Lapping prop to shaft

[zoidberg]

I came across this 2-part video...

...which details the process of lapping-in a prop. I haven't encountered this before and, as I have a prop to refit shortly, I'm wondering if this is another task I didn't anticipate. It's an old prop going back onto the same shaft as before. Is this really necessary?

I won't be able to remove the shaft to a workshop. Is there a reasonable way to achieve a reasonable result 'by hand'....?

C'mon, engineers. What would you do?

 

[Tranona]

If it fitted before, it will fit again! Just clean the taper and key and makes sure there are no burrs then slide it on and tighten the nut. It won't come off.

 

[penberth3]

If it fitted before, it will fit again! Just clean the taper and key and makes sure there are no burrs then slide it on and tighten the nut. It won't come off.

Yes. But if you're really worried wouldn't a squirt of "bearing-fit" do the same job for a lot less effort? Don't ask me for the Loctite number, you can google it.

 

[Plum]

I came across this 2-part video which details the process of lapping-in a prop. I haven't encountered this before and, as I have a prop to refit shortly, I'm wondering if this is another task I didn't anticipate. It's an old prop going back onto the same shaft as before. Is this really necessary?

I won't be able to remove the shaft to a workshop. Is there a reasonable way to achieve a reasonable result 'by hand'....?

C'mon, engineers. What would you do?

Not necessary if the two tapers have been machined correctly which, as they worked together before, will be good. Certainly do not use any sealing/bonding compounds on the taper, Just make sure the key is fitted correctly so it drives the prop but does not restrict the prop from engaging on the taper.

Www.solocoastalsailing.co.uk

 

[Graham_Wright]

Not necessary if the two tapers have been machined correctly which, as they worked together before, will be good. Certainly do not use any sealing/bonding compounds on the taper, Just make sure the key is fitted correctly so it drives the prop but does not restrict the prop from engaging on the taper.

Www.solocoastalsailing.co.uk

Why a key?

 

[Stemar]

Why a key?

It's what turns the prop. Essentially, it's a bit of metal that fits in a slot in the shaft and engages with a slot in the prop. Without it, you risk the shaft spinning away and the prop slipping, so not putting the power - bad for the prop & shaft and bad for you on a lee shore.

 

[solent clown]

Why a key?

key joints are one of the oldest pieces of technology in engineering. Propshafts typically have a parallel key rather than a woodruff. Ask two engineers and you will get two different nuanced answers on the function. The key ties the two parts together to prevent any movement that could lead to the joint failing. The lapping mentioned above is to ensure the faces are a fit and able to transmit drive rather than shake, and mill each other to bits. A bad fit or a loose prop can lead to the key shearing.

 

[Graham_Wright]

It's what turns the prop. Essentially, it's a bit of metal that fits in a slot in the shaft and engages with a slot in the prop. Without it, you risk the shaft spinning away and the prop slipping, so not putting the power - bad for the prop & shaft and bad for you on a lee shore.

That's not "why", that's "what".

If the tapers are precisely matched, it is unnecessary.

 

[solent clown]

That's not "why", that's "what".

If the tapers are precisely matched, it is unnecessary.

the key really comes in to play at sudden changes of state, overrun, reversal of direction etc where the friction of the joint may be overcome by the flywheel effect of the prop

 

[langstonelayabout]

It's what turns the prop. Essentially, it's a bit of metal that fits in a slot in the shaft and engages with a slot in the prop. Without it, you risk the shaft spinning away and the prop slipping, so not putting the power - bad for the prop & shaft and bad for you on a lee shore.

Rather amusingly the prop fell off a nearby charter boat as they tried to reverse into their last but one night's mooring.

The skipper was looking (diving) for it for hours and I'm sure never found it.

Certainly the next day was a flat calm and we saw them being towed towards their next port. Never underestimate the value of Loctite or a correctly fitting woodruffe key...

 

[RichardS]

key joints are one of the oldest pieces of technology in engineering. Propshafts typically have a parallel key rather than a woodruff. Ask two engineers and you will get two different nuanced answers on the function. The key ties the two parts together to prevent any movement that could lead to the joint failing. The lapping mentioned above is to ensure the faces are a fit and able to transmit drive rather than shake, and mill each other to bits. A bad fit or a loose prop can lead to the key shearing.

