Step Gearing with continuous bearing teeth is a gear design where the meshing teeth are replaced by hardened cylindrical pins or rollers that turn freely on bearings, engaging a profiled mating wheel through rolling contact rather than sliding contact. Conventional involute spur gearing relies on a small sliding contact patch that wears, scuffs and pits under shock load. Step Gearing converts that sliding action into rolling action across a continuous engagement zone, sharing torque across multiple pins at once. The outcome is a drive that handles dirty, slow, high-torque service — bucket conveyors, kiln drives, mill ring gears — at efficiencies above 96% and service lives measured in decades.
Step Gearing Interactive Calculator
Vary pin diameter, H7 bore allowance, and engaged bearing pins to see bore fit, lubrication size, and torque sharing in a rolling step gear mesh.
Equation Used
This calculator uses the article fit example for a rolling bearing pin: the pin diameter sets the minimum bore, the H7 allowance sets the maximum bore, lubrication passage diameter is at least 25% of pin diameter, and torque is assumed to divide evenly across the engaged pins.
- Bearing pin diameter is the nominal lower bore size.
- H7 clearance allowance is entered in micrometres.
- Load is shared equally by the engaged pins.
- Cycloidal pocket profile is assumed held to about +/-0.02 mm.
How the Step Gearing (continuous Bearing Teeth) Works
Step Gearing solves the problem you hit when you scale a normal involute spur gear up to mill or kiln sizes — the sliding velocity at the tooth flank turns the gear face into a wear surface, and you spend the rest of the gearbox's life chasing pitting, scuffing and lubrication failures. Replace the sliding tooth flank with a hardened pin running on a needle bearing and the contact becomes rolling, not sliding. The mating wheel uses a cycloidal or trochoidal profile cut to match the pin diameter exactly, so as the pinion rotates, each pin rolls into and out of its pocket without ever scrubbing the wall. We see this referred to as pin gearing, lantern pinion drive, or continuous-tooth-contact rolling element gearing depending on the industry — it is all the same idea executed at different scales.
The geometry is unforgiving. The bore that holds the bearing pin must be sized to roughly H7 — a 30 mm pin gets a 30.000 to 30.021 mm bore, no looser, or the pin walks under load and the contact stress jumps 40% on the leading edge. Pin centres on the pinion plate must hold true position within 0.05 mm or the contact ratio drops below 1 — meaning at some point in the rotation only one pin carries the entire torque, and you get periodic chatter you can hear from across the plant. The mating wheel profile must match the pin radius within ±0.02 mm. If the profile is cut undersize the pin bottoms in the pocket and you get clunking. Cut it oversize and you lose the conjugate action that gives Step Gearing its smooth output.
Failure modes are predictable. Pin bearings seize first if grease relief is undersized — the pin stops rotating, the bearing pin becomes a sliding tooth, and you destroy the wheel profile in a few hundred hours. Wheel pocket spalling shows up next when shock loading exceeds the case-hardened layer depth, typically 1.5 to 2.5 mm on a properly carburised wheel. Last is plate fatigue — the pinion plate sees a cyclic bending load every time a pin engages, and a poorly radiused pin bore acts as a stress riser.
Key Components
- Bearing Pin: Hardened cylindrical pin, typically 52100 bearing steel through-hardened to 60-62 HRC. Diameter ground to h6 tolerance. The pin is the working tooth — every other component exists to let it roll cleanly into the wheel pocket.
- Needle Bearing or Bushing: Sits between the pin and the pinion plate bore, allowing the pin to rotate freely under load. Drawn-cup needle bearings rated to 5,000 N dynamic load are typical at module 4-6 sizes. Without this bearing the pin slides and the mechanism loses its main advantage.
- Pinion Plate: Carries the bearing pins on a precise pitch circle. Pin position tolerance ±0.05 mm true position. Plate thickness sized so bending stress at the pin bore stays below 30% of yield to avoid fatigue cracking after 10⁷ cycles.
