Overview
Friction-Type Torque-Shear Bolts are engineered fasteners critical for structural integrity in high-stress applications. Unlike standard bolts, they rely on friction between clamped surfaces to transfer loads, eliminating slippage risks. Their unique shear-off design ensures precise tensioning by snapping the bolt tail once the target torque is achieved. These bolts are indispensable in modern steel construction, particularly for joints subject to dynamic loads or seismic activity. They comply with international standards like ASTM A325/A490 and ISO 898-1, guaranteeing consistent performance in safety-critical projects such as skyscrapers and suspension bridges.
Structure and Working Principle
The bolt comprises three key components: a high-strength shank, a specially designed nut, and a breakaway torsion section. During installation, electric torque wrenches rotate the nut until the torsion section shears off at the calibrated torque value, indicating proper preload. This mechanism creates a slip-critical connection where friction between steel surfaces (typically with 0.35–0.50 friction coefficient) bears the load rather than bolt shear strength. The breakpoint is precisely calculated to achieve optimal clamping force without overstressing the joint materials, typically ranging from 70% to 90% of the bolt's yield strength.
Key Features
1. **Torque Control**: Shear-off design eliminates guesswork in tensioning, achieving ±5% preload accuracy compared to ±25% with conventional methods. 2. **Corrosion Protection**: Common coatings include hot-dip galvanizing (50–80μm) and mechanical zinc plating (5–15μm), with optional Dacromet for harsh environments. 3. **Quality Assurance**: Each batch undergoes tensile testing (minimum 120 ksi yield strength for A325) and hardness verification (HRC 22–32). Field verification uses direct tension indicators or ultrasonic measurement, ensuring compliance with AISC and EN 14399 standards. The bolts' fatigue resistance is 2–3 times higher than standard bolts in vibration-prone applications.
Application Areas
Primary applications include moment-resistant frames in skyscrapers, where they connect beams to columns in seismic zones. Bridge construction utilizes them for splice plates in girder webs and flanges, with typical diameters of 20–30mm for main members. In industrial settings, these bolts secure heavy machinery bases subjected to dynamic loads, such as turbine generators or press machines. Wind turbine towers extensively use M24–M36 sizes due to their vibration damping characteristics and maintenance-free service life exceeding 25 years.
Maintenance and Precautions
Installation requires ASTM F959-compliant washers and calibrated tools—either electric shear wrenches or manual torque multipliers. Surface preparation is critical: mill scale must remain intact, and oil/grease contamination should be below 5mg/cm². Post-installation inspections check for proper shearing (break plane should be flush with nut top) and use tension calibration devices on 10% of bolts. Avoid reuse; even unbroken bolts may have undergone yield stress from over-rotation. In corrosive environments, apply zinc-rich paints over installed bolts, excluding friction surfaces.
B2B Procurement Guide
Specify: 1) ASTM/EN standard, 2) diameter/length (e.g., M24×120), 3) coating type, 4) batch test certificates. Lead times range from 4–8 weeks for custom orders. Bulk discounts apply at 10,000+ units, with MOQs typically 500 bolts per size. Reputable manufacturers include Hilti, Unbrako, and structural specialists like Nucor Fastener. For projects requiring traceability, request bolts with laser-etched IDs matching mill test reports. Spot-check 2% of deliveries for dimensional compliance (per ANSI B18.2.6) and coating thickness using magnetic gauges.
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