Shaft Alignment Question

[Roniszoro]

Hi All;

Can one tell if a drive shaft is aligned correctly without disconnecting the coupling to check? I believe that the answer is "no", but I'd like confirmation.

Thank you in advance for your input.

Ron

[motion30]

No it must be disconnected

[Reefmagnet]

Unless I'm thinking of something different, you should only need to loosen the coupling. A method of checking misalignment is to poke a feeler gauge between the coupling and gearbox flange at various points, with variations in the gap indicating the direction of misalignment.

[Sailmonkey]

Like reefmagnet says, loosen the bolts just enough to get feeler blades in there.

But you can learn a lot by rolling the shaft by hand and observing the engine movement.

[nhschneider]

I find the concept of aligning the prop shaft coupling to .004” to be ridiculous. Sure, it needs to be a close alignment but regardless of how close it’s adjusted, as soon as the transmission is shifted into gear, the torque from the prop is going to ‘dis-align’ it instantly. How much does your engine flex on its mounts when you’re running the engine? It’s very hard to measure but I can guarantee you that it’s a lot more than .004”. Just think about all of the production boats where the engine/transmission/coupling/shaft/prop are installed before the deck is. Do you think they go through the process of re-aligning the engine after the boat is splashed? They should but they don’t. Regardless, do those boats have a high incidence of transmission or cutlass bearing failure? I don’t think so.

Of course, if you think you have a problem, it should be checked. When I replaced the cutlass bearing last year (it had over 4,000 hours on it), I also replaced the motor mounts and spent far too long getting the engine-coupling alignment to within .004”, knowing all along that it was probably a waste of time.

Fair winds and calm seas.

[Q Xopa]

Originally Posted by nhschneider

I find the concept of aligning the prop shaft coupling to .004” to be ridiculous. Sure, it needs to be a close alignment but regardless of how close it’s adjusted, as soon as the transmission is shifted into gear, the torque from the prop is going to ‘dis-align’ it instantly. How much does your engine flex on its mounts when you’re running the engine? It’s very hard to measure but I can guarantee you that it’s a lot more than .004”. Just think about all of the production boats where the engine/transmission/coupling/shaft/prop are installed before the deck is. Do you think they go through the process of re-aligning the engine after the boat is splashed? They should but they don’t. Regardless, do those boats have a high incidence of transmission or cutlass bearing failure? I don’t think so.

Of course, if you think you have a problem, it should be checked. When I replaced the cutlass bearing last year (it had over 4,000 hours on it), I also replaced the motor mounts and spent far too long getting the engine-coupling alignment to within .004”, knowing all along that it was probably a waste of time.

Fair winds and calm seas.

Agreed that alignment changes when the engine moves on its flexible mounts.
Also agree that although its going to change when running at full power starting at a reasonably close point to give it a good shot.
It depends on who you have doing it and the equipment they have and the understanding they have.
The usual method using feeler gauges has some other limitations.
As asked and replied above this method requires atleast partial disassembly.
Its not 'live' as in you have to measure, then adust.
Feeler gauges cant tell if a gap is shaft misalignment or coupling misalignment or what combination of either.
Also feeler gauges only tell the angle of the two shafts, but not the axis of the shafts are coaxial.
My preferred method is using a double shaft clamp bracket and dial test indicators set up. Obviously this is more expensive than a set of feeler gauges.

[Empty Pockets]

Originally Posted by Q Xopa

Also feeler gauges only tell the angle of the two shafts, but not the axis of the shafts are coaxial.

^^^ This. The faces can be parallel but the shafts wildly out of line. My flanges have a male/female centering flange. A high center in the output flange fits in a recessed socket in the shaft flange. Only when you have the flanges aligned axially so those two slide together do you start measuring and adjusting the faces to parallel. And you re-check the centers after adjusting the faces.

[Cheechako]

Originally Posted by Reefmagnet

Unless I'm thinking of something different, you should only need to loosen the coupling. A method of checking misalignment is to poke a feeler gauge between the coupling and gearbox flange at various points, with variations in the gap indicating the direction of misalignment.

You need to draw it apart more on many of them. There is a machined recess/step that centers the two coupling halves. You not only need to know if the alignment is out as you mention, but also if out axially. Once apart you may find the two halves are far out of being concentric to each other.
That's the hard part about alignment; you get the angle right and the concentricity is off, you get it concentric and the angle's off!

