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I. Overview
Workpieces for batch hot‑dip galvanizing are of various types, and racking is the primary process in batch galvanizing. Early practices adopted hoisting whole bundles for secondary racking, which suffered from low efficiency. Modern factories adopt environmentally‑friendly enclosed collection of acid fumes and flue dust, imposing higher requirements on the racking process.
Racking refers to neatly suspending workpieces onto rack beams, which are then transported to various process tanks by cranes. It ensures equipment and personal safety, and improves production efficiency, output and coating quality.
II. Racking Devices
1. Supports and Rack Beams
Supports: Bear rack beams and workpieces. They are divided into fixed‑height type (simple structure, low cost) and adjustable‑height type (electric/pneumatic/hydraulic adjustment, compatible with diverse workpieces, high cost). Sufficient strength and stable foundation must be guaranteed.
Rack beams: Used for suspending workpieces. They shall possess adequate rigidity, with dimensions matching the zinc bath and racking requirements.
2. Three Categories of Racks
① Steel wire: The most commonly used in the industry, with low zinc carry‑over. Drawbacks: High labor intensity for manual binding, mostly for single‑time use. The mainstream choice is No. 9 annealed steel wire (ASTM A853).
Steel wire loses strength at high temperatures, and its load limit is far lower than its tensile strength at ambient temperature. Knot‑tying methods directly affect load‑bearing capacity, with the third knot type delivering the highest load capacity. Multi‑strand binding enables even load distribution, and tail folding‑back prevents slipping off.
② Hooks and chains: Suitable for large‑size workpieces and reusable.
Defects: Prone to hydrogen embrittlement, corrosion and thermal damage. High‑strength alloy chains are prohibited for use in pre‑treatment prior to hot‑dip galvanizing. Wear and corrosion shall be inspected before use, and a dezincing tank shall be equipped to remove residual zinc.
③ Special‑purpose racks: Custom‑built for mass‑produced workpieces of identical specifications to boost efficiency, safety and galvanizing quality.
Note: Nylon slings shall not be used in hot‑dip galvanizing processes.
III. Racking Procedures
Production rhythm: Prepare workpieces for pre‑treatment in advance and reserve workpieces to be galvanized in the galvanizing area to guarantee production capacity.
1. Pre‑racking inspection: Inspect surface defects, vent and drain holes, and structural deformation risks of workpieces to verify compliance with galvanizing design specifications.
2. Classified placement: Classify workpieces by weight, solid/hollow structure, corrosion severity and deformation susceptibility. Prioritize racking workpieces with similar material, shape and surface condition together.
3. Select appropriate racks: Choose racks according to workpiece conditions; combined use is allowed, and contact marks caused by racks shall be minimized as much as possible.
4. Suspension onto rack beams
For symmetrical workpieces, lifting points shall be positioned at the 1/4 positions of both ends to avoid deformation;
Vent holes shall be placed at the highest positions and drain holes at the lowest positions. Vertical suspension is preferred to facilitate venting and draining;
Strictly control the total weight, which shall not exceed the rated load of rack beams. Maintain a minimum clearance of 100 mm between workpieces and the bottom and walls of the zinc bath;
Maintain proper spacing between workpieces to prevent adhesion and scratching while avoiding space waste;
Hollow enclosed components must be provided with vent and drain holes; otherwise, explosions may occur.