Prop shaft vibration

[Malcolm FraryParticipant @malcolmfrary95515]

The two ends of the center link look nice when in direct line, but in reality one should be at 90 degrees to the other to cancel out the phase shift when the joint goes through an angle instead of multiplying it. When a joint changes angle, the driven part moves in an ellipse. Having the two ends at right angles restores circular motion. Varying the rotation speed along the drive train at a rate that corresponds to the natural resonance of the bow of the rotating shaft will increase the vibration at that speed. The closing shots in the link show a correctly assembled center link with a nice yellow line to highlight the relationship between the two ends.

The long unsupported length outboard of the hull won't help, either. As others have suggested, a support similar to what the real thing would have had is wanted. With a long shaft (usually longer than yours), it is not uncommon to insert a bush to near the center of the tube – near but not quite so that the two halves can't form their own resonance. Does the vibration happen without the props? With the slightest imbalance out there, any movement will be allowed to maximize itself with the lack of support. The tube by itself is not enough.

[JeremyParticipant @jeremy15845]

What am I mssing here? The couplings are in line with what is shown as correct phasing.

https://www.google.co.uk/search?q=universal+joint+alignment&biw=1920&bih=945&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwiy-7e-vuHNAhXsAMAKHYHGACIQsAQINA

I will support the tube where it exits the hull. The model calls for an insert between the hull and the tube right up to its end. When I get the new prop shafts, I will test with and without props.

[Colin BishopModerator @colinbishop34627]

Long shafts can be a problem – see my thread on my Brenda model. As Malcolm says, supporting the shaft/tube at the prop end will almost certainly be a big help as long as it doesn't introduce any bending, Just loosely place the support in position and let the glue fix it.

Colin

[ashley needhamParticipant @ashleyneedham69188]

The Titanic has fully supported shafts outboard, Jeremy has not yet got this far on the build, but they will be there.

I would wait until this has happened and test again, if it still vibrates, get new shafts.

Coupling phasing…whew…! I did not realise this existed.

Ashley usual disclaimer

[Mark Jarvis 2Participant @markjarvis2]

Hi Jeremy,looking at your picture of the shafts coming through the bulkhead to the U/J's, the shaft end is not supported and seems to have a long reach from the U/J to the next bulkhead support. the shaft may be whipping around the U/J area ie flexing and moving, try adding a support behind the the U/J and around the shaft.

Hope im clear

Mark

[JeremyParticipant @jeremy15845]

Problem solved! I have replaced the Caldercraft prop shafts with Raeboesch water tight shafts, slid in laterally through slots cut into the frames and then secured with wooden fillets covering the slots. No vibration and the added benefit, hopefully, that there will be no water coming up the shafts.

The Raboesch shafts are fairly tight within the tubes and, hopefully, will benefit from running in. The instructions state categorically that lubrication is unneccessary and I have limited this to a drop of oil both ends. I am reluctant to pull the shafts out to add oil or grease because the instructions tell you to take due care to avoid damaging the G ring. As this is internal to the bearing housing, I don't see how you can take care. Best left alone?

Thanks for all the advice

Jeremy

[Neil Powell 1Participant @neilpowell1]

Posted by ashley needham on 08/07/2016 08:35:58:

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

Why is it that a double coupling is better than a single one? Could you explain. I don't disbelieve you, I just don't understand. I'm very new to this game and only got involved through wanting to make my own bait boat rather than buying something off the market(currently on v3). I also find that when I run it dry it does make a fair bit of noise.

Thanks

Neil

[Malcolm FraryParticipant @malcolmfrary95515]

Posted by Neil Powell 1 on 19/12/2019 08:12:24:

Posted by ashley needham on 08/07/2016 08:35:58:

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

Why is it that a double coupling is better than a single one? Could you explain. I don't disbelieve you, I just don't understand. I'm very new to this game and only got involved through wanting to make my own bait boat rather than buying something off the market(currently on v3). I also find that when I run it dry it does make a fair bit of noise.

