Figure 3-9.-A pointer’s handwheel.Figure 3-10.-Developing a torque.Look at figure 3-9. When this gun pointer pulls on onehandle and pushes on the other, he’s producing a couple.If he cranks only with his right hand, he no longer has acouple—just a simple first-class lever! And he’d haveto push twice as hard with one hand.A system of gears-a gear train-transmits themotion to the barrel. A look at figure 3-10 will help youto figure the forces involved. The radius of the wheel is6 inches—1/2 foot-and turns each handle with a forceof 20 pounds. The moment on the top that rotates thewheel in a clockwise direction is equal to 20 x 1/2 = 10ft-lb. The bottom handle also rotates the wheel in thesame direction with an equal moment. Thus, the totaltwist or torque on the wheel is 10 + 10 = 20 ft-lb. To getthe same moment with one hand, apply a 20-poundforce. The radius of the wheel would have to be twiceas much—12 inches—or one foot. The couple is aconvenient arrangement of the wheel-and-axlemachine.SUMMARYHere is a quick review of the wheel and axle-factsyou should have straight in your mind:A wheel-and-axle machine has the wheel fixedrigidly to the axle. The wheel and the axle turntogether.Use the wheel and axle to magnify your effort or tospeed it up.You call the effect of a force rotating an objectaround an axis or fulcrum a moment of force,or simply a moment.When an object is at rest or is moving steadily, theclockwise moments are just equal and oppositeto the counterclockwise moments.Moments of force depend upon two factors: (1) theamount of the force and (2) the distance fromthe fulcrum or axis to the point where the forceis applied.When you apply two equal forces at equal distanceson opposite sides of a fulcrum and move thoseforces in opposite directions so they both tendto cause rotation about the fulcrum, you have acouple.3-6
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business
