Hydraulic Pumps
A variety of different kinds of hydraulic pumps exist, but all of them have at least one thing in common ... pressure. A hydraulic pump works by creating pressure to move a liquid, which in most cases is water. Some different types of hydraulic pumps include those that work off of gears, those that work off of screws and those that use simple gravity to create a water flow. Additionally, a water pump can be reverse-engineered to drive a motor with the use of pressurized liquid. Whatever the method, the key is that pressure creates movement.
      Water Flow
Water has weight, and that's what a hydraulic ram pump operates off of. With these pumps, an uphill reservoir allows water to flow through a main pipe and build up force. When enough force builds up, a valve quickly closes in the pipe. The increase in pressure forces a valve that leads to a second pipe (which leads upward from the pump) to open, and the water then defies gravity and shoots up. This continues until the built-up pressure drops; then the original valve reopens to start the water flowing, beginning the cycle again.
      Alternate Pressure Makers
A hydraulic ram pump requires no power to function, but other forms of hydraulic pumps work differently. Whether it's the spinning of gears, the pounding of pistons or the turning of screws, these mechanical energies can all be used to create pressure in a fluid-filled chamber. Once that pressure is created, the fluid moves upward just like in a hydraulic ram pump. However, it takes another form of power to move the screws, pistons or gears, and that usually requires an electric motor.