A Driving Wheel with Internal Rim Guard is an intermittent-motion device where a driver wheel carries a single tooth (or short tooth segment) plus a concentric arc-shaped rim that locks against the driven wheel between engagements. It indexes the driven wheel one tooth per driver revolution, typically at 30 to 300 RPM in mechanical counters, while the internal rim arc holds the driven wheel rigidly stationary the rest of the cycle. We see this geometry inside Veeder-Root tally counters and classic odometer carry stacks, where positive locking between counts is non-negotiable.
Driving Wheel with Internal Rim Guard Interactive Calculator
Vary the driven tooth count and locked cycle fraction to see the rim guard dwell angle, index window, and one-tooth counter step.
Equation Used
The calculator converts the locked fraction of one driver revolution into the rim guard arc angle, with the remaining angle used for indexing. The driven wheel advances one tooth per driver revolution, so its angular step is 360/N and its revolution ratio is 1/N.
- One driving tooth advances the driven wheel by one tooth per driver revolution.
- Rim arc percent is the locked portion of one 360 deg driver cycle.
- Driven wheel motion during index is ideal with no backlash or tooth profile correction.
How the Driving Wheel with Internal Rim Guard Works
The mechanism splits one full rotation of the driver into two distinct phases: a short engagement window where a single driving tooth pushes the next tooth of the driven wheel forward by one pitch, and a long locked window where a smooth internal rim arc — concentric with the driver's axis — sits hard against the gullets of the driven wheel and prevents it from rotating in either direction. That arc is the rim guard. It does the same job a Geneva drive's locking disc does, but packaged inside a single driver blank instead of a separate plate.
The geometry has to be tight. The rim arc radius must match the driven wheel's gullet radius within roughly 0.05 mm on a typical 20 mm counter wheel — too loose and the driven wheel rocks back and forth between counts (you see digit flicker on a register), too tight and the driver binds or skips. The single tooth on the driver enters the driven wheel's tooth space at a precise tangent angle, usually 14.5° or 20° involute, and must clear the next gullet by 0.1 to 0.2 mm before the rim arc takes over the locking duty. If that handoff is mistimed — say the tooth disengages before the arc engages — the driven wheel free-wheels for a few degrees and the count goes wrong.
Failure modes are predictable. Worn rim arcs let the driven wheel rotate under vibration, which in a Veeder-Root style register shows up as digits creeping between counts. A burred driving tooth tip will skip teeth on the driven wheel, missing counts entirely. And if the driver and driven shafts aren't parallel within about 0.1° over their span, the tooth engages off-centre and chews the driven flank.
Key Components
- Driver Wheel Blank: The rotating disc that carries both the single driving tooth and the concentric rim arc. Typically machined from one piece of brass or hardened steel so the tooth and arc share the same datum within 0.02 mm. On a 25 mm-diameter blank the rim arc usually spans 300° to 330° of the circumference.
- Single Driving Tooth: The protruding tooth that engages one tooth space of the driven wheel per driver revolution. Profile is a standard 14.5° or 20° involute, with the tip relieved by 0.05 to 0.1 mm to ease entry. This is the only feature that transmits index motion.
- Internal Rim Guard Arc: The concentric arc on the driver that mates with the gullets of the driven wheel during the dwell phase. Radius tolerance is typically ±0.05 mm — looser than that and the driven wheel develops backlash, tighter and you get binding. Length of the arc directly sets the dwell-to-index ratio.
- Driven Counter Wheel: The wheel that advances one tooth per driver revolution. Tooth count sets the count ratio: a 10-tooth driven wheel gives the classic decade counter used in odometers and tally registers. Gullet radius must match the rim guard arc radius.
- Shaft Bearings: Plain bronze bushings or pivot jewels that fix the driver-to-driven shaft spacing. Centre distance tolerance of ±0.025 mm is standard for counter-grade work — looser bushings let the rim arc lift off the gullets and the counter loses its locking action.
Industries That Rely on the Driving Wheel with Internal Rim Guard
The Driving Wheel with Internal Rim Guard shows up wherever you need a small, cheap, robust mechanical count with positive between-count locking. It's lighter and shorter than a Geneva drive for the same indexing job, which is why it dominated the mechanical counter market for most of the 20th century before electronic counters took over. You still find it in mechanical equipment that has to count without a battery — fuel pumps, water meters, turnstile heads, vintage adding machines, and analogue automotive odometers.
- Industrial Counting: Veeder-Root Series 1432 mechanical tally counters, where the rim guard locks each digit wheel between counts to prevent vibration-induced miscounts on production lines.
- Automotive: Smiths Industries mechanical odometers fitted to MGB and Triumph dashboards from the 1960s, using a stack of driving-wheel-with-rim-guard pairs to carry the tens, hundreds, and thousands digits.
