A collar bearing and step is a pair of plain-bearing surfaces that carry a vertical shaft — the collar bearing supports radial load and lifts the shaft against gravity through one or more shoulders machined onto the shaft, while the step (or footstep bearing) supports the bottom end and absorbs the remaining axial thrust. Robert Willis described and classified the arrangement in his 1841 Principles of Mechanism. The collar transmits load through a series of bronze rings sliding on lubricated faces; the step takes the residual point load at the shaft foot. The combination kept vertical mill shafts, turbine spindles, and line-shaft uprights running for decades before rolling-element thrust bearings displaced them.
Collar Bearing and Step Interactive Calculator
Vary total thrust, load-carrying faces, and residual step share to see axial load split through a collar bearing and footstep.
Equation Used
The calculator divides the axial thrust W between the collar faces and the step. If the step share s is zero, the collar faces carry all the thrust equally, so two loaded faces give W/2 on each face, matching the article diagram.
- Collar faces share the carried thrust equally.
- Step load is the selected residual thrust not carried by the collar faces.
- Dynamic effects, oil-film stiffness, wear, and misalignment are not included.
Inside the Collar Bearing and Step
The collar bearing carries axial load by spreading it across multiple sliding faces. You machine one or more raised collars — basically integral shoulders — onto the vertical shaft, then sandwich each collar between two bronze rings held in a split housing. Oil floods the gap. As the shaft rotates, each collar face slides against its mating ring, and the friction torque is small because the relative speed at the collar radius is modest and the oil film keeps metal off metal. Stack 3 or 4 collars and you divide the total thrust load by the number of faces, which is the only way you could carry a 20-ton vertical waterwheel shaft on plain bearings in 1850.
The step sits at the bottom. It is a hardened bronze or lignum vitae block let into a cast-iron seat, with the shaft foot — usually a hardened steel pivot pin pressed into the shaft end — bearing on it. The step takes whatever thrust the collars do not, and it locates the shaft radially at the foot. The step block runs in an oil bath. If you let the oil level drop, you lose the hydrodynamic film at the pivot first because the rubbing speed there is near zero — the centre of the pivot has no relative motion at all — and the bearing transitions to boundary lubrication. That is when you see scoring and the characteristic blue-black wear pattern radiating from the centre of the pivot.
Tolerances matter. The collar faces must run square to the shaft axis within roughly 0.05 mm per 100 mm of collar diameter or you load one side of each ring and lose the multi-face load sharing. The step seat must be parallel to the collar faces within similar limits. Get either wrong and the shaft will hammer once per revolution, the oil film will collapse on the loaded side, and you will be replacing bronze inside a month.
Key Components
- Shaft Collars (integral): Raised shoulders machined directly onto the vertical shaft, typically 25-50 mm thick and projecting 20-40 mm from the shaft OD. Each collar transmits a share of the axial load to its mating bronze ring. For a heavily loaded shaft you machine 3 to 5 collars spaced along the upper bearing housing.
- Bronze Bearing Rings: Split bronze rings (typically SAE 660 leaded tin bronze) clamped in the housing, with a flat face that mates against the collar face. Running clearance on the flat is held to roughly 0.1-0.2 mm initially so the oil wedge can form. The rings wear, not the shaft.
- Step Block: The bottom support — a flat or slightly dished bronze block, sometimes lignum vitae for water-flooded service, seated in a cast-iron pocket. Carries the residual axial thrust and locates the shaft foot. Renewable as a wear part.
- Pivot Pin: A hardened steel pin, often 60-62 HRC, pressed into the bottom of the shaft. The pin contacts the step block. Hardened so the wear concentrates in the cheaper bronze block, not in the shaft itself.
- Oil Bath / Reservoir: Surrounds the entire collar stack and the step. Oil viscosity sized so the collar Sommerfeld number sits in the hydrodynamic regime — typically ISO VG 100 to VG 220 mineral oil for mill service. Level must stay above the top collar.
- Housing and Cap: Cast-iron split housing that holds the bronze rings and provides bolted clamping force on the rings. The cap is shimmed to set the running clearance on the collar faces.
Real-World Applications of the Collar Bearing and Step
You find collar bearings and steps wherever a vertical shaft carries a heavy axial load and the equipment predates rolling-element thrust bearings — or where rolling elements would not survive the environment. Plenty of these installations are still running because the design is simple, repairable with hand tools, and tolerant of contaminated lubricants in ways a tapered-roller thrust bearing is not.
- Grain Milling: The vertical runner-stone spindle on a Meadows 30-inch stone mill is supported by a step bearing in the bridge tree, with the upper collar bearing carrying the radial load and most of the stone weight.
- Hydroelectric Heritage Plants: The 1895-installed Stanley Mills turbine-generator at Cossipore used a collar thrust bearing stack to carry the 18-ton vertical Francis turbine and rotor assembly before Kingsbury tilting-pad bearings replaced it in the 1920s.
- Sugar Cane Crushing: Vertical-shaft cane mills of the Cuban and Louisiana plantation era ran the king-roll spindle on a bronze step bearing in oil bath, with collar rings taking the upward thrust from the crushing reaction.
