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Propeller Shaft Machining Advice?

05/29/2010 5:41 AM

I would like to repair a damaged stainless steel hollow propeller shaft. It is damaged where the cutlass bearing runs.

Can I turn the shaft down to take a sleeve and heat fit the oversize sleeve to the shaft. Then turn the "new shaft area" with oversize sleeve down to its correct size? What allowance for shrink fitting would be suitable for a 2.625" od shaft? (The sleeve needs be 8-9" long and cannot be more than 3 mm wall thickness when finished.)

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#1

Re: machining properller shaft

05/29/2010 8:22 AM

The first consideration is to be the vessels class society (if any). For instance, I am assuming this is a vessel (boat of some kind).

If so is it ever inspected by ABS, USCG, LLOYDS or any other society? All of these have different repair procedures depending upon damage severity, exact bearing journal location in relation to load and working RPM, etc. If this is the case, you will avoid problems later on by getting these people involved to document and specify exact repairs to be carried out, they will witness the repairs as well.

Your proposed repair is common for light damage to bearing journals and can be accomplished with the parameters you have mentioned. In more severe cases the shaft will be repaired by cladding via specialized welding process to avoid distorting the shaft.

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#4
In reply to #1

Re: machining properller shaft

05/29/2010 8:46 PM

Thanks Tim for a very complete answer. It is a prop shaft on a pleasure boat and it has a hollow shaft to facilitate a rod in the center to vary the prop pitch. It is somewhat over engineered so I can afford to loose a couple of mm of the dia to accommodate the sleeve/journal. Also, the journal can be finished larger than the original shaft dia.

The vessel is not under survey. My problem is how much expansion-contraction I should allow for if the sleeve was to be 3mm thick? I was thinking in the region of 0.006"

Do you think that would provide sufficient "lock" without to much stress on the sleeve?

Thanks to all for the advice, better come for a sail if your ever in southern thailand.

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#2

Re: machining properller shaft

05/29/2010 11:14 AM

Will turning the shaft OD down weaken it?

Maybe Tim's (GA) suggestion of building the surface up and machining back to size would be the better approach?

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#6
In reply to #2

Re: machining properller shaft

05/29/2010 9:05 PM

Thanks for reply, I guess it will weaken a tad but I think it will be OK. The "remetalling" option is not viable in this area. cheers

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#10
In reply to #6

Re: machining properller shaft

05/30/2010 1:29 AM

jonpadang -- This is a tricky problem involving not only some calculations with Lame's thick cylinder formulas; but also an understanding of metallurgy and consideration of the skill level and equipment available to the mechanics who do machining and fitting.

What kind of stainless steel do you have to make that sleeve out of and how long is it going to be?

Is your boat going to be used in salt or brackish water for any length of time? Improper heating of the stainless steel sleeve combined with the residual tensile stress needed to maintain its shrink fit may result in stress corrosion cracking in the presence of chlorides (salt).

Do you have some way of controlling the temperature the sleeve gets heated to? Temperature controlled oven or an infrared thermometer?

It's going to take a pretty good machinist with a good lathe to hold a decent tolerance on machining that sleeve with 3mm thick walls. Else your .006" fit (sounds like a reasonable starting point short of doing some calculations) could easily be more like .003 or .009.

You suggested some inconsistency in past results of others in your area trying similar fixes. Do you know how and why some of those attempts didn't work out well?

I've got a feeling you are in for some "cut and try" experimentation.

Milo or Andy Germany -- Either of you guys have your "ears on"?

Ed Weldon

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#15
In reply to #10

Re: machining properller shaft

05/30/2010 8:10 AM

Your surely correct there, a lot of "cut and try" I am in a pretty basic boatyard and mostly wooden fishing boat technology. I hope to start with a tube that is oversize OD and, after fitting, turn it down (on the shaft) to the required size. They are used to turning big shafts here.

The boat runs in salt (sea) water and gets sand and mud into the equation so wear is always a problem. Why it is not common practice to have replacable sleeves on the shafts escapes me as shaft wear is a very common problem. Perhaps, sleeving the ss shaft will create some electrolosis problems. Unfortunately I am not sure the grade of ss the shaft is but planning to use 304 or 316 depending on availablity for the sleeve. Apart from being water lubricated, it is common to force feed water into the stern tube at the inboard end (usually from the engine exhaust cooling water) to prevent oxygen deprivation and consequent electrolosis. Thanks for your interest and advice,

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#22
In reply to #10

Re: machining properller shaft

05/31/2010 2:17 AM

Hello Ed,

We use a mandrel for holding thin wall tubes to accurately machine the O.D.

