A Centrifugal Lubricating Device is a shaft-mounted oil reservoir that uses rotational speed to throw oil outward onto a crankpin, bearing, or gear surface. Unlike a wick-fed or drip-feed oiler that meters oil by gravity, a centrifugal lubricator only delivers oil while the shaft spins, scaling oil flow with RPM. The purpose is to keep highly loaded rotating joints — particularly steam engine crankpins — fed with fresh oil under load without operator attention. Result: a self-regulating feed that delivers more oil at higher speed, exactly when bearing pressure-velocity demand is highest.
How the Centrifugal Lubricating Device Actually Works
The device is mounted on the rotating part it lubricates — typically a crankpin, eccentric, or shaft collar. Oil sits in a small reservoir cast or screwed into the rotating body. As the shaft turns, centrifugal force pushes the oil outward through a small drilled passage that emerges directly at the bearing surface. At rest the oil stays put because gravity alone is not enough to push it past the metering hole or weighted plug. Spin the shaft past a threshold speed — usually 60 to 120 RPM on a slow stationary engine — and oil flows continuously down the bore onto the journal.
The geometry matters. The metering hole is drilled radially outward from the centre of rotation; oil pressure at the hole exit equals ρ × ω² × r × Δr, where Δr is the radial distance from the oil's free surface to the bearing. If the hole is too large the reservoir empties in minutes and the bearing runs dry. Too small and oil starves at the high-load corners of the stroke. On a Crossley or Tangye horizontal engine the typical metering hole is 1.0 to 1.6 mm — not 2 mm, not 0.5 mm. Drill it wrong and you either flood the engine room with oil mist or wipe the brass.
Failure modes are predictable. Sludge from old oil blocks the metering hole — symptom is a hot crankpin smelling of scorched grease within 20 minutes of start-up. A loose or missing weighted plug causes oil to siphon out at rest, leaving the reservoir empty next morning. And if the reservoir cap leaks at speed, oil flings outward in a fine mist that coats the flywheel and the engine room walls. You see this on neglected gas and steam engines — a tell-tale oily ring on the floor under the crankpin's path.
Key Components
- Oil reservoir body: A bored or cast cavity in the rotating part, typically holding 50 to 250 ml of oil depending on engine size. Wall thickness must withstand both centrifugal hoop stress and the cyclic bending the crankpin sees — for a 100 mm diameter reservoir at 250 RPM, wall stress is modest but fatigue at the threaded cap matters.
- Metering orifice: A radial hole 1.0 to 1.6 mm diameter drilled from the reservoir floor outward to the bearing surface. Diameter sets the oil delivery rate; deburr both ends or sharp edges will shear the oil and starve the journal.
- Weighted plug or non-return ball: Sits over the inner end of the metering hole and lifts only when centrifugal force exceeds its weight. Prevents drain-down at rest. A typical plug is brass, 6 to 10 grams, sized so it lifts at roughly 60 RPM.
- Filler cap: Screwed plug with a fibre or copper washer for sealing under centrifugal pressure. Must be tight — a finger-loose cap will weep oil mist at speed and leave the engine running on whatever sits in the bottom of the reservoir.
- Oil delivery groove: Shallow channel cut into the bearing journal that distributes the oil emerging from the metering hole around the full pin circumference. Without this groove oil reaches only one quadrant of the brass and the opposite side wipes.
Who Uses the Centrifugal Lubricating Device
Centrifugal lubricators show up wherever a heavily loaded bearing rotates with the shaft itself, making it impractical to feed oil from a stationary drip cup. They were standard equipment on stationary steam and gas engines from roughly 1880 through the 1940s, and you still find them on heritage machines and a handful of niche industrial applications today.
- Heritage Steam Engines: Crankpin oilers on Robey, Marshall, and Ruston horizontal mill engines preserved at sites like Kew Bridge Steam Museum and Markfield Beam Engine.
- Stationary Gas Engines: Crossley Brothers hot-tube ignition engines used the same centrifugal cup design on the crankpin, visible on running examples at the Anson Engine Museum near Stockport.
- Marine Auxiliary Machinery: Vibrating-pattern feed pumps and small auxiliary engines on Edwardian steam launches, where a stationary oiler could not reach the rotating eccentric strap.
- Textile Mill Line Shafts: Shaft-collar oilers feeding plummer block bearings on the long horizontal line shafts at preserved Lancashire cotton mills like Queen Street Mill in Burnley.
