Roots Vacuum Pump Working Principle — How a Positive Displacement Pump Pulls Vacuum

If you’ve ever stood next to a tissue paper machine, a vacuum packaging line, or a pneumatic conveying system, you’ve heard a roots vacuum pump working — even if you didn’t know that’s what it was. It’s the machine that quietly holds a sheet of paper against a forming roll, that lifts a product off a conveyor, that evacuates a tank. Ask most people how it creates suction, and you’ll get a vague answer about “it sucks air out.” That’s true, but it hides the interesting part.

A roots vacuum pump doesn’t suck in the way a household vacuum cleaner does. It’s a positive displacement machine — it traps a fixed volume of gas and physically carries it from the inlet to the outlet, one pocket at a time. Understanding that single idea — positive displacement — is the key to understanding every other thing about how a roots vacuum pump works, where it belongs, and where it doesn’t.

This article explains the roots vacuum pump working principle from the rotor geometry up: the counter-rotating lobes, the timing gears that keep them apart, the micron-scale clearance that makes the pump oil-free, and the internal slip that sets its ultimate vacuum limit. We’ll also compare it head-to-head with liquid ring and rotary vane pumps so you can see exactly why a roots unit is the right — or wrong — choice for your vacuum duty.

What Is a Positive Displacement Pump?

Before the roots mechanism makes sense, you need the broader category it sits in: positive displacement.

A pump moves gas (or liquid) by one of two strategies:

• Dynamic (kinetic) pumps — a centrifugal blower, a turbo blower, a fan — accelerate the fluid with a spinning impeller and let the housing geometry convert that velocity into pressure. Flow and pressure are linked by a performance curve. Change the back-pressure and the flow slides along the curve.
• Positive displacement (PD) pumps — a roots blower, a gear pump, a piston compressor — trap a discrete volume of fluid in a cavity, isolate it, and move it from the inlet side to the outlet side. Each cycle displaces a known volume. The flow is essentially independent of the discharge pressure (until the machine runs out of power or mechanical strength).

This distinction matters because it explains the roots pump’s defining behavior: its pumping speed (throughput) stays nearly constant as the vacuum deepens, right up until it hits its mechanical compression limit. That’s the opposite of a centrifugal or liquid ring machine, whose throughput falls as vacuum deepens.

A positive displacement blower and a positive displacement vacuum pump are the same animal viewed from two ends. Run the same host machine so it pushes air out against resistance, and it’s a blower (pressure mode). Connect its inlet to a vessel and let its outlet sit at atmospheric pressure, and it pulls that vessel toward vacuum (vacuum mode). On Slovdson’s LC and SLV series, one dual-mode host does both — the rotor set, the timing gears, and the housing are identical; only the porting and the drive direction determine which mode you get.

Roots Vacuum Pump Working Principle — Inside the Housing

Here is the actual mechanism, step by step.

1. Two counter-rotating three-lobe rotors

Inside a precision-machined housing sit two rotors, each with three lobes. They turn in opposite directions (“counter-rotating”), timed so the lobes mesh past each other but never touch. On modern machines, the three-lobe profile has replaced the older two-lobe design because it gives smoother, lower-pulsation delivery.

As the rotors turn, each lobe sweeps a fixed volume of gas into the pocket formed between the lobe, the adjacent lobe, and the housing wall. The gas is carried around inside the casing from the inlet to the outlet and released. Because the geometry is fixed, every revolution displaces the same swept volume. That’s the “positive displacement” in action — the pump is a volumetric machine, not a velocity machine.

2. Timing gears keep the rotors apart

The rotors don’t mesh by contact. They’re synchronized by a pair of timing gears (the “dual gearbox” you’ll see in the specifications) mounted on the rotor shafts, outside the gas path. The gears set the precise phase relationship so the lobes pass with a controlled gap and never collide. Because the rotors are gear-driven rather than fluid-coupled, the speed is exact, and the phase never drifts — important when the vacuum pump runs for thousands of hours a year.

3. Non-contact clearance — why the pump is oil-free

This is the detail that makes a roots pump special. The clearance between the rotor tips, the rotors, and the housing is only a few tenths of a millimeter — tight enough to keep most gas moving forward, but the rotors and housing never physically touch. There’s no oil film in the gas path, no seals dragging, no lubricated contact surface to wear. The result is an oil-free, contamination-free vacuum on the process side. What little clearance exists is the pump’s only “seal” — and that clearance is also its defining limitation, as we’ll see next.

4. How vacuum is actually created

In vacuum mode, the pump’s inlet connects to the vessel or process you want to evacuate, and its outlet discharges to atmosphere. As the rotors carry gas pockets from the inlet around to the outlet, they reduce the gas density in the inlet line. Gas flows from the higher-pressure vessel into the now-lower-pressure inlet, where it gets trapped, carried, and dumped at atmospheric pressure at the outlet. Repeat that thousands of times a minute and the vessel pressure drops — you’ve created a vacuum.

The pumping speed (how many cubic meters per minute the pump can move) is set by the rotor size and speed and stays nearly constant as the vacuum deepens. What doesn’t stay constant is the pump’s ability to compress — and that brings us to the most misunderstood part of the roots vacuum pump working principle.

The Critical Limitation: Internal Slip Sets the Ultimate Vacuum

Because the rotors don’t touch and the only seal is a tiny clearance, a small amount of gas leaks backward from the high-pressure (outlet/atmospheric) side to the low-pressure (inlet) side through that clearance. Engineers call this internal slip (or back-leakage).

