Alternative tightening methods
July 21, 2025
By Bob Lund | Engineering Manager
By Bob Lund | Engineering Manager
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In recent discussions, we’ve explored the complexities of using torque to tighten your bolts. While torque remains the most common method, many of you have shared that it’s not always practical—especially in field service environments. Fortunately, torque isn’t the only option. In the world of structural bolting, several alternative methods are used to ensure consistent and reliable preload. Some of these techniques may be adaptable to your own applications.
There are two primary bolts in this world: ASTM A325 and ASTM A490. While their tensile strengths align with SAE J429 Grade 5 and Grade 8 bolts respectively, they are not interchangeable. Structural bolts are designed differently. They feature heavy hex heads for better load distribution and a shorter thread lengths, increasing the chance of a larger cross-sectional area in a shear plane. ASTM committees write the A325 and A490 standards, while the Research Council on Structural Connections (RCSC) provides guidance on their proper use in structural joints. In applications such as pre-tensioned or slip-critical connections, bolts are installed to within 70% of their minimum tensile strength—a higher preload than the 75% of proof load often recommended in industrial settings. For these connections, there are four primary methods of tightening for achieving proper preload. Each offers unique advantages and may inspire solutions for industrial or field-based applications: 1. Turn-of-Nut Method This widely used method involves tightening the nut to a snug condition, then rotating it a specified number of degrees (e.g., 1/3 or 2/3 turn) based on bolt size and joint conditions. The key advantage is repeatability—the angle of rotation correlates directly with bolt elongation and clamp load, regardless of friction or lubrication. This method is especially useful in field service or tight spaces where torque wrenches are impractical. It’s also commonly used in robotic assembly, where torque is applied to establish snug tightness, followed by angle-controlled tightening. The key for repetition here is to make sure the pieces being clamped are indeed together and the nug snug tight before measuring the angle for tightening. 2. Tension Control Bolts These specialty bolts feature a breakaway spline that shears off once the correct tension is reached. While highly effective, they require specialized tools and are best suited for high-volume applications due to the cost of equipment. 3. Direct Tension Indicating (DTI) Washers DTI washers have raised bumps that compress under a specified load. A feeler gauge or visual indicator (in the case of “squirter” washers) confirms when the correct pre-engineered load is achieved. These washers can be engineered for industrial use, offering a visual confirmation of proper tightening—ideal for quality assurance in the field. 4. Calibrated Wrench Method This method uses a torque wrench, but with a twist: installers must test multiple bolts using a skidmore prior to their shift to account for variability in fastener friction. This approach acknowledges the limitations of torque and ensures more consistent clamp loads. Across all applications, improper clamp load remains the leading cause of fastener failure—accounting for an estimated 85% of issues we see. While torque is the most accessible method, these alternative techniques offer more reliable and repeatable results in many scenarios. Engineering, sales, purchasing, and production often approach fasteners from different perspectives. While sales may focus on volume and purchasing on cost, engineering and quality teams have a unique opportunity to influence total product performance and lifecycle cost. By selecting the right fastening method and ensuring proper installation, you can reduce assembly time, extend product life, and minimize warranty claims. At Fastenal, we’re committed to helping you find cost-effective, performance-driven solutions—even if that means recommending fewer or lower-cost parts. Our goal is to build long-term partnerships based on trust and technical support. Engineering support and trainingFastenal’s engineering team is here to support your fastener-related needs—from torque guidance and material selection to training and troubleshooting. We offer in-person sessions, online resources, and direct consultation to help you make informed decisions and avoid costly mistakes.
For more information, visit Fastenal Engineering & Design Support or contact our engineering team at [email protected]. Fastenal Company disclaims all warranties, both express and implied, relating to the information, reports, opinions and analysis disclosed to the Customer by Fastenal Company. Fastenal Company shall not be liable for any errors or omissions or any losses, injuries or damages arising from the use of such information, reports, opinions and analysis by the Customer.
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