Rolling-contact Gears with Forked Catch are a paired drive where two friction-rolling discs transmit motion continuously while a forked catch on one disc engages a single pin or lobe on the other to enforce a positive index step. They solve the problem of getting reliable, repeatable single-step advance out of a smooth friction drive without slip. The forked catch grabs the driven pin once per input revolution, indexes it through a fixed angle, then releases for the rolling contact to coast through the dwell. You get clean intermittent indexing — common on light-duty labelling, dial feeders and textile take-up rolls running 30 to 200 cycles per minute.
Rolling-contact Gears with Forked Catch Interactive Calculator
Vary the number of index positions, driver speed, and catch engagement angle to see step angle, dwell time, and average output speed.
Equation Used
The fork captures the driven pin once per driver revolution. For equally spaced index positions, the driven step angle is 360 divided by the number of positions. The catch angle sets how much of each driver revolution is spent indexing; the remaining time is dwell.
- One fork engagement occurs per driver revolution.
- Index positions are equally spaced around 360 degrees.
- Catch engagement angle is measured on the driver disc.
- Rolling contact is ideal with no slip during dwell.
Inside the Rolling-contact Gears with Forked Catch
The mechanism is built around two ideas working together. The first is a rolling-contact friction pair — two profiled discs (often non-circular or lobed) running tangent so their surfaces roll without slip. The second is a forked catch, a small Y-shaped fork mounted on the driver, which captures a single pin or stub-tooth on the driven disc once per revolution. During the catch phase the fork drives the pin through a defined arc — that's your index step. Outside that arc the fork is clear of the pin and the rolling surfaces simply track each other through the dwell, holding the driven shaft stationary or moving at a programmed slow rate.
The geometry is fussy. The fork mouth must clear the pin diameter by 0.05 to 0.15 mm on each flank — too tight and you get jamming on entry as thermal growth closes the slot, too loose and you get backlash that you'll see as a 1 to 3° wobble at the indexed position. The pin diameter has to match the fork mouth to a slip fit of around H8/f7. The rolling surfaces themselves carry no positive engagement, so contact pressure must stay below the Hertzian limit for the disc material — typically 600 to 900 MPa for hardened tool steel against itself. If the contact pressure goes too high you get pitting on the driver disc within a few hundred thousand cycles, which then changes the rolling ratio and throws the catch timing off by milliseconds per cycle.
The usual failure modes are predictable. The fork tips chip if the pin hits them off-axis — that's almost always a parallelism error between the two shafts greater than 0.1 mm over 100 mm. The pin galls or shears if the index step is loaded beyond the fork's bending capacity. And the rolling surfaces glaze and slip if oil contamination gets onto a friction-only design. Run the mechanism dry, or with a tacky lubricant only on the catch fork — never on the rolling face.
Key Components
- Driver disc with forked catch: The input wheel carrying the Y-shaped fork on its periphery. The fork mouth is typically 1.5 to 4 mm wide depending on pin size, with a 5 to 10° lead-in chamfer on each tine to guide the pin into engagement. Made from hardened steel, 58 to 62 HRC at the fork tips.
- Driven disc with catch pin: The output wheel carrying a single hardened dowel pin standing 4 to 8 mm proud of the rim. The pin sits at a defined radius so its tangential velocity matches the fork's tangential velocity at the moment of capture — mismatch by more than 5% and you'll hear a knock on each engagement.
- Rolling-contact surfaces: The friction-rolling band on each disc, ground to a surface finish of Ra 0.4 µm or better. These surfaces transmit no torque during the index step (the catch takes that load) but maintain phase between the discs through the dwell so the next catch lines up correctly.
- Shaft pair and bearings: Two parallel shafts on rolling bearings — typically deep-groove ball bearings for light service, angular contact for higher cycle rates. Shaft parallelism must hold within 0.05 mm over the disc face width or the fork loads unevenly on its tines.
- Index stop or detent (optional): On precision builds a sprung detent locks the driven disc during the dwell phase to absorb any rolling-contact slip. Common on dial-feeder applications where the indexed position must repeat to ±0.1°.
Who Uses the Rolling-contact Gears with Forked Catch
You see this mechanism wherever a designer wants the smooth phasing of a rolling-contact drive but needs one positive index step per cycle to position a part. It's a niche between full Geneva drives (which give pure intermittent motion) and continuous gearing (which gives no dwell). The forked catch variant shows up most often in light-duty machinery where shock loads are low, the index angle is small (typically 30 to 90°), and packaging or assembly throughput sits in the 30 to 200 cycles per minute range.
