A maintenance engineer at a machine tool plant spent three months arguing for a variable piston pump on a 140 bar circuit. His reference table said single-acting vane pumps stop at 7 MPa, and the numbers appeared to leave him no choice. A supplier engineer asked one question that changed the decision: which single-acting pump, the simple one or a pressure-compensated one? The compensated single-acting pump he had dismissed carried a catalog rating of 250 bar.
That reversal sits at the heart of the most misunderstood pump in fluid power. A single-acting vane pump isn’t a low-pressure design by nature. It’s an unbalanced design by nature, which is a different thing entirely, and the distinction decides what you can specify and at what cost.
This article covers what an unbalanced vane pump is, why the circular cam ring forces one pumping cycle per revolution, how that produces a radial side load that has to go somewhere, what the real pressure ceiling is today, and why variable displacement comes free with this architecture. It also covers where the design wins and where it should not be used at all.
This is the design deep dive. For the full family picture, start with our complete guide to hydraulic vane pumps.
Specifying a pump for a specific pressure and flow? Request a technical specification sheet, and our engineers will confirm whether an unbalanced or a balanced design is the right call for your duty.
What Is a Single-Acting (Unbalanced) Vane Pump?
A single-acting vane pump is a rotary positive-displacement pump in which each chamber between adjacent vanes draws in fluid once and discharges it once per rotor revolution, because the rotor turns eccentrically inside a circular cam ring.
That is the whole definition, and it carries two consequences. Because the circular bore allows only one region of expansion and one of contraction per revolution, the pump is also called an unbalanced vane pump. Discharge pressure acts on one side of the rotor and inlet pressure on the other, with no opposing pressure zone to cancel it out.
The component set is small and familiar. A rotor carries radial slots. Vanes slide in those slots and press against the cam ring, or stator ring, which on this design is a plain cylinder. A port plate carries the inlet and outlet windows, and two side plates close the rotor and ring into a sealed volume. The port plate has two windows only, one suction and one discharge, roughly opposite each other.
A naming trap worth stating early. The term “single-acting” also describes sliding-vane transfer pumps used for fuel, LPG, refrigerants, and chemical duty. Those are different machines, running at different clearances, viscosities, and pressure regimes, and they are not hydraulic power pumps. This article covers the hydraulic vane pump only.
For the step-by-step mechanics of suction, transport, and discharge, see how a vane pump works.
The Geometry: One Cycle per Rotor Revolution
The behavior of the design follows from one geometric fact. A circular bore offset from a circular rotor produces a crescent-shaped chamber whose radial height varies from zero at the contact point to twice the eccentricity, 2e, at the point diametrically opposite.
Each pocket between two vanes therefore passes through a fixed sequence as the rotor turns. It grows through the suction arc and draws fluid in. It crosses a sealing arc where volume stays roughly constant. It contracts through the discharge arc and pushes fluid out. Then it does it again, exactly once, on the next revolution.
A circular bore makes one cycle possible. A two-lobe, elliptical bore by definition produces two, which is the entire basis of the balanced design. That contrast is the reason the two architectures diverge on pressure, noise, and controllability.
The Displacement Formula for This Geometry
Because displacement tracks geometry directly, a variable design can be described exactly. Theoretical displacement per revolution is:
V_D = 2π × e × L × (D_C − e), equivalently π × e × L × (D_C + D_R)
where D_C is the cam ring bore, D_R is the rotor diameter, e is the eccentricity, and L is the vane width. For small eccentricities, it simplifies to roughly 2π × e × L × D_C, which is why eccentricity is the single lever that controls flow.
The Vane Set: Tilt, Count, and the Pulsation Trade-off
Two details in the vane set are widely misreported.
Vanes are set at a backward tilt rather than radially. The reason is starting torque. Tilting the vane backward aligns the combined tangential inertia and centrifugal force with the slot axis at start-up, so the vane is thrown outward instead of self-locking in its slot. Published values sit in the 13 to 24 degree range depending on the design.
