A modified eccentric with elongated yoke is a reciprocating drive where a circular eccentric disc rides inside a yoke whose slot is longer than the eccentric's diameter, producing a stroke with built-in dwell at one or both ends. You see this exact mechanism on Bruderer high-speed stamping press feed units and on older Heidelberg cylinder presses. The elongation lets the disc travel inside the slot without moving the yoke, holding the output stationary while the input shaft keeps rotating. Designers use it to add a timed pause to a simple eccentric drive — without the cost of a cam.
Modified Eccentric With Elongated Yoke Interactive Calculator
Vary disc diameter and slot length to see free travel, estimated dwell angle, and the animated yoke response.
Equation Used
The key article calculation is the free travel inside the elongated slot: slot length minus disc diameter. The dwell angle shown is a compact one-end estimate scaled to the worked example, where 10 mm of free travel on a 40 mm disc gives about 60 degrees of dwell.
FIRGELLI Automations - Interactive Mechanism Calculators
- Slot length is measured along the yoke travel direction.
- Disc diameter is the effective rolling or contact diameter.
- Dwell angle is a quick one-end estimate matched to the worked example.
- Negative clearance is treated as zero free travel and zero dwell.
How the Modified Eccentric with Elongated Yoke Works
The mechanism starts as a standard scotch yoke — an eccentric disc pinned off-centre on a rotating shaft, captured inside a rectangular slot in a sliding yoke. In a plain scotch yoke the slot width matches the disc diameter exactly, so every degree of shaft rotation drives the yoke. Elongate that slot along the direction of yoke travel, and you change the rules. Now the disc can roll along the inside of the slot without pushing the yoke at all — the yoke stops moving, even though the shaft keeps spinning. That stop is the dwell.
The dwell length depends on one number: slot length minus disc diameter. If your eccentric disc is 40 mm diameter and your slot is 50 mm long, you get 10 mm of free travel inside the slot — that translates to roughly 60° of shaft rotation where the yoke sits still at one end of its stroke. Get the slot length wrong and the mechanism misbehaves in predictable ways. Slot too short and the dwell vanishes, leaving you with a plain harmonic reciprocator. Slot too long and the disc rattles between the slot ends at the transition, producing an audible knock that hammers the contact faces and erodes them within hours of running.
The dwell-rise-dwell motion comes free with the geometry, but it isn't gentle. The follower velocity profile shows a sharp acceleration spike when the disc first contacts the slot end after the dwell — that's the cost you pay versus a true cam. You manage it by hardening both contact faces of the slot to 58-62 HRC and running the eccentric on a sealed needle bearing rather than a plain bushing. Skip the bearing and the slot faces gall within the first few thousand cycles.
Key Components
- Eccentric Disc: A hardened steel disc, typically 30-80 mm diameter, pinned on the input shaft with its centre offset from the shaft axis by the throw radius. Surface finish on the disc OD must hit Ra 0.4 µm or better to avoid scoring the yoke slot. Concentricity between bore and OD held to 0.02 mm.
- Elongated Yoke: The reciprocating output member, machined with a rectangular slot whose length exceeds the eccentric disc diameter by the desired free-travel distance. Slot faces hardened to 58-62 HRC and ground parallel within 0.01 mm to prevent the disc from binding diagonally.
- Needle Roller Bearing: Mounted between the eccentric disc and a hardened sleeve that contacts the slot. Drops sliding friction by roughly 80% versus a plain bushing and stops the slot faces from galling. Standard INA NK-series bearings work for shaft speeds below 800 RPM.
- Yoke Guide Rails: Two parallel linear bearings or hardened rod guides that constrain the yoke to pure linear motion. Lateral play above 0.05 mm lets the yoke cock under load and forces the eccentric to climb the slot wall, multiplying contact stress.
- End-Stop Bumpers: Optional polyurethane or aluminium bumpers fitted at slot ends to absorb the transition shock if the design runs above 300 RPM. A 90 Shore A urethane pad cuts peak transition acceleration by roughly 40%.
Industries That Rely on the Modified Eccentric with Elongated Yoke
The elongated yoke shows up wherever a designer needs a brief pause built into a continuously rotating drive — but doesn't want to pay for a cam, follower, and return spring. It's the cheapest way to add dwell to a reciprocator. You find it in metal stamping feeders, textile take-up motions, printing press inkers, and any machine where a part needs to sit still for a defined window while a process happens around it.
