Anti-Cavitation Valve Applications

By Robert Post, CFPHS, Consultant to Caterpillar via Actalent Services
and Lauren Schmeal, Editor, Fluid Power Journal

Contamination and high temperatures are well known for causing damage to hydraulic components and systems. Fortunately, there are many articles about those topics. Cavitation is another form of contamination and is detrimental to a safe, efficient, and dependable hydraulic system. Cavitation is most often associated with the suction line connecting a reservoir to a pump but can be found in several other places within a hydraulic system including motors or double-acting cylinders. A system designer can counteract the negative effects of cavitation on those two location types by being aware of potential problem areas and taking steps to eliminate cavitation.

An Introduction to Cavitation

Cavitation occurs when the static pressure of the hydraulic fluid drops below the vapor pressure, forming small vapor-filled cavities. When those cavities implode, neighboring materials are damaged. This article focuses on avoiding cavitation in hydraulic motor or cylinder circuits. Both of those circuits will be explored individually to expand on the application of anti-cavitation check valves.

You may be wondering how anti-cavitation check valves differ from regular check valves. The answer is simple: there is no difference. Anti-cavitation check valves are valves applied in a specific manner. Check valves prevent the flow of hydraulic fluid in one direction. Consider the inversion of that idea, which is to allow flow in the other direction under the right conditions. Hydraulic motor and cylinder circuits have the same remedy with the proper application of anti-cavitation check valves to counteract induced loads from kinetic energy or the application of external forces. We will review each application of the anti-cavitation check valve individually.

Anti-Cavitation in Motors

Hydraulic motors have many applications on mobile and industrial equipment, each with unique needs. Those applications can be grouped into two basic categories: those that stop promptly when the energy input is removed, and motors that gradually coast to a stop.

Motors that depend on hydraulic power to provide torque to power winches, drive wheels, or cranes do not continue to rotate when the flow is terminated because the energy supply is removed when the valve closes. In those applications, coasting could be undesirable and may even dictate the need for a brake to safely hold position when power is removed. Those circuits do not need anti-cavitation valves.

Alternatively, machines with a large amount of angular momentum are allowed to coast to a stop, releasing kinetic energy slowly. It may take several minutes to coast to a stop. Machines such as large fans, stump grinders, and large rotating brooms (Fig 1) require a circuit that allows fluid to flow after the directional control valve closes.

Motor spools or anti-cavitation valves allow fluid to flow after the valve returns to neutral. The first method, a motor spool in the directional control valve, (Fig 2) allows the hydraulic fluid to recirculate from the tank port to the pressure port while the valve is in neutral and the motor coasts to a stop. This is a common solution for machines with dedicated attachments, including forestry equipment with a single purpose.

A motor spool is not always installed on the host machine. Sometimes, a closed center valve is installed for compatibility with other attachments. Off-road utility machines use quick disconnects to provide a fast and easy way to replace attachments. The attachment may not always use a motor; perhaps the next attachment uses cylinders. It is common for utility vehicles, such as wheel loaders or skid steer machines, to have closed-center directional control valves to hold a cylinder in position when the valve is in neutral.

For a host machine with a closed center directional control valve, an anti-cavitation valve (Fig 3) provides a path for fluid to leave the motor while allowing make-up fluid to return to the pressure port, thus avoiding cavitation. Without the anti-cavitation check valve, closing the directional control valve abruptly stops the flow of fluid to the motor. Damage to the motor or driveline can occur if a hydraulic motor is stopped while the inertia of any device with high kinetic energy attempts to continue to rotate to drive the motor forward until the load coasts to a stop. When that happens, the motor behaves like a pump and can generate very high pressures in the return lines. This simultaneously causes a vacuum at the pump inlet and can have potentially devastating effects.

Motor cavitation is easy to detect and sounds similar to pump cavitation. Loud whining, thumping, or banging can be heard as the motor is forced to continue rotating from the kinetic energy of the spinning load. To avoid this problem, anti-cavitation check valves are installed to allow fluid leaving the outlet port to return to the inlet port. Until now, these simple examples showed a motor without a case drain. More efficient motors, e.g. a piston motor, have a case drain line that sends relatively small amounts to flow to the reservoir. When coasting to a stop, the case drain continues to send fluid out of the case drain port and must be replenished to avoid shaft seal damage. Hydraulic accumulators are commonly used (Fig 4) in such an application to replenish the fluid leaving the motor via the case drain line. Starting the motor refills the accumulator to ready it for the next shutdown. Accumulators can be large so as to provide enough fluid to accommodate case drain losses while the motor coasts to a stop, sometimes over several minutes.

