A Variable Throw Traversing Bar is a reciprocating linkage that converts rotary input into a back-and-forth linear stroke whose length can be changed without stopping the machine. Universal Winding Company commercialised the principle on yarn winders in the 1910s, with patents under John E. Atwood. The bar runs off an adjustable crank or sliding-block pivot, and shifting that pivot changes the stroke. It lets one machine wind packages of different traverse lengths — typical bobbin winders cover 80 mm to 280 mm strokes from a single drive shaft.
Variable Throw Traversing Bar Interactive Calculator
Vary the short and long crank pin radii to compare the resulting traverse strokes and see the reciprocating bar motion.
Equation Used
The traversing bar follows the horizontal projection of an adjustable crank pin. Moving the pin radius r changes the total stroke directly, so the end-to-end stroke is S = 2r.
- Scotch-yoke style geometry with horizontal bar motion.
- Crank pin radius is measured from the disc center.
- Clearance, wear, and lost motion are not included.
The Variable Throw Traversing Bar in Action
The mechanism takes a steady-speed rotary input — usually a gear or pulley turning at 200 to 600 RPM — and drives a bar that slides along a fixed guide. The crank pin sits in a slot or sliding block, and the radial position of that pin sets the throw. Move the pin closer to the crank centre and the stroke shortens. Move it out and the stroke lengthens. You can change the throw while the shaft is still spinning, which is why winder operators love it on long production runs.
The geometry matters. The traversing bar is a cousin of the scotch yoke, but with an adjustable crank radius. If the slot in the bar is not parallel to the guide axis within about 0.05 mm over its length, you get side loading on the slider and the bar starts to chatter. We have seen worn slot edges add 1 to 2 mm of lost motion at each end of the stroke — that shows up on the wound package as soft, sloppy ends. The pin-to-slot fit needs to be a running clearance of around 0.02 mm; any tighter and it galls under thermal expansion, any looser and the yarn guide rattles audibly at full RPM.
What goes wrong? Three things, mostly. The adjustment screw that locates the crank pin can creep under vibration, so the stroke walks longer or shorter over a shift — you fix that with a jam nut or a Belleville stack. The slider bushing wears unevenly because the load reverses every half turn, and at 400 RPM that is 800 load reversals per minute, which fatigues bronze bushings faster than people expect. And if you skip lubrication on the slot, you get fretting that locally roughens the surface and doubles the bar's drag in under 200 hours.
Key Components
- Adjustable Crank Disc: Carries the crank pin in a radial T-slot. Loosening the lock screw lets the operator slide the pin from minimum to maximum throw — typically 20 mm to 140 mm radius on a winder-class unit. The disc must run true within 0.03 mm TIR or the stroke ends become asymmetric.
- Crank Pin and Slider Block: Hardened steel pin, usually 12 mm to 20 mm diameter, riding in a bronze or PTFE-faced slider block inside the bar's slot. The pin needs surface hardness ≥ 58 HRC and a Ra finish below 0.4 µm. Anything rougher and the slider scores within a few hundred hours.
- Traversing Bar with Slot: The reciprocating member. Steel or aluminium bar with a precision-ground straight slot along its length. Slot parallelism to the bar's guide axis must hold within 0.05 mm over the slot length to keep side loads off the linear bushings.
- Linear Guide Bushings: Two bushings constrain the bar to a single axis of motion. On a 400 RPM winder these see 800 load reversals per minute, so we spec linear ball bushings or graphite-impregnated bronze rated for at least 10 million reversals before measurable wear.
- Throw Adjustment Lock: Jam nut, T-handle clamp or Belleville-preloaded screw that locks the crank pin radius. Without preload the pin creeps under vibration and the stroke length drifts — a 1 mm drift over 100 mm stroke is enough to push a yarn package out of spec.
- Yarn Guide or Tool Carrier: The end-effector mounted to the bar. On a coil winder it is a wire guide eyelet; on a textile machine it is a porcelain yarn-guide ring. The carrier mass directly affects the inertial load — every 100 g of carrier adds roughly 4 N of peak inertial force at 400 RPM and 100 mm stroke.
Where the Variable Throw Traversing Bar Is Used
You find Variable Throw Traversing Bars wherever a machine needs to lay material back and forth across a winding core or substrate, and the stroke length needs to change between products. The mechanism beats a fixed-stroke cam because you can run mixed product on one frame, and it beats a servo-driven traverse because it costs a fraction as much and never needs a controller. Where stroke uniformity within ±0.5 mm is acceptable and stroke changes happen between batches rather than mid-cycle, this is the right call.
- Textile Winding: Universal Winding Company yarn cone winders use a variable throw traversing bar to wind packages from 80 mm to 280 mm traverse without changing cams.
