Shaft Alignment

[Danny Perez]

shaft rake

Shaft rake, the angle at which the shaft exits the hull, is the first thing to consider. The degree of rake is determined by the boat designer and must fit into the maximum and minimum angle of tilt for the engine manufacture. This shaft rake is generally set for you by the boat manufacturer through the engine bed and/or the length of the strut outside the boat. Therefore, aligning the shaft should be as simple as matching the face of the transmission coupling with that of the shaft coupling while adjusting the engine legs until the coupling faces are parallel top to bottom and left to right. All the alignment is done while maintaining the designed shaft rake - well, this is what we are taught in US Navy schools and those engineers are much smarter that I am.

 

[jibes138]

Question regarding system design

I think Ross hit on a subject that I've often wondered about. The world invented a way to rotate misaligned shafts a long time ago called a universal joint. In a sense a flexible couple provides a little give but a U-joint allows significant misalignment. Why don't they use a u-joint between the engine and the shaft? Understood space is often an issue, but if you can get rathet wrenches with u-joints putting one on a boat shaft would be easy. What am I missing in this thought process?

Secondly, someone noted the science of vibrations in this discussion of thump. Any motor and shaft system has natural frequencies. These are RPM's where the system will vibrate severely then when you pass through these RPM ranges it will settle back down. Designers will try to design products so the operating range of the system is not near the natural frequencies. There are several modes of vibration so as you increase a motor from idle slowly you will often feel certian RPM's there you hit these frequencies where the vibration is really high, then it will go away until you hit the next mode. This is a function of the design of the system and not something you will make go away. The worst thing that can happen is to have a natural frequency right in the speed range you want to be. It will eat up your system. So if you have a lot of vibration try a little more or less RPM's and see if it goes away.

 

[RichH]

Why don't they use a u-joint between the engine and the shaft? Understood space is often an issue, but if you can get rathet wrenches with u-joints putting one on a boat shaft would be easy. What am I missing in this thought process?

Because U joints dont rotate at constant velocity when at more than a few small degrees of misalignment. Constant velocity joints have a slightly larger tolerance of 'mis-alignment' but will quickly destroy themselves when run constantly at more than the 'spec'. misalignment limits.

 

[Rich Stidger]

Engine Shaft Alignment Form

This seems to be a good spot to post an alignment form that I made up this Spring when I was doing my own shaft alignment.

After you assure that the flange surface is perpendicular to the shaft and that the shaft flange is in the correct position, you then need to measure with feeler gauges and make adjustments to bring the shaft and engine flange halves parallel with each other. The accepted maximum tolerance is 0.001 for every inch in diameter of the flange. Thus for a 4" diameter flange the maximum error would be 0.004".

This form helps to organize your measurements and actions so that you can make steady progress to a final alignment. You may need multiple pages of this form if the alignment is off is a couple directions.

Use the form as follows:

Pushing the flanges together, measure the clearance at the top, bottom, port and stbd edges and record the number of thousandth's in the circles nearest the flange in the center of the form.

In space #2 write in your action to correct the clearance errors. Then enter the next set of measurements in the triangles moving outward from the center. Continue as required.

The attached file has the blank form on page one and a filled-out sample on page two.

Note that it is not really necessary to push the flanges together so that one of the measurements is zero. All you need to do is to get all of the clearances to be equal.

 

[Ed A]

Its easy to get into an argument here about the techniques and potential problems.

I would disconnect the cupling jump in the water and check for play in the cutlass. If its got play its shot. simple deal. it has to be replaced and possible to do in the water on most boats. but a pain to do.


Then Rich has the sound deal. center and block up the shaft and align the engine to in. keep

Adjust untill you have the same measurement on ALL THE WAY AROUND the cupling only then bolt up. yes check it again after about10 hours.

Yes the mounts move vibrate and shake the trick is when they do you dont want the shaft to contact the boat.
'So logic says put it in the middle.

This ususally does not create a knock or bump. but its still has to be right. Then the prop has to be clean, true and be balanced.

If you don,t go thur this correctly it will cause problems.

 

[Rick2413]

Thank you to everyone for so much input. I had a very successful day today on the boat. I know it's not exact but I moved the shaft around until it rotated easily in what felt like a center position and blocked it as suggested then aligned the engine to the coupler. The thumping is gone and all seems great. I will re-do it properly on haul out in the Fall.

 

[Ross]

Some rhetorical questions for the men who would couple shafts to engines. We all agree that it is desirable to center the shaft in the stern tube. The shaft is then supported in a fixed position as the engine is brought into position and the coupling is completed. Isn't this counter intuitive for most men? isn't the usual procedure to align the shaft to the engine while the engine attempts to align its self with the shaft? Doesn't this sometimes result in a very successful coupling but a serious misalignment problem? In this problem isn't it essential the the shaft be held in position while the engine is brought into position? There have been references to strange noises but these usually resolve themselves as the alignment problems are corrected.

 

[MKirk51]

jibes,
I know I'm resurrecting an old thread but I'm considering replacing a saildrive with a diesel and shaft drive, and I've been researching everything related to shafts, struts, stuffing boxes, etc. In reference to u-joints, it did seem odd to me also that you don't see them used. RichH mentioned the issue about destorying them by running them with a misalignment that's more than 'spec'. If I misunderstood what I read in another source, please correct me, but another problem arises with the thrust from the propeller. U-joints aren't meant to handle that kind of force. If a thrust bearing is employed to 'protect' it, it might help, but now you're adding even more complexity. It might help to think about the drive shaft in rear wheel drive autos. It only rotates. There's nothing really 'pushing' on it.
I generally only take away knowledge from you folks, so I hope I got that right and can impart some for a change.

 

[tommays]

You have Aquadrive CV units for this but it costs

 

[BobM]

....and you won't have room...

There isn't room to mount an Aquadrive CV unit in a 28 footer unless you mount the engine in the middle of the cabin LOL. You will have fun just fitting in a normal coupler and stuffing box. My 30 footer has maybe ten inches for the whole shooting match and my friend's Freedom 28 has less.

 

[MKirk51]

Bob and Tom, (hey, sounds familiar!)
I wasn't thinking of using u-joints in my conversion. You're right, there's hardly room for a regular setup, let alone somethin' fancy. I was just trying to add some more info regarding jibes' question about not seeing much in the way of u-joints and prop shafts.

I'd seen one or two references to Aquadrive but never took a good look until now. It seems to be a pretty cool unit. I see it does have the thrust bearing to take the loads away from the u-joint (or CV joint rather), or to be more accurate, to take the loads away from the engine. Again, pretty cool.

 

[BobM]

My other ride is...

Bob and Tom, (hey, sounds familiar!)
I wasn't thinking of using u-joints in my conversion. You're right, there's hardly room for a regular setup, let alone somethin' fancy. I was just trying to add some more info regarding jibes' question about not seeing much in the way of u-joints and prop shafts.

I'd seen one or two references to Aquadrive but never took a good look until now. It seems to be a pretty cool unit. I see it does have the thrust bearing to take the loads away from the u-joint (or CV joint rather), or to be more accurate, to take the loads away from the engine. Again, pretty cool.

My other ride is a 1964 Dodge Dart GT convertible...so perhaps more like Tom and Ray from Car Talk, but Tommy owned the Dart rag top and his was a '63.

http://www.cartalk.com/content/testdrives/Reviews/dodge-dart-1963.html