Pumps create flow. They aspirate fluid at an inlet and displace it to an outlet. Displacement can take place in two ways. A nonpositive displacement pump picks up fluid and moves it. Examples would be a water wheel or the coolant pump used on a typical diesel engine.
A positive displacement pump not only creates flow but backs it up. Fluid picked up by a positive displacement pump is sealed and held while the pump turns. Most hydraulic circuits only use positive displacement pumps and we can divide these into two general categories:
• Fixed-displacement pumps
• Variable-displacement pumps
When studying pumps, you should remember that hydraulic pumps only create flow, not pressure. Pressure results from creating resistance to flow.
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