An Actual machine is that in which there is a loss of energy in some manner. In an actual machine, the work output is less than the work input, i.e., efficiency of an actual machine is always less than 100%.

In Actual machines, mechanical advantage is less than velocity ratio.

Reversible machine If a machine is capable of doing some work in the reversed direction, after the effort is removed, then the machine is known as reversible machine. The condition for a machine to be reversible is that its efficiency should be more than 50 %.
Non- reversible or self locking machine If a machine is not capable of doing some work in the reversed direction, after the effort is removed, then the machine is known as non-reversible or self locking machine. The condition for a machine to be non-reversible or self locking is that its efficiency should be less than 50 %.

A machine having an efficiency greater than 50 percent, is known as reversible machine.

A machine having an efficiency less than 50 percent, is known as irreversible machine.

A machine having an efficiency equal to 100 percent, is known as ideal machine.

Non-reversible Machine If a machine is not capable of doing some work in the reversed direction, after the effort is removed, then the machine is known as non-reversible or self locking machine. The condition for a machine to be non-reversible or self locking is that its efficiency should be less than 50%.
A screw jack used for lifting the load is a non-reversible machine or self locking machine.