Fastener Cap Screws are closely controlled to meet the dimensions of ASME (ANSI) B18.2.1. Cap Screws are proportioned and tolerances set to assure full and proper loading when assembled into a tapped hole. Unlike a standard bolt, a washer face is provided under the head to provide a bearing surface when tightened. Also unlike standard bolts, which have much looser tolerances and allow the body to be larger than the nominal diameter, Cap Screws have a body which is never larger than the nominal diameter to ensure fit-up into tapped holes without counterboring. The end of the Cap Screw is chamfered to aid in inserting into a tapped hole. They are cold formed on precision high-speed formers, utilizing in-process quality systems.
SAE VS. ASTM Specifications
With minor differences, SAE Grade Cap Screws have equivalent versions in ASTM standards. The Grade 2 Cap Screw is comparable to an ASTM A307 Grade A, the Grade 5 Cap Screw is comparable to an ASTM A449 and the Grade 8 Cap Screw has the same strength as the ASTM A354 Grade BD. Our SAE Grade 8 and ASTM Grade BD Cap Screws are nearly identical, with the only exception being the ASTM required “BD” stamp on the head of the cap screw. Allowed steel chemistry is another difference, for example, SAE permits plain carbon 1541 grade steel for small diameter Grade 8 products, while ASTM A354 Grade BD requires alloy steel. Required head mark differences are the primary reason that most grades can no longer be dual certified. Recently, the ASTM A449 standard changed to a required “A449” head mark from the SAE three radial lines head mark. Dimensions, mechanical properties, and steel chemistry are still nearly identical, but, head marks aren’t, ending the allowance for dual certification.
Mechanical Properties
The mechanical properties of finished Cap Screws are covered in the Society of Automotive Engineers (SAE) J429, “Mechanical and Material Requirements for Externally Threaded Fasteners Specification”. This specification covers the materials, manufacturing methods, mechanical properties, and testing requirements for cap screws. Grade 2 (A307A) Cap Screws are low strength fasteners made from carbon steel, achieving their strength primarily from cold forming. Grade 5 (A449) Cap Screws are medium strength fasteners that achieve their strength from the medium carbon steel and a quench and temper heat treatment. Grade 8 Cap Screws (A354 GR BD) are high strength fasteners made from alloy steels that also achieve their strength from the alloy steel and a quench and temper heat treatment. Standard mechanical tests are performed in our A2LA (American Association for Laboratory Accreditation) Accredited Laboratory.
Applications
Cap Screws can be used in tapped holes or with a nut (like a bolt). Most designs using Cap Screws, where a clamp load is specified, require the Cap Screw to be tightened to about 75% of the proof load to ensure a safe working range. The single shear strength of a Cap Screw is about 60% of the ultimate tensile strength times the shear area, so, working loads should be lower than this breaking strength. The fatigue strength of Cap Screws as measured by the endurance limit will vary widely depending upon the amplitude, frequency, and type of applied load. Endurance limits of 10-20% of the tensile strength have been reported for high strength Cap Screws.
Suggested Torque Values
All torque/tension relationships should be viewed with a cautious eye since no one table can indicate the range of conditions expected to be experienced by a fastener. Torque is only an indirect indication of tension. The torque value to use in an application is best obtained by using a calibrated torque wrench (or transducer) and a Skidmore-Wilhelm type load indicating the device to equate actual torque to desired tension. Nearly all of the torque/tension tables which have been developed, including the one shown in Table 3, are based on the following formula:
T=(k*d*P)/12 where T = Torque (ft-lbs), d = nominal diameter (inches), P = Tension (lbs), and k is the “torque coefficient” or “nut factor” (dimensionless)
The value of k is a dimensionless “fudge factor” which includes variables such as friction, thread conditions, etc. The value of k can range from 0.10 for a well-lubricated connection to greater than 0.30 for a rusted assembly. Normally, k is approximately 0.20 for plain steel (increase by about 10% for zinc plated and decrease by about 25% for parts which have been well lubricated). The torques shown below represent starting values for plain cap screws (k = 0.20) at 60% – 90% of the proof load tension: