Introduction    Factors    Values    What is Torque    Bolted Joints    Glossary

Torque is the application of a Force acting at a radial Distance and tending to cause rotation.

In this section, learn more about Torque and how it is measured, how to specify the Torque value of fasteners, and how to correctly tighten Bolted Joints. There is also a handy guide to Torque Units and conversion factors, a Torque Glossary and a Torqueleader Torque conversion programme to help you convert between the different types of torque units.

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Torque Introduction

Torque Conversion Programme

Torqueleader Torque Converter

DOWNLOAD FILE - TORQUE CONVERTOR

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Torque Factors

Torque units - Torque conversion factors

Torque units
Torque is the application of a Force acting at a radial Distance and tending to cause rotation.

The International Standards Organisation (ISO) recommend the use of the Newton metre (N.m) to denote the ‘base' torque unit.

The following note and the table (fig1) are taken from International Standards ISO 1000 - 1981 (BS 5555; 1981) Confirmed December 1987.

The Newton metre should be written as N.m with prefixes to denote multiples of the with prefixes to denote multiples of the base unit e.g. MN.m (mega Newton metre). Divisions of the base unit are also denoted by prefixes e.g. cN.m (centi Newton metre) - see table (fg1) for full list of prefixes.

An interesting note: the use of capitals to denote an ISO unit is reserved for those named after their Inventor or Founder, for example, capital N is used after Isaac Newton.

Factor	Prefix	Symbol
1018	exa	E
1015	peta	P
1012	tera	T
109	giga	G
106	mega	M
103	kilo	k
102	hecto	h
10	deca	da
10-1	deci	d
10-2	centi	c
10-3	milli	m
10-6	micro	μ
10-9	nano	n
10-12	pico	p
10-15	femto	f
10-18	atto	a

 

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Torque Values

A guide to specifying torque values for fasteners

Introduction
The following notes are given as a guide only. It is recommended that torque values derived from formulae should not be used without comparison to figures obtained using practical tests.

Generally, the reliability of the joint is dependent upon the bolt's ability to clamp the parts together. Adequate clamping prevents relative motion between parts of the joint and leakage from joints containing gaskets. Measuring a bolt's clamp force is difficult, especially under production assembly conditions. The preload generated by a bolt can be indirectly controlled by regulating the applied torque. This method, known as torque control, is by far the most popular method of controlling a bolt's preload. There is a relationship between the torque applied to a bolt and the resulting preload. A problem exists in that friction has a large influence on how much torque is converted into preload. Besides the torque required to stretch the bolt, torque is also required to overcome friction in the threads and under the nut face. Typically, only 10% to 15% of the torque is used to stretch the bolt. Of the remaining torque, typically 30% is dissipated in the threads and 50% to 55% under the nut face. Because friction is such an important factor in the relationship between torque and preload, variations in friction have a significant influence on the bolt's preload. Different bolt surface finishes have different friction values.

The torque required for a socket headed screw will not be the same as that required for the same size standard hexagon head bolt. The larger bearing face of the standard hexagon bolt will result in an increased torque being required compared to a socket headed screw. This is because more torque is being dissipated between the nut face and the joint surface.

Stresses induced into a bolt
When a bolt is tightened the shank and thread sustain a direct (tensile) stress due to it being stretched.

In order to effectively utilise the strength of the bolt, yet leave some margin for any loading the bolt would sustain in service, an equivalent stress of 90% of the yield stress is commonly used.

Background
The following information is provided to assist Engineers wishing to establish the theoretical torque value for a particular fastener. Caution should be exercised when using theoretical values because the preload and torque is dependent upon the friction values selected.

Terms used in the formulae below
Care should be taken to use consistent units throughout.

T Tightening torque to be applied to the fastener.

F The preload (or clamp force) in the fastener.

σ_E Equivalent stress (combined tensile and torsional stress) in the bolt thread. A figure of 90%

of the yield or proof stress of the fastener is usual.

σ_T Tensile stress in the fastener.

d2 Pitch diameter of the thread.

d3 Minor (or root) diameterof the thread.

P Pitch of the thread.

μ_T Thread friction coefficient.

μ_H Underhead coefficient of friction.

D_f The effective friction diameter of the bolt head or nut.

D_o Outside diameter of the nut bearing surface.

