A two-pawl continuous rectilinear ratchet bar is a toothed straight bar advanced step-by-step by a pair of pawls — one driving, one holding — instead of a rotating wheel. The driver pawl pushes the bar forward by one tooth on each input stroke while the holding pawl locks the bar against reverse travel during the return stroke. This gives you continuous net advance from a reciprocating input, which is why you find it inside lever jacks, hand-operated lifting tables, and theatre fly bars where load must hold between strokes.
Two-pawl Continuous Rectilinear Ratchet Bar Interactive Calculator
Vary tooth pitch, stroke rate, stroke effectiveness, and target travel to see the ratchet bar advance rate and time to distance.
Equation Used
The ratchet bar advances one tooth pitch for each effective input stroke. With stroke rate entered in strokes per minute, the linear speed is v_bar = p * n * eta / 60, where p is tooth pitch and eta is the fraction of strokes that fully engage.
- Each successful input stroke advances the bar by one tooth pitch.
- Stroke frequency n is entered in strokes per minute.
- Efficiency eta represents missed teeth, slip, or incomplete engagement.
- The holding pawl prevents reverse travel between strokes.
How the Two-pawl Continuous Rectilinear Ratchet Bar Actually Works
The bar itself is a straight rack with asymmetric ratchet teeth — a steep loading face on one side and a shallow ramp on the other. Two pawls bear on the same tooth row but at different points along the bar. The driving pawl rides on the input lever and translates with it; the holding pawl is grounded to the frame. Push the lever, the driving pawl engages a tooth flank and shoves the bar forward by one pitch. Release the lever, the driving pawl skips back over the ramped tooth backs while the holding pawl drops into the next tooth and locks the bar in place. Net result: every stroke advances the bar one tooth, and the bar never falls back.
Why two pawls instead of one? Because with a single pawl on the driver, the moment you start the return stroke the load wants to push the bar back the way it came. A grounded holding pawl is what makes this a continuous advance — without it, you have a one-shot pusher, not a ratchet. The geometry that matters most is the tooth pitch p and the pressure angle on the loading face. We typically run a loading-face angle of 3-7° off vertical so the load self-seats the pawl deeper rather than camming it out.
Get tolerances wrong and the failure modes are predictable. If pitch on the bar drifts more than about ±0.05 mm from the pawl spacing, the driving pawl starts landing partway up a tooth and skips under load — you feel a sudden lurch and the lever bottoms out. If the holding pawl spring is undersized (below roughly 0.5 N seating force on a 10 mm pitch bar), the pawl floats during the return stroke and the bar walks back a fraction of a tooth each cycle. Worn tooth tips, rounded below 0.2 mm radius from the original sharp edge, are the third common killer — the pawl rides up and over instead of dropping in.
Key Components
- Ratchet Bar (Rack): Straight steel bar machined with asymmetric teeth on a fixed pitch, typically 5-25 mm pitch depending on load. The loading face sits at 3-7° off vertical for self-seating; the back face ramps at 30-45° so the pawl can skip over on the return.
- Driving Pawl: Mounted on the moving lever or carriage. On the forward stroke it engages a tooth and translates the bar by one pitch. Spring loading is light — 0.3 to 1 N — just enough to keep the tip seated against the bar.
- Holding Pawl: Grounded to the frame. Drops into the next tooth as the bar advances and prevents reverse travel during the return stroke. Must take the full static load of whatever the bar is holding, so it sits on the steep loading face with full tooth engagement.
- Pawl Springs: Light torsion or compression springs that keep both pawls in contact with the bar. Sized so seating force exceeds friction and pawl inertia by about 3×. Below that and the pawls float during fast cycling.
- Lever or Actuator Input: Reciprocating input that drives the driving pawl. Stroke length must equal or slightly exceed one tooth pitch — undershoot by even 0.2 mm on a 10 mm pitch and you skip every other stroke.
- Release Mechanism: Manual cam or trigger that lifts both pawls clear of the bar so it can be retracted. Critical safety detail: on a load-bearing jack, the release must require two-handed action so it cannot be bumped open under load.
Where the Two-pawl Continuous Rectilinear Ratchet Bar Is Used
You see this mechanism wherever someone needs to advance a straight load step-by-step and have it hold between strokes without an external brake. The defining feature is that the bar holds its position passively under load — power off, hand off, lever released, the load stays put. That makes it useful anywhere a leadscrew would be too slow, a hydraulic ram too expensive, and a winch too floppy.
- Vehicle Recovery: The Hi-Lift Jack, made in Bloomfield Indiana since 1905, uses a two-pawl rectilinear ratchet bar as its core lifting element — the standard 48-inch model lifts up to 7,000 lbs one tooth at a time.
