A Centrifugal Crank-pin Oiler is a small oil reservoir bolted to the rotating crank web that uses centrifugal force to feed lubricant outward through a drilled passage into the crank-pin big-end bearing. Joseph Whitworth's workshops popularised the cup form on machine tools in the 1850s, and steam engine builders like Robey and Tangye adopted it by the 1880s. The cup spins with the crank, oil migrates outward against the cup wall, and a metered hole at the outer rim delivers a steady drip to the bearing surface — no external pump, no tubing, no shaft seal to fail.
Centrifugal Crank-pin Oiler Interactive Calculator
Vary crank speed, oiler radius, and oil density to see the centrifugal head and pressure that drive oil toward the crank-pin bearing.
Equation Used
The calculator converts crank speed to angular velocity, then estimates centrifugal oil head at the oiler radius. Rim pressure is found from P = rho g h, equivalent to P = 0.5 rho omega^2 r^2 for oil rotating from the crank center to the cup rim.
- Oil rotates with the crank cup at steady speed.
- Head is calculated from crank center to the metering-orifice radius.
- Pressure is gauge pressure at the cup rim before orifice and viscosity losses.
- Flow restriction, felt clogging, and bearing back-pressure are not included.
How the Centrifugal Crank-pin Oiler Actually Works
The principle is simple — anything heavier than air, including oil, pushes outward when you spin it. Bolt a small cup of oil onto the crank web, drill a fine passage from the cup's outer wall through to the crank-pin journal, and you have a self-pumping lubricator. Once the engine is running above maybe 50 RPM, the centrifugal pressure at the rim of the cup is high enough to drive oil through a 0.8-1.2 mm metering orifice and into the brass big-end bearing at a controlled rate. The faster the engine runs, the harder the oil is flung — which is exactly what you want, because bearing wear scales with shaft speed too.
Geometry matters. The metering hole sits at the outer wall of the cup, the cup itself is offset from the crank centreline by 60-150 mm depending on engine size, and the drilled passage runs radially inward to the crank-pin oilway. If the cup is mounted too close to the crank centre, centrifugal pressure drops with the square of radius and you get oil starvation at the bearing — the classic symptom is a hot big-end and a knock that gets worse as load comes on. If the metering orifice is too large, the cup empties in 20 minutes and the engineman is climbing onto the flywheel guard every shift to refill it. If it's too small, the bearing runs dry under heavy load. Most builders settled on a refill interval of 4-8 hours of running, which matched the watch pattern on stationary mill engines.
The other failure mode is contamination. The cup is open at the top to allow refilling and atmospheric pressure equalisation, so grit, condensate, and lint find their way in. A worn felt strainer on the orifice lets a chip of swarf block the passage, the bearing wipes, and you're regrinding a crank-pin. Keep the cup covered between fills, use a fine-mesh gauze on the orifice, and inspect the felt every overhaul.
Key Components
- Oil cup body: Cast brass or steel reservoir, typically 30-80 ml capacity, bolted or screwed to the crank web with two M6 or M8 fasteners. The inside wall is machined smooth so oil migrates cleanly outward under rotation rather than churning.
- Metering orifice: A drilled hole 0.8-1.2 mm diameter at the outermost point of the cup wall. The exact diameter sets the feed rate — too large drains the cup in under an hour, too small starves the bearing. Most factory drawings spec ±0.05 mm on this dimension.
- Radial oilway: Drilled passage running from the metering orifice through the crank web and into the crank-pin journal, typically 3-5 mm bore. Cross-drilled to intersect the bearing surface at the point of lowest hydrodynamic pressure, usually 90° off the load line.
- Felt or gauze strainer: Fine filter pressed into the orifice to stop grit and lint from blocking the passage. Replace at every overhaul — a clogged strainer starves the bearing as effectively as no oil at all.
- Cap or cover: Hinged or screw-on lid that keeps debris out between fills while still allowing the cup to vent to atmosphere. On enclosed-crankcase engines this becomes a slinger ring instead.
Industries That Rely on the Centrifugal Crank-pin Oiler
You find centrifugal crank-pin oilers wherever a builder needed to lubricate a rotating big-end without running an external pressure feed. They were standard on Victorian and Edwardian stationary engines, common on early gas engines, and persisted on traction engines and small marine plant well into the 1930s. Modern restorations rebuild them as original because nothing else fits the period and nothing else is as quietly reliable.
- Stationary steam plant: Robey horizontal mill engines built at Lincoln from the 1880s used twin centrifugal oil cups on each crank web to feed the big-end bearings of cross-compound layouts.
