Prop Shaft Runout

[Hermit Scott]

I set up my new prop shaft on a bench with a dial indicator. I checked it at the 9 O'clock position and the 12 O'clock position.
The runout is .004" in the center of the shaft. I was suprised at how easily these shafts flex. Pushing down with my finger on the threads by the prop it is very easy to get the shaft to flex out .010" to .020" in the center 24" away from the threads. Just the weight of the prop being mounted bends the shaft .001" when measured in the center of the shaft at the 12 O'clock position.
The idea of having run out to with in .002" in the center of the shaft is impossible while mounted on a boat. And once it's running it would have to be pretty far out, measured in 1/16" instead of thousands.
Has anyone measured the run out on a shaft out or in their boat? What are the specs for runout at the coupling with everything mounted in the boat?

 

[tommays]

You need V-blocks as the thomson bearings will make it hard to figure out were its really bent

I would say you need less than .001 or you will get a vibration

We dont have a special press BUT set up shafts in the lathe with a 4 jaw chuck and a steady rest that we place were the support bearing would be (we do mixer shafts)

The hard part is how learning how much you have to bend it so it ends up true ,its the art part of it

 

[frank balcer]

Hermit.....

Hermit, .004 thousandths is not too bad. Before sucombing to the lure of big oil money, I spent 13 years in a job shop that specialized in marine running gear. For small shafts when we did run out checks we would use soft jaws to hold the coupling end of the shaft and indicate zero here. If it was a small shaft with no center we would run the steady rest at the bearing journal area. we usually shot for 2 to 3 thousandths runout. Small shafts are a pain to straighten. We used heat to straighten all shafts. Do a runout check, put the bend up, heat up and make the bend actually worse then quench with water, as high side cools faster than the rest of the shaft the metal cantracts and pushes (or pulls) the shaft back in the opposite direction. The easiest sizes to straighten were 8 inch shafts and bigger. We had a shipset of shafts for a high speed gov. boat that had 3 inch X 16 feet shafts and they required .0015 max runout. I got them straight but it took some time. Man, as I reminiss about my old job, I do miss the challenges we were given back then...... Frank

 

[sailaway78]

Shaft runout

Hermit, I think you will be fine with this. Of course I can give you a way to make it better.
You need a couple rolls of duct tape. What kind of place do you have there? I don't see a single roll of duct tape, and no baling wire anywhere. No wonder you have run out.

 

[Hermit Scott]

Bailing wire?

Do you think I am made of money, I can't afford bailing wire. I use coat hangers.
I was standing on the duct tape so I could get a better angle for the picture. Duct tape is good for more than just water proofing hull/keel joints.

 

[Maine Sail]

To answer..

To answer your question more accurately, based on industry specific guidelines, I am referencing ABYC P-06 Propeller Shafting Systems

This is from Table III - Straightness Tolerances (supports placed at ends of bar)

Permissible variations in throw in one revolution from straightness.

Up to 3' = .0025"
Over 3' = .0025"
Over 4' = .003"
Over 5' = .003"
Over 6' = .003"
Over 7' = .003"

 

[Bob S]

Hermit,

Just curious, how long is that shaft? Years ago I had to straighten bars after they where heat treated. I used V blocks and a small arbor press with an indicator. We applied pressure on the high points until we brought it into an acceptable tolerance. You move the V blocks closer to the bend then move them further apart and recheck. It takes patients but it can be done. I did do it on a granite surface plate. I'm not sure how flat the surface your working from should be. Looks like a welded plate used as a steady rest for your saw. Is that a Jet? If you have a Bridgeport you can use the table and use the quill as your arbor press.

 

[Hermit Scott]

The shaft is almost 5'. The table is an outfeed table for my jet bandsaw. It's an I-beam structure. The bearings are Igus solid bearings. I just got back from the bearing company with 2 pillow block roller bearings. I guess I do need some V-blocks to do this right. The table will not flex. The bearings are self aligning, but this shaft is so flimsy if the bearings have to align at all it will flex the shaft.
When I spin it by hand I have to be careful not to put pressure laterally at all or the shaft flexes. This is not acurate enough. I will try something else and see how it affects the outcome.

 

[Ross]

Hermit, Are you discribing shaft whip rather than straightness? Machinery's handbook should give you the allowable shaft lengths between supports.

 

[frank balcer]

Maine Sail

Three thousandths sounds really tight for a seven inch shaft. We always used the ABS standards (American Beureo of Shipping) unless we were working a goverment contract, and I can't for the life of me remember the standard for diameters that large, im thinking .004 to .007 for low speed applications. We had a crusty old ABS inspector that served our area (Gulf Coast) and boy was he a stickler for the rules. Some of the fellows called him "Last Word" as in a last word indicator that is used to get to tenths and smaller. Man those were the days!

 

[Maine Sail]

Three thousandths sounds really tight for a seven inch shaft. We always used the ABS standards (American Beureo of Shipping) unless we were working a goverment contract, and I can't for the life of me remember the standard for diameters that large, im thinking .004 to .007 for low speed applications. We had a crusty old ABS inspector that served our area (Gulf Coast) and boy was he a stickler for the rules. Some of the fellows called him "Last Word" as in a last word indicator that is used to get to tenths and smaller. Man those were the days!

Frank,

If you were referring to the ABYC standards referenced above that was not for a 7" diameter shaft but rather for7 foot long shaft. A 7' or 7 foot long shaft is suggested to be within .003"..

