Precision Shoulder Screws

Overview on description, manufacture and use:

What is a precision shoulder screw?

Shoulder screws are different from regular machine screws in a variety of ways. In appearance, the unthreaded shank (or shoulder) portion is larger than the thread diameter. The difference between the two is usually on the order of one so that, for instance, a 3/8” diameter shoulder will be accompanied by a 5/16” thread. The larger diameter shoulder allows for greater shear strength across that unthreaded portion and permits its use in a wide variety of applications, including pulley attachment, heavy machinery support and motion guiding. When describing a shoulder screw, the diameter and length of the shoulder are the primary dimensional characteristics. The heads are usually round and knurled. The standard drive types are hex socket.

Precision shoulder screws take this technology one step further by introducing several additional features to the ASME B18.3 standard. For one, the shoulder diameter tolerance is twice as exact on a precision shoulder screw. The permissible range in the diameter of a standard shoulder screw is usually .002 inches, while on a precision shoulder screw the tolerance is cut in half, to a hair-thin .001. This closely-held diameter allows for use in applications where extremely tight linear or rotary movement within assemblies requires a high level of accuracy in its component parts to continue to operate in a safe and consistent manner.

Another improvement introduced with precision shoulder screws is the number of different drive types available in addition to the hex socket. Slotted, Phillips, six-lobe (Torx) and six-lobe tamper-resistant are some examples.

The thread class is also different between the two types of shoulder screws. The threads on precision shoulder screws are normally rolled to a 2A thread tolerance, reducing the chance of cross threading during installation and permitting secondary coatings to be applied without exceeding the thread allowance. The military grade versions consist of the following specifications –

MS51575 – Slotted Stainless Steel
MS51576 – Socket Stainless Steel
MS51975 – Socket Alloy/Cad plate
NAS1297 – Hex Head


How are precision shoulder screws made?

Precision shoulder screws generally start out as round bar and can be made from any number of materials, including Austenitic Stainless (303, 316), Martensitic Stainless (416), High-heat/High-strength Stainless (A-286), High-carbon heat-treated Alloy Steel, Low-carbon Steel or Ledloy (a type of steel that uses lead as an alloying material in order to improve its machining characteristics).

The bar is run through a Swiss screw machine, which creates a screw blank by cutting away material from the bar. Very large diameter shoulder screws are manufactured using Brown and Sharp screw machines. After the blank has been made it moves to several secondary operations, including mill slotting or broaching, thread-rolling, then washing and finishing. Different materials may require heat treatment as a part of the manufacturing process.

What are the other features of a precision shoulder screw?

Precision shoulder screws can be supplied with nylon locking elements applied to the threads, ensuring a tight, secure fit in high-vibration environments. In applications where the shoulders are subjected to continuous wear factors, Titanium coatings can be applied to increase the usable life span of the screw. In fact, nearly any available metal finish is possible, thanks to both the thread allowance and the close shoulder tolerance. Most importantly, most of the dimensions can be altered for a particular application. That means not having to engineer a project around standard-typefasteners. This is where the manufacturer that you use becomes a key to the project. Richard Manno & Company knows precision shoulder screws better than any other manufacturer. Whether it’s a basic version shipped from our vast inventory, or an intricate design manufactured to custom specification, you can be sure that it will be the right part for your application.