JUNE-JULY 2018 Nov-Dec 2017 - Copy - Page 90

BOOK IN SERIES (8) facilitates piston maintenance and disassembly as well as oil change. These pumps are usually directly connected to a small tank matched with the pump itself. Figure 4.4 Figure 4.3 Since manual operated systems cannot deliver a substantial flow, lever pumps are employed only in simple applications with single-acting short-stroke cylinders (return results from the load thrust); yet, this is for sure the most widespread pump worldwide because it is used in countless fields of application. A pressure of 700 bar can be generated with a lever carefully dimensioned vis-à-vis the fulcrum and it can be even as much as almost 2800 bar in some special systems. In double-acting manual pumps (Figure 4.4), when the lever is operated, the fluid continuously flows out from the outlet due to the recurrence of the inlet/outlet phases of the two pumping parts. By acting on the by-pass valve grip, the outlet (which is double in this case due to a possible second actuator) connects to the inlet, putting the actuator on stand-by. The pistons suck and deliver the fluid alternately when the by-pass is closed and the lever is operated. 90 Two-stage versions with two coaxial pistons are available for the applications characterised by a long phase of approach to the load condition without resistance (low pressure and maximum flow) and a major stress during the final stage (maximum pressure and low flow): in the first stage the larger piston ensures the maximum flow whereas, when a small lever stops it, the second piston (whose bore is dramatically smaller) helps reaching the maximum pressure. Special stainless steel systems for water applications are available. v The following list includes only few applications of manual pumps: v Jacks for general lifting purposes (short stroke) v Pullers v Bolt cutters v Parallelogram platforms v Emergency pump in series with automatic pump (for instance, if there is no tension the machine must be handled for safety reasons) v Tensioning of steel ropes v Trolley jacks (essential in repair shops) v Portable cranes (for short-range movements of engine test benches,containers,etc.) v Small presses for occasional deep drawing or bending v Flexible hose fittings v Benchtop pipe cutters, bending machines and tube closures v Wine torques (for small-sized businesses because large- sized companies use complex pneumatic systems) v Laboratory tests on pressures up to 2000 bar External gear pumps Apart from manual pumps, which are used (albeit widely) only for simple flying applications, external gear pumps are the simplest and most common oil hydraulic pumps operated by a motor. Their success is accounted for by a number of advantages, like their extreme lightness, mechanical simplicity, wide-range viscosity tolerance, optimum suction, their wide range of flow rates, adaptability to any position and space and, last but not least, their cost, which makes them one of the cheapest types on the market. www.ghmediabusiness.com