The direct acting relief valve is designed for controlling the fluid pressure in hydraulic system. Nearly every hydraulic system needs relief vavle to protect or control the system, the direct acting relief is common equipped in the machines as well. –

1. Spool Of Direct Acting Relief Valve Types:

The spool type direct acting relief valve is a kind of spool type, pipeline (threaded) connection direct acting pressure relief valve. There two oil ports of P inlet port and T outlet port on the right and left side of valve housing, connected to hydraulic system with hydraulic connectors, therefore it belongs to threaded connection valve. As you can see from the Picture A, there is an inner drain port “e” in the valve housing, four-way through at the side of spool ”f”, axial damping channel ”g” and taper bore ”c”.

 The hydraulic force generated on the lower side area of valve spool compares with the spring force, the spool is beginning to open when fluid pressure is larger than the spring presetting force, then the hydraulic oil flows to reservoir through outlet port T.

The orifice “g” is dynamic hydraulic damping, which can improve the stability of the valve, but it is no effect when the valve is a pressure steady state, the channel “e” is used to drain the leak oil stored in spring chamber back to the oil reservoir (Inner drain). Rotating the part 3 in 180 ° degree, to remove the screw bolt at L port, then the internal drain type is converted to external drain if the L port connects the oil pipeline where to oil reservoir directly.

PleaseDirect-Acting-Relief-Valve-Tpye-symbol-Pictur check the Picture B-1: internal drain direct acting relief valve & Picture B-2: external drain direct acting relief valve. The spool type direct acting relief valve controls the inlet pressure at main port by adjusting the presetting pressure
of spring, but the valve is hard to regulate the system pressure if the control pressure is
high, hence, it is suitable used for the low-pressure hydraulic system.


– Spool Type Of Direct Acting Relief Valve –

1. Adjusting knob (bolt)  ;  2. Adjusting spring  ;  3. Front Cover  ;  4. Spool   ;   5. Valve Housing

2. Poppet Type Of Direct Acting Relief Valve:
The Picture C-1 is the normal poppet type of direct acting relief valve with subplate connection and small flow rat, the part “3” is poppet type. This valve is used as remote control valve, safety valve or like a pilot part equipped in pilot pressure relief valve (Like Pressure relief valve, Reducing Valve, Sequence Valve). The pressure adjustment range of valve is be able to altered by changing the different required spring part “4”, the orifice “2” is mainly used to improve the valve operation stability.
– Poppet Type Of Direct Acting Relief Valve –

Direct Acting Relief Valve

1. Valve Housing  ;  2. Valve seat  ;  3. Poppet  ;  4. Adjusting spring  ;  5. Screw bolt

The poppet type of direct acting relief valve is small size, the soft materials of adjusting spring is accepted for easily pressure regulation, therefore, this kind of valve is available for higher hydraulic pressure system.

3. Damping Type Of Direct-Acting Relief Valve
The damping type of direct-acting relief valve is a damping piston direct-acting relief valve, which is a new direct-acting relief in recent years. If compared with poppet type of direct-acting relief valve, the damping type valve is used especially for the hydraulic system requires high-pressure and high flow rate, with a stable pressure and flow characteristics under high hydraulic pressure and large flow rate, low noise.

Finotek DBD series is the damping type of direct acting relief valve, there are three types of connection: threaded connections, subplate mounting and cartridge type connections, with the size from 6-30mm option. The DBD series valve is be able to reach the maximum pressure of 40MPa and flow rate to 300L/min. Please check the DBD technical sheet for more information about it.

In Picture D-1 shows the direct-acting relief valve (Cartridge style) with ball-type cushioning spool. The part “4” cushioning spool and steel ball part “2” is not rigid connection, but the cushioning spool “4” touches the ball with spring part “5” (When the pressure force keeps balance across the cushioning spool.), the movement of ball spool is be able to smooth and stable due to the damping position of cushioning spool.

The dynamic fluid resistance of ball-type cushioning spool is equal to the pressure on both side when in steady state. The enclosed chamber pressure of the valve is relatively varied while the cushioning spool moves, the fluid resistance generates at the moment will be able to improve the operation stability of the ball-type cushioning spool direct-acting relief valve. 

– Direct-Acting Relief Valve With Ball-Type Cushioning Spool –

1. Adjusting spring ; 2. Ball head ; 3. Valve seat ; 4. Cushioning spool ; 5. Damping Spring