Efficiency. The efficiency of a machine is defined as the ratio of work supplied to work put into it. Although oiling any sliding or rotating components helps reduce friction, all machines create some friction. Because of internal friction, simple machines always have an efficiency less than 1.0. However, a motor is more efficient than an engine. The efficiency of a motor is usually expressed by **its torque** divided by its rotational speed. A motor with high torque for its size and low rotational speed is very efficient.

Examples: An electric drill has a maximum efficiency of about 95 percent because it converts only 5 percent of the electrical energy into mechanical work. A battery-powered tractor has a maximum efficiency of about 70 percent because it converts about 30 percent of the chemical energy in the batteries to mechanical work. These numbers show that even though simple machines are easy to construct, they are not very efficient. This limits their use to applications where cost is not important and alternative methods are available.

The efficiency of **any machine** will decrease if it is used continuously instead of being rested occasionally. This is because continuous operation requires constant maintenance by either replacing parts that wear out or adding more fuel to keep up with **increased heat production**. Machines used regularly may need additional insulation, while those used rarely can be built from **double walls** or insulated containers to save on operating costs.

Simple machines can be used together to increase their efficiency.

The efficiency of any machine is determined by how much friction and other variables lower the machine's actual work output from **its theoretical maximum**. A frictionless machine would be completely efficient. In practice, however, machines are not perfectly smooth, and some energy is lost due to friction. The amount of loss is very small but it adds up over time, so that over many cycles the total loss becomes significant.

In general, the efficiency of a machine is expressed as a percentage. This means that if you double the number of hours that a machine runs, then you should get an increase in **its actual work output** of **100 times** (or 2%, or 0.5%). If the machine was inefficient, then this would not be the case: if it were twice as efficient as it actually is, then its efficiency would be 50%, not 100%.

For example, let's say that a machine can lift 10 pounds for 1 hour and 20 minutes without losing any energy. This would be represented by the equation W = 10 lb × 60 min ÷ 1 hr. 20 min = 0.45 kW or 450 watts. The machine is said to have an efficiency of 45% because it produces 90% of its possible output. If we doubled the number of hours that the machine ran, we would expect it to produce 180 watts instead.

A machine with a 20% efficiency produces just one-fifth of its potential output. A standard motor for a refrigerator/freezer has an efficiency of about 90%. A standard household washing machine has an efficiency of about 40%.

Efficiency can also be described as the useable power of a system divided by the total power input to **that system**. That is, it is the ratio of actual work done to potential work available from its source. For example, if a motor drives a fan which in turn blows air into a room, then the efficiency of the fan is defined as the rate at which it moves air compared to its possible movement; that is, it is the number of rotations of the fan per unit time (its speed) divided by the total number of rotations of **its shaft** over time (its torque). Here, the fan is moving **more air** per rotation but because it is rotating more slowly, its efficiency is less than 100%.

In practice, the efficiency of most machines is not exactly 90% or 40%, but rather between these values. The main reason for this is that some of the energy put into the machine is lost in heat, radiation, or both. Other factors reducing efficiency include friction, incorrect gearing, etc.

By minimizing friction, the efficiency of a basic machine may be boosted. However, oiling and greasing the machine is an effective approach to boost its efficiency. Additionally, using high-quality parts will provide better performance with less maintenance.

The three most common types of machines are motors, pumps, and generators. Motors use electricity to turn a shaft, which in turn turns other components such as fans or lights. Pumps use energy from outside sources (such as springs) to lift water or other fluids. Generators produce electricity from **mechanical movement** (rotation) such as that produced by a motor or flowing water. All electric power generators require some form of moving part (usually a spinning disk or rotor) to convert mechanical movement into electrical power. Smaller scale generators used in **home appliances** and electronics need not be efficient, but they do need to be able to function with very low output levels when needed. Larger scale generators used in power plants need to be highly efficient because even small losses can add up over time.

Electricity is the flow of electrons through a conductor such as a copper wire. Electric circuits inside equipment use these flows to operate components such as motors, heaters, and lights.