A slotted connecting rod with rest at end is a modified slider-crank where the connecting rod carries a straight slot that engages the crank pin, producing reciprocating motion with a built-in dwell — the output stops momentarily at one end of its stroke while the crank pin travels along the slot. You see this geometry in the indexing tables of older Acme Gridley multi-spindle screw machines. The slot length sets dwell duration, the crank radius sets stroke. It gives you a hold-time at the working end without adding cams, clutches, or electronic timing.
Slotted Connecting Rod with Rest at End Interactive Calculator
Vary crank radius, slot length, and RPM to see dwell angle, dwell time, stroke, and the animated rest-at-end motion.
Equation Used
The slot length Ls sets the crank angle over which the pin slides in the slot while the output rests. With a long connecting rod, total stroke is approximately 2R, and dwell time is the dwell-angle fraction of one crank revolution.
- Connecting rod is long enough that obliquity is negligible.
- Stroke is approximated as S = 2R.
- Slot length is measured along the rod axis.
- If Ls exceeds 2R, the arcsin input is clamped to 1 for calculation.
Inside the Slotted Connecting Rod with Rest at End
The trick is in the slot. A normal slider-crank has the crank pin pinned to the rod, so the slider follows a continuous sinusoidal motion — no pause, no rest. Replace the pin joint with a straight slot machined into the rod, and the crank pin now slides inside that slot for part of the rotation. During the slot-engaged portion, the rod stays at one extreme of its stroke while the pin tracks along the slot length. That stationary period is the rest, and it falls at one end of the stroke — typically the working end where you need a tool to dwell on the workpiece.
Geometry sets everything. Slot length Ls measured along the rod axis directly controls dwell angle θd through the relation θd ≈ 2 × arcsin(Ls / 2R), where R is crank radius. If you cut the slot 0.2 mm too long, you steal stroke from the return side and the slider undertravels at the back end. Cut it too short and the dwell collapses — you'll get a brief pause that won't hold a punch on the work long enough to form metal. The slot width must match the crank pin diameter within H7/g6 — typically 0.013 mm clearance on a 12 mm pin. Tighter and the pin galls, looser and the rod chatters audibly at the moment the pin transitions from slot-end contact to free travel.
Failure modes cluster around the slot ends. The transition points where the pin enters and leaves the slot see impact loading every cycle, and the slot ends will peen, mushroom, or spall after a few million cycles in hardened steel. You'll hear it before you see it — a sharp tick at the start of dwell, growing into a clack. Hardened slot inserts, or case-hardening the rod to 58-62 HRC at the slot, push service life from roughly 2 million cycles up past 20 million.
Key Components
- Crank: Rotates at constant input speed and carries the crank pin at radius R from the shaft centre. R is fixed by the required total stroke — stroke equals 2R when the rod is long enough that obliquity is negligible.
- Crank Pin: The pin transmits motion from the crank into the slotted rod. Diameter and surface hardness drive wear life — typical builds use a 10-16 mm hardened ground pin at HRC 60+ with Ra ≤ 0.2 µm to keep slot-face wear predictable.
- Slotted Connecting Rod: The rod with the straight slot machined along its length. Slot length sets dwell duration, slot width sets pin clearance. The slot edges must be hardened or fitted with a hardened insert to survive the impact at slot-end engagement.
- Slider or Output Block: Carries the working tool — punch, indexing pawl, feed finger. The slider rides in a guide that takes side load off the rod, otherwise the rod bends and the slot wears unevenly along its length.
- Guide / Slideway: Constrains the output block to pure linear motion. Without a positive guide, the rod sees side load that levers the crank pin against one slot face and you get one-sided wear within 50,000 cycles.
Who Uses the Slotted Connecting Rod with Rest at End
You find this mechanism wherever a constant-speed input shaft must produce a reciprocating output that pauses at the working end of its stroke — without the cost of a cam or the complexity of a Geneva. Old shop machinery used it heavily before pneumatics and servos took over. It still earns its keep where electronics are unwelcome: dusty environments, high-temperature zones, and equipment that must run for decades on lubrication alone.
- Screw Machine Manufacturing: Stock-feed dwell on Acme Gridley RB-6 and RB-8 multi-spindle bar machines, where the bar must hold still while the collet closes.
- Printing Machinery: Ink-fountain ductor roller dwell on Heidelberg KORD 64 sheet-fed offset presses, holding the roller against the fountain for a defined contact time.
