Machines are not completely efficient since part of their labor is spent to overcome friction. Work output is therefore always less than work intake. Mechanical efficiency, which is work output divided by work input, is used to calculate machine work. The mechanical efficiency of a machine depends on its design and how well it functions. Machines can have different levels of mechanical efficiency: high, medium, or low.
The most important factor affecting the mechanical efficiency of a machine is the ratio of the number of degrees of freedom (DOFs) to be moved or acted upon to the number of motors or other actuators that can operate on them. This ratio is called the "DOF/actuator" value and it should be as high as possible. For example, a machine with one axis of rotation and one linear motion device acting on it has a 2-DOF machine. A machine with two axes of rotation and two linear motion devices would be a 4-DOF machine.
As we move up in DOF/actuator values, mechanical efficiency increases. However, this increase becomes very small after about 10 DOF/actuators. After this point, adding more DOF/actuators does not significantly improve mechanical efficiency.
Another factor affecting mechanical efficiency is friction.
Machine Effectiveness Because some of the input work is utilized to overcome friction, the output work is always less than the input work. As a result, efficiency is never 100 percent. The closer a machine's efficiency is to 100 percent, the better it is in reducing friction.
Because the machine must use some of the effort to overcome friction, a machine always accomplishes less work on an item than the user does on the machine. The percentage of effort put into a machine by the user (input work) that is converted into work done by the machine is known as efficiency (output work). For example, if a person pushes a rock up a hill using a wheelbarrow and then rolls it back down, the person has done only 20% of the work required to move the rock up the hill. The rock will be slightly worn after being used for this task.
Efficiency varies between machines but is usually very low. A common example is the human-powered tractor, which turns many parts of our farm over each year. However, even these tractors are more efficient than most people realize. They use large engines that can lift heavy weights with little loss of energy. These engines could not be used effectively in other ways because they run on oil or gas which are expensive to buy and transport to farms across America.
The engine of a tractor is an excellent example of a high-efficiency motor. It uses almost all of the energy supplied to it and converts this energy into useful work in the form of speed. In fact, modern tractor motors are so efficient that one study estimated that the average tractor uses about 14% of the fuel consumed by a diesel car by weight!
Because a machine's output is always less than its input, it can never be entirely efficient. A certain percentage of work done on a machine is wasted due to friction and lifting some of the machine's moving elements.