JUNE-JULY 2018 Nov-Dec 2017 - Copy - Page 89

BOOK IN SERIES HYDRAULICS in Industrial and Mobile Applications CHAPTER 4 OIL HYDRAULIC PUMPS Get your copy of this book : http://www.assofluid.it/en/publications-requestt Oil hydraulic pumps are classified first as fixed or variable displacement pumps. E ven if pressure reaches the maximum limit and the prime mover can generate the power needed, flow in fixed displacement pumps is constant as long as the rotational speed of the mechanical generator stays the same. Flow rate changes when the number of revolutions on the transmission shaft declines: flow diminishes as revolutions per minute decrease and vice versa; pressure does not change (under specific conditions and if the load is constant). Special mechanisms inside the pump casing in variable displacement pumps decrease displacement from the maximum limit to zero flow (see chapter 5). The external intervention can be carried out automatically or manually by means of a hand wheel. In the first case, the dedicated controllers can be controlled with a mechanical spring, load sensing, electromechanical or electroproportional system. Figure 4.1 Like any other piston pump, their operating principle is based on the reciprocation of suction and delivery (Figure 4.2). Variable flow pumps (not all types are suitable) that can reverse the flow are available: their outlet becomes the inlet while their prime mover has a constant direction, which is the prerequisite for closed circuit applications like wheel and track drive or hoist and winch handling. FIXED DISPLACEMENT PUMPS Manual lever pumps Manual pumps (Figure 4.1) consist of a piston sliding inside its cylinder and operated by a lever that can be moved by hand or with a spring return pedal during suction depending on the applications. www.ghmediabusiness.com Figure 4.2 By pulling the lever (10) upward, the rod (6) moves the piston (1) during suction: the inlet valve (2) opens allowing the oil to flow into the cylinder; the intermediate valve (3) prevents the delivered fluid from flowing back into the pump casing (9). At the upper dead centre, delivery is started by pushing the lever downward: the inlet valve (2) closes preventing the fluid to flow back into the tank, the intermediate valve (3) allows the fluid to flow during the delivery. Seals (5) block leakages between the pump casing (9) and the rod; the plug 89 The hydraulic pumps described in this paragraph can have only a fixed displacement for a number of mechanical reasons.