- Textile Manufacturing: Picker-stick reset dwell on Draper Northrop X-3 automatic looms where the shuttle pause coincides with weft insertion.
- Mechanical Punch Presses: Slide rest-at-bottom on small open-back inclinable presses like the Bliss C-22, giving the punch a moment to set the form before lifting.
- Glass Bottle Making: Plunger dwell on Hartford-Empire IS machines for the press-and-blow forming cycle, where the plunger must hold position while glass flows.
- Packaging Equipment: Carton tucker-flap dwell on Bosch Sigpack HCM cartoners, holding the flap closed while the upstream glue sets.
The Formula Behind the Slotted Connecting Rod with Rest at End
The core design number is the dwell angle — the fraction of crank rotation during which the output sits still. At the low end of typical designs you'll target 30° of dwell, just enough for a quick tool kiss. The sweet spot for most punch and indexing applications sits around 60-90°, giving a clear hold without losing too much working stroke. Push past 120° and you lose so much stroke that the return becomes too fast — accelerations spike and the rod tries to throw itself off the guide. The formula below ties slot length, crank radius, and dwell angle together so you can size all three at once.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| θd | Dwell angle — crank rotation during which output stays at rest | degrees or radians | degrees |
| Ls | Length of the slot machined in the connecting rod, measured along rod axis | mm | in |
| R | Crank radius — distance from crank shaft centre to crank pin centre | mm | in |
| S | Total output stroke (S = 2R for long rod) | mm | in |
Worked Example: Slotted Connecting Rod with Rest at End in a vintage horological escapement test rig
You are commissioning the slotted-rod feed drive on a refurbished Schwermann KS-3 watch-balance staking press at a Swiss horology training workshop in La Chaux-de-Fonds. The crank turns at 45 RPM nominal, crank radius is 18 mm, and the slot is machined to 14 mm. The press needs a dwell of roughly 0.18 seconds at the bottom of stroke for the staking punch to fully seat a balance staff into a wheel collet without rebound. You need to verify the dwell angle and the dwell time at three operating points across the workshop's typical 30-60 RPM range.
Given
- R = 18 mm
- Ls = 14 mm
- Nnom = 45 RPM
- Nlow = 30 RPM
- Nhigh = 60 RPM
Solution
Step 1 — compute the dwell angle from slot length and crank radius:
Step 2 — convert dwell angle to dwell time at nominal 45 RPM. One revolution takes 60/45 = 1.333 s, so each degree takes 1.333/360 = 0.00370 s:
That's within 6% of the 0.18 s target — close enough that a small slot-length tweak from 14 mm to 14.4 mm would land it dead on. At nominal speed the punch sits firmly on the staff for long enough to seat it without bounce, and the operator hears one clean tick per cycle.
Step 3 — at the low end of the range, 30 RPM:
At 30 RPM the dwell stretches to a quarter second. That's overkill for a balance staff but useful for the heavier wheel-mounting work the same press does — the operator can adjust the staking force mid-dwell without rushing.
Step 4 — at the high end, 60 RPM:
At 60 RPM you've dropped under 0.13 s of contact. For a delicate balance staff that's borderline — push the speed any higher and the punch starts to lift before the staff fully seats, and you get the characteristic intermittent rejects that horologists call a 'shy stake'.
Result
Nominal dwell time at 45 RPM is 0. 170 s, with dwell angle 45.8°. At 30 RPM the dwell extends to 0.254 s — generous, comfortable, slightly slow for production. At 60 RPM it falls to 0.127 s, which is the practical upper speed before the staff fails to fully seat. If you measure dwell time noticeably shorter than predicted at any speed, the three usual culprits are: (1) slot ends peened over from impact loading, effectively shortening Ls by 0.3-0.6 mm, (2) crank pin worn undersize so the pin makes contact with the slot face later in the cycle, or (3) the slider guide loose enough that the output starts moving before the pin actually leaves the slot end — check guide clearance with a 0.02 mm feeler before blaming the slot.