In the earlier examples, the motors only rotate in one direction. Attempting to send flow in the reverse direction will not produce the desired performance from the motor. This is due to the fluid’s freedom to bypass the motor through the anti-cavitation check valve. For reversing motors with high energy loads, a more sophisticated circuit is required and falls beyond the scope of this introductory article.

One final aspect of anti-cavitation valves to consider is valve selection and sizing. The valve must have a flow rating equal to or greater than the flow going to the motor. The cracking pressure must be high enough to prevent instability if elevated return pressures are present. An anti-cavitation valve with low cracking pressure can be unstable and may cause chatter or other instability.

Anti-Cavitation in Cylinders

Cylinders exposed to induced loads can also cavitate. While the formation of small vapor-filled cavities in cylinders is possible, there are elements of a cylinder that are weaker by comparison. Piston rod seals can allow air to enter the cylinder and cause aeration. Both cavitation and aeration are detrimental to a hydraulic system, so an effective countermeasure is needed.

In this example, consider the cylinder on the thumb of an excavator (Fig 5). A thumb is the additional structure, painted black in this image and used to apply force opposed to the bucket motion. The excavator’s thumb is used to clamp or control a load that is larger than the bucket. At equilibrium, the bucket and thumb are applying forces to the rock in opposite directions. When clamping material with the excavator, it is common for the thumb to remain stationary while the bucket is curled toward the operator, pushing against the thumb mechanism and forcing the thumb cylinder to retract. The cylinder will withdraw safely because the pressure at the cap port is already controlled to limit extending forces; however, conditions at the rod end are equally important.

The thumb cylinder and the external force can be represented as shown in the schematic in Fig 6. When sufficient external force is applied, a relief valve will open to safely control the cap port pressure and the piston will begin to move toward the cap end of the cylinder. Only a very small amount of piston movement is necessary to generate a vacuum at the rod port. As the piston moves toward the cap, cavitation will occur on the rod side of the piston in a tightly sealed cylinder (2 to 3 psia, or 14 to 20 kPa absolute pressure). It is more common that air is drawn into the cylinder past the rod seals which are designed to resist pressure from the inside. The internal negative pressure allows atmospheric pressure to push air into the cylinder around the rod seals or wipers. After the air has entered the rod port, it may return to the reservoir under the right conditions, or it may remain in the cylinder and cause a springy or spongey motion.

Cavitation in cylinders is difficult to detect because it does not have a characteristic noise like a motor. In some ways, very low cylinder pressure can cause even more damage. Contamination can be forced under rod wipers and seals, allowing water or debris to enter the cylinder. Such contamination can cause corrosion or scoring inside the cylinder or the directional control valves connected to the cylinder.

To counteract negative pressure in the rod port, anti-cavitation check valves are added to the circuit. Some directional control valves include anti-cavitation checks in the work port relief valves. When that is not the case, supplementary valves must be added. Figure 6 shows an integral anti-cavitation check on the right and a supplementary check valve on the left. The supplementary valve can be connected to a case drain or another source of make-up fluid. When low pressure is present in the rod port, cavitation is avoided because fluid is drawn through the check valve to fill the void in the rod side of the cylinder.

Applications in Select Industries

Beyond traditional heavy-duty and industrial applications, the principles of cavitation control are critical in other sectors. Let’s explore how anti-cavitation is utilized in this issue’s focal areas, the medical, food processing, and plastics industries. In the medical industry, hydraulic systems are integral to devices such as surgical robots, imaging equipment, and patient support systems, where even minor fluid inconsistencies can jeopardize performance and patient safety. Effective anti-cavitation measures ensure these systems operate smoothly and maintain cleanliness standards for sterilization.

In food processing, hygienic and efficient fluid handling is paramount. Hydraulic components in food production lines—used in packaging, filling, and sorting—must prevent contamination that could compromise food safety. Employing anti-cavitation check valves helps maintain continuous and leak-free operation, thus reducing the risk of microbial contamination and costly cleanups while supporting compliance with rigorous sanitary standards.

Specific to plastics, particularly in processes like injection molding and extrusion, maintaining stable hydraulic pressure is essential for consistent product quality. Cavitation-induced fluctuations can lead to defects, increased cycle times, and higher scrap rates. Anti-cavitation solutions not only ensure reliable pressure control and minimize leaks, but they also contribute to improved efficiency, reduced waste, and higher-quality molded parts.

By incorporating these anti-cavitation strategies, system designers can extend the reliability and performance of hydraulic systems across diverse industries, ensuring safe, efficient, and contamination-free operations.

Conclusion

To summarize, cavitation can take place in unexpected places and cause damage in remote areas. This makes identification of the root cause difficult at times. Armed with this knowledge, system designers can anticipate cavitation problems and avoid this less familiar contamination source.