Pravahi Hydraulics - Counterbalance Valve

Figure: 4-port vented load load-reactive Counterbalance Valve Diagram

Description:

They are modulating valves which allow free flow into the actuator and then block the reverse flow until they feel a pilot pressure inversely proportional to the load in the pilot line. They employ a check valve for the free flow direction, a relief valve to control the flow in the reverse direction and a pilot piston which lowers the relief valve setting. They can lock loads in a leak free mode and they are well suited for many clamping applications or to prevent negative loads from falling down in case of hose failure. For load lowering, they improve motion control in most systems because they compel the directional control valve to always meter positive pressure, also under overrunning load conditions. Counterbalance valves can be used with paired cylinders: pilot pressure will open first the valve of the most heavily loaded cylinder; this will cause load transfer to the other cylinder and the related valve, still closed, will require less pilot pressure for opening. For best safety, they should be fitted close to the actuator, either flange mounted or connected through metallic pipe.

• Functions of counterbalance valve modules

  • A – Free upstream flow through the check valve for load lifting.
  • B – Locking of reverse downstream flow when the directional valve is not operated, or the pump is stopped.
  • C – Load lowering metered by the piston of the relief valve opened and controlled by the pilot pressure of the “lowering” flow delivered upstream by the directional control valve, also when the load tends to overrun.
  • D – Pressure relief (with open centre directional spools) for pressure surges in the actuator caused by the inertia of the load, by oil expansion due to heating, or by external forces.

• Why Use: Provides precise control and safety for overrunning loads, with features like pressure relief protection.

• Example:

  • Sun Hydraulics CB**, CA**, CW** series
  • HydraForce VB** series
  • Walvoil VO** series

Pressure setting of the relief function

The relief section of the valve must have a pressure setting (Pt) high enough in order to be capable to fully re-close the piston in a leak free condition and stop any downstream flow also under maximum load induced pressure (Pmax). For this purpose the pressure setting (Pt) must be at least 30% higher than P max, and this is expressed by the following basic formula:

Pilot Pressure Required to Open the Valve

The pilot pressure \(P_{\text{pil}}\) required to open a counterbalance valve depends on the load-induced pressure \(P_{\text{load}}\), the cylinder ratio \(\phi\), and the pilot ratio \(R\) of the valve. The cylinder ratio and pilot ratio are defined as:

The following sections detail the pilot pressure calculations for different configurations.

Load Pushing on the Cylinder Rod (Valve Fitted to the Full Bore Side)

For a counterbalance valve fitted to the full bore side of a double-acting cylinder, with no back-pressure in the oil return line (\(V_2 - T\) line) and negligible seal friction, the pilot pressure (\(P_{\text{pil}}\)) or cracking pressure is given by:

This can often be simplified as:

Load Pulling the Cylinder Rod (Valve Fitted to the Annular Chamber)

When the load pulls the cylinder rod and the valve is fitted to the annular chamber, the load-induced pressure (\(P_{\text{load}}\)) generates a force in the opening direction, opposing the spring load. This reduces the pilot pressure required to start opening the relief piston. In the ideal case of no back-pressure and no seal friction, the pilot pressure is calculated as:

Counterbalance Valve Fitted to an Equal Area Actuator or Motor

For an equal area actuator or hydraulic motor where \(\phi = 1\), the cracking pressure is determined by:

Different pilot ratios are available and, as a general indication, it can be stated that:

High pilot ratio (R ≥ 8 : 1): allows load lowering with reduced pilot pressure for a faster machine operation and with energy saving. It is best suited for applications where the geometry of the structure maintains the load induced pressures approximately constant during motion (example: extension of a straight boom).

Low pilot ratio (R ≤ 4 : 1): requires a higher pilot pressure for load lowering but it permits a more precise and smooth control of the motion. It is recommended for applications where the geometry of the structure determines high changes of the load induced pressure during motion with resulting instability of the machine (example: cylinder controlling a pivoting arm).

Relief compensated type counterbalance valves

These valve modules have a special configuration of the relief piston that allows the relief opening independently from any back pressure whereas the piloted opening remains subject to additive pressure at port V2.

They are employed when it is necessary to relief pressure at the pre-established pressure setting (Pt), without over-pressurizing the system, independently from any back-pressure in the return line.They are normally fitted in conjunction with main control valves having closed centre spools equipped with port relief valves.

Vented type counterbalance valves

These valve modules have a fully vented spring chamber and both the relief opening and the piloted opening are independent from back-pressure at port V2. Venting is often open to atmosphere and, whenever possible, is connected to tank or to a low pressure line. They must be used only in conjunction with main control valves having closed centre spools and equipped with port relief valves.

This type of “fully balanced” valves is necessary in a few typical applications and exactly: a) When the piloted opening determines a reverse flow toward a highly pressurized line (example: regenerative circuits for cylinders, where the oil from the annular chamber is recycled into the line feeding the full bore side, or series type circuits, where the oil unloaded by an actuator is employed to power a second actuator). b) When pressure surging in the oil return line could cause oscillations of the relief piston which would amplify flow instability and fluctuations. c) When the pilot opening is controlled through a joystick which delivers a “low pressure signal” and the relief piston needs to maintain stable open positions also with strong pressure fluctuations. d) When the counterbalance valve is part of a closed loop circuit with pressure upstream and downstream.

Dampening of the pilot piston, for stable pilot opening and smooth load lowering

The operation of a counterbalance valve while controlling load lowering is affected by many variables among which the input of the operator, the opening areas of the directional valve, the load induced pressure, the viscosity of the fluid, etc., and, most of all, the fluctuations of the pilot signal. Insufficient dampening of counterbalance valves is often the cause for undesired oscillations; Bosch Rexroth Oil Control has developed counterbalance valve modules with a variety of special devices in order to achieve a more controllable and stable pilot function for better load lowering. Here are the most commonly used.

