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Force, Torque, and Shaft Power Measurement

By

Aj. Somchai Triratanajaru M.Eng. KMUTT,

B.EngKMITL

Term 1/2553

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LECTURE OUTLINE

1. Standards and Calibration2. Basic Methods of Force Measurement3. Characteristics of Elastic Force Transducers4. Resolution of Vector Forces5. Torque Measurement on Rotating Shafts6. Shaft Power Measurement (Dynamometers)7. Gyroscopic Force and Torque Measurement8. Vibrating-Wire Force Transducers

*Major part of this presentation are from Measurement Systems, Ernest O. Doebelin

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1.Standards and calibration

Force, (F=ma); thus a standards for force depends on standards of mass and acceleration.

Mass standards-> cylinder of platinum-iridiumcalled International Kilogram.Acceleration -> drive from standards of lengthand time.g -> sea level and 45o latitude 980.665 cm/s2

If not , must be compensated.

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Force standards

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2. Basic methods of force measurement

There are seven methods introduce in thereference text book. Some methods are onlyuse in lab. We will focus on weighting andtorque measuring instrument But keep in mindthat it is still have so many methods out there.Use this information as the guide line for furtherstudy.

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2.1 Balancing it against the known gravitational force on a standard mass, either directly or through a system of levers.

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2.2 Measuring the acceleration of a body of known mass to which unknown force is applied.

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2.3 Balancing it against a magnetic force developed by interaction of a current-carrying coil and a magnet.

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2.4 Transducing the force to a fluid pressure and then measuring the pressure.

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2.5 Applying the force to some elastic member and measuring the resulting deflection.

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Strain-Gauge Load Cell

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Strain-Gauge Beam Transducers

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Young's modulus

Young's modulus, E, can be calculated by dividing the tensile stress by the tensile strain:where

E is the Young's modulus (modulus of elasticity)F is the force applied to the object;A0 is the original cross-sectional area through which the force is applied;ΔL is the amount by which the length of the object changes;L0 is the original length of the object.

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Young's modulus..

Young's modulus is the ratio of stress, whichhas units of pressure, to strain, which isdimensionless; therefore Young's modulusitself has units of pressure.

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Young's modulus…

Material Modulus (GPa)

Aluminum 69Copper 117Steel 207

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LVDT Force Transducer

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Piezoelectric Load Cells

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Torque Measurement of Rotating Machines: Dynamometers

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Chassis Dynamometer

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Servo-controlled Dynamometer

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2.6 Measuring the change in precession of gyroscope caused by an applied torque related to the measured force.

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2.7 Measuring the change in natural frequency of a wire tensioned by the force.

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Further reading and more exercises

Text.Process control Instrumentationtechnology

Curtis D. Johson