How to determine the installation torque for screws?

The determination of screw installation torque is a systematic engineering decision-making process. Its core objective is to generate an appropriate, stable, and reliable preload (clamping force) within the bolt, so as to ensure that the connected components do not loosen, leak, or suffer fatigue failure under operating loads.

01 | Main Methods for Determining Installation Torque

1. Theoretical Calculation Method (the most fundamental method)

This method is based on the torque-preload formula:

T = K × d × F

Where:

• T = target installation torque

• K = torque coefficient (the most critical and variable parameter, typically in the range of 0.1–0.3)

• d = nominal bolt diameter (e.g., for M10, d = 10 mm)

• F = required target preload

How to determine F (preload)?

The preload is determined based on the functional requirements of the joint (e.g., sealing, resistance to separation) and the yield strength of the bolt material. Typically, the target preload is set at 70%–90% of the lower limit of the bolt material's yield strength for high‑demand connections, or lower for general connections, to ensure an adequate safety margin.

How to determine K (torque coefficient)?

The K value comprehensively reflects the friction conditions in the thread pair and under the bearing surface, and is greatly influenced by lubrication, coatings, and surface roughness. An accurate determination of K requires experimental measurement (i.e., measuring the torque and the resulting bolt axial force under specific conditions, and back‑calculating K). In the initial design phase, empirical values or supplier data can be used as references.

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2. Look‑up Table Method (the most common and practical method)

In engineering practice, the most widespread approach is to directly consult authoritative standards (such as ISO, DIN, GB, JIS), mechanical design handbooks, or torque recommendation tables provided by bolt/fastener suppliers. These tables have already taken into account the bolt strength grades (e.g., 4.8, 8.8, 10.9, 12.9), diameter, material, and common friction conditions (e.g., unlubricated, oiled), and have built‑in safety factors.

Note: When using the look‑up table method, you must ensure that your application conditions (especially the lubrication state) are consistent with the assumptions made in the table.

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3. Experimental Verification Method (critical for safety‑related joints)

For safety‑critical connections such as those in engines, aerospace, and powertrains, it is essential to ultimately determine and verify the installation torque through physical testing. The procedure involves using bolt axial force sensors or ultrasonic preload measurement instruments on prototype parts, repeatedly testing to find the torque range that consistently produces the target preload. This method can directly calibrate the effects of friction scatter and yield the most reliable process parameters.

 

02 | Key Factors Affecting Installation Torque

1. Bolt‑related factors

• Strength grade: Higher grades (e.g., 12.9 vs. 8.8) can withstand higher installation torques to generate greater preload.

• Diameter and pitch: Diameter is a direct multiplier in the torque formula and has the greatest influence. Fine‑pitch threads can produce slightly higher preload at the same torque (due to minor differences in friction).

• Surface treatment and coating: Coatings such as zinc plating, Dacromet, and phosphating significantly change the coefficient of friction, thus greatly affecting the torque coefficient K.

2. Friction condition (the most dominant factor affecting the torque coefficient K)

• Lubrication: Whether lubricated, and the type of lubricant used (engine oil, molybdenum disulfide, special thread grease), have an enormous effect on the K value. Lubricated torque can be only 50 %–70 % of the dry torque.

• Surface roughness: Smoother thread and bearing surfaces result in lower friction, and thus higher preload for a given torque.

3. Factors related to the connected parts

• Material and hardness: Soft materials (e.g., aluminium, plastics) require care – excessive torque can cause thread stripping or deformation of the parts. It may be necessary to use a lower torque grade or add washers.

• Clamping length: An excessively short clamping length reduces joint flexibility, making it more prone to loosening under dynamic loads; a higher preload may be required.

4. Service conditions and environmental factors

• Type of load: Bolts subjected to dynamic, alternating, or shear loads need higher preload to prevent fretting and loosening.

• Temperature: High or low temperatures alter material properties and friction coefficients, potentially requiring torque adjustments or the use of special materials.

• Corrosive environment: A higher initial preload may be needed to compensate for future preload loss.

5. Assembly method and tooling

• Tightening strategy: Whether using torque control, torque‑angle control (more precise), or yield‑point control (utilising material yield). The latter two methods provide more direct control over preload and reduce the influence of friction.

• Tool accuracy: Pneumatic tools, manual torque wrenches, and electric servo tightening spindles differ in accuracy and repeatability.

• Tightening speed: Excessive speed can lead to inaccurate measurements or temperature increases that affect friction.

 

03 | Summary: Engineering Practice Steps for Determining Installation Torque

1. Clarify requirements: Define the function of the joint (sealing, pressure‑bearing, positioning), the types of loads it will be subjected to, and the operating environment.

2. Select the bolt: Choose an appropriate strength grade, diameter, and material based on the requirements.

3. Preliminarily determine the torque:

   • First choice: Consult standard torque tables that best match your application conditions.

   • Alternatively: Perform theoretical calculations if the required preload F and an estimated torque coefficient K are known.

4. Consider correction factors: Make reasonable adjustments to the table‑based or calculated values according to the actual lubrication condition, surface treatment, material of the connected parts, etc. Correction is especially important when friction conditions differ from the standard assumptions.

5. Test and verify (essential for critical joints): Conduct tightening tests on prototype parts, measure whether the actual preload meets the requirements, and optimise the torque value accordingly.

6. Establish the process specification: Document the final installation torque value, lubrication requirements, tightening sequence, tool type, and accuracy in the assembly process documentation.

Core idea: Installation torque is not an isolated number, but rather the result of a systematic process. The ultimate goal is to indirectly and stably obtain the preload – the final parameter that ensures joint performance – by controlling the torque, which is a relatively easy‑to‑measure process parameter.