Fundamentals of shoulder screwsRyan Clancy | February 11, 2022
There is a huge variety of fasteners on the market today and they are all used by manufacturers to assemble most if not all of the products that we use from day to day. Shoulder screws might not be found in every local hardware store, but they are ubiquitous in nearly every industry as they are very versatile and provide some specific features when used.
Shoulder screws are sometimes referred to as stripper bolts or shoulder bolts and feature three sections: the head, the shoulder (unthreaded) and the threads. This type of screw has integral threads that can be seen on less than half of the length of the screw, with the rest of the shaft having a smooth surface and a larger diameter than the threads. This allows the material that is being bolted to move around or rotate around the axis of the screw. There are two different varieties of shoulder screws: precision and commercial.
Precision shoulder screws
Precision shoulder screws, often referred to as tight tolerance shoulder screws, do not fall under the governance of standard sizing like commercial screws. The thread class is 2A, and the diameter of the shoulder can measure from 3/32 inch to 1/2 inch. The tolerance on the shoulder can range from +0.000/-0.001 to -0.0005 to -0.0015. The length of the shoulder on a precision screw will also have a smaller tolerance than a commercial, coming in at -0.000 to +0.002.
Commercial shoulder screws
Commercial shoulder screws all fall under the ASME B18.3.3 standard (metric sizes) and ASME B18.3 standard (imperial sizes). The thread class is 3A, and the diameter of the shoulder for commercial screws will be from 1/4 inch to 2 inches. The shoulder tolerance is slightly more than precision shoulder screws at +0.005/-0.005.
What are shoulder screws used for?
All screws have the common purpose of holding objects and/or materials together and keeping them in a specific position. Shoulder screws, however, are designed to be used for parts that include a joint, pivot, sliding motion, shaft or mounting pin. The items/objects that shoulder screws are used for include but are not limited to:
- Machinery support
- Motion guiding
- Precision spacing
- Vacuum systems
Vacuum systems generally use a type of vented precision shoulder screw that allows faster, more effective pump down of these systems. They do this by making a path through the systems so that the volumes of air that are trapped inside can be removed from the vacuum chamber. For specific applications like these, choose between multiple types of shoulder screws.
What type of shoulder screws are used for various applications?
In order to select the correct shoulder screw for an application, a number of specifications are needed. The threads, shoulder, head and material that the screw is made out of is needed to ensure the correct type is chosen. In general, shoulder screws are defined first by their shoulder diameter, and then by their shoulder length. The entire length of the shoulder screw will include the head height as well, which is important to note.
There are many different heads that a shoulder screw can sport, including a Phillips, square, slotted, hex, Robertson or a number of special hex shapes. In general, the diameter of the head is twice the height, and about 30% to 50% bigger than the shoulder diameter. It has a flat top and there will typically be an undercut between the bottom of the head and the shoulder to allow objects or materials to mount flush to the head when installed.
The shoulder diameter must be bigger than the threads of the shoulder screw. In general, the threads are quite short, but the threads can be made to any length depending on the specific application. The space between the threads and the shoulder, called the thread neck, is generally undercut to again allow for flush mounting. The thread neck is also the weakest part of the screw, and any over-tightening or over-torqueing can lead to the screw fracturing or breaking here. The depth of the undercut can be specified by the user when the screw is being manufactured, along with the fillet dimension and the maximum width. The depth of the undercut is not standardized on precision shoulder screws, but they are standardized under ASME B18.3 and ASME B18.3.3M in commercial shoulder screws.
Shoulder screws can be constructed of all of the regular materials that other screws are made of. However, they are almost always made out of a type of steel. The most common materials that are used are as follows:
- 303 stainless steel
- 316 stainless steel
- 17-4 PH stainless steel
- 416 stainless steel
- Alloy steel
- Mild steel
Engineers utilize shoulder screws as they provide lots of flexibility; the shoulder length, head type, thread length and all of the other specifications can be modified to suit the application perfectly. It is quite common that bespoke shoulder screws are made to order for a specific application, and in this case, the undercut that is usually featured can be omitted. The threaded hold can then start with a chamfer which is sure to increase strength and increase the tightening limits. Screws like these that are simpler than the original are easier to manufacture and therefore cost less.
So, what do you think of shoulder screws? Do you use them often, or have had any bad experiences with them? Engineering360 would love to hear your thoughts in the comments below!