A Knott Rotary Pump is a sliding-vane positive-displacement pump used in trailer overrun brake actuators, where an eccentric rotor with spring-loaded vanes converts drawbar push-rod motion into hydraulic pressure for the wheel cylinders. Typical units run at low rotational speeds — under 50 RPM during a brake application — and deliver line pressures of 60-100 bar through a displacement of roughly 8-15 cm³ per revolution. The design replaces a conventional master cylinder on heavier trailers, giving smoother braking modulation. Knott GmbH of Eggstätt, Germany has fitted variants of this pump to thousands of European braked trailers since the 1970s.
Knott Rotary Pump Interactive Calculator
Vary pump displacement, shaft speed, brake pressure, and volumetric efficiency to see flow, torque, power, and leakage change in a rotary vane pump.
Equation Used
This calculator uses the positive-displacement pump relationship for a sliding-vane Knott rotary pump. Displacement multiplied by shaft speed gives theoretical flow; volumetric efficiency reduces that to delivered flow. Pressure and displacement set the ideal shaft torque required to generate brake-line pressure.
- Displacement is geometric pump displacement per revolution.
- Pressure is outlet gauge pressure relative to inlet.
- Volumetric efficiency accounts for internal leakage only.
- Torque is ideal hydraulic torque and excludes bearing and seal friction.
Inside the Knott Rotary Pump
The Knott Rotary Pump sits inside the overrun brake assembly on a trailer drawbar. When the towing vehicle slows, inertia pushes the trailer forward against the drawbar, and that linear push-rod motion gets converted by a rack-and-pinion or lever arrangement into rotation of the pump shaft. Inside the pump body, an eccentric rotor carries 4 to 6 sliding vanes in radial slots. As the rotor turns inside the offset bore, the vanes slide outward under spring pressure and centrifugal force, sweeping a crescent-shaped chamber that grows on the inlet side and shrinks on the outlet side. That shrinking chamber is what generates hydraulic pressure to the wheel cylinders.
The geometry is unforgiving. Vane tip clearance against the housing bore must sit between 5 and 15 µm — go tighter and you get galling on cold mornings when the steel housing contracts faster than the brass vane carrier; go looser and internal leakage drops the line pressure below the threshold needed to pull the brake shoes against the drum. If you notice the trailer brakes feel spongy or lag the tow vehicle by half a second, the most common cause is exactly this — vane tip wear past 20 µm clearance, often combined with a weakened vane spring that no longer holds the tip against the bore at the start of rotation. Other failure modes you would see in a typical Knott KF or KR-series unit are inlet check-valve seat erosion, which shows up as the pedal feel going soft after the trailer sits parked for a week, and rotor shaft seal weep, which contaminates the brake fluid with axle grease and turns the fluid milky.
The sliding vane pump architecture is chosen here over a gear or piston pump because the rotary positive displacement principle gives near-linear pressure response to drawbar travel, which is what the driver actually feels through the tow vehicle. A gear pump would pulse, a piston pump would need a complex cam, and neither would tolerate the long parked dwell times typical of a caravan.
Key Components
- Eccentric Rotor: The hardened steel rotor sits offset from the housing bore centreline by 2-3 mm, creating the crescent chamber that does the pumping work. Rotor surface hardness must hit 58-62 HRC because the vane slot edges see fatigue loading every revolution.
- Sliding Vanes: 4 to 6 vanes made from sintered bronze or composite, sliding freely in radial slots. Each vane carries a small coil spring at its base to keep the tip against the bore at low rotation speeds — without that spring, centrifugal force alone is too low at the 30-50 RPM brake-application range.
- Pump Housing Bore: Honed steel cylinder, surface finish Ra ≤ 0.4 µm. The bore must be round to within 8 µm — any ovality and the vanes lose contact at two points per revolution, dumping pressure across the chamber.