D_i Inside diameter of the nut bearing surface.

Calculation Procedure
The formulae used are applicable to metric and uinified thread forms which have a thread flank angle of 60°. The calculation procedure distinguishes between thread and underhead friction as well as differences which can be caused by bearing face diameter variations. The procedure comprises the following steps:

Step 1
Fastener Details. Dimensions and strength grades are specified in various standards (see download PDF - mechanical properties of fasteners below) Table 1 presents information on strength grades of bolts; the most common grade for metric fasteners is grade 8.8. Estimating the appropriate friction coefficient can be problematical. Tables 2 and 3 may be used as a guide when other information is not available. Tables 4 and 5 provide relevant information relating to thread dimensions.

Step 2
Establish the preload. The preload F is related to the direct tensile stress σT by:

F = A_S × σ_T

The stress area of the thread AS represents the effective section of the thread. It is based upon the mean of the thread pitch and minor diameters. It can be obtained from tables or calculated using the formula:

Step 3

To determine the tensile stress in the fastener, first establish what proportion of the fasteners yield strength you wish to use. Normally a figure of 90% of the yield strength is acceptable but may be varied to suit the application.

The following formula can be used to determine the tensile stress in the fastener.

 

Step 4
As can be seen from tables 2 and 3, upper and lower limits to friction values are stated. Traditionally a mean value of friction is used when calculating the tightening torque and preload value. Be aware however, that for other conditions remaining constant, the higher the value of friction - higher is the required tightening torque and lower is the resulting preload. Determine the tightening torque. The relationship between tightening torque T and bolt preload F is:

 

If units of Newtons and millimeters are being used, T will be in N.mm. To convert to N.m, divide the value by 1000.The effective friction diameter Df can be determined using the following formula:

For a standard hexagon headed nut, Do is usually taken as the across flats dimension and Di as the diameter of the bolt's clearance hole.

Example calculation

As an example, the above formulae will be used to determine the preload and tightening torque for a grade 8.8 M16 hexagon headed bolt.

Step 1. Establishing the dimensions and friction conditions. The data Establishing the dimensions and friction conditions. The data below is to be used

d₂ = 14.701mm d₃ = 13.546mm P = 2mm
μ_T Taken as 0.11 μ_H Taken as 0.16

Step 2. Taking the stress area as 157mm², gives the bolt preload F to be 77087N.

Step 3. Calculating the tensile stress in the fastener. Using 90% of 640N/mm2 gives σ_E=576N/mm2, substituting values into the formula gives σ_T = 491N/mm².

Step 4. Determination of the tightening torque T

i) The effective friction diameter. Taking Do = 24mm and Di = 17.27mm gives Df = 20.6mm.

ii) Using the values calculated gives a tightening torque T of 350437Nmm, that is 350Nm.

DOWNLOAD PDF - MECHANICAL PROPERTIES OF FASTNERS

Bolt Science Website

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What is Torque

What is Torque? How do we measure Torque?

What is Torque?
Torque is the application of a Force acting at a radial Distance and tending to cause rotation.

Torque is used to create tension.

How?

Referring to the diagram below, it can be seen that as the nut and bolt are tightened the two plates are clamped together. The thread converts the applied torque into tension in the bolt shank. The amount of tension created in the bolt is critical.

Why?

The tension in the bolt creates a "clamping" force between the two parts. If the clamping force is too low, the fasteners can work loose due to vibration or movement between the component parts. If a clamping force is too high, the fastener may permanently stretch and no longer apply the required clamping force. In severe cases the fastener may fail in assembly or during use when under load. Clamping force is generally referred to as the preload.

 

How is torque calculated?

Torque is the result of multiplying the value of Force applied by the Distance from the point of application.

Comparing the two examples it will be noted that the same resultant Torque can be achieved with a lower Force if the Distance from the nut/bolt is increased.

It should also be realised that some torque wrenches are "length dependant" which means that the actual torque applied to the fastener varies if the hand position on the wrench is varied - even with the wrench pre-set! This occurs if the pivot point of the wrench mechanism is not coincidental with the point of application of torque.

It should also be realised that some torque wrenches are "length dependant". This means the actual torque applied to the fastener is dependant on the position of the operator hand on the wrench. This occurs if the pivot point of the wrench mechanism does not coincide with the axis of the fastner.

