Socket Set Screws are Designed for Compression Only

  1. Abstract

    Specifications which define requirements for socket set screws (SSS), such as ASTM F912 or ISO 898-5, state this style of product shall be used for compression applications only. Socket set screws that meet industry standard specifications have been known to fracture due to tensile loads. If the SSS ever engages two internally tapped components at the same time there is a risk of tensile loads being applied to the SSS.

  2. Specifications

    ASTM F912, Clause 1.3: “These set screws are intended for compression applications only and are not customarily subjected to embrittlement tests. For tensile applications, consult with the manufacturer for proper alloy and hardness.”

    ISO 898-5, Clause 1: “Fasteners in conformance with this part of ISO 898 are classified to specified hardness classes and are intended for use under compressive stress only.”

  3. Discussion

    Each fastener made to an industry specification has been designed to serve a specific purpose. For example, structural bolts such as ASTM F3125 Grade A325 are designed for high strength steel-to-steel connections; i.e. structures for bridges, buildings, and stadiums. A designer would not use a structural bolt for a wood-to-concrete application. The same holds true for SSS; they are designed for the purpose of preventing relative motion of two components, e.g. a collar with a tapped hole and a shaft.

    Through hardened SSS are typically made from medium carbon low alloy steel that have been quench and tempered to 45-53 HRC. Although no specific minimum tempering temperature is required for SSS specifications, a common temper is 650 °F. With all companies looking to lower costs, the economy alloys used today require a low tempering temperature to assure hardness requirements are still met. This leaves the SSS material with little toughness/ductility yet excellent compressive capabilities.

  4. Conclusions

    When SSS are used in tension a common delayed failure mode is hydrogen induced cracking (HIC). In this case, though the failure mode of the SSS is HIC, the fundamental root cause is: this fastener is not designed for the intended application, i.e. tension loads.

  5. Solution

    If a SSS geometry is necessary for a particular application involving tensile loads on the screw, then, a unique fastener must be designed and fabricated. The alloy and material conditions need to reflect the design criteria and both purchaser and supplier need to agree to the requirements and test procedures of the part. At times, standard SSS can be re-tempered to lower their hardness to SAE J429 Grade 8 hardness range (33-39 HRC), or, ISO 898-1 Class 10.9 hardness range (32-39 HRC). Re-tempering to Grade 8 or Class 10.9 improves ductility and toughness enabling the SSS to withstand tensile loads.

Tyler Olson
Fastener Enthusiast
Polycrew Contributor
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