- Inlet/Outlet Check Valves: Spring-loaded ball or plate valves separating the suction port from the high-pressure delivery line. Crack pressure typically 0.3-0.5 bar; seat leakage above 1 cm³/min causes pedal-pressure decay during long parked periods.
- Drive Shaft and Seal: Connects the drawbar lever mechanism to the rotor. Lip seal rated to 100 bar dynamic, with a backup wiper to keep road grit out. Seal leakage past 0.05 cm³/hr means brake fluid contamination within a year.
- Reservoir Tap: A small port that lets the pump draw replacement fluid from the master reservoir on the return stroke, compensating for shoe wear take-up volume — typically 1-2 cm³ per stroke.
Industries That Rely on the Knott Rotary Pump
You find the Knott Rotary Pump and its close cousins on practically every European-market braked trailer above 750 kg gross weight. The mechanism is favoured because it survives years of outdoor parking, handles long towing intervals between brake applications, and gives the driver the smooth, progressive pedal feel that an inertia-only mechanical brake cannot match. The rotary positive displacement principle scales cleanly from a 1,300 kg utility trailer up to a 3,500 kg twin-axle horsebox.
- Caravan & Leisure: Hobby and Fendt touring caravans use Knott KF-series rotary pump overrun couplings on chassis from AL-KO and BPW, sized for 1,800-2,500 kg gross.
- Equestrian Transport: Ifor Williams HB511 and HB506 horseboxes fit the Knott KFG35 overrun assembly with integrated rotary pump for the front axle hydraulic brake circuit.
- Boat Trailers: Brenderup and Snipe boat trailers above 1,500 kg run Knott rotary pump actuators because the sealed hydraulic circuit tolerates saltwater immersion better than mechanical rod linkages.
- Plant and Construction: Indespension tipper trailers and Nugent plant trailers carrying mini-excavators use the Knott KR30 series for 3,000-3,500 kg rated hydraulic braking.
- Agricultural Trailers: Krone and Conow farm trailers up to 3,500 kg fit Knott overrun couplings on light grain and silage carriers where a full air-brake system would be overkill.
- Motorsport Transport: Brian James race-car transporters use the Knott KFG27 with integrated rotary pump and reverse-lock-out solenoid, common at British circuits like Silverstone and Donington.
The Formula Behind the Knott Rotary Pump
The core sizing question is line pressure as a function of rotor displacement and shaft speed during the brake application. At the low end of typical operating range — say 20 RPM during a gentle highway slowdown — the pump barely moves enough fluid to take up the wheel cylinder shoe-clearance volume, which is why some drivers feel a brief delay on the first soft brake pedal touch. At the nominal operating point of 40-50 RPM during normal deceleration, the pump hits its design pressure of 70-80 bar within 0.3 seconds. Push the drawbar hard during an emergency stop and shaft speed can spike to 80-100 RPM, which is where check-valve dynamics and vane spring response start limiting flow rather than displacement geometry.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Q | Volumetric flow rate delivered to brake lines | cm³/s | in³/s |
| Vd | Pump displacement per revolution (vane chamber volume × number of chambers) | cm³/rev | in³/rev |
| N | Rotor shaft speed during brake application | rev/s | RPM |
| ηv | Volumetric efficiency, accounting for vane tip leakage and check-valve losses | dimensionless | dimensionless |
| P | Hydraulic line pressure delivered to wheel cylinders | bar (or Pa) | psi |
| Fdrawbar | Inertial force pushing drawbar forward into pump | N | lbf |
| Apiston | Effective rotor pumping area (related to displacement and rotor stroke) | cm² | in² |
Worked Example: Knott Rotary Pump in a 2,500 kg twin-axle horsebox
You are sizing the rotary pump output for a Knott KFG35 overrun coupling fitted to a 2,500 kg twin-axle horsebox built by Equi-Trek at their Yorkshire works. The pump has 6 vanes, a chamber volume of 2.0 cm³ each (giving Vd = 12 cm³/rev), and a volumetric efficiency ηv of 0.92 when new. You want to verify line flow across the realistic operating range during braking from highway speed.