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#3

Re: Propeller Shaft Machining Advice?

05/29/2010 8:38 PM

I don't know if anyone does it this way, but it would make sense for the shaft to be a uniform diameter throughout, with a renewable press-on journal (sleeve) at the cutlass bearing, and also at the shaft seal area.

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#5
In reply to #3

Re: Propeller Shaft Machining Advice?

05/29/2010 8:57 PM

Thanks for the reply Tornado, I am in Southern Thailand at present and some of the fishing boats here do this sort of procedure on bigger shaft but with mixed success.

It seems there is not a standard procedure used by them.

The shaft is oversize even for commercial survey so I can afford to loose a few mm.

Thanks to all for the advice, just need to decide now how much undersize the journal id needs to be before heating and placing? The shaft is 2.625" OD

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#12
In reply to #5

Re: Propeller Shaft Machining Advice?

05/30/2010 4:21 AM

just need to decide now how much undersize the journal id needs to be before heating and placing? The shaft is 2.625" OD

between 0.0005" and 0.0015" is sufficient interference for this application. Packing the shaft in ice and hang the sleeve from a wire passed through it and heat gently and uniformly with a rosebud tip, it won't take much heat to accomplish.

Is the cutlass bearing pressed fit or seated in epoxy?

Just curious, the shaft should be wearing out the bearing not vice versa?

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#13
In reply to #12

Re: Propeller Shaft Machining Advice?

05/30/2010 7:28 AM

Thanks a lot for the advice, The Cutlas will be seated in Epoxy to achieve a degree of "self alignment" Because of some corrosion in the stern tube I need to bore the tube out a little. This will also give an opportunity to correct some small miss-alignment of the tube.

The machining sizes you recommend are quite small. Do you feel sure that is enough to lock the sleeve securely?

Wearing of the shaft is common due to sand and mud that runs in the bearing faces and usually wears the cutlas first but over a few seasons the shaft gets rough and then the cutlas wears quickly. The cutlas bng is 10.5" long and same as the pic from one kind person.

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#19
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Re: Propeller Shaft Machining Advice?

05/31/2010 1:54 AM

Yes I am aware of the grit/sand issue effecting the bearing surfaces. Often I have repaired other equipment used in similar circumstances by relieving the end of the shaft and then using a replacement sleeve of either graphite impregnated nylon or Rolon with great success.

Yes the interference values are appropriate.

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#21
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Re: Propeller Shaft Machining Advice?

05/31/2010 2:13 AM

Also you may machine the O.D. of the sleeve to within 0.030"-0.040" of finished size prior to assembly.

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#7

Re: Propeller Shaft Machining Advice?

05/29/2010 10:54 PM

This is making a few assumptions but, If you can allow for machining the shaft down in size, would it not make more sense to do the machining and find a suitable bearing that would fit the outer bearing race dimension and the new targeted new "machined" O.D. of the shaft "Inner race I.D."? Bearing manufacturers are making bearings for "Fixed" shafts all the time.

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#8
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Re: Propeller Shaft Machining Advice?

05/29/2010 11:00 PM

This is a cutlass bearing.

More of a bushing than a bearing.

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#9
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Re: Propeller Shaft Machining Advice?

05/29/2010 11:58 PM

As we can see in the pictures, the grooves in the rubber lining of this bearing allow water to migrate through it, which contributes the lubricating film. Metallic ball or roller bearings would fare poorly in this environment, so this is a pretty good approach. However, water is not as good a lubricant as oil, so this design isn't bullet-proof. That's why I suggested a renewable shaft sleeve in an earlier post. The name of this bearing sometimes appears as "Cutlass" and sometimes as "Cutless", due perhaps to trademarking.

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#14
In reply to #9

Re: Propeller Shaft Machining Advice?

05/30/2010 7:44 AM

Thanks Tornado and Lynlynch, Your suggestion of replaceable sleeve is great. In the process of making this one I hope to make at least one replacement sleeve.

