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Shoulder screws have a cylindrical head with hexagon socket, a precision undersized shoulder and a short externally threaded tip. Between the shoulder and threads, a small undercut area ensures a close fit to the mating part. They are used as guides for stripper springs; as clevis pins, hinges and trunnion mountings; as pivots for linkages and swing arms; as shafts for cam rollers, pulleys, sprockets and other rotating parts; and as spacers. They are also known as:
stripper bolts, axle bolts, shoulder bolts, hexagon socket head shoulder bolts.
Shoulder screws are versatile and used with both nuts and internally threaded (tapped) holes.
Threads, specified as the number of threads per inch, are Class 3A, standard right-hand and UNC series (Unified National Coarse); Class 3A "threads provide for applications where closeness of fit and/or accuracy of thread elements are important". The screw point is flat or slightly concave, and chamfered.
Shoulder screw sizes, in fractional inches, commonly range from 1/4" to 1 1/4" in diameter, and lengths span from about 1/4" to 8" and longer. Size refers to the diameter of the shoulder and length is the shoulder length as measured from under the head to the far edge of the shoulder. Thread size is an additional specification and is related to and smaller in diameter than the shoulder—common thread sizes range from 10-24 for 1/4" diameter shoulders to 7/8-9 for 1 1/4" shoulders. Thread length is also based on shoulder diameter and remains the same regardless of shoulder length.
"Shoulder refers to the enlarged unthreaded portion of the screw, the diameter of which serves as the basis for derivation of the nominal size." The precision shoulder is undersized and is between -0.002" and -0.004" of the nominal shoulder size. As an example, the shoulder diameter of a 1/2" (0.500") shoulder screw will be 0.498" maximum to 0.496" minimum.
The cylindrical head has a flat top with a chamfered or radiused perimeter and the side may be plain or knurled. Shoulder screws have a hexagon socket so use a hex key or hex bit to drive. For sizes 1 1/4" and smaller, hex socket size is half of the shoulder diameter; for 1 1/2" to 2" sizes, it's a little larger than half. Internal wrenching allows close spacing and installation in a counterbored recess.
Alloy steel and 18-8 stainless steel are common materials. Black oxide is a typical finish for the head and threads of alloy steel shoulder screws. There is no surface treatment to the shoulder to inhibit rust.
Shoulder screws are available with a nylon patch or nylon pellet in the threaded area to provide prevailing torque and resist loosening.
Refer to American Society of Mechanical Engineers Standard ASME B18.3, Socket Cap, Shoulder, Set Screws and Hex Keys (Inch Series), for specifications relating to shoulder screws.
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