GB922-1986 Standard Threads for Wood Screws

The Comprehensive Guide to GB/T 922-1986: Wood Screw Thread Standards

As a leading specialist at Ever Power, I understand that the structural integrity of wood-based assemblies relies heavily on the thread geometry of the fasteners. The GB/T 922-1986 standard defines the critical dimensions for wood screw threads, ensuring optimal pull-out resistance and ease of installation.

Understanding Wood Screw Thread Geometry

Unlike machine screws, wood screws are designed to displace and compress wood fibers to create a strong mechanical bond. The GB/T 922-1986 standard specifies the relationship between the major diameter (the nominal diameter) and the minor diameter ($d_1$), along with the pitch ($P$) and crest width ($b$).

Key Dimensional Parameters

• Major Diameter (Nominal Value): The maximum diameter over the threads.
• Minor Diameter ($d_1$): The core diameter at the root of the thread, critical for calculating the screw’s shear strength.
• Pitch ($P$): The distance between adjacent thread peaks, which determines how much wood is “bitten” per rotation.
• Crest Width ($b$): The flat portion at the top of the thread.

Technical Specifications and Dimensional Data

For engineers and procurement professionals, selecting the right screw size requires a deep dive into the dimensional limits. Below, we have categorized the GB/T 922 data across the full range of nominal diameters from 1.6mm to 20mm.

Precision Range (Small Diameters 1.6mm to 3.5mm)

For delicate wood assemblies and precision carpentry, the standard provides small-diameter options. A 1.6mm nominal screw has a minor diameter ($d_1$) ranging from a maximum of 1.2mm to a minimum of 0.95mm, with a pitch ($P$) of 0.8mm and a crest width ($b$) of 0.25mm. As the diameter increases to 3.5mm, the minor diameter ($d_1$) sits between 2.5mm and 2.1mm, with a 1.4mm pitch and a 0.3mm crest width.

Standard Construction Range (4mm to 8mm)

This range is common in furniture manufacturing and general construction.
For a 4mm screw, the minor diameter is 2.8mm (Max) and 2.4mm (Min), with a 1.6mm pitch.
Stepping up to the 6mm screw, we see a core diameter of 4.2mm (Max) to 3.72mm (Min), featuring a 2.5mm pitch and a 0.3mm crest width.
The 8mm variant increases its core diameter to 5.6mm (Max) to 5.12mm (Min) with a 3mm pitch.

Industrial and Structural Range (10mm to 20mm)

For heavy-duty structural timber work, larger diameters are required.
A 10mm nominal screw features a minor diameter ($d_1$) of 7.2mm (Max) and 6.62mm (Min) with a 3.5mm pitch.
The largest size in this standard, the 20mm wood screw, has a robust minor diameter of 15mm (Max) and 14.3mm (Min), utilizing a 6mm pitch and a 0.4mm crest width to maximize its load-bearing surface area.

Material Selection and Metallurgy

The performance of a wood screw is not solely dependent on its dimensions but also on the material used. GB/T 922-1986 highlights several standard materials:

• Carbon Steel: Typically Q215 or Q235 (per GB 700), providing high tensile strength and cost-efficiency for indoor applications.
• Copper and Copper Alloys: H62 or HPb59-1 (per GB 4424/4425), preferred for aesthetic appeal and superior corrosion resistance in specific environments.

While the 1986 standard focuses on these materials, Ever Power frequently provides upgrades to Stainless Steel (304/316) for marine and outdoor applications to ensure lifetime durability against rust and oxidation.

Essential Technical Requirements & Surface Quality

The geometry specified in GB/T 922 includes several performance-related mandates that affect how the fastener interacts with wood fibers:

1. Needle Point and Shank Geometry

The standard dictates that the screw tip must reach a “needle point” (针尖) to facilitate easy initial penetration into the wood. The shank itself can be cylindrical or conical within the thread length, but inverse cones are strictly forbidden as they significantly reduce the pull-out force.

2. Surface Roughness and Finish

While surface roughness is not strictly regulated for the minor diameter or the tip, the thread flanks must maintain a roughness of Ra=12.5. This ensures a balanced coefficient of friction—enough to stay secure, but smooth enough to drive into the wood without excessive torque.

3. Structural Integrity and Defects

Quality control per GB/T 922 ensures that screws are free from surface defects such as cracks, folds, or rust. Furthermore, for screws with more than 10 total thread turns, the thread run-out must be between 3 and 5 turns, ensuring a gradual transition to the unthreaded shank.

Expert Insight: The Rolled vs. Cut Thread Debate

According to the standard (Section 2.5), wood screws can be produced via rolling (辗制). If rolling is used, the diameter of the unthreaded portion of the shank is allowed to be smaller than the major diameter of the thread. This is a common manufacturing choice as rolling improves the grain structure of the metal, leading to a stronger thread than traditional cutting.