Have some differing opinions on the amount of undersize to make the sleeve, partly perhaps because of the thickness. I should mention that the initial OD may be larger than the finished size. After fitting it was my intention to then turn it down on the shaft to the required OD. Therefore I have to find a suitable piece of tube to start the process, hopefully an ID that will not need machining, only the shaft. The workshop here is able, and regularly does turn shafts and prop tapers.

Lots of sound advice from many people so thanks to all of you for the help. Great site and glad i found it.

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#18
In reply to #14

Re: Propeller Shaft Machining Advice?

05/30/2010 11:30 AM

If you have a facility that is familiar with refurbing shafts, I'm sure they know about flame spraying powdered metal the same as the shaft to build up the worn ares. If not, contact a welder manufacturer/supplier/representative in your area. I'm sure they can direct you to a shop that can do flame spraying.

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#11
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Re: Propeller Shaft Machining Advice?

05/30/2010 1:55 AM

What is the tolerance for wear on a bearing like this? A very little can be taken off a shaft with emery cloth? Just for cleaning the shaft up rather then turning down by 1-2 mm.

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#17
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Re: Propeller Shaft Machining Advice?

05/30/2010 9:36 AM

That is excellent advise, the OP said he could afford to loose a few mm off the shaft without compromising the power transmission. If it to be machined down for a sleeve which will not add any any great degree of torsional strength, there is no reason why it could not be machined down to 2.5" which is the next standard size for a cutlass bearing,this is a reduction of 3.25mm on the diameter. This is a lot cheaper and easier than heat shrinking a sleeve which must have an exact clearance to avoid jamming prematurely during insertion. In any case a sleeve should be an ambient press fit as it does not have to carry any power transmission its basic requirement is to act as a shaft support. It could be pressed on with a loctite seal if this approach was taken which would aid assembly and seal the contact area against chloride ingress, this would be more than tight enough to resist bearing rotational friction.

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#23
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Re: Propeller Shaft Machining Advice?

05/31/2010 7:46 AM

I think you are 100% correct on the 2.5" bearing for this go around, but what about the next time? This repair is just about as common as painting the hull and will certainly need to be done again.

Marine growth, rope or even fishing string in some cases is enough to restrict the water flow and prematurely wear the bearing and shaft. Poor alignment is also a major player in this situation.

Some vessels by there design wear these bearings more than others as well due to factors that have been built into them like the shaft angle or length as compared to the bearing length. The natural flexing of the hull plays a major part as well as the alignment.

Be sure the vessel is well supported with many blocks along the keel as well as the transom and side shell to simulate its natural load when afloat when installing the cutlass bearings. It is common practice to install these bearings with a very loose fit and setting them with resin of some kind (chak-fast orange) while centering them by means of a piano wire tightly stretched through all the bearing I.D.'s. If at all possible, always use the stuffing box (sealing area) as a centering point for this alignment

Do not securely re-connect the shaft to the gear box until the vessel is in the water resting at its normal draft in calm water. At this time you need to verify proper gear/shaft alignment to correctly complete the job.

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#20
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Re: Propeller Shaft Machining Advice?

05/31/2010 2:01 AM

Not if the assebly is similar to that of a Chevy camshaft

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#16

Re: Propeller Shaft Machining Advice?

05/30/2010 8:23 AM

Here we do it like you are indicating only we freeze the shaft with nitrogen liquid bath and slide the repair sleeve over the shaft. this allows the shrinkage to expand to the sleeve size and lock into place. Then machine the sleeve to the desired diameter. Heating the sleeve and freezing the shaft will make the fit very tight and will withstand the tendency to slip later.

Trying to make a sleeve that is thin and put it on is a mistake. Give yourself a thicker sleeve to work with and then remove the extra material to the thickness you need after it is fitted to the shaft.

You can make a coffer dam out of anything and fill the inside of the shaft with liquid nitrogen to make the freeze in the area of the repair and to the short end only. Note: you will need to do this in a room that is very dry. Once the sleeve is in place allow it to warm up to ambient temperature without disturbing the parts. Time will depend on mass. I leave them sit overnight before machining. You could speed the process up some but would not advise applying heat to the shaft.

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