Given
- Vd = 12 cm³/rev
- ηv = 0.92 dimensionless
- Nnominal = 45 RPM
- Nlow = 20 RPM
- Nhigh = 90 RPM
Solution
Step 1 — convert nominal shaft speed from RPM to rev/s:
Step 2 — compute nominal flow at the design operating point during normal motorway slowdown:
That is enough to fill the wheel-cylinder take-up volume of roughly 2.5 cm³ per axle in under 0.3 seconds — fast enough that the driver feels the trailer pulling back almost in sync with the tow car's brake pedal.
Step 3 — at the low end of the typical operating range, 20 RPM during a gentle deceleration:
At this flow you get a noticeable lag — the wheel cylinders take roughly 0.7 seconds to reach effective pressure, which is why a soft pedal touch on a long downhill feels like the trailer is briefly absent before catching up. Knott tunes the drawbar damper specifically to mask this lag, but you cannot eliminate it.
Step 4 — at the high end, 90 RPM during a panic stop:
In theory. In practice the inlet check valve cannot reseat fast enough above ~75 RPM, so ηv collapses from 0.92 down to about 0.70, and real flow tops out near 12.6 cm³/s. That ceiling is exactly why the Knott couplings include a mechanical bump-stop on the drawbar — it caps shaft speed before the pump goes into cavitation.
Result
Nominal pump flow comes out at 8. 28 cm³/s at 45 RPM, the design point for a 2,500 kg horsebox during normal motorway braking. That number means the brakes engage with about a 0.3-second build-up — quick enough to feel synchronised with the tow vehicle but soft enough to avoid jackknife snatch. Across the range, the pump delivers 3.7 cm³/s at the lazy 20 RPM end (noticeable lag, masked by the drawbar damper) and tops out near 12.6 cm³/s during a panic stop because check-valve reseating limits the high end well below the 16.6 cm³/s the geometry would predict. If your measured line pressure builds slower than this — say 0.6 seconds to reach 70 bar instead of 0.3 — check vane tip clearance first (worn past 20 µm causes internal leakage that flattens flow), then inspect the rotor shaft seal for fluid weep, and finally pressure-test the inlet check-valve seat for backflow leakage above 1 cm³/min, which kills the first stroke after a long parked period.
Choosing the Knott Rotary Pump: Pros and Cons
The Knott Rotary Pump is one of three serious options for trailer overrun braking. The choice usually comes down to gross trailer weight, how often the trailer is parked, and how much pedal-feel quality the driver demands. Here is how the rotary pump stacks up against a conventional master cylinder and a purely mechanical rod-and-cam system.
| Property | Knott Rotary Pump | Master Cylinder (linear) | Mechanical Rod Linkage |
|---|---|---|---|
| Operating speed range | 20-90 RPM rotor | Linear, 0-150 mm/s rod | Linear, drawbar-direct |
| Line pressure delivered | 60-100 bar | 40-80 bar | N/A (mechanical force only) |
| Pedal feel modulation | Smooth, progressive | Slightly stepped near piston cup seal | On/off character, no modulation |
| Tolerance to long parked periods | Good — vanes self-seat on first revolution | Poor — piston cup takes a set, soft first pedal | Excellent — no fluid involved |
| Typical service life | 100,000+ km or 10 years | 60,000-80,000 km, cup seal limits life | Indefinite, but adjustment every 5,000 km |
| Maintenance interval | Fluid change every 2 years | Fluid change yearly, cup overhaul at 5 years | Cable/rod adjustment every 5,000 km |
| Maximum trailer GVW | 3,500 kg | 2,700 kg practical | 1,500 kg legal limit (EU) |
| Relative cost (replacement unit) | £280-420 | £180-260 | £90-150 |
| Cold-weather performance | Stable to -25 °C | Stiffens below -10 °C | Best — unaffected |
Frequently Asked Questions About Knott Rotary Pump
That is almost always inlet check-valve seat drying, not pump wear. When a trailer sits for months, the brake fluid drains slowly past a marginal check-valve seat back into the reservoir, leaving the pump chambers air-bound on the first stroke. The first brake application then spends its displacement just refilling the chambers rather than building line pressure.
