Vertical Load Assembly Concept Generation:

i) Design 1

This design utilizes springs to apply the load.  A central gear on top of the apparatus would be manually turned.  The central gear would then rotate two smaller gears at the same rate.  These gears would each then turn a threaded rod.  The rods would cause a small drilled and tapped block to move closer to the top of the rod.  The spring is attached to the bottom of the small block and to the side of the flange mounted bearings, so when the block moves up the springs are stretched.  In this way, a vertical load is applied to the specimen.  A diagram of this procedure can be seen in Figure: 3-9.

Disadvantages of Design 1:

-A taller tower, which would likely be more susceptible to fatigue, would be required for operation.

-Springs may fatigue over course of testing.

-Load would be measured by spring displacement which would be prone to user error.

-Several moving parts would add to complexity.

Advantages of Design 1:

-Light weight.

-Springs are known to be used in existing products similar to ours.

-Load can be varied in extremely small increments.

ii) Design 2:

This design utilizes weights to apply a downward force on the specimen.  A container located above specimen can be loaded with weights.  The load is free to move downward.  Force is applied via a single ball bearing in contact with the specimen.

Disadvantages of Design 2:

-Direct contact with specimen could deform specimen surface.

-Loading increments are limited by the weights available.

-Loose weights would require extra dampening to prevent vibration.

-Additional loading increases overall mass of platform, which is moved for tensile loading.

Advantages of Design 2:

-Easily determinable load application.

-Simple design.

iii) Design 3

This design utilizes a screw driven system to lift the flange mounted bearings which house the end of the specimen.  The screw applies a downward load on a force transducer.  The end of the specimen experiences an equal load in the opposite direction.  Figure: 3-12 diagrams this process.

Disadvantages of Design 3:

-Screw could loosen during operation thus decreasing the load.

-Tight tolerances required for smooth operation.

Advantages of Design 3:

-Simple design.

-Light weight.

-Load cell allows for data acquisition as well as accurate determination of load.

-Load can be varied in extremely small increments.

Vertical Load Assembly Selection: Decision Matrix

All three vertical load assembly concepts were evaluated on mass, reliability, practicality and expected performance.  A decision matrix (Figure: 3-13) was then used to determine the design to be pursued.

Mass (.25):  Mass was a defining factor for the reason that the vertical load assembly would need to be translated for the tensile loading portion of the tester.

Reliability (.25):  Reliability was based on the projected life of each system.  Moving parts or parts that are susceptible to fatigue were cause for concern when rating this particular category.  Reliability was considered crucial due to the duration of the tests that are to be performed.

Practicality (.15):  Practicality was based on the overall simplicity of each system.

Performance (.35):  Variability of each system was important in the rating of each assemblies performance.  The ability to vary applied loads is considered important for precise testing.     

Decision: 

            After compiling scores in the decision matrix design 3 was the clear choice.  The overall simplicity of design 3 along with the ability to precisely vary load were key in this selection.

Decision Matrix:

                      Figure: 3-13