Brown’s movement 70 uses a continuously rotating driver, a tappet and a studded output disk. Its outer rim surface meets adjacent studs during the stationary part of the cycle, while one opening provides clearance during indexing. The animation uses ideal point contacts to show how the stud spacing and shaft positions determine the timing.
External Stop Rim · Stud Drive Geometry & Timing
Inspect the outer retaining rim and one clearance opening. Adjust the stud layout and speed; contact geometry sets the engagement interval. Point contacts and stationary output after release are idealizations.
Equation Used
- Equally spaced ideal contacts on a rigid output disk.
- Separate fixed shaft centers.
- A rigid radial tappet advances one pitch per input revolution.
- Selected rim thickness and explicit clearance openings.
- Output assumed stationary after release; no inertia or load model.
- Driver web and studded plate occupy separate axial planes.
Brown 70: outer retaining surface and one clearance opening. Marker size does not represent a physical roller.
A tappet advances one stud pitch
The driver and output have separate fixed shafts. The output carries equally spaced studs. When the tappet reaches the active contact, its radial face pushes that contact through one output pitch. The next adjacent pair then occupies the stationary positions.
The retaining surface is the outer circumference of the rim. A single opening in the retaining rim clears the driven contact as it moves inside the circle and returns to the rim. The other contacts stay outside the outer retaining surface.
The gray band is the rim itself; its openings are actual gaps in that band. The pale driver web behind it is in a different axial plane. Orange marks the active ideal contact, and a small blue dot identifies one output stud so you can follow successive steps.
Compare geometry and timing
Stud count sets the output step. Orbit radius sets the scale, while the ratio between shaft spacing and orbit radius changes the engagement angle. Input speed determines the time per cycle and average output speed.
The timing is obtained from the geometry shown. It is not an independent arbitrary drive-angle or lock-percentage input. The drawing uses ideal contact points and illustrative proportions; it does not establish a load rating.
Pause or scrub near engagement to inspect the tappet and rim clearance. Autoplay is slowed so a brief indexing interval remains visible.
Pitch and contact construction
For N equally spaced contacts on radius r, let α=π/N and let d be the shaft center distance. Place the output center at (0,0) and driver at (d,0). The stationary contacts lie at angles −α and +α, so their distance from the driver center is R=√(d²+r²−2dr cosα).
Define γ=atan2(r sinα,d−r cosα). The tappet turns through 2γ radians while the driven contact moves from −α to +α. Driving input angle is 2γ converted to degrees; stationary input angle is 360° minus that value.
During engagement, the input face angle θ decreases from π+γ to π−γ. Distance along the face is t=−d cosθ−√(r²−d²sin²θ). The near intersection (d+t cosθ,t sinθ) lies on the stud circle. The output advances 2α=360°/N per driver revolution.
For input speed n rpm, average output speed is n/N. Stationary time is the stationary input angle divided by 6n seconds. At zero input speed the drawing remains at its selected position and timing reads stopped.
Example: ten studs and 200 mm shaft spacing
With N=10, r=100 mm and d/r=2, the step is 36° and the retaining contact radius is about 109.35 mm. Contact geometry determines the engagement interval; the output remains stationary for the rest of the modeled cycle.
Increasing input speed changes time and average speed but preserves the shape and angles. Scaling the orbit radius and shaft spacing together preserves the timing angles. Increasing stud count reduces the angle advanced per cycle.
Contact idealization and limits
The circular markers are contact locations, not finite stud diameters. The selected rim band is 0.08r thick. The single opening is centered on the tappet’s radial face and provides angular clearance for the active point.
A practical mechanism needs actual stud shapes, compatible tappet and notch profiles, axial clearances and load data. This model does not calculate impact, inertia, bounce, friction or load-holding capacity. It assumes the output stays at its stationary position after release; it does not certify positive locking under all applied loads.
The ideal contact law has abrupt changes in output velocity at pickup and release. The animation follows this contact law.
Rim-drive questions
Which surface retains the studs?
The outer circular surface of the rotating rim. Brown 70 shows one clearance opening for passage during indexing.
Is this a slotted Geneva wheel?
No. A Geneva wheel uses slots to guide a driving pin; this arrangement transfers motion through studs, a tappet and a retaining rim.
Why is drive angle calculated?
Shaft spacing and stud pitch determine the contact path and therefore the input angle needed for each output step.
Are the dots the size of real studs?
No. They identify ideal contact points. Finite stud and notch sizing require a separate profile and clearance design.
Primary reference
Henry T. Brown, 507 Mechanical Movements, movement 70: the outer retaining surface, tappet, studs and one clearance opening.
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