A Stop Motion is a mechanical device that automatically halts a machine the instant a monitored condition fails — a broken thread, a missing part, a slipping belt. Lancashire power looms made the device famous through the warp and weft stop motions that triggered a knock-off lever the moment a yarn parted. The purpose is to protect the workpiece, the tooling, and the operator before damage compounds. A well-tuned stop motion drops the running member inside one machine cycle, often under 100 ms.
Stop Motion Interactive Calculator
Vary machine speed and stop-motion delay times to see total response time, stopping angle, and one-cycle margin.
Equation Used
The calculator adds the sensor, lever, and brake delays, converts machine RPM to angular velocity, then multiplies by total response time to estimate how far the shaft turns before stopping.
- Machine speed is constant until braking begins.
- Brake time is entered as the full time from brake engagement to zero speed.
- One cycle equals one full shaft revolution.
- Delays are summed in series: sensing, lever trip, then braking.
Inside the Stop Motion
A stop motion sits between a sensing element and the prime mover's clutch or brake. The sensor — a drop wire, a feeler arm, a photocell, a tension finger — rides on or near the monitored variable. As long as the variable behaves, the sensor stays parked. The moment it deviates, the sensor falls, swings, or switches, and that motion trips a knock-off lever which releases the clutch and applies the brake. On a Northrop loom, a drop wire weighing about 4 grams lands on a vibrating reed within roughly 8 ms of a warp end breaking, and the loom is fully stopped within one pick — typically under 200 ms at 180 RPM.
Geometry matters here. The drop wire's slot must clear the warp yarn by a specific gap — on a typical cotton warp running 30 ends per inch, you want 0.3 to 0.5 mm clearance. Tighter than that and the wire false-trips on healthy yarn vibration. Looser and a fine yarn break goes undetected because the wire never falls far enough to engage the reed. Same logic applies to the weft fork: prongs must enter the shed at exactly the right pick angle, usually 280° to 300° of crank rotation, or you get nuisance stops on every pick.
What fails? Three things, mostly. The knock-off lever's pivot bushing wears, adding latency that lets the loom run on for several picks after a break — you see this as repeating defects in the cloth. The brake band glazes, doubling stopping distance. And the drop-wire bank fouls with lint, holding wires up that should have fallen. A monthly de-lint and a quarterly bushing check keep the system inside spec.
Key Components
- Sensor element (drop wire, feeler, photocell): Tracks the monitored variable in real time. On warp stop motions the drop wire sits over each end with 0.3-0.5 mm slot clearance. Mass is held to 3-5 g so that response time stays inside one pick at running speed.
- Knock-off lever: The mechanical trigger that converts a small sensor displacement into clutch release. Pivot friction must stay low — a worn bushing adding 0.2 N·m of drag is enough to delay knock-off by 30-40 ms, which on a high-speed machine means several wasted cycles.
- Clutch and brake assembly: Disengages the prime mover and arrests the running mass. On a Sulzer P7100 projectile loom the brake must dissipate around 1.2 kJ per stop. Brake-band glazing or oil contamination doubles stopping distance and shows up as longer fault marks in the fabric.
- Reset latch: Holds the stop motion armed and only releases on the verified fault signal. The latch must reset within a defined window — typically 50 ms — or the operator gets a false 'cleared' indication while the fault is still present.
- Indicator or signal lamp: Tells the operator which channel tripped. On modern looms each drop wire row maps to a specific lamp or display segment so the weaver finds the broken end in seconds rather than scanning 4,000+ ends manually.
Where the Stop Motion Is Used
Stop motions appear anywhere a continuous process can ruin a workpiece or damage tooling in milliseconds. Textile machinery is the historical home, but film projection, packaging, paper converting, and modern automated assembly all run stop motions in some form. The common thread is simple — a fault that can't wait for a human reaction time of 250 ms.
- Textile weaving: Picanol OmniPlus-i air-jet looms run electronic warp stop motions with up to 8 drop-wire bars feeding individual end-break detection at 1,000+ RPM.
