torque measurements metrology

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  • TORQUE MEASUREMENTS Vikiram Sanjeev 14M262 Vivek 14M263 R.Mukhil 14M439 M.Aravindhan 15M432 A.M.Dev Prakash15M432

  • TORQUE MEASUREMENTSTorque measurements is often associated with determination of mechanical power(either power required to operate the machine or power developed by the machine)Torque measuring devices are commonly referred to as dynamometersWhen so applied both torque and speed must be determinedAnother important reason for measuring torque is to obtain load information necessary stress or deflection analysis.

  • DEFINITION OF TORQUE:Mathematically torque is defined as the cross product of the position vector of the point where the force is applied (distance vector) and the force vector, which tends to produce rotation.Loosely speaking torque is a measure of the turning force on an object such as bolt or a flywheel. TORQUE= r*F= r*F sin().

  • TYPES OF DYNAMOMETERSAbsorption dynamometersa) Mechanical dynamometerb) Hydraulic dynamometerc) Electric dynamometerDriving dynamometersTransmission dynamometers

  • Absorption Type DynamometersIt measures and absorbs the power output of the engine to which they are coupledThe power absorbed is usually dissipated as heatExampleProny brake dynamometerHydraulic dynamometer

  • a) Mechanical dynamometer Prony break dynamometerThe mechanical energy is converted into heat through dry friction between wooden brake block and flywheel(Pulley) The attempt is to stop the engine by means of a brake on the flywheel and measure the weight which is hanging, as it tries to rotate along the flywheel

  • LimitationsFriction co-efficient varies due to wear of wooden blocks. So, it is not suitable for large powers when it is used for long periodsCooling is to be ensured to limit the temperature rise

  • ApplicationsIt is used to find the torque in automobile engines.It is used for measuring and recording the torque on a rotating system such as -Crankshaft -Gearbox -Transmission motor -Bicycle crank -Electric motors -Cap torque tester03/02/16

  • b) Hydraulic dynamometerIt works on principle of dissipating the power in fluid friction rather than dry friction The vortices and eddy current are set up in the water and tend to turn the dynamometer cannons in direction of rotationThis is resisted by the brake arm and the balance system measures the torque

  • AdvantagesLess space is required for high absorption capacity No water is required for cooling Cost efficient

  • c) Electric dynamometerConstruction and Working Principle of Electrodynamometer Type WattmeterDynamometer type wattmeter works on very simple principle and this principle can be stated as "when any current carrying conductor is placed inside a magnetic field, it experiences a mechanical force and due this mechanical force deflection of conductor takes place" It consists of following partsThere are two types of coils present in the electrodynamometer. They are :(a) Moving coil : Moving coil moves the pointer with the help of spring control instrument. A limited amount of current flows through the moving coil so as to avoid heating. So in order to limit the current we have connect the high value resistor in series with the moving coil. The moving is air cored and is mounted on a pivoted spindle and can moves freely. In electrodynamometer type wattmeter, moving coil works as pressure coil. Hence moving coil is connected across the voltage and thus the current flowing through this coil is always proportional to the voltage.

  • (b) Fixed coil: The fixed coil is divided into two equal parts and these are connected in series with the load, therefore the load current will flow through these coils. Now the reason is very obvious of using two fixed coils instead of one, so that it can be constructed to carry considerable amount of electric current. These coils are called the current coils of electrodynamometer type wattmeter. Earlier these fixed coils are designed to carry the current of about 100 amperes but now the modern wattmeter are designed to carry current of about 20 amperes in order to save power.(c) Control system: Out of two controlling systems i.e.(1) Gravity control(2) Spring control, only spring controlled systems are used in these types of wattmeter. Gravity controlled system cannot be employed because they will appreciable amount of errors.(d) Damping system: Air friction damping is used, as eddy current damping will distort the weak operating magnetic field and thus it may leads to error.(e) Scale: There is uniform scale is used in these types of instrument as moving coil moves linearly over a range of 40 degrees to 50 degrees on either sides.

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  • Advantages of Electrodynamometer Type WattmeterFollowing are the advantages of electrodynamometer type wattmeters and they are written as follows:(a) Scale is uniform upto certain limit.(b) They can be used for both to measure ac as well dc quantities as scale is calibrated for both.

  • Errors in Electrodynamometer Type WattmeterFollowing are the errors in the electrodynamometer type wattmeters:(a) Errors in the pressure coil inductance.(b) Errors may be due to pressure coil capacitance.(c) Errors may be due to mutual inductance effects.(d) Errors may be due connections.(i.e. pressure coil is connected after current coil)(e) Error due to Eddy currents.(f) Errors caused by vibration of moving system.(g) Temperature error.(h) Errors due to stray magnetic field.

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  • Driving DynamometersThey measure torque and also provide energy to operate the devices to be tested They are used in torque measurements of compressors and pumpsThis type is not commonly used

  • Transmission DynamometersThese dynamometers are located at a particular place in the machine and the sense the torqueThey do not supply or receive energy Transmission dynamometers are also called as torque meters

  • DC DynamometersIt comes under absorption as well as transmission Torque is measured by a balancing force at a fixed known torque armWhen it is used as Absorption dynamometers. It acts as a DC generatorIt acts as a DC motor, when it is used as Transmission dynamometers.

  • ApplicationIC engine testingSteam turbines Pumps

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