Quick diagnostic: pump the overrun coupling by hand 4-5 times before driving off after a long park. If the pedal firms up by stroke 3, the check valve is the cause. Fix is usually a £40 valve cartridge replacement — Knott sells these as a service kit for the KF and KFG series.
Vane count drives pressure ripple, not peak pressure. A 4-vane pump delivers the same nominal flow but with roughly 18% pressure pulsation between vane crossings, while a 6-vane pump drops that to about 8%. On a heavy single-axle trailer the 4-vane ripple is invisible to the driver, but on a twin-axle horsebox carrying live cargo, the 6-vane is worth the cost premium because the smoother pressure curve avoids triggering the horse to brace and unbalance the load.
Rule of thumb: under 2,000 kg or inert cargo, 4-vane is fine. Over 2,500 kg or live/fragile cargo, spec the 6-vane every time.
A 30% pressure shortfall on a unit that otherwise functions points to one of two things. First, drawbar lever ratio drift — the linkage that converts drawbar push to rotor torque uses a sliding pivot that can wear an oval hole over years, dropping mechanical advantage. Inspect the pivot bushing; if the bore has gone from 12.0 mm to 12.4 mm, you have lost roughly 4% of output per 0.1 mm of wear.
Second, vane spring fatigue. The little coil springs behind each vane lose 15-20% of their preload after 80,000 km, and at low rotation speeds (under 30 RPM) centrifugal force alone is not enough to seal the vane tips. New vane spring kits are cheap; replace all of them as a set, never piecemeal.
Yes, but the displacement maths changes. Disc-brake calipers need much less fluid volume per stop than drum wheel cylinders — typically 0.4 cm³ per caliper versus 2.5 cm³ per drum cylinder. Pair a standard Knott KFG35 (sized for drums) with disc calipers and the pump over-pressures the system in the first 50 ms of a stop, which feels like a stab brake to the driver.
The fix is either a smaller-displacement pump variant (Knott offers a 8 cm³/rev version) or fitting an in-line pressure damper. Most aftermarket disc-brake conversion kits for European trailers include the damper specifically for this reason.
Because the drawbar sees additional gravitational pre-load on a downgrade. The trailer's own weight component along the slope pushes the drawbar forward independent of the tow vehicle's braking, and the rotary pump cannot tell the difference between inertial push and gravity push. On a 10% downgrade, a 2,500 kg trailer adds roughly 2,450 N of extra drawbar force before the tow vehicle even touches the brake.
This is why every Knott coupling includes a reverse-lockout and why downhill snatching is normal at low speeds. If the effect is severe enough to cause snatching above 30 km/h, the drawbar damper is likely worn — the gas damper inside the coupling tube limits how fast the pump shaft can spin during a transient push, and damper failure removes that filtering.
The pump itself is fine — it does not run unless the drawbar pushes. The damage happens at the wheel cylinders and shoes. With the handbrake partially on, the wheel cylinders sit pressurised by the mechanical handbrake linkage, so any extra hydraulic input from the rotary pump during normal road speed undulations stacks on top. Brake-fluid temperature can hit 200 °C within 20 km, and that boils moisture in the fluid into vapour. The next time you actually need the brakes, the pump pushes against compressible vapour instead of liquid, and you get no braking.
Diagnostic clue: smell the wheel hubs after a short tow. If they smell like overheated brake fluid (sweet, slightly chemical), check the handbrake release before you drive another metre.
References & Further Reading
- Wikipedia contributors. Rotary vane pump. Wikipedia
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