Know your shoulder screws

Headnote

The basics of shoulderscrew grades, dimensions, materials, and options can help designers get the best product for their applications.

Shoulder screws, also known as shoulder bolts and stripper bolts, are machine screws with an integral shoulder or journal between the head and thread. Although they are not readily available in local hardware stores, shoulder screws are used extensively in industries from aerospace to consumer products because of their versatility and unique attributes when installed.

Shoulder screws have three main sections: head, shoulder, and thread. The head has the largest diameter, the shoulder is described by its diameter and length, and the thread has a major diameter slightly smaller than the shoulder diameter.

When the thread of the shoulder screw is fully installed, the unthreaded shoulder extends beyond the surface into which the thread embeds. The shoulders give the screws their versatility by acting as shafts or dowels for rotating items such as bearings and bushings, axles for rolling parts, guides for sliding elements, and pivot points or mounting pins.

Shoulder screws are often used in punch-and-die mechanisms or plastic-injection-mold sets. The screws act as linear slides to accurately align the die or mold halves as they open or close. In these applications, they are commonly called stripper bolts.

What makes shoulder screws unique is the mechanical components the shoulders can accommodate. Bearings mounted on shoulders create simple cam followers. Pulleys, gears, and sprockets can rotate at moderate speeds on shoulders. Shoulder-mounted springs can preload components traveling axially along the length of the shoulder.

Specifying shoulder screws

Shoulder screws are specified by shoulder diameter and then by shoulder length. For instance, a 1A x 1 -in. shoulder screw has a shoulder diameter of Yt in. and a l-in.-long shoulder. This differs from machine screws, where the same specification yields a screw with a threaded section Va in. in diameter and 1-in. long. The overall length of the shoulder screw includes the head height, the 1-in. shoulder length and the thread length.

The screws come in a wide range of shoulder diameters and lengths in both inch and metric sizes. Many manufacturers offer custom or made-to-order screws.

Commercial-grade shoulder screws in standard sizes - as specified by ASME B 18.3 for inch sizes and ASME B18.3.3M...