Stupid question about engine/shaft alignment

[skyflyer]

Can anyone with a marine engineering background explain to me why we go through the process of aligning gearbox output shaft to prop shaft within literally thousands of an inch, when (a) as the engine moves and vibrates on it's mountings the alignment will shift anyway and (b) a CV or simple universal joint would do a great job of coping with quite serious misalignment?

 

[NickRobinson]

I too feel that feeler gauges to thous' seems extreme, even with solid stuffing boxes when the boat is supposed to change shape on launch/drying out, never mind when its rough enough for the crew to be clipped on.

I've redone more than one boat, mounts/shafts/cutless/stuffing box rebedding (low power admittedly) by puttin everything back 'as was' and being reassured that I could turn over the shaft by hand with no binding.

Don't forget though when considering automotive; Hard Spicers/Rotoflex/QH CVJs etc; that not all can cope with thrust

 

[prv]

a CV or simple universal joint would do a great job of coping with quite serious misalignment?

Indeed they would, and some people fit them:

Rare in production boats because of the extra cost.

Pete

 

[sailorman]

Can anyone with a marine engineering background explain to me why we go through the process of aligning gearbox output shaft to prop shaft within literally thousands of an inch, when (a) as the engine moves and vibrates on it's mountings the alignment will shift anyway and (b) a CV or simple universal joint would do a great job of coping with quite serious misalignment?

all very well but the vessel is pushed forward via the engine mounts unless you fit a thrust bearing & bulkhd

 

[Tranona]

Can anyone with a marine engineering background explain to me why we go through the process of aligning gearbox output shaft to prop shaft within literally thousands of an inch, when (a) as the engine moves and vibrates on it's mountings the alignment will shift anyway and (b) a CV or simple universal joint would do a great job of coping with quite serious misalignment?

As sailorman says the thrust is direct from the shaft through the engine mounts, so if the coupling is solid it needs to be perfectly aligned. You can indeed fit a universal joint but these do not take thrust so you need something like the Aquadrive that prv shows to transmit the thrust direct to the hull and isolate it from the engine.

However there are several flexible couplings on the market that will allow a small amount of misalignment and still take the thrust, but often these have a length penalty over a solid coupling and in many installations there is not room without moving the engine forwards (as you also often need to do to fit an Aquadrive).

 

[mtb]

It's all about thrust loading you must fit a decent coupling
I'm just about to fit a centaflex coupling, having not long fitted a yanmar 2qm15. I fitted new vetus engine mountings and find because the engine moves about quite a bit there is no way you can set it up to run true.
I'm considering fitting a new shaft seal as well to help with the misalignment rather than the standard set up
Unless you can hard fix an engine to solid beds there will always be movement
It's more about setting up the couplings with the vessel afloat to try and elimate vibration noise and wear to the stern tube / cutless bearings

 

[Jcorstorphine]

I used a Cardan shaft with two Hardy Spicer couplings similar to the attatched link at £100

http://img.hisupplier.com/var/userI...4_Cardan_shaft_for_Ford_Motor_Ben_Volvo_s.jpg

I also fitted a simple forward and reverse thrust bearing which cost about £50 (ish) to take the prop thrust

Not had any problem with drive from a 40 HP Ford diesel with 1.5 :1 reduction.

In a previous boat, I fitted a Cardan shaft with two rubber donuts similar to the link below and that was in a boat I built about 35 years ago and they are still working OK

http://www.classicalfa.com/product_images/m/gb031_1__53394_zoom.jpg

Makes engine installation very easy but you should still try and get the engine fairly well aligned.

 

[Birdseye]

Can anyone with a marine engineering background explain to me why we go through the process of aligning gearbox output shaft to prop shaft within literally thousands of an inch, when (a) as the engine moves and vibrates on it's mountings the alignment will shift anyway and (b) a CV or simple universal joint would do a great job of coping with quite serious misalignment?

There are such arrangements but they arent necessary on a small boat. By the time you allow for a 3 ft long shaft, the tiny movements of the engine on its bearers doesnt matter. Simple to work out but a 1mm movement in your engine would alter the anle of the shaft by 0.05 degrees, something which the rubber cutlass bearing copes with easily

But engine alignment isnt about that small movement at all. Its about making sure that the engine and the shaft are effectively in a straight line. If the two are not in a straight l;ine then when they rotate its a bit like a skipping rope. And it vibrates.

The big plus for the UJ and thrust bearing typoe arrangement is that it takes the thrust of the prop direct to the hull rather than putting it through the engine mounts which in turn means you can use softer engine mounts to cut vibration and noise transmission

 

[gordmac]

UJ would do angular misalignment but not radial.

 

[vyv_cox]

There is a rule of thumb that when there are three possible flexible elements, i.e. coupling, stern gland and engine mountings, only two of them can be flexible. If all three were free to move laterally it would be impossible to prevent the shaft from colliding with the stern tube. In order to use the universal joint shown in the OP perhaps the easiest way would be to have the stern gland, which will act as a support bearing, or an intermediate bearing known as a plummer block, firmly attached to the top of the stern tube. The other alternative would be to have the engine solidly mounted but this leads to high levels of transmitted vibration and noise.

