Detailed Explanation of Make-Up Gear Pumps**
#### **I. Basic Definition**
A make-up gear pump is a small-displacement gear pump whose primary function is not to act as the main power source, but to **replenish hydraulic fluid** in a hydraulic system's main circuit (especially in closed-loop circuits). It compensates for internal leakage that occurs during system operation, maintains system pressure, and provides a necessary control fluid source.
Simply put, if the main hydraulic pump is the "heart" of the system, then the make-up pump is the "auxiliary pacemaker" that ensures the heart does not "run dry" or "cavitate."
#### **II. Core Function and Necessity**
Make-up pumps are primarily used in **closed-loop hydraulic systems**. In a closed-loop system, the inlet and outlet ports of the main pump (typically an axial piston pump) are directly connected to an actuator (like a hydraulic motor), and the fluid circulates within a closed circuit. This type of system faces several issues:
1. **Internal Leakage:** Inevitable internal clearances in the main pump, motor, and valves cause fluid to leak from the high-pressure side to the low-pressure side.
2. **Volumetric Efficiency:** The system cannot be 100% sealed; some fluid is needed to fill the voids created by leakage.
3. **Cavitation Risk:** Without fluid replenishment, the suction side of the main pump can develop a vacuum due to fluid loss, leading to cavitation, which can severely damage the pump and motor.
4. **Lack of Control Fluid Source:** The variable mechanism (swashplate) of the main pump requires a source of low-pressure fluid to operate; this control fluid comes from the make-up pump.
5. **Heat Dissipation and Flushing:** The fluid circulating within the closed loop heats up due to power losses. Cooler, new fluid needs to be introduced to replace some of the hot fluid for cooling.
**The role of the make-up pump is precisely to solve these problems:**
* **Compensate for Leakage:** Continuously supplies fluid to the low-pressure side of the main circuit, preventing cavitation and air ingestion.
* **Provide Control Pressure:** Supplies a stable pilot pressure for the main pump's variable mechanism.
* **Maintain Low-Side Pressure:** Ensures the main pump's inlet always has sufficient pressure, improving its efficiency and service life.
* **System Cooling and Filtration:** The replenishing fluid passes through a cooler and filter, carrying away heat and maintaining fluid cleanliness.
#### **III. Structure and Working Principle**
1. **Structure:**
* Make-up gear pumps are typically **external gear pumps** due to their simple structure, low cost, high reliability, and good self-priming capability.
* They are usually **co-axially connected** to the main pump, i.e., mounted on the main pump's rear end cover and driven by the same drive shaft. This integrated design is very compact and requires no separate power source or connections.
* Main components include: drive gear, driven gear, pump body, front and rear covers, etc.
2. **Working Principle:**
* When the engine or motor drives the main pump shaft to rotate, the gears of the make-up pump also turn.
* As the gears rotate, the volume in the suction port (where gears unmesh) increases, creating a partial vacuum that draws fluid from the reservoir through a filter.
* The fluid is carried in the gear tooth spaces to the discharge port (where gears mesh).
* At the discharge port, the meshing of the gears reduces the volume, squeezing the fluid out to create a flow with a certain pressure.
* This pressurized flow is directed through dedicated passages to the main pump's make-up port, supplying the closed-loop circuit.
3. **Operation within the System:**
* The oil from the make-up pump first passes through a **make-up relief valve** (typically set at 15-25 bar). This valve determines the make-up pressure.
* The oil then passes through two **check valves** connected to both sides of the main circuit (Port A and Port B). Due to the nature of check valves, the oil only flows to the side with lower pressure, achieving automatic replenishment.
* The system also includes a **flush valve** (or hot oil shuttle valve), which releases a portion of the hot fluid from the high-pressure side of the main circuit back to the tank, while the make-up pump supplies an equal amount of cool fluid, achieving system cooling.
#### **IV. Typical Applications**
Make-up gear pumps are widely used in all equipment employing closed-loop hydraulic drives:
* **Construction Machinery:** Travel systems of excavators, mobile cranes, wheel loaders, rollers.
* **Agricultural Machinery:** Travel drive systems of combine harvesters.
* **Port Machinery:** Container handlers, reach stackers.
* **Industrial Equipment:** Injection molding machines, winches, test benches.
* **Wind Power:** Hydraulic drives for pitch control systems.
#### **V. Key Characteristics**
**Advantages:**
* **Simple Structure, Ruggedness:** Gear pump technology is mature and offers strong contamination resistance.
* **Low Cost:** Relatively inexpensive compared to the critical role they play.
* **High Integration:** Co-axial mounting with the main pump saves space and requires no separate drive.
* **Good Self-Priming Capability:** Reliably draws fluid from the reservoir.
**Disadvantages/Limitations:**
* **Fixed Displacement:** They are usually fixed displacement pumps; their flow is proportional to shaft speed and cannot be adjusted.
* **Efficiency and Noise:** Their volumetric efficiency and noise performance are generally inferior to vane and piston pumps, but as small-displacement make-up pumps, these shortcomings are often acceptable.
* **Pressure Pulsation:** Inherent pulsation exists in the output flow and pressure.
#### **VI. Selection and Maintenance Points**
1. **Key Selection Parameters:**
* **Displacement (cc/rev):** Calculated based on the total leakage of the main pump and motor, control fluid requirements, and cooling needs. Typically 10%-25% of the main pump's displacement.
* **Rated Pressure:** Must meet the system's required make-up pressure (set by the make-up relief valve).
* **Speed Range:** Must match the main pump's operating speed range.
* **Connection Method:** Must ensure correct mounting and sealing to the main pump's rear end cover.
2. **Common Failures and Maintenance:**
* **Insufficient Make-up Pressure:**
* **Causes:** Worn make-up pump (excessive axial or radial clearance), low or stuck relief valve setting, clogged suction filter, air leak in the inlet line.
* **Symptoms:** Slow and weak system operation, abnormal noise from the main pump (cavitation sound).
* **Abnormal Noise:**
* **Causes:** Poor suction (clogged filter, high fluid viscosity at low temperature), air leak in the inlet line, internal pump wear.
* **Maintenance Recommendations:**
* Maintain hydraulic fluid cleanliness; change fluid and filters regularly.
* Ensure the reservoir fluid level is correct and the suction line is sealed properly.
* Regularly check system pressures; promptly investigate any abnormalities in make-up pressure.
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### **Summary**
The make-up gear pump is a seemingly insignificant but vital auxiliary component in a closed-loop hydraulic system. Utilizing a simple and reliable gear pump structure, it performs multiple tasks: "compensating for leakage," "providing control power," and "cooling the system." It is a key component that ensures the normal operation of the main pump and actuators and extends the service life of the entire system. During selection and use, focus on ensuring its displacement and pressure match the main system, and perform routine maintenance diligently.