- Theatre Rigging: Counterweight fly bar locking systems on older stage rigs use a two-pawl ratchet bar as the secondary safety on arbor travel, holding the load if the rope lock slips.
- Construction Formwork: Acrow-style adjustable shoring props use a coarse-pitch ratchet bar with twin pawls to set deck height, then take fine adjustment with a screw at the head.
- Furniture Manufacturing: Adjustable workbench lifts and the mechanism inside many hospital bed frames use a small-pitch ratchet bar — typically 8 mm pitch — to step the deck up under patient load.
- Marine Hardware: Sailboat boom vang and outhaul tensioners on smaller dinghies sometimes use a linear ratchet bar instead of a cam cleat when the load needs to hold positively against shock loads.
- Material Handling: Pallet-positioner foot pumps on hand pallet trucks like the Crown PTH50 use a two-pawl ratchet bar internal to the pump linkage to step the forks up to working height.
The Formula Behind the Two-pawl Continuous Rectilinear Ratchet Bar
The number that matters most for sizing is net advance per unit time — how fast the bar moves under repeated stroking. That tells you whether the mechanism keeps up with the operator or the input actuator. At the low end of typical hand-operated stroking — roughly 20 strokes per minute on a heavy jack under load — advance is slow but predictable. At the nominal cruise rate of 60 strokes per minute on a light load, you hit the design sweet spot where pawl seating is reliable and the operator isn't fighting fatigue. Push past 100 strokes per minute and the pawls start to float on the return because spring force can't reseat them fast enough — you get missed teeth and the advance rate plateaus or drops.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| vbar | Net linear advance rate of the ratchet bar | mm/s | in/s |
| p | Tooth pitch (centre-to-centre distance between teeth) | mm | in |
| n | Stroke frequency of the input lever | strokes/s | strokes/s |
| η | Engagement efficiency (fraction of strokes that advance one full tooth) | dimensionless | dimensionless |
Worked Example: Two-pawl Continuous Rectilinear Ratchet Bar in a residential elevator service jack
Sizing the lift rate on a refurbished two-pawl ratchet jack used by a residential elevator service company in Surrey BC for cab levelling during pit work. The bar has 12 mm tooth pitch, the technician strokes the lever at 60 strokes per minute under a 400 lb load, and engagement efficiency runs about 0.95 with new pawls.
Given
- p = 12 mm
- nnom = 60 strokes/min
- η = 0.95 dimensionless
Solution
Step 1 — convert nominal stroke frequency to strokes per second:
Step 2 — compute nominal bar advance rate at 60 strokes/min:
That's 0.68 m per minute — the cab moves visibly but slowly, about the pace where the operator can read the level gauge and stop on a tooth without overshoot. This is the design sweet spot for this kind of work.
Step 3 — at the low end of the typical operating range, 20 strokes/min while lifting near rated load:
That's about a quarter inch per second — slow enough that you can stop on a specific tooth by feel, and the operator's arm doesn't fatigue under the heavier lever force needed at full load.
Step 4 — at the high end, 100 strokes/min on a light load with worn pawls (η drops to about 0.80):
In theory you'd hit 19 mm/s at η = 0.95, but pawl float at this stroke rate drops effective engagement and the bar advance plateaus. Push faster and you hear the driving pawl chatter on the tooth backs without dropping in — the bar barely advances at all.
Result
Nominal bar advance rate is 11. 4 mm/s, which translates to about 28 seconds to lift the cab 320 mm — the typical levelling range for elevator pit work. At the low end the bar creeps at 3.8 mm/s under heavy load, fast enough to be useful but slow enough to stop precisely; at the high end you nominally reach 16 mm/s but pawl float and engagement loss cap real-world performance well below that. If you measure significantly less than 11.4 mm/s at nominal stroking, check three things in order: (1) driving pawl spring fatigue — a spring delivering under 0.3 N seating force lets the pawl skip teeth, especially on the upstroke; (2) lever stroke length shortened by linkage wear so each stroke advances less than a full pitch — measure the actual stroke at the pawl tip, not at the handle; (3) bar tooth-tip rounding from years of service, which drops η below 0.85 and shows up as the lever feeling soft at the top of each stroke.
