The Brownell crank motion is a modified crank-and-rocker linkage that converts steady rotary input into an output stroke with a built-in dwell — a moment where the driven part holds nearly still while the input keeps turning. Early Brownell-built textile carding and roving frames used it to advance fibre during the still phase and pause during the active phase. The purpose is intermittent feed without a Geneva drive or cam. The outcome is smoother, quieter feed at higher cycle rates than a comparable cam-follower.
Brownell Crank Motion Interactive Calculator
Vary crank radius, coupler length, and pivot offset to estimate dwell angle, stroke, link ratio, and rotation clearance.
Equation Used
This calculator uses the Brownell worked-example geometry as the reference: crank radius r, coupler length Lc, and pivot offset e set the estimated dwell zone. The stroke is evaluated from an offset crank-slider position equation over one full revolution.
FIRGELLI Automations - Interactive Mechanism Calculators
- Planar offset crank-slider approximation of the Brownell crank motion.
- Dwell angle is estimated from the tuned coupler and offset geometry.
- Positive clearance means the model can complete a full rotation without geometric lockup.
Inside the Brownell Crank Motion
The Brownell crank motion takes a standard crank and adds a secondary link — usually a slotted lever or a curved coupler — that bends the output's velocity profile so it slows almost to zero at one extreme of its travel. The crank keeps spinning at constant RPM, but the driven slider or rocker pauses for roughly 60-90° of crank rotation depending on link proportions. That pause is the dwell. During the dwell, a fibre web, a print substrate, or a feed roller can be advanced or stamped without the output member fighting it.
The geometry is a non-uniform crank motion variant — same family as the offset crank slider, but with the coupler length and pivot offset tuned so the output velocity passes through near-zero at the dwell point. Get the offset wrong by even 2-3 mm on a 100 mm crank and the dwell either disappears or turns into a hard reversal that hammers the bushings. The pivot-to-pivot tolerance on the coupler must hold within ±0.05 mm on a precision feed application — sloppy pin holes show up immediately as positional jitter at the dwell, which on a textile carding frame means uneven sliver weight.
Failure modes are predictable. Worn pivot bushings let the dwell drift several degrees per revolution, so timing relative to a downstream cutter or stamp goes off. A bent coupler — common after a jam — flattens the dwell entirely and the output starts moving when it shouldn't. And if the crank speed climbs past the design point, inertia in the output link overshoots the dwell and the mechanism becomes a noisy rocker with no useful pause at all.
Key Components
- Driving crank: The constant-speed input arm, typically 40-150 mm in industrial Brownell-style feeds. Runs at the machine's main shaft speed, often 60-300 RPM. Bore concentricity to the main shaft must hold within 0.02 mm or the dwell timing wanders.
- Coupler link: Connects crank pin to the rocker or slider. Length and curvature set the dwell duration — a coupler 1.4-1.6× the crank length gives the cleanest dwell. Pin-hole tolerance ±0.05 mm; anything looser shows up as 1-2° of timing scatter.
- Output rocker or slider: Carries the driven element — feed roller pawl, cutter linkage, or stamping head. Its pivot offset from the crank centre is the critical design dimension; typical values 0.3-0.5× crank length depending on dwell angle target.
- Pivot bushings: Bronze or needle-bearing bushings at all three pin joints. Wear here is the number-one cause of dwell drift in service. Replace when radial play exceeds 0.1 mm — beyond that, dwell timing scatters by more than 3° per revolution.
- Adjustable pivot block: On tunable Brownell variants, the rocker pivot mounts on a slotted block so the operator can fine-tune dwell angle in the field. Movement range typically ±5 mm, locked with two M8 cap screws torqued to 25 Nm.
Who Uses the Brownell Crank Motion
You see Brownell crank motion wherever a machine needs an intermittent advance from a continuously rotating shaft and a Geneva drive would be too jerky or a cam too maintenance-heavy. The mechanism shows up most in legacy textile equipment, but the principle has crossed into packaging, printing, and small-component assembly. Why use it instead of a cam? Lower part count, no follower spring to fatigue, and far quieter operation at speeds above 200 RPM where cam-follower contact starts to chatter.
