Pump

What does the pump capacity used in a dump truck depend on? Is it all related to the weight lifted by the dump truck?

The displacement of the hydraulic pump used in dump trucks (primarily gear pumps; plunger pumps are less commonly used in small and medium-sized dump trucks) is indeed a key design factor, but it is not directly or solely determined by the lift weight.

Simply put: the lift weight determines the required system operating pressure, while the lift speed (time) directly determines the required pump displacement.

Let's break down the key factors that determine the pump displacement size for dump trucks:

Core Determinant: Flow Requirement

The pump's displacement (cc/rev) determines the amount of hydraulic fluid (flow rate) the pump can deliver per unit time. The greater the flow rate, the faster the lift cylinder can extend and retract, and the shorter the lift time.

Flow (L/min)  Displacement (cc/rev) × Engine Speed (rpm) / 1000

Therefore, to achieve the desired lift speed, a pump displacement that meets the required flow rate is required.

Specific factors influencing pump displacement selection (from most important to least important):

Lifting Time Requirement (Most Critical Factor):

This is a key performance indicator for construction vehicles. Users typically want the vehicle body to be lifted into position as quickly as possible (for example, reaching its maximum angle within 20-30 seconds).

The greater the load to be lifted, the greater the cylinder thrust required, but the cylinder diameter must also be designed accordingly. A larger diameter cylinder requires a higher flow rate to fill quickly. Therefore, to meet the required lifting time, even at the same engine speed, a larger pump displacement is required for larger tonnage dump trucks.

Lifting Mechanism Type and Cylinder Size:

Dump trucks have a variety of lifting mechanisms, including front-end (single/twin), twin-cylinder mid-lift, and twin-cylinder rear-lift.

The cylinder diameter and stroke are key parameters. The larger the cylinder diameter and the longer the stroke, the greater the total volume of hydraulic fluid required to complete a lift. To deliver this much fluid in a short period of time, a pump with sufficient flow is essential. The calculation formula is:

Required flow (L/min) = (Cylinder volume × Number of lifts) / Lifting time (min)

Therefore, the cylinder size directly determines the required flow, and thus the pump displacement.

Power take-off (PTO) speed ratio and engine operating speed:

The hydraulic pump in a dump truck is typically connected to the transmission via a power take-off (PTO) and driven by the engine.

The PTO speed ratio determines the relationship between the pump speed and the engine speed (for example, a 1:1 or 1:0.8 ratio).

The engine typically operates at a specific speed during lifting (such as an economical speed of 1500-1800 rpm). The actual pump output flow must be calculated based on this operating speed when designing the system.

If the PTO speed ratio is smaller or the engine lifting speed is lower, a larger pump displacement must be selected to compensate for the same flow.

System operating pressure (strongly related to the lift weight):

This answers the second half of your question. While the weight being lifted is indeed crucial, it primarily affects the system's operating pressure, not the pump's displacement.

The greater the lift, the greater the thrust the cylinder must generate. According to the formula Thrust = Pressure × Area, for a given cylinder area, a higher system pressure is required.

System pressure is primarily determined by the hydraulic pump's rated pressure and the pressure-carrying capacity of the cylinder, valve, and tubing. Gear pumps typically have a rated pressure limit (e.g., 16MPa, 21MPa, 25MPa, etc.). If the lift is significant, it's important to select pumps and components with higher pressure ratings rather than simply increasing the displacement.

In summary: Weight determines pressure, speed determines displacement.

Vehicle Compatibility and Heat Dissipation Capacity:

Large-displacement pumps operating at high pressure consume significant engine power (power = pressure × flow). This requires sufficient engine power reserve, otherwise the engine will stall during lift.

High-flow, high-pressure operation generates significant heat, placing higher demands on the hydraulic fluid's heat dissipation system (tank, radiator). Insufficient heat dissipation can lead to excessive oil temperatures, accelerating oil and seal degradation.

Example:

Scenario 1: Light-Duty Dump Truck (8-ton lift)

A single-stage front lift cylinder with a small bore might be used. The required lift time is 20 seconds.

After calculating the required flow rate, a 40cc gear pump might be selected, providing approximately 72 L/min at 1800 rpm. A system pressure of only 14 MPa might be required to meet the required lift force.

Scenario 2: Heavy-Duty Dump Truck (30-ton lift)

A multi-stage front lift cylinder (commonly known as a "five-section stick") with a larger bore and longer stroke might be used. The required lift time is also 25 seconds.

Due to the large cylinder volume, a very high flow rate is required to quickly fill it. Calculations suggest a gear pump with a displacement of 100cc or even larger, capable of delivering a massive 180 L/min at 1800 rpm, might be required. At the same time, to generate sufficient lifting force, the system operating pressure is often designed to be higher, such as 21 MPa or 25 MPa.

Conclusion

Selecting the displacement of a dump truck hydraulic pump is a systematic process, dependent on:

Direct factors: desired lifting speed (time) and cylinder size.

Important related factors: lift weight (which determines system operating pressure), engine speed, and PTO ratio.

Critical factors: engine power and system heat dissipation capacity.

Therefore, the displacement of a dump truck pump primarily depends on the user's required lifting speed and cylinder specifications. While lift weight does not directly determine displacement, it serves as a foundation for determining system operating pressure and cylinder size, indirectly influencing displacement selection. Larger vehicles, which typically require larger cylinders and faster lifting speeds, naturally require larger pumps.