Note: Most Torqueleader wrenches are not length dependant.

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Bolted Joints

Guidelines on the tightening of bolted joints

The guidelines presented below have been included so that an Engineer is aware of the potential pitfalls relating to the tightening of bolted joints. They are based upon experience and the results of published tests and research findings completed by organisations over many years. The guidelines are of a general nature, and not necessary specific to a particular industry.

1. Use a Calibrated Torque Wrench

Ensure that a calibrated torque wrench is used and that a torque value is specified on the tightening specification. Be aware that certain automatic tightening tools, such as impact wrenches, can result in significant variations occurring in the torque value and a bolts preload. A calibrated torque wrench should therefore be used for the final tightening operation.

2. Specify the correct tightening torque

Whenever feasible, specify the tightening torque based upon actual test results rather than a theoretically calculated value. Experimental determination of the tightening torque can be established by measurement of bolt extension, strain gauges or by the use of a load cell embedded in the joint.

3. Specify a tightening sequence

The majority of joints consist of more than one bolt and bring together surfaces which are not completely flat. The sequence of tightening bolts can have a major influence on the resulting preloads. With such joints, consideration should be given to specifying the sequence in which the bolts are to be tightened. Because the joint surfaces compress, tightening one bolt in the vicinity of another, will affect the preload generated by the first bolt tightened.

A good tightening sequence is one which ensures that an even preload distribution is achieved in the joint. Because joints containing conventional gaskets have a comparatively low compressive stiffness, bolt preloads in such joints are particularly sensitive to the tightening sequence. Based on experience, if the bolts are in a circular pattern, a cris-cross tightening sequence would normally be specified. For non-circular bolt patterns, a spiral pattern starting at the middle would normally be specified. On critical joints, a tightening pattern which tightens the bolts more than once can be specified to ensure an even preload distribution.

4. Be cautious with the use of plain washers
Use caution when specifying plain washers. Clearance between the bolt shank and the washer hole can result in relative lateral motion occurring. This can change the friction surface from nut and washer, to washer and joint surface during tightening. This affects the torque-tension relationship and will lead to large variations in preload. In some situations, such as to cover slots or to reduce the surface pressure under the bolt head, plain washers are traditionally specified. In such circumstances, ensure that they are of sufficient thickness and hardness and that they are a good fit to the bolt shank.

5. Flange Headed Bolts
On relatively soft materials, or when high tensile bolts are used, consideration should be given to the use of flange headed bolts and nuts. Such fasteners reduce the surface pressure under the nut surface reducing the amount of preload lost due to embedding. Because of the larger diameter bearing faces, generally a higher tightening torque is required because more torque is dissipated by friction.

6. Gaskets
Conventional gaskets creep; this results in a reduction in the bolts preload over time. The majority of such creep usually occurs shortly after installation. To reduce the effect of such problems, re-tightening of the bolts is frequently completed after allowing a period of time to elapse after initial tightening.

7. Embedding
Embedding is plastic deformation which occurs in the threads of the fastener and in the joint itself. It is caused by high stresses generated by the tightening process. Such embedding results in a loss of bolt extension and hence preload. Typically, preload loss due to embedding is in the region of 10%. It increases with the number of joint surfaces being clamped and with the roughness of those surfaces. High surface pressures under the bolt head can also be a cause of excessive embedding. This can be due to the use of high tensile fasteners in relatively soft materials. Hardened washers or the use of flanged fasteners can reduce such effects. Caution should also be exercised in the use of short bolts clamping several interfaces together. In such joints the small amount of bolt extension can be significantly reduced by the large amount of embedding which can be anticipated. Joint relaxation is a term often used to describe the combined effects of embedding and gasket creep.

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Torque Glossary

Glossary of terms

Torque - Torque is a ‘turning' or ‘twisting' force and differs from tension which is created by a straight pull. Torque can be thought of informally as 'rotational force'. The force applied to a lever multiplied by its distance from the levers fulcrum, is the torque.

