Contact Height

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.

 

Method of determining ycontact

 

            ycontact is just the i.d.

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

 


            Method of determining ycontact

 

            By similar triangles:

 

 

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

 

                    Method of determining ycontact

 

 

 

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