Prop shaft nuts

[Daydream believer]

Milling machines & lathes have tapers to hold various chucks. Many have drawbars to pull them up
In my mill I have to be super carefull not to overtighten the drawbar otherwise I have a job to free the taper. None have a keyway.
The morse tapers on my lathe just require the chuck to be pushed home by hand & they grip immediately

 

[Moodysailor]

If the two tapers match properly a light tap into position using a block of wood will stop it revolving on the shaft.

If it does not, dont fit the prop until the mis-match has been rectified.

On a previous boat the tab washer located on the prop boss, between two of the three blades, and was subsequently locked against the flat of the nut.

The keyway is also used to provide drive, the taper on a prop shaft exists to increase the surface area and assist with friction. The tapers should be lapped in, but actually don't want to be too perfect otherwise they will bind permanently. This is undesirable in this application, hence a keyway is also used. It's actually quite common practice in mechanical engineering to have slightly mismatched taper angles to ensure removal is possible.

The tab washer setup you describe is also acceptable, the way it works is the same.

 

[rotrax]

The keyway is also used to provide drive, the taper on a prop shaft exists to increase the surface area and assist with friction. The tapers should be lapped in, but actually don't want to be too perfect otherwise they will bind permanently. This is undesirable in this application, hence a keyway is also used. It's actually quite common practice in mechanical engineering to have slightly mismatched taper angles to ensure removal is possible.

The tab washer setup you describe is also acceptable, the way it works is the same.

Not so.

The little square bit of steel bar - 1/4 X 1/4 - would handle but a fraction of the 110HP our Yanmar is rated for before shearing off..

Tapers ARE designed to become a very tight fit, so a perfect fit is desirable. The steering ball joints in cars, for example - dont want those coming loose, and, as noted by Daydream Believer, morse tapers on lathes, mills and pillar drills. A key and keyway makes it a trickier job to fit a prop correctly.

Easy to break a tight taper if you know the way.

Can be a black art, but sound reasons why the taper breaks.

Usually distortion, hitting both sides of a steering arm together with the ball joint taper inside and the nut loose will do it.

My prop comes off with pressure from a three legged puller plus heating a small area on one side with a MAPP gas torch. Applied pressure plus heat distortion does the rest. The puller - the largest I have - wont do it without the heat.

I leave the nut loose otherwise it would fly off!

I have almost 60 years experience with tapered shafts and driven items. All I offer is from my ONC and direct experience.

Keeping a Triumph motorcycle clutch on the gearbox shaft after getting 64 BHP from First Mates Methanol burning racing engine taught me a lot - not only did the power double, the torque did too!

 

[Moodysailor]

Not so.

The little square bit of steel bar - 1/4 X 1/4 - would handle but a fraction of the 110HP our Yanmar is rated for before shearing off..

Tapers ARE designed to become a very tight fit, so a perfect fit is desirable. The steering ball joints in cars, for example - dont want those coming loose, and, as noted by Daydream Believer, morse tapers on lathes, mills and pillar drills. A key and keyway makes it a trickier job to fit a prop correctly.

Easy to break a tight taper if you know the way.

Can be a black art, but sound reasons why the taper breaks.

Usually distortion, hitting both sides of a steering arm with the ball joint taper inside and the nut loose will do it.

My prop comes off with pressure from a three legged puller plus heating a small area on one side with a MAPP gas torch. Applied pressure plus heat distortion does the rest. The puller - the largest I have - wont do it without the heat.

I leave the nut loose otherwise it would fly off!

I have almost 60 years experience with tapered shafts and driven items. All I offer is from my ONC and direct experience.

Keeping a Triumph motorcycle clutch on the gearbox shaft after getting 64 BHP from First Mates Methanol burning racing engine taught me a lot - not only did the power double, the torque did too!

The example you cite works in a completely different way to the rotation of a prop shaft, shear is not the same as rotation as I am sure you well know.

A tight taper is desirable. However a perfect taper is not. The difference between a tight taper and a perfect one is that one can be broken loose, the other you would wreck the prop or shaft trying to remove it. A loose taper is undesirable in any circumstance.

