This document covers design and process requirements for bolt torque, common bolt tightening methods, standard tightening tools, and torque inspection procedures.
Bolted connection is the fundamental assembly method in machinery manufacturing and widely adopted across industrial production. It is also one of the most critical joining forms for engines. The quality of bolted joints directly determines engine power performance, operational safety, and sealing performance of all connected positions.
Controlling tightening torque is the most reliable, precise and efficient approach to guarantee stable bolted connections.
1. Design and Process Requirements for Bolt Torque
1.1 Tightening torque requirements for general standard bolts 1.2 Tightening torque requirements for standard pipe thread plugs 1.3 Tolerance grade requirements for general standard bolts
Uppercase letters on the left correspond to torque values marked at the top-left corner of installation indicators, representing the nominal torque of the bolt. Lowercase letters on the right denote the torque tolerance grade of the bolt.
2. Common Bolt Tightening Methods and Torque Control
Multiple tightening techniques are available. Widely adopted options include the torque method (T), torque-angle method (TA), yield point control method (Y1/Y2), and bolt elongation method.
3. Selection of Tightening Tools for Assembly
To achieve error-free tightening, tools meeting design torque requirements shall be selected according to product structural features and assembly processes. Tools fall into two categories by operation mode: handheld and stationary.
3.1 Handheld Screwdrivers
Handheld screwdrivers include multiple types with mechanical torque limitation. For basic models, tightening torque is restricted by motor power and impact force. High-end configurations are equipped with torque-controlled clutches, delivering precision up to ±3% (based on standard deviation).
3.2 Stationary Screwdrivers
Both pneumatic screwdrivers with high-precision torque-set clutches and electric servo screwdrivers can be fixed for stationary operation. Torque and angle measurement sensors can be fitted to collect measurement data. Pneumatic screwdrivers adopt mechanical torque setting; their measurement system serves for monitoring rather than closed-loop control. Electric servo-controlled screwdrivers use measurement signals to directly regulate the tightening process, adjusting motion according to real measured values to reach the set target. Measurement data feeds back into the control loop to synchronize assembly outcomes, enabling precision better than ±1% (based on standard deviation).
High tightening accuracy imposes strict limits on rotation speed. Extremely low rotating speed is commonly programmed to secure high-precision fastening results.
3.3 Vector-Controlled Electric Torque-Fastening Tools
A complete electric torque wrench system consists of: one electric torque wrench, one DL control box, connecting cables, operation switch monitor, auxiliary tightening fixture (with workpiece positioning sensor and signal output device), custom lifting rings and special sockets supplied with the tool set.
Tool precision: ±5%. The system features missing-tightening alarm and reminder functions. The fixture unlocks only when all bolts meet torque specifications and all bolt positions are fully fastened. If operators forcibly open the fixture to remove workpieces during operation, the system triggers an alarm and locks the wrench. The tool supports torque control, total rotation angle monitoring and follow-up rotation angle monitoring throughout one-step tightening. Rotation speed can be freely programmed according to actual needs.
Other built-in functions include batch counting with fixture linkage to prevent missing bolt installation, wrong thread detection, impact overload detection, real-time torque display, tool self-diagnosis, fault alerts and maintenance prompts. If tightening torque fails to meet standards, the tool locks automatically for manual troubleshooting to block defective products from flowing to subsequent stations. Comprehensive torque feedback is available: the control box displays real tightening torque, while the wrench has indicator lights (green for qualified, yellow/red for unqualified). The status monitor also provides visual light prompts.
3.4 Sensor-Based Stationary Tightening Spindles
Stationary electric servo CNC tightening spindles with built-in sensors support fully programmable screw tightening processes. Within the performance range of the spindle, torque, rotation speed, dwell time and rotation direction can be freely set and adjusted to match diverse tightening processes for precise fastener installation.
High precision and comprehensive monitoring functions maximize assembly reliability. Brushless motor drive cuts operational costs and extends service life while maintaining stable power output and peak torque, making these spindles ideal for CNC automatic fastening. Built-in torque and angle sensors enable precise closed-loop control and full real-time recording of all critical tightening parameters. Supported by servo control technology, these spindles sustain torque precision within standard deviation less than ±1% over millions of tightening cycles.
4. Four Common Static Torque Inspection Methods
Torque Method (T)
Torque plus Angle Method (TA)
Yield Point Method (Y1/Y2)
Bolt Elongation Method
5. Torque Decay (Preload Loss)
The essence of threaded fastening relies on clamping force to clamp mating components together. Torque decay refers to the gradual drop of clamping force after tightening, categorized into relaxation and self-loosening.
Relaxation
Clamping force declines over time relative to the initial tightened value, with no rotational movement of bolts or nuts.
Self-Loosening
Reduced clamping force causes bolts or nuts to rotate in the loosening direction, leading to further loss of clamping force. In extreme cases, fasteners may fully loosen. This phenomenon must be eliminated for safety-critical connections.
Embedment
High contact stress induces plastic deformation on mating surfaces, resulting in clamping force decay.
Multi-Bolt Connections: Torque Decay of U-Bolts
Two major causes and corresponding solutions are listed below:
Both leaf springs and spring seats feature curved contact surfaces, creating tiny clearances after tightening. Solution: Redesign contact areas between leaf springs and spring seats to flat surfaces.
Leaf spring deformation and friction lead to: (1) Reduction of internal elastic potential energy; (2) Gradual decrease of friction between spring laminations. Solution: Apply pre-compression treatment to leaf springs.
6. Conclusion
Rising quality requirements for threaded joints demand proper selection and standardized use of tightening tools. Effective bolt torque control directly determines product reliability, safety and sealing performance for sealed threaded assemblies. Stable compliance of all torque indicators is essential for consistent finished product quality.