- Traction engines: Burrell and Fowler showman's road locomotives carried 50 ml brass oil cups bolted to the crank webs, feeding the crank-pin through a drilled web passage.
- Early internal combustion: Crossley and Tangye gas engines from the 1890s used the same crank-web cup arrangement before pressurised splash troughs became standard.
- Small marine engines: The Stuart Turner 5A and similar launch engines fitted miniature centrifugal oilers to crank webs in open-crankcase configurations.
- Heritage restoration: The working beam engines at Kew Bridge Steam Museum retain original centrifugal crank-pin oilers on their auxiliary engines as part of the museum's commitment to operational authenticity.
- Industrial machine tools: Whitworth and Smith & Coventry planing machines used cup-style centrifugal oilers on their crank-driven tables, the design later migrating onto steam engine practice.
The Formula Behind the Centrifugal Crank-pin Oiler
The useful number is the centrifugal pressure developed at the metering orifice — that's what drives oil through the passage and into the bearing. At low engine speeds the pressure is small and feed rate is barely a drip; at the high end of the typical operating range the pressure climbs with the square of speed and oil flow can become excessive if the orifice is sized for full-load running. The sweet spot for most stationary engines sits around 80-95% of rated speed, where the centrifugal head is high enough to maintain a steady film but not so high that the cup drains before the next refill round. This formula gives you the pressure head at the orifice in metres of oil column.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| h | Centrifugal pressure head developed at the orifice, expressed in metres of oil column | m | ft |
| ω | Angular velocity of the crankshaft | rad/s | rad/s |
| r | Radial distance from crank centreline to the metering orifice | m | ft |
| g | Gravitational acceleration | 9.81 m/s² | 32.2 ft/s² |
Worked Example: Centrifugal Crank-pin Oiler in a restored Crossley horizontal gas engine
You are rebuilding the centrifugal crank-pin oiler on a 1902 Crossley horizontal hot-tube gas engine at a heritage gasworks museum in Fakenham, Norfolk. The engine runs at a nominal 180 RPM, the oil cup orifice sits at r = 0.120 m from the crank centreline, and you want to confirm the centrifugal head is sufficient to drive SAE 30 oil through a 1.0 mm metering orifice with a felt strainer in place. The original drawings call for a feed rate of roughly one drop every 4 seconds at rated speed.
Given
- N = 180 RPM (nominal)
- r = 0.120 m
- g = 9.81 m/s²
- dorifice = 1.0 mm
Solution
Step 1 — convert nominal engine speed to angular velocity in rad/s:
Step 2 — compute centrifugal pressure head at the orifice at nominal speed:
That's 0.26 m of oil column, roughly 2.3 kPa — a gentle but reliable push, exactly the order of magnitude needed to maintain a steady drip through a 1.0 mm orifice with a felt strainer fitted. Plenty of head to overcome strainer resistance, not enough to atomise the oil or empty the cup in minutes.
Step 3 — at the low end of the typical operating range, idle speed of about 90 RPM:
At 90 RPM the head drops to 65 mm of oil — barely enough to push past the felt strainer, and feed rate falls to maybe one drop every 15-20 seconds. The engine will run, but if you idle it for an hour at low speed the big-end runs marginal. This is why old enginemen warned against extended idle on cup-fed engines.
Step 4 — at the high end of the typical operating range, governor-cut overspeed of around 220 RPM:
At 220 RPM the head climbs to 0.39 m and feed rate roughly doubles versus nominal. The cup empties quicker, but the bearing is well-fed. The takeaway — orifice sizing should be done at nominal RPM, never at idle or peak.
Result
Nominal centrifugal head at 180 RPM and r = 0. 120 m comes out to 0.261 m of oil column, equivalent to about 2.3 kPa at the orifice — a steady, gentle feed that matches the original Crossley drawing's one-drop-per-four-seconds spec. At 90 RPM idle the head collapses to 65 mm and feed becomes marginal; at 220 RPM peak it climbs to 390 mm and the cup drains noticeably faster — sweet spot is firmly in the 160-190 RPM band. If your measured feed rate runs lower than predicted, check three things: (1) the felt strainer may be glazed with old oil varnish — replace it, glazing can halve flow, (2) the orifice may have been drilled undersize during a previous overhaul — verify with a 1.0 mm pin gauge, (3) the cup may be bolted to a non-original boss that puts the orifice closer to the crank centreline than 0.120 m, and head drops with the square of radius so even a 10 mm error matters.