However see http://www.ybw.com/forums/showthread.php?498173-Prop-nuts post #10 for a alternative view.

Richard

 

[zoidberg]

Big 'Ifs'......

Yes, the prop was pulled from it's existing propshaft. The old boat's new to me, so I've no idea how well or otherwise it was fitted, or whether there was any vibration in operation.
I am told by the very helpful T Norris Marine that the prop, at 12 x 8, is slightly undersize for the new Beta 14 engine - but that it is an non-significant issue not worth shelling out more than £200 to change. 'Try it and see' was the advice.

So I'll try the pen-marker technique shown in the above video, and photograph the results. Should significant 'high spotting' be indicated, it's been suggested I take a fine-grade belt from a belt sander, invert it, coat the carbide with thin oil, and apply some 'hand lapping' with fingertip pressure to the high spots. I'm a bit wary of that idea.....

 

[RichardS]

So I'll try the pen-marker technique shown in the above video, and photograph the results. Should significant 'high spotting' be indicated, it's been suggested I take a fine-grade belt from a belt sander, invert it, coat the carbide with thin oil, and apply some 'hand lapping' with fingertip pressure to the high spots. I'm a bit wary of that idea.....

I've not watched the video ..... but can't you lap it in for a few minutes with fine valve grinding paste. If it's anything like engine valves after a few minutes you will see from the colour whether there is a problem or not and can carry on lapping until it looks right.

I lapped these valves in at the weekend and then poured in white spirit:

Richard

 

[Tranona]

You are worrying unnecessarily. How many years was the prop in place?, and what have you done to it that would lead you to think that it will not now fit securely? Are there any signs of fretting or damage to the key or keyway in the prop? did it just fall off when you undid it or did you have to pull it?

Plenty of other useful things to do to fill your time. Just make sure both faces are clean and put it all back together again.

BTW your prop is on the small side if you have the standard 2:1 reduction and you may well find that revs are on the high side and you might not be able to achieve hull speed. You should be able to get the prop repitched for less than a new prop.

 

[earlybird]

Why a key?

The normal prop-shaft tapers of ~1:10 are rather steep for good drive characteristics, more of a location taper. Industrial drive tapers such as the Morse series are nearer 1:20. Also, a taper is not entirely reliable when the axial load is reversed when going astern, the nut has to be really tight.

 

[RichardS]

The normal prop-shaft tapers of ~1:10 are rather steep for good drive characteristics, more of a location taper. Industrial drive tapers such as the Morse series are nearer 1:20. Also, a taper is not entirely reliable when the axial load is reversed when going astern, the nut has to be really tight.

Indeed. The camshaft sprockets on my V8 have no key and the taper alone is transmitting 300BHP through 16 valve springs which are so hard that the engine cannot be turned by hand even with no spark plugs in it. That kind of taper is usually called an interference fit and those sprockets nuts are tight beyond normality. :ambivalence:

Richard

 

[scottie]

Try brasso liquid and gently lap it in

 

[Graham_Wright]

The normal prop-shaft tapers of ~1:10 are rather steep for good drive characteristics, more of a location taper. Industrial drive tapers such as the Morse series are nearer 1:20. Also, a taper is not entirely reliable when the axial load is reversed when going astern, the nut has to be really tight.

Time (and revs) will tell. The taper is 1:10 and when I slid the prop on gently by hand it took a fair few revs forward and astern (out of the water) before it could be removed.

 

[reeac]

Indeed. The camshaft sprockets on my V8 have no key and the taper alone is transmitting 300BHP through 16 valve springs which are so hard that the engine cannot be turned by hand even with no spark plugs in it. That kind of taper is usually called an interference fit and those sprockets nuts are tight beyond normality. :ambivalence:

Richard

Those must be mighty big camshafts if it takes 300 bhp to drive them ....what output is the engine?

 

[<152587>]

What exactly are we building here, Hubble telescope? Super precision machine tool? Oh, just a boat. Do you imagine every yacht builder fitting props to this accuracy you are demanding? To get the job really accurate build yourself a Class A clean room, temperature controlled, laminar flow, then you could control the fit to a few micron.