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2. Consider the load on the system
The output is defined by the load which the system element is driving ie: the next element in the power transmission system. The input is defined by the need to provide power to the element under consideration. General characteristics include dimensional, structural, environmental, operational and commercial factors.

Although it might seem more logical to define the element input first, it is usually the output which is more important. The reason for this is that the power transmitted is determined by the load which must be driven. A correctly designed system will allow the load to draw the power required for normal operation from the power source.

The correct approach to selecting the first element in the rotary power transmission system (the motor) is to consider first the output characteristics of torque, speed, power etc. and the variations of these with each other and with time.

This means that the rotary source should not be selected on the basis of its being electrical, hydraulic, pneumatic etc. but only on the basis of its output. This procedure will allow the designer lo keep options open, in terms of energy source, in an application where the traditional 'electric motor' approach may not be appropriate.

An example might be found in a system which requires short bursts of high torque at low speed. A hydraulic motor, driven by energy stored in accumulators might provide a better solution than loading the electrical supply with sudden surges. Although the element will be selected primarily on the basis of its output characteristics there are many applications where its input parameters are important. Such an element is a gearbox where the input is an integral part of the matching process.