A single-acting hydraulic power unit powers a cylinder in one direction and allows gravity or another external force to return it. A double-acting hydraulic power unit powers the cylinder in both directions. On a dump trailer, that usually means power-up/gravity-down versus power-up/power-down.
The choice affects your cylinder, hoses, electrical demand during lowering and reservoir size. Primary Mover supplies both Primary Mover and KTI power units. The right configuration starts with how your equipment needs to move.
Single Acting vs. Double Acting: A Quick Comparison
| Feature | Single Acting | Double Acting |
|---|---|---|
| Cylinder movement | Hydraulic power in one direction; external force returns the cylinder | Hydraulic power extends and retracts the cylinder |
| Typical dump operation | Power up, gravity down | Power up, power down |
| Working hoses to one cylinder | Usually one | Usually two |
| Motor during lowering | Normally off; the return valve is electrically operated | Normally running during powered lowering |
| Reservoir oil exchange | Supplies the oil needed to extend the cylinder and receives it back | Oil returns from one chamber while the pump supplies the other |
| Common applications | Compatible gravity-return dump beds and single-acting telescopic hoists | Compatible powered-return hoists, hydraulic jacks, tilt systems and equipment |
These descriptions cover typical circuits. Some power units add gravity-down or other control options. Confirm the exact model and hydraulic diagram.
How a Single-Acting Hydraulic Power Unit Works
Pressing UP starts the motor and pump, sending pressurized oil through the working line to extend the cylinder. Releasing the button stops the motor, and the circuit's check or holding arrangement retains the oil.
Pressing DOWN opens the return path. The weight of the dump body pushes oil out of the cylinder and back through the hose and valve to the reservoir. The motor normally does not run during this gravity-lowering function, although the electrically operated valve still needs power.
Single acting describes the cylinder's powered direction, not the number of valves inside the power unit. The manifold can contain a relief valve, check valve, lowering valve and flow-control components.
Choose this configuration when the cylinder and equipment are designed to return under gravity or another suitable external force.
How a Double-Acting Hydraulic Power Unit Works
A double-acting cylinder has a working connection to each chamber. During extension, the pump supplies the base end while oil from the rod end returns to the reservoir. During retraction, the valve reverses those connections: the pump supplies the rod end, and base-end oil returns to the tank.
The power unit therefore provides powered movement in both directions. In this conventional arrangement, returning oil goes through the reservoir; it does not simply pass directly from one cylinder port to the other.
Double acting is useful when the application requires powered retraction, such as a hydraulic trailer jack, or a hoist designed for powered lowering. Proper load-control valves and compatible plumbing remain important when gravity assists movement. Hydraulic holding does not replace the approved mechanical support required for service beneath a raised body.
Why Reservoir Sizing Is Different
Single Acting: The Tank Supplies the Cylinder's Working Oil
As a single-acting cylinder extends, oil leaves the reservoir and remains in the cylinder until it retracts. The tank must supply that working volume while retaining enough oil over the pickup. When the cylinder retracts, it must accept that oil without overflowing.
Long-stroke telescopic cylinders can require a substantial oil volume. Use the cylinder manufacturer's oil requirement, together with reserve oil, fill limits and installation angle. Stroke length alone does not establish the correct tank size.
Double Acting: Oil Exchanges Between Two Filled Chambers
In a conventional single-rod, double-acting cylinder, both chambers contain oil after the system is filled and bled. As the cylinder moves, one chamber returns oil while the other receives it.
The rod occupies space, so the two chambers hold different volumes. Across a full stroke, the net reservoir-level change equals the rod's displaced volume:
Net oil-volume change = rod cross-sectional area × stroke.
This often allows a smaller reservoir than a comparable single-acting arrangement. However, rod displacement is only the volume change to accommodate. It is not the complete tank-sizing requirement: the system also needs initial fill oil, reserve over the pickup, air space, room for thermal expansion and suitable cooling and air separation.
An Example of the Difference
Consider a conventional cylinder with an 8-inch bore, 3-inch rod and 30-inch stroke:
- The base-end swept volume is approximately 26.1 US quarts.
- The rod-end swept volume is approximately 22.4 US quarts.
- The difference is approximately 3.7 US quarts, which is the net reservoir exchange during a full stroke in the double-acting arrangement.
That explains the potential reservoir savings. It does not establish that a nominal 4- or 6-quart tank is adequate. Check usable capacity and the manufacturer's required operating levels. Multiple cylinders and telescopic designs require their own calculations.
