A few questions about prop shaft alignment
- Thread starter Project_Mayhem
- Start date
Don’t disconnect the coupler for this part.
This centering doesn’t have nearly the same tolerance as the final alignment. An inspection mirror and/or a pair of calipers helps a lot. Chances are, if you’re just replacing the shaft, that it’s already OK and you don’t need to do anything. But no guarantees: the mounts could have vibrated loose over the years, or maybe been adjusted wrong previously.
Alternatively, find a short length of hose that has an O.D. matching your shaft log and an I.D. matching your shaft; split it lengthwise and use it to hold your shaft in the center of the log while disconnected, then move the engine until you can connect it.
Centering the shaft in the log is quite easy with a split collar (3D print) fastened around the log and shaft with a hose clamp. Happy to share..
Last edited:
It's kind of interesting seeing a thread take a life of its own.
I actually designed a bushing with flange for the log but the split shaft design would definitely save some time. How much tolerance is recommended for the OD and ID?
It seems to me that the best way to do this is to disconnect the couplers, line up the shaft with the center of the log and then adjust the engine to the shaft.
I doubt the flanges on the couplers have the same OD so how does one know if the engine is lined up laterally? When the two aren't on the same plane it's easy to measure. When they are on the same plane, it seems like the only point that can be measured is the OD of the coupler flanges.
How do you know if it's best to adjust the front or back of the engine?
I actually designed a bushing with flange for the log but the split shaft design would definitely save some time. How much tolerance is recommended for the OD and ID?
It seems to me that the best way to do this is to disconnect the couplers, line up the shaft with the center of the log and then adjust the engine to the shaft.
I doubt the flanges on the couplers have the same OD so how does one know if the engine is lined up laterally? When the two aren't on the same plane it's easy to measure. When they are on the same plane, it seems like the only point that can be measured is the OD of the coupler flanges.
How do you know if it's best to adjust the front or back of the engine?
Something like this ? Oh, I know what you mean.
You must have been looking at mine. There is about 1/8" difference in diameter between the two flanges. I check the vertical and horizontal alignment just by fingertip feel. When you feel how little force is required to move the engine 1/8" to compensate for any lateral misalignment, you see how unimportant this part of the alignment is. It's all in the angularity (if there is such a word).
Last edited:
Noted. I think this is an application where a dial gauge could be used. You could measure the differences on opposite sides and adjust until it's even all around. Since you mentioned that this is measurement isn't crucial, I could use the end of my cheap caliper. I'm sure it's more accurate than my fingers.
jssailem
SBO Weather and Forecasting Forum Jim & John
- Oct 22, 2014
- 24,454
My understanding (not a mechanical engineer) is that from the point of the propeller through the cutlass bearings and the shaft log, the shaft is centered. The shaft is engineered out of very stiff material and is machined dead-straight. The shaft coupling is machine-fitted and faced on the prop shaft so that the shaft is centered in the coupling. The cutlass bearings constrain the shaft in every direction but permit the shaft to rotate freely. The bearings act like precision bushings on a machine-tool spindle.
The transmission coupler and the shaft coupler only need to face together, and feeler gauges are used to measure the gap between the two faces as they are bolted together. The engine is moved on its mounts to adjust the angle of the transmission coupler to ensure that the two faces are aligned and the transmission is centered on the shaft.
Of course, that's far more accurate. While you're doing that, give the Xmission flange a gently push and see how little force is required to move it this small leateral error. This is one of the reasons for the rubber mounts, the other bigger one of course is vibration elimination.
I remember seeing large steam turbines being mated to equaly large alternators. Eveybody solid as a brick
house. And lateral and angular error being aligned +/_ 0.0000 or better. No forgiveness at all.It's only the angular to worry about on our little sewing machine engines. Imagine a good angular error between the flange faces and then trying to swing the engine around on its rubber mounts until it's lined up. OUCH ! lots of force required there and lots of vibration to go along with it.
If you read the document posted above, originally supplied by Ralph, you will notice that shaft alignment is not a two axis exercise.
