2005/2006 I. Hydraulic and Pneumatic Systems1 Valves If the pump is the heart of a hydraulic system then the valve is the brain. Valves are used to perform a large variety of governing and controlling functions. Form: Function: pressure control valves flow control valves check valves (non-return valves) directional control valves a) Spherical b) c) d) e) Conical f) Plate g) Spool valve Poppet valve Aspects of classification

2005/2006 I. Hydraulic and Pneumatic Systems2 Valves Pressure control valves: pressure relief valves pressure reducing valves Pressure relief valve Has the task to limit the pressure in a hydraulic system or in a part of the system. The pressure can rise in a hydraulic system if: pressure difference valve pressure ratio valve -the flow rate from the pump is larger than the flow rate through the actuator -the volume of a closed system is reduced -the load of the actuator rises -heat is introduced into a closed system -the hydraulic resistance of the system rises

2005/2006 I. Hydraulic and Pneumatic Systems3 Valves Pressure relief valve x A k C F spring p sy s QpQp Symbol

2005/2006 I. Hydraulic and Pneumatic Systems4 Pressure relief valve Theoretical basics Force equilibrium: (1) Curtain surface: Circumference ∙ height = k ∙ x = A c Bernoulli equation: (2) (3) (4) Q rv depends nonlinearly on x or if we express x from (1) then:

2005/2006 I. Hydraulic and Pneumatic Systems5 Pressure relief valve Theoretical basics: (5) Let be the pressure when the valve just opens (x = 0)  subst. to (4) (5) is valid from x = 0 to x = x max, x max being the displacement when the spring is completely compressed If p > p max :

2005/2006 I. Hydraulic and Pneumatic Systems6 Pressure relief valve Theoretical basics: Q rv negative part: physically impossible p max p sys belongs to x max

2005/2006 I. Hydraulic and Pneumatic Systems7 Pressure relief valve Hydraulic aggregate: The simplest hydraulic system Pump + pressure relief valve M Q rv Q ag user QpQp p sys reservoir

2005/2006 I. Hydraulic and Pneumatic Systems8 Hydraulic aggregate Let us derive the characteristics of the complete aggregate. Parallel circuit so: Q rv Q pump Q ag1 Q ag2 p sys n = n 1 < n = n 2 = = = = Q rv Q pump

2005/2006 I. Hydraulic and Pneumatic Systems9 Pressure relief valve The pressure relief valve always has to be matched with the pump. If for example the rotational speed is increased (orange curve)  then there will be flow through the aggregate even with higher pressure. Wrong! The last section of the curve has to be at the negative Q plane.

2005/2006 I. Hydraulic and Pneumatic Systems10 Pressure relief valve Versions: There are various versions of pressure relief valves: 1.Directly operated → ← pilot operated 2.Self-operated → ← externally operated The self-operated valve is controlled by its own pressure, whereas the externally operated valve is controlled by an outside pressure. Directly operated – already explained.

2005/2006 I. Hydraulic and Pneumatic Systems11 Pilot operated pressure relief valve Valve constant has to be increased: Pilot operated is applied when large flow rates have to be controlled. Consists of two valves: At higher flow rates the losses at a directly operated valve would be very large. Another problem is that with high flow rate the dynamics of the valve gets worse. The area can be increased but this increases size and costs and worsens the sensitivity of the control. The spring constant can be decreased but this increase against the size. (Large preloading is necessary.) Again bad for dynamics. Solution: two valves: pilot operated valve -Main valve (low c) -Pilot valve (high c)

2005/2006 I. Hydraulic and Pneumatic Systems12 Pressure relief valve Pilot operated: p 1 is the system pressure that has to be limited. The valve is closed as long as the limit pressure is not reached at the pilot valve. x main valve spring 1 A p1p1 Q p2p2 pilot valve Thr