This is generally achieved with a special shaped screw is fitted in the pilot line between the pilot port and the pilot piston of the valve. The pilot flow enters along the long, narrow helical space formed between the damper screw and the female thread in the valve body. The restriction level can be adjusted by varying the number of threads, or the length of the screw which enters the female thread. The pilot oil enters as a smooth steady laminar flow which moves the piston, and maintains the signal very stable. A check valve is often incorporated inside the screw so that, when pressure drops in the system, the pilot pressure is released quickly and the valve blocks immediately, that is a safety must.

Pilot flow enters through an orifice and is partially drained to tank through a second orifice; by selecting the dimensions of the two orifices, the pilot pressure reduction can be calibrated. The combined effect of the inlet restriction and the restriction to tank enables to maintain the signal very stable also in combination with load sensing main control valve.

The above arrangements are very effective for pilot dampening but are sensitive to viscosity; changes in oil temperature can produce variations in response time, independent from the operator’s input. The target of the latest developments has been to design dampening devices which combine both sufficient dampening good responsiveness.

To overcome the delay problem, a small normally open by-pass (VEM) has been added in parallel with the damper screw. The VEM closes and quickly locks any further flow inlet when the pressure setting is reached (see scheme). When the lowering is started slowly by the operator, the pilot oil enters immediately the pilot chamber and pressurizes it up to almost cracking point; only the final pressurization is achieved through the dampening screw, and this happens quickly without delay.

In case of quick lowering, the VEM locks almost immediately and the pressure increase in the pilot chamber is controlled by the dampening screw. When a quick closing of the counterbalance valve must be achieved, the pilot oil can be discharged through the VEM itself which operates as a check valve in the reverse direction.

Heat and Power Losses

Load holding valves introduce energy losses:

• Heat Generation:
  • Pressure drops during metering (counterbalance valves) or pilot opening (check valves) generate heat.
  • Vented valves reduce heat by mitigating back pressure effects.
• Power Losses:
  • Counterbalance valves consume power during flow modulation, especially with high flow or frequent cycling.
  • Pilot-operated check valves have lower losses due to simpler operation.
• Mitigation:
  • Use pressure-compensated or vented designs.
  • Match valve flow capacity to actuator needs to minimize throttling.
  • Incorporate cooling systems for high-cycle applications.

Tuning to Avoid Jerky Movements

Jerky (chattering) movements result from improper settings.

• Tuning Steps:
  • Pilot Ratio: Lower ratios (<4.5:1) enhance stability, reducing setting by less per pilot pressure increase (e.g., 3:1 ratio).
  • Spring Tension: Start at maximum, reduce incrementally to achieve smooth lowering without runaway.
  • Flow Matching: Ensure valve flow matches actuator requirements to avoid cavitation.
• Testing: Monitor for chattering during slow load lowering, adjusting settings for smooth modulation.

Incompatible Valve Types

Incompatible selections include:

• Pilot-Operated Check Valves in Overrunning Loads: Lack modulation for dynamic lowering.
• High Pilot Ratio Valves (>4.5:1) in Low-Pressure Systems: Require excessive pilot pressure, causing delays or jerkiness.
• Non-Vented Valves in Back Pressure Systems: Back pressure increases effective setting, risking instability.
• Non-Pressure-Compensated Valves in Load-Sensing Systems: Disrupt flow regulation.

Impact of Spool Position

Spool position affects performance:

• Pilot-Operated Check Valves: Binary (open/closed), driven by pilot pressure.
• Counterbalance Valves:
  • Closed: Blocks flow, holding load via spring and load pressure.
  • Metering: Modulates flow during lowering, controlled by pilot pressure. Incorrect positioning causes chattering or runaway.
  • Open: Allows free flow during lifting via check valve.
  • Load-Reactive Design: Combines spool control (modulation) and poppet leakage (low, <5 drops/minute), with damping sleeve for smooth closure and opening.

Important Patents

Key patents include:

• US 4,006,664 (1977, Rexroth): Pilot-operated counterbalance valve for cranes.
• US 5,542,618 (1996, Sun Hydraulics): Compact counterbalance valve with integrated functions.
• US 7,234,489 (2007, HydraForce): Proportional counterbalance valve with LVDT feedback.

Internal Working of Load Holding Valves

• Pilot-Operated Check Valve:
  • Components: Poppet/ball, pilot piston, spring.
  • Operation: Free flow to raise load; reverse flow blocked unless pilot pressure unseats check.
• Counterbalance Valve:
  • Components: Check valve, pilot-assisted relief valve (spool/poppet), spring, pilot line.
  • Operation:
    • Lifting: Free flow through check valve.
    • Holding: Spool/poppet blocks reverse flow.
    • Lowering: Pilot pressure shifts spool, metering flow. Feedback ensures stability: if load outpaces flow, pilot pressure decays, closing valve.

Key Questions for Engineers

1. What is the maximum load-induced pressure, and does the valve exceed 1.3–1.5 times this value?
2. What flow rate is required, and does the valve match?
3. Is back pressure a concern? Choose vented valves for downstream restrictions?
4. What pilot ratio balances stability and efficiency?
5. Does the valve comply with ISO 8643 and Machinery Directive for leakage and safety?
6. Are ride control or manual lowering required? Verify compatibility.
7. What are the space and mounting constraints? Consider cartridge valves.
8. How critical is efficiency? Opt for vented or low-pressure-drop designs.
9. What maintenance is feasible? Choose robust, adjustable valves.