- Cycloidal Mating Wheel: Cast or machined wheel with cycloidal pockets profiled to match pin diameter within ±0.02 mm. Case-hardened to 1.5-2.5 mm depth, core hardness 30-35 HRC for shock resistance. This is the high-cost, long-lead component.
- Pin End Caps: Retain the pin axially in the plate. Usually circlips in machined grooves, or threaded end caps on larger drives above module 8. Axial play held to 0.1-0.3 mm — tighter binds the bearing, looser lets the pin walk and chew the wheel face.
- Lubrication Channel: Cross-drilled passage feeding grease or oil to the pin bearing. Undersized channels are the number-one cause of premature bearing seizure. Rule of thumb is channel diameter no less than 25% of pin diameter.
Industries That Rely on the Step Gearing (continuous Bearing Teeth)
Step Gearing earns its place wherever load is high, speed is low, and the operating environment punishes ordinary gears. You will not find it in a high-speed reducer or a precision indexing head — the contact ratio and backlash do not support that. You find it instead in mill drives, lock gates, mine hoists, kiln girth drives, and heavy positioning systems where a 50-year service life and tolerance for grit and shock load matter more than precision.
- Mining: Pin-and-rack drives on slewing systems for large rope shovels — the P&H 4100XPC electric mining shovel uses pin-rack geometry on the swing drive to handle the shock loading from a 65 m³ dipper crowding into a muck pile.
- Cement: Girth gear pinion drives on rotary kilns at FLSmidth installations, where the kiln ring carries module 30 cycloidal teeth meshing with a pinion running on rolling element pin contact for 25-year service intervals.
- Marine & Locks: Miter gate drives on USACE navigation locks like the Soo Locks at Sault Ste. Marie — slow, massive, intermittent duty under freezing water and grit, exactly where pin gearing outlasts conventional spur drives.
- Heavy Manufacturing: Indexing turntables on shipyard panel-line robots at facilities like Hyundai Heavy Industries, where 200-tonne plates rotate through welding stations on a Stiebel pin-rack ring drive.
- Theme Park Rides: Vertical lift drives on Intamin accelerator coasters where Nedschroef-style pin gearing on the catch-car retrieval cable winch handles 1.5 million cycles per season without measurable wear.
- Steel Mill: Roller table drives on hot strip mills at facilities such as the SMS group plant rebuilds, where scale and water rule out conventional spur gearing and only enclosed pin-and-wheel drives survive the duty cycle.
The Formula Behind the Step Gearing (continuous Bearing Teeth)
The core sizing calculation for Step Gearing is the per-pin contact force, because that is what determines whether your pins, bearings and wheel pockets survive the duty cycle. At the low end of the typical operating range — say 30% of rated torque — only the leading pin or two carry meaningful load and the rolling contact stress sits well below the wheel hardness, giving you essentially infinite life. At nominal load the calculated number of engaged pins matches the design contact ratio (typically 2 to 3 simultaneously meshed), spreading torque across multiple rolling contacts. Push past 100% rated torque into shock or stall and the leading pin sees disproportionate load because elastic deflection of the pinion plate redistributes force forward — and that is where pin bearing seizure and pocket spalling start. The sweet spot for a long-life Step Gearing drive sits between 40% and 80% of calculated rated torque.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Fpin | Tangential force per engaged pin | N | lbf |
| T | Transmitted torque at the pinion shaft | N·m | lbf·ft |
| Dpc | Pin centre pitch circle diameter | m | ft |
| ne | Number of pins simultaneously engaged (contact ratio) | — | — |
| Kd | Load distribution factor (1.0 ideal, 0.6-0.8 typical) | — | — |
Worked Example: Step Gearing (continuous Bearing Teeth) in a paper mill yankee dryer drive
Sizing the pin gear pinion driving the yankee dryer cylinder on a Valmet OptiFlo tissue machine rebuild. The 4.5 m diameter yankee turns at 25 RPM at full machine speed, drawing 750 kW through a 2-stage reducer that delivers 220 kN·m at the output pinion shaft. The pinion has a 0.85 m pitch circle diameter and runs 14 hardened bearing pins of 80 mm diameter against a cast cycloidal wheel keyed to the dryer journal. You need to confirm the per-pin contact force across the operating range so the bearing pin specification and wheel case depth survive a 30-year service life.