- Utility Metering: Neptune T-10 residential water meter register packs, where the carry mechanism between sweep, tens, and hundreds wheels uses single-tooth drivers with internal rim arcs.
- Fuel Dispensing: Gilbarco mechanical gallon totalisers in pre-electronic petrol pumps, where positive locking between counts was required for trade-approved metering.
- Access Control: Perey turnstile head counters at sports stadiums and subway entrances, counting passengers per rotation with a tooth-and-rim driver feeding a 4-digit register.
- Office Equipment: Burroughs Class 3 adding machines from the 1920s through 1950s, using driving-wheel-with-rim-guard pairs in the carry train of the accumulator register.
The Formula Behind the Driving Wheel with Internal Rim Guard
The key design number is the dwell-to-index ratio — what fraction of the driver's revolution the driven wheel sits locked vs. moving. At the low end of the typical range (say a 240° rim arc), you spend two-thirds of every revolution locked, which is fine for slow tally counters but cuts into the time available for indexing at high speed. At the high end (320° to 340° rim arc), the driven wheel is locked over 90% of the time and the indexing happens in a sharp pulse — which is what an odometer carry needs, but it demands a much faster acceleration of the driven wheel during the short engagement window. The sweet spot for general-purpose mechanical counters sits around 300° to 315° of rim arc.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| θindex | Driver rotation angle during which the single tooth engages and indexes the driven wheel | degrees (°) | degrees (°) |
| θrim | Angular span of the internal rim guard arc on the driver | degrees (°) | degrees (°) |
| Rdwell | Fraction of each driver revolution that the driven wheel is locked stationary | dimensionless | dimensionless |
| ωdriven,peak | Peak angular velocity of the driven wheel during the index window | rad/s | rad/s |
Worked Example: Driving Wheel with Internal Rim Guard in a vintage gas-pump gallon totaliser
You are restoring a 1958 Tokheim 39 gas-pump gallon totaliser and need to verify the rim arc and single-tooth geometry on the units-digit carry wheel. The driver runs at 120 RPM at maximum dispense rate, the driven wheel has 10 teeth on a 24 mm pitch diameter, and the original rim arc spans 306°. You want to confirm the dwell ratio and check that the peak driven-wheel velocity stays within what the brass digit drum can take without overshooting the next position.
Given
- θrim = 306 degrees
- Ndriver = 120 RPM
- Zdriven = 10 teeth
- Ddriven = 24 mm
Solution
Step 1 — at nominal 120 RPM, work out the index window angle and the dwell ratio:
So during every driver revolution the driven wheel is locked rigid by the rim guard for 85% of the cycle and only moves during the remaining 54°. That is the dwell-to-index split a trade-approved gas-pump totaliser needs — the digits sit dead still long enough to read clearly between counts.
Step 2 — convert driver speed to driven-wheel index time at nominal 120 RPM. One driver revolution is 0.5 s, so:
The driven wheel must rotate one tooth pitch (360° / 10 = 36°) in that 0.075 s window, giving a mean angular velocity of 36° / 0.075 s ≈ 480°/s ≈ 8.4 rad/s during indexing.
Step 3 — at the low end of the operating range, 30 RPM (slow trickle dispense), one driver revolution is 2.0 s:
That is a leisurely index — the digit drum visibly clicks over and the brass takes essentially no impact. At the high end, 240 RPM (fast emergency dispense some pumps allow), one driver revolution is 0.25 s and the index window collapses to 0.0375 s, pushing peak driven-wheel velocity over 16 rad/s. In a 1958 Tokheim drum the brass starts to overshoot the rim-arc capture point above roughly 200 RPM, which is exactly why the pump's flow regulator caps the totaliser drive around 150 RPM in service.
Result
At the nominal 120 RPM, the driven wheel is locked 85% of every cycle and indexes in a 0. 075 s pulse — fast enough to keep up with full-rate dispensing, slow enough that the brass digit drum doesn't overshoot. At 30 RPM the index is a relaxed 0.30 s click; at 240 RPM the 0.0375 s pulse exceeds what a 65-year-old brass drum can capture cleanly, and you'll see digits land between positions. If your restored pump shows the units digit rocking back slightly between counts, the rim arc has worn below its 12.0 mm nominal radius — measure with pin gauges and replace if it's under 11.95 mm. If digits skip entirely at high flow, the single driving tooth tip is burred or has lost its 0.05 mm relief and isn't catching the next gullet. And if the digit lands cleanly but then drifts during dwell, your driver-to-driven shaft centre distance has opened up past 18.05 mm, usually from a worn front pivot bushing.