 

[rogerthebodger]

UJ would do angular misalignment but not radial.

A single UJ will only do angular misalignment but a double UJ and in the OP pic will allow radial, the amount depends on the distance between the two UJ's.

My setup is a rigid bearing just astern of the PSS seal and a deep grove ball bearing in a self aligning flange house taking the thrust direct to the hull. Then a flexible coupling to the gearbox and a flexible mounted engine and I have no hull noise due to engine vibration.

 

[rotrax]

Dont forget that when a simple Hookes Joint-UJ- is driving out of line the components change speed while this is happening in a fast-slow fast-slow manner.

Constant velocity joints were developed in the automotive world to eliminate this.

The speed increase/decrease causes premature wear and vibrations.

Triumph Hearald/ Vitesse models suffered severe "body boom" in the sixties-they had several simple UJ's in the rear suspension.

On one car the problem went away only after a sun roof-Webasto type-was fitted.

The vibes were still there but the "boom" was acceptable.

 

[charles_reed]

Most small marine engines have "soft" mounts, the force from the screw goes straight in to be taken by the engine mounts.
If there's sufficient room many fit an additional bearing and do just as you propose - most boats don't have enough room and, in any case, the problem is past history as the majority of production boats are fitted with saildrives.

 

[Avocet]

Dont forget that when a simple Hookes Joint-UJ- is driving out of line the components change speed while this is happening in a fast-slow fast-slow manner.

Constant velocity joints were developed in the automotive world to eliminate this.

The speed increase/decrease causes premature wear and vibrations.

Triumph Hearald/ Vitesse models suffered severe "body boom" in the sixties-they had several simple UJ's in the rear suspension.

On one car the problem went away only after a sun roof-Webasto type-was fitted.

The vibes were still there but the "boom" was acceptable.

You can reduce that considerably by making sure the yokes of the two universal joints are in-line. Then the slow-fast-slow bit only happens to the short shaft in the middle and the input and output shafts maintain a more constant velocity (if they are roughly parallel to each other).

The other thing is that with a single cylinder diesel, there are pretty vicious crank sped fluctuations as it fires and then has to carry itself over the exhaust, induction and compression strokes before firing again. The rubber joints (like Centaflex) are very good for absorbing that, whereas Hook joints or CV joints aren't as good.

 

[Neil_Y]

There are such arrangements but they arent necessary on a small boat. By the time you allow for a 3 ft long shaft, the tiny movements of the engine on its bearers doesnt matter. Simple to work out but a 1mm movement in your engine would alter the anle of the shaft by 0.05 degrees, something which the rubber cutlass bearing copes with easily

But engine alignment isnt about that small movement at all. Its about making sure that the engine and the shaft are effectively in a straight line. If the two are not in a straight l;ine then when they rotate its a bit like a skipping rope. And it vibrates.

The big plus for the UJ and thrust bearing typoe arrangement is that it takes the thrust of the prop direct to the hull rather than putting it through the engine mounts which in turn means you can use softer engine mounts to cut vibration and noise transmission

Some might find this hard to believe...
This video link shows very clearly the difference and problems with UJ's
https://www.youtube.com/watch?v=UEvaOg7glKk

I believe that rubber shaft bearings accentuate any problems rather than cope with movement. As the shaft will try to flex under thrust loads and will then whip, the bearings are there to hold the shaft in line and rubber being rubber doesn't prevent this movement. A hard bearing results in less noise and vibration and also less wear as it only allows movement up to its running clearance.

We have supplied cardan shaft couplings for some commercial vessels (port of London pilot boats using shaft in oil with thrust bearings) these still need to be set up accurately (coplanar) so that the joints cycle evenly which extends the life. (still had one fail) The big difference between boats and cars though is that in a boat the drive is always pushing, it is similar to driving a car driving uphill for its whole life, you'd find your car system would have a much shorter life if this was the case. The UJ as it is not constant velocity uses up energy and gets hot as it cycles and moves with quite a surprising amount of friction. The Thompson coupling gets over this problem and these have been used in boats to great effect, but they are not cheap.

In my view it is more important to get the shaft bearings lined up accurately as these reduce shaft whip and flex, the engine however moves with the flex of the shaft between the forward bearing and the coupling. Still important as any large movements or permanent offsets will rapidly wear the forward bearing again allowing shaft flex and noise/vibration. Bearing carriers out of alignment will give problems that people often associate with engine alignment.

There are also now two types of CV joint that will take thrust loads one from Italy and another from Power Train Europe, a very nice coupling we used to sell but now we're too busy with bearings.

 

[Neil_Y]

Cardan shafts with two UJ's need an offset...explained here better than I can. but this is how we had marine systems installed.

http://thealignmentblog.com/blog/2014/01/22/understanding-offset-shaft-alignment/

 

[chrishelen]

That Thompson coupling looks the biz.