When to Use a Slotted Connecting Rod with Rest at End and When Not To
Slotted rod with rest at end is one of three classical ways to get a dwell out of a constant-speed input. Each has a clear application zone — pick the wrong one and you'll fight it forever.
| Property | Slotted Connecting Rod with Rest at End | Cam with Dwell Profile | Geneva Drive |
|---|---|---|---|
| Typical operating speed | 10-200 RPM | 10-1500 RPM | 20-600 RPM |
| Dwell precision (timing repeatability) | ±2° crank angle | ±0.2° crank angle | Exact by geometry |
| Dwell duration adjustability | Fixed by slot length — change requires rod swap | Re-cut cam profile | Fixed by number of slots |
| Manufacturing cost (small batch) | Low — straight slot, common materials | High — profile grinding, hardening | Medium — indexed slots and lock |
| Service life before slot/contact wear | 2-20 million cycles depending on hardening | 10-100 million cycles with hardened cam | 5-50 million cycles |
| Load capacity at output | Medium — limited by slot-end impact | High — full cam contact | Medium-high — full pin engagement |
| Shock noise during dwell transition | Audible tick at slot-end engagement | Quiet if profile blended | Sharp click at lock entry |
| Best application fit | Single dwell at one stroke end, simple machinery | Multiple dwells, complex motion laws | Equal-step indexing |
Frequently Asked Questions About Slotted Connecting Rod with Rest at End
The formula assumes the crank pin contacts the slot end the instant rotation drives it there. In practice, any radial clearance between pin and slot width adds a transition zone where the slider has already started moving but the pin hasn't fully engaged the opposite slot face. A 0.05 mm slot-width clearance can shave 3-5° off effective dwell.
Measure pin-to-slot clearance with a feeler. If it's above H7/g6 fit (roughly 0.02 mm on a 12 mm pin), the slot is worn or was originally cut loose. Re-bushing the slot with a hardened insert ground to fit restores dwell and stops the audible tick that comes with sloppy clearance.
Slot in the rod gives you dwell at one end of stroke — the working end. Slot in the crank arm (the Whitworth quick-return geometry) gives you a non-uniform velocity ratio between forward and return strokes, not a true dwell. They look similar on paper but solve different problems.
If you need the output to truly stop and hold, put the slot in the rod. If you need a fast return and slow working stroke without an actual stationary period, the crank-arm slot is correct. Mixing them up is the most common selection mistake we see on first-time builds.
The pin sees its peak contact stress at the slot ends, where engagement happens with finite velocity. Treat it as a Hertzian line contact between the pin OD and the slot face. For typical machine-tool applications running 50-200 RPM with output loads of 200-2000 N, hardened pins of 10-16 mm at HRC 58-62 give acceptable life.
Rule of thumb: pin diameter in mm ≈ √(output load in N) / 4. So a 1600 N load wants roughly a 10 mm pin. Below this, slot-end peening accelerates and you'll be re-grinding within months instead of years.
Chatter at dwell entry usually means the slider has momentum at the moment the pin reaches the slot end, and the slot end has to absorb that kinetic energy as an impact. If the slot face isn't hard enough, it deforms slightly under the impact, the pin rebounds, and you get a vibration ringdown that lasts 5-20 ms into the dwell.
Two fixes work. First, harden the slot face to 58 HRC minimum or fit a hardened insert. Second, add a small radius (0.5-1.0 mm) at the slot ends so the pin transitions smoothly rather than bottoming abruptly. The radius costs you a degree or two of dwell but eliminates 90% of the chatter.
You can run higher, but the slot-end impact energy scales with speed squared. Past 200 RPM the impact at each slot transition starts driving stress waves through the rod that fatigue the slot ends regardless of hardness. Heritage Acme Gridley screw machines ran these mechanisms up to about 250 RPM with hardened slot inserts, but every commercial high-speed application above 300 RPM uses a cam-with-dwell instead.
If you're stuck at 250-400 RPM, a hardened slot insert plus a needle-roller crank-pin sleeve buys you another 50% in life, but you're patching a fundamentally noisy solution. Above 400 RPM, accept the cam.
On a hardened-slot build (58-62 HRC, ground finish), measurable peening starts around 2-5 million cycles and timing shift becomes visible — meaning more than 1° of dwell-angle drift — at roughly 10-15 million cycles. On an unhardened mild-steel slot it can happen inside 100,000 cycles.
The early symptom is timing drift in whatever the dwell synchronises with — glue not setting fully, punch not seating fully, paint application short on coverage. By the time you can hear the impact, you're already past the point where downstream quality has shifted measurably. Inspect slot ends with a 10× loupe every 6 months on production equipment.
References & Further Reading
- Wikipedia contributors. Slider-crank linkage. Wikipedia
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
