How to correctly calculate the bolt tightening torque: tables

What is Torque?

Torque (or tightening torque) is a physical quantity that defines the force applied to a threaded connection (bolt or nut) during tightening. It is measured in Newton meters (N·m) and determines the degree of bolt tension that holds the connected parts together.

Proper bolt tightening is critically important to ensure the reliability and safety of threaded connections in various structures and mechanisms. Incorrect torque can lead to loosening of the connection or even its failure. This article will cover the key aspects of selecting the correct tightening torque for bolts and provide tables for different strength grades and thread diameters.

The Importance of Proper Torque

The tightening torque determines the force with which the bolt presses the connected parts together. Insufficient tightening can cause loosening under vibrations or loads, while excessive tightening can lead to deformation or failure of the bolt or thread. Therefore, it is crucial to adhere to the recommended torque values for each type of bolt and operating condition.

Why is it important to follow the correct tightening torque?

The correct tightening torque ensures:

• Optimal load distribution – prevents uneven wear of parts.
• Prevention of thread damage – excessive torque can strip the thread or break the bolt.
• Operational safety – particularly critical in the automotive and aerospace industries.

Factors Affecting the Choice of Torque

When determining the optimal tightening torque, the following factors should be considered:

• Bolt strength grade: A higher strength grade allows for a greater tightening torque.
• Thread diameter and pitch: As the bolt diameter increases, the recommended tightening torque also increases.
• Material of the bolt and connected parts: Different materials have different mechanical properties, affecting the allowable tightening torque.
• Presence of lubrication or coating: The use of lubrication reduces friction in the threaded connection, allowing the required tightening to be achieved with lower torque.

Torque Table for Metric Bolts

Below is a table of recommended tightening torques for bolts of different strength grades and thread diameters. These values are approximate and may vary depending on specific operating conditions. Always refer to the manufacturer's technical documentation for precise data.

Pre-tightening force and torque table for threaded connection with coarse thread pitch and friction coefficient 0.14

Nominal thread diameter	Thread pitch, P	Nominal cross-sectional area As, mm2	Pre-tightening force Q, H					Torque Mkr Hm
			4,6	5,6	8,8	10,9	12,9	4,6	5,6	8,8	10,9	12,9
М4	0,7	8,78	1280	1710	4300	6300	7400	1,02	1,37	3,3	4,8	5,6
М5	0,8	14,2	2100	2790	7000	10300	12000	2,0	2,7	6,5	9,5	11,2
М6	1,0	20,1	2960	3940	9900	14500	17000	3,5	4,6	11,3	16,5	19,3
М8	1,25	36,6	5420	7230	18100	26600	31100	8,4	11	27,3	40,1	46,9
М10	1,5	58	8640	11500	28800	42200	49400	17	22	54	79	93
М12	1,75	84,3	12600	16800	41900	61500	72000	29	39	93	137	160
М14	2,0	115	17300	23100	57500	84400	98800	46	62	148	218	255
М16	2,0	157	23800	31700	78800	115700	135400	71	95	230	338	395
М18	2,5	193	28900	38600	99000	141000	165000	97	130	329	469	549
М20	2,5	245	37200	49600	127000	181000	212000	138	184	464	661	773
М22	2,5	303	46500	62000	158000	225000	264000	186	250	634	904	1057
М24	3,0	353	53600	71400	183000	260000	305000	235	315	798	1136	1329
М27	3,0	459	70600	94100	240000	342000	400000	350	470	1176	1674	1959
М30	3,5	561	85700	114500	292000	416000	487000	475	635	1597	2274	2662
М33	3,5	694	107000	142500	636000	517000	605000	645	865	2161	3078	3601
М36	4,0	817	125500	167500	427000	608000	711000	1080	1440	2778	3957	4631
М39	4,0	976	151000	201000	512000	729000	853000	1330	1780	3597	5123	5994

Torque Calculation Formula

Mkr = K * P * d,

where:

• K – tightening coefficient applied to the bolt (average value from the manufacturer’s certificate is used),
• P – controlled preload force of the bolt (excluding relaxation losses), kN,
• d – nominal diameter of the bolt’s threaded part, mm.

Bolt Tightening Methods

1. Tightening with a torque wrench – the most accurate method, allowing precise torque control.
2. Tightening "by feel" – suitable for small connections but lacks precision.
3. Tightening by the angle-turn method – after initial tightening, the bolt is rotated by a specific angle for more precise tension.

Recommendations for Bolt Tightening

• Use a torque wrench: This ensures precise torque control and prevents exceeding permissible values. The torque wrench must be properly calibrated.
• Check the thread condition: Ensure that the thread is clean and undamaged before tightening.
• Use lubrication when required: If specified in the technical documentation, apply the appropriate lubricant to reduce friction and achieve the necessary torque.
• Follow the tightening sequence: Especially important for flange or multi-bolt connections. Even tightening ensures better load distribution.

Use of Special Tools for Torque Control

Using a standard wrench does not guarantee torque accuracy, as the applied force depends on the mechanic's skill and perception. Therefore, special tools such as torque wrenches are used in critical applications.

When selecting a torque wrench, consider its maximum permissible load. Ideally, the tool should exceed the average required tightening torque by 25–30%.

How to Properly Tighten Bolts with a Torque Wrench?

1. Clean bolts and threaded connections from dirt, rust, and debris.
2. Set the required tightening torque on the wrench, following the instructions.
3. Start tightening, applying force gradually.
4. Wait for the click (in a limit-type wrench) or the appropriate signal (in a digital wrench).
5. Ensure the connection is secure and avoid over-tightening.

Consequences of Improper Bolt Tightening

Excessive Force

• Bolt failure due to excessive stretching.
• Thread stripping, making the hole unusable for future use.
• Deformation of connected parts.

Insufficient Tightening

• Vibration and gradual loosening of the fastening.
• Fluid leakage in systems where sealing is crucial (e.g., pipelines).
• Accelerated wear of components due to micro-movements.

How to Determine the Correct Torque?

To determine the optimal tightening torque, special tables are used, based on experimental data.

The formation of these tables occurs by gradually increasing the tightening torque until the bolt fails. The optimal value is considered to be 75–80% of the failure torque.

The correct tightening torque is a crucial parameter for the reliable operation of threaded connections. Using torque wrenches and following recommended torque values helps prevent premature wear of components, fastener damage, and critical failures.