TORQUE
Dictionary meaning of Torque
A force that tends to cause rotation – noun
Apply torque or a twisting force to (an object) – verb
(Example - I gently torqued the bottle cap)
Just pull and push as a linear force, torque can be thought at a rotary force or twisting force.
Torque is a product of force and distance at which the force is applied.
Torque = Force x Distance
Torque Calibration
Definition as per British Standard BS7882 - Torque with traceability derived from
national standards of mass, length and time, and of specified uncertainty of measurement, which can be
applied to the torque measuring device.
Torque – Product of tangential force and length applied about a known centre of rotation.
The SI unit for torque is the newton metre (N⋅m or Nm).
Measure Torque
Depending on need and application several methods have been applied to acquire torque reliably and
accurately. Torque can be measured from static object or constantly moving object. Measuring torque
from Static object is relatively simple, compared to torque measuring from moving object like vehicle
or a motor.
A torque sensor or transducer converts torque into an electrical signal.
The most common torque transducer is a strain gauge that converts torque into a proportional
electrical signal which can be read out using some indicator. The strain gauge is bonded to a beam
or structural member that deforms when a torque or force is applied. Deflection induces a stress that
changes the resistance of strain gauge. A Wheatstone bridge converts the resistance change into an
electrical signal output.
Static torque sensors or reaction torque sensors are used in case of
limited rotation application, where rotation will not cause damage to torque transducer cable.
Advantage of such application of torque measurement minimizes the error attributable to the
inertia of rotating components.
Application can be
Torque wrench calibration
Measuring torque on bolted joint, which is also known as brake away torque.
Measuring torque required to rotate object in its axis.
Torque required to open a cap.
Other widely accepted methods for measuring torque rely on mounting torque transducers in the
machine on the rotating shaft. Generally these methods use strain gauges torque sensors. Each
method has advantages and limitations, many of which are application dependent. In-depth understanding
is required for best suitability on application.
However many latest techniques have been developed during the years like coil type torque
transducer, strain gauge torque transducer, contact type torque transducer, non-contact type
torque transducer just to name few. Type of torque transducer are selected depending of need
for accuracy and application.
Strain gauges bonded to the shaft, the shaft becomes torque transducer. This than is calibrated,
process that usually involves loading the shaft in a predetermined manner and record results.
However care should be taken to mount the gauges on the shaft is required technique and understanding
on subject matter. Though it sounds very simple of mounting the gauge to make torque transducer
it is not an easy task.
Torque transducers are available in various shaft combination
Hex shaft torque transducer
Round shaft torque transducer
Square shaft torque transducer
Flange type torque transducer
Torque transducers are also available in many other type of shaft / coupling
In some case when torque transducer shaft is used is of hex or square type, torque Transducer shaft can be male
on one side and female on other side.
TORQUE CALIBRATION
Torque Transducer Calibration
Calibration of torque transducer that or Torque calibration transducer is carried out as per
following two standard methods.
BS 7882 – British Standard
Method for calibration and classification of torque measuring devices
DIN 51309 – German Standard
Materials testing machines - Calibration of static torque measuring devices
At Prescience Calibration we follow British Standard BS 7882 : 2008.
British Standard specifies requirements for the calibration classification of torque measuring devices,
including those used for the calibration of hand torque tools. PCPL in total follows the method of
calibration, calculation of calibration results and the classification of the torque measuring device
in a static mode
Torque measuring device – System comprising an electrical, mechanical, hydraulic or optical torque
measuring device, with associated instrumentation, including the automated logging of data when part of
the device.
Torque tools, torque transducers go out of calibration with use or whenever it goes faulty.
Torque calibration of torque transducer has to be performed to ensure that the torque measuring
device is working with in its specified limits / tolerance.
Calibration of torque transducer also ensures the measurement traceability to national / international
standard.
Frequency of calibration – Torque measuring device or torque transducer shall be recalibrated
at least every 12 months and whenever it suffers any damage or has been subjected to any repair.
According to BS 7882.
Torque creep – Change in output which occurs with time while a torque transducer is subjected to a
constant torque and with all environment conditions and other variables remaining constant.
Torque creep recovery – Change in output which occurs with time after the torque applied to the
torque transducer has been removed, with all environmental conditions and other variables remaining constant.
Static torque calibration – a process where discrete torque increments are applied
in the stepwise manner with time allowed for stabilisation of the transducer at each applied torque
Continuous torque calibration - a process where applied torque is applied in a
continuous linear manner up to the maximum calibration torque. The process takes a fraction of the
time of a static torque calibration and data from the transducer can be recorded continuously. The
applied torque, applied by the torque machine is usually measured by a calibrated reference torque
transducer.
Source
BS 7882 – Calibration and classification of torque measuring devices
DIN 51309 – Calibration of static torque measuring devices.
National Physical Laboratory UK
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