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To determine the spring force at a deflection other than the rated travel, two different methods are available. The following graph is easily constructed, allowing the Test Engineer quick access to the approximated force. The preload force and rated travel force are supplied for all Smiths Connectors probes on the catalog page.

To construct a spring force graph simply plot the forces on a scaled chart and draw a straight line.
Deflection Chart
 
Should a more accurate force be required, the following formulas can be used:

Formula 1 - Spring Force Constant
K = (Rated Force - Preload Force) \   Rated Travel

Formula 2 - Force at deflection Point .xxx
F @ .xxx = K(.xxx) + Preload Force
Example: Find the force at a deflection of .140" for the S-100, 3.5 oz. spring.
   Preload force = 1.5 oz    Rated force = 3.5 oz.
   Rated travel = .170"  
K   = (Rated Force - Preload Force) \ Rated Travel
     = (3.5 - 1.5) \ .170
     = 2 (.170)

K = 11.76 oz./in.

F @ .140 = K(.140) + Preload Force 
                = 11.76 (.140) + 1.5 
                = 1.6 + 1.5
                = 3.15 oz. at .140 deflection