Gravity-Down Speed Depends on the Return Path
A single-acting system can raise a large dump body successfully yet lower it slowly. On the way down, the available gravity-generated pressure must move oil through the cylinder port, hose, fittings, manifold passages and lowering valve. Restrictions anywhere in that path affect descent speed.
Primary Mover single-acting units use an SAE #8 work port. The KTI single-acting models we stock use an SAE #6 work port. Both directions use that working connection. Port size matters, but it is only one part of the complete flow path.
We offer standard PFP-SA and high-return-flow PFP-SAHO configurations. Selected high-flow packages for large-volume applications use a larger lowering-valve passage to reduce return restriction. Confirm the exact configuration in your kit rather than assuming every single-acting unit has the same return components.
Where an adjustable lowering-flow control is fitted, it can be set to suit the approved operating speed. More motor horsepower alone will not make gravity lowering faster when the motor is off. Return capacity, fluid viscosity, load, plumbing and valve design all matter.
Does Double Acting Lift More Weight or Move Faster?
Neither circuit type automatically lifts more. Available cylinder force depends on pressure acting over the effective piston area, including any opposing pressure. Actual dump capacity also depends on the hoist, mounting geometry and load.
Speed depends on delivered flow and the cylinder volume being filled. Compare flow at the required working pressure, not just a maximum-PSI label or no-load GPM figure. Battery voltage under load, motor and pump selection, wiring and restrictions also affect performance.
Our current standard Primary Mover 12V single-acting and double-acting units use a 3 HP (2.2 kW) motor. Choose the circuit for the job, then confirm pressure, working flow, reservoir capacity and duty-cycle requirements.
For more detail, read Why Hydraulic Pressure Matters in a Scissor Hoist System and Why Your Scissor Dump Isn't Dumping—and How to Fix It.
What Are Single Double and Dual Double Power Units?
In our Primary Mover combination-unit lineup:
- Single Double: one single-acting circuit and one double-acting circuit. A typical application is a gravity-down dump bed plus a powered-up/powered-down jack.
- Dual Double: two double-acting circuits. A typical application is a powered-up/powered-down dump bed plus a double-acting jack circuit.
Primary Mover also supplies KTI configurations for these applications. KTI combination-unit stock is more limited, so confirm availability. Naming varies between models; check the port diagram and functions rather than relying on the words alone. Separate circuits do not necessarily provide simultaneous operation.
Explore the Primary Mover Single Double power unit and Primary Mover Dual Double power unit.
Poly and Steel Reservoir Options
Across our power-unit range, poly options include 4, 5, 6, 8 and 13 quarts, with narrow rectangular and square configurations sold under the 13-quart name. Steel options include 6, 8, 10, 15 and 20 quarts, plus 6-, 8- and 10-gallon tanks.
Sizes and availability vary by brand and circuit. A catalog tank name is not a guarantee of usable oil volume. Check dimensions, mounting orientation, fill limits and the selected unit's actual usable capacity.
Which Hydraulic Power Unit Should You Choose?
- Choose single acting for a compatible cylinder and mechanism designed to return under gravity or another external force.
- Choose double acting when the cylinder needs hydraulic power in both directions.
- Consider a combination unit when you need separate dump and jack functions from one power unit.
- Size the complete system: confirm cylinder type, oil-volume requirements, pressure, loaded flow, electrical supply, mounting and controls.
Browse 12V DC hydraulic power units or all hydraulic power units. For help selecting a Primary Mover or KTI unit, call 985-888-6554 with your cylinder model, application and electrical-system details.
Common Questions
Does a Double-Acting Power Unit Use Two Pumps?
Not necessarily. A conventional unit can use one pump and directional valving to supply either cylinder chamber. Double acting describes powered movement in two directions.
Does a Single-Acting Power Unit Use Battery Power While Lowering?
The motor normally stays off, but the electrically operated lowering valve and controls still draw power. That usually reduces lowering-related battery use compared with running the motor for powered descent.
Can I Convert a Single-Acting System by Changing the Power Unit?
A pump swap alone does not turn a single-acting cylinder into a double-acting cylinder. Check the cylinder design, ports, hoses, valve arrangement and load controls. For adding a separate double-acting jack while retaining a compatible single-acting dump unit, see our X-Block guide.
Does a Double-Acting System Prevent Reservoir Overflow?
No. Overfilling, trapped air, inadequate usable capacity and load-driven movement that outruns the supply can still cause problems. Low voltage or an unsuitable control arrangement can contribute. Diagnose the cause before adding oil or changing valves.