A crank-pin lubricator is a small oil reservoir mounted on the rotating crank web of a steam or gas engine that feeds oil outward to the big-end bearing using centrifugal force. The arrangement was standardised across British mill-engine practice by builders like Robey of Lincoln from the 1880s onward. As the crankshaft turns, oil in the cup is flung along a drilled passage in the crank pin and into the brass, maintaining a hydrodynamic film. Properly sized, a single charge runs an 8 hour shift on a mill engine without intervention.
Crank-pin Lubricator Interactive Calculator
Vary shaft speed, pickup radius, discharge radius, and oil density to see centrifugal oil delivery pressure and flow direction.
Equation Used
The calculator applies the centrifugal pressure head developed between the oil pickup radius r1 and discharge radius r2. A positive result means the bearing outlet is farther from the rotation axis than the pickup, so centrifugal force drives oil outward.
- Oil is treated as incompressible.
- Pressure shown is ideal centrifugal head only.
- Viscous losses, burrs, sludge, and drilling restrictions are neglected.
- Positive delivery requires r2 greater than r1.
Operating Principle of the Crank-pin Lubricator
The big-end bearing on a steam engine sits on a rotating pin, which makes feeding it oil awkward. A stationary drip lubricator can't reach a moving target — anything you drop on the crank pin gets flung off before it enters the brass. The crank-pin lubricator solves this by riding with the pin. You bolt a small reservoir to the crank web, drill a passage from the bottom of the reservoir radially outward through the crank pin to the bearing surface, and let centrifugal force do the pumping. At running speed the oil sits as a ring against the outer wall of the cup, and the drilled passage taps that ring at a known radius.
Geometry matters more than people realise. The pickup point inside the cup must sit at a smaller radius than the discharge point at the bearing surface — otherwise the oil never moves outward. The pressure head is ρ × ω2 × (r22 − r12) / 2, so doubling the speed quadruples the delivery pressure. If you notice the brass running hot after a rebuild and the oil cup still has charge in it, the problem is almost always that someone drilled the feed passage at the wrong radius, or left a burr at the inlet that throttles flow below the bearing's demand.
Failure modes cluster into three buckets. First, sludge — old mineral oil leaves a varnish that blocks the radial passage, and a 1.5 mm hole becomes a 0.8 mm hole over a season. Second, plug-loss — the threaded filler plug backs out under vibration if it isn't pinned, and the cup empties in minutes. Third, under-charging — operators top up to the sight glass at standstill, not realising the oil ring climbs up the cup wall under rotation and the actual usable charge is roughly 60% of the static volume.
Key Components
- Reservoir cup: A turned brass or cast-iron pot bolted to the crank web, typically 50–90 mm diameter and holding 80–250 ml of oil. The cup must be balanced against an opposite mass on the crank or you introduce a vibration couple at running speed.
- Filler plug: A threaded plug, usually 3/8 BSP or 1/2 BSP, with a knurled head for hand tightening. Must be cross-pinned or wired — vibration alone backs out an unsecured plug within an hour at 200 RPM.
- Radial feed passage: A drilled hole, typically 1.5–3.0 mm diameter, running from the inside wall of the cup through the crank web and along the axis of the crank pin to a cross-drilling at the bearing. Burrs at either end choke flow disproportionately because flow scales with d4.
- Pickup tube or wick: On larger engines a short brass tube projects inward from the outer wall, setting the pickup radius r1. The difference (r2 − r1) determines how aggressively the oiler delivers — too small and you starve the bearing, too large and you empty the cup before the shift ends.
- Sight glass or telltale: A small glass window or dipstick port letting the engineman check level without stopping. On Robey horizontal engines this sits flush with the static oil line so a quick wipe-and-look during a slow turn confirms charge.
Real-World Applications of the Crank-pin Lubricator
You see crank-pin lubricators on almost every reciprocating engine built between 1860 and the 1950s where the big-end ran in a brass and the operator wanted to walk away from the engine for hours at a time. The mechanism is mechanically simple, requires no external pump, and scales from small workshop engines up to colliery winders. Modern restoration projects keep using the original design because alternatives — pressurised oil galleries through hollow crankshafts — require machining work that's out of scope for most heritage budgets.
- Heritage textile mills: The 1907 J & E Wood horizontal cross-compound at Queen Street Mill in Burnley uses original brass crank-pin oilers, charged once per 4 hour demonstration run.
- Preserved gasworks: Crossley horizontal hot-tube gas engines like the 1902 unit at Fakenham Gasworks Museum rely on centrifugal crank-pin oilers as their primary big-end lubrication.
- Colliery winding engines: The Markham winder at Astley Green Colliery Museum, a 1912 Yates and Thom twin-cylinder design, runs centrifugal oil cups on both crank webs.