2005/2006 I. Hydraulic and Pneumatic Systems13 Pressure relief valve Pilot operated: In this case the pressure p 1 acts on both sides of the valve and the valve is in equilibrium: p 1 = p 2. In the pressure rises above the limiting pressure of the pilot valve then the pilot valve opens. A flow starts through the throttling valve so that p 1 > p 2. If A(p 1 -p 2 ) > F S01 (preload force of spring 1) then the main valve also starts to open. This is more favourable because it allows a softer spring in the main valve. The pilot valve can be spatially separated from the main valve – the control can be exercised from a distance. More favourable static characteristics Better dynamic characteristics

2005/2006 I. Hydraulic and Pneumatic Systems14 Pressure relief valves Pilot-operated pressure relief valve 1 - főszelep, 2 - elővezérlő szelep, 3 - főtolattyú, fojtás, vezérlő vezeték, 8 - szeleptest, 9 - rugó, 15 - tehermentesítés 1 - Main valve 2 - Pilot valve 3 – Main spool Throttle Operation line 8 - Valve body 9 - Spring 15 - Discharging

2005/2006 I. Hydraulic and Pneumatic Systems15 Pressure relief valve Simplified symbol of a pilot operated pressure relief valve Applications of pressure relief valves: -Safety valve (most common) -Overflow valve (pressure source always open produces large losses, should be used only for small power) -Sequence valves (turn-on or turn-off) p1p1 Q rv directly operated pilot operated ideal Characteristic curves of pressure relief valves

2005/2006 I. Hydraulic and Pneumatic Systems16 Pressure relief valve Safety valve and overflow valve: RV p plpl Safety valve Overflow valve (pressure source) QpQp QsQs p RV p plpl

2005/2006 I. Hydraulic and Pneumatic Systems17 Pressure relief valve Sequence valves: “Turn-on” valve is used when two or more users are fed by the same pump. When one working step is finished, the pressure rises and a second user is switched through the valve. “Turn-off” valve is applied when two pumps (one high pressure, one low pressure) work in parallel. If the pressure is high enough then only the high pressure pump works. If the pressure drops below the limiting value of RV1 then the valve closes and the pump delivers into the system again.

2005/2006 I. Hydraulic and Pneumatic Systems18 Pressure relief valve Sequence valves: Sequence valve (turn off) RV1 Sequence valve (turn on) RV2 pl1pl1 pl2pl2 Q p > Q S Here the RV1 is an externally operated valve. Check valve RV1 RV2 pl1pl1 pl2pl2 HPP LPP Q

2005/2006 I. Hydraulic and Pneumatic Systems19 Pressure relief valves Dynamic behaviour of pressure relief valves: Sudden opening or closing leads to oscillations of the valve. Stationary characteristics depend only on the valve, dynamics characteristics depend on the whole system. The dynamic behaviour can be tested by a sudden application of the pressure on the valve.

2005/2006 I. Hydraulic and Pneumatic Systems20 Pressure relief valve Dynamic behaviour of pressure relief valves: First linear rise, then damped oscillation. There is a differential equation system which can be solved to simulate the process. V is the volume of fluid between pump and valve → capacity and inductivity changes. DV p1p1 Q rv Test rig QpQp RV p t p1p1 V1V1 V2V2 V3V3 V 1 < V 2 < V 3

2005/2006 I. Hydraulic and Pneumatic Systems21 Big pictures End of normal presentation Beginning of big pictures

2005/2006 I. Hydraulic and Pneumatic Systems22 Valves Form a) Spherical b) c) d) e) Conical Poppet valve f) Plate g) Spool valve

2005/2006 I. Hydraulic and Pneumatic Systems23 Valves Pressure relief valve

2005/2006 I. Hydraulic and Pneumatic Systems24 Valves Pressure relief valve

2005/2006 I. Hydraulic and Pneumatic Systems25 Valves Pressure relief valve

2005/2006 I. Hydraulic and Pneumatic Systems26 Valves Pressure relief valve

2005/2006 I. Hydraulic and Pneumatic Systems27 Valves Pressure relief valve x A k C F spring p sy s QpQp Symbol