Prop shaft vibration

Advert

New Replies

Report Bug

Help/FAQs

Prop shaft vibration

This topic is empty.

Viewing 25 posts - 1 through 25 (of 39 total)

1 2 →

Author

Posts
7 July 2016 at 12:28 #66361
Jeremy
Participant
@jeremy15845
I am getting quite serious vibration at certain speeds from the prop shafts of my Amati Titanic. The shafts turn easily by hand without any apparent resistance points. As shown in the photo, the prop shafts are connected to the motor by a double coupling as specified in the RC conversion instructions. The shafts themselves are firmly glued to the three frames though which they pass. Any thoughts or suggestions would be appreciated.

Jeremy
Advert

7 July 2016 at 12:28 #2607
Jeremy
Participant
@jeremy15845
7 July 2016 at 12:35 #66362
Dave_P
Participant
@dave_p
Looking at your photo I would suggest your problem is due to the couplings not in line. This would cause vibration at different speeds.
7 July 2016 at 13:08 #66363
Jeremy
Participant
@jeremy15845
That, I did not want to hear . If the couplings have to be in line, I will somehow have to extricate the prop shafts and level them up. Any thoughts on how to do this, particularly where they run through two frames to which they are firmly epoxied.
7 July 2016 at 13:23 #66364
Dave_P
Participant
@dave_p
Sorry to be the bearer of bad news.

I would leave the shafts alone and move the motor. Could you not elongate the holes for screws and motor shaft and move the motor up to meet the shaft centre line?

I am assuming (dangerous I know) that you have not planked the hull yet and looking at the picture I would think it not be too difficult to do.

Edited By Dave_P on 07/07/2016 13:25:41
7 July 2016 at 14:06 #66366
Jeremy
Participant
@jeremy15845
Unfortunately, I think it's the shafts which are incorrect. The motors are in the right place and it would anyway be difficult to access the screws fixing them to the frame. I now realise that the shafts should run horizontally and not slope down towards the stern. I should be able to extricate the shafts even at the expense of writing them off. I may need to patch the two frames they go through to make new holes but all will be hidden once planked. Wish me luck! Will I ever learn……?
7 July 2016 at 14:23 #66367
Dave_P
Participant
@dave_p
OK I now see what you are describing. Shafts will hopefully come away without too much damage.

I have everything crossed for you.

Don't worry we've all been their.

Dave
7 July 2016 at 14:49 #66368
Keith Long
Participant
@keithlong89920
Jeremy – while you've got the shafts out you might try aligning the uj's rotationally so that they are "in -phase" rather than your arrangement which is "out-of-phase" the pictures on the following link should explain it better. In-phase should give a smoother drive if the joints are running angled.

Keith
7 July 2016 at 19:01 #66369
Jeremy
Participant
@jeremy15845
Prop shafts came out easily without damage, thanks to a tip I found on another web site – a hot soldering iron degrades the epoxy allowing, in this case, the shafts to slide out. Keith – thanks for the in-phase suggestion. I will make sure I do this although, if I get it right, the drive should be virtually straight.

I think that the conversion required a double coupling for dimensional reasons.. The prop shaft tube is 9" long and the second coupling adds 1.5". if a 10.5" shaft had been available, I could have done without the second coupling. However, 10" or 11" were the only options. It is a pity, however, because the junction between the coupling and the prop shaft will be inaccessible. The lift-off section of the superstructure does not go back quite far enough, at least according to the plans.

Jeremy
7 July 2016 at 19:59 #66370
Dave_P
Participant
@dave_p
Good tip Jeremy, thanks for sharing.
7 July 2016 at 23:43 #66371
Dodgy Geezer 1
Participant
@dodgygeezer1
Are you testing this layout at full revs with no drag on the propeller?

You will find that vibrations which occur when the prop is running in air are damped down considerably – perhaps non-existent – when the prop runs in water…
8 July 2016 at 08:35 #66372
ashley needham
Participant
@ashleyneedham69188
ALSO. Although the couplings may be a bit out of line, its what they are made for. I have several boats with alignment much worse than this using double couplings and they run sweet as the proverbial nut.

As DG says, vibration out of the water is more likely than in.

Could try replacing the couplings with silicone tube. Out of balance prop and /or bent propshaft are the first things I look at. Amazing what a change of prop can accomplish;'

Ashley
8 July 2016 at 10:02 #66373
Jeremy
Participant
@jeremy15845
The prop shafts are out now and I will re-align them to be horizontal (as they should be) and that will make the couplings virtually straight. The prop shafts seem OK to me. The shafts turn smoothly with no tight spots, so can I assume that they are not bent? As to the props, they are Rivabo brass and I don't have any others to try. The vibration only occurred at certain speeds (not top speed), so maybe the alignment of the coupling was a contributing factor. I am probably a couple of years away from having the boat in the water, so a test of vibrations under those conditions will have to wait.

I will post the out-of-water vibration situation when I have finished making the changes. Thanks to all for your help.

Jeremy
8 July 2016 at 11:35 #66374
ashley needham
Participant
@ashleyneedham69188
Rolling the shafts on a dead flat surface is the only way to tell if they are bent (Bob would probably put them on a lathe and mike them up)

Couple of years away from water…I couldn't wait that long!!

