SEMS Screws.

What are SEMS Screws?

SEMS screws, also known as screw and washer assemblies or pre-assembled washer screws, are fasteners from Accu's Precision Screws range supplied with washers permanently held on the shank, so the screw and its washers fit as a single part. They look like an ordinary socket cap, hex or pan head screw with a flat and a spring washer sitting directly under the head, loose enough to turn but unable to slide off the thread. Because nothing needs adding at the bench, an assembler picks up and drives one component instead of three, which speeds up fitting and removes the risk of a missing or dropped washer.

They are used wherever fixings are fitted in volume, from electrical enclosures and sheet metal panels to electronics, appliances and general machine building. Every screw in Accu's range pairs a flat washer with a spring washer, an arrangement often called a double SEMS, combining a wider bearing face with some elastic take-up in the joint.

How SEMS Screws Work.

They are made by placing the washers on the screw blank before the thread is rolled. Rolling raises the thread above the original blank diameter, so once it is formed its outside diameter is larger than the washer bore and the washers can no longer slip off, yet they remain free to rotate under the head. In use, the flat washer spreads the clamping load over the part being fixed, while the spring washer, compressed as the screw is tightened, stores a little energy that helps the joint keep its preload. Where vibration is a real concern, Accu's pre-applied thread locking patches add a second line of defence.

Picture an assembly line building steel electrical enclosures, where operators fix thin sheet panels to the frame with M4 Phillips pan head screws on powered drivers. With separate parts, each fixing means picking up a screw, a spring washer and a flat washer, and a dropped washer can end up inside an enclosure among the wiring. A Phillips pan head SEMS screw arrives with both washers in place, so every joint is a single pick and drive, and the flat washer spreads the load on the panel as the screw tightens. Across hundreds of fixings per shift, that saves handling time and takes loose washers out of the process entirely.

Materials SEMS Screws Are Made From

Accu's range is manufactured from engineering-grade metals, with screw and washers paired for strength, corrosion resistance or weight:

• Stainless Steel - High Tensile (A2-70) / Stainless Steel - (A2): An A2-70 screw with A2 washers, the main stainless pairing for socket cap head SEMS from M3 to M6, including the large flat washer version.
• Stainless Steel - (A2-50) / Stainless Steel - (A2): An A2-50 screw with A2 washers, used across the hex head and Phillips pan head ranges from M2 to M12.
• Stainless Steel - (316L): Low-carbon molybdenum stainless steel for chemical and corrosion resistance, in socket cap head SEMS from M3 to M6 with a smaller flat washer.
• Titanium - (Grade 2) / Titanium - (Grade 2): Commercially pure titanium screw and washers, light and highly corrosion resistant, in socket cap head SEMS from M4 to M6.
• Titanium - (Grade 1) / Titanium - (Grade 1): The most formable commercially pure titanium grade, offered at M3 in socket cap head SEMS.
• Steel - (4.8): Property class 4.8 steel, with a nominal tensile strength of 400 MPa, used for the sheet metal Phillips pan head and security Torx pan head SEMS.

Every stainless and titanium SEMS screw is available in a Natural finish, and apart from the 316L socket cap heads, which are natural only, most sizes can also be supplied with a pre-applied patch: Thread Locking - AccuLock 360, Thread Locking - Anu-Lok 180 or Thread Locking - precote 80. The steel screws come in Trivalent Chromate, with Anu-Lok 180 also available on the security Torx range.

Sizes & Types of SEMS Screws Available

The range is available in four head styles and a wide spread of metric sizes:

• Thread Size: M2 (2 mm) to M12 (12 mm).
• Thread Pitch: 0.4 mm to 1.75 mm.
• Length (L): 4 mm to 50 mm.
• Head Height (H): 1.3 mm to 8 mm.
• Flat Washer Width (OD): 5 mm to 26 mm.
• Spring Washer Thickness (Max) (T): 0.7 mm to 3 mm.

Socket Cap Head SEMS Screws cover M3 to M6 in stainless steel and titanium, including a large flat washer version for slotted holes and a 316L version with a smaller washer for compact fixings. Hex Head SEMS Screws run from M4 to M12 in A2-50 with A2 washers, the largest sizes in the range. Phillips Pan Head SEMS Screws span M2 to M6 with PH1 to PH3 drives, plus a steel version for sheet metal panels of 0.8 to 1.2 mm. Security Torx Pan Head SEMS Screws use a pin-in hexalobular drive from T6 to T20, in M2 to M4, to deter tampering.

Availability varies by type and material. Titanium is limited to socket cap heads from M3 to M6 in lengths of 8 to 30 mm, with Grade 1 at M3 alone, while the stainless socket cap, hex and Phillips ranges go up to 50 mm. The M2 and M2.5 Phillips screws take fewer patch options than larger sizes, and the steel Phillips and security Torx screws stop at M5 and M4 respectively.

As an alternative where vibration resistance matters more than a washer, Thread Locking Screws carry a pre-applied patch on a standard screw, and where a fixing must stay in a removable panel, Captive Screws keep it from being lost.

FAQ

Q: What does SEMS stand for?

A: SEMS is generally traced to Illinois Tool Works, whose machines produced pre-asSEMbled screw and washer combinations. The word has since become a generic term for screw and washer assemblies of every head and drive type.

Q: What are SEMS screws used for?

A: They suit high-volume assembly where speed matters, such as electrical enclosures, sheet metal fabrication and appliance or electronics production. Because the washers come pre-assembled, there are fewer parts to handle per joint, no washer can be left out and none can fall loose into the product.

Q: What types of SEMS screws are there?

A: They are grouped by their head and drive style as well as by the washers fitted. Accu's range covers socket cap head, hex head, Phillips pan head and security Torx pan head styles, all in the double SEMS format with both a flat and a spring washer. Single-washer versions, with just one or the other, also exist but are not part of this range.

Q: Are SEMS screws reusable?

A: Generally, yes, as the washers stay on the screw when it is removed. The spring washer can flatten slightly after repeated tightening and a pre-applied thread locking patch grips less well each time it is reused, so inspect the screw, washers and any patch before refitting it in a critical joint.

Bespoke SEMS Screws Manufacture.

High precision, bespoke manufacture of SEMS Screws to customer specification. State of the art facilities specialising in both small batch prototyping and large scale manufacture.