- Metal Stamping: Roll-feed dwell units on Bruderer BSTA high-speed stamping presses, where the strip must hold position while the punch enters the die
- Printing: Ink-form roller traverse on legacy Heidelberg KORD cylinder presses, providing a brief stop at each end of the lateral oscillation
- Textile: Cloth take-up dwell mechanism on Sulzer projectile weaving looms, holding the fabric stationary during shed change
- Packaging: Carton flap-folding plunger drive on Bosch Doboy cartoners, dwelling at full extension to let the glue set under pressure
- Sheet Metal Forming: Wire-feed indexing on Wafios spring coilers, dwelling while the cutoff blade severs the formed wire
- Food Processing: Biscuit-cutter reciprocating head on Baker Perkins rotary moulders, holding the cutter at the bottom of stroke for clean release
The Formula Behind the Modified Eccentric with Elongated Yoke
The dwell angle is what matters here. Practitioners size the slot length to hit a target dwell window — too short and the downstream process doesn't finish, too long and the rise phase compresses into a violent acceleration spike. At the low end of the typical range (10-20° dwell) the mechanism behaves nearly like a plain scotch yoke with a hint of pause. The sweet spot lives between 40° and 90° of dwell at each end, where you get a useful stationary window without crushing the rise-phase velocity. Push past 120° total dwell and the rise-acceleration peak doubles, beating the contact faces apart.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| θdwell | Total dwell angle per revolution at one slot end | degrees | degrees |
| Lslot | Length of the elongated yoke slot in the direction of travel | mm | in |
| Decc | Outside diameter of the eccentric disc (or bearing OD if using a roller) | mm | in |
| r | Throw radius — distance from shaft centreline to eccentric disc centre | mm | in |
Worked Example: Modified Eccentric with Elongated Yoke in a strip-feed dwell unit on a high-speed stamping press
You are sizing the elongated-yoke dwell unit that drives the gripper-feed slide on a Bruderer BSTA-30 high-speed stamping press running at 600 strokes per minute. The strip must sit still for about 60° of crankshaft rotation while the punch enters the die, then advance 25 mm before the next punch cycle. Eccentric disc OD is 40 mm, throw radius is 12.5 mm. You need to find the slot length that produces 60° of dwell, then check what happens at the low and high ends of the typical operating window.
Given
- Decc = 40 mm
- r = 12.5 mm
- θdwell, target = 60 degrees
- Stroke = 25 mm
- N = 600 RPM
Solution
Step 1 — rearrange the dwell formula to solve for the free travel inside the slot at the nominal 60° target:
Step 2 — plug in the 60° nominal dwell and the 12.5 mm throw radius:
That's a 3.35 mm elongation past the disc diameter — small, but it gives you a clean 60° pause at each end of the stroke. At 600 RPM that pause lasts 16.7 ms — long enough for a punch to enter the die and start the cut.
Step 3 — check the low end of the typical operating range. If you back off to a 20° dwell target (the minimum useful pause for most stamping work):
Only 0.38 mm of slot elongation. The dwell window collapses to about 5.6 ms at 600 RPM — barely enough to settle the strip before the punch arrives. You'd see strip whip and feed-position scatter on long parts.
Step 4 — check the high end. If you push to a 120° dwell target (aggressive — used on draw-form work where the punch needs a long bottom-dwell):
The slot is now 12.5 mm longer than the disc. Total dwell across both ends eats 240° of the cycle, leaving only 120° for the rise. Peak rise-velocity climbs to roughly 1.7 m/s and peak acceleration roughly triples versus the 60° case. At 600 RPM you'll hear the slot ends knock and the eccentric needle bearing will pit within 50 hours of running.
Result
The nominal slot length comes out to 43. 35 mm for a 60° dwell at each stroke end — the sweet spot for high-speed stamping work. The low-end 20° case (40.38 mm slot) feels almost like a plain scotch yoke with an imperceptible pause and lets the strip whip on long blanks; the high-end 120° case (52.5 mm slot) gives you a luxurious dwell but compresses the rise phase into a hammer-blow that destroys bearings. If your measured dwell drifts shorter than predicted, check three things in order: (1) the eccentric disc bore-to-OD concentricity — anything above 0.05 mm runout shaves the effective slot clearance and shortens dwell, (2) yoke slot ground-parallelism, because a tapered slot pinches the disc mid-travel and starts the yoke moving early, and (3) shaft endplay above 0.1 mm, which lets the disc cock and contact one slot wall prematurely.