Thanks

Neil

A single joint needs perfect alignment such that the center line of both the motor shaft and the prop shaft intersect at the middle of the "spider". If this is not the case, one or other of the shafts will try to move to keep it in line, as the requiired crossing point moves as the shafts rotate, giving vibration. Either that, or the center universal part will try to slide on its splines to make up the difference. Something has to give. The result is noise, vibration and loss of power as well as wear on the joint.

A double joint, correctly phased, cancels that out. Incorrectly phased, it increases the problem. Any system using "standard" universals only really works when the shafts either side are parallel, or very nearly so, if an angle has been introduced, a constant velocity type coupler is needed. For examples, look underneath cars – a rear wheel drive prop shaft is effectively a double joint with its outer ends parallel to each other. The drive shafts on FWD cars incorporate CV joints so that steering is possible. Some rather nice CV types at **LINK** , about pare 20.

[Gareth JonesParticipant @garethjones79649]

Neil,

A single 'universal joint' coupling can only accommodate angular misalignment between the input and output shafts. It cannot accommodate any lateral offset, i.e. side to side or up and down misalignment of the shafts. A pair of couplings can accommodate lateral offset as well, the amount of offset being dependent on the length of the shaft between the couplings. However with a double universal joint, the couplings act as the locating bearings for the centre section of shaft. If there is any play in these bearings the centre section of shaft is liable to vibrate.

The geometry of universal joints is complex and subtle. For example many joints are not constant velocity, i,e, if the input shaft is rotating at constant speed, the output shaft will rotate at a speed which varies with angular position as the coupling rotates. This can lead to noise, vibration and wear. Obviously these effects are more significant with systems that are transmitting high powers at high speeds.

If at all possible its best to arrange the propshaft in your boat to be as straight as possible with the absolute minimum offset and angular misalignment.

Gareth

[Neil Powell 1Participant @neilpowell1]

Thanks team, i'll get some ordered.

[ashley needhamParticipant @ashleyneedham69188]

I bought some thick walled silicone tube a while ago and have replaced loads of u/j with bits of it. It is a tight fit on the brass splines, and although not super flexible, provides an alternative. Using slightly longer-than a single coupling would provide more resilience against small misalignments.

i Would just repeat me previous 3 year old statement about double couplings working for me whatever the theory.

Ashley

[Chris FellowsParticipant @chrisfellows72943]

Yes, that's what they are for anyway.

Chris

[Richard SimpsonParticipant @richardsimpson88330]

An interesting thread, despite its age, and one that has a lot of valuable potential for builders.

As has been mentioned a double UV joint should not cause vibration problems in itself and is the only mechanical way to ensure correct transmission if the two shafts are parallel but offset.

What noticeably doesn't seem to have come up in the discussion are two significant points.

First vibration is caused by an out of balance force, not necessarily misalignment. This could be anything from a poorly manufactured UV joint to a badly machined or cast propeller. These should have been changed out individually to see if the vibration disappears, thereby eliminating the source. As an example I have seen UV joint grub screws replaced by excessively long hex headed stainless steel screws in the past. Realignment should not have been necessary. Being basic if you have to perfectly align a UV joint, what is the point in having it?

Secondly I am surprised that vibration is being caused in a transmission system that is powering a model of the Titanic. At the revolutions the shafts should be rotating I would not have expected much vibration to have been experienced so I suspect that the shafts were rotating too fast. Bearing in mind scale speeds and performance, even with a little poetic licence to get out of trouble, shaft speeds should not generally create vibration unless the out of balance weight is considerable. Speed will amplify the vibration, it does not cause it. The cause is back to the first point, find the out of balance force by a process of elimination and remove it.

[Neil Powell 1Participant @neilpowell1]

Finally finished my bait boat and tested it in my local reservoir yesterday. I can confirm 2 pieces of advice that i picked up on here.

1. when the boat is in the water the vibrations are dampened down considerably. In fact i was really surprised how quietly it runs.

2. Using a 2 blade s type prop reduces vibration. I tested this on the bench and a 55mm 2 blade prop was so much quieter that a 40mm 3 blade.

thanks again for all the advice

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