- Steam launches and yachts: Restored Edgar Allen and Sissons launch engines on Lake Windermere use small turned-brass crank-pin cups, typically 60 ml, recharged at every landing.
- Traction and portable engines: Aveling & Porter and Burrell road locomotives feed the big-end via a Stauffer-style screw-down crank-pin cup that combines centrifugal feed with manual advance during the day.
- Heritage pumping stations: The James Watt rotative engines at Crofton and Kew use crank-pin lubricators on the rotative crank, separate from the parallel-motion pin oilers.
The Formula Behind the Crank-pin Lubricator
What the practitioner cares about is delivery rate — millilitres of oil per hour reaching the bearing — because that determines the run time between charges and whether the brass stays cool. The driving pressure scales with the square of angular velocity, so a slow-running engine at 60 RPM delivers very differently from one at 240 RPM. At the low end of mill-engine practice (around 60 RPM) you're relying on a generous radial offset to get any meaningful flow at all. At the nominal middle of the range (150–200 RPM) the formula gives you a comfortable head and the design forgives small drilling inaccuracies. At the high end (300+ RPM, like a small launch engine) the cup empties faster than the operator expects, and pickup-tube placement becomes critical to avoid starving in the last quarter of the charge.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Q | Volumetric oil delivery rate to the bearing | m3/s | in3/s |
| Cd | Discharge coefficient of the radial passage, typically 0.6–0.7 for a clean drilled hole | dimensionless | dimensionless |
| A | Cross-sectional area of the radial feed passage | m2 | in2 |
| ω | Angular velocity of the crankshaft | rad/s | rad/s |
| r2 | Radius from crank centreline to discharge point at bearing surface | m | in |
| r1 | Radius from crank centreline to oil pickup point inside the cup | m | in |
Worked Example: Crank-pin Lubricator in an 1894 Marshall portable engine at a heritage threshing demonstration
You are sizing the brass crank-pin oiler for a recommissioned 1894 Marshall single-cylinder portable engine being prepared for working threshing demonstrations at a steam fair in Lincolnshire. The crank radius is 152 mm, the pickup tube projects inward to a radius of 90 mm, the radial feed drilling is 2.0 mm diameter, and the design speed under governor is 180 RPM. The bearing demand sheet calls for 8 ml/hour minimum to keep the brass below 60 °C above ambient under a 12 BHP load.
Given
- r2 = 0.152 m
- r1 = 0.090 m
- d = 2.0 mm
- N = 180 RPM
- Cd = 0.65 dimensionless
- ρoil = 880 kg/m3
Solution
Step 1 — convert the nominal 180 RPM to angular velocity:
Step 2 — compute the passage area for the 2.0 mm drilling:
Step 3 — compute the nominal delivery rate at 180 RPM:
That is far above the 8 ml/hour bearing demand, which tells you the cup acts as a metered reservoir — the bearing only accepts what the clearance film can carry, and the rest stays in the radial passage as a standing column. In practice, oilfilm uptake at the brass throttles actual transfer to roughly 10–15 ml/hour, which gives you a working margin.
Step 4 — at the low end, suppose the engine is being run gently at 90 RPM during a slow demonstration:
Half the speed gives half the delivery — the head scales linearly with ω because we're inside a square root. Still well above bearing demand, so the brass stays fed. At the high end, suppose the governor lets go and the engine runs away to 300 RPM:
At runaway speed the cup empties in under 25 minutes if the operator doesn't notice — and that's the failure case to watch for, not under-supply.
Result
Nominal delivery at 180 RPM is 6. 0 ml/min, comfortably above the 8 ml/hour bearing demand. Across the 90–300 RPM operating range delivery scales from 3.0 ml/min at the slow demonstration end up to 10 ml/min near runaway — the sweet spot sits at 150–200 RPM where a 200 ml cup runs roughly 4–5 hours between top-ups. If your measured cup empties faster than this prediction, the most common causes are: (1) the filler plug not seating fully and oil escaping past the threads under centrifugal pressure, (2) the cup mounting flange warped so the gasket leaks down the crank web, or (3) the discharge cross-drilling at the bearing being oversized from a previous reaming, dumping oil straight through the brass without forming a film.
