Nylon Bush

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I'm turning a nylon bush for an onboard project. The bush will get wet but is not actually immersed and needs an ID of 1.25 ins. Now, I know that when a metal bush expands the hole gets bigger, but what happens to a nylon bush when it absorbs a bit of moisture ? I think the hole will get smaller ??, but does anyone know roughly by how much so I can allow a bit of clearance to avoid jamming but retain a reasonably close fit ? Thanks guys, Phil
 
My guess is also that the hole will get smaller, but I don't know how much. However, if you haven't already bought the nylon, perhaps you should get acetal (aka Delrin) instead?

Pete
 
The hole may get bigger.

If the inner and outer diameter are nearly the same - like a slice of a piece of tubing- then the circumference will expand a lot, compared with the thickness, leading to a larger diameter overall.

On the other hand, if is is like a small hole on a disc , then i guess the hole would get smaller.

IMHO, depends on the balance between diameter and thickness
 
davidej said:
The hole may get bigger.

If the inner and outer diameter are nearly the same - like a slice of a piece of tubing- then the circumference will expand a lot, compared with the thickness, leading to a larger diameter overall.

On the other hand, if is is like a small hole on a disc , then i guess the hole would get smaller.

IMHO, depends on the balance between diameter and thickness
Click to expand...

If you heat a metal disc with small hole in it the hole gets bigger. I dont see why this should be any different. Gut feeling is that it will be though!
 
VicS said:
If you heat a metal disc with small hole in it the hole gets bigger. I dont see why this should be any different. Gut feeling is that it will be though!
Click to expand...

You are probably right but i would like to see the maths of this.

Presumably the co-efficient of expansion and of elasticity come into this.

Long time since i did my physics
 
davidej said:
You are probably right but i would like to see the maths of this.

Presumably the co-efficient of expansion and of elasticity come into this.

Long time since i did my physics
Click to expand...

All I know is that if you make nylon bushes instead of Delrin, when water immersed they seize up. This would suggest the hole gets smaller. Why take the risk? Just use Delrin.
 
geem said:
All I know is that if you make nylon bushes instead of Delrin, when water immersed they seize up. This would suggest the hole gets smaller. Why take the risk? Just use Delrin.
Click to expand...

As I was reading the post I was just about to say the same, also it depends on the grade of nylon as there are a good few
 
Ok, here's how I understand it. If a metal bush expands due to a temperature increase, a nylon bush will behave the same.
The rate of expansion will be different for different materials. Therefor the outside dia will increase and so will the inside dia.
However certain materials when immersed in a liquid, will absorb some of the liquid and swell. This process is very different to expansion. I would imagine the outside dia to increase and the inside dia to decrease.
Couple this to the idea that most nylon bushes are a push fit inside a metal bore, the outside dia is unable to expand due to swelling up so the inside dia can be reduced by a surprising amount.

This is my first post, please be gentle if I'm wrong.
 
I suspect the hole will get smaller IF the water is absorbed from the central bore. The uptake of water would be unlikely to be equally spread throughout the bulk of the fitting so the inner mass would expand but be constrained by the outer mass. Maybe.
 
If the outside of the bush is constrained, and the volume of material increases, the thing will deform and the hole will be smaller.
If the thing is in free space and expands isotropically, the hole will get bigger.
Delrin is better!
 
lw395 said:
If the outside of the bush is constrained, and the volume of material increases, the thing will deform and the hole will be smaller.
If the thing is in free space and expands isotropically, the hole will get bigger.
Click to expand...

I'm not sure what isotropically means, but I suspect it probably doesn't apply. Every cubic millimetre of nylon will not expand uniformly at the same time; the water that makes it happen will diffuse in from the outside. So the walls of the hole will push inwards, against the as-yet-unaffected bulk beyond them.

Pete
 
prv said:
I'm not sure what isotropically means, but I suspect it probably doesn't apply. Every cubic millimetre of nylon will not expand uniformly at the same time; the water that makes it happen will diffuse in from the outside. So the walls of the hole will push inwards, against the as-yet-unaffected bulk beyond them.

Pete
Click to expand...

Isotropic means uniform in all directions, unlike wood for example which has a grain. Nylon may have structure to it too, depending on how it's been made.
I think you are right, it will deform in all sorts of ways.
 
This is getting right down to the nuts and bolts of many bearing issues...whilst you're at it, the thermal expansion is often greater than the moisture related change, I run calculations based on all the materials involved and their individual thermal expansion rates and then add the moisture to get clearances. Trouble is once a bush gets tight it generates heat which amplifies the problem.

I use this table as a source of material data when doing comparisons.
http://www.machinist-materials.com/comparison_table_for_plastics.htm
 
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