- Coil Winding: Marsilli and Meteor coil winders use the principle on bobbin and toroid lines where coil bobbin lengths vary between 15 mm and 90 mm across the product family.
- Wire and Cable: Niehoff bunchers and Reelex packaging lines use adjustable-throw traverses to lay copper bunches onto take-up reels of varying flange spacing.
- Paper Converting: Slitter-rewinders on Black Clawson and Goebel machines use variable-stroke traverses on edge-trim guides where slit width changes between jobs.
- Shaper and Slotter Machines: Cincinnati shapers use adjustable-throw bull-gear linkages — geometrically the same mechanism — to set ram stroke between roughly 50 mm and 600 mm depending on the workpiece.
- Filament Winding: McClean Anderson filament winders use variable-throw traverses on resin-impregnated tow placement heads when winding pressure vessels of varying length.
The Formula Behind the Variable Throw Traversing Bar
The core question is: given a crank radius r and rotational speed N, how fast and how far does the bar travel? Stroke is twice the throw — that part is geometry. But peak velocity and peak acceleration scale very differently with crank radius and speed, and that is where builders get caught. At the low end of typical operation — say 200 RPM and a 30 mm throw — peak velocities are gentle and the slider sees almost no inertial load. At the nominal 400 RPM, 80 mm throw, the bar runs at human-eye-blur speed and the inertial loads start to matter. Push to the high end at 600 RPM and 140 mm throw and you are deep into the territory where slider-block fatigue, not geometry, governs life.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| vpeak | Peak linear velocity of the traversing bar (occurs at mid-stroke) | m/s | ft/s |
| apeak | Peak linear acceleration of the bar (occurs at stroke ends) | m/s² | ft/s² |
| S | Total stroke length, end to end | m | in |
| r | Crank throw radius (centre of crank disc to crank pin) | m | in |
| N | Crank rotational speed | rev/s | RPM |
Worked Example: Variable Throw Traversing Bar in a precision toroidal coil winder
Sizing the variable throw traversing bar on a Marsilli-style precision toroidal coil winder where the input shaft turns at 400 RPM nominal off a flat-belt drive, and the wire-guide carrier (160 g including eyelet and bracket) must lay 0.2 mm enamelled copper across coil bobbins ranging from 30 mm to 140 mm in length. You need to know peak velocity, peak acceleration, and inertial load at the slider block across the full operating range so you can spec the slider material and adjustment lock.
Given
- Nnom = 400 RPM
- rnom = 0.040 m (for 80 mm stroke)
- rmin = 0.015 m (for 30 mm stroke)
- rmax = 0.070 m (for 140 mm stroke)
- mcarrier = 0.160 kg
Solution
Step 1 — convert nominal speed to rev/s:
Step 2 — peak velocity at the nominal 80 mm stroke (r = 0.040 m):
Step 3 — peak acceleration at nominal:
That is roughly 7 g of acceleration at the stroke ends. With a 0.160 kg carrier, peak inertial force is F = m × a = 0.160 × 70.2 ≈ 11.2 N. A bronze slider handles that comfortably.
Step 4 — at the low end of the operating range, 30 mm stroke (r = 0.015 m, same 400 RPM):
Gentle. The bar barely loads the slider — you would not hear it over the spindle motor. This is where small-bobbin work lives, and it is forgiving of slop in the adjustment lock.
Step 5 — at the high end, 140 mm stroke (r = 0.070 m, same 400 RPM):
Now you are at 12.5 g and nearly 20 N reversing 800 times per minute. A plain bronze slider will fret-wear within 300 hours at this duty. You either drop to graphite-impregnated bronze, swap to a needle-roller slider block, or de-rate the top speed to 300 RPM when running long-stroke jobs.
Result
At nominal 400 RPM and 80 mm stroke the bar peaks at 1. 68 m/s with 11.2 N of inertial load on the slider — comfortable territory for a standard bronze block, and the wire guide tracks cleanly enough to lay 0.2 mm enamelled copper without overlap errors. The low-end 30 mm stroke runs at 0.63 m/s and 4.3 N (effortless), while the 140 mm stroke pushes 2.93 m/s and nearly 20 N — which is where slider-block life starts collapsing and you should consider de-rating speed for long-stroke product. If your measured peak velocity comes in 15% below 1.68 m/s, suspect three things in this order: a slipping flat belt at the input pulley losing real RPM under load, a crank pin that has crept inward under vibration because the lock screw lacks Belleville preload, or a slot-and-pin clearance opened beyond 0.05 mm where the bar lags the crank through the stroke ends. Check belt tension first — that fixes it 60% of the time.