- Locomotive Side-Rod Bearings: Crank-disc oilers on side rods of narrow-gauge industrial locomotives, including Bagnall and Hunslet types used at preserved tin mines and quarry railways.
- Agricultural Steam: Crankpin lubricators on Burrell and Fowler traction engines that compete in road runs at events like the Great Dorset Steam Fair.
The Formula Behind the Centrifugal Lubricating Device
The useful number to compute is the oil delivery pressure at the metering hole exit, which determines whether oil flows to the bearing or sits in the reservoir. At low shaft speed the centrifugal pressure is barely enough to lift the weighted plug — the engine starts dry and depends on residual film for the first revolutions. At nominal running speed the pressure climbs into a comfortable working range and oil flows steadily. Push past the design speed and pressure rises with the square of RPM, which is why an over-speeded engine slings oil everywhere. The sweet spot is whatever pressure delivers your design oil flow without atomising the oil into mist.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| P | Centrifugal pressure at the metering hole exit | Pa | psi |
| ρ | Oil density (typical steam cylinder oil ≈ 900 kg/m³) | kg/m³ | lb/ft³ |
| ω | Shaft angular velocity (= 2π × N / 60 where N is RPM) | rad/s | rad/s |
| r2 | Radius from shaft centre to metering hole exit | m | in |
| r1 | Radius from shaft centre to oil free surface inside reservoir | m | in |
Worked Example: Centrifugal Lubricating Device in a restored Robey horizontal mill engine crankpin oiler
You are sizing the centrifugal crankpin oiler reservoir on a 1908 Robey 12-inch by 24-inch horizontal mill engine being restored at a preserved rope works in Chatham. Crank throw is 305 mm so the crankpin centre orbits at r = 0.305 m. The reservoir is bored into the crank disc with the oil free surface at r<sub>1</sub> = 0.180 m and the metering hole exit at r<sub>2</sub> = 0.305 m. Engine design speed is 140 RPM, with a typical operating range of 90 to 180 RPM depending on load. Oil is medium steam cylinder oil at ρ = 900 kg/m³.
Given
- ρ = 900 kg/m³
- r1 = 0.180 m
- r2 = 0.305 m
- Nnom = 140 RPM
- Nlow = 90 RPM
- Nhigh = 180 RPM
Solution
Step 1 — convert nominal RPM to angular velocity:
Step 2 — compute the radial term (r22 − r12):
Step 3 — calculate centrifugal pressure at the metering hole at nominal 140 RPM:
That is a comfortable working pressure — enough to push oil steadily through a 1.2 mm metering hole and lift a 7 g brass plug. Now check the low end of the range, 90 RPM:
At 90 RPM the pressure has dropped by more than half because pressure scales with the square of speed. The plug still lifts, but oil flow is sluggish — on a cold start at light load you may see the crankpin running drier than ideal for the first few minutes. Now the high end, 180 RPM:
At 180 RPM oil flow nearly doubles over nominal. This is what you want under heavy load — but if your filler cap is not properly sealed with a fresh fibre washer, you will see oil misting from the cap at this pressure and a fine spray coating the flywheel rim within an hour.
Result
Nominal centrifugal pressure at the metering hole is roughly 5,860 Pa (0. 85 psi) at 140 RPM with a 1.2 mm orifice. At that pressure oil flows steadily and the crankpin runs cool to the touch after a 30-minute warm-up. Across the operating range, pressure climbs from 0.35 psi at 90 RPM to 1.41 psi at 180 RPM — a four-fold spread, with the sweet spot sitting between 130 and 160 RPM where flow is generous but the cap is not yet weeping. If you measure a hot crankpin or scorched-oil smell despite a full reservoir, the three usual culprits are: (1) sludged metering hole — pull the plug, run a 1.0 mm drill through by hand and re-prove flow with thin oil; (2) wrong oil grade — winter-weight oil drops viscosity and the radial pressure formula assumes the oil reaches the orifice, which it cannot do if it has thinned to a film clinging to the reservoir wall; (3) reservoir filled past the inner edge meaning r<sub>1</sub> sits closer to the shaft than designed, reducing Δr<sup>2</sup> and choking flow until enough oil has been thrown out to expose the proper free surface.
