Calculating Stresses on Inserts 1

[daintysaint]

Does anyone know of a calculation to utilize for estimating the strength of inserts in plastics. The inserts I am speaking of are the ones to be thermally or ultrasonically installed. I want to know if I have to take into account the thread stress area or not. Seems like many variables to account for, looking for a formal way of analyzing. Any suggestions?

 

[diamondjim]

Your fastener manufacture must have specs
defining the basis of these inserts.
Contact them first and go from there.
They must have many references in their
sales pitches.

 

[diamondjim]

daintysaint,
I would expect the manufacturer would have
the information that you are requesting.
Have no idea if you are talking about a press
fit application or is the insert threaded on
the od? If so I would use that as the basis
for the plastic pull out force to be calculated.

 

[chicopee]

First you have to determine what loads would be imposed on the inserts; is it shear, tensile, bending or external torque. Do these loads act singly or are they combined? Obtain either the yield or working stresses from the insert manufacturers. If they provide yield stresses then you have to assign safety factors to determine working stresses. it will be then up to you to determine the actual stresses on these inserts to determine if you are within the working stresses. I do not believe that these inserts would be under cyclic loading so I wont approach this design aspect.
If the loads mentioned above are combined, your stresses may have to be calculated by using the Morh circle concept.

 

[daintysaint]

Thank you for your responses. I have the strength of the brass material and the sterngth of the plastic material, I guess I was looking for a way to predict failure strength without testing to a particular margin of error. How do I go about assigning a safety factor. I also know the thread stress area of the botl and would conversely be able to kno wthe stread stress area of the insert. The inserts I am speaking of are threaded inserts which are molded-in, thermally inserted or ultrasonically inserted into plastics. Most fastener manufacturers of these kinds of hardware list strength data, but for specific materials. I am would be deigning a new part with a different material, I thougth there would be a way to estimate within a margin of error what it would take to pull insert out or rotate it in the plastic. I will check out the Mohr's circle method. Thanks for your inputs.

DS

 

[CoryPad]

daintysaint,

Normally you don't have to consider thread stress area since the inserts are much stronger than the polymer/insert interface. Most inserts have surface features to promote physical interlocking. This prevents easy calculation/analysis. A way to start is to calculate the interface shear area A:

A = &[ignore]pi[/ignore]; D L

where

&[ignore]pi[/ignore]; = 3.141 592 654
D = insert outer diameter
L = insert height

Then, the axial force resistance F is:

F = A &[ignore]tau[/ignore];_i

where

&[ignore]tau[/ignore];_i = the interface strength

The interface strength usually is unknown, but could be approximated using the polymer shear strength &[ignore]tau[/ignore];_p.




Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[diamondjim]

You stated plastic as the base material
brass for the insert
what is the grade and material of the bolts?
and also the torque values you wish to impose?

There is a section in Machineries Handbook
that shows how to calculate the strength of
internal threads or how long they should be
in contact for different strength materials.
I assume you are using the thread locking
type insert keensert or solid inserts rather
than heli-coil style inserts. I also assume
the length is 2 times the diameter and you
are only concerned about the static calculations
and that the parts are not subjected to much
of a dynamic type loading. Everyone should
test their product for liability reasons as I
assume the strength of plastic is quite
variable as well as the surrounding structure
from design to design. I assume there is
enough material around the keensert to ensure
it will not fail from that condition. I would
expect material to exceed a circle at least one
and one half times keensert od.

 

[chicopee]

the factor of safety s/b based on the importance of the fasteners to human life; type of loading ie, constant,reverse or cyclic; material ability to redistribute stress;effects of temperature, accepted industry practices.