Check-Q-Meter

A check-Q-meter is a controlled, leak-free check valve for hydraulic systems that regulates backflow from a consumer regardless of the load and simultaneously acts as a pipe rupture safety device. It controls the outflow rate based on the inflow rate on the opposite side of the actuator, allowing suspended or moving loads to be moved in a controlled manner without overshooting. In industrial and mobile hydraulic systems, it protects cylinders and motors from uncontrolled acceleration.

Operating Principle and Design of the Locking Q-Meter

The Check-Q-Meter combines two basic hydraulic functions in a single component: leak-free blocking of backflow and flow-dependent regulation of outflow. This combination distinguishes it significantly from simple check valves, which merely block flow in one direction and allow it to pass freely in the other.

Pilot-Controlled Backflow Regulation

The core function of the Check-Q-Meter is to control the outflow from the consumer in such a way that it always maintains a defined ratio to the inflow on the opposite side. In cylinders, the main piston opens in the flow direction with minimal pressure loss. On the return side, the valve meters the opening so precisely that the actuator’s movement is not accelerated by the load. The control process is pilot-operated: A pilot section and a pilot piston initiate the release, while a follow-up spool adjusts the opening behavior to the applied control flow.

Main Components and Their Functions

A shut-off Q-meter consists of several functional assemblies that together produce the controlled shut-off and throttling effect:

• Housing: Houses all components and provides the pressure connections. Depending on the design, SAE flanges, subplate connections, or cartridge versions are available.
• Main cone or main piston: Ensures free flow when open and closes leak-free when shut off.
• Pilot unit: Generates the pilot-controlled unlocking pressure that lifts the main cone.
• Pilot spool: Directs the control pressure to the control surface of the main piston to initiate controlled opening.
• Follow-up spool: Couples the backflow control to the inlet flow and ensures that the valve opens in proportion to the supplied volume.
• Control damping: Prevents oscillations and ensures a smooth, stable response under varying load conditions.

Load-Independent Speed Control

The key feature of the Locking Q-Meter is its load-independent control of the consumer’s speed. Without this valve, a pulling load would accelerate the cylinder or motor as soon as the hydraulic resistance in the return line is insufficient to slow the movement. The Locking Q-Meter prevents this so-called “runaway” by allowing return flow only to the extent permitted by the inlet on the opposite side.

Function in Hydraulic Cylinders

In cylinders, the check-Q-meter operates between the return line and the tank or bypass path. When the cylinder extends, the pressure oil flows through the valve into the piston chamber, while the valve on the annular space side regulates the outflowing volume flow. Since the effective areas on the piston side and the annular chamber side are different, the valve must adjust the outflow rate to match the area ratio of the cylinder. The sliding spool performs this task by using the control flow from the opposite side to proportionally adjust the opening of the main cone.

Function in Hydraulic Motors

In hydraulic motors, the check Q-meter maintains the rotational speed under varying loads. If, for example, the control signal is lost or the supply pressure drops, the valve closes immediately and holds the motor in its position. This prevents the motor from running away if the load becomes a pulling force and the return flow would otherwise flow out unchecked. Typical applications include winches, travel and swing drives on mobile machinery, where load dynamics fluctuate significantly.

Pipe Rupture Protection as an Integrated Safety Feature

In addition to regulating the return flow, the Shut-Off Q-Meter functions as a pipe rupture protection device. If a line in the return path ruptures, the pressure on the consumer side drops abruptly. The valve detects this pressure difference and closes the return flow path, preventing the load from dropping or moving away uncontrollably. This protective function is particularly important for lifting equipment and work machines, where a line rupture can endanger people and property.

Supplement with a Secondary Pressure Relief Valve

In some models, such as the FB variant of the Rexroth FD series, a secondary pressure relief valve is additionally integrated. It protects the consumer side from impermissible pressure spikes that can result from external forces or inertia during valve closure. Depending on the size, the adjustment ranges for this valve are 200, 300, or 400 bar.