Single-acting pumps then use an odd vane count, commonly 13 or 15. Adding vanes does reduce flow pulsation, but the return flattens quickly: published analyses of sliding-vane pumps put the useful gain at up to about six vanes, with only marginal improvement beyond that. So the count is not really a pulsation decision. It is driven by sealing and by how finely the pressure distribution is spread around the ring, and the odd number is the convention because an even count in this geometry produces a larger and more symmetric ripple.
The parity rule is usually stated backwards. Odd vane counts belong to the single-acting architecture. Balanced double-acting designs use an even count, typically 10 or 12, because their two-lobe symmetry demands it.
Where the Unbalance Comes From: The Radial Side Load
Here is the mechanism that everything else follows from. Every pocket in the discharge arc sits at discharge pressure. Every pocket in the suction arc sits at inlet pressure. The rotor therefore sees a net hydraulic force pushing it to one side, and that force travels through the rotor, then the shaft, then the bearings.
The magnitude is worth estimating, because almost no published description puts a number on it. The resultant is approximately the discharge-to-suction pressure difference acting on the rotor’s projected area over the loaded arc:
F ≈ Δp × D_R × L × k
Take a mid-size pump with a 70 mm rotor, 30 mm vane width, and a 70 bar pressure differential. The product Δp × D_R × L comes to about 14.7 kN, and with a coverage factor of roughly 0.7 to allow for pressure transition zones, the net side load lands near 10 kN. Treat that as an order-of-magnitude estimate with clearly stated assumptions, not a catalog figure. The point is that a kilonewton-scale sideways load on the end of a shaft isn’t a minor effect.
Two consequences follow directly. Bearing load becomes the design’s limiting factor rather than the pumping elements. And shaft deflection under that load distorts the running clearance, which is a leading cause of vane tip wear and side plate scuffing.
Tomás learned this the expensive way. A repair shop he worked at rebuilt the same unbalanced pump three times in fourteen months, replacing vanes and side plates each time. The wear pattern was always on one side of the cam ring. The pump was doing exactly what the geometry requires; a coupling misaligned by roughly 0.3 mm was adding mechanical load on top of the hydraulic side load. Realigning the drive and moving that duty to a balanced pump ended the cycle.
How Designers Manage the Unbalance
The load can’t be eliminated in a single-acting pump. It’s unbalanced by definition. What engineers can do is reduce it, and four families of techniques do the work.
Port-plate geometry. The most direct approach. A discharge port groove in one plate is mirrored by a substantially identical balancing groove in the opposite plate, interconnected at their leaving extremities, so flow is active in the balancing groove opposite the direction of vane travel (U.S. Patent 2,786,422). Computational fluid dynamics work on single-action vane pumps also shows that optimizing port plate angle and adding V-shaped cavities reduces radial force sharply.
Hydrostatic bearings. Offsetting the lubricant bay from the load line shifts the shaft’s running line and improves stability and load capacity, letting the bearing carry more load at the same pressure without a larger frame.
Shaft stiffness. Deflection falls as stiffness rises, so stiffening the shaft is a direct remedy. One documented design adds a rotor barrier inside a hollow shaft, a change the patent reports as raising shaft stiffness by nearly 15 times and reducing rotor deflection during start-up by about the same ratio (U.S. Patent 6,837,688).
Vane-level solutions. Intra-vane and pin-vane constructions apply system pressure only to a small controlled area, keeping tip loading light while still sealing. Twin-lip vanes with balancing bores reduce cam ring loading by orders of magnitude compared with single-lip vanes. Dual-vane designs put two nearly balanced vanes in each slot.
The honest summary is that these techniques buy pressure and life, and none of them changes the architecture. A single-acting pump remains a side-loaded pump.
The Pressure Ceiling: 7 MPa and the Modern Exception
This is where most published descriptions stop being useful. The classic figure is real, and the modern figure is very different.