 

[Ross]

I have been checking the reference books that I have here and the tolerances that have been refered to seem to relate to diameters and not straightness. We always checked for straight by rolling the shaft on a known flat surface. It is not posible to find a straight piece of 3/4 inch shaft 6 feet long by supporting the ends only. It will sag under its own weight.

 

[Hermit Scott]

It does sag under it's own weight BUT it sags uniformly. So as it rolls it should be in the same position +/- it's runout.
Ross what's whip?

 

[Maine Sail]

I have been checking the reference books that I have here and the tolerances that have been refered to seem to relate to diameters and not straightness. We always checked for straight by rolling the shaft on a known flat surface. It is not posible to find a straight piece of 3/4 inch shaft 6 feet long by supporting the ends only. It will sag under its own weight.

Ross,

The only thing I did not include in the above ABYC recomendations is the following statement.

Specified diameter of shafting - over 15/16 inch (23.8 mm) to eight inch (203.2 mm) incl.
Specified lengths of 20 ft. (6.0960 m) and less.
Supports placed at ends of bars or 42" intervals.

Those measurements I posted above above are in feet not shaft diameter as that is covered in the 15/16 to 8" comment above. Of course if you have a 7/8" or 3/4" shaft I don't know how ABYC wants it measured for run out as they don't specify.

ABYC uses the SAE J755 standard as their reference for this perhaps Hermit is an SAE member and can get this standard as a reference.to shed more light...

Shaft whip as I understand it is the resulting throw or wobble in a shaft, that occurs between supports or bearings. In a perfect world/situation with a perfectly straight and balanced shaft there would be no shaft whip if the bearings and shaft are not to far apart and the shaft is of a large enough diameter. Shaft whip is more pronounced with any mis-alignments, propeller balance/alignment issues, bent or un-true shaft, failing zinc, or smaller diameter shafts etc. etc.. I think you'd be hard pressed to find a sailboat that does not experience some shaft whip to some extent. Even a perfectly aligned and dialed in boat will exibit some form of shaft whip at a certain speed. To minimize shaft whip a balanced prop that has been lap fit to the shaft, a proper alignment and a coupling that has been fit and faced are all necessary componets of the mix... It is also important that the exposed space between the prop and strut does not exceed the diameter of the shaft it's self..

 

[Ross]

As you run a shaft with no load it will run smoothly if it is well supported. When you load the shaft it should run smoothly unless the torque is pulling it out of straight. A shaft with a long over hang can start to whip or flail on the end if it is over loaded or out of balance. Sometimes the flexibility of a shaft is used for minor misalignments with proper bearing supports on each end.

 

[Ross]

Mainesail , I think we agree that no matter how hard we try perfection is always just out of reach. ;D

 

[Hermit Scott]

I am not an automotive engineer brother. I have never had a job outside my own company so I miss out on alot of things that are obvious to those even newly indoctrinated in a company that already has engineers working at it.
I think I understand the whip concept; the little bit of mass that is outside of center of radial balance is multiplied times the speed and is a force that is outward and perpendicular to the shaft. So when the speed increases the whip increases?
I am just turning this by hand as slowly as possible.
I need to take this prop and have it checked for balance. This is one of those things where I have no trust in anyone that does this. They can tell me it needs some magic powder sprinkled on it and how would I know the difference.
Is there a dynamic balance for props or do they just use some kind of static thing like an air bubble in a level?
All this seems kind of futile with that single cylinder clatter box driving the 'perfect' shaft and 'perfectly' balanced prop.

 

[Ross]

You could set it up on knife edges with the prop installed and get static balance. You could take it to a drive shaft shop and they could run a dynamic balance for you. In the end you have an engine on rubber mounts, your alignment can be perfect while the boat sits motionless, but when you put the transmission in gear and open the throttle everything changes. I have learned how to make things nearly perfect from one master and have learned just how bad things can be and still work well from another master. Do the best you can and it will be better than almost every installation out there.

 

[BobM]

"All this seems kind of futile with that single cylinder clatter box driving the 'perfect' shaft and 'perfectly' balanced prop."

You hit that nail right on the head. I had all my parts replaced or trued up by a marine machine shop assembled everything, lined everything up and ultimately concluded that the relationship between the engine beds and the shaft log is woefully out of alignment. So hopefully you checked your engine mounts when you pulled the engine.

The problem I had could have been poor alignment of the shaft log straight from the manufacturer, or due to degradation of the beds of epoxy putty S2 used under the 4 x 4's they laid down to avoid having to do anything else to deal with the curved surface they were attaching to. I went absolutely nuts for weeks trying to figure out how to align everything correctly, thought I got pretty close, then had a whistling sound which was probably shaft related. I paid a "mechanic" for an evaluation for the opinion "I don't know."

Then the noise, which only occurred at low rpm, disappeared after my 3 day cruise home. She was hauled and the orientation through the newly replaced cutless bearing...now doesn't look much different than through the old one I was told I had to replace. Due to the flexibility of hulls on the hard I think I will judge cutless bearing wear, at least on my boat, on movement and not orientation through the bearing in the future.

 

[Tom S]

Hermit - is your shaft bronze

I assume it is. Bronze is MUCH more flexible than Stainless Steel.


Also what was alluded to above is that you are measuring runout with the weight of the flexible shaft only supported on the ends when you measure it. You C30 will have supports with open unsupported sections much shorter. I would try your measurements on the bench with the same distance as what you have on the C30 and see what you measure. I bet it would be 1/2 what you observed