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330:148 (g)Machine Design

2. Force, Work and PowerNageswara Rao Posinasetti

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Objectives

Understand the difference between force, work, and power.

Recognize and be able to convert between mass and weight.

Convert between English and metric units for force, work,and power.

Understand the basic principles of fluid mechanics as theyapply to hydraulic and air cylinders or similar products.

Apply the principles of work, force, and power to movingmachines.

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Weight, Force and Mass

Weight, W = m gW = weight, lb or N

M = Mass, lb or kgg = acceleration due to gravity,

32.2 ft/s 2, 9.81 m/s 2

Force, F = m aa = acceleration, m/s 2

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Work and Power

Work = Force Distance ft-lb or N mWork done, W = F d

Power is rate of doing work

Power, P =

t = time, s

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t

W

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Power and Speed

1 hp = 550 ft-lb/s= 6600 in-lb/s= 33,000 ft-lb/min= 396,000 in-lb/min

Power in SI Units is Watts1 hp = 746 W = 0.746 kW

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Power and Speed

P = T P = Power, ft-lb/s or WT = Torque, ft-lb or N m

= Rotational speed in radians/secondT =

=

n = revolutions per minute

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P

60

n2

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Torque

Torque is a twisting momentRotates a part in relation to other

T = F dF = force appliedd = distance

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Calculate the amount of torque in ashaft transmitting 750 W of power

while rotating at 183 rad/s.

P = T Torque = 750 / 183 = 4.09836 N-m

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Power and Rotational Speed

P = T Since, =

We get, P =

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60

n2

60nT2

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tHW

TimeWork

Power

min1R2W

nT2Power

396,000nT2

powerHorse

63,000nT

powerHorse

T = torque (in-lb)

n = rotational speed, rpm

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Pressure, Force and Area

Pneumatic and Hydraulic CylindersForce = Pressure Area

Work done = Force Distance movedDistance moved = stroke length

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Moments of Inertia and

Section ModulusProperties of beamsMoment of Inertia

Section Modulus =

See Appendix 3 (p 469) for different sections

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axisneutralthefromDistance Inertiaof Moment

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