One of the key challenges in bolted assembly is achieving accurate preload through appropriate tightening methods. Insufficient preload caused by inaccurate tightening processes is a common cause of bolt connection failure. Designers must be familiar with the principles and characteristics of mainstream bolt tightening techniques. This article systematically introduces six primary methods for bolt preload control. It should be noted that a certain degree of preload scatter is inevitable regardless of the tightening method adopted, which belongs to normal process characteristics.
Six mainstream methods are widely used to control the preload of threaded fasteners: torque‑controlled tightening, angle‑controlled tightening, yield‑point controlled tightening, bolt stretching tightening, thermal tightening, and tension‑indicating tightening.
1. Torque‑Controlled Tightening
Torque‑controlled tightening is the most commonly used preload control method in industrial applications. The rated tightening torque required for bolt assembly can be obtained from standard torque tables or calculated according to the correlation between tightening torque and bolt preload.
During tightening, the bolt shank bears both axial tensile stress and torsional shear stress. Conventional torque reference tables generally ignore torsional stress and assume that 75% of the total stress contributes to effective axial tension. Under high‑friction conditions, torsional stress increases significantly. When superimposed with axial stress, the equivalent stress may exceed the material yield strength and lead to bolt failure. For higher accuracy, the process is controlled such that the combined equivalent stress reaches approximately 90% of the yield strength, ensuring safe and reliable assembly by balancing axial and torsional stress coupling.
Torque‑dominated self‑locking fasteners such as nylon lock nuts and mechanical lock nuts are widely used in vibration‑prone scenarios to prevent loosening. These fasteners significantly increase torsional stress in the bolt shank during tightening and alter the torque‑to‑preload conversion efficiency. Therefore, torque values must be adjusted accordingly during parameter setting.
The major limitation of torque‑controlled tightening is that 85% to 95% of the applied torque is consumed to overcome thread and bearing‑surface friction. Minor variations in friction conditions can result in substantial preload deviation. Applying friction stabilizers on fasteners can reduce friction dispersion and improve tightening accuracy.
Standard measures to improve torque tightening accuracy are summarized as follows:
1. Avoid using plain flat washers. Relative slippage between washers and contact surfaces changes the effective friction radius and distorts the torque‑preload relationship. If a larger bearing area is required to reduce surface pressure, flange bolts and flange nuts are preferred.
2. Determine tightening torque through actual calibration. Strain gauges can be attached to the bolt shank for on‑site tightening calibration under real assembly conditions. Load cells installed under bolt heads provide auxiliary measurement but are less accurate than strain gauges due to altered contact characteristics.
3. When experimental calibration is unavailable, calculate the standard tightening torque comprehensively based on fastener machining tolerance, nut bearing dimensions, and locking torque characteristics. The professional torque calculation program TORQUE developed by Bolt Science can realize accurate calculation.
4. Specify tightening torque values on assembly drawings with a standard tolerance of ±5%. For critical joints, use calibrated torque wrenches and conduct post‑assembly torque verification.
Different tightening methods result in distinct preload dispersion levels, among which pure torque control presents relatively large deviation.
2. Angle‑Controlled Tightening
Angle‑controlled tightening, also known as the nut rotation method, was introduced shortly after World War II for manual assembly and is now compatible with power tightening equipment. The process first tightens the bolt to a seating torque and then rotates the nut by a predetermined angle to bring the bolt into partial plastic deformation. This method effectively reduces preload fluctuation caused by yield strength tolerance and provides better preload consistency than pure torque tightening.
The main disadvantages include the requirement for experimental calibration of tightening angles (no universal applicable parameters) and limited reusability. Repeated tightening after plastic deformation easily causes fatigue failure of fasteners.
3. Yield‑Point Controlled Tightening
Developed by SPS and also known as the joint control method, this technique delivers one of the highest preload accuracies. It minimizes the influence of friction coefficient variation and achieves highly consistent bolt preload.
Derived from the manual judgment of tightening feel, this method adopts a high‑precision electronic control system to collect real‑time torque and rotation angle data. By monitoring the slope change of the torque gradient, the system accurately identifies the bolt yield point and terminates tightening immediately to avoid overloading.
Despite improved stability, minor preload scatter still exists due to thread friction. Higher thread friction increases torsional stress and reduces axial preload under the same yield threshold. This method accurately detects bolt yielding under the combined effect of tension and torsion.
Yield‑point controlled tightening is widely used for critical components such as cylinder head bolts and connecting rod bolts, providing stable high preload and allowing optimized bolt sizing. However, the high cost of dedicated intelligent tightening tools limits widespread application. Reliable high‑precision preload cannot be guaranteed in field maintenance without matching professional equipment.
4. Bolt Stretching Tightening
Large‑diameter bolts require extremely high tightening torque, which is difficult for conventional tools. For bolts above 20 mm in diameter, hydraulic stretching is commonly adopted to replace hydraulic torque wrenches, avoiding excessive torque reaction force and limited operating space.
Working principle: A dedicated hydraulic cylinder is fitted over the nut. The bolt or stud thread fully extends through the nut and connects to the stretching fixture. Hydraulic pressure stretches the bolt axially to produce elastic elongation. While maintaining the stretching load, the nut is manually rotated and locked in place using a socket and torque bar.
This method provides extremely high preload accuracy through precise hydraulic pressure control. However, minor preload loss occurs after pressure release due to elastic rebound. Dismantling corroded or seized nuts remains a major operational difficulty.
5. Thermal Tightening
Thermal tightening utilizes the thermal expansion characteristics of metal. The bolt is heated and elongated, and the nut is rotated and positioned. During cooling, the bolt attempts to contract but is longitudinally constrained by the clamped components, generating stable axial preload.
Common heating methods include direct flame heating, heating jacket coils, and carbon resistance heating elements. The process is time‑consuming, and precise strain measurement requires full cooling to ambient temperature, resulting in low efficiency. It is rarely used in general industries and is only applicable for ultra‑large heavy‑duty bolts.
6. Tension‑Indicated Tightening
This method adopts specialized load‑indicating fasteners, load‑indicating washers, or deformation detection techniques to indirectly measure bolt elongation and preload, covering a variety of high‑precision assembly solutions.
Typical tension‑indicating bolts include Rotabolt and HiBolt. Rotabolt features a central drilled hole with an internal measuring pin and a rotatable clearance structure. The pin remains unloaded during elastic stretching; once full preload is achieved, the clearance disappears and rotation is locked, indicating correct tightening. HiBolt adopts a similar principle, in which slight bolt shrinkage clamps the central pin to reflect tension status.
Load‑indicating washers are widely used in civil engineering steel structures. Pre‑fabricated protrusions on the washer surface undergo plastic deformation under load. The correct preload is verified by measuring the residual gap with a feeler gauge. This type of washer is rarely used in precision mechanical assembly.
Bolt elongation can be directly measured using micrometers or precisely detected via ultrasonic testing. Elongation values can be converted into real‑time bolt preload through calibration or calculation. For ultrasonic measurement, the bolt head and end surface require grinding to ensure optimal acoustic reflection and testing accuracy.
