Helical extension springs exert a force by pulling or stretching them.  Usually, they are made from round wire and are close-wound with the initial tension between the coils.  Their ends can be formed with loops in many varieties to attach to their applications.

Extension Springs .040 Zin plated music wire extension for marine products

Extension Springs .040 Zin plated music wire extension for marine products

Material data sheet:

Common Spring Materials

Max
Recommended %
Tensile

St

Sb

Music Wire

45

75

Chrome Silicon

45

75

Chrome Vanadium

45

75

302 Stainless Steel

35

55

316 Stainless Steel

35

55

17-7 Stainless Steel

45

75

The following tables give tolerances that can be used as a reference.  Actual manufacturing tolerances will depend on the spring specifics.
 

Outside Diameter Tolerances:

Wire
Diameter
(in)

Spring Index, D/d

4

6

     8            10           12

14

16

0.015

0.002 0.002 0.003 0.004 0.005 0.006 0.007

0.023

0.002 0.003 0.004 0.006 0.007 0.008

0.01

0.035

0.002 0.004 0.006 0.007 0.009 0.011 0.013

0.051

0.003 0.005 0.007

0.01

0.012 0.015 0.017

0.076

0.004 0.007

0.01

0.013 0.016 0.019 0.022

0.114

0.006 0.009 0.013 0.018 0.021 0.025 0.029

0.171

0.008 0.012 0.017 0.023 0.028 0.033 0.038

0.25

0.011 0.015 0.021 0.028 0.035 0.042 0.049

0.375

0.016

0.02

0.026 0.037 0.046 0.054 0.064

0.5

0.021

0.03

0.04

0.062

0.08

0.1

0.125

 

Spring Free Length Tolerances

.5 in or less

0.02

0.5″ to 1.0″

0.03

1″ to 2.0″

0.04

2″ to 4″

0.06

4″ to 8″

0.093

8″ to 16″

0.156

16″ to 24″

0.218

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Katy Spring & Mfg. Inc.