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View Poll Results: shaft hub replacement questions
If you use a split hub - it has to be fit and faced first 1 100.00%
If you use a split hub - it doesn't have to be fit and faced 0 0%
If you use a split hub - you can reuse it if need down the road 1 100.00%
If you use a split hub - you still need to replace it each time 0 0%
A split hub is worth the etxra money. 1 100.00%
I split hub isn't worth the extra money. 0 0%
Multiple Choice Poll. Voters: 1. You may not vote on this poll

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Old 02-02-2013, 12:04   #1
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Replacing Shaft Hub

So I need to replace my shaft seal which means I have to remove the shaft hub which means I need to replace the shaft hub.

So if I replace it with with s Split hub, does it really need to be fit and faced?

If I use a split hub is it really reusable if I ever have to take it apart again?

And if you can not reuse a split hub down the road is there really any reason to spend the extra money for one?
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Old 02-02-2013, 12:36   #2
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Re: replacing shaft hub

The only advantage to a split hub is it will fit tighter on the shaft, to a point. The problem being is the hub in relation to the shaft diameter. Any hub that is a bit loose will need the face re-machined while it's mounted on the shaft.

A hub should not be any more then .002" loose for a 1-2" shaft. .001" would be ideal. This allows for a snug fit with antiseize between the two.

If a solid hub is a bit loose it will cock a bit when tightening the set screws. With a split hub a bit loose it clamps tight on the split end but still loose at the flange, so it still cocks when tightening the set screws, just differently.

Personally, I prefer the standard solid hub. The best thing is to make sure everything is real clean on assembly. The hub should slide on the shaft real smooth with some lubricant. If it has to be forced on you've got trouble. I've found hubs that were slightly small and had to machine out a .002".

It's best to take the shaft with when you buy the hub. I'm a Machinist so I can buy most anything anywhere, and make it fit.

The secret to removing a hub is how it's installed in the first place. You have to keep out the corrosion. Paint the hub, lube the shaft and keep it dry.

Also it's good to preinstall the hub on the shaft and run a dial indicator over the face and register. The shaft will have to be mounted in a lathe to do that.
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Old 03-02-2013, 08:30   #3
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Re: replacing shaft hub

for a group that normally has lots of opinons etc not much input on this

In other reading what I read was that no lube should be used between the hub and shaft and that the fit should be an interference fit that should require light tapping to drive the hub onto the shaft

so I have 2 oppostive positions already
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Old 03-02-2013, 09:25   #4
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Re: replacing shaft hub

Interference fits are for devices under high stress and accuracy. Boat shafts hardly qualify for that but do need a "close fit" to avoid becoming loose. And that's where the set screws come in.

Interference fits require heat and or mechanical tooling to install or extract an assembly, which is hardly doable on a coupling down in a bilge while standing on your head (in a lot of applications).

Getting a rusty steel hub off of a soft SS shaft is bad enough w/o it being pressed on. That's why you want antiseize between the two surfaces, or at least something to prevent the steel next to the shaft from rusting over time.

And that is the secret to getting them off. The set screws placed half the length of the hub at 90 and secured in place with dimples in the shaft, with safety wire keeping them from turning has been a tried and true method for many years.

Split couplings can add a little extraction ease if wedges are forced into the split, providing it is not rusted either. But they are a longer unit which requires more available shaft and if rusted, even harder to get off.

When the steel rusts, it creates little pockets of iron oxide which grow, forcing micro dimples into the SS shaft, locking them together. IAW creating it's own interference fit with rough edges. Like wrapping your hand around a rough cement shaft.

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Old 03-02-2013, 09:42   #5
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Interference fits between shafts (male) and any form of collar (female) have been standardised in terms of fit for decades. Just refer to the Machinist's handbook. You specify the fit, either sliding, transition or interference and the nominal size, either hole or shaft and the tables will give you the final machined dimensions you need to achieve.

For interference fits you normally don't lube as the fit can destroy the lube, even extreme pressure lubes which then either embrittle the material or corrode. Dissimilar merals can also be an issue. Typically you wanr the collar to be more sacrificial than the shaft if the shaft also carries seals. It also means you can replace the collar while leaving the shaft in situ. Think about how important this may be.

Often warming a collar and freezing a shaft allow an interference fit to be assembled without special tools when a press is not available.