When to Use a Modified Eccentric with Elongated Yoke and When Not To
The modified eccentric with elongated yoke sits between a plain scotch yoke and a true disc cam. It buys you dwell cheaply, but you give up smooth motion at the transitions. Here's how it stacks up against the two mechanisms it usually competes with on a designer's drawing board.
| Property | Modified Eccentric with Elongated Yoke | Plain Scotch Yoke | Disc Cam with Roller Follower |
|---|---|---|---|
| Maximum practical speed | Up to 800 RPM with hardened slot, knocks above that | Up to 1500 RPM with proper bearings | Up to 3000 RPM with profiled cam |
| Dwell capability | Built-in dwell at one or both ends, 10-120° tunable | Zero dwell — pure sinusoidal motion | Any dwell pattern — set by cam profile |
| Motion smoothness at transitions | Sharp acceleration spike entering rise phase | Smooth sinusoidal — no transition shock | Smooth — designer chooses cycloidal or polynomial |
| Manufacturing cost | Low — turn the disc, slot the yoke | Lowest — simplest geometry possible | Highest — requires CNC profile grinding |
| Service life of contact faces | 3-5 million cycles before slot ends pit | 10+ million cycles, only the disc OD wears | 20+ million cycles with hardened roller follower |
| Tolerance sensitivity | Slot length ±0.05 mm sets dwell accuracy | Slot width ±0.02 mm — tight but simple | Cam profile ±0.005 mm — most demanding |
| Best application fit | Low-cost dwell-rise-dwell on packaging and stamping | Pure reciprocation — pumps, compressors | Precision indexing — assembly, weighing |
Frequently Asked Questions About Modified Eccentric with Elongated Yoke
The knock comes from the eccentric disc accelerating across the free-travel gap before it contacts the slot end. With nothing constraining it during dwell, the disc drifts to one wall, then has to slam to the opposite wall when the yoke needs to reverse direction. You hear the impact.
Two fixes work. First, fit a light preload spring or a urethane bumper at the slot ends to absorb the transition energy — a 90 Shore A pad cuts peak impact roughly 40%. Second, drop your shaft RPM by 20% and listen again — if the knock disappears, you're running above the speed where the inertia of the yoke itself causes the issue, and you need either softer end pads or a redesign with a profiled cam.
At 400 RPM and 50° dwell, both will work. The decision is cost versus motion quality. The elongated yoke is roughly one-fifth the manufacturing cost of a profiled cam — you turn the disc on a lathe and slot the yoke on a mill, no profile grinding needed. But the rise phase will have an acceleration discontinuity at the dwell-to-rise junction.
Rule of thumb: if the load on the yoke is under 200 N and there's no precision-positioning requirement at the transition, use the elongated yoke. If you're driving anything that needs to settle within ±0.1 mm at the end of rise — or if the load exceeds 500 N — pay for the cam. The cam's smooth profile won't excite the structure, and your downstream tolerance budget will thank you.
This is almost always disc-to-slot-wall contact happening earlier than geometry predicts. The most common culprit is a slot whose end-walls aren't perpendicular to the travel direction — a 0.5° taper at each end effectively shortens the free-travel zone by roughly the wall-height times the tangent, which on a 20 mm tall slot kicks the disc into early contact.
Pull the yoke and put it on a surface plate. Sweep the slot ends with a dial indicator referenced to the guide rail axis. If you see more than 0.02 mm deviation across the slot height, the slot was milled with a worn end mill or the part flexed during machining. Re-grind the ends square and the dwell will return.
Not from a symmetric slot, no. The geometry forces equal dwell at both ends because the eccentric disc traces the same arc going in and coming out. To get asymmetric dwell you need to offset the slot relative to the shaft centreline — machine the slot so its midpoint sits, say, 2 mm displaced from the shaft axis along the travel direction.
That offset puts more free travel on one side than the other and gives you tunable asymmetry. Watch the stroke length though — offsetting the slot also reduces the effective stroke on the long-dwell side. For a 25 mm stroke design, offsets above 3 mm start eating into the working stroke noticeably and you'll need to grow the throw radius to compensate.
Because the bearing carries continuous radial load through every degree of rotation, while each slot face only sees load during one half of the cycle. The bearing accumulates fatigue cycles at twice the rate of the slot.
On top of that, during the dwell phase the bearing rolls along the slot wall under the inertia of the yoke and any process load — that's pure rolling fatigue concentrated on a small contact patch. INA NK-series bearings rated for 800 RPM continuous typically last 2000-3000 hours in this duty before brinelling shows up. If you're seeing earlier failure, check radial load — anything above 60% of dynamic rating drops L10 life cubically, so a 20% overload halves bearing life.
Oversizing it works for dwell length but creates a positioning problem. With excess slot clearance the yoke has no defined position during dwell — it can drift under any external force. On a horizontal application that drift is small, gravity-driven; on a vertical application or anywhere with process load on the yoke, the yoke can move a millimetre or more during what should be a stationary phase.
The fix is either to size the slot to the exact computed length within ±0.05 mm and accept the geometric dwell, or oversize generously and add a separate detent or magnetic latch to hold the yoke during dwell. Don't split the difference — a sloppy slot with no latch gives you the worst of both worlds: imprecise position and a knocking transition.
References & Further Reading
- Wikipedia contributors. Scotch yoke. Wikipedia
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