For this week's Seek & Geek, I wanted to find a cool mechanism and, after some browsing, started looking at different types of pumps.
Just a few days back, for example, there was an event at one of the machine shops on campus (MIT MakerWorks), where some students were building their own pocket-sized impeller pumps. These are the kind you most often use for air mattresses.
Gear pumps are pretty common in hydraulic systems, and as far as I know them, they typically provide a constant displacement. The issue with this type of system, however, is that if you are actuating pistons, for example, you will have to deal with the oil that the pump is moving around even after the cylinder has filled up. In a simple system, you would do this thru a relief valve. But, doing so isn't terribly efficient and tends to heat up the oil pretty quickly - even if you have a sizeable reservoir. I've seen some machines that were built this way and didn't accommodate such a scenario. If you remember how oil gets thin as it heats up, and sometimes even starts burning up (think of butter, olive oil and vegetable oil in the kitchen), you might notice how this can go badly for a machine too.
Of course, there are also pumps that supply variable displacement. The swash plate piston pump is one of these, tho I wasn't terribly familiar with its mechanism. I set out to understand how these devices work, including how you can build feedback into them, and to find a good reference I could share with you all!
Here are two links that I thought illustrated this mechanism fairly well:
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