Internal slip grows as the pressure difference across the pump grows. At shallow vacuum, the slip is a small fraction of the throughput, so the pump is extremely effective. As the vacuum deepens and the pressure ratio rises, slip eats into the net pumping speed until, at some point, the gas leaking backward equals the gas being carried forward. At that point the pump can’t pull the vessel any deeper. That pressure is the roots pump’s ultimate vacuum — and it’s why a standalone roots vacuum pump is a high-pumping-speed, medium-vacuum device rather than a deep-vacuum device.

For Slovdson’s roots vacuum pumps, the practical working vacuum is around −45 kPa (roughly 0.55 bar absolute) as a standalone unit. For deeper vacuum — into the low-millibar range — a roots pump is used as a booster staged behind a backing pump (typically a liquid ring or rotary vane pump) that handles the final compression to atmosphere. The roots stage contributes enormous pumping speed at the intermediate pressure; the backing pump contributes the deep compression. Together they cover a range neither achieves alone.

This is the single most important takeaway for selection: a roots vacuum pump wins on throughput at medium vacuum, not on ultimate depth. If your process needs −45 kPa fast and clean, it’s ideal. If you need 1 mbar, you need it backed — or a different technology.

Vacuum Mode vs Pressure (Blower) Mode — Same Machine

It’s worth stressing, because it confuses buyers: the roots vacuum pump and the roots blower are the same mechanism. Flip the porting and drive direction and the vacuum pump becomes a pressure blower (and vice versa). Both are positive displacement, both use the same counter-rotating three-lobe rotors and timing gears, both are oil-free on the gas path.

• Blower (pressure) mode: outlet pushes against system back-pressure (aeration basin, pneumatic line); inlet draws from atmosphere.
• Vacuum (pump) mode: inlet draws from the process vessel; outlet discharges to atmosphere.

Slovdson’s LC energy-saving series is built around exactly this dual-mode idea — one PMSM + VFD-driven host that an operator switches between pressure and vacuum duty. The detailed drivetrain engineering (why a permanent magnet synchronous motor plus a variable frequency drive cuts power 30–65%) lives on our LC Series Energy-saving Roots Blower Vacuum Pump; the point for this article is simply that the vacuum half of that machine works by the positive displacement principle described above.

Roots Vacuum Pump vs Other Vacuum Technologies

“Roots” isn’t the only way to make vacuum. Here’s how it compares to the technologies you’ll actually be choosing between.

The practical read:

• Choose a roots vacuum pump when you need to move a lot of gas at moderate vacuum, cleanly and reliably — paper sheet transport, pneumatic conveying, central vacuum systems, tank evacuation.
• Choose a liquid ring pump when the gas is wet, dirty, or carries condensate, and ultimate depth matters more than throughput — common in chemical and process plants. (See our liquid ring vacuum pump.)
• Choose a rotary vane pump when you need deep vacuum and can accept oil in the path — labs, molding, vacuum packaging. (See our single-stage rotary vane vacuum pump.)
• For large, continuous, energy-sensitive duty, a magnetic levitation or air-suspension turbo vacuum pump delivers deep vacuum with the lowest running cost — see our maglev and air-suspension vacuum units.

→ Browse all Vacuum Pumps →

Where Roots Vacuum Pumps Are Used

The medium-vacuum, high-throughput profile fits a specific set of jobs:

• Tissue and paper converting. The application where Slovdson’s roots vacuum pumps are most often specified. In a tissue folding line, the vacuum pump grips each sheet against a suction belt and carries it through the fold geometry — napkin folders, facial tissue lines, and paper towel machines all depend on this. Unlike a paper machine wet end (which uses vacuum for dewatering), tissue converting uses vacuum for sheet transport at the folding section. Throughput stability — the PD characteristic — matters far more than ultimate depth, because a half-second vacuum dip at line speed means a misfed sheet and a stopped line. For tissue mills running 4–6 folding machines, the [37kW LC vacuum pump] is our most commonly sold model; for 6–8+ machines, mills typically specify the [45kW LC]
• Pneumatic conveying. Moving powder, pellets, or grain through a line needs steady vacuum against a pressure that rises as the line loads. A roots pump’s constant pumping speed holds the line moving.
• Packaging and material handling. Vacuum lifting, carton erecting, bottle handling, pick-and-place — all need clean, reliable medium vacuum on demand.
• Central vacuum systems. Hospitals, laboratories, and factories with many vacuum points benefit from a high-throughput roots booster backed by a smaller pump.
• Tank and vessel evacuation. Rapidly pulling a vessel from atmospheric to medium vacuum before a deeper pump takes over, or for processes that only need moderate vacuum anyway.

Sizing a Roots Vacuum Pump — Three Numbers

When we size a roots vacuum pump for a customer, three numbers decide everything:

1. Required pumping speed (m³/min or m³/h). How much gas must be removed, and how fast? Set by the process — web width, line speed, number of suction points, leakage.
2. Working vacuum (kPa or mbar). How deep must the vacuum be at the point of use? If it’s around −45 kPa or shallower, a standalone roots pump does it. Deeper, and we stage a backing pump.
3. Duty hours and electricity rate. A roots pump running 24/7 is worth equipping with the PMSM + VFD drivetrain (the LC series) — the 30–65% power saving compounds across thousands of operating hours. For intermittent duty, the cost-effective classic SLV (fixed-speed) is often the right call.

→ See the classic three-lobe roots vacuum pump (SLV) for the fixed-speed option, or the LC energy-saving series for the PMSM + VFD dual-mode version.

Frequently Asked Questions