- Knitting: Mayer & Cie circular knitting machines use needle-detector stop motions that halt the cylinder within one revolution if a needle latch breaks, preventing a cascade of holed fabric.
- Film and projection: The Maltese Cross intermittent movement in a 35 mm projector pairs with a film-break stop motion that kills the lamp shutter and feed sprocket the moment loop tension fails.
- Paper converting: Bobst die-cutters use sheet-detection stop motions that halt the platen if a sheet feeds skewed by more than 2 mm, protecting the cutting die.
- Packaging: Bosch Pack 403 cartoning lines use missing-leaflet stop motions that halt the carton flight chain within one station if the optical sensor sees no leaflet at the insert position.
- Bottling: Krones Contiform blow-moulders use preform-presence stop motions that prevent the mould from closing on an empty station, avoiding a 15,000 EUR mould repair bill.
The Formula Behind the Stop Motion
The number that decides whether your stop motion is fit for purpose is the total stopping distance — how far the monitored member travels between fault occurrence and full halt. At the low end of the typical range, a slow benchtop machine running 30 RPM gives you forgiving timing and almost any stop motion works. At the high end, a 1,200 RPM air-jet loom punishes any latency in milliseconds. The sweet spot is matching sensor response, lever inertia, and brake torque so the total stopping angle stays under one full machine cycle.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| θstop | Total angular travel of the prime mover from fault to full stop | radians | degrees |
| ω | Running angular velocity of the prime mover | rad/s | RPM |
| tsense | Sensor response time from fault occurrence to mechanical trip | seconds | milliseconds |
| tlever | Knock-off lever travel time from trip to clutch release | seconds | milliseconds |
| tbrake | Brake engagement and deceleration time to zero speed | seconds | milliseconds |
Worked Example: Stop Motion in a high-speed cigarette maker stop motion
You are commissioning the paper-web break stop motion on a Hauni Protos M5 cigarette maker running at 16,000 cigarettes per minute. The garniture drum runs at 533 RPM and a paper-break sensor (mechanical tension finger) trips the clutch and brake. You need to verify the stopping angle so a paper break does not cause more than half a drum revolution of damaged cigarettes piling into the tipping section.
Given
- ω = 533 RPM
- tsense = 12 ms
- tlever = 8 ms
- tbrake = 35 ms
Solution
Step 1 — convert running speed to angular velocity in degrees per second so the answer comes out in degrees of drum rotation:
Step 2 — sum the latencies and compute the nominal stopping angle:
That is just under half a drum revolution at nominal — within the 180° budget but with very little margin.
Step 3 — at the low end of typical operating range, run the same machine at 350 RPM (start-up creep) with the same latencies:
At creep speed the stop motion looks lazy — the operator sees the drum coast through about a third of a turn, which feels safe and is. Now the high-end case: same machine pushed to its mechanical ceiling of 600 RPM:
198° exceeds the half-revolution budget. In practice the tipping section sees a small pile of broken-paper rejects every time the stop motion fires at top speed. The fix is to drop tbrake below 25 ms by replacing a glazed brake band, or accept that 533 RPM is the real production ceiling for this stop motion configuration.
Result
Nominal stopping angle is 176° at 533 RPM — inside the 180° design budget by a slim 4° margin. At the 350 RPM creep speed the drum stops in 116° and the operation feels comfortably safe; at 600 RPM the angle balloons to 198° and you start dumping rejects into the tipping line, so 533 RPM is the real ceiling unless brake performance improves. If you measure stopping angle longer than predicted, check three things in order: (1) brake-band glazing, which typically adds 15-20 ms and shows up as a shiny band surface on inspection; (2) clutch oil contamination from a leaking gearbox seal letting the clutch slip during disengagement; (3) tension-finger spring fatigue raising tsense above 20 ms — a quick check is to swap in a fresh spring and time the trip with a high-speed phone camera at 240 fps.