The simplest, lowest-cost single-acting vane pumps are commonly rated at a maximum working pressure around 6 to 7 MPa, or 60 to 70 bar (roughly 870 to 1,015 psi). That number isn’t arbitrary. It’s what the unbalanced radial load allows when the pump is built to a low price point with ordinary bearings and a standard shaft.
Modern pressure-compensated single-acting pumps run far above it:
- Bosch Rexroth PV7 adjustable vane pumps are catalogued at a maximum operating pressure of 100 bar in the direct-controlled version and 160 bar in the pilot-operated version, over a drive speed range of roughly 1,000 to 1,800 rpm.
- Duplomatic RV1P variable displacement vane pumps cover 16 to 120 cm³/rev with a catalogue maximum operating pressure of 250 bar, and pressure compensator adjustment ranges from 20 to 250 bar on the smaller sizes. The data sheet specifies 800 to 1,800 rpm on sizes 016 through 063, fluid temperatures of 15 to 60 °C, a recommended viscosity of 22 to 68 cSt, and a maximum drain port pressure of 1 bar. It also states plainly that radial and axial shaft loads are not permitted on the pump shaft.
What changed isn’t the geometry but the hardware and the hydraulic design around it: intra-vane construction, pressure-compensated side plates that close the axial gap as pressure rises, heavy-duty bearings, short and stiff shafting, double hydrostatic axial compensation, and on the RV1P a response fast enough to eliminate a separate relief valve.
There is one more clarification that no vendor page makes, and it matters for procurement. The vane pump families catalogued at 175 to 320 bar don’t get there by pushing single-acting geometry harder. They get there by abandoning it. Eaton Vickers documents the V and VQ series as fixed-displacement balanced vane pumps, with the VQ rated to 210 bar and the VQH to 241 bar. Parker documents the Denison T6, T67, and T7 series as high-pressure balanced vane pumps reaching up to 320 bar. The industry’s answer to the radial side load was two-lobe geometry, not a stronger unbalanced pump.
| Design | Typical Maximum Pressure | Why |
|---|---|---|
| Classic simple single-acting (unbalanced) | 6 to 7 MPa / 60 to 70 bar | Low-cost bearings and shaft absorb the side load |
| Modern compensated single-acting (variable) | 100 to 250 bar | Compensated side plates, heavy-duty bearings, stiff shaft |
| Balanced double-acting (fixed) | 210 to 320 bar | Two opposing pressure zones cancel the radial load |
| Piston pumps | 350 to 450 bar and above | Different architecture entirely |
So the correct answer to “what is the maximum pressure of a single-acting vane pump” depends on which single-acting pump you mean, and the spread across the range is more than fourfold.
Need pressure and flow data against a real duty cycle? Contact our engineering team for a sizing review before you commit to a pressure class.
The Payoff: Why Variable Displacement Comes Free
Every disadvantage above has a mirror image that explains why the design survives. The cam ring is a plain circle, and a plain circle can be moved.
Because displacement is proportional to eccentricity, pivoting the ring sideways changes flow steplessly while the pump runs. Push the ring to zero eccentricity and displacement falls to zero. Manufacturers group the automatic versions into constant-pressure, constant-flow, and pressure-limiting types, and adjustment can be manual or automatic. That mechanical simplicity is the entire reason variable vane pumps exist, and it is why almost every variable vane pump on the market is a single-acting one.
A two-lobe balanced ring can’t be moved to change displacement without destroying the symmetry that balances it. The circular bore, in other words, is simultaneously the source of the pressure limit and the source of the controllability.
There is a second capability that follows from the same geometry. Reversing the direction of eccentricity swaps the suction and discharge circuits. Flow direction reverses with no valve change and no external plumbing. Neither a balanced vane pump nor a gear pump offers that, and on bidirectional machinery, regenerative circuits, or equipment that has to be reversed in the field, it can decide the specification on its own.
