What material should shaft keys be made from?

[steve77]

Just went through a repair involving a new propshaft and propeller. My question has to do with the keys for the shaft.

I thought they were always made of a soft material, like bronze. However, I understand some people use stainless keys.

So, I have a stainless propshaft, bronze prop, and mild steel shaft coupling. What would you suggest for keys for the prop and coupling?

Thanks!

Steve

[a64pilot]

I'll go out on a limb and say I believe most are stainess, but I've seen cad plated regular steel key stock being used in my local yard, cheap stuff you buy at the local home supply store.
I'd rather not use that.


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[atoll]

I would go with 316 stainless,bronze shears too easily,and mild steel will rust out after some time

[model 10]

You could probably find bronze somewhere but don't use mild steel or plated steel or brass for that matter. You may not need a key to hold the prop on but you don't want the key to rust away in a year either. I put a plain steel key in our water pump not thinking, and it rusted out in 18 months.

[Ziggy]

Originally Posted by Guy

You could probably find bronze somewhere but don't use mild steel or plated steel or brass for that matter. You may not need a key to hold the prop on but you don't want the key to rust away in a year either. I put a plain steel key in our water pump not thinking, and it rusted out in 18 months.

A bronze key. I'd prefer to shear a key than to destroy a transmission.

[a64pilot]

316 is preferred, but seems 304 is the norm for better stuff, I don't know what average SS is but it seems to be a poorer grade than 304.
I've had a hard time finding 316 myself, seems to be just a little less rare than Monel.
In theory you could use a wood key, key is just a positioning aid, but I have a hard time with that theory, a keyway is a stress riser adding fatigue, a failure point and a weak spot in say a shaft, it it wasn't needed I have a hard time believing that a competent Engineer would spec it?
Is bronze key stock easy to find?

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[model 10]

Originally Posted by a64pilot

?
Is bronze key stock easy to find?

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Not to bad. In the SF Bay area there are several places to get bronze square key stock.
Get this, after our water pump key failure I went to the best water pump shop in SF to get a proper stainless key for our Sherwood pump. We went through every key in his box with a magnet, all of them were magnetic. I had to order one.

[atoll]

Originally Posted by Ziggy

A bronze key. I'd prefer to shear a key than to destroy a transmission.

I've lost count of the times I have had to dive over the side mid ocean and cut away rope,netting or plastic after the engine has been stopped dead,never had a problem with a transmission due to stoppage.

I would hate to have to try replace a key way on a bobbing boat every time something got in the prop

[Ziggy]

Originally Posted by atoll

I've lost count of the times I have had to dive over the side mid ocean and cut away rope,netting or plastic after the engine has been stopped dead,never had a problem with a transmission due to stoppage.

I would hate to have to try replace a key way on a bobbing boat every time something got in the prop

I too have had the engine stopped dead by lobster pot warp on more than one occasion, and my bronze key hasn't sheared, or even deformed. However, if the prop hits a heavy hard object like a log, the torque could be high enough to damage the transmission. In fact some people install a plastic coupling like Drivesaver or similar to function as a "mechanical fuse" and protect the tranny. Personally, I think these couplings fail prematurely and a bronze key is much more robust but still provides some protection. In addition, there is some risk of galling with a stainless key in a stainless shaft.

See this thread for a discussion of Drivesavers and the like: http://www.cruisersforum.com/forums/f114/drivesaver-good-or-bad-27096.html#post290591

[redsky49]

Bronze for several reasons.

First, you want to avoid any possibility of galling.

Second, and most important, you can relatively easily adjust the size of bronze key stock. File it, if is too large, peen it, if it is too small. The key should be a tight fit in the shaft key slot, an easily sliding fit on the prop. A perfect fit can be achieved with the bronze key, ensuring a tight non-loosening fit of the propeller.

Third, it may protect your transmission to some extent.

For a detailed examination of propeller shafts, go to Turn Wright Machine Shop on Youtube where Chuck Feener explains how to properly lap a propeller to the shaft, among other tips and tricks.

[redsky49]

On the above post #10, I meant to refer to Keith Fenner. My apologies.

[Wotname]

AFAIK, the functions of the key are quite different between the prop and the coupling.

The prop key doesn't assist with torque transfer, the taper does that. The key is only to assist in tightening the prop nut.
EDIT The taper must be properly laped to achieve it's desired function.

The coupling key is required to transfer torque and should be the same material as the shaft.

[tdh1849]

I have always been told that stainless is the only key to use. If you have a problem locating one, call, " Frank and Jimmy's Prop Shop, the biggest shop in Ft.Lauderdale and ask them to get you one.

[Mikado]

Originally Posted by redsky49

Bronze for several reasons.

First, you want to avoid any possibility of galling.

Second, and most important, you can relatively easily adjust the size of bronze key stock. File it, if is too large, peen it, if it is too small. The key should be a tight fit in the shaft key slot, an easily sliding fit on the prop. A perfect fit can be achieved with the bronze key, ensuring a tight non-loosening fit of the propeller.

Third, it may protect your transmission to some extent.

For a detailed examination of propeller shafts, go to Turn Wright Machine Shop on Youtube where Chuck Feener explains how to properly lap a propeller to the shaft, among other tips and tricks.

I agree 100% with Redsky49

[Cadence]

Originally Posted by Ziggy

A bronze key. I'd prefer to shear a key than to destroy a transmission.

I'll second that train of thought.