Physics question

Flossdog

Flossdog

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Definitely boaty this one! If you heat up a prop drive flange or even a propeller, i.e. a ring of metal with a hole in it, will the hole get larger as the metal expands outwards or smaller as the metal expands into the hole! Probably depends on the relative dimensions of the id to od but is there a general sort of answer?
 
Larger - the metal expands around its circumference, this forces it outwards, expanding the hole. Standard way to fit bearing rings is to heat them up.
 
Flossdog said:
Definitely boaty this one! If you heat up a prop drive flange or even a propeller, i.e. a ring of metal with a hole in it, will the hole get larger as the metal expands outwards or smaller as the metal expands into the hole! Probably depends on the relative dimensions of the id to od but is there a general sort of answer?
Click to expand...
Suppose it depends on the relative expansion coefficients of the metals in question? Although as you heat the prop it's likely to heat faster than the shaft inside it, so should free itself in good time I would imagine.
 
As said always larger. The gedanken experiment..... Imagine there was no hole - it was simply filled up with metal. It's obvious in that case that it'd get larger. Why should the metal around it do anything different if you make a hole.
 
A caveat; if you are heating to separate a stainless steel shaft from a mild steel drive flange, as in the OP, the relative expansions are roughly in the ratio of 3:2; ie as temperature increases, the fit gets tighter.
Moral is to heat the flange quickly, don't let heat soak into the shaft.
Brass/ bronze & SS are about equal.
 
Last edited:
earlybird said:
A caveat; if you are heating to separate a stainless steel shaft from a mild steel drive flange, as in the OP, the relative expansions are roughly in the ratio of 3:2; ie as temperature increases, the fit gets tighter.
Moral is to heat the flange quickly, don't let heat soak into the shaft.
Brass/ bronze & SS are about equal.
Click to expand...

Coefficients of expansion;-

Phosphor bronze 16.7
Stainless steel 10.4.

Fit gets looser with heat?
 
Get off as much paint and detritis as you can then if you warm the hub gently with a blow torch from the outer diameter it will loosen and come off OK. Do this before much heat gets into the shaft
Ideally remove the whole assembly and suspend it with the shaft vertical and the prop at the bottom. The prop shopuld then simply fall off under its own weight when the prop is warmed.
This has worked for me a few times
Martin
 
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