Category: Lifting Machines

In the case of self-locking machine, after load removal friction force overcomes the work done by the load in reverse direction. Thus for an irreversible of self-locking machine efficiency should be less than 50%. Example 13.1: In a lifting machine, it was found that the effort was moved by 300 mm to lift a load by…

  Frictional loss = Input – Output = P × y – W × x   When effort is removed, i.e., P = 0. The work done by the load exceeds the frictional loss and load starts to move in reverse direction. Thus for a reversible machine the efficiency must be greater than 50%.

The machine which starts to move in reverse direction after removal of effort, is called reversible machine. Simple rope and pulley drive is a reversible machine. The machine, which does not move in reverse direction after removal of effort, is called irreversible or self-locking machine. Screw jack, worm and worm wheel, and winch crab are examples of irreversible machine.

From law of machine For maximum mechanical advantage denominator should be minimum. Therefore,  will be minimum when W will be very large in comparison to C, i.e.,  is negligible. Hence, maximum mechanical advantage,  Maximum efficiency,  The graphical representation of relationship between load and efficiency is shown in Figure 13.3. Figure 13.3 Relationship Between Load and Efficiency

Frictional loss is the part of the inputs or work done by the effort used to overcome the friction of the machine. No machine can be 100% frictionless; therefore, some parts of the inputs are always used to overcome the friction of the machine. Thus, the efficiency of the real machine always lies below 100%.…

Velocity Ratio (VR): It is ratio of distance moved per unit time by effort to the distance moved per unit time by load. , where y is distance moved by the effort (P), and x is distance moved by the load (W). Mechanical Advantage (MA): It is ratio of load lifted and effort applied on a lifting machine. Its significance is…

A machine is a device, which is capable to do some work on application of some effort. Here, effort means input in the form of force or energy supplied to the machine to do work. The work may be explained in the form of load lifting. An illustrative example of a lifting machine is explained…