SELF – LOCKING MACHINE
Sometimes, a machine is not capable of doing any work, in the reversed direction, after the effort is removed. Such a machine is called a non-reversible or self – locking machine. A little consideration will show, that the condition for a machine to be non –reversible of self-locking is that its efficiency should not be more than 50%.
FRICTION IN A MACHINE
In previous articles, we have not taken the friction of the lifting machines into account. Or in other in words, we have assumed the machines to be friction-less. But this is a rare phenomenon, as every machine cannot be friction-less. It has been observed that there is always some amount of friction present in every machine, which can be expressed on a graph effort and load. If we record the various values of efforts required to raise the corresponding loads and plot a graph between effort and load we shall get a straight line AB as shown figure.
It may be noted from the graph that the intercept OA represents the amount of friction present in the machine. The machine friction may be expressed either on the effort side or on the load side. If expressed, on the effort side, the friction force. But if expressed on the load side, the friction may be defined as the additional load that can be lifted or the additional resistance that can be overcome.