Choosing the Driving Wheel with Internal Rim Guard: Pros and Cons
The Driving Wheel with Internal Rim Guard sits in the same design space as the Geneva drive and the simple ratchet-and-pawl. They all give you intermittent motion with a locked dwell, but they trade off differently on size, cost, speed, and accuracy. Here's how to pick.
| Property | Driving Wheel with Internal Rim Guard | Geneva Drive | Ratchet & Pawl |
|---|---|---|---|
| Typical operating speed | 30 to 300 RPM | 30 to 600 RPM (4-slot up to 1000 RPM) | 10 to 200 RPM |
| Indexing accuracy | ±0.5° between counts (rim-arc limited) | ±0.1° between stations (slot-and-pin limited) | ±1° to ±3° (pawl drop-in slop) |
| Dwell-to-index ratio | Adjustable via rim arc, 70% to 95% | Fixed by slot count (75% for 4-slot, 67% for 6-slot) | Up to 99%, set by drive cam profile |
| Package size for same count ratio | Smallest — single driver blank | Medium — driver disc plus locking disc | Largest — separate ratchet wheel plus pawl arm |
| Manufacturing cost | Low — one machined blank per side | Medium — two precision parts per side | Lowest — stamped pawl, simple ratchet |
| Reverse-rotation lock | Yes, full lock both directions during dwell | Yes, full lock both directions during dwell | One-way only — slips in reverse |
| Best fit application | Mechanical counters, odometers, tally registers | High-speed indexing tables, film projectors, packaging carousels | Hoists, winches, hand tools, jacks |
| Typical service life | 10 to 100 million counts before rim wear matters | 100,000 to 10 million indexing cycles | 100,000 cycles in heavy duty, more in light tools |
Frequently Asked Questions About Driving Wheel with Internal Rim Guard
That's almost always rim arc wear, not driving tooth wear. The rim guard radius has dropped below the driven wheel's gullet radius, leaving a small clearance the driven wheel can rotate within when vibration hits it. On a 12 mm nominal arc radius, even 0.08 mm of wear gives you about 0.4° of free rotation per count — enough to make a digit visibly wobble.
Pin-gauge the rim arc at three points around its span. If any reading is under spec, the part needs replacing or building up — you cannot shim a worn arc back to size.
Pick on package size and count rate. The Driving Wheel with Internal Rim Guard packs into a thinner stack because the locking arc lives on the same blank as the driving tooth — a Geneva needs a separate locking disc behind or in front of the slotted wheel. For a multi-digit register where you stack 4 to 8 wheels on a common shaft, that thickness saving compounds.
If you need indexing above 400 RPM or you need a fixed integer count ratio greater than 1:10, the Geneva wins because its locking geometry handles higher driven-wheel accelerations cleanly. Below 300 RPM and at 1:10 or simpler ratios, the rim guard is smaller, cheaper, and just as accurate.
Check shaft parallelism before anything else. If the driver and driven shafts are off-parallel by more than about 0.1° over a 20 mm span, the single driving tooth contacts the driven flank at an angle and either skates over the tip without engaging or loads the flank corner so hard it chips. Either way, you lose counts.
Quickest field check: blue the driving tooth, run the mechanism slowly by hand for 10 revolutions, and inspect the wear pattern on the driven flanks. Even contact across the full face means parallelism is good. Contact biased to one end means a bushing has worn unevenly and the shaft has tilted.
Mechanically you can extend the rim arc to about 350°, but the index window collapses to 10° and the driven wheel has to make a full tooth pitch in that tiny rotation. Peak angular velocity of the driven wheel scales inversely with the index window, so going from 54° to 10° multiplies driven-wheel acceleration by more than 5×. Brass and zinc digit drums start to overshoot and bounce back off the rim-arc capture, which gives you exactly the wrong outcome — phantom counts.
The practical ceiling for a counter-grade mechanism is around 320° to 325° of rim arc. Above that, switch to a cam-driven indexer if you genuinely need 95%+ dwell.
You've doubled the rim-arc contact load without changing the bushings. Each driven wheel pushes back radially on the rim arc through its gullets, and that radial load is reacted by the driver shaft bushings. Two wheels mean roughly twice the bushing load, the driver shaft deflects slightly, and the rim arc lifts off the first wheel's gullets at the far end of the stack — that's where you see the skip.
Either fit a second support bushing between the two driven wheels, or step up the shaft diameter. Most multi-digit registers carry an intermediate bushing every 2 to 3 wheels for exactly this reason.
Treat it as an Archard wear problem on the rim arc. Wear volume scales with sliding distance × normal load / hardness. For a hardened-steel driver against a hardened-steel driven wheel running dry in a counter, you typically see 0.01 to 0.05 µm of arc wear per million counts. Brass-on-brass wears 10× to 50× faster — fine for a domestic water meter doing a few thousand counts a day, marginal for a turnstile doing 50,000.
Rule of thumb: if the application sees more than about 10 million lifetime counts, specify hardened steel for both the driver blank and the driven wheel and accept the cost. Below that, brass is cheaper and quieter.
References & Further Reading
- Wikipedia contributors. Geneva drive. Wikipedia
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