A crank-pin lubricator supplies oil to a bearing that moves with the crank. Historical designs include stationary cups with wiping pickups and centrifugal feed arrangements. This calculator illustrates the centrifugal form and calculates the ideal pressure difference between two radii in oil rotating with the shaft.
Related actuator design tools
This calculator concerns oil pressure in a rotating crank passage. For a separate actuator-driven crank linkage, calculate stroke, linkage angle and peak force from that linkage’s geometry; the oil-pressure result does not size its drive.
For crank motion, review the throw radius and the changing angle between the actuator and driven link. Those two details decide whether the actuator is pushing efficiently or fighting the geometry.
If the application needs synchronized motion, feedback should be treated as a requirement rather than an accessory. Two open-loop actuators wired together can drift apart when friction, load, or mounting geometry differs between sides.
- Map actuator stroke to the actual output travel before choosing a model.
- Check for binding, changing leverage, and peak load near the ends of travel.
- Decide whether timed motion is acceptable or position feedback is required.
For a FIRGELLI design review, calculate the load case with the FIRGELLI actuator force calculator and plan the switching or controller arrangement with the linear actuator wiring diagram generator. If an actuator or cylinder is already installed, the linear actuator replacement finder helps keep the comparison tied to real dimensions.
Crank-pin Lubricator Interactive Calculator
Adjust shaft speed, the two comparison radii and oil density. The calculator reports the signed centrifugal pressure difference and shows the crank-pin drive in motion.
Equation Used
- Oil rotates with the passage at the entered shaft speed.
- Constant oil density.
- Only the centrifugal contribution to the pressure difference is calculated.
- No flow-rate, loss, viscosity or bearing-film calculation.
- Drawing thickness and axial separation are schematic.
Pressure-head model only: flow rate, priming and bearing adequacy are not calculated.
A rotating oil passage
The schematic shows a fixed cup feeding the shaft through an axial inlet. A passage in the rotating crank carries oil toward the crank-pin bearing. The connecting rod follows the crank pin while its other end remains in a straight guide.
The two marked radii are pressure-comparison locations in the rotating oil. The calculation assumes the liquid follows the shaft rotation. The drawing is an explanatory section rather than a manufacturing drawing; the feed inlet, rotating member and connecting rod occupy different axial planes.
A wiping lubricator is a different arrangement: a moving pickup collects oil from a stationary cup. It should not be analyzed as though it were the centrifugal pressure model used here.
Moving-bearing lubrication
Crank-pin bearings require a feed arrangement that reaches the moving joint. This calculator explores the centrifugal contribution to pressure in such an arrangement. It does not determine the required oil delivery or prove that a bearing receives adequate lubrication.
Pressure difference between two radii
For liquid in steady rotation, radial equilibrium gives dp/dr = ρω²r. Integrating from r₁ to r₂ gives p₂−p₁ = ρω²(r₂²−r₁²)/2, where ω=2πN/60. The calculator converts entered millimetres to metres and reports pressure in kPa.
Equivalent oil head is (p₂−p₁)/(ρg). Density cancels from that head expression. If r₂ is smaller than r₁, the pressure difference is negative; this is a signed comparison, not a prediction that oil must flow inward.
Example pressure head
At 200 rpm, r₁=30 mm, r₂=60 mm and oil density 900 kg/m³, the ideal pressure increase is about 0.533 kPa, equivalent to 60.4 mm of oil. Doubling speed multiplies this contribution by four. Changing density changes pressure in direct proportion but leaves equivalent oil head unchanged.
What this result covers
The result includes only the centrifugal pressure contribution. Passage resistance, viscosity, gravity, priming, leakage, inlet pressure and bearing-film behavior are outside the calculation. Those effects determine actual delivery and require additional information.
The animation shows the connected moving parts. The pressure equation does not provide an oil flow rate.
Lubricator questions
Is the displayed pressure the actual bearing supply pressure?
No. It is the ideal centrifugal contribution between two selected radii, without inlet pressure or losses.
Why does oil density not change the head result?
Pressure is proportional to density, and conversion from pressure to oil head divides by the same density.
What does a negative result mean?
The second comparison radius is inward of the first. In steady rotation its centrifugal pressure contribution is lower.
Does the calculator predict oil flow?
No. Flow also depends on passage geometry, viscosity, pressure boundaries and bearing conditions.
Reference
Gardner D. Hiscox, Mechanical Movements, Powers, Devices and Appliances, 1901 edition, printed page 90, figures 322–324. The plate distinguishes wiping and centrifugal crank-pin lubrication arrangements. The pressure expression follows independently from radial equilibrium of rotating liquid.
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