Measuring the amount of initial tension in an extension springs can be tricky and an often misunderstood process for those unfamiliar with spring manufacturing. Sometimes incoming inspectors will extend the spring until they can "see daylight" between the coils and then measure the load. Some will use other unorthodox methods developed and passed on by inspectors or engineers who they themselves are unfamiliar with spring design.
The proper and most accurate method used by the professional spring maker to determine initial tension is to extend the spring to a given length (L1) insuring coil separation. The spring is then extended an equal distance to (L2). The correct amount of initial tension is equal to two (2) times the load achieved at (L1) minus the load at (L2).
P1 = Load @ L1
P2 = Load @ L2
As an example:
If an extension has a load of 10 pounds when it is extended .500 and a load of 15 pounds when it is extended to 1.000 (and additional .500 of deflection), then the initial tension would be 5 pounds.
Please contact the Newcomb Spring facility nearest you for more information on initial tension and other considerations of spring design.
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