Ashley
8 July 2016 at 15:14 #66383
Dave_P
Participant
@dave_p
Fair comment regarding vibration due to propos out of water but out of line couplings don’t help. I maybe being over cautious here but my career as a diesel engineer tells me otherwise. You may get away with misalignment but it does not make it right.
8 July 2016 at 21:53 #66389
Kev.W
Participant
@kev-w
Jeremy, if you want to eliminate the second coupling, there are firms that will make you a shaft of any length you want.

You could give Mobile Marine Models a ring, they made me a new shaft for my Billings boat, I'm sure other members will know of other shops that will do this too.
9 July 2016 at 10:14 #66403
Malcolm Frary
Participant
@malcolmfrary95515
A single universal joint, unless properly aligned, will induce vibration. An arrangement with two universals, if properly phased, will cater for a range of misalignment without introducing vibration. (Look under a rear wheel drive front engine car for arrangement to allow a fixed motor to drive an axle that bounces up and down). A CV type joint will be good, as will a flexible sleeve type join.

My bet is on the root of the problem being a not perfectly straight shaft.
9 July 2016 at 17:58 #66406
Jeremy
Participant
@jeremy15845
Prop shafts re-installed in the correct geometry, although they are not yet epoxied in place. The couplings are close to being straight and they are in phase. I have used Teflon washers flush fitted at both ends to try and restrict water ingress. The shafts are horizontal and below the water line, so I need as much protection as I can get.

Result is that there is quite a lot of vibration out of the water. I have taken the shafts out of the tubes and rolled them down an inclined mirror. This produces some rattling which suggests that they are not 100% true or maybe the mirror is not flat? Presumably, there must be some manufacturing tolerance in a mass-produced item. How straight is a perfectly straight shaft? The same must be true of the props. I could try plastic oines for running but would prefer to retain the brass ones.

So what next? Do I plough on and hope that all will be well in the water or do I need to investigate this problem further. I could buy new prop shafts (Caldercraft fine-line as the Graupner ones were not available) but would it make any difference other than to my wallet?

Jeremy
9 July 2016 at 18:25 #66407
ashley needham
Participant
@ashleyneedham69188
Generally I would think that a mirror would be dead flat.. Some rattling is a bit dodgy, both or just one?s

Other culprit could be one coupling is slightly off.

I think you have taken sensible precautions in the assembly and should glue it up. If there is an issue in the water then a change of shaft(S) may be warranted , coupling(s), prop(s) or even a motor. It may simply be because the hull does not yet possess sufficient stiffness?

Ashley
9 July 2016 at 19:31 #66408
Jeremy
Participant
@jeremy15845
Both, but one slightly more than the other. I understand that the shaft rests on bearings at either end of the tube so its straightness is not so critical. Is that right? Maybe I could just get replacement shafts. The couplings look OK.

A change in shaft or coupling after assembly may be difficult. The conversion to RC involves making part of the superstructure lift off. The problem is that this only goes as far as the frame aft of the couplings and, as you will see in the photo, the attachments to the shafts will be just out of reach. Until I get on to building the superstucture, I won't know if there is any way I can add one extra section.

The stiffness of the hull is compromised by the cut-outs for the RC. It will have 5 costs of fibreglass cloth and epoxy after the first planking to stiffen it up and the instructions emphasise the need to angle the edges of theframes to achieve optimum contact with the planking.

Jeremy
10 July 2016 at 10:50 #66414
Malcolm Frary
Participant
@malcolmfrary95515
If the image in the mirror looks right, the mirror is flat.

The two elements as pictured will induce vibration, they should be at 90 degrees to cancel properly. Look at https://www.youtube.com/watch?v=gmV4qwLfOMY and freeze about 1 minute in to see how it should be done.

Any bow in a long narrow shaft will give an imbalance when it rotates due to there being more weight one side of the actual center line than the other. If its rotation speed is being varied by an incorrectly arranged pair of U/Js, a cyclic resonance will develop.

The washers will help, but to keep water out, fit lubricators and add a water resistant grease. Or arrange things so that the shafts can be serviced.

Edited By Malcolm Frary on 10/07/2016 10:54:37
10 July 2016 at 15:20 #66423
shipwright
Participant
@shipwright
Jeremy,

Looking at the photos it appears that the starboard coupling alignment is not as straight as the port side – that might just be an effect of the angle of the photo. The other observation that I would make is that you seem to have a relatively long unsupported length of prop tube (at the stern. I would have thought that a "P" bracket or "A" frame support would have been required in the design). Also when you trialled the motors driving the shafts were the prop tubes glued/fixed to the frames of the boat ?
10 July 2016 at 17:33 #66428
Jeremy
Participant
@jeremy15845
On further examination, the prop shafts are clearly not straight and I will get them replaced. The prop tubes will be glued to the aft frame which they appear to just touch in the photo. The shafts were not glued in place for the trial but are securely held in place through the frames and I applied pressure at the contact points with the aft frame. I am pretty sure now that the distorted prop shafts are the main cause of the vibration but we will see when I fit the new ones.

The coupling alignments are as close to identical as I can get them – hopefully close enough.

Jeremy
10 July 2016 at 23:27 #66443
Malcolm Frary
Participant
@malcolmfrary95515
When you fit the new ones and couple up the U/Js, have a look at the link I posted earlier. An improperly phased double joint will cause uneven drive which will induce vibration. Having two improperly phased will just leave both creating vibration.
11 July 2016 at 08:39 #66446
Jeremy
Participant
@jeremy15845
Malcolm – The couplings are in phase as recommended by Keith which he iluustrated with a link. I have to admit not understanding your video link.. What am I still doing wrong?

Jeremy
Author

Posts