Given
- Tnom = 220,000 N·m
- Dpc = 0.85 m
- ne = 3 pins
- Kd = 0.75 —
- Pin diameter = 80 mm
Solution
Step 1 — at nominal full-machine torque, calculate the per-pin tangential force:
That is the design point. 230 kN spread across an 80 mm pin running on a needle bearing is well within the dynamic capacity of an INA NK80/35 needle assembly rated near 380 kN dynamic — you have safety margin but no extravagance, which is correct for a 25 RPM long-life drive.
Step 2 — at the low end of the operating range, machine running at start-up creep speed at roughly 30% rated torque:
At this load the Hertzian contact stress between pin and pocket sits around 600 MPa — well below the 1,400 MPa case-hardness threshold of a properly carburised cycloidal wheel. The drive is essentially loafing. This is the load condition during morning warm-up and threading, and it accumulates the majority of operating hours over the machine's life.
Step 3 — at the high end, an emergency stop or sheet-break shock load hitting 180% rated torque, and load distribution Kd collapses to 0.5 because elastic deflection forces the leading pin to carry most of the spike:
Now you have a problem. 621 kN exceeds the static rating of the needle bearing and pushes the pin-pocket Hertzian stress past 1,800 MPa — into the regime where the case layer cracks and spalls. The wheel will tolerate maybe a few hundred such events before you see visible pocket damage. This is why yankee drives use shear-pin couplings on the input side: the coupling fails at 130% rated torque and protects the pin gearing from exactly this load case.
Result
The nominal per-pin force comes out to 230 kN at full rated torque. At that load the drive runs cool, the pins roll cleanly in their pockets, and case fatigue accumulates slowly enough to project a 30-year service life. At 30% load (start-up) per-pin force drops to 69 kN — essentially zero wear contribution. At 180% shock load it spikes to 621 kN, which is the regime where case spalling and bearing brinelling start, so the upstream torque limiter must be set below this. If you measure pin contact-mark width wider than 25 mm on the wheel pocket after commissioning, the most likely causes are: (1) pin true-position error above 0.1 mm shifting the engagement geometry, (2) cycloidal wheel profile cut oversize causing the pin to climb the pocket flank instead of seating, or (3) a seized needle bearing on one pin converting it from rolling to sliding contact and dragging the wear pattern off-axis.
Step Gearing (continuous Bearing Teeth) vs Alternatives
Step Gearing competes against involute spur gearing, planetary reducers, and direct cycloidal drives in heavy slow-speed service. The choice usually comes down to torque density, environmental tolerance, and how much you care about service life vs initial cost. Here is the honest comparison on the dimensions buyers actually care about:
| Property | Step Gearing (Pin Gear) | Involute Spur Gear | Cycloidal Reducer |
|---|---|---|---|
| Typical operating speed | 1-100 RPM at output | 10-3000 RPM | 5-500 RPM at output |
| Peak load capacity | Very high — module 30+ practical | High — limited by tooth bending | High — limited by housing size |
| Backlash | Moderate, 0.1-0.5° at output | Low, 0.05-0.15° (precision ground) | Very low, <0.05° |
| Tolerance to grit and contamination | Excellent — open running tolerated | Poor — requires sealed enclosure | Good — sealed housing required |
| Service life at rated load | 20-50 years industrial | 10-25 years industrial | 15-30 years industrial |
| Initial cost (relative) | High — custom wheel profile | Low — standard catalogue parts | Medium-high — precision components |
| Best application fit | Slow heavy mill / kiln / lock drives | General machine drives, gearboxes | Robotic joints, indexing tables |
| Repair strategy | Replace individual pins on-site | Replace gear pair, often offsite | Replace full unit |
Frequently Asked Questions About Step Gearing (continuous Bearing Teeth)
That is almost always a single pin bearing that has either seized or is dragging — when one pin in a 12 or 14 pin set stops rolling, it converts to sliding contact for the duration of its engagement arc, and that engagement happens once per revolution at the same angular position. The hum is the sliding pin scrubbing the cycloidal pocket flank.