Newton - A Newton is the amount of force required to accelerate a mass of one kilogram at a rate of one meter per second²

Dyne - In physics the dyne is a unit of force specified in the centimetre-gram-second (cgs) system of units, symbol 'dyn'. One dyne is equal to exactly 10.5 Newtons. Further, the dyne can be defined as 'the force required to accelerate a mass of one gram at a rate of one cm per second2

Kilogram force - The deprecated unit kilogram force (kgf) or kilopond (kp) is defined as the force exerted by one Kilogram of mass in standard Earth gravity. 1 kgf= 9.80665 Newton's.

Pound force - Is a non Si unit of force or weight lbf or lbF. The pound force is equal to a mass of one avoirdupois pound (which is defined as exactly 0.45359237 kg) multiplied by the standard acceleration due to Earth gravity.

Dynamometer - A device used to measure RPM and torque from which power produced by an engine or other rotating device can be calculated.

Clicker tools - When the pre-set torque value is reached the operator will hear an audible 'click'. And feel a small impulse, as there is approximately 3 degrees of movement at this point.

Breaking tools - On reaching the pre-set torque value these tools 'break' at a specific point along the tools length. The break is at a greater angle (20 degrees) than a ‘click' type wrench. Thereby reducing the possibility of over-tightening.

Slipping tools - When the pre-set torque value is reached a mechanism in the tool causes the application of torque to cease and the tool 'slips' free.

Friction - A force that resists the relative motion of two bodies in contact.

Transducer - A device (or medium) that converts energy from one form to another. The term is generally applied to devices that take a physical phenomenon (torque, pressure, temperature, humidity, flow etc) and converts it to an electrical signal.

Deflection - The change in twist or length along the primary axis between no load and rated load conditions.

Full scale/rated scale - The maximum value that a torque transducer is designed to measure.
First peak torque - When a 'click type' torque wrench signals that the torque has been achieved, the applied torque will momentarily drop before climbing again.

Peak torque - In the case of a 'click' or 'break' type torque wrench, this may be higher than the actual break point if the wrench continues to be loaded beyond the break. Consequently, peak torque is more useful for calibrating devices without a break signal such as, a dial or electronic wrenches.

Compression - This, as you would expect, describes a 'squeezing' action or force on an object when being fastened or torqued.

Tension - The opposite of compression, a 'stretching' action or force on an object.

Stress - A measure of force per unit area.

Strain - A measure of deformation or elongation of a material, its units are inch per inch, it is the ratio of a change in length to the original length of a specimen, and as such has no units.

Strength - The stress value at which a sample of material fails.

Torque Calibration - A set of operations that establish, under specified conditions, the relationship between the values of quantities indicated by a torque measuring instrument or torque measuring system, and the corresponding values realised by standards.

UKAS - The United Kingdom Accreditation Service, is the UK national body responsible for assessing and accrediting the competence of organisations in the fields of measurement, testing inspection and certification of systems products and personnel.

Metrology - The science of measurement.
Torque load cell - A torque transducer typically employing strain gauges to measure elastic deformation. 

TALS - Torque activated logging system.

TSC - Torque wrench slipper calibrated.

TSP - Torque wrench slipping preset.

ESD - Electrostatic discharge protected.

PSE - Preset torque limiting screwdrivers with ergonomic grip.

CRS - 'Clean room' torque limiting screwdrivers.

IFR - Impact free re-setting cams.

TCR - A ratchet type adjustable 'clicker' torque wrench.

TCS - A range of adjustable 'clicker' torque wrenches with 16mm spigot type carrier.

TCP - A range of pre-set, production clicking type torque wrenches with 16mm spigot type carrier.

WSTT - Weld stud test tool.

TLS - Torque limiting screwdrivers.

TWD - Torque wrench digital.

ISO - (International standards organisation), an international organisation working with the United Nations that maintain standards for all applications of technology for global industry. Wrench/spanner - A wrench having a hook, hole, or pin at the end for meshing with a related device on another object.

Torque wrench - A torque wrench is a wrench used to precisely apply or measure the torque of a fastening such as a nut or bolt. It is usually in the form of a socket wrench with special internal mechanisms. A torque wrench is used were the tightness of screws and bolts is crucial. This permits proper tension and loading of all parts.

Torque Screwdriver - Precision tool, perfect for assembly operations where accurate torque values are required.

Torque limiter - A mechanical overload protection device designed to protect mechanical equipment from damage.

Torque tool - A device used to precisely set the torque of a fastening such as a nut & bolt.

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