Shoulder Bolts - Shoulder Screws

Shoulder bolts, also known as shoulder screws, are a type of machine screw that are designed to act as a shaft or dowel for the rotation or sliding of attached parts, including pulleys, gears, and mechanical assemblies. These screws feature an unthreaded shoulder, which is a smooth, cylindrical section of the screw that is larger in diameter than the threaded portion. The shoulder determines the nominal size of the screw and is the part that extends beyond the surface into which the thread is driven.

The smooth surface of the shoulder allows for a tight fit and prevents the parts being attached from rotating or sliding around the screw. This feature makes shoulder bolts ideal for use in precision applications where accurate positioning is critical.

Shoulder bolts are commonly used in industries such as automotive, aerospace, and manufacturing. They are available in both metric and imperial sizes, with inch sizes being specified by the shoulder diameter and length, followed by the overall length, while metric sizes are denoted by shoulder diameter, thread diameter, and shoulder length.

In addition to their use in attaching pulleys and gears, shoulder bolts are also frequently used to attach stripper plates in injection molding machines. Because of this, they are sometimes referred to as stripper bolts.

Shoulder bolts are a versatile and reliable fastening solution that can provide precise positioning and alignment in a variety of applications.


How to Measure Socket Head Shoulder Screws / Shoulder Bolts

 How to measure socket shoulder screws

Inch Shoulder Screws:

1. List shoulder diameter, then

2. List thread diameter/size, then

3. List shoulder length

Example: 1/4"-10-24 x 7/8" = a shoulder screw with a 1/4" diameter shoulder, #10-24 thread size and 7/8" shoulder length

Thread size is one standard size smaller than the shoulder diameter and doesn't need to be specified, except for:

  • A screw with a 1/2" diameter shoulder has a 3/8"-16 thread.
  • A 5/8" diameter shoulder has a 1/2"-13 thread.

Metric Shoulder Screws:

1. List shoulder diameter, then

2. List thread size, then

3. List shoulder length

Example: M6-M5 x 20 mm = a shoulder screw with a 6 mm shoulder diameter, a 5 mm thread diameter and 20 mm shoulder length

Commercial Grade Shoulder Screws vs. Precision Shoulder Screws

Commercial grade shoulder bolts primarily possess a hex socket drive that is driven into place via a hex key or allen wrench. A socket head is beneficial when installing the screw in tight spaces where there is not enough clearance for regular driving or wrenching. Hex socket shoulder screws are manufactured from either alloy steel with a black oxide finish or uncoated stainless steel.

Precision shoulder screws are available with either a hex-socket drive or a slotted drive. Additionally, some precision screw varieties feature special tamperproof drives including Torx and six-lobe styles. Manufactured from stainless steel, precision shoulder screws are useful as a precision spacer and bearing surface for a wide variety of applications. Undercuts on each side of the shoulder ensure perfect seating for precise low friction assembly. Finally, the tolerance on the shoulder diameter of the precision screw is far more exact than that of a commercial grade screw, which makes them vital in assemblies that require a high level of accuracy in their component parts.

Threadlocking Shoulder Screws

AFT offers stock sizes of threadlocking shoulder screws that feature either a nylon-pellet or a nylon-patch, which are nylon locking elements applied to the threads of the screw that increase the holding power of the screw and ensure a tight fit in high-vibration applications. All of our should screws and bolts come in alloy steel or stainless steel.

As always, if you don't see the size, finish or threadlocking feature you are looking for, let us know. We specialize in custom parts

Shoulder Screw Resources