Answers, Remarks and Hints to Points to Ponder

Point to Ponder: 1

A.      Can you draw an inlet pressure flow characteristics of the ball type check valve?

Ans: Let a positive pressure drop correspond to the forward flow direction. Then, because of spring force, a minimum positive pressure drop would be needed to establish full flow. Similarly, below a minimum pressure drop, no flow would take place. For negative pressure drop, the flow is zero.

B.      What should be the nature of the spring used in the check valves, hard or soft?

Ans:  With a hard spring, because of spring force, the minimum positive pressure drop needed to establish full flow would be high. Therefore the spring should be soft, only ensuring that when the pressure drop across the valve is very small, the flow is cut off.

Point to Ponder: 2

A.      In Fig, what is the position of the valve when there is no pressure at both the pilot ports?

Ans: The valve shown is a two-position four-way valve. The spring symbol indicates that if no pilot pressure exists at either port, the position is determined by the spring. Since the spring is shown at the left, the left box shows the position of the valve when pilot pressures at the ports are zero.

B.      Is it possible to connect directional control valves in series and/or parallel? Can anything be achieved thereby? Are there conditions in which this is possible?

Ans: The valves can easily be connected in parallel, provided the pump is capable of delivering the required flow. In case they are connected in series, the tank port of the first valve is connected to the pump port of the other. For two position valves this is possible. However for three position valves, for independent operation of the two loads, the valve should be open center type, that is the pump and tank ports should be connected. If these ports remain plugged then independent operation of loads would not be possible.

Point to Ponder: 3

A.      Consider the type 4 and type 6 centre conditions of the 3-position 4 way valve shown above. Imagine that a double acting hydraulic cylinder is connected across the ports A and B. Comment on the possible motion of the load when the valve is at the center.

    Ans: For the type 4 valve, the load is locked, since both the ports A and B are plugged. For the type 6 valve, the load is floating, since both the ports A and B are connected and are at tank pressure. Therefore with any pressure applied on the load, the cylinder can shift without resistance.

B.      Can you justify, why the bourdon tube is perhaps the most suitable of the pressure sensor sin this application?

    Ans: For hydraulic circuits, continuous pressure monitoring is not needed. Generally intermittent manual monitoring is needed for maintenance and trouble shooting. Therefore gauges such as diaphragm gauges, which can operate at similar ranges of pressure and can provide continuous electrical signals is a more expensive alternative. Bourdon gauges are inexpensive, rugged and fairly accurate for the application.