Flow and pressure controls, part II: flow dividers - Hydraulic Systems Trends

Diesel Progress North American Edition, Jan, 2003 by Russ Henke

In our last discussion, we examined flow and pressure controls with an overview of valve control of flow rates and methods of valve control. Now let's take a look at flow dividers and pump control of flow rates.

Flow Dividers

A flow divider valve is a form of pressure-compensated flow control valve which received one input flow and splits it into two output flows. The valve can deliver equal flow rates in each stream or a predetermined ratio of flow rates. Fig. 1 shows how a flow divider could be used to roughly synchronize two cylinders in a meter-in circuit. It should be pointed out that flow dividers, like all pressure and flow control devices, operate over a narrow bandwidth rather than at one set point. Thus, there will be a variation of flow rates in the secondary branches, and precise synchronization cannot be achieved with a flow divider valve alone.

Flow dividers can also be used in meter-out circuit configurations. Bleed-off does not affect flow divider valves. They can also be "cascaded, i.e., connected in series, to control multiple actuator circuits as in Fig. 2.

Rotary Flow Dividers

Another technique for dividing one input flow into proportional, multiple-branch output flows is the rotary flow divider (Fig. 3). A rotary flow divider consists of several hydraulic motors connected together in parallel by a common shaft. One output fluid stream is split into as many output streams as there are motor sections in the flow divider. Since all motor sections turn at the same speed, output stream flow rates are proportional to the sum of displacements of the motor sections. Rotary flow dividers usually have larger capacities than available flow divider valves.

The pressure drop across each motor section is relatively small because no energy is delivered to an external load, as is usually the case with a hydraulic motor. However, the designer cannot overlook pressure intensification generated by a rotary flow divider. If for any reason the load pressure in one or more branches drops to some lower level or to zero, full differential pressure will be felt across the motor sections in the particular branch or branches. The section thus pres surized will act as hydraulic motors and drive the remaining sections as pumps. This results in an elevated or intensified pressure in these circuit branches. Caution must be exercised in applying rotary flow dividers to minimize the potential for pressure intensification. Rotary flow dividers can also integrate multiple branch return flows into a single return flow.

Pump Control Of Flow

Pump control of flow rate presupposes the use of a variable displacement pump. Of the three basic types of pumps most commonly used in fluid power applications -- gear, vane and piston -- only the vane and piston types are currently made in variable designs.

Non pressure compensated pumps require an auxiliary control to shift the pumping element to vary the pump's displacement. These auxiliary controls are available in hydraulic, pneumatic, mechanical, and electrical versions to match the needs of most control applications. Though pressure-compensated pumps are usually considered to be pressure control devices, the designer must remember that control is a chieved by reducing the displacement of the pump when a set pressure level is reached. Thus, a change in flow rate is involved. If this change occurs while the actuator is still in motion, it is reflected as a change in actuator speed.

The purpose of flow control is speed control. All the devices discussed in this chapter control the speed of the actuator by controlling the rate of flow to it. Flow rate also determines rate of energy transfer at any given pressure. The two are related in that the actuator force multiplied by the distance through which it moves (the stroke) equals the work done on the load. The energy transferred must also equal the work done. The speed of the actuator determines the rate of energy transfer (i.e., horsepower), and speed is thus a function of flow rate, Fig. 4.

Direction Control

The third basic control area in fluid power technology is directional control. Directional control does not deal primarily with energy control, but rather with directing the energy transfer stream to the proper place in the system at the proper time. Directional control valves can be thought of as fluid switches which made the desired "contacts," that is, they direct the high energy input stream to the actuator inlet, and provide a return path for the lower energy oil.

That this is an important function can be inferred from the scores of different directional control valve configurations available in the marketplace. Moreover, it is of little consequence to control the energy transfer of the system via pressure and flow controls, if the flow stream does not arrive at the right place at the right time. Thus a secondary function of direction control devices might be defined as the timing of cycle events. Since fluid flow can be throttled in a directional control valve, some measure of flow rate or pressure control can also be achieved with these valves.