plastic sleeve interference fit problem 1

[pointline]

Hello, I am new to this form stuff so here we go.......

I am looking for a better sizing interference fit
for a plastic called 'gs nylontron'.
We are pressing this sleeve 3.500" long by 1.040" O.D. w/.960" I.D. into a steel tube that is reamed out to a 1.027" - 1.030" I.D.. The sleeve is put around a
steel shaft that is .880" O.D. with a hard plastic end cap. This is put into the steel tube that is reamed out. We press the sleeve with a 12-ton press machine.
The problem we are having is that we push the sleeve in about 2.750" to 3.000" in the tube in length. Then it stops and the end cap start to mushroom the rest of the material outside the tube. Sometimes we can push it in with out a problem. But if it mushrooms out the material....well we got @^&2???[] problems.
I feel the O.D. of the sleeve is way to big and my boss says no. I feel if we had a 1.034" O.D. & .920 I.D. would be better.
The sleeve is only use as a guide for the shaft and stop the bearing asm. at the end of the shaft, (inside the tube), from coming out.
?? what sizes should we be designing for so we can press we ease and not come out of the tube.

Michael.

 

[desertfox]

Hi pointline

The interference fit should be based on the loads it will see in service and any enviromental conditions that the assembly may be subjected too.
Can you provide any more information about the service conditions.

Regards desertfox

 

[unclesyd]

I don't have my data on Nylatron handy but one problem we hads was that some of the compounds were bad about absorbing water.

 

[pointline]

Thanks for the replies. As far as Nylatron with water issues, we have not had any come up yet. for unclesyed
We really do not have any loads per say. The Nylatron sleeve is use as a guide for a shaft you can turn by hand. Designed to have .008" clearance to rotate. Not a problem there. We do not want someone to pull out the bearing asm. out of the tube. The sleeve is to provide a stop. We are having problems pressing it into the tube. Tube 1.027" I.D., sleeve is 1.040" O.D. and
mushrooms out about .750"lg. of material left to go. And yes we need the tube to be 3.500" long finish.
Guess for your service we be is a shaft around 30"lg. in a steel tube 35" that can easily slide back and forth & rotate 360 degrees by hand. for desertfox

pointline

 

[MikeHalloran]

Try keeping the plastic sleeves in a freezer until just before you press them.



Mike Halloran
Pembroke Pines, FL, USA

 

[pointline]

Thank's Mike will try that also.

 

[desertfox]

Hi pointline

Herewith the formula you can use to estimate the required axial force to drive your sleeve in but you will need material properties like Modulus of Elasticity and Poissons
Ratio for the plastic.
You mention that sometimes they fit together okay and other times the plastic bulges, my approach would be to estimate the axial force required to push the plastic sleeve into the steel with the maximum interference using the formula below. Notice that the axial force required to push home the sleeve increases with engagement length and I am wondering if with maximum interference and at the engaged length of 2.75" - 3.00" the axial force required to drive it further causes a compressive stress failure in the plastic and hence the bulging.This may not be the case with
minimum interference and hence would explain the variation.
If I am correct about the compressive stress failure then you can use the formula to work out a better interference which avoids this situation.

THE FORMULA:- Pa= f*3.142*d*L*Pc

where f=friction coefficient
Pc=contact pressure between the two members
d= nominal shaft dia
L=length of external member.
Pa= axial force required to interference fit
to calculate Pc for a given interference use the formula:-

Pc=x/[Dc*[((Dc^2+Di^2)/(Ei(Dc^2-Di^2))+....................
((Do^2+Dc^2)/(Eo*(Do^2-Dc^2))-((Ui/Ei)+
Ui/Eo))]



where x = total interference
Dc=dia of shaft
Di = dia of inner member(this is zero for solid shaft)
Do= outside dia of collar
Uo=poissons ratio for outer member
Ui=poissons ratio for inner member
Eo=modulus of elasticity for outer member
Ei=modulus of elasticity for inner member

This formula for Pc will simplify if the materials are the same.
Hope this helps

regards desertfox

ps:- I am not 100% sure that you can use the formula for plastics as the do not behave the same as metals but its the only approach I can think of.

 

[unclesyd]

I hate to post this after the effort put forth by desertfox.

Since you bushing isn't carrying any real bearing load to account for, what is the possibility of installing the bushing in two parts. I have done this on foot bearings on large slow speed agitators.

 

[pointline]

Thanks, I will look into trying that formula if I
can find the properties of nlyatron. And also look
into pressing two smaller parts.

pointline

 

[dimjim]

Couldn't you make a jig that would
enclose the tube before pressing in
the nylon sleeve?

 

[corus]

An alternative to Mike's suggestion would be to heat the outer tube. It depends on which material has the higher thermal expansion coefficient.

corus

 

[pointline]

Hello, We will not able to press 2 bushing. We can
looking heating the tube up but I do not know what
will happen to the finish powder-coated tube.

pointline

 

[zifos]

Have you tried adding a little lubricant??
Has made a world of difference for me when trying to press fit parts. Water may even work and won't leave any oily/greasy residue. A soap also works good, I used to use windex to get wake boarding boots on and the water kind of rinses away the windex so its no longer slippery.

Michael (Mech-E)