Torque wrenches - basics and calibration

Physical basics

Torque

Torque (also moment of force, from Latin momentum motive force) describes the rotational effect of a force on a body.

Torque is a physical quantity in classical mechanics and corresponds to the force for rectilinear movements - but for rotational movements. A torque can accelerate the rotation of a body and bend (bending moment) or twist (torsion moment) the body.

In drive shafts, torque, together with speed, determines the transmitted power - a crucial quantity for evaluating the performance of a motor vehicle, for example.

The internationally used unit of measurement for torque is the newton meter.

If a force acts at right angles on a lever arm, the amount M of the torque is obtained by multiplying the force F by the length of the lever:

What is simply expressed by this formula can only be implemented in practice by taking numerous influences into account.

In practice - e.g. when using a torque wrench - the force does not usually act at right angles on the lever arm. In theoretical terms, the force application can be drawn into a force parallelogram. This is referred to as a pair of forces. Conversely, in statics, each torque can also be described by a force pair

Von Pietz - Eigenes Werk, CC BY-SA 3.0,

https://commons.wikimedia.org/w/index.php?curid=10621446

Newtonmeter

The Newton meter (Newton x meter) is

the unit for the vectorial quantity torque and

for the scalar quantities (mechanical) energy and work.

According to the formatting rules of the International System of Units, the two unit signs N and m must always be separated by a space or a multiplication point, but in practice this space is not used. The order of the factors must not be interchanged, this would mean mN = milliNewton!

In e.g. Word, N ∙ m can be entered by typing N then alt + 0183 then m.

Screw- or bolted connections

A screw connection is a detachable connection of two or more parts by one or more screws. Bolted joints are the only commercially used joining technique that can be undone. All other techniques - such as welding, soldering, riveting, gluing and others cannot be undone without destruction without effort.

A bolted connection can be mentally reproduced by imagining the components between the bolt head and the nut as a tension spring. All the components that are in between - washers, sheet metal, but also loss components such as dirt, rust, unclean thread run - should be pulled together / held together by this spring.

However, it is precisely these components that act against the spring and can be assumed to be a compression spring from the point of view of these components.

By tightening the screw connection, the screw is stretched over the range of the components.

This force is referred to as the preload force. Accordingly, preload force is generated by the internal tension of the components.

With

VDI 2230 sheet 1 therefore prescribes: Bolts shall be sized to withstand the operating forces encountered and to perform the function of the joint formed.

The yield strength - also called the yield point - indicates how far a bolt can be stretched: A cheap spring will serve as an example to illustrate the processes:

If this is stretched in the intended operating range, it returns to its original shape after the stretching stops. If this spring is stretched too far, it loses its original shape; the coiled wire stretches or breaks off. Steel behaves in a very similar way.

The yield strength is reached from the point where the metal still forms back to its original shape after being stretched. If this is no longer the case, the limit has been exceeded and the screw may no longer be used..

Influencing variables on bolted joints

A bolted connection has a wide variety of influences that must be taken into account for a strong connection:

Occurring operating forces

Axial forces

Transverse forces

Bending moments

Torsional forces

Load change

Significant influencing variables are also introduced (around) the screw itself:

Strength class

Friction

The setting behavior

The tightening method

Size

Geometry of the screw.

The tightening of a bolted joint is a complex process: Tightening a bolt with the nominal torque is not yet a guarantee for correct bolting over a time span that is as unlimited as possible. The force that holds the two parts together is the preload force within the joint. The preload force stretches the bolt and is the force that prevents the connection from loosening.

The actual target value of preload force cannot be measured during assembly at present! This is only possible in individual cases (laboratory, test).

Bolt classification

Screws are subject to classification.

One finds

1. digit:

This digit represents 1/100 of the minimum tensile strength.

2nd digit:

This digit represents the ratio of the material yield strength to the tensile strength.

Example:

A bolt is marked 8.8

1st digit:

1/100 of the minimum tensile strength:

8 = 800 N/mm2

e.g. 8.8 = 640 N/mm².

Torque and friction

There are always frictional influences during the bolting process. These are unavoidable, but must be recognized and controllable.

Reasons for friction (among others)

Production tolerances (surface, geometry)

Roughness and foreign particles (e.g. rust)

Lubrication condition: dry, oiled or greased

Material pairing and surface condition

Molecular adhesion and temperature influence

Coating

Comparison suit DM / DM and