Let's start with some definitions.
In practical terms, hydraulic actuators are usually motors, or cylinders. Cylinders are also called linear actuators, since their motion is in a straight line. And hydraulic motors, of course, can also be referred to as rotary actuators.
Even though it's more to say, sometimes the term "rotary actuator" can be less confusing than "motor", especially when electric motors and heat engines start to enter the conversation.
The Rotary Actuators family is a little bigger than just hydraulic motors. It also includes semi-rotary actuators, like helical, rack and pinion, and vane actuators.
Let's look at a simple hydraulic system that uses hydraulic motors.
For the sake of this example, only the right side driving circuit is shown.
This Generic Brand™ skid steer loader relies on a diesel engine to turn the hydraulic pump. The pump pushes hydraulic fluid through the propulsion and steering system, into the hydraulic motor.
Energy starts out at a prime mover (engine, or electric motor) and is converted multiple times, only to end at the shaft of a hydraulic motor. Many hydraulic systems begin with rotational energy, and after several energy-losing conversions, arrive back at rotational energy once more. Why bother with the whole hydraulic system and going to a rotary actuator, anyway? Isn't that inefficient?
The conversion to the hydraulic motors is very necessary. If the wheels were driven directly by the engine, the operator would be deeply unhappy with the skid steer loader’s performance! Both sets of wheels could only go in the same direction, and reverse would not be an option.
The schematic symbol for a motor is a circle, with an arrow (or arrows) indicating the direction of flow. The arrow(s) will always point inward, representing flow into the motor.
Just like other schematic symbols, extra symbols can be "stacked" onto the basic motor symbol to indicate extra functionality.
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