SEMs Screw

A SEMS screw, also known as combination or pre-assembled screws, is a type of fastener that consist of a screw or bolt combined with one or more washers that are permanently attached to it.

The washer is preassembled onto the screw and is typically captive even when not tightened. This design helps simplify assembly processes and reduces the risk of losing or misplacing washers during installation. The washers in SEMS screws can have various configurations, such as flat washers, lock washers, external tooth, internal tooth, split lock washer, or combination washers, depending on the specific application requirements. The washers are positioned under the screw head and along the threaded portion. 

Benefits of using SEMS screws:

• Cost - effective Solutions:  While SEMS may cost more upfront than separate screws and washers, they often save costs through improved assembly efficiency and reduced errors.
• Reduced Assembly Error Risks:  SEMS screws guarantee washer placement and secure attachment, minimizing washer omission or misplacement.
• Ensured Performance Consistency:  SEMS screws maintain washer security for dependable, consistent performance, preventing loosening or detachment.
• Enhanced Assembly Efficiency:  By pre-assembling washers, the need to handle and place individual washers is eliminated, streamlining the production process.
• Customized to Unique Requirements:  SEMS screws can be customized with different washers to suit various application needs, including flat, lock, or internal tooth washers.

1) Here are some common materials and strength class: 

• Carbon steel - The most common material for screws and bolts, relatively inexpensive and suitable.
• Stainless steel - 304 & 316 well known for their corrosion resistance, making them an excellent choice where exposure to moisture or corrosive substances is a concern.
• Alloy steel - Made from steel combined with other elements to enhance its mechanical properties, including strength and hardness. 
• Class 4.8 / 5.8 / 6.8 - Low to medium strength, commonly used in general-purpose applications where high strength is not a primary requirement. 
• Class 8.8 / 10.9 / 12.9 - Alloy steel, offers increasing levels of tensile and yield strength.

2) Recommeded types of finish and pre-applied chemical patches: 

• Finish - zinc, zinc nickel, zinc flake, phosphate, nickel, copper, black oxide, coatings per automotive standards (RoHs compliance)
• Pre-applied chemical patches - Three Bond, Precote, Loctite, 3M, nylon patch (approved source)

3) Types of head shape:

• Flat head / Round head / Oval head / Pan head  / Truss head / Cheese head 
• Binding head / Fillister head / Button head / Hex washer head / Hexagon head

4) Types of drives:

• Hexlobular drive (ISO 10664) - Six lobe shaped socket
• Phillips drive - Cross recessed type H
• Pozi drive - Cross recessed type Z
• Hexagon socket - Hexagonal socket in the head, often used with hex keys (Allen wrenches) 
• Slot drive - Single slot in the screw head
• Tamper resistant 6 lobe - Have a pin in the center of the 6 lobe recess
• Hexlobular head (DIN 34800)

5) Most commonly used washers for assembly include:

• DIN 6902 - Plain washers for screw and washer assemblies
• DIN 6903 - Plain washers for tapping screw and washer assemblies
• DIN 6904 - Curved spring washers
• DIN 6905 - Spring lock washers
• DIN 6907 - Serrated lock washers with external teeth
• DIN 6908 - Conical spring washers