As I said in an earlier post, a fair bit of misapplied knowledge here - for example, comparing a morse taper with a prop shaft. The materials and finish of each are completely different, and one is designed as a standard which specifies materials and finishes. The other has different materials, different finishes, and different standards of taper.

The keyways on prop shafts are not there to provide alignment, it is there to assist with drive. It does not have the sole purpose of transmitting drive and that is not what I suggested. Tapers were originally added to prop shafts to increase the surface area in order to increase stiction. Keyways are fitted to provide extra security so that the prop will still drive in the case that the taper does not hold.

I do think your understanding of the material is a bit off though, as the 1/4" key way is strong enough in shear. It is fretting that often destroys keyways, this happens when things come loose - if all is fitted and tight as designed then there is no problem.

I don't think you would find any prop shop or manufacturer who will advise to install a propeller without a keyway, even if they are supplying the shaft & prop, so my advice is also supported by the industry. It is more costly and time consuming to machine a keyway, so if it could be done without then this would have been already. But the different materials in use and the application this is used in requires a level of mechanical security beyond just a taper.

As the marketing for one company I worked for stated - "you can't just get out and walk home at sea"

 

[rotrax]

Having dealt with hundreds of sheared woodruff keys over the years it is very clear the rotation of the driven item shears the key at the point it is above the machined slot in the shaft. Often, on a motorcycle, by a sudden stop of the engine - perhaps locking the rear wheel with the clutch engaged. The driven item, normally a rim heavy flywheel magneto, keeps going, leaving half the key in the flywheel, the other half in the drive shaft.

Call it what you will, but it is clearly sheared!

Another poster stated his shaft and prop is not keyed.

Island Packet use a different taper, longer and slower than many European boats.

Very hard, by comparison, to get the prop off.

 

[Moodysailor]

Having dealt with hundreds of sheared woodruff keys over the years it is very clear the rotation of the driven item shears the key at the point it is above the machined slot in the shaft. Often, on a motorcycle, by a sudden stop of the engine - perhaps locking the rear wheel with the clutch engaged. The driven item, normally a rim heavy flywheel magneto, keeps going, leaving half the key in the flywheel, the other half in the drive shaft.

That is a completely different application and setup. The keyway on a propeller shaft is significantly larger in relation to the rotational mass. This is why it is important to focus on the specific use -case and not get caught in the weeds discussing general engineering.

The island packet example is one where they have evidently designed around a larger surface area - this makes sense from an engineering perspective.

I can't add much more to this thread at this point, it's an entertaining discussion but we are detracting from the OP's question around what prop nut to use.

 

[DownWest]

Milling machines & lathes have tapers to hold various chucks. Many have drawbars to pull them up
In my mill I have to be super carefull not to overtighten the drawbar otherwise I have a job to free the taper. None have a keyway.
The morse tapers on my lathe just require the chuck to be pushed home by hand & they grip immediately

Slight difference. Morse tapers are designed to transmit the torque, but be released easily. Prop shafts just need to drive, hence getting off can be 'slightly' more difficult. I always understood that the taper did the drive, the keyway was just the locater, usually because of angular considerations, like timing.
Of the last three shaft connecter clamps, off the gearbox, only one had a keyway. Of course, these are not tapers, but just rely on friction. Back in physics days, pressure on the joint was more important than material.
The idea of heating the prop a bit , before tightening it up, makes me wonder how big a hydraulic puller one has to hand to get it off next time...

Still, in a recent thread, I related how my Renault timing belt toothed crank pulley has no keyway, just relies on friction . A slightly critical joint? No taper involved either.

 

[oldmanofthehills]

Slight difference. Morse tapers are designed to transmit the torque, but be released easily. Prop shafts just need to drive, hence getting off can be 'slightly' more difficult. I always understood that the taper did the drive, the keyway was just the locater, usually because of angular considerations, like timing.
Of the last three shaft connecter clamps, off the gearbox, only one had a keyway. Of course, these are not tapers, but just rely on friction. Back in physics days, pressure on the joint was more important than material.
The idea of heating the prop a bit , before tightening it up, makes me wonder how big a hydraulic puller one has to hand to get it off next time...

