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Non-return balancing and brake valves

Subjects:

  • check valve
  • Balance valve, brake valve

check valve:
A non-return valve allows the flow of liquid in one direction unimpeded and blocks the flow of liquid in the opposite direction.

The check valve in the following diagram is located in the return oil filter housing. At rest, the spring presses the ball valve onto its seat. When the fluid pressure above the ball valve is high enough to overcome the spring force, the ball valve opens and fluid can flow past the check valve to the reservoir. The non-return valve serves here as a protection against too high a pressure, which can occur when the filter element is so contaminated that the liquid flow is impeded.

We also find the non-return valve as a safety device in hydraulic systems of, for example, a tipping trailer. The valve is mounted in the hydraulic cylinder and prevents the bucket from coming down uncontrollably in the event of a sudden breakage of the hose. Instead, the ball immediately shuts off the volumetric flow as a result of the thrust, so that the cylinder with the load comes to an immediate standstill.

The check valve is not only found in security systems, but also in circuits where we want the hydraulic oil to flow through a component in one direction. An example of this can be seen in the following image.

Between the control valve and the supply of the cylinder we see a flow control valve in a Graetz circuit. This circuitry allows the fluid to flow through the flow control valve in the arrow direction, regardless of how the cylinder is operated. This is because the flow control valve only operates in one flow direction and such a circuit prevents multiple flow control valves in one line.

When extending the piston, the lower left and upper right check valves are pushed off their seats to allow the flow of fluid through. The other two check valves remain closed.

The moment the control valve is set to the far right position, the fluid flow direction to and from the cylinder reverses. The return fluid leaves the cylinder and presses the upper left and lower right ball valves from their seats to flow – via the control slide – to the reservoir.

In both situations, the fluid flows in the direction of the arrow through the flow control valve.

Graetz circuit

Balance valve, brake valve:
The balancing valve, also called a brake valve, is a precisely controlled check valve. It is suitable for controlled braking of a large load. The balancing valve is used in smaller cranes, such as truck-mounted cranes.

The balancing valve is in fact an extension of a “normal” check valve. The difference between these two valves is that the controlled non-return valve has to be opened against the load pressure and is therefore load pressure dependent and sensitive. The normal non-return valve is sensitive to jerky movements, which the balancing valve does not suffer from. The required control pressure for the balancing valve depends on the preset spring pressure.

The hydropump, pressure relief valve and filter have been omitted in this drawing for convenience. The control slide is in the correct position to steer the cylinder. The liquid flows through the check valve (in the balancing valve) to the cylinder. 

After switching the control slide (extreme right position) the cylinder is sent in again. The oil is now supplied on the rod side (via connection B). The built-up pressure pushes the balancing valve open. The oil flows back to the reservoir through the balancing valve.

The moment the cylinder descends faster than oil is supplied to the rod side (we call this leading the way), the pressure on the rod side drops and so does the control pressure on the balancing valve. As a result, the valve is pushed towards “closed” by the spring. As a result, the volume flow decreases and comes into balance with the volume flow supplied on the rod side.

The balancing valve can also function as a pressure protection in the event of overloading of the consumer due to external forces or by suddenly operating the control slide: oWhen the control slide is suddenly centered during the lowering of the cylinder, the balancing valve closes immediately. Due to the pressure that is then built up in the cylinder, the balancing valve is pressed open again from the cylinder side. The balancing valve thus also limits the pressure in the cylinder.

balancing valve