Prop shaft run-out

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seaesta

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I am replacing my prop and engine. The existing propshaft seems to be in good order (solid 316 stainless). I have put the prop shaft in a lathe and "clocked" the run-out with a dial guage. Initially this was 1 mm. I have applied some gentle pressure and the run-out is down to 0.2mm (about 8 thou in imperial). Can anyone advise whether this is likely to give problems with noise resonance etc?. The shaft is about 4 feet 6 inch long with about two feet unsupported at the flex coupling point. Max r.p.m of the shaft will be 1800 r.p.m.
Cheers
Martin

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Replace it for certain !!!!

A cheap scape answer is to re cut the taper, but the proper answer is to replace.


<hr width=100% size=1>David
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I am inclined to say that if you are going to the expense of replacing your engine and prop the additional cost of new shaft and bearings would be a good investment.
However, 0.2 mm TIR on the shaft (assumed that is between centers) does not render it unservicable in itself, it depends where the "bend" is . If you have access to a big enough lathe (which you obviously do) the correct check would be hold the shaft true in the chuck at the engine end and put a 3 point steady on the shaft at the point where the rear bearing runs and clock the shaft at the taper. If it is within .01 mm it's only just acceptable for the RPM you are talking about. Ideally it should be less. If the "bend" is aft of the bearing its difficult to correct (but not impossible). If it is between the chuck end and the steady try correcting it by pressure and re-check the taper. Then check at the point where your inner water seal is. Assuming this is flexibly mounted this is not too big a problem but should still run reasonably true to avoid vibration. If these two points are OK run out elsewhere (i.e between the two points mentioned) of 0.2mm should not cause out of balance vibration at that speed.
If you can't get it running true this way either re-machine the taper or give up and make a new one. If you have a lathe you can do it yourself!


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Basically I agree with you, but re cutting the taper in itself often makes matters worse if the shaft still runs out at the point of the bearing. If the shaft is true at the engine end and the bearing point it is quite acceptable to re machine the taper but doing so between centres will not make the prop run true if the shaft runs out at the bearing point. A common oversight!

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IMO the best option is to renew the shaft.
I also have misgivings about 2' of unsupported shaft between the inner seal and the flex coupling. I would weigh up the possibility of an extra bearing if it is reasonably practicable.

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