Surface finish is important as too rough a finish will often result in a sloppy fit after assembly. Cleanliness and assembling shaft and coupling square is crucial.
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Old 03-02-2013, 10:49   #6
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Re: replacing shaft hub

Not sure why you have to destroy the flange to get it off, maybe you have tried and it is stuck? Heating helps and using a socket between the fanged while tightening with bolts will usually get it off. Some yards will have small hydraulic presses to do this as well. Heating with a map gas torch will help as well. No matter if you are using an old or new flange you should have it fit and trued to the shaft this would apply to both solid and split couplings. Split are easier to get off so if you think you may doing this again they I would go that route. You can use anti seize as well, but in the long run keeping the flange clean and free of rust will help.
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Old 03-02-2013, 11:04   #7
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Re: replacing shaft hub

Quote:
Originally Posted by Don Lucas View Post
for a group that normally has lots of opinons etc not much input on this

In other reading what I read was that no lube should be used between the hub and shaft and that the fit should be an interference fit that should require light tapping to drive the hub onto the shaft

so I have 2 oppostive positions already
In the real world, the .001 clearance mentioned is nearly an interference fit anyway....
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Old 03-02-2013, 11:32   #8
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Re: replacing shaft hub

Quote:
Originally Posted by Cheechako View Post
In the real world, the .001 clearance mentioned is nearly an interference fit anyway....
With .001" clearance the hub would be able to rotate and slide on the shaft by hand, but not wiggle at all. With .002" clearance the hub would be able to spin and slide freely on the shaft with a slight bit of wiggle.

A standard class 3 interference fit is -.001" plus -.001" for every inch of diameter of the shaft. IAW a press fit for a 1" shaft would be -.002"

e.g. coupler= .998, shaft = 1.000"

The problem with an interference fit on stainless steel is the gall factor. Once you gall the SS it's done for! Until one applies a lot more pressure, causing it to gall even more. SS is a gummy material, if you can imagine dragging two pieces of clay together, it starts to roll up a little ball in between the surfaces. And like a snow ball, it just gets bigger as it goes.
I can't count how many SS parts I've had to machine off shafting. Even after using heat to expand the SS to get it off. Shrinking parts with liquid nitrogen has been about the best method for assembling SS parts that I've experienced. The -300 F shrinks SS pretty good.
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Old 03-02-2013, 11:33   #9
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Re: replacing shaft hub

Additional question...should the key be loaded first or tapped in after? Years ago, I sold lots of anti seize for high power HVAC drives with similar flange/coupling arrangements. It may be important to the future removal of the coupling without having an open flame from a torch in the engine space or needing to rent a hydraulic puller. I think Del is correct about the fit.

Todd
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Old 03-02-2013, 11:41   #10
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Re: replacing shaft hub

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Originally Posted by IdoraKeeper View Post
Additional question...should the key be loaded first or tapped in after? Years ago, I sold lots of anti seize for high power HVAC drives with similar flange/coupling arrangements. It may be important to the future removal of the coupling without having an open flame from a torch in the engine space or needing to rent a hydraulic puller. I think Del is correct about the fit.

Todd
A key should be a 0 to 0 fit in the shaft. IAW be able to lightly tap the key into the shaft keyway. All broaches are designed to give key slots .001" to .002" clearance so a coupler can slide on over a keyed shaft, pressed on or not.

If one were to drive in the key afterwords it could broach part of the key if the KW's are not lined up. But there are tapered keys for special applications, which have a head on them for extraction.
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Old 03-02-2013, 12:43   #11
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Normally the key, which is usually a softer material will be installed in the slot in rhe shaft first then. The key will deform slightly. It is important to ensure good alignment when installing the female component.

Loctite products and freezing the shaft or heating the female component will help with galling and pickup. A good surface finish or even polishing will also help with the more problematic stainless grades.

You typically only get one shot at the install.
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Old 03-02-2013, 13:34   #12
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Re: replacing shaft hub

Quote:
Originally Posted by Don Lucas View Post
for a group that normally has lots of opinons etc not much input on this

In other reading what I read was that no lube should be used between the hub and shaft and that the fit should be an interference fit that should require light tapping to drive the hub onto the shaft

so I have 2 oppostive positions already
That is because no one really knows what the flip you are asking.
Do you have a few picture of your problem to show us?
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Old 03-02-2013, 14:31   #13
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Re: replacing shaft hub

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That is because no one really knows what the flip you are asking.
Do you have a few picture of your problem to show us?
I get it! And, actually, I think it's been pretty much answered.
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Old 03-02-2013, 14:52   #14
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Re: replacing shaft hub

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Originally Posted by Cotemar View Post
That is because no one really knows what the flip you are asking.
Do you have a few picture of your problem to show us?
thanks for the input!
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Old 17-03-2013, 15:23   #15
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Re: Replacing Shaft Hub

Well in the end I couldn't get the shaft out without dropping the rudder. And I couldn't drop the rudder without having the boat lifted higher. And I couldn't lift the boat higher till other boats in the yard got moved!

So I cleaned up the shaft and hub. The hub had no rust on it (both outside and inside) and I could tell it had been installed with grease between the two. It still took a gear puller to get it off. Once both cleaned up I could just dry slide the hub onto the shaft. So I decide to just reuse it! Once I put a thin coating of grease on the shaft when I went to install it I had to lightly tap in on.

So all the thought, worry and concern were of no use!
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