tension Springs
An extension spring is a helical elastic element with tightly wound coils designed to withstand axial tensile loads and provide restoring force. Wire diameter typically ranges from 0.2 to 16 millimeters, and outer diameter varies between 4 and 150 millimeters. Initial tension is generated through internal twisting of the wire and defined as 5 to 35 percent of rated load. The shear modulus of spring steel is approximately 79 gigapascals, and allowable tensile stress ranges from 500 to 1000 megapascals. The spring index is generally selected between 6 and 14 to reduce stress concentration. The spring rate is calculated using k equals Gd to the fourth power divided by 8nD to the third power.
The structure consists of a close wound helical body with German, English, or full loop end hooks. Free length may range from 15 to 600 millimeters with 5 to 25 active coils. Stress relief heat treatment is performed between 200 and 420 degrees Celsius. Final hardness of carbon steel after treatment ranges from 40 to 50 HRC. Maximum allowable extension is typically limited to 20 to 45 percent of free length. Quality control includes initial tension testing, fatigue evaluation up to 10^6 cycles, and dimensional inspection with 0.01 millimeter precision.
