ics 2.force and torque measurement notes

8/2/2019 Ics 2.Force and Torque Measurement Notes

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Multiple - lever systems


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PNEUMATIC LOAD CELL

Load cellA load cell is a transducer that is used to convert a force into electrical signal. This conversion isindirect and happens in two stages. Through a mechanical arrangement, the force being senseddeforms a strain gauge. The strain gauge converts the deformation (strain) to electrical signals. Aload cell usually consists of four strain gauges in a Wheatstone bridge configuration
http://en.wikipedia.org/wiki/Transducerhttp://en.wikipedia.org/wiki/Forcehttp://en.wikipedia.org/wiki/Electrical_signalhttp://en.wikipedia.org/wiki/Strain_gaugehttp://en.wikipedia.org/wiki/Strain_(materials_science)http://en.wikipedia.org/wiki/Wheatstone_bridgehttp://en.wikipedia.org/wiki/Wheatstone_bridgehttp://en.wikipedia.org/wiki/Strain_(materials_science)http://en.wikipedia.org/wiki/Strain_gaugehttp://en.wikipedia.org/wiki/Electrical_signalhttp://en.wikipedia.org/wiki/Forcehttp://en.wikipedia.org/wiki/Transducer


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BASIC PRINCIPLE OF STRAIN GAUGE LOAD CELLWhen steel cylinder is subjected to a force, it tends to change in dimension. On this cylinder, ifthe strain gauges are bonded, the strain gauge also is stretched or compressed, causing achange in its length and diameter. This change in dimension of the strain gauge causes itsresistance to change. This change in resistance or output voltage of the strain gauge becomes ameasure of applied force.

CONSTRUCTION OF STRAIN GAUGE LOAD CELLThe main parts of the strain gauge load cell are as follows. They are a cylinder made up of steelon which four identical strain gauge are mounted and out of four strain gauges, two of them (R1and R4) are mounted along the direction of the applied load(vertical gauges). The other twostrain gauges (R2 and R3 Horizontal gauges) are mounted circumferentially at right angles togauges R1 and R 4

OPERATION OF STRAIN GAUGE LOADCASE 1When there is no load (force) on the steel cylinder, all the four gauges will have the sameresistance. As the terminals N and P are at the same potential, the wheat stone bridge isbalanced and hence the output voltage will be zero.


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CASE 2Now the load (force) to be measured (say compression force) is applied on the steel cylinder.Due to this, the vertical gauges R1 and R4 will under go compression and hence there will be adecrease in resistance. At the same time, the horizontal gauges R2 and R3 will under go tensionand there will be an increase in resistance. Thus when strained, the resistance of the variousgauges change.

Now the terminal N and P will be at different potential and the change in output voltage due to theapplied load (force) becomes a measure of the applied load force when calibrated.

TEMPERATURE SENSIVITY


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TORQUE MEASUREMENT

Dynamometer

A dynamometer, is a device for measuring force, moment of force (torque), or power. For

example, the power produced by an engine, motor or other rotating prime mover can be

calculated by simultaneously measuring torque and rotational speed


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Water brake type absorber

The water brake absorber is sometimes mistakenly called a "hydraulic dynamometer." Water

brake absorbers are relatively common, having been manufactured for many years and noted

for their high power capability, small package, light weight, and relatively low manufacturing

cost as compared to other, quicker reacting "power absorber" types.

Their drawbacks are that they can take a relatively long period of time to "stabilize" their load

amount and the fact that they require a constant supply of water to the "water brake housing"

for cooling. The schematic shows the most common type of water brake, the variable level

type. Water is added until the engine is held at a steady RPM against the load. Water is then

kept at that level and replaced by constant draining and refilling, which is needed to carry

away the heat created by absorbing the horsepower. The housing attempt to rotate in

response to the torque produced but is restrained by the scale or torque metering cell that

measures the torque
http://en.wikipedia.org/wiki/Water_brakehttp://en.wikipedia.org/wiki/Water_brake


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HEENAN-FROUDE DYNAMOMETER

The construction and operation of the Heenan-Froude dynamometer may briefly be

described as follows.It consists of a rotor keyed to a shaft carried by bearings in an

outer casing. The casing in turn is mounted on anti-friction rollers, so that it is free toswivel, within limits, about the same axis as the shaft. The shaft, and rotor, are arranged

for coupling to the engine shaft, and revolve continuously inside the casing, through

which water is circulated.

Both rotor and casing are provided with a series of semielliptical cups or pockets. These

are formed by semi-circular vanes in a circular channel. Water is introduced into these

cups by means of a suitable inlet pipe connected with a channel formed at the back of the

cup;- in the casing, the water passing into the cups from this channel through holes

going right through the vanes or cup-walls on the casing. On the roior being revolved

with the casing full of water, the latter in the rotor cups is thrown outward, by

centrifugal force, into the casing cups, the action of which on the water in the latterbeing such as to form a vortex in the two cups.

It will readily be seen that when the rotor is in motion the water flowing from the cups of

the casing into those of the rotor has a considerable momentum imparted to it, and the

rate of change of momentum which occurs forms a definite force acting on the rotor,

tending to prevent its motion ; while, on the other hand, water leaving the rotor and

entering the cups on the casing has its momentum in the plane of rotation destroyed, and

consequently strives to rotate the casing. Owing to the manner in which the vanes or cup

wallsare inclined, the above action continues until the frictional resistance is just

sufficient to balance the force set up by acceleration of the water discs, and the higher

the velocity the greater will be the resisting torque. It is so that this vortex motion of thewater in the cups should not be disturbed that the water is introduced into the latter

through the holes in the cup walls previously mentioned.

In the Standard brake the capacity is controlled by- regulating the amount of water

retained in the brake cups. This is effected by means of valves at inlet and outlet,

automatically operated by the motion of the lever arm due to variation in load on the

engine under test. The load is applied in both types at the end of a lever arm rigidly fixed

to the brake casing, in the D.P. machines taking the form of a spring balance and fixed

weight, while in the standard machines by the suspension of dead weights. These weights

are varied to suit the engine, fine adjustment in the load being made by means of a

steelyard and poise provided for the purpose. The power absorbed is ascertained bymeans of the usual formula, but to render this in a simple form the lever arm


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is made of certain length. The formula therefore reads :

B.H.P. =

where W = load on the arm, N = revolutions per minute, K = constant.

ics 2.force and torque measurement notes

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