Still, in a recent thread, I related how my Renault timing belt toothed crank pulley has no keyway, just relies on friction . A slightly critical joint? No taper involved either.

Heating the prop to any degree before fitting would probably render it immovable forever. It is how they fit rims on railway wheels and the outer casing of canon. Neither need any other fixing after that. The compression from outer casing of canon is enough to modify the underlying structure. Differently heating of component once fitted is almost impossible

 

[DownWest]

Heating the prop to any degree before fitting would probably render it immovable forever. It is how they fit rims on railway wheels and the outer casing of canon. Neither need any other fixing after that. The compression from outer casing of canon is enough to modify the underlying structure. Differently heating of component once fitted is almost impossible

One could argue that a bronze prop can be removed by heating, as it expands at a higher rate than the SS shaft. Not sure I would want to try it, if it was fitted hot.

 

[oldmanofthehills]

One could argue that a bronze prop can be removed by heating, as it expands at a higher rate than the SS shaft. Not sure I would want to try it, if it was fitted hot.

I have seen it done on lesser items where localised heating was possible but heating up large prop to any degree in situ is a bit of a risk to bearing, hull etc. Maybe if prop only had minor heating before fitting

 

[DownWest]

A while back, replaced the undercarriage leg on a Piper Comanche. Put the chromed steel tubular leg in the freezer and the ally wheel yoke in the oven. You only get one go..

 

[rotrax]

I have seen it done on lesser items where localised heating was possible but heating up large prop to any degree in situ is a bit of a risk to bearing, hull etc. Maybe if prop only had minor heating before fitting

Never any need to fit a prop to a shaft by heating the prop if both the external and internal tapers are good. You risk stretching the internal taper.

See post #23 - my large bronze prop is easily removed by applying load with a large 3 leg puller and then directing heat from a MAPP gas torch in just one spot.

The distortion by heating in just one spot plus the puller pops it off a treat.

The puller - plus a larger one borrowed from the yard - will not do it alone.

 

[Graham_Wright]

Slight difference. Morse tapers are designed to transmit the torque, but be released easily.

Unfortunately, this can be a disadvantage. On a manual machine, the operator will notice that a drill that has snatched in the hole it was drilling and has pulled out of its MT socket can rectify the situation. On CNC lathes, that can spell disaster. Some operators take to locking the drill in its holder.

 

[oldgit]

There is a little bit of misunderstanding about the role of tapers & keyways in this context on this thread - some of the information is correct in theory but misapplied, and some is a little bit misunderstood.

In answer to the OP, go with tried & tested approaches for prop nuts - a physical lock using a (new) tab washer, castle nut & cotter pin or double nut & cotter pin. I would not advise a nyloc not or thread locker for a prop shaft nut.

Have you told T Norris. ?
Stern Gear for Yachts & Motorboats

 

[Graham_Wright]

Have you told T Norris. ?
Stern Gear for Yachts & Motorboats

Brass!!!!

 

[Moodysailor]

Have you told T Norris. ?
Stern Gear for Yachts & Motorboats

I wish I'd had the idea to setup a prop shop. Especially when I worked in Florida - props & shafts were like consumable items over there.. ?‍

Unfortunately I'm just an engineer.

 

[ean_p]

There is a little bit of misunderstanding about the role of tapers & keyways in this context on this thread - some of the information is correct in theory but misapplied, and some is a little bit misunderstood.

In answer to the OP, go with tried & tested approaches for prop nuts - a physical lock using a (new) tab washer, castle nut & cotter pin or double nut & cotter pin. I would not advise a nyloc not or thread locker for a prop shaft nut.

I lost count of the number of Nyloc nuts that have come loose on stainless fixings in the past to the extent that I stopped fitting them to anything that was in any way exposed to vibration etc. It's only anecdotal but I think stainless is too smooth/ slippery for the nylon ring to 'grip' on to!