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Stress and stress risers are words that we often hear in our shops.
Usually when bad things have happened to our work. Here are 5 ideas to
reduce stress in precision machined products and a brief tutorial on
what it is when the engineers say "stress."
Looking at this, someone is going to say "stress riser".
Stress (when used by designers, engineers, and metallurgists) refers
to the measurement of load on a part or test specimen related to the
area under that load. Stress can be considered to be have three modes,
axial (in line) , bending (you know what that means), or torsional
(twisting or torque). The following graphic illustrates some stress
states our parts may encounter.
Diagram of Simple Stress State
Residual stress can be considered to be a kind of "internal pressure"
in the material which may act in the same direction as the stress
applied to the part. Because of this, it can actually reduce the load
carrying ability of the part. This is what usually results in failures.
Characteristics of the part may also contribute to the concentration of
these internal stresses, leading to premature failure of the parts once
in service and subjected to load.
Here are 5 ideas to reduce stress in precision machined parts.
- Assure a smooth surface.
- Use a larger not smaller diameter for threading.
- Always maximize the fillet or radius between section or diameter changes.
- Provide both pads and relief areas on parts where applicable.
- Be alert to the fact that some materials are particularly notch sensitive, especially in the transverse direction.
Some things never change . . .
Assure a smooth surface. The creation of a smooth
surface prevents the concentration of internal stresses at sharp
changes in surface. Parts with smooth surface finish are much less
likely to fail than parts where deep grooves, tool marks or pits can
allow stresses to build up.
Use a larger not smaller diameter for threading.
This is both related to the strength of the additional material as well
as to the geometry and radii between change of dimensions. The more
generous radius possible with the larger diameter for threading can
improve the endurance limit of the part substantially. In heat treated
4340, the increase in radius from 0.015″ to 0.090″ increase the
endurance limit from 34,000 to 65,000 psi.
Always maximize the fillet or radius between section or diameter changes.
Any design which allows stress to concentrate locally will promote
fatigue failure. Generous radii and fillets are inexpensive insurance
against premature failure.
Make sure that the designer has provided both pad and releif areas on parts joining perpendicularly.
Instead of having a single point or locus for the change in forces to
be distributed through the part, pads and relief areas diffuse the
stresses that would otherwise be concentrated, improving the
performance of the part.
Be alert to the fact that some materials are particularly notch sensitive, especially in the transverse direction. Many
of the materials that we prefer to machine are resulfurized, and in
these steels, the manganese sulfides can in fact lower the steel's
transverse mechanical properties. Also, cold drawing and or forging
prior to machining can influence grain flow which can enhance the
ability of the material to carry the load. The material the designer
selected could be a large reason for the material's ability to handle
stress, or not.
There you have it. Stress = Load. Don't give it places to concentrate on your precision machined parts.
Editor's Note: CR4 would like to thank Milo for sharing this blog entry, which was originally posted here.
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