Shuttle Valve — Only One Can Win

This type of valve belongs to the on-off valve category. There are many varieties, but they share common characteristics: at least three ports (two inlets, one outlet); the spool position (flow path opening/closing) is determined by the pressure at the two inlets; at steady state, only one inlet is connected to the outlet. They can be divided into two types: high-pressure-through and low-pressure-through.

1. Low-Pressure-Through

The spool moves under the opposing action of the two inlet pressures, connecting the lower-pressure inlet to the outlet. This is commonly used in closed circuits to discharge hot oil, hence also called a “hot oil discharge valve.”

The graphic symbols for hot oil discharge valves come in various forms; some show 3 states, with dashed lines indicating the middle is a transitional state.

In closed circuits: Auxiliary pump P2 draws cold oil from the tank, which enters the low-pressure side of the circuit (motor return side, usually hotter) through a check valve; the hot oil discharge valve connects the low-pressure side to the relief valve; since this relief valve’s set pressure is lower than the other relief valve, it opens to discharge hot oil; the other relief valve serves only as a safety valve to protect pump P2.

Because continuous hot oil discharge is needed, there are requirements for the pressure differential-flow characteristics of this valve, but response sensitivity requirements are not high.

2. High-Pressure-Through

The spool moves under inlet pressure action, connecting the higher-pressure inlet to the outlet (“fence-sitter” characteristic). Hereafter, “shuttle valve” specifically refers to this type.

ISO 1219-1:2006 recommends corresponding graphic symbols; some manufacturers use different forms to represent ball spools or spool valves, with some spool valves having larger openings.

(1) Types

1. Spool forms: ball, cone, spool. 2) Spring configuration: most shuttle valves have no spring; spring-loaded shuttle valves function similarly to a combination of two check valves. 3) Biased type: pressure at one port must exceed the other port’s pressure plus the spring preload to connect to the outlet. 4) Mounting methods: commonly screw-in type, also embedded type. 5) Multi-inlet type: available in three-inlet, four-inlet, and other forms.

(2) Applications

1. Difference from check valve combinations: A shuttle valve has only one spool, so one port is always connected; outlet pressure cannot simultaneously exceed both inlet pressures. A combination of two check valves has two spools and may simultaneously block both inlets, causing outlet pressure to “stagnate” and fail to accurately transmit inlet pressure.
2. Brake circuits: The shuttle valve connects the brake to the high-pressure side of the motor; when pressure exceeds the brake spring force, the brake releases; when the directional valve is in neutral, the motor has no pressure, and the brake spring locks.
3. Crane hoist drum circuits: An additional pilot-operated 2-position 3-way valve is added so that the brake releases only when the drive pressure simultaneously exceeds both this valve’s spring preload and the brake spring force (improving reliability).
4. Load-sensing circuits: Used to select the highest value from multiple load pressures and transmit it to the variable pump displacement control mechanism (as the basis for pump outlet pressure); almost every load-sensing valve section is equipped with a shuttle valve.

For example, in a system driving 4 actuators, load pressures must pass through multiple shuttle valves, easily causing sluggish response; excavator circuits (7-8 actuators) require multiple standard shuttle valves; using multi-inlet shuttle valves can improve the series-connection sluggishness problem.

(3) Characteristics

When selecting shuttle valves, pay attention to: allowable pressure, pressure differential-flow characteristics, internal leakage. Different scenarios have different requirements:

• Load-sensing circuits: Response sensitivity is the core requirement; the spool should be light, and the opening cannot be too small (a small flow is needed to transmit pressure);
• Brake circuits: Flow requirements exist, but response sensitivity requirements are low.