When to Use a Stop Motion and When Not To
Stop motions are not the only way to stop a machine on fault. Pure electronic safety circuits and full servo-controlled drives both compete with classical mechanical stop motions, and each wins on different axes.
| Property | Mechanical stop motion | Electronic safety relay + brake | Servo-drive controlled stop |
|---|---|---|---|
| Total response time | 40-80 ms | 20-50 ms | 5-15 ms |
| Maximum running speed before stopping angle exceeds one cycle | ~1,200 RPM | ~2,500 RPM | Limited only by drive bandwidth |
| Capital cost per axis | Low (~$200) | Medium (~$800) | High ($2,500+) |
| Maintenance interval | Quarterly bushing/brake check | Annual relay test | Firmware updates only |
| Diagnostic feedback to operator | Lamp or flag — channel-level | PLC alarm code | Full data log with fault timestamp |
| Typical lifespan before rebuild | 10-15 years | 15-20 years | 20+ years |
| Best application fit | Textile, paper, legacy mechanical machines | Packaging, light assembly | High-speed coordinated motion (SMT, robotics) |
Frequently Asked Questions About Stop Motion
This is almost always a drop-wire mass and slot-clearance mismatch. If the wires are too heavy or the slot too tight, normal warp vibration drops them onto the reed and you get nuisance stops. Lighten the wires to 3-4 g and open slot clearance to 0.4 mm and false stops drop sharply.
The miss-real-breaks half of the same problem usually comes from lint bridging the slot. A wire that should fall stays suspended on a film of fly. De-lint the bank weekly on cotton and twice-weekly on wool blends.
Run the stopping-angle calculation at the machine's top speed first. If a mechanical stop motion's typical 55-70 ms total latency keeps you inside one cycle of rotation with margin, the mechanical solution wins on cost and parts availability. If you are already at 80-90% of the angle budget, you have no headroom for normal wear and you should go electronic.
The other deciding factor is diagnostic granularity. If the operator needs to know which of 4,000 ends broke, channel-level mechanical drop wires with indicator flags still beat most electronic systems on cost per channel.
Latency creep beyond predicted is almost always in the clutch-to-brake transition rather than the sensor. Check whether the clutch is fully releasing before the brake engages — a worn clutch throw-out bearing causes the clutch to drag for 20-30 ms while the brake fights it, and you burn time and brake material simultaneously.
The diagnostic check: pull the brake feed momentarily and time clutch release alone. If clutch release is over 15 ms on its own, the throw-out bearing or clutch fork is the culprit, not the stop motion sensor or knock-off lever.
Optical and inductive sensors detect presence, not condition. A crushed carton still reflects light or breaks an inductive field, so the stop motion sees no fault. You need a profile-based sensor — a laser micrometer or vision check — that measures the carton's actual dimension or shape rather than just whether something is in the station.
Rule of thumb: if the fault you care about leaves the workpiece roughly the same size as a healthy one, presence sensors will miss it every time. Move to dimensional sensing.
Sometimes, but the limit is not what the calculation says — it's brake thermal capacity. Stopping energy scales with the square of speed. Push a loom from 600 to 720 RPM and stopping energy goes up 44%, which the brake band has to dissipate as heat. The band glazes faster, fades during repeated stops, and within weeks your stopping angle has drifted up by 30-50%.
If you want to run faster reliably, upgrade the brake (sintered band, larger contact area) before you upgrade anything else in the stop-motion chain.
Two thermal effects compound. Brake-band friction coefficient drops 10-20% from cold to operating temperature on most organic friction materials, so the same brake torque produces longer deceleration. Second, gearbox oil thins as it warms, so any clutch dragging through oil sees less viscous resistance and the drag time extends slightly.
This is why qualification tests should be run hot, not cold. A stop motion that passes a cold acceptance test by a small margin will routinely fail an hour into a production shift.
References & Further Reading
- Wikipedia contributors. Loom. Wikipedia
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