Single-Acting vs Double-Acting Vane Pumps: A Buyer’s Comparison
| Attribute | Single-Acting (Unbalanced) | Double-Acting (Balanced) |
|---|---|---|
| Cam ring shape | Circular bore, rotor eccentric | Elliptical two-lobe, rotor concentric |
| Port plate windows | 2 | 4 |
| Cycles per revolution | One suction, one discharge | Two suctions, two discharges |
| Vane count | Odd, typically 13 or 15 | Even, typically 10 or 12 |
| Radial load on shaft | High side load, one direction | Net zero |
| Displacement | Fixed or variable | Fixed only |
| Pressure, classic form | 60 to 70 bar | 175 to 210 bar, to 320 bar heavy-duty |
| Pressure, modern compensated form | 100 to 250 bar | Not applicable (fixed design) |
| Volumetric efficiency | About 92% | Above 94% |
| Flow pulsation | About 5 to 10% | Below 2% |
| Noise | Higher, especially variable versions | Low, comparable to internal gear |
| Flow reversal | Yes, by reversing eccentricity | No |
| Contamination tolerance | Lower | Lower than a gear pump either way |
| Relative cost | Lower than a variable piston pump | Similar to a fixed vane pump |
One more naming distinction. “Double-acting” describes the balanced internal geometry of a single cartridge. “Double vane” describes two or three cartridges stacked on one shaft. They are different things and the terms get swapped constantly. For detailed information, see our guide on double-acting vane pumps. For the multi-cartridge picture, see our gear pump vs vane pump comparison and the types of hydraulic pumps explained overview.
Where Single-Acting Vane Pumps Win
The design’s advantages map onto a narrow but valuable set of duties:
- Variable-flow circuits at low to medium pressure, where flow must track demand rather than run flat out
- Hydraulic cylinder circuits with fast and slow motion, where a variable pump reduces power loss, lowers oil heating, simplifies the circuit, and removes components
- Machine tools, presses, plastic and rubber processing machinery, die-casting, and metallurgical equipment where a variable pump is wanted below the cost of a variable piston pump
- Bidirectional and reversible-duty machinery, using the flow-reversal property
- Low-viscosity, clean-fluid transfer duty, with the caveat that this is a different machine class and a different buying decision
The limitations belong in the same list, because they are equally real. Flow pulsation of 5 to 10% is substantial. Noise is higher than a balanced design, particularly on variable versions. Volumetric efficiency sits several points below a double-acting pump. Contamination tolerance is poor. And there is a speed floor below which the design stops working, which brings us to the operating limits.
Operating Limits and How to Specify One
Speed has both a floor and a ceiling. Vane tips seal against the cam ring primarily through centrifugal force, so below roughly 600 to 900 rpm the vanes may not extend at all. The usable suction-speed window is often quoted around 8.3 to 25 r/s, and general products run at 1,500 to 2,000 rpm. Above the ceiling, fill becomes the limit and cavitation begins.
Amara hit the floor on an oversized power unit. Her machine only needed a fraction of the pump’s capacity, so the drive ran near 400 rpm. The vanes never fully extended; they bounced against the ring, and over a few thousand hours they wore a rippled pattern into the cam ring bore, commonly called washboarding. By the time the noise was investigated, the ring was past its wear limit and the whole pump needed replacing rather than a cartridge kit.
Viscosity and cold start matter for the same reason. Thick oil slows vane extension before the pump reaches its minimum speed, which is why cold-start behavior is a specification point on this design and rarely one on a gear pump.
Filtration is tighter than for a gear pump. Vane sealing is line contact at the tips, and the leakage path opens and closes with clearance, so 10 to 25 µm absolute is the working target. Cleanliness targets of ISO 4406 18/15 up to 140 bar and 17/14 for 140 to 210 bar are typical, working toward 18/16/13 on critical duty. An inlet strainer is generally not recommended.
