I have worked in large industry with many different type pumps.
One of the simplest pumps I have ever worked on, is chainsaw oiler pumps.
Most are a tube with a close tolerance solid shaft inside that has a shallow flat machined across the outside diameter on the end and is rotated by a gear. Inline with the flat, on either side of tube, is a feed fitting and a discharge fitting.
I have never understood how, when the shaft is rotated, they can pump oil from the oil tank to the chain and bar, with no check valves of any type involved.
One of the simplest pumps I have ever worked on, is chainsaw oiler pumps.
Most are a tube with a close tolerance solid shaft inside that has a shallow flat machined across the outside diameter on the end and is rotated by a gear. Inline with the flat, on either side of tube, is a feed fitting and a discharge fitting.
I have never understood how, when the shaft is rotated, they can pump oil from the oil tank to the chain and bar, with no check valves of any type involved.