- Marine Capstans: Royal Navy steam-driven capstans up through HMS Warrior used a footstep bearing of lignum vitae at the deck, with a collar bearing in the gun-deck above to carry the vertical pinion shaft.
- Textile Mill Line Shafting: Vertical jackshafts in 19th-century New England cotton mills — Lowell, Manchester, Pawtucket — used collar-and-step bearings to transfer power from waterwheel shafts on the wheel floor up to belt-driven horizontal line shafts on the production floors.
- Foundry Cupola Blowers: Belt-driven vertical blower spindles on positive-displacement Roots-type cupola blowers used a step bearing at the casing base and a collar bearing at the top drive sheave.
The Formula Behind the Collar Bearing and Step
The friction torque in a collar bearing is what determines how much power you waste turning the shaft and how hot the oil bath gets. At the low end of the operating range — light loads, slow speeds — you sit comfortably in the hydrodynamic regime and friction torque scales with viscosity and speed, not with load. At the high end — heavy thrust, low speed — you risk dropping out of the hydrodynamic regime entirely and metal-to-metal contact spikes the friction by a factor of 5 or more. The sweet spot is when each collar face carries a unit pressure of roughly 0.5-1.0 MPa (70-150 psi) on bronze, and rubbing speed sits between 0.5 and 3 m/s at the mean collar radius.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Tf | Total friction torque resisting shaft rotation | N·m | lbf·ft |
| μ | Coefficient of friction at the collar/ring interface | dimensionless (typ. 0.005-0.05) | dimensionless |
| W | Total axial load on the shaft (per collar face = W / nc) | N | lbf |
| Rm | Mean radius of the collar face = (Router + Rinner) / 2 | m | ft |
| nc | Number of collar faces sharing the load | integer | integer |
Worked Example: Collar Bearing and Step in a restored vertical millstone spindle
A heritage gristmill in Vermont is recommissioning the vertical runner-stone spindle on a 1872 Munson Brothers stone mill. The spindle carries a 4-foot French buhr runner stone weighing 1,800 lb, plus the dynamic lift component from the meal flow. Total axial load on the spindle is 9,800 N. The collar bearing has 3 collars, each with outer radius 75 mm and inner radius 50 mm. Bronze-on-steel running in ISO VG 150 oil, μ ≈ 0.015 in the hydrodynamic regime. The mill runs at a nominal 90 RPM stone speed, with a typical operating range of 60-130 RPM depending on grain feed.
Given
- W = 9800 N
- nc = 3 faces
- Router = 0.075 m
- Rinner = 0.050 m
- μ = 0.015 —
- Nnom = 90 RPM
Solution
Step 1 — find the mean collar radius:
Step 2 — compute friction torque at nominal load and 3 collar faces. Each face carries 9,800 / 3 = 3,267 N, but the total friction torque sums across all faces, so use total W:
Note: the nc = 1 multiplier appears because each face carries W/nc but there are nc faces — the total is independent of nc when μ stays in the hydrodynamic regime. The reason you use 3 collars is to keep unit pressure low so μ stays low. Check unit pressure on each face:
Step 3 — at 90 RPM nominal, mean rubbing speed and friction power loss:
Step 4 — at the low end of the operating range, 60 RPM, rubbing speed drops to 0.39 m/s and friction power drops to 57.7 W. The collar still floats on a hydrodynamic film at this speed because unit pressure is well below 1 MPa, and the oil bath stays cool to the touch. At the high end, 130 RPM, rubbing speed reaches 0.85 m/s and friction power climbs to 125 W — the oil bath warms noticeably, but you are still inside the safe envelope for SAE 660 bronze with VG 150 oil. Push the spindle past about 180 RPM and the oil churning losses dominate the friction losses, the bath temperature climbs past 60 °C, and viscosity falls fast enough that μ starts climbing again.
Result
Nominal friction torque is 9. 19 N·m, costing roughly 87 W of parasitic power at 90 RPM. That is small compared to the 4-7 kW the millstone consumes grinding wheat, so you barely notice it on the waterwheel side. At 60 RPM the bearing wastes 58 W and runs cool; at 130 RPM it wastes 125 W and the oil bath sits warm but well within spec — the sweet spot is 80-110 RPM where unit pressure, rubbing speed, and oil viscosity all line up. If you measure friction torque higher than 15 N·m on a stall test, suspect one of three things: (1) the collar faces are not parallel to the bronze rings — shim the cap until you have uniform 0.15 mm clearance all around, (2) the oil has emulsified with flour dust and lost viscosity, which shows as milky discharge when you drain it, or (3) one collar is carrying the entire load because the others have worn back, which you'll see as a deep groove on one bronze ring and pristine surfaces on the others.
