LEEA foundation part-3.pdf - The Lever and Moments of Force At the beginning of this module we mentioned that a lever may be considered as the most

LEEA foundation part-3.pdf - The Lever and Moments of Force...

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©LEEA Academy Foundation Step Notes Jan 2017 v1.5 39 The Lever and Moments of Force At the beginning of this module, we mentioned that a lever may be considered as the most basic of machines. There are many examples of the Lever in our everyday life, some are obvious others not so obvious, e.g. a spanner is an obvious example but a less obvious example is a jib crane. When a force is applied to a Lever it gives it a turning effect, which is known as the Moment of Force or Turning Moment . Moment of Force = Force x shortest distance to the line of action of the force When we apply a force (F) on the spanner at a distance (d) from the centre of the bolt, the turning moment on the bolt is F x d Torque Torque is important to the examiner and tester. For example, the nuts of wire rope grips must be set to the correct torque, as must the foundation bolts of crane structures. These are set using a Torque Wrench, which allows the nuts to be tightened to a known torque. In the course of our duties it is often necessary to check that nuts have been correctly tightened to the required torque. Notes:
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©LEEA Academy Foundation Step Notes Jan 2017 v1.5 40 The Couple This type of Turning Moment is known as a Couple, where two equal and parallel forces are applied symmetrically about a point. Here they act about a bolt centre (0). The forces here are acting anti clockwise and would cause the bolt to turn. If however the forces were applied in opposite directions the lever would remain stationary, that is in Equilibrium, as the Anti clockwise Moment about point 0 = Clockwise Moment about point 0. Point ‘0’ is the Pivot Point or Fulcrum about which the forces act. Mechanical Advantage Any increase in the effort will move the load. In more complicated machines, e.g. a hand chain hoist, the effort required to move the load is usually much smaller than the load. Their relationship is known as the Mechanical Advantage. The mechanical advantage allows the device to perform the task for which it was designed. If we consider the Mechanical Advantage of a simple winch, we can see that by increasing the lines of cable between the winch and the vehicle being pulled, we can pull more than the working load limit of the winch, e.g: 1t WLL winch and 1 line of cable = 1t Max. tow weight 1t WLL winch and 2 lines of cable = 2t Max. tow weight 1t WLL winch and 3 lines of cable = 3t Max. tow weight (ignoring friction) Notes:
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©LEEA Academy Foundation Step Notes Jan 2017 v1.5 41 A chain hoist is operated by hand. An operator will pull down on one of the chain loops on one side of the chain. This will turn a pulley mechanism inside the chain hoist housing. When this pulley turns, it will lift up the end of the other chain which usually has a hook on the end.
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