[Q Xopa]

Originally Posted by Cheechako

You need to draw it apart more on many of them. There is a machined recess/step that centers the two coupling halves. You not only need to know if the alignment is out as you mention, but also if out axially. Once apart you may find the two halves are far out of being concentric to each other.
That's the hard part about alignment; you get the angle right and the concentricity is off, you get it concentric and the angle's off!

Correct, maybe we did not use exactly the same words but we said the same thing, re angular and concentric/ coaxial.

But as I mentioned because you are measuring the coupling faces how can you be sure the shafts, which is what we are trying to align, are aligned. The couplings maybe misaligned on the shafts.

But that said as also mentioned Im confident it is more than close enough because it more than likely changes when the motor is outputting torque, the boat is healing and the engine is moving on its mounts.

But the feeler gauge method is how it is usually done and recommended by most builders.

This might be a good time to mention most boats engines are in different positions ie higher on the hard than in the water.

[markbarendt]

I spent many years doing alignments on industrial engines and yes, it can be done without disassembly but it requires special tools, specifically dial indicators and or other measuring tools. For the small engines and couplings typical on most recreational sail boats it's not a practical choice. Disassembly is easier and you can get close enough using feeler gauges and such.

That is not to say one should be sloppy just because everything is going to twist out when under load.

The reason for a tight/small tolerance, say .004 while static in this case, is not that the coupling can't handle more, it's that when the engine is under load and the mounts are flexing, the coupling and shafts will stay within the working limits.

While doing an alignment can be a bit tedious and frustrating a failure as a result of a poor alignment can be catastrophic.

[Q Xopa]

Originally Posted by markbarendt

I spent many years doing alignments on industrial engines and yes, it can be done without disassembly but it requires special tools, specifically dial indicators and or other measuring tools. For the small engines and couplings typical on most recreational sail boats it's not a practical choice. Disassembly is easier and you can get close enough using feeler gauges and such.

That is not to say one should be sloppy just because everything is going to twist out when under load.

The reason for a tight/small tolerance, say .004 while static in this case, is not that the coupling can't handle more, it's that when the engine is under load and the mounts are flexing, the coupling and shafts will stay within the working limits.

While doing an alignment can be a bit tedious and frustrating a failure as a result of a poor alignment can be catastrophic.

Agree with you on all of this.

0.004" as you say is a very good limit to start from even if it will move some when running. Obviously much better than starting from a complete unknown which 0.200" (or any other number).

In fact in my opinion as you say even asking this question you are aware of this issue and the need to do it puts you way ahead of a vast majority out there.

[Q Xopa]

Originally Posted by Q Xopa

Agree with you on all of this.

0.004" as you say is a very good limit to start from even if it will move some when running. Obviously much better than starting from a complete unknown which 0.200" (or any other number).

In fact in my opinion as you say even asking this question you are aware of this issue and the need to do it puts you way ahead of a vast majority out there.

Ooops forgot to mention that if you have the DTIs it is easier and quicker and more accurate to use them. But most arent done with this and are fine.

[markbarendt]

Originally Posted by Q Xopa

Agree with you on all of this.

0.004" as you say is a very good limit to start from even if it will move some when running. Obviously much better than starting from a complete unknown which 0.200" (or any other number).

In fact in my opinion as you say even asking this question you are aware of this issue and the need to do it puts you way ahead of a vast majority out there.

If .004 is the manufactuer’s specification, for the coupling in question, great but I only used .004 as an example because it was the number used above.

The manufacturer of the coupling or boat supplies the tolerance limits and it varies depending on type and size.

For a given type of coupler, a 4” diameter version may have a tolerance of .004 and a 6” version might have a .006 tolerance. Both numbers represent the same angular error.

A different type of coupling may have a .040 tolerance for both sizes.

Find the proper tolerance numbers through proper sources if at all possible.

[Cheechako]

If I got one within 004 or 005 I was ecstatic. Often .010 was fine for me. I knew it was a snapshot in time and a week later it wouldn't be that close anyway. But I've had the couplings drop .25 above/below each other when decoupled. So felt pretty good about correcting that!

[Empty Pockets]

Originally Posted by Cheechako

So felt pretty good about correcting that!

So did the bearings in your transmission!