Choosing the Collar Bearing and Step: Pros and Cons
Collar-and-step bearings dominated vertical-shaft thrust applications for a century, but rolling-element thrust bearings and hydrodynamic tilting-pad bearings have largely displaced them in new equipment. The choice today is mostly about whether you are restoring heritage machinery, working in a contaminated environment, or designing for the lowest possible cost on a low-speed shaft.
| Property | Collar Bearing and Step | Tapered Roller Thrust Bearing | Kingsbury Tilting-Pad Thrust Bearing |
|---|---|---|---|
| Typical speed range (RPM) | 10-300 | 10-3,000 | 100-15,000 |
| Coefficient of friction (steady state) | 0.005-0.05 | 0.0015-0.005 | 0.001-0.003 |
| Load capacity per face | 0.3-1.0 MPa unit pressure | high — concentrated rolling contact | very high — distributed pad area |
| Tolerance to contaminated oil | excellent — wears bronze gracefully | poor — rollers brinell and spall | moderate — pads can recover |
| Typical service life before rebuild | 10-30 years on bronze rings | 5-15 years on rollers | 20-40 years on pads |
| Repair cost and complexity | low — re-cast bronze on site | medium — replace bearing assembly | high — specialist refurbishment |
| Initial cost | low (custom machined) | low to medium (catalogue) | high (engineered) |
| Best application fit | heritage mills, slow vertical shafts, dirty service | modern industrial vertical drives | large turbines, generators, marine thrust |
Frequently Asked Questions About Collar Bearing and Step
One-sided heating means one or two collars are carrying nearly all the load instead of the load sharing across all 3 or 4 faces. The usual cause is that the bronze rings on the cool side have worn back faster than the loaded side — or were shimmed unevenly during the last rebuild — so when the shaft sits down under load, only the tight faces actually contact.
Pull the cap, set up a dial indicator on each collar face, and lift the shaft 0.5 mm with a jack. The clearance you read on each ring should be within 0.05 mm of the others. Anything more and you re-shim or re-face the high rings until the load shares evenly.
Bronze for oil-lubricated service indoors. Lignum vitae for water-flooded service or anywhere the bearing is submerged or splash-fed with water — marine capstans, vertical pump shafts, and waterwheel kingposts traditionally used lignum vitae because the wood self-lubricates with its own resin and does not corrode.
The crossover rule is simple: if the bearing sees clean mineral oil, bronze gives you 3-5x the life. If it sees water, mud, or grain dust slurry, lignum vitae will outlast bronze by a similar margin because bronze galls and corrodes in those environments.
Work backwards from unit pressure. Compute the load each face would carry with 1 collar — W divided by the annular area π × (Ro2 − Ri2). If the result is below 1.0 MPa for bronze on steel in oil, one collar is fine. If it lands above that, add collars until each face sits in the 0.3-0.7 MPa band, which is the sweet spot for hydrodynamic film stability over decades of service.
Heritage mill shafts often used 3-5 collars not because the static load demanded it but because shock loads from grain feed or stone dressing could spike the unit pressure 3-4x. Sizing for the spike, not the average, is why those bearings still run today.
Stick-slip in the boundary regime. At zero RPM the oil film is squeezed out, and as the shaft starts turning the bronze grabs and releases the steel collar repeatedly until rubbing speed builds up enough to lift onto the hydrodynamic wedge. You hear it as a chirp or whistle that disappears within a few seconds.
If it persists past 30 seconds of running, your oil viscosity is too low (move from VG 100 to VG 150 or VG 220), or your collar surface finish is too rough — you want Ra below 0.4 µm on the steel collar face. Hand-scraped or lapped bronze rings paired with ground steel collars almost never chatter once oil reaches the gap.
The formula assumes hydrodynamic lubrication with μ around 0.005-0.02. At zero and very low speed there is no oil wedge yet, so you are in boundary lubrication where μ for bronze on steel sits at 0.08-0.15 — about 5-10x higher. That is exactly the spike you measured.
This is why you never spec a motor based on running torque alone for a collar bearing. Size the motor breakaway torque at 6x the calculated steady-state friction torque, or fit a barring gear to break the shaft loose before the main drive engages. Old mill engineers knew this — that is why nearly every large vertical mill shaft had a hand-cranked turning bar bracket cast into the housing.
Sometimes, but think it through first. A roller thrust bearing concentrates the entire axial load into a small ring of rolling contact instead of spreading it across multiple bronze faces. Two problems show up: first, the original housing is rarely stiff enough to keep the roller race flat under full load — it deflects and the rollers skew. Second, any ingress of mill dust, grain, or moisture brinells the rollers within months because rolling contacts hate contamination.
If the application is clean, dry, and the housing is rigid, retrofit works. If it is a dusty mill, leave the collar bearing alone — it was the right answer in 1880 and it is still the right answer today.
Aim for 0.10-0.20 mm total axial clearance across the full collar stack when the shaft is jacked up off the step. Tighter than 0.08 mm and the oil cannot find its way in cleanly during startup — you'll get the chatter described above. Looser than 0.30 mm and the shaft hammers vertically each time the load reverses, which beats the bronze faces flat within months.
Set it with shims under the cap, not by tightening cap bolts unevenly. Uneven bolt torque cocks the bearing and causes the one-sided heating problem from the first FAQ.
References & Further Reading
- Wikipedia contributors. Plain bearing. Wikipedia
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