When to Use a Variable Throw Traversing Bar and When Not To
You have three reasonable ways to drive a reciprocating traverse on a winding or laying machine: a variable throw traversing bar, a fixed-stroke cam, or a servo-driven linear actuator. Each wins on different axes and the choice depends on how often stroke changes, what stroke accuracy you need, and how much budget you have.
| Property | Variable Throw Traversing Bar | Fixed-Stroke Barrel Cam | Servo-Driven Linear Actuator |
|---|---|---|---|
| Typical operating speed | 200–600 RPM | 300–1200 RPM | 0–500 strokes/min, electronically limited |
| Stroke accuracy | ±0.5 mm typical | ±0.05 mm (cam-defined) | ±0.01 mm with encoder feedback |
| Stroke change time | Seconds — adjust on the fly | Hours — swap cam | Instant — software |
| Capital cost (relative) | 1.0× | 0.7× | 4–8× |
| Maintenance interval | Slider re-grease every 500 hr | Cam follower service every 2000 hr | Effectively zero (sealed servo + ballscrew) |
| Lifespan at rated duty | 10–20 million reversals | 50+ million reversals | Limited by ballscrew, ~100 million strokes |
| Best application fit | Mixed-product winders, batch changeovers | Single-product high-volume winding | High-precision filament/wire placement |
| Mechanical complexity | Low — single linkage | Low — but cam is custom-machined | High — drive, controller, encoder, feedback loop |
Frequently Asked Questions About Variable Throw Traversing Bar
This is almost always the lock screw losing preload under reversing vibration. A plain hex screw clamping the crank pin in a T-slot will back off measurably at 400 RPM with 800 reversals per minute — we have seen 0.5 mm of radial creep in under 2 hours.
Fix it with a Belleville washer stack under the lock screw, or replace the screw with a serrated-flange bolt and a jam nut. If the drift is asymmetric (one stroke end grows but the other shrinks), check whether the crank disc itself is shifting axially on its shaft — a missing or worn key is the next suspect.
The decision usually comes down to layer-to-layer pitch accuracy. A variable throw bar holds about ±0.5 mm at the stroke ends, which is fine for 0.2 mm wire on a 30 mm bobbin (you get clean layers up to maybe 8–10 layers). For finer wire — 0.1 mm and below — or for orthocyclic winding where wire turns must nest in the previous layer's grooves, the mechanical traverse loses to a servo every time.
Rule of thumb: if your wire diameter is greater than 0.15 mm and you are running standard random or wild winding, the mechanical bar is more reliable and a quarter the cost. Below 0.15 mm or for any precision-layer pattern, go servo.
At small crank radii the slider block spends more time near the slot ends, where any slot-end roughness or burr from manufacturing concentrates the wear. The slot is usually finished with the assumption that the slider sweeps across most of its length — at 15 mm throw on a 200 mm slot, you are using less than 8% of the slot face.
Inspect the slot ends with a fingernail or a magnifier. Any visible step, burr, or score line at the small-throw working zone is your problem. Stone the slot lightly and re-lubricate; the notch usually disappears.
Calculate peak inertial force at your worst-case stroke and speed: F = m × (2π N)² × r, where m is carrier mass and r is the longest throw you will run. Bronze sliders handle around 15 N continuous reversing load before fretting becomes a problem within 500 hours. Above 15 N you should look at graphite-impregnated bronze (good to about 25 N) or needle-roller slider blocks (good to 80 N+).
The other warning sign is audible — if you hear a faint metallic click at each stroke reversal even with fresh grease, the slider is already pounding the slot face and you are within weeks of measurable wear.
Soft package ends mean the yarn guide is dwelling too long at the stroke reversals — either the bar is decelerating slower than it should, or the operator has set the throw slightly larger than the package needs and the yarn lays past the package edge before reversing.
Check the throw setting first — measure the actual stroke with a dial indicator on the carrier and compare to the desired traverse. If the stroke matches the package length within 1 mm, then look at the slot-and-pin clearance; clearance above 0.05 mm lets the bar overshoot at the reversal points and pile yarn at the ends.
Geometrically yes, mechanically no. Peak acceleration scales with N² × r, so doubling RPM at full throw quadruples the inertial load on the slider and the linear bushings. At 800 RPM and 140 mm stroke on the worked example above, peak acceleration jumps to about 490 m/s² and slider load passes 78 N — bronze fails in tens of hours, not hundreds.
If you genuinely need both long stroke and high RPM, the geometry is telling you to switch mechanism — either a barrel cam (which has profile-controlled deceleration and lower peak acceleration) or a servo. Pushing the variable throw bar past its envelope is a false economy.
References & Further Reading
- Wikipedia contributors. Scotch yoke. Wikipedia
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