- Textile machinery: Brownell & Co. carding and roving frames used the linkage to advance fibre sliver during the dwell phase while drafting rollers turned continuously.
- Printing presses: Sheet-fed letterpress feed tables — including some Heidelberg Original platen variants — use a related dwell-crank linkage to hold the sheet stationary during impression.
- Packaging machinery: Cartoning machines like the older Bosch Sigpack lines use modified crank-dwell linkages to index cartons under glue heads at 120-150 cycles per minute.
- Sewing equipment: Industrial bartack machines use a dwell-crank arrangement on the work-clamp feed so the clamp pauses while the needle bar penetrates fabric.
- Small-component assembly: Pin-insertion machines on connector assembly lines use a Brownell-type linkage to hold the pin still during the press stroke without a separate dwell cam.
- Wire forming: Spring-coiling machines use the linkage to advance wire by a fixed pitch during the cutter-open phase.
The Formula Behind the Brownell Crank Motion
The useful number to compute is the dwell angle β — the portion of crank rotation during which the output velocity stays below a chosen threshold (typically 5% of peak output velocity). At the low end of the typical design range, with a coupler ratio of 1.2 and small offset, you get only 30-40° of useful dwell — barely enough for a fast index. At the high end, coupler ratio 1.8 with large offset, you can stretch dwell to 110° but the output stroke shrinks and the return motion gets violent. The sweet spot for textile and packaging feeds sits around coupler ratio 1.5 and offset 0.4× crank length, giving ~75° of dwell with a usable stroke and tolerable return acceleration.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| β | Dwell angle — crank rotation over which output velocity stays below 5% of peak | degrees | degrees |
| Lc | Coupler link length | mm | in |
| r | Crank radius (driving crank length) | mm | in |
| e | Pivot offset from crank centre to rocker pivot | mm | in |
Worked Example: Brownell Crank Motion in a label-applicator indexing drive
You are designing the indexing drive for a pressure-sensitive label applicator that places labels on 50 ml glass vials moving on a starwheel. The starwheel must dwell while the label is wiped on, then index 60° to the next pocket. Main shaft runs at 180 RPM. You picked a Brownell crank with r = 50 mm and want to know what coupler ratio and offset to set.
Given
- r = 50 mm
- Lc = 75 mm
- e = 20 mm
- N = 180 RPM
Solution
Step 1 — at the nominal design point with coupler ratio 1.5 (Lc = 75 mm) and offset e = 20 mm, plug into the dwell-angle formula:
That 51.6° of crank rotation is your dwell window. At 180 RPM the crank turns 1080°/s, so the dwell lasts 51.6 / 1080 ≈ 48 ms — long enough to wipe a label cleanly, since a typical foam roller wipe needs 30-40 ms.
Step 2 — at the low end of the typical operating range, drop coupler ratio to 1.2 (Lc = 60 mm) and shrink offset to e = 12 mm:
The dwell looks longer on paper but the velocity profile is mushy — the output is creeping, not stopped, and a label wipe at this setting smears the leading edge.
Step 3 — at the high end, push coupler ratio to 1.8 (Lc = 90 mm) with offset e = 30 mm:
Now you have nearly 87 ms of dwell at 180 RPM, but the return stroke acceleration roughly doubles, and the rocker pivot bushing sees peak loads around 2.4× the nominal case. On a 24/7 line, that means bushing replacement every 4-6 months instead of the 18-month interval you would get at the nominal setting.
Result
Nominal dwell is 51. 6° of crank rotation, or 48 ms at 180 RPM — comfortably inside the 30-40 ms label-wipe window with margin for shaft-speed variation. The low-end setup gives a misleading 60° on paper but only ~28° of genuinely-stopped output, which produces smeared labels. The high-end setup buys you 93.6° of dwell but punishes the rocker bushings with 2.4× peak load and cuts service interval by 3-4×. If you measure dwell on the assembled machine and get 45° instead of the predicted 51.6°, check three things in order: (1) coupler pin-hole wear — if radial play exceeds 0.1 mm at either end, the dwell flattens; (2) crank set-screw slip on the main shaft — even 1° of slip shifts the dwell window relative to the starwheel cam; (3) rocker pivot block creep — the M8 lock screws back off under vibration and the offset drifts, killing dwell symmetry.
