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  • Understanding Hydraulic Systems: Two-Line vs. Three-Line

Maximizing Efficiency in Two-Line vs. Three-Line Hydraulic Systems 

When it comes to hydraulic systems, understanding the nuances between two-line and three-line hydraulic systems can make a significant impact on productivity. We will dive into the key components, highlighting their differences and impact on productivity enhancements offered by each system. While both systems can power end dump systems, the setup and flow of hydraulic fluid differ significantly, impacting performance and longevity.

Get more work done, improved productivity, faster cycle times, more uptime.

Dump Truck PTO/ Pumps - Typical Components

The typical components utilized in both two-line and three-line hydraulic systems consist of:

  • Reservoir
  • PTO
  • Dump Pump
  • Quick Disconnect
  • Cylinder

Each component in the hydraulic system serves an essential purpose.

Reservoir

The tank or reservoir typically holds 50-70 gallons of fluid and can be made from materials like plastic (poly), steel, or aluminum. Steel and aluminum tanks are commonly used and are often mounted behind the cab in end dump wet systems. 

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The dump pump is the central component of the hydraulic system. It serves as the “heart” by pumping hydraulic fluid throughout the system. Within the dump pump, you’ll find the Gear Pump, Control Valve (often air-shifted), Main system pressure relief valve (the “brains” of the operation), and the Quick Disconnect.

Power Takeoff (PTO)

PTOs transmit power from the vehicle’s engine or transmission to the hydraulic pump.

In end dump systems:

  • PTOs are typically mounted to the bottom of the transmission.
  • They operate at around 120% of engine speed.
  • Most modern PTOs are air-actuated, although some still use cables.
  • Usually, PTOs are directly connected to the pump.
  • Occasionally, driveshaft variations are encountered.

Dump Pump: The Heart of the System

The dump pump is the central component of the hydraulic system. It serves as the “heart” by pumping hydraulic fluid throughout the system. The dump pump consists of several components:

Gear Pump: Responsible for pumping hydraulic fluid
Control Valve: Manages flow direction and pressure
Air Shifting: Many dump pumps use air to shift valves
Relief Valve: Maintains system pressure within safe limits

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Quick Disconnect

Usually, a wing-type quick coupler is used, and they screw together securely, minimizing resistance. They also allow smooth box movement and are reliable for this application.

Hydraulic Cylinder

The cylinder has three to five stages (depending on end dump size). When fully extended, it holds approximately 30 to 50 gallons. Note that the cylinder is usually part of the trailer, not the wet kit itself.

How a Two-Line System is set up

 1. Two-Line System

Inlet and Return Hose: The inlet hose also serves as the return hose, and it goes from the tank to the pump and then to the control valve.

Pressure Line: A separate pressure line connects to the cylinder.

Operation: When you engage the PTO (power take-off), the pump starts spinning. Oil is then drawn from the tank, passes through the pump, and returns via the return line. The oil will circulate, keeping the pump cool. To raise the cylinder, the spool shifts, directing oil through the pressure line. Then, in the hold position (neutral), flow from the cylinder to the pump is cut off, maintaining the lowered position.

2. Concerns in Two-Line Systems:

Oil Accumulation: When the pump spins in neutral, oil can’t flow anywhere. About half a cup of oil remains trapped inside the gear section. Over time, this can cause pump distortion and catastrophic failure.

Spreading Gravel Scenario: If you raise the box halfway and put it in neutral while pulling forward, the pump continues spinning. Prolonged spinning generates heat, potentially damaging the pump.

Main Impact on Productivity: Benefits of a 3-line system

Faster Cycle Times and Reduced Maintenance

Operators handling high-load cycles can gain substantial benefits from faster cycle times. When lowering the box, the impact of this system becomes evident. Shifting the PTO into the lower position allows oil to flow down through the pressure line, back through the valve, and then through both the return and suction lines. This design minimizes resistance, resulting in quicker box descent.

Time Savings and Load Efficiency

Imagine your box coming down 15 seconds faster each time. If you handle around 50 loads per day, those saved seconds accumulate. For operators paid per load, the time saved compensates for the additional third hose needed in the three-line system. 

Filter Option on the Return Line

In the three-line system, you have the option to include a filter on the return line. This filter effectively removes sediment and contaminants from the hydraulic oil. While the oil continues to flow through both the return and suction lines, the filter ensures that every second cycle time, 100% of the oil is filtered. Even during idling with the PTO engaged, the filtered oil circulates back. The benefits of this filter include reduced sediment, increased component life, and an extended oil lifespan. 

Two-Line vs. Three Line FAQs

How do you switch from a two-line to a three-line? 

Install the Sleeve: Locate the suction port on your pump and insert a sleeve into this port. The sleeve is a key component for transitioning to a three-line setup.

Modify the Return Line: Remove the cap from the return line and connect a hose from the return line to the bottom of the hydraulic tank. This completes the three-line configuration.

How hot does the oil get when it gets recirculated when it's in the whole position for an extended period on a single act on the two-line system?

The oil temperature can vary significantly based on operating conditions, system design, and the specific fluid being used. In extreme cases, the oil temperature can reach up to 2300 to 4000 degrees Fahrenheit (approximately 126 to 204°C). When the oil gets too hot, it can cause damage such as burning off paint from the pump housing. In severe cases, the pump’s internal components may also be affected, leading to reduced pumping efficiency or complete failure.

How can you tell if your pump is starting to go bad?

Gradual Decline in Pump Performance: Operators may notice the pump working well in the morning but weakening as the day progresses. As oil warms up, its viscosity decreases and internal bypassing becomes easier, reducing efficiency.

Shaft Seal Leakage: If oil or fluid leaks around the pump’s shaft area, the seal may be compromised. Address shaft seal leaks promptly to prevent further damage.

What are the generalized costs for adding a filter and what are the benefits of both with it. If you choose not to do that, what are some of the risks that you could face?


The cost of adding a filter to a hydraulic system can vary based on factors such as the type of filter, system size, and brand. However, as a general estimate: The filter itself typically costs around $80 and additional expenses may include fittings, hoses, and installation labor. Some benefits included by adding a filter include sediment removal, prolonged pump life, improved system efficiency, and maintenance savings. However, risks included of not using a filter are increased wear and tear, reduced efficiency, higher maintenance costs, and risk of system failure. Overall, adding a filter is a relatively straightforward process with clear benefits. It’s a small investment that pays off in terms of system longevity and reliability. 

  • Two-Line vs. Three-Line Summary

The choice between a two-line and three-line hydraulic system isn't merely a matter of preference, it's a strategic decision that can significantly impact operational efficiency and bottom-line performance.

By grasping the nuances outlined in this discussion, operators can make informed decisions to maximize productivity and minimize downtime in their mobile power operations.

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