Diagnostic check — pull the pinion cover and rotate each pin by hand. A healthy pin spins with light finger pressure. A dragging pin will need a wrench. The cause is usually grease starvation in the cross-drilled lubrication channel or a needle bearing that has brinelled from a shock event. Replace the pin and bearing as a set, and inspect the matching pocket for the start of spalling before you put it back into service.
The decision hinges on three things: shock load environment, expected service life, and whether you can afford downtime for repair. Planetary reducers are more compact and cheaper up front, but a kiln drive sees thermal expansion shock and refractory chunks dropping into the kiln. That puts torque spikes through the drivetrain that fatigue planet bearings.
Step Gearing wins when service life and field-repairability matter. You can change a single bearing pin on a Sunday shutdown without pulling the drive. You cannot do that with a planetary unit — you swap the whole reducer. Run the numbers on hourly downtime cost vs cost differential, and for any plant where one hour of downtime exceeds about $5,000, Step Gearing pays back inside a decade.
Nine times out of ten the load distribution factor Kd in your calculation was too optimistic. Textbook examples assume Kd = 1.0, meaning load shares evenly across all engaged pins. In reality, elastic deflection of the pinion plate, manufacturing tolerance on pin position, and cycloidal wheel profile error mean the leading pin in the engagement arc carries 40-60% of total load even when nominally three pins are engaged.
For a real installation use Kd between 0.6 and 0.8 unless you have measured your specific build with strain gauges on the pins. The other common error is treating contact ratio as a fixed integer — at light loads only one pin may actually carry torque even though geometry says three are engaged, because backlash takeup is incomplete.
Sometimes — but the geometry rarely matches without machining the entire ring. The cycloidal pocket profile of a Step Gearing wheel cannot be cut into the involute root form of a spur ring gear; you would need to machine off the existing teeth and re-cut the cycloidal pockets, which usually means removing the ring entirely.
If the slewing bearing raceway is healthy and you only need to replace the toothed face, a specialist gear works can profile-cut a new cycloidal face on the existing ring. Below module 10 or so this is rarely economic compared to a full ring replacement. Above module 20 — typical of mining shovels and large cranes — the ring is worth the rebuild because the bearing race underneath is the truly expensive part.
End-loaded wear means the pinion plate and the cycloidal wheel are not parallel — there is a misalignment between the pinion shaft axis and the wheel axis, so the pins contact the pocket on one edge instead of across the full face. Common causes are foundation settlement under the wheel, bearing wear on the pinion shaft letting the pinion tilt under load, or thermal distortion of the housing during commissioning.
Check pinion shaft runout at the bearing journals first — anything above 0.1 mm TIR per metre of shaft length will produce visible end-loading on the pins within 5,000 hours. Correct alignment to within 0.05 mm/m and the wear band will reseat across the full pin length over the next few hundred hours of operation.
Less than you would expect, because the engagement is rolling, not sliding. A typical Step Gearing design runs a contact ratio of 2 to 3 — comparable to a well-cut spur gear — and the rolling contact eliminates the sliding-velocity-induced torque ripple that conventional spur gears produce at the start and end of each tooth engagement.
What you do see is a small ripple at pin-passing frequency (pin count × shaft RPM). On a 14-pin pinion at 25 RPM that is 5.8 Hz — well below any structural resonance and usually invisible in the torque trace. If you need lower ripple than that — for instance in a precision indexing application — that is when you step up to a true cycloidal reducer with continuously profiled teeth instead of discrete pins.
References & Further Reading
- Wikipedia contributors. Lantern gear. Wikipedia
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