Figures 15-1 through 15-5 show the schematic symbols for several fixed-displacement pumps. Use fixed-displacement pumps in simple, one- or two-cylinder circuits that never stop under pressure. Also use them for single-speed motor circuits, or circuits where several cylinders operate simultaneously but never stop and hold at full pressure. Fixed-displacement pumps always move a set volume of fluid at a pressure between that dictated by resistance and the maximum relief valve setting. Blocking the outlet of a fixed-displacement pump sends excess flow through the relief valve to tank. When fluid goes across the relief valve at pressure, all of the input energy generates heat.

Fig. 15-1Fixed-displacement pumps may be gear, gerotor, vane, or piston types. The most common are gear and vane. They are relatively inexpensive, very reliable, and generate little heat when used correctly.

Gear and vane pumps come in a wide variety of configurations. Figures 15-1 through 15-3 show one or more pumps in a single housing. The pumps may share a common inlet or have multiple inlets. Most combination pumps have separate outlets for use in different sub-circuits. The flow from each pump in the combination may be the same or different.

Fig. 15-2Figure 15-4 shows the symbol for a self-contained double pump for a high-low circuit. Flow from both pumps moves an actuator to and from the work at low pressure. The high-volume pump unloads through an integral unloading valve at work contact. This leaves all motor horsepower to drive the low-volume/high-pressure pump. This circuit usually consumes less horsepower without sacrificing cycle time. The packaged pump represented here is compact and inexpensive, but any double pump with the correct valves can supply a high-low circuit.

Fig. 15-3

Many manufacturers produce thru-drive pumps like the one shown in Figure 15-5. A double-shafted electric motor normally operates both pumps. With a thru-drive pump, a second pump is bolted to and driven by the shaft of the first pump. When connecting more than two pumps, consider some possible problems: will the shaft of the first pump handle the torque of additional pumps; will additional pumps result in too much overhung load from too many pumps.

Fig.15-4Fixed-displacement pump circuits
Figure 15-6 shows a schematic circuit for a fixed-displacement pump operating a single cylinder. At rest, the pump unloads through a tandem-center valve at minimum pressure. When the cylinder extends, pressure is whatever it takes to stroke the cylinder. When the cylinder contacts the work, pressure increases to whatever it takes to perform the work. As the cylinder retracts, pressure is whatever it takes to return the cylinder and load. At no time does the relief valve dump oil to tank. Therefore, this circuit operates with little heat and should not require a heat exchanger when using high-efficiency parts.

Fig. 15-5Figure 15-7 depicts one way to use fixed-displacement pumps in a multiple-cylinder circuit. Each of the three cylinders in this example has a separate pump, relief valve, and directional valve. The actuators move at the desired speed and force because each pump’s flow and relief valve settings match their cylinder’s work requirement. Because there are no flow controls, the relief valves never dump excess fluid, allowing all input energy to do useful work. Heat should not be a problem in this circuit.
















Two pumps supply CYL3 to stroke it rapidly. This circuit works best when CYL2 does not cycle at the same time CYL3 does.












It takes time to design efficient circuits, but the results pay off in future savings. The high-low circuit in Figure 15-8 — which cycles a large fast-stroking cylinder — saves on both first cost and operating cost. If a single 60-gpm pump operating at 3000 psi were used, a 120-hp motor would be required. By substituting a double pump with 60- and 10-gpm sections, the motor size can be reduced without sacrificing cycle time. The big difference occurs because moving the cylinder at say 450 and 500 psi, only requires 20.4 hp. When the cylinder meets resistance and pressure builds to about 500 psi or higher, the 60-gpm pump section unloads at no pressure while the 10-gpm pump does the work. The 10-gpm pump at 3000 psi requires 17.5 hp. Although work speed is slower, travel time is faster. With a little figuring, it’s easy to save money on the electric motor and controls up front, and reduce energy cost for the life of the machine.

Fig. 15-8


















In Figure 15-9, a fixed high-displacement thru-drive pump, coupled with a low-displacement, pressure-compensated pump, creates a different kind of high-low circuit. This circuit provides fast travel and then maintains clamping pressure for extended periods with little heat generation. The circuit operation is the same as Figure 15-8. It requires no special electric controls because the unloading valve automatically dumps the high-displacement pump at any pressure above 400 psi. The low-displacement’ pressure-compensated pump reduces energy cost and heating. This pump arrangement takes the place of a large pressure-compensated pump in certain applications.

Fig. 15-9