What does a Shaft Key do ?

[alec]

I am in the process of putting my prop back on the shaft (bronze prop on stainless shaft).

I notice that the key needs to be replaced . It looks as though part of the key has come in contact and split .

Am I right in thinking that the taper should take the strain and not the keyway ?

If this is the case, what does the keyway actually do ?

Personally, I would much rather the prop spin if caught by a rope rather than damage the engine beds which has happened before. Unfortunately, there is no space for a rope cutter. Do I still need a key ?

Should there be a slight gap between the key and prop ?

Ant thoughts would be very welcome.

Thanks in advance.

 

[fireball]

The key just stops the prop from slipping on the shaft - judging by how tough it was to remove our prop from the tapered shaft I guess the key is only of use for a short time ....

 

[nimbusgb]

If the shaft does spin inside the taper the prop nut backs off, there is a faint 'clunk' and the engine races as is experiences the release from the prop loading. With a bit of luck you are in shallow enough water for a diver to retrieve your bronzeware!
/forums/images/graemlins/laugh.gif

 

[fireball]

ah - but the nut had a keep on it - so it wouldn't undo .... what with that and the key the bludy thing wasn't budging!
Took a hydraulic puller to get the prop off (having remove the nut!) ...

 

[earlybird]

I would definitely retain the key, which should be a good fit width-ways. In astern gear, the prop is being pulled off the taper. If the nut is not absolutely tight, drive will be lost. I've seen this happen. Take your point about ropes, but in forward gear, a well fitting taper is capable of very high torque, and the prop might well not slip under those conditions anyway.

 

[mikemonty]

The tapers purpose is to accurately locate and centre the prop on the shaft, the keys purpose is to stop it from rotating.
Its not unusual for a taper fitting (prop or coupling) to need pulling - or worse - and often the friction is sufficient to stop rotation (paritcularly if the nut has been horsed up excessively) but it shouldn't be relied upon to prevent rotation - that's what the key is for.
It's unlikely to "fire up" when it the water - but not impossible that without a key it might get fouled in a rope or similar and friction-weld to the shaft.
If you are completely stuck for a replacement key, get a set of files, a tin of engineers blue and a lump of metal and make a key.
Its the sort of thing they used to get us marine engineer apprentices to do!

 

[Poignard]

[ QUOTE ]
Its the sort of thing they used to get us marine engineer apprentices to do!

[/ QUOTE ]

... and accurately square a mild-steel round bar using a hammer and cold chisel, and for the final face the instructor would make us to hold the chisel in the right hand and the hammer in the left [or vice-versa for lefthanders]. Bruised knuckles and blood everywhere. /forums/images/graemlins/frown.gif

 

[TigaWave]

Some cutters don't actually need any space between the prop and the bearing carrier, it depends on the shape of your prop hub, deadwood blunt stern tubes can also be catered for in most cases.

The taper does hold the prop in some designs which don't have a key, but these are hydraulically fitted. With a threaded shaft to pull up the prop you need the key.

 

[billskip]

the key should be a "tight slide fit" "sideways" in the shaft and prop to prevent rotation "rocking" but there should be a clearence fit "top to bottom" to allow the taper to seat correctly.

If your prop rotates on the shaft it will most likley damage the prop (and possibly the shaft) and this will be expensive.

which ever way you go key or no key the prop has to be secure on the shaft and any forign body cought up ie rope will try to stall your engine and something will "give"

My advice is to stick with the design as intended for your boat and fit a new key.

 

[alec]

[ QUOTE ]
the key should be a "tight slide fit" "sideways" in the shaft and prop to prevent rotation "rocking" but there should be a clearence fit "top to bottom" to allow the taper to seat correctly.

If your prop rotates on the shaft it will most likley damage the prop (and possibly the shaft) and this will be expensive.

which ever way you go key or no key the prop has to be secure on the shaft and any forign body cought up ie rope will try to stall your engine and something will "give"

My advice is to stick with the design as intended for your boat and fit a new key.

[/ QUOTE ]


Could you clarify the 'clearance' bit again please - can't quite get my head round it without have the prop etc in front of me.

Does this tie in with the 'shine a torch or see some daylight ?

Many thanks for all the responses.

 

[PuffTheMagicDragon]

[ QUOTE ]

Could you clarify the 'clearance' bit again please
.

[/ QUOTE ]

Basically it means that the width of the keyway (the groove) is exactly the same as the width of the key, BUT the combined depth of the two 'grooves', the one in the prop PLUS the one on the shaft taper, need to be slightly more than the thickness of the key itself.

 

[alec]

[ QUOTE ]
[ QUOTE ]

Could you clarify the 'clearance' bit again please
.

[/ QUOTE ]

Basically it means that the width of the keyway (the groove) is exactly the same as the width of the key, BUT the combined depth of the two 'grooves', the one in the prop PLUS the one on the shaft taper, need to be slightly more than the thickness of the key itself.

[/ QUOTE ]

Brilliant !

Very nicely explained for a simple soul like myself.

Thanks,

 

[PetiteFleur]

One of my first 'tasks' as an apprentice was to file a piece of hexagon mild steel bar 6" long into a rectangular key...... and it had to be spot on!

 

[catmandoo]

unlocking a chastity belt

 

[catmandoo]

key trans mits torque from shaft to hub , gear or wahtever or vice versa

 

[Sailfishing]

[ QUOTE ]
[ QUOTE ]
Its the sort of thing they used to get us marine engineer apprentices to do!

[/ QUOTE ]

... and accurately square a mild-steel round bar using a hammer and cold chisel, and for the final face the instructor would make us to hold the chisel in the right hand and the hammer in the left [or vice-versa for lefthanders]. Bruised knuckles and blood everywhere. /forums/images/graemlins/frown.gif

[/ QUOTE ]


I was particularly fond of blueing mild plate and hand scraping high spots!