July 23, 2014
What the heck are Precision Shoulder Screws?
Here we go again, with our “what the heck is that” series, where we discuss some of the unique fasteners and electronic hardware that manufacturers use in the design and assembly of their products. Today, we will introduce you to the Shoulder Screw.
What is a Precision Shoulder Screw?
Shoulder Screws, which are sometimes called Shoulder Bolts or Stripper Bolts, are (generally) hex socket screws with an enlarged, unthreaded cylindrical shoulder under the head. They are comprised of 3 main parts: the head, the shoulder and the thread. They come in two basic categories, “Commercial” and “Precision”.
Commercial Shoulder Screws are made to inch standard ANSI ASME B18.8.2. The size range for the shoulder diameter is from ¼ to 2”. The thread class is 3A and the tolerance on the shoulder is +.005 / -.005.
Precision Shoulder screws are sometimes referred to as “tight tolerance” shoulder screws. They are not governed by any official ANSI or ASME standard. The size range for the shoulder diameter is from 3/32 to ½. The thread class is 2A and the tolerance on the shoulder diameter varies from +.000/-.001 to -.0005 to -.0015 depending upon the manufacturer. Now that’s precise!
Who uses Shoulder Screws?
Just as Dowel Pins and Retaining Rings are used to locate or hold parts together within a precision assembly or a fixture, so are Precision Shoulder Screws.
Yet shoulder screws are more versatile. When installed, the unthreaded shoulder acts as a shaft for rotating items such as bearings and bushings, precision spacing, machinery support, and motion guiding. Precision Shoulder screws exacting tolerances make them ideal for use with other precision components.
They are broadly used across various applications in many industries, including: aerospace, electrical motors, hydraulic equipment, instrumentation, tooling and fixtures, machinery, military and many more.
What do I need to consider when selecting Shoulder Screws?
Although Shoulder Screws are offered in a multitude of materials and styles, when it comes to the main commercial and precision offerings, there are a few main issues to consider.
Which Drive and Head Style?
Precision shoulder screws are readily available in hex socket and slotted drives. Philips drives are available in certain sizes. Torx and Star are available for special order. Typical head styles are a modified fillister with a flat top where the head diameter is about twice the head height. Low head fillisters are available in certain sizes
Which material do I need?
Precision shoulder bolts are readily available in Type 18-8/303 Stainless Steel and 416 Stainless Steel. 316 Stainless Steel, Alloy and other exotic materials are available for special order.
Are there other factors to consider?
When ordering Precision Shoulder screws, specify the Shoulder Diameter, Shoulder length, thread size, material and drive type. For example, an example of a typical Precision Shoulder screw is a ¼ shoulder x ½ long x 10-32 thread in SS18-8 with a hex-socket drive. Non-locking is the most popular style, but locking may also be available.
Precision Shoulder Bolts often show up under their brand name. Some of the most popular brands include: BERG, Concord, Globe, PIC, Lyn-tron and RAF. A comprehensive selection of Shoulder Screws is also offered by McMaster-Carr (tight tolerance series), MSC and Grainger.
Finally, Precision Shoulder Screws can also show up under their Mil-spec numbers, which include MS51575 & MS51576 and are DFARS compliant with full paperwork.
Precision Shoulder Screws at MF Supply
The full range of commercial and mil-spec parts, including parts that are DFARS and ROHS compliant.
Brand names and generic equivalents.
Custom made parts per print in non-standard and exotic materials and finishes.
Inch and Metric sizes.
For more detailed information on Precision Shoulder Screws, visit us at http://www.mfsupply.com/SearchResults.asp?Cat=1922
And if you don’t see it listed, as always, ask us. Finding the right screw for you is our tag line after all!
Welcome to the second edition of our “What the heck is that” series, where we discuss some of the unique fasteners and electronic hardware that manufacturers use in the design and assembly of their products. If you sell to the military either directly or indirectly, then you are probably aware of today’s topic: “DFARS” and “Mil-spec fasteners”.
What does Mil-spec mean anyway?
