Written by Dan S. There are three classes of lens to
mount interfaces. Each has their benefits and drawbacks. In my experience
(with medium volume industrial lens systems) if the tolerance on lens
position is not too high and stress in the lens at the contact surface is not
a major concern then the sharp corner should be sufficient. However, a trade
study and/or sensitivity analysis should be made to determine the important
factors of any design.
Interface Types
Sharp Corner
This is the simplest type of
interface from both a design and a manufacturing standpoint, at least from a
theoretical standpoint. It assumes that the edge that is in contact with the
lens has a infinitely small radius. This is not realistic for two main
reasons.
Benefits:
- Very
simple calculation
- It's
easy to determine the effect of tolerances on the lens position
Drawbacks
- Not
realistic; An infinitely small corner radius is not realistic
- From a
stress standpoint this would mean that the stress in the metal would also be
infinite. But the metal will yield at
a certain stress level, making the analysis one of a non-linear phenomenon.
When this yielding happens the corner is no longer sharp, resulting in an
error in the position of the lens.
-
From a
manufacturing standpoint after the part is made the edges will have burrs
that need to be removed, this alone will blunt the edge
But despite these practical issues
this is still probably the most common assumption made when calculating the lens
position. It generally works because the effect from this actual radius on
the edge is small enough that its effect on the position of the lens is small
enough for most designs as to be negligible.
;)
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Method of determining ycontact
ycontact is just the i.d.
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Toroidal
In this interface type the more
realistic case of the edge having a radius.
Benefits:
- More realistic
- When doing stress calculations this
is the most general way to treat the interface (see Contact Stress page for
further explanation)
Drawbacks
- If the contact radius must be very
small manufacturing may have problems confirming the actual radius
- More complicated calculation to
determine the nominal lens position
- More complicated to determine the
effect of tolerances on the actual lens position
;)
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Method of determining ycontact
By similar triangles:
;)
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Tangential
Benefits:
- More realistic
- Lowest contact stresses of any
interface type
Drawbacks
- More complicated calculation to
determine the nominal lens position
- Even more complicated to determine
the effect of tolerances on the actual lens position than for the toroidal
interface
- The effect of the tolerances on
individual features is very sensitive to how the interface is dimensioned
- It is hard for manufacturing to
control/measure the angle of a very short face
;)
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Method of determining ycontact
;)
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