Mounting and drive. Do not apply radial or axial load to the pump shaft; the RV1P catalog forbids it outright, and the 1 bar maximum drain pressure is part of the same requirement. Coupling alignment is commonly held within about 0.1 mm in field guidance and 0.05 mm in some OEM service literature. Bleed air at start-up.
When you write the RFQ, the useful data set is: required flow at each duty point and the ratio between them, continuous system pressure rather than relief setting alone, speed at the drive, fluid and viscosity window, ambient and cold-start conditions, control type, mounting and shaft form, port positions, rotation direction, and whether the circuit must be reversible.
Cost sits between a fixed vane pump and a variable piston pump. The decision usually comes down to duty cycle: the more hours a machine spends at reduced or zero flow, the faster the compensator pays for itself.
Frequently Asked Questions
What is a single-acting vane pump?
A rotary positive-displacement pump whose rotor turns eccentrically inside a circular cam ring, so each chamber between adjacent vanes draws in and discharges fluid once per revolution. It uses two port windows, one suction and one discharge.
Why is a single-acting vane pump called an unbalanced vane pump?
Because discharge pressure acts on one side of the rotor and inlet pressure on the other, with no opposing pressure zone. The pressure difference creates a net radial force that loads the shaft and bearings in one direction.
What is the maximum pressure of a single-acting vane pump?
It depends on the build. Simple low-cost designs are commonly rated around 6 to 7 MPa (60 to 70 bar). Modern pressure-compensated single-acting pumps are catalogued at 100 bar (Bosch Rexroth PV7 range) and up to 250 bar (Duplomatic RV1P).
Can a single-acting vane pump be made variable displacement?
Yes, and it’s the only vane architecture that can. Displacement is proportional to eccentricity, and a circular cam ring can be pivoted to change eccentricity steplessly while the pump runs. A two-lobe balanced ring cannot be moved without losing its balance.
How many vanes does a single-acting vane pump have?
Usually an odd number, commonly 13 or 15. More vanes reduce pulsation, and an odd count gives lower pulsation and less radial force fluctuation than an even count. Balanced double-acting pumps use an even count, typically 10 or 12.
Can a vane pump reverse its flow direction?
A single-acting vane pump can. Reversing the direction of eccentricity swaps the suction and discharge circuits, so flow direction changes with no valve or plumbing change. Balanced vane pumps and gear pumps can’t do this.
What causes the radial load, and how is it reduced?
It’s the discharge-to-suction pressure difference acting on the rotor’s projected area, producing a net side force of roughly Δp × D_R × L. Designers reduce it with balance grooves in the port plate, port angle optimization and V-shaped cavities, hydrostatic bearings, stiffer shafts, and intra-vane or twin-lip vane constructions. None of these eliminates it.
Is a single-acting vane pump the same as a single-acting sliding vane transfer pump?
No. They share a name and a sliding-vane principle, but a transfer pump moves low-viscosity fluids at low pressure with different clearances and materials. A hydraulic vane pump is a power component built for a hydraulic circuit.
Conclusion
A single-acting vane pump is defined by a circular cam ring and an eccentric rotor, which together produce one suction and one discharge per revolution. That same geometry produces a one-sided pressure distribution, and the resulting radial side load is what limits simple designs to around 6 to 7 MPa and makes bearings, not pumping elements, the critical component.
Three things follow, and they are what should drive a specification decision. The load scales directly with pressure, cam ring diameter, and vane width. The circular bore is why the design can be made variable displacement and why it can reverse its own flow. And the pressure ceiling is a function of how the pump is built, not of the architecture, spanning 60 bar on a simple unit to 250 bar on a compensated one, while the 210 to 320 bar families reach those numbers by switching to balanced geometry.
Match the pump to the duty. If flow varies and pressure stays moderate, the unbalanced design is often the most economical answer. If flow is constant or pressure is high, a balanced or piston design usually wins.
Request a technical specification sheet or an OEM quotation → Our engineers will confirm the pressure class, control type, and filtration spec for your circuit, and we supply vane, gear, and piston families from one verified source with performance testing on every unit.