Technical Specifications and Sizes

Standard Q-meter shut-off valves, such as the Rexroth FD series, cover a wide range of industrial and mobile applications. Sizes 12, 16, 25, and 32 correspond to the nominal diameters commonly used in hydraulic systems. The maximum operating pressure at the ports is 350 bar, and the maximum flow rate is 560 l/min. Pilot-operated release requires a control pressure in the range of 20 to 50 bar.

Connection Options and Designs

Manufacturers offer various designs to allow for flexible integration of the valve into existing circuits:

1. Cartridge version (KA): A screw-in valve for block mounting; compact and space-saving.
2. SAE flange without secondary pressure relief valve (FA): For direct connection to SAE flanges, without an additional pressure relief valve.
3. SAE flange with secondary pressure relief valve (FB): Same as FA, but with an integrated secondary pressure relief valve for enhanced protection.
4. Subplate (PA): For mounting on subplates, without a secondary pressure relief valve.

Operating Conditions and Fluid Requirements

Shut-off Q-meters are designed for use with mineral oils in accordance with DIN 51524 (Type HL and HLP). Some models also support flame-retardant fluids of the HFD-R group. The operating temperature ranges from -20 to +80 °C, and the permissible viscosity range is between 10 and 800 mm²/s. Since this is a pilot-operated precision valve, the cleanliness of the pressurized fluid is critical. Manufacturers require oil contamination in accordance with ISO 4406, Class 20/18/15 or better.

Distinction from Other Valve Types

Shut-off valves and check valves are among the fundamental components in hydraulic systems, but their mode of operation differs significantly from that of the shut-off Q-meter.

A simple check valve prevents backflow and opens in response to a control pressure. It holds the load but does not affect the speed of movement. The Check-Q-Meter goes a step further by actively regulating backflow and thereby controlling the actuator’s movement. It is this combination of holding and regulating that makes it suitable for applications with high safety requirements.

Typical Applications

Check-Q-Meters are used wherever loads must be moved and held in a controlled manner without the movement speed depending on the load. Applications range from mobile machinery to lifting equipment and stationary industrial systems.

Mobile machinery

In excavators, crane booms, wheel loaders, and forestry machines, lock-Q meters ensure the controlled lowering of loads. Particularly when used on lifting cylinders that span great heights, the valve prevents the cylinder from retracting uncontrollably when the supply pressure drops. On mobile machinery, it keeps the speed of hydraulic motors stable, even when the load changes.

Industrial Lifting and Conveying Technology

In industrial material handling, Sperr-Q-Meters ensure the proper positioning of lifting platforms, roller conveyors, and clamping devices. Crane systems use the valve to synchronize the lowering of loads with the pump’s set flow rate. If a line ruptures, the valve closes and prevents the attached load from dropping.

Presses and Injection Molding Machines

On presses and injection molding machines, check-Q-meters ensure the smooth operation of clamping and ejection cylinders. The pilot-operated control system ensures that movement remains smooth even under high counterforces. The secondary pressure relief valve protects the consumer side from pressure surges that can occur during sudden closure.

Maintenance and Commissioning

Maintenance of a Sperr-Q-Meter focuses on the wear parts in the pilot-operated section as well as on the sealing elements. Proper commissioning ensures proper function and prevents damage caused by incorrect operation.

Inspection of Wear Parts

The wear parts of a shut-off Q-meter include the sliding surfaces of the main piston, the drag spool, and the pilot spool, as well as the seals in the pilot section. Signs of wear, such as scoring, pitting, or increased clearance, manifest as erratic control behavior, load pulsation, or sluggish valve closure. Regular inspection and timely replacement of wear parts prevent consequential damage to the cylinder and motor.

Cleanliness of the Hydraulic Fluid

Since the pilot-operated function is sensitive to contamination, it is crucial to comply with the oil purity class specified by the manufacturer. The orifice holes in the pilot section are narrow, and even small particles can impair function. The orifice inserts are available in several sizes to adapt the control characteristics to the specific application. For example, depending on the size, the diameters are 0.3 mm for sizes 12 and 16, 0.4 mm for size 25, and 0.6 mm for size 32. These fine cross-sections underscore the need for effective oil filtration.