167 Lessons Learned  

While the spring proved successful in the end, there were several drawbacks throughout the process that  should be noted.  

Vendor  Selection   There was a limited selection of vendors that were willing and able to manufacture our custom volute   springs . In the production of this spring,  multiple requests were submitted but only  two vendors  were willing  to quote. The first vendor lacked extensive experience with volute springs and they did not fully understand  how the different spring parameters (inner coil diameter, helix angle, end configuration) affect ed the final  spring force output curve. Their initial batch of springs produced a force vs. deflection curve that was   significantly different than what was requested and t hey were unable to adjust the pr ocess or spring  parameters to provide a spring t hat met the design requirements. The second vendor , who also had limited  experience but displayed a much better understanding  of volute springs,  yielded better overall results . The  test sample spring as well as the first three batches of springs all satisfied the design requirements with  minimal dropout at the vendor .    The vendor ’s experience and understanding of how the volute spring parameters affect the characteristics  of the design is the most critical factor in selecting a vendor . Choosing the right vendor will reduce the  dropout rate which will ultimately reduce products overall unit price  on future orders as well as the schedule  risks associated with having to re- manufacture new springs . 

Manufacturing Process   The process for fabricating standard wire  compression springs  consists of feeding a wire into an automatic  coiler which consistently and accurately sets the pitch and diameter of the spring. The process, which both  vendors employed, to fabricate volute springs is much more labor intensive. Due to a lack of standard  equipment/tooling,  the vendors had to feed the strip material onto the rotating mandrel by hand, which made  the process very workmanship dependent . This inherently will reduce the consi stency of the spring  characteristics on a unit to unit and lot to lot basis.    For large order  quantities or anticipated reccurring orders,  it is strongly recommended that the vendor  develops a standard process/procedure and uses the appropr iate equipment/t ooling to reduce spring  variation and overall dropout . For small order quantities or one time purchases, it is recommended that  a  standard process/procedure is used for manufacturing at a minimum .  

Spring End Configuration  Another  critical issue encountered during the development process was the configuration of the ends  of  springs (unaltered vs. ground vs. closed) and how they affected the spring’s overall performance.    The first configuration tested had unaltered ends which caused  the spring to  have rectangular points on the   inner and outer coil  (Figure 8) that interfaced directly with the adjacent parts . This resulted in an unstable  deployment  which caused the adjacent parts  to cant . This is an undesirable situation because it makes the  final deployed position less stable and  less predictable. Another issue observed during testing was that  the  rectangular point on the end caused the i nner, most powerful  coil to begin expanding  when the spring was  being compressed. This caused an increase in friction between the inner coil and the adjacent coil . The  increased friction had a negligible effect  on output force when the deflection was small  but as the spring  got closer to the solid height , the impact grew exponentially . When the spring was close to the solid height,  the other coils, all fully compressed, forced the inner coil closed to its original diameter . This caused  excessive friction between the inner coil and the adjacent coil and the rectangular point on the inner coil and the fitting compressing the spring. This caused false output force readings of ~18 lb (80 N) on a spring  with only ~3 lb (13 N)  of potent ial. Although this did not affect the spring to the same degree in the  deployment direction, it made it very difficult to get an accurate compress ed output force measurement.