Centrifugal Crank-pin Oiler vs Alternatives
Centrifugal crank-pin oilers are not the only way to lubricate a big-end bearing. Splash systems, pressure-fed pumps, and sight-feed displacement lubricators all compete for the same job. Pick the wrong one for the application and you either overcomplicate a simple engine or starve a hard-working bearing.
| Property | Centrifugal Crank-pin Oiler | Pressure-fed Oil Pump | Splash/Dipper Lubrication |
|---|---|---|---|
| Speed range (RPM) | 50-600 RPM, falls off at low speed | Effective from cranking speed upward, no lower limit | 150-1500 RPM, needs minimum splash velocity |
| Feed rate control | Set by orifice diameter, ±20% repeatability | Tightly metered, ±5% with relief valve | Uncontrolled, dependent on sump level |
| Refill interval | 4-8 hours typical, manual fill | Continuous from sump, no manual fill | Continuous from sump, no manual fill |
| Mechanical complexity | Very low — drilled cup and orifice only | High — pump, drive, relief valve, gallery | Medium — dipper, trough, level control |
| Cost (small engine) | £20-£80 for a brass cup and oilway | £300-£1500 with pump and plumbing | £100-£400 for sump and dipper |
| Lifespan / reliability | Decades if strainer is maintained | : pump wear, seal failure, gallery blockage | Long life, sump sludge accumulation |
| Best application fit | Open-crank stationary and traction engines | Enclosed crankcase, high speed, high load | Enclosed crankcase, moderate speed |
Frequently Asked Questions About Centrifugal Crank-pin Oiler
Two causes — the cup may not be fully sealed at the cover, and gravity alone is driving a slow weep through the orifice when the orifice happens to stop at the bottom of its rotation. If the cover gasket is hardened or missing, atmospheric breathing also drives slow drainage as the cup cools and contracts overnight.
The fix is to position the crank for shutdown with the orifice at top dead centre of its arc — most enginemen barred the engine over to a known stop position before knocking off. A fresh cork or leather cover gasket eliminates the breathing weep entirely.
Not by linear scaling. The centrifugal head goes with ω²×r², so a doubled radius gives you four times the head — but the orifice also needs to flow proportionally more oil to feed a larger bearing surface area. Bearing oil demand scales roughly with projected bearing area (D × L), which goes up linearly with engine size, not as a square.
The rule of thumb — fix the radius based on geometry available on the crank web, then size the orifice to deliver the bearing's specified drip rate at nominal RPM. For a doubled big-end size, expect to drill the orifice about 1.4× the original diameter, not 2×.
Keep the cup unless you have a documented reliability problem. A pressure-fed conversion means a driven pump, plumbing through the crank, a rotating seal at the crankshaft end, and a relief valve — all of which are failure points the original design doesn't have. The cup is dead simple and works.
The exception is a working-museum engine that runs 8+ hours daily with no engineman. There the manual refill interval becomes a labour problem, and a small auxiliary pump feeding a stationary drip cup over the crank-pin (not into a rotating cup) preserves the look while removing the refill chore.
Feed rate at idle is not diagnostic — what matters is feed rate at the loaded speed where the bearing is actually working. If you've sized the orifice from a low-speed observation, the bearing is being starved at running speed because hydrodynamic film thickness needs more oil under load, not less.
Diagnostic check — measure the cup level drop over a 30-minute run at nominal RPM under load. Calculate ml/hr. Compare to the original drawing or to a Stribeck-curve estimate of bearing oil demand at that load. If feed is below 60% of demand, the orifice is undersized or the strainer is restricting.
You've overfilled. The cup has a designed maximum fill line, usually 60-70% of capacity, because the oil surface inside the spinning cup forms a paraboloid — at speed the inner surface rises up the cup wall and the outer surface presses against the rim. Overfill and the oil surface reaches the cover, finds the breather slot, and gets thrown out as a fine mist.
The fix is straightforward — fill only to the marked line, or to a depth equal to half the cup wall height if no line exists.
Drain the cup, clean it, and refill with a measured volume of paraffin or thin flushing oil. Run the engine at nominal RPM for a fixed time — say 15 minutes — and measure how much fluid remains in the cup. Compare to the predicted flow through a clean 1.0 mm orifice at that head, which is roughly 8-15 ml/min depending on viscosity.
If your measured flow is less than 50% of predicted, the oilway is partially blocked, typically with carbonised oil residue at the cross-drilling intersection. The only fix is dismantling and rodding the passage with a soft brass wire and solvent.
References & Further Reading
- Wikipedia contributors. Splash lubrication. Wikipedia
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