Standards, specifications, and technical parameters of combination screws (including GB9074.4, GB9074.13, and GB9074.8)

The Master Guide to SEMS Screws: Understanding GB9074.4, GB9074.8, and GB9074.13 Standards

In the world of high-efficiency assembly, SEMS screws (also known as pre-assembled combination screws) are a game-changer. These fasteners come with a pre-attached spring washer and a flat washer, held in place by the screw’s threads. At Ever Power, we specialize in manufacturing high-precision SEMS screws that not only meet but often exceed national standards for durability and performance.

1. Anatomy of SEMS Screws: The 3-in-1 Advantage

A typical SEMS screw consists of three distinct components: the primary screw, a spring washer (usually GB9074.26), and a flat washer (GB9074.24 or GB97.4). The washers are trapped on the shank during the thread-rolling process, meaning they can rotate freely but cannot fall off. This design drastically reduces assembly time and prevents lost components in complex machinery.

2. GB9074.4 vs. GB9074.8: Pan Head Precision

GB9074.4: Standard Pan Head Combination

This is the most common variant, based on the GB818 cross-recessed pan head screw. It is designed for general-purpose electronics and appliance assembly where a reliable bearing surface is required.

• M3 Specifications: Pitch of 0.5mm, a maximum head diameter (dk) of 5.6mm, and a head thickness (k) of 2.4mm. Uses a No. 2 cross recess.
• M5 Specifications: Pitch of 0.8mm, dk max of 9.5mm, and k max of 3.7mm. Requires a No. 2 driver.
• M8 Specifications: Large-scale assembly with a 1.25mm pitch, dk max of 16mm, and k max of 6mm. Uses a No. 4 cross recess.

GB9074.8: Small Pan Head Combination

Based on the GB823 standard, the GB9074.8 features a head that is both narrower and thinner than the GB9074.4. This is ideal for compact spaces.

• Comparison: For an M5 screw, the GB9074.4 has a dk max of 9.5mm, while the GB9074.8 is trimmed down to 9.0mm. Similarly, the head thickness (k) is reduced to 3.45mm.
• Drive Detail: Notably, the M5 size in GB9074.8 uses a No. 3 cross recess, differing from the No. 2 used in the standard pan head.

3. GB9074.13: The Hexagon Solution for Heavy-Duty Use

Known as the Cross Recessed Hexagon Head SEMS Screw (based on GB29.2), this standard is built for strength. It is frequently used in high-vibration environments like railway construction and bridge building.

• M6 Specifications: Pitch of 1.0mm, a nominal head thickness of 4mm (max 4.24mm), and a dk max of 10mm.
• M8 Specifications: Pitch of 1.25mm, nominal thickness of 5.3mm (max 5.54mm), and dk max of 13mm.

4. Material Science: Stainless Steel vs. Carbon Steel

At Ever Power, we primarily focus on SUS304 and SUS316 Stainless Steel. These materials provide a beautiful, bright finish upon cleaning and require no additional plating. SUS316 offers superior resistance to chloride corrosion, making it perfect for marine environments.

Carbon Steel Options: For high-tensile requirements, we use 1010, 1018, or 10B21 carbon steel. These screws are available in Grades 4.8 and 8.8. Unlike stainless steel, carbon steel SEMS screws must undergo a complex hardening process, followed by de-hydrogenation (to prevent embrittlement) and electroplating to prevent rust.

5. Specialized Washer Technical Data

The performance of a combination screw relies on the quality of its washers. We do not use “market-standard” washers; we use custom-manufactured components designed specifically for SEMS integration.

GB9074.26 Spring Washers

These washers provide the tension necessary to prevent loosening under vibration. For an M6 screw, the spring washer has a minimum diameter of 5.41mm and a thickness (s) between 1.5mm and 1.7mm. For M10, the thickness increases to a range of 2.35mm to 2.65mm.

GB9074.24 Flat Washers

The flat washer distributes the load. An M6 flat washer has a maximum outer diameter (d1) of 12mm and a thickness of 1.6mm. For larger M12 sizes, the d1 expands to 24mm with a 3mm thickness to protect the mounting surface.

6. Beyond the Standard: Custom Lengths by Ever Power

While the national standard (GB) typically limits lengths between 6mm and 60mm, Ever Power’s manufacturing capabilities allow us to produce SEMS screws as short as 5mm and as long as 90mm. This flexibility makes us a preferred partner for custom industrial projects.