on my 27mhz gear a treat but as you have found, GHZ equipment is pretty bulletproof.
I'm pleased to say that my old 27Mhz Macgregor gear has been gracefully retired (anyone remember that make?) so no more boats disappearing off into the distance following taxi cabs for me!!!!!!  :D

The original question still makes me ponder a bit though and even if it is only dodgy couplings, does anyone else suffer from narrow regions of vibration at specific speeds? {:-{

Paul.
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tobyker

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Re: Curious vibration
« Reply #24 on: June 19, 2010, 12:45:49 AM »

Yup those elegant belt drives were just the sort of thing I meant.
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