Centrifugal Lubricating Device vs Alternatives
Centrifugal lubrication is one of three classical ways to feed oil to a moving bearing on a steam or gas engine. Each suits a different combination of speed, accessibility, and operator attention. Here is how it stacks up against the gravity drip feed and the mechanical force-feed lubricator.
| Property | Centrifugal Lubricator | Gravity Drip Feed | Mechanical Force-Feed Lubricator |
|---|---|---|---|
| Operating speed range | 60–600 RPM, oil flow scales with RPM² | 0–any RPM, flow independent of speed | Any RPM, flow set by ratchet drive |
| Suitable bearing location | Rotating with the shaft (crankpin, eccentric) | Stationary bearings only | Any bearing — stationary or via flexible feed pipe |
| Oil delivery accuracy | Coarse — depends on RPM, oil temperature, fill level | Moderate — depends on level and viscosity | Precise — metered drops per minute, repeatable |
| Cost and complexity | Cheapest — a hole and a plug | Cheap — sight glass and needle valve | Expensive — multi-feed pump body, ratchet drive, check valves |
| Failure mode if neglected | Sludged orifice → wiped brass within an hour | Empty cup → silent dry start, scoring | Broken ratchet pawl → silent zero feed to one bearing |
| Typical service interval | Refill each shift, clean orifice yearly | Refill several times per shift | Refill weekly, rebuild every 5 years |
| Application fit | Steam and gas engine crankpins, eccentrics | Cylinder oilers, stationary plummer blocks | High-value cylinders, modern industrial engines |
Frequently Asked Questions About Centrifugal Lubricating Device
Centrifugal pressure is fine — your problem is almost certainly distribution, not delivery. Without an oil groove cut around the journal, oil emerges from the metering hole and clings to one quadrant of the brass, leaving the opposite side dry. On a steam crankpin the load reverses every half revolution, so a journal that is oiled on only one side will scuff on the unfed side regardless of how much oil you throw at it.
Pull the brasses, scribe a shallow helical groove around the pin (1 mm wide, 0.5 mm deep is plenty), and the heat will disappear within ten minutes of restart.
Rule of thumb: aim for the reservoir to last one shift — typically 8 to 10 hours of running. For a 200 ml reservoir at 140 RPM you want roughly 0.3 ml/min, which a 1.2 mm hole delivers comfortably with medium cylinder oil. Increase to 1.6 mm and the same reservoir empties in under three hours.
The cleanest way to prove it is bench-test: rig the crank disc on a slow-speed motor, fill with the actual oil grade you will use at operating temperature, and time the drain. Don't trust calculation alone — orifice discharge coefficient varies wildly with edge condition and oil viscosity.
If the engine was originally fitted with centrifugal oilers, keep them — a Wakefield or Mollerup force-feed pump on a Victorian crank disc looks wrong and forces you to route a feed pipe across a rotating part, which never ends well. Centrifugal oilers are also forgiving of the slow, dirty starts a heritage engine sees during cold demonstrations.
Force-feed makes sense when the engine runs continuously at high duty and you cannot afford the variable feed rate. For most preservation work — running a few hours on demonstration days — the original centrifugal oiler is the right answer, both authentically and operationally.
Your weighted plug or non-return ball is missing, stuck open, or sized wrong. At rest, the only thing stopping oil from siphoning down the metering hole and out onto the bearing is the plug sitting on the inner seat. If the plug is too light it will not seal; if it is corroded to one side of the bore it will hang open.
Pull the plug, weigh it (typical is 6–10 g brass), check the seat is clean and the plug drops freely under its own weight. A common preservation-era error is replacing a lost brass plug with a steel ball bearing — steel is denser, the plug now lifts at a higher RPM, and the engine runs dry through start-up.
It matters more than people think. The pressure formula assumes a continuous oil column from the free surface to the orifice — but at low speed and low temperature, a thick steam cylinder oil (ISO VG 460 or heavier) will not flow across the reservoir floor fast enough to feed the orifice continuously. You get pulsing delivery and dry intervals.
For preservation engines running below 60°C oil temperature, drop to ISO VG 220 or a dedicated steam cylinder oil rated for the temperature range you actually operate in. Save the heavy compound oils for engines that run long enough to reach proper operating temperature.
Above roughly 1 psi reservoir pressure, any leak path in the filler cap atomises oil into a fine spray that the rotating crank disc throws outward like a centrifuge. The oiler is working — the cap is failing.
Replace the fibre or copper washer under the cap with a fresh one cut to the correct OD, and torque the cap properly. Don't rely on a worn washer that has been compressed flat; the centrifugal pressure pushes oil through micro-channels in the squashed fibre that are invisible at rest.
References & Further Reading
- Wikipedia contributors. Lubrication. Wikipedia
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