Diaphragm operated devices are essentially pistons operating within a cylinder. They transfer pressure from one sealed area to another. In the case of hydraulic and pneumatic cylinders, the moving piston is sealed off by a sealing device such as an o'ring, cup, vee packing, or a similar device. A diaphragm merely limits the stroke and provides a frictionless seal. Diaphragms generally cost less, have more tolerance range, and do not require hardened super finishes or lubrication.