Before we get into the nitty gritty of DFARS, let’s discuss what makes a “Mil-spec” fastener. Mil-spec is the informal name for the military standard the U.S. Department of Defense uses in the production of military equipment. For example, the MS24693C pictured above is a fastener produced according to military standards.
Mil-spec fasteners look a lot like their civilian cousins, but, these little guys are made to a specific standard that defines in detail, the screw’s attributes, including: dimensions, tensile strength, hardness, threads, drive type, material qualities and plating, just to name a few. Mil-spec fasteners are even traceable by lot to designated manufacturers. The government maintains a list of factories and suppliers who are qualified to manufacture and distribute Mil-spec fasteners. These suppliers have met all the government’s requirements and pass the highest quality control standards. This designation is called Qualified Suppliers List for Distributors [QSLD] and the Qualified Suppliers List for Manufacturer’s [QSLM].
AKA – Also known as…..
Mil-spec fasteners are usually designated by an AN, MS, NAS or NASM prefix followed by a part number. Each of these prefixes is an abbreviation: NAS stands for National Aerospace Standard and MS stands for Military Standard. An example of one of the most common and popular Mil-spec machine screws is the MS24693C. Its commercial equivalent is a Phillips Flat 100 degree Machine Screw in 300 series stainless steel. Below, we’ll tell you more about this popular fellow.
Who uses Mil-spec Fasteners?
Mil-spec fasteners are used by anyone manufacturing or servicing military equipment. Often, the bill of materials calls for specific Mil-spec grade fasteners in accordance with a part number, drawing or procurement requirement. Some of the most common uses of Mil-spec fasteners include aerospace and naval vessels.
Now, tell me about DFARS…
Almost a decade ago, the term “DFARS Compliant Material” came to the forefront for companies supplying parts and services to the government. The original Federal Acquisition Regulation (FAR) clause 252.225-7014 specified “A Preference for Domestic Specialty Metals.” What that means in our world is that for fasteners to be DFARS compliant, the metal used to fabricate them must be melted or manufactured in the United States or a qualifying country.
Qualifying countries include: Australia, Austria, Belgium, Canada, Czech Republic, Denmark, Egypt, Finland, France, Germany, Greece, Israel, Italy, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland, Turkey, United Kingdom of Great Britain and Northern Ireland.
Important Note: DFARS only pertains to fasteners made from “specialty metals” including: stainless steel, high alloy steel like Grade-BD, or Grade 5 Chromium steel with high chromium content.
To keep up to date with DFARs requirements, visit www.acq.osd.mil/dpap/dars/dfarspgi/current/index.html
An example of a DFARS Mil-Spec Screw please!
As we’ve learned, the MS24693C is actually Phillips Flat 100 degree Machine screws in 300 series stainless steel. The MS24693-C4 is a 4-40 x 3/8 Phillips Flat 100 Degree Machine Screw in 302 Stainless Steel, and it is one of the most widely used sizes. Interestingly, the MS24693C superseded the AN662C series, so you might come across the AN662C on an older print of bill of materials. Keep in mind, with any design or manufacturing decision, the reason for using a specific screw generally rests in the hands of engineering.
Features of the MS24693C
– Strict military standards ensure performance and reliability
– Made/melted in the USA or other DFARs-approved country
– Materials and dimensions meet Federal standard FF-S-92
– Inspected per Mil-I-45208
– Full lot traceability, manufacturer’s certification and chemical and physical certification
– Available in diameters from #0 through 1/4 inch in coarse and fine thread and various lengths
Lessons Learned: Considerations when purchasing MS24693C or other Mil-spec DFARs fasteners
- Always ask for “full certifications”. This includes part name, part description, date of manufacture, lot number, chemical composition of material, and treatment of material including plating or passivation.
- At point of purchase, make sure the screws are DFARS. If you do not specifically ask for DFARS-compliant screws, you might receive screws that don’t meet the DFARS requirements, in which case, they won’t meet military standards. We’ve learned this the hard way so hopefully you don’t have to!