When to Use a Two-pawl Continuous Rectilinear Ratchet Bar and When Not To
A two-pawl rectilinear ratchet bar is one option for stepwise linear advance with passive load holding. The honest comparison is against a leadscrew with thrust bearing and a hydraulic ram with check valve, because those are the mechanisms a designer actually picks between when the requirement is "step it up and hold it."
| Property | Two-pawl ratchet bar | Leadscrew with thrust bearing | Hydraulic ram with check valve |
|---|---|---|---|
| Linear speed under load | 10-20 mm/s typical hand-stroked | 1-5 mm/s typical hand-cranked | 20-100 mm/s pumped |
| Position resolution | One tooth pitch (5-25 mm) | Continuous, sub-mm | Continuous, sub-mm |
| Static load holding | Passive, full bar capacity | Passive via thread friction | Passive via check valve |
| Load capacity range | 100-30,000 lbs | 50-5,000 lbs hand-driven | 500-100,000+ lbs |
| Cost per unit (single shop build) | Low — $50-300 in parts | Medium — $150-800 | High — $400-3,000 |
| Maintenance interval | Lubricate teeth every 100-500 cycles | Re-grease screw every 50-200 cycles | Inspect seals every 6-12 months |
| Failure mode if neglected | Tooth-tip rounding, pawl float | Thread galling, backlash | Seal leak, sudden load drop |
| Best application fit | Manual jacks, fast coarse advance | Precision positioning under load | Heavy lifting, long holds |
Frequently Asked Questions About Two-pawl Continuous Rectilinear Ratchet Bar
Almost always the input stroke at the handle is not the same as the stroke at the driving pawl tip. Lever pivots, clevis pins, and linkage joints accumulate slop over time, and 0.5 mm of play at each of three pivots eats 1.5 mm off the pawl-tip stroke. On a 10 mm pitch bar that's a 15% shortfall — enough that the pawl drops into the same tooth it just left on every second stroke.
Measure the stroke directly at the pawl tip with a dial indicator while you work the handle through its full range. If the pawl tip travels less than tooth pitch plus 1 mm of overtravel, tighten or replace the worn pivots before blaming the pawls or the bar.
Offset twin rows — sometimes called a vernier ratchet — halve the effective advance increment without halving the pitch. You run two driving pawls 180° out of stroke phase, each on its own row. Useful when you need finer position resolution than a single coarse row gives but you still want the load-bearing tooth depth of a coarse pitch.
The cost is doubled pawl count, doubled spring count, and a much tighter manufacturing tolerance on the row-to-row offset — typically ±0.02 mm. If you don't need sub-pitch resolution, stick with a single row. The Hi-Lift Jack has used a single 19 mm pitch row for over a century and the resolution is fine for 4×4 recovery.
No. The holding pawl spring only needs to seat the pawl into the next tooth as the bar advances — 0.3 to 0.6 N is plenty on a typical hand-jack scale. Once seated, the load itself wedges the pawl deeper through the loading-face geometry. Oversize this spring and you fight it on every advance.
The driving pawl spring is the one that earns its keep. It must reseat the pawl on the return stroke fast enough to engage the next tooth before the next forward stroke begins. Size for roughly 3× the pawl-tip mass × peak return-stroke acceleration. On a typical lever jack stroking at 60 strokes/min that works out to 0.5-1.0 N. Spring it lighter and you get pawl float at speed — the symptom is the lever stroking faster than the bar advances.
Horizontal works, but the failure mode shifts. On a vertical bar the load self-seats both pawls — gravity does free work for you. On a horizontal bar you lose that, and any side load on the bar guide tries to lift the holding pawl off its tooth. The pawl pivot axis must be perpendicular to the side-load direction, not parallel, or you're relying on spring force alone to hold the load.
If you're getting unexpected back-walking on a horizontal install, check that the bar guide bushings haven't worn enough to let the bar tilt — even 1° of tilt can lift a pawl tip by 0.3 mm and disengage it under shock load.
The mechanism itself is reliable. The danger is the release lever — flipping it from up to down under load disengages both pawls simultaneously and the bar drops at full load weight. Operators have been killed when the handle whips up as the bar releases. This is a mechanism-level safety issue, not a wear issue: it's inherent to having a single release that lifts both pawls together.
If you're designing a new two-pawl ratchet for load-bearing service, build the release so it requires sequential action — drop the holding pawl onto a tooth first, then lift the driving pawl, then release the holding pawl through a separate trigger. That converts a sudden drop into a controlled descent.
Around 25-30 mm for hand-operated work. Beyond that, two problems hit at once. First, the lever throw needed to advance one pitch becomes long enough that the operator can't make a clean stroke without repositioning their grip — you lose stroke rate. Second, the load steps become coarse enough that you can't stop near a target height — you're always one tooth above or below where you want to be.
If you genuinely need long advance per stroke, gear the lever to multiply input stroke into bar travel rather than coarsening the pitch. Keeps tooth strength up and resolution fine.
References & Further Reading
- Wikipedia contributors. Ratchet (device). Wikipedia
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