In many engineering disciplines, properly tightened bolts are critical for ensuring the structural integrity and performance of assembled components. The sequence in which bolts are tightened plays a key role in achieving the desired preload or clamping force in the joint.
We provide an overview of bolt tightening sequences, exploring the fundamentals, benefits, and best practices.
What is a Bolt Tightening Sequence?
A bolt tightening sequence refers to the specific order in which bolts or threaded fasteners are tightened in an assembly. The goal of following a proper sequence is to evenly clamp parts together while minimizing distortion. There are several common sequences:
- Circular: Tightening progresses circularly around the joint.
- Diagonal: Bolts are tightened in a crisscross diagonal pattern.
- Spiral: Tightening follows an inward spiral pattern.
- Star: Bolts are tightened in a star pattern from the center outwards.
The optimal sequence depends on factors like the number of bolts, geometry, materials involved, and desired clamping force.
Why Follow a Sequence?
Following a proper tightening sequence provides several key benefits:
Even clamping
Following a sequence allows tightening to progress gradually across the joint, spreading force evenly instead of concentrating it in one area. This prevents lopsided distortion of the parts. Even clamping ensures proper contact between surfaces and an optimized joint.
Controlled preload
Sequencing the order of tightening allows the clamping force or preload to be built up slowly in a controlled manner. This achieves the target preload precisely without any sudden spikes that can overload the joint. A controlled preload is essential for creating an optimal clamping force.
Reduced stress
Tightening in an organized sequence eliminates random or haphazard tightening, which can create localized stress concentrations and fatigue in the joint. Following the recommended pattern minimizes chances of over-tightening any single bolt, reducing stress.
No torque loss
Random tightening causes uneven settling and deformation as bolts displace materials they are embedded in, leading to preload loss as the torque relaxes. An organized sequence prevents uneven embedding and deformation, maintaining the achieved preload.
Best Practices for Bolt Tightening Sequences
To maximize the benefits of sequencing, engineers should follow these practices:
- Consult specs: Always check OEM specs for the recommended sequence and torque values.
- Torque control: Use a calibrated torque wrench to control preload precisely.
- Alternate direction: Tighten diagonally opposite bolts to balance forces.
- Recheck torque: Make a final pass, checking all bolts for proper torque.
- Consider friction: Factor in friction, which affects torque-tension relationship.
Applications and Joint Types
Proper sequencing works for flange joints, bracket joints, equipment mounts, structural frames, machine joints and more. Some of the applications where proper sequences is crucial include:
Structural
Following recommended sequences in structural joints is critical when assembling buildings, bridges, towers, dams, and other infrastructure. This prevents uneven loading and dangerous structural failures.
Mechanical
In mechanical equipment like engines, pumps, compressors, and machining tools, sequencing ensures stable, leak-free joints between components subject to vibration, pressure, and fatigue over time.
Aerospace
The extreme environments in aerospace systems like aircraft, rockets, and satellites demand proper sequencing to create robust, fail-safe joints on components.
Automotive
Automotive assemblies such as engines, transmissions, and suspensions require sequencing to optimize clamping force on parts subject to intense vibration and loading.
ALLTORC’s Solutions for Bolt Testing
Following the right tightening sequence allows engineers to achieve uniform preload, minimize distortion, control clamping forces, and ultimately create safer, more robust joints.
ALLTORC provides bolt testing equipment, including hydraulic torque wrenches, bolt tensioners, flange spreaders and tool calibration services to facilitate precision tightening during bolt sequencing.
Our quality torque tools allow engineers to apply exact preloads and recheck values, giving you confidence in creating leak-free, distortion-free joints. Contact us today to discover how ALLTORC can deliver repeatable, accurate torque control for your critical bolting needs.