- Packaging: Carton flap-closing dial on a Bosch Pack 102 cartoner, where each station needs a 60° index then a long dwell while glue is applied
- Textile machinery: Take-up roll indexing on a Karl Mayer warp knitting machine, advancing the fabric one course per stroke at 600 strokes per minute
- Watchmaking and instruments: Date-wheel advance in mechanical movements like the ETA 2824-2, where a forked catch on the hour wheel indexes the date disc once per 24 hours
- Assembly automation: Pin-insertion station on a Gefit rotary assembly machine, indexing a 12-position dial with 30° steps between insertion strokes
- Printing: Plate-cylinder phase reset on a Heidelberg Speedmaster auxiliary drive, where a single-pin catch re-references the cylinder to the master timing belt once per print cycle
- Food processing: Cup-feeder dial on a Hassia THL form-fill-seal machine indexing yogurt cups at 80 cycles per minute
The Formula Behind the Rolling-contact Gears with Forked Catch
What you need to predict is the index angle the driven disc sweeps through during one fork engagement, because that's the number that drives station spacing on a dial or stride length on a take-up roll. The formula links the radius ratio of the two discs, the fork's effective contact arc on the driver, and the resulting index step on the driven side. At the low end of the typical range — radius ratio near 1:1 with a short fork arc — you get a small, precise index of 10 to 20°. At the high end — radius ratio of 1:4 with a long fork arc — the driven disc swings through 90° or more in a single catch, but you start losing positional repeatability because any slip in the rolling contact during the catch phase amplifies through the gear ratio. The sweet spot for most packaging and textile work sits at a 1:2 ratio with a 60° index step.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| θindex | Angle swept by the driven disc during one fork engagement | degrees (°) | degrees (°) |
| Rdriver | Effective rolling radius of the driver disc at the fork pitch line | mm | in |
| Rdriven | Effective rolling radius of the driven disc at the catch pin | mm | in |
| φfork | Angular contact arc of the fork on the driver — how many degrees of driver rotation the fork stays engaged with the pin | degrees (°) | degrees (°) |
Worked Example: Rolling-contact Gears with Forked Catch in a vial-capping dial on a pharmaceutical filler
You're designing the cap-presentation dial on a Marchesini ML 421 vial capper, running 120 vials per minute. The dial holds 6 caps on a 90 mm pitch circle and must index 60° per cycle, then dwell while a pick-and-place head lifts the cap. You've chosen rolling-contact gears with a forked catch because the line runs dry (no oil mist allowed near open vials) and you need the dwell phase to be vibration-free. The driver disc has Rdriver = 30 mm and the driven dial sits at Rdriven = 60 mm. You need to set the fork contact arc φfork to deliver exactly 60° of index.
Given
- Rdriver = 30 mm
- Rdriven = 60 mm
- θindex (target) = 60 °
- Cycle rate = 120 cycles/min
Solution
Step 1 — solve the formula for the required fork contact arc at the nominal 60° index target:
So the fork stays in contact with the catch pin through 120° of driver rotation. That's a long engagement — the driver is essentially carrying the pin for one-third of every revolution, which means you must size the fork tines to handle the full index torque, not a peak transient.
Step 2 — at the low end of the operating range, suppose the line slows to 60 cycles/min during a changeover and you tighten the index to 45° for a finer station pitch. The required fork arc becomes:
That's a comfortable arc — the fork is engaged for a quarter revolution, loads are modest, and positional repeatability holds within ±0.1° because the rolling-contact phase has plenty of dwell to settle.
Step 3 — at the high end, push the index to 90° for a 4-station dial running at the line's max 200 cycles/min:
Now the fork is engaged for half of every driver revolution. In theory the kinematics still work, but at 200 cycles/min the catch entry impact rises with the square of speed and the fork tip stress climbs past 400 MPa for a typical 12 mm-wide steel fork. You'll see fork-tip rounding within 2 to 3 million cycles and the index angle drifts by 0.5° as the worn fork lets the pin enter late. The practical sweet spot for this geometry is the nominal 60° index at 120 cycles/min.
Result
The fork contact arc must be 120° to deliver the target 60° index on the dial. In practice you'll feel this as a crisp, repeatable advance with the dial settling within about 50 ms after each catch — fast enough that the pick-and-place has 450 ms of clean dwell to lift the cap. Comparing the three operating points: the 45° low-end index gives the longest fork life and tightest repeatability but limits you to 8-station dials, the nominal 60° index hits the sweet spot, and the 90° high-end index works but eats fork tips inside a year of two-shift operation. If your measured index angle drifts from the predicted 60°, the three most likely causes are (1) catch-pin wear flattening one face of the dowel — check pin diameter against the original H8 spec and replace if it's lost more than 0.05 mm, (2) shaft parallelism drift after a bearing change loading one fork tine harder than the other, which you'll see as asymmetric wear on the fork mouth under a loupe, or (3) rolling-surface glazing from accidental oil contamination, which lets the discs slip during the dwell so the next catch enters at the wrong phase.
