key loose in shaft coupling

[Will]

43' DC with 485 HP 6-71 TIs.
I noticed about 1/4 in. of the shaft key sticking out behind the coupling to the trans. and was able to push it back in without resistance. The center nut holding the coupling to the shaft seems to be tight. I'm told it's the coupling to shaft taper that takes the load, not the key.
What are the thoughts from the forum?
Will

 

[Avenger]

Right. It is the taper that does the job. If the key is bound up in the coupling, or the prop, then the coupling may be riding partially on the key which means that you don't have full contact with the surface of the taper. This almost inevitably leads to failure. You should be fine the way you are.

 

[Craig]

I totally agree with the above, however, a loose key IMO indicates a possible preblem. The key should be snug in the keyway not bound up, but snug enough not to come out. I believe the key way is machined at the same angle as the taper thus becomming in effect a part of the taper. A loose fey would indicate a possibe wear problem.

 

[Avenger]

As usual, since I cannot see it from here I'm assuming that the statement about the nut being tight is accurate and the coupling is correctly seated on the taper. In which case the key could be removed entirely with no ill effect. OTOH if the key has become loose because the coupling is moving and wearing the key, then that's a problem. However, if it is loose it probably would have sheared the key a long time ago.

Actually, maybe one of our experts can tell us why we even bother to key these things, since the keyway is a known and common failure point. If it's only there to prevent the coupling or prop from spinning when installed we should probably start looking for a better way to install these things.

 

[Boatsb]

As usual, since I cannot see it from here I'm assuming that the statement about the nut being tight is accurate and the coupling is correctly seated on the taper. In which case the key could be removed entirely with no ill effect. OTOH if the key has become loose because the coupling is moving and wearing the key, then that's a problem. However, if it is loose it probably would have sheared the key a long time ago.

Actually, maybe one of our experts can tell us why we even bother to key these things, since the keyway is a known and common failure point. If it's only there to prevent the coupling or prop from spinning when installed we should probably start looking for a better way to install these things.

The key is for alignment of the shaft and coupler and should be snug not loose. Some couplers have a nut. Others use a set screw and some clamp on the shaft but all are friction fit. If the key is lose try to pull the coupling and check the key way for distortion or wear. Even a slight it of rotation could distort the key or keyway and indicate a need to tighten the coupling.

 

[Colwyn D]

I agree the taper takes all the rotational forces. Certainly when I put the props on and after tightening the nuts I use a small hammer and drift to move the key back a touch, this ensures the keys have not become bound up. Assuming they were a slightly looser fit then yes I could easily move them as you have found.
Good point from Scrod. The last time I removed the props which had been lapped on eight weeks earlier I found the prop nuts had loosened to hand tight. Amazed when it required a big puller to get them off, they were locked on real tight.

 

[Craig]

As stated before the key is for alignment. The shaft taper handles the force. If the key is loose then either the keyway or key is worn, I have even seen the wrong size keys installed. Depending on how concerned you want to get with it. You could disassemble the unit, check for wear, measure the key and keyway for size. Maybe replace the key with a new one. If you don't have any vibrations you could just be sure the coupling is tight.

The taper takes care of the torque only if the keyway keeps the coupling in line.

Also the above applies to the props as well, the same taper theory is used. Lots of people change props underwater, and it works, however lapping the couping in with a compound insures correct seating and alignment, most divers I know do not lap the props.

 

[StratPlan61]

Just a side note to the last comment regarding prop installation and lapping. I'm curious to hear from the Hatteras University attendees AFTER they attend. Last year the machinist fellow who spoke about shafts/props installations had a whole new perspective regarding lapping. Hope you guys share . . . . )

 

[Avenger]

The taper takes care of the torque only if the keyway keeps the coupling in line.

Sorry, you've lost me here.

 

[Avenger]

Also another note before we head off in this direction: Keying the coupling for alignment makes sense if the coupling is lathe-turned when installed on the shaft. Then the key is needed to be sure that the coupling is re-oriented correctly when the shaft is installed in the boat. I'm not aware of any such need on props.

 

[Craig]

The key is to aid in alignment, to insure the taper in on squarely, as so I was taught. The props also have a taper and a key and way for the same purpose. Lapping is important to get the best fit possible.

It seems like the key would take some torque also, but the engineers that design these things say no.

I'm not totally sure, due to an old memory, but I think this method is called a dodge taper lock.

 

[LENZ]

The shaft key is there for drive/shear purposes. When installing a prop or coupling up a transmission you should move the prop or trans towards the rotation (pre-load). Gear reducers/Blowers use nothing but taper fits in order to set clc. My 2 cents..

 

[krush]

Did ya'll know the studs on your wheels are only loaded in tension?

 

[Avenger]

Oh, excuse us mister ungineer! I've never seen any kind of boat where the wheels were held on with studs!

 

[captddis]

Oh, excuse us mister ungineer! I've never seen any kind of boat where the wheels were held on with studs!

Yea but which nut goes first?

 

[krush]

Studs don't make boats go, hot sexy women do!

 

[saltshaker]

Yea but which nut goes first?

Never seem to remember this, but I think it is the large nut first and the second as a lock nut. Am I right (I've got a 50/50 shot at it)?

 

[saltshaker]

Studs don't make boats go, hot sexy women do!

I thought it was big bucks, which also helps with the hot sexy women

 

[Canuck Dennis]

Another dumb question, if the key was left off the prop shaft, would the prop still spin when the shaft does ??

Man. I love the warm sunny days in Ft Myers (at last)

 

[jim rosenthal]

I've heard of situations where the key was lost from the prop and propshaft and the shaft would just spin, and not turn the prop. But I haven't experienced this first hand, and everytime I've pulled the props it was a LOT of work to get them off, even with the Walters prop puller that I have. STILL a lot of work. I can believe they are a friction fit.

Regarding wheels and studs, loaded only in tension and not in shear, I can believe it. The studs and nuts just hold the wheel on. Something else turns the wheel. Like the protrusions that the wheel locks onto when the nuts are tightened down onto the studs. (or the drive pins on my Halibrands, which fit into holes on the back of the wheel center)