thermal expansion on of materails on a "sliding" fit

[c0rd0bes]

Hi gents wondering if anyone can help please.


I have a ground steel SKF bearing shaft O.D. 25mm diam.
passing through a small Nylon 66/ MoS2 filled shaft I.D. 25mm O.D. 30mm. this was all machined and slid no problems in the machine shop with no lateral play.

We have a triangular JIB section where we have two 1/2" thick Aluminium 6082t6 with a 30mm hole at 30 degrees plates. These plates are bolted to the inside wall of the triangular jib. And the Nylon66 is lightly pressed into them.

Leaving a triangulare jib section, with aluminium plates bolted either side and a Nylon shaft passing through and extending either side.

Now we assembled everything up (with the fits etc..) and had to ream a bit at the end to take account of the aluminium fit on the Nylon, passed the SKF shaft through and all was good. shaft could be removed and inserted no problem. We then driled and slotted each end of the Nylon and inserted over it one of these cam lock shaft clamps. So in effect they could inset the bar and have no lateral movement and clamp up gradually or clamp fully to have a fixed shaft if they liked. All seemed good

I came in this mornign to be "told" the shaft had siezed in the Nylon/mos2 sleeve and the extended Nylon ends broken. (crap I know - there is more to this!).

What I am asking is what temperature differences would be roughly required to enalbe this to happen (if at all possible) given the materials used.

 

[desertfox]

Hi

A sketch would be useful.

desertfox

 

[c0rd0bes]

My appologies here is a very rough sketch if you rotate the view anticlockwise you have it.

 

[desertfox]

Thanks for the sketch, I'll study it a while and give you my thoughts however does the assembly see any temperature?

 

[hydtools]

Were the shaft clamps tightened and left in place as someone tried to move the shaft?

Ted

 

[Oldhydroman]

It's just a thought ... but don't forget that Nylon 66 is hygroscopic and the dimensions of a machined part will change in response to how much water it absorbs or desorbs. The Nylon part of the shaft may well have swelled overnight (cool air - higher humidity) and your clearance fit unfortunately became an interference fit.

DOL

 

[c0rd0bes]

Hi there

to Desert fox - only normal temperature differences nothing extreeme in any way. it will be getting used outside in the wet

to hydtools - well that is the multi million dollar question, I think yes, but getting someone to admit what they done.

to oldhydroman - thats what i am keen to find out and very roughly what temp differences / humidty levels would change that type of fit.


thanks people I appreciate it

 

[c0rd0bes]

meant to add

to Desert fox - the bar (not shown in my sketch) can be removed and put in with the variant hot / cold mixes on every day temps.

 

[Oldhydroman]

In answer to your question about the effects of humidity; unless your polymer is one chosen specifically for low water absorption (and I don't think MoS2 filled Nylon 66 is one of those) then you can expect a water absorption as much as 7% by weight and this will cause dimensional changes as high as 2%.

DOL

 

[desertfox]

Its difficult to calculate the temperature differences that would cause the failure as you describe without the exact clearences/dimensions between the assembled components. However looking at the coefficients of expansion if you consider the steel shaft as having a dia of 24.9mm and the nylon sleeve an internal diameter of 30mm, then for the steel to achieve the same diameter as the nylon sleeve assuming only the steel was heated would need a temperature difference of 355K.

 

[c0rd0bes]

Thankyou gents you have gave me something to consider.
I was emailed yesterday by one of the users who reported that it was used outside, wind and rain etc and all was fine.

we are currently making a new insert so will adjust the tolerances based on the info given

thanks again