Rolling-contact Gears with Forked Catch vs Alternatives
The forked catch variant sits between a Geneva drive and a continuous gear pair. Each option suits a different combination of index angle, cycle rate and shock tolerance. Pick based on the dwell ratio you actually need and the load the index step must carry.
| Property | Rolling-contact gears with forked catch | Geneva drive | Continuous gear pair |
|---|---|---|---|
| Typical cycle rate (cycles/min) | 30 to 200 | 10 to 300 | no limit (continuous) |
| Index repeatability | ±0.1° to ±0.3° | ±0.05° to ±0.1° | n/a (continuous motion) |
| Index step range | 20° to 120° | 60° to 90° (fixed by slot count) | any (no dwell) |
| Shock load capacity at the catch | Low to medium (fork-tip bending limited) | High (full slot engagement) | High (full tooth engagement) |
| Lubrication needs | Dry or tacky grease on fork only | Oil bath or grease on slot | Oil bath required at speed |
| Cost and complexity | Low — two discs, one pin, one fork | Medium — slotted wheel + drive crank | Low — but no dwell capability |
| Best fit application | Light-duty indexing with smooth phasing needs | Heavy-duty intermittent indexing | Any application without dwell requirement |
Frequently Asked Questions About Rolling-contact Gears with Forked Catch
Almost always a tangential velocity mismatch at the moment of capture. The fork tip and the catch pin must have the same linear speed at the instant the fork mouth closes around the pin — if they don't, the pin slams into one side of the fork and you hear it.
Check that Rdriver at the fork pitch line matches the fork's actual tip radius (people forget the fork tip sits a few mm proud of the disc rim). A 5% velocity mismatch produces an audible knock; 10% will chip the fork within a few hundred thousand cycles. Trim the fork pitch radius or shift the catch pin radially until the knock disappears.
Single pin gives one index per driver revolution — clean, simple, and you control the dwell ratio purely by the fork arc. Multi-pin (2 or 3 pins evenly spaced on the driven disc) gives multiple indexes per driver revolution and shortens the dwell, which only makes sense when your station count divides evenly into your cycle ratio.
Rule of thumb: stay with single pin unless you specifically need two or more index events per driver rev. Multi-pin builds need every pin radius matched within 0.02 mm or the index angle varies cycle-to-cycle and you lose the repeatability that pushed you toward a positive catch in the first place.
The rolling-contact surfaces are still in contact during the dwell — that's by design, they hold phase between the discs. If the driver is rotating during the dwell (which it is, because the input motor doesn't stop) the friction roll transmits a small velocity to the driven disc unless the rolling ratio is exactly 1:1 with a stationary phase built into the disc profile.
For a constant-radius rolling pair the driven disc will track the driver during dwell, just at a programmed slow rate. If you need a true zero-velocity dwell you need either a non-circular rolling profile with a flat-tangent dwell zone, or a sprung detent that locks the driven disc once the fork releases. Most production builds use the detent — it's simpler than machining a non-circular disc.
Around 200 to 250 cycles per minute for a typical 50 to 100 mm disc with a 12 mm-wide steel fork. Above that, two things go wrong. First, the catch entry impact scales with the square of speed and fork-tip stress climbs into the fatigue range — you get fork rounding inside a year. Second, the rolling-contact surfaces start to skid on entry because the fork accelerates the pin faster than the rolling friction can transmit, which glazes the discs.
Above 250 cycles/min switch to a Geneva drive or a cam-driven indexer. Below 30 cycles/min the forked catch is overkill — a simple ratchet-and-pawl is cheaper.
Thermal growth in the fork-mouth-to-pin clearance. A typical machine warms 15 to 25°C over a shift, and steel grows about 12 µm/m/°C. On a 30 mm fork-pitch radius that's 5 to 10 µm of growth in the fork mouth — enough to shift the engagement timing by a fraction of a millisecond, which translates to 0.5 to 2° of index error at 120 cycles/min.
Two fixes: spec the fork mouth with a slightly looser cold clearance (0.10 to 0.15 mm per flank instead of 0.05 mm) so warm operation closes to the design clearance, or add a sprung detent to absorb the drift. The detent fix is more common because it also handles bearing wear over time.
Not directly — the forked catch alone delivers ±0.1 to ±0.3° at best, because the pin-to-fork-mouth clearance creates an irreducible backlash window. For inspection-grade indexing you need a secondary precision element.
The standard approach is to use the forked catch for the gross index motion and add a sprung taper-pin detent or a Curvic coupling that engages at the end of each index step. The catch gets the dial to within ±0.2° and the taper pin pulls it into a hard mechanical seat that repeats to ±0.01°. This is the same trick used on watchmaker's lathes and on precision rotary indexers from companies like Hirschmann.
References & Further Reading
- Wikipedia contributors. Intermittent mechanism. Wikipedia
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