ASTM A125
Helical Spring Testing
Standard Specification for Steel Springs, Helical, Heat-Treated - PDF
ASTM A125 is the standard specification for heat-treated helical steel springs: hot-coiled compression and extension springs made from carbon or alloy steel bar and then quenched and tempered. It sets the material, heat-treatment, hardness, dimensional and load requirements a spring must meet before it is accepted, and it is widely used for heavy industrial, railroad, off-highway and machinery springs.
What ASTM A125 Requires
Steel bar of the specified carbon or alloy grade, coiled and heat-treated (quenched and tempered)
Hardness within the range given in the specification, checked on the finished spring
Free height, solid height, coil diameter and number of coils within tolerance
Ends squared and ground as ordered, with the spring standing square and parallel
Load at a specified test height (or spring rate) within the ordered tolerance
No significant permanent set after the spring is compressed to solid
How Springs Are Tested to ASTM A125
Load and set checks are run in compression on a universal testing machine or spring tester fitted with flat, parallel compression platens.
Measure and record the free height of the spring
Compress the spring to solid (set-out), usually several times, then release
Re-measure free height; the loss is the permanent set
Compress to the specified test height and record the load
Record load at two heights within the working range to calculate spring rate
Check hardness and dimensions to the specification
Spring Rate Formula and Worked Example
Spring rate (stiffness) from two test points: k = (F2 − F1) / (L1 − L2)
Example: a spring carries F1 = 8,000 N at a length L1 = 150 mm and F2 = 12,000 N at L2 = 130 mm. The rate is k = (12,000 − 8,000) / (150 − 130) = 4,000 / 20 = 200 N/mm.
Permanent set example: free height 250.0 mm before set-out and 249.2 mm after, so set = 0.8 mm (0.32 %). Compare the result with the limit in your order or the specification.
For design checks, the theoretical rate of a round-wire compression spring is k = G·d4 / (8·D3·n), where G is the shear modulus (about 79,300 MPa for steel), d the bar diameter, D the mean coil diameter and n the number of active coils. With d = 20 mm, D = 120 mm and n = 6, k ≈ 153 N/mm.
Equipment for ASTM A125 Testing
Universal testing machine or dedicated spring tester with enough capacity for the solid load
Large, flat, hardened compression platens (larger than the spring outside diameter)
Displacement measurement accurate enough to read spring length at the test heights
Hardness tester for checking the heat-treated spring
ASTM A125 FAQ
What does ASTM A125 cover?
Heat-treated helical compression and extension springs hot-coiled from carbon or alloy steel bar, including material, hardness, dimensions, load and set requirements.
How is spring rate measured?
Compress the spring between flat platens, record the load at two lengths in the working range, and divide the change in load by the change in length.
What is permanent set?
The loss in free height after the spring has been compressed to solid. A125 springs are set out during manufacture so that little further set occurs in service.
What machine do I need?
A compression testing machine or UTM with capacity above the spring's solid load and platens larger than the spring diameter. Confirm the exact test heights, tolerances and hardness range against your copy of ASTM A125 and the purchase order.