You are not just aligning top to bottom and side to side on the parallel plane of your flanges, you are also aligning the east, west, north, south, orthogonal axis to that plane. Correct alignment has those two planes becoming orthogonal to each other and to share the same origin.
In that paper, it actually states that's what being done and says the derivation is beyond the scope of that document. It's actually really slick!
As an aside, you'd be amazed at how accurate you can feel. During the age of apprenticeships, like 16th, 17th, 18th centuries - to advance from being an apprentice blacksmith to journeyman blacksmith some guilds required the aspirant to forge a cube where each side had to be within 0.001" - measured by feel - and that was an accurate test...
dj
Can you imagine whaling away on this red hot cube until you think you're getting close and then taking your shocked and shaking hands and trying to feel it's dimensions for something close to 0.001" +/_.
I remember pulling into a little coastal town up the coast called Sointula where they were putting on a medieval festival with jousters on horseback, fetching maidens of exceedingly suitable endowment, and the ubiquitas blacksmith's shop. Everybody and anybody welcome to give it a try.
Hell, I know a little bit about metallurgy. This'll be piece of cake. Yeah, not so much. All I can say is that I didn't smash my thumb in the process. I don't know if this smithy ever made the journeyman level but he was good and fast in pounding out beautifully decorative coat hangers and the like.
We have our specialists today and they had theirs back then. Equally impressive.
Boy I can bore you to death with these kinds of trivia... So just one more - back in the second half of the 1800's when the railroads were coming on strong. New York city needed a lot of blacksmiths. You couldn't do background checks back then. No pieces of paper saying blah blah blah.... So how could they know if this person walking in applying for what at the time was a high paying job was actually qualified? Simple - they gave them one single test. Take a piece of flat stock - I think they used 11/4" or 1 1/2" by like 3/8" but I don't recall exactly - and the aspirant had to forge a ring using only the anvil and as the ring came around - the two ends had to meet. Now if you've ever tried to blacksmith - that is a brilliant test! That is really hard to achieve! But it was clearly evident if the aspirant knew blacksmithing or not in just a few minutes... Simple, fast, effective.
In today's world of blacksmithing, most are artists - not really technical blacksmiths. But hey, no need today for that kind of precision forging. Nobody would even know to appreciate it...
Right now I'm setting up a blacksmith shop here in my back yard. I'm going to start forging stainless steel fittings for my boat as the commercially available fittings are mostly crap, and if not crap, bloody expensive! Plus, given my boats age, it's almost impossible to find replacement parts that match what I've got. I'm going to get into casting bronze too - that's next after I get the forge all set up. Hahaha - as they say, going back to my roots - blacksmithing is how I started in my career...
dj
Quotes aren't working for me at the moment.
Forging a ring with just a hammer and anvil definitely sounds like a challenge!
I suspect that there will be a bit of uneven wear on the cutlass if the shaft isnt parallel with the tube/bearing. Of course it will take quite a while so I believe everyone is correct that it's not a crucial alignment. I'll likely print a split bushing to line it up close to center and just use my hands to figure out when the couplers are lined up laterally.
Forging a ring with just a hammer and anvil definitely sounds like a challenge!
I suspect that there will be a bit of uneven wear on the cutlass if the shaft isnt parallel with the tube/bearing. Of course it will take quite a while so I believe everyone is correct that it's not a crucial alignment. I'll likely print a split bushing to line it up close to center and just use my hands to figure out when the couplers are lined up laterally.
The small dial counts the number or rotations, the larger dial shows how off the shaft is. If the shaft were completely straight, there would be no needle movement with the larger dial. Of course, you probably will not get that accurate, but this will show you how much the shaft is out of alignment. You can find YouTube videos on dial gauges used in different applications. Obviously this is not something you'd hold in your hand. It has to be anchored (as shown) somehow to get an accurate reading.
Now I see the problem ........... a simple matter of semantics.
What you refer to as "off" is what we with the greasy fingernails refer to as "runout." It is only found on a single shaft.
What the OP was referring to is called "alignment" which is the comparison of straightness between two or more shafts.