When to Use a Crank-pin Lubricator and When Not To
Crank-pin lubrication can be done several ways, and the choice depends on engine speed, run-time expectations, and how much restoration budget you have for crankshaft machining. The centrifugal cup is the heritage default — but ring oilers, splash systems, and full-pressure feed all have their place. Compare them on the dimensions that actually matter when you're specifying a rebuild.
| Property | Centrifugal crank-pin lubricator | Splash lubrication | Pressure-fed oil through hollow crankshaft |
|---|---|---|---|
| Suitable speed range | 60–400 RPM | 200–1500 RPM | Any speed up to 6000 RPM |
| Run time per charge | 4–8 hours typical | Unlimited (sump-fed) | Unlimited (pump-fed) |
| Installation cost on heritage rebuild | Low — bolt-on cup | Medium — sealed crankcase needed | High — requires crank re-drilling and pump fitting |
| Reliability against operator error | Moderate — depends on charging discipline | High — automatic | High — interlocked with oil pressure switch |
| Maintenance interval | Top up every shift, strip yearly | Oil change every 200–500 hours | Filter change every 100–250 hours |
| Authenticity for pre-1950 restoration | Period-correct | Period-correct on later engines only | Anachronistic on heritage builds |
| Bearing load capacity supported | Up to ~5 MPa projected | Up to ~8 MPa projected | Up to ~25 MPa projected |
Frequently Asked Questions About Crank-pin Lubricator
Delivery to the cross-drilling is not the same as film formation at the bearing surface. If the discharge port at the brass sits in the loaded zone of the bearing, the rotating film carries oil away before it can spread axially across the brass. The fix is to position the discharge in the unloaded zone — typically 90° before the load vector — so oil enters where clearance is largest and gets dragged into the loaded arc by shaft rotation.
A second cause is oil grade. A SAE 30 mineral steam oil at 60 °C has a viscosity around 80 cSt; at 90 °C it drops to 30 cSt and the hydrodynamic film collapses. If the brass is running hot enough that the oil thins below the minimum film thickness for your bearing clearance, no amount of delivery will save it. Step up to a steam cylinder oil or a compounded SAE 50.
Flow scales with the area of the drilling, so a 3.0 mm hole delivers roughly four times the flow of a 1.5 mm hole at the same speed. Bigger isn't better here — the bearing only takes what the clearance film carries. Anything beyond that rate just empties the cup faster and gets thrown out as fog inside the engine room.
The rule we use: size the drilling so the cup runs for the full intended shift between charges. For a 200 ml cup at 180 RPM serving an 8 hour shift, you want roughly 25 ml/hour effective transfer, which a 1.5 mm drilling delivers comfortably. Reserve 3.0 mm drillings for slow engines (under 80 RPM) where you need extra area to compensate for low ω.
Almost always the filler plug. Centrifugal pressure inside a cup at 200 RPM and 150 mm radius is around 6 kPa — modest, but enough to weep past a plug that isn't fully seated or that lacks a fibre washer. Pull the plug, fit a new fibre or copper washer, torque it firmly, and cross-pin or wire-lock it.
If the plug is sound, check the gasket between the cup and the crank web. A warped flange or a gasket trimmed too narrow lets oil escape between the cup base and the web, and centrifugal force flings it radially outward in a fine spray that coats everything within a 2 m radius.
You can, but the design rules change. Above ~400 RPM the oil ring inside the cup climbs almost vertically up the outer wall, and any pickup tube placed too close to the wall draws air instead of oil once the level drops past it. You'll see this as intermittent feed — the brass runs cool for the first hour and then suddenly heats up.
For high speed, either use a deeper cup with the pickup at less than 50% of the wall radius from the centre, or switch to pressure feed through a hollow crank. The Crossley high-speed gas engines of the 1920s, running 350–400 RPM, sat right at the edge of where centrifugal cups remain practical — most builders moved to pressurised systems above this.
Compare the actual cup empty-time against the calculated delivery rate. Charge the cup to a known volume at standstill, run for a measured period, stop, and re-measure. If the cup empties at less than 50% of the predicted rate, you have flow restriction in the passage — typically varnish from old oil or carbon from overheating.
A quick clearing technique: drain the cup, charge it with a 50/50 mix of paraffin and fresh steam oil, run for 20 minutes at half speed, drain, and refill with clean oil. This shifts most varnish without requiring a teardown. If empty-time still doesn't recover, the passage needs reaming — book the strip.
The pickup radius r1 sets how much of the cup volume is usable. A pickup at 90% of r2 gives you a small head difference and slow delivery but uses nearly the full charge. A pickup at 50% of r2 delivers aggressively but the cup runs dry once the oil ring thins to that radius — leaving up to 60% of the static charge stranded.
For typical mill-engine practice (150–250 RPM) we set r1 at 60–65% of r2. That gives roughly 70% usable charge and a delivery head sufficient to overcome any minor passage roughness. Slower engines push the ratio toward 50% to compensate for low ω; faster engines push it toward 75% to slow delivery and extend run time.
References & Further Reading
- Wikipedia contributors. Lubricator. Wikipedia
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