Choosing the Brownell Crank Motion: Pros and Cons
Brownell crank motion lives in the same design space as Geneva drives and barrel cams. Each one buys intermittent motion in a different way, and the right choice depends on cycle speed, dwell-to-index ratio, noise constraints, and how much maintenance the line will tolerate.
| Property | Brownell crank motion | Geneva drive | Barrel cam (indexing cam) |
|---|---|---|---|
| Practical speed range | 60-400 RPM input | 30-200 RPM input (jerk-limited) | 30-1500 RPM input |
| Indexing accuracy | ±0.5° at output | ±0.05° at output | ±0.01° at output |
| Dwell-to-index ratio (typical) | 1:1 to 3:1, tunable | Fixed by tooth count (3:1 to 5:1) | Fully programmable, any ratio |
| Initial cost (mid-size machine) | $200-600 | $400-1,200 | $2,500-8,000 |
| Service life before rebuild | 18-24 months at 180 RPM | 5-10 years (low wear) | 10+ years |
| Noise at 200 RPM | Low — 65-70 dB | Moderate — 75-85 dB (driver pin impact) | Low — 60-70 dB |
| Failure mode | Bushing wear flattens dwell | Driver pin impact damage | Follower roller pitting |
| Design complexity | Moderate — 3-link synthesis | Simple — standard tables | High — custom cam profile |
Frequently Asked Questions About Brownell Crank Motion
Most likely the velocity threshold you used to define dwell is being violated by elastic deflection in the coupler link, not a geometry error. A long, slender coupler — anything with a length-to-diameter ratio above 20:1 in steel — flexes under inertial load and lets the output drift several tenths of a millimetre during the supposed dwell. The fix is to thicken the coupler or switch to a box section. A second cause is backlash at the rocker pivot — even 0.05 mm of pin-hole clearance turns into measurable creep when the rocker reverses direction.
Adjust the rocker pivot offset (e). Most production Brownell-style frames mount the rocker pivot on a slotted block exactly so you can shift it 2-5 mm in the field. Moving the pivot away from the crank centre lengthens dwell but shrinks output stroke, so you trade one for the other. Mark the original position with a punch before you move anything — and always re-time the downstream cam after, because shifting offset changes when in the crank cycle the dwell starts, not just how long it lasts.
Pick the Brownell when input speed is above 150 RPM, when noise matters, or when you need a tunable dwell-to-index ratio. The Geneva wins on indexing accuracy — its locking arc holds output position to a few arc-minutes — but it gets loud and beats up its driver pin above 150-200 RPM. Pick Geneva for low-speed, high-precision indexing like turret tool changers; pick Brownell for higher-speed continuous feed jobs like label applicators, carton indexers, and textile slivers where ±0.5° at the output is acceptable.
Thermal expansion of the coupler link. A 75 mm steel coupler grows about 0.07 mm per 10 °C rise, and on a hot textile floor the linkage can climb 30 °C from cold-start to steady state. That 0.2 mm length change is enough to shift dwell timing 1-2°. If your machine indexes against a fixed downstream cam, the relative timing slips and the output starts hitting at the wrong moment. The cure is either an Invar coupler on critical machines or a 30-minute warm-up cycle before timing-critical production runs.
Yes for the dwell angle β, no for dynamic behaviour. The angle depends only on the ratios Lc/r and e/r, so geometric scaling preserves it. But inertial loads scale with mass times length, which goes as the fourth power of linear scale. Doubling every dimension at the same RPM means rocker pin loads go up roughly 16×, and bushing life drops accordingly. Either reduce input RPM or upsize the bearings — don't just scale and run.
Inconsistent advance with consistent input speed almost always traces to the pawl-and-ratchet interface downstream of the linkage, not the linkage itself. The crank is delivering the same stroke every cycle; it's the pawl that's slipping a tooth or skipping engagement. Check pawl spring tension first, then ratchet tooth wear. If the pawl side checks out clean, then look at the crank pin set-screw — a slipping crank pin will give you a stroke that varies by 1-2 mm cycle to cycle and mimics pawl slip almost exactly.
References & Further Reading
- Wikipedia contributors. Straight-line mechanism. Wikipedia
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