In hydraulic system design and maintenance, there are two groups of components that frequently appear, yet are easy to confuse: first, the “check valve” versus the “pilot-operated check valve”; and second, the “external-gear pump” versus the “internal-gear pump.” Understanding their structures, operating principles, and appropriate application scenarios is crucial for system safety, reliability and performance optimization.
1. Check Valves vs Pilot-Operated Check Valves: from preventing back-flow to controlled release
Role and mechanism of the check valve A check valve is a very basic component in hydraulic systems. Its role is to allow hydraulic fluid to flow in only one direction, and automatically shut when the flow reverses, thereby preventing fluid back-flow or pressure rollback. It is broadly described as permitting free flow in one direction while blocking in the opposite direction. Typical applications include locations such as the pump outlet, back-oil return lines, or oil return isolation. When fluid enters from the permitted side, the valve element is pushed open; when reverse flow is attempted, the reversing pressure or spring force prompts the valve to close, thus protecting the system from issues such as reverse flow, back pressure, or pump return. Because the mechanism is simple, structure straightforward and cost low, the check valve has long been an important passive protective component in hydraulic systems.
Extended functions of the pilot-operated check valve The pilot-operated check valve builds on the basic check valve by incorporating a pilot (control) port. Its logic is: when no signal is applied to the pilot port, it functions like a standard check valve—only one direction is allowed. But if the pilot port receives hydraulic pressure, the pilot force pushes the valve element to release the sealed state, thereby permitting flow in the previously blocked direction or unlocking a locked oil circuit. Therefore, the pilot-operated check valve combines “back-flow prevention” with “controlled release/lock holding” functions. It is often used in systems requiring position-holding or release control, such as hydraulic cylinders being locked, preventing downward slipping, or lifting mechanisms.
Comparison and selection advice From a control perspective: the check valve is a typical passive component—its response is entirely driven by fluid flow direction and internal spring or valve element mechanisms. The pilot-operated check valve is a hybrid active/passive component—it still provides passive back-flow prevention, but can also actively switch flow via an external pilot signal. In terms of selection: if the system only needs to prevent hydraulic fluid back-flow, avoid pressure rollback or hydraulic pump reversal, then a standard check valve will normally suffice. But if the system also requires “cylinder stop then lock position,” “ensure no downward slip,” or “release the oil circuit at a specific moment,” then a pilot-operated check valve is more suitable. It is also worth noting that although a pilot-operated check valve can be used for load holding, in situations where the load may “over-speed downwards” or the environment is extreme, relying on a pilot check valve alone may not be adequate; often a counterbalance valve or other safety control element needs to be used in conjunction.
2. External-Gear Pumps vs Internal-Gear Pumps: structural differences lead to performance differences
Structure and features of external-gear pumps An external-gear pump consists of two identical interlocking gears, each mounted on its own shaft, enclosed within a rigid pump casing. During operation, the gears generate an expanding volume on the suction side to draw in fluid, and on the discharge side the volume decreases to force the fluid out, thereby accomplishing fluid transfer. Its advantages include: relatively simple structure, lower cost, stronger contamination tolerance (because the gear clearances are comparatively larger, making it more tolerant of impurity-containing fluid). However, because of the external gear meshing and larger clearances, its flow pulsation may be stronger and noise may be higher; in systems with high precision or low noise demands, its performance may be somewhat inferior.
Structure and features of internal-gear pumps An internal-gear pump features a pair of gears of different sizes: a larger outer gear (the drive) that envelops a smaller inner gear (the driven), which are eccentrically meshed; between them is typically a crescent-shaped partition separating the suction region from the discharge region. This structure offers advantages in terms of operational smoothness, noise control, suction performance and volumetric efficiency. However, its manufacturing complexity and cost are relatively higher, and it places greater demands on oil cleanliness, impurity control, and maintenance.
Application scenarios and selection advice In practical system design, if the equipment is operating in a harsh environment, with potentially high impurity levels and where contamination resistance is the primary concern, and when flow pulsation or noise requirements are not extremely strict, then an external-gear pump is usually a more reliable choice. On the other hand, if the system requires low noise, high precision and smooth flow, and the equipment’s maintenance conditions are favorable, then an internal-gear pump is more appropriate. Simultaneously, selection should also consider fluid viscosity, system pressure, usage frequency, maintenance convenience and budget. In other words, factors such as viscosity, pressure, operating conditions and manufacturing requirements must all be considered.
Summary
From the foregoing analysis, we can clearly see the key contrasts between these two component/pump pairs:
In terms of valves: the check valve is the typical passive component for preventing back-flow, whereas the pilot-operated check valve extends that to include “active release/lock” capability.
In terms of gear pumps: the external-gear pump embodies the advantages of “simple structure, contamination resistance, low cost,” while the internal-gear pump emphasizes “low noise, high precision, smooth flow.” In hydraulic system design, component selection and maintenance, understanding these differences and judging their respective advantages accordingly can effectively improve system performance, extend service life and reduce fault occurrences.
