A Skip Jack is a hand-operated mechanical lifting jack that uses a lever, a toothed rack, and a spring-loaded pawl to raise heavy loads in discrete increments and hold them against gravity between strokes. The pattern traces back to British railway and farm-implement makers in the late 1800s, with companies like Tangyes of Birmingham producing the early industrial versions. Each handle stroke advances the rack by one tooth, and the pawl drops into the next notch to hold the load while the operator resets. The result is a compact, self-locking lifter that moves 2-10 tonne loads with hand effort alone — still standard kit on rail gangs, recovery crews, and bridge erectors.
Skip Jack Interactive Calculator
Vary rack tooth pitch, stroke count, load, and tooth flank angle to see lift per ratchet action and pawl force demand.
Equation Used
The calculator uses the Skip Jack article principle that each completed handle stroke advances the rack by one tooth pitch. It also estimates the vertical load force and the pawl tooth normal force from the selected tooth flank angle.
FIRGELLI Automations - Interactive Mechanism Calculators.
- Rack advances exactly one tooth pitch per completed handle stroke.
- Holding pawl carries the full vertical load between strokes.
- Tooth flank force is estimated from a simple cosine resolution using the flank angle.
- Friction, impact, wear, and dynamic shock loads are not included.
How the Skip Jack Actually Works
The Skip Jack converts a long handle stroke into a small vertical rise of a toothed lifting bar — the rack — and uses a spring-loaded pawl to lock that rise in place between strokes. You push down on the handle, a drive pawl engages a tooth on the rack, the rack lifts by one pitch (typically 8-12 mm depending on the build), and a holding pawl on the body drops into the next tooth as the drive pawl resets. The load never falls back. That's the whole trick — a ratchet and pawl arrangement scaled up with a Class 2 lever to give you mechanical advantage on the order of 30:1 to 60:1.
Geometry matters more than people think. The pawl tip must seat fully on the tooth flank — typically a 60° flank angle — and the body casting has to hold the pawl pivot square to the rack within roughly 0.5 mm. If the pawl pivot wears or the spring weakens, the pawl will skip a tooth under load, and you'll feel the handle drop suddenly mid-stroke. That's the classic failure mode and it's how people get hurt. The rack teeth also need to stay clean — packed grit on a railway jack will hold the pawl off its seat by half a millimetre and that's enough to shear the tooth corner under a 5 tonne lift.
Lowering is where most operators get caught out. A proper Skip Jack uses a separate release lever or a reversed pawl orientation to drop the load one tooth at a time. You never just kick the holding pawl out under load — the rack will run free and the handle will whip. The self-locking ratchet is what makes the jack safe; the controlled release is what keeps it that way.
Key Components
- Toothed Rack (Lifting Bar): The vertical steel bar with cut teeth on one face that carries the load upward. Teeth are typically 10 mm pitch with a 60° flank angle, hardened to around 45 HRC so the pawl tip doesn't peen them under repeated 5-tonne lifts.
- Drive Pawl: Spring-loaded pawl coupled to the handle that engages a rack tooth on each downstroke and lifts the rack by one pitch. The pawl tip-to-flank contact must be full-face — partial engagement of less than 70% of the tooth height shears the corner off in service.
- Holding Pawl: Second spring-loaded pawl mounted to the jack body that drops into the next tooth as the drive pawl returns, locking the load in place between strokes. Spring force is usually 15-25 N — enough to seat reliably but not so stiff it resists the release lever.
- Lever Handle: Long steel handle, typically 800-1200 mm, that gives the operator the mechanical advantage to lift multi-tonne loads with around 200-400 N of hand force. Handle length sets your lifting capacity for a given hand effort — shorter handle, less rise per stroke and harder pumping.
- Body Casting: Cast or fabricated steel housing that holds the pawl pivots square to the rack and resists the bending moment from off-centre loads. Pivot bore tolerance of around H8 keeps the pawls from cocking and skipping under load.
- Release Lever: Separate control that disengages the holding pawl in a controlled way to lower the load one tooth at a time. On a properly built Skip Jack the release is dead-man style — let go and the pawl drops back to lock.
- Foot or Toe: Lifting point at the base of the rack — either a flat foot for under-axle lifting or a side toe for getting under low rail flanges. Toe lifts shift the load eccentrically, so the body casting has to take a bending moment, not just compression.
Where the Skip Jack Is Used
Skip Jacks turn up wherever you need to lift a heavy thing in a confined space using only hand effort, with the load held safely between strokes. Rail crews carry them by the dozen for tamping and sleeper replacement. Bridge erectors use them to set girders. Farm and forestry crews lift stuck vehicles and tractors. The common thread is no power, no hydraulics, no compressor — just a lever, a rack, and two pawls.
- Rail Maintenance: Network Rail track gangs in the UK use Tangye-pattern 7-tonne track jacks to lift rail and sleepers during ballast tamping and sleeper replacement on regional lines.
- Vehicle Recovery: The Hi-Lift Jack — a North American Skip Jack derivative — is standard kit on Land Rover and Jeep off-road recovery rigs for lifting axles out of mud and changing wheels on uneven ground.
- Bridge Erection: Steel bridge crews use 10-tonne Skip Jacks to incrementally raise girder ends onto bearing pads during span placement, where hydraulic jacks would be too tall to fit under the beam.
- Agriculture and Forestry: Logging crews in British Columbia carry Skip Jacks on skidder trucks to lift bunched logs and free pinched chokers — a hand jack works where there's no room for a hydraulic bottle.
- Heritage Railway Restoration: Volunteer crews on the Bluebell Railway and Severn Valley Railway use period-correct Tangye and Duff-Norton Skip Jacks for re-railing wagons and lifting tender frames during overhaul.
- Building Movers: House-moving contractors use banks of synchronised Skip Jacks under timber-frame structures for short controlled lifts during cribbing and shoring before the hydraulic system takes over.
The Formula Behind the Skip Jack
The core question on a Skip Jack is what hand force you need at the handle to lift a given load. The relationship comes from balancing the moment about the drive-pawl pivot. At the low end of the typical operating range — a 1 tonne lift on a long-handled rail jack — hand effort runs around 80-120 N, which feels like a firm push. At nominal — a 5 tonne lift — you're at 200-300 N, comfortable two-handed pumping. Push to the high end — a 10 tonne lift — and you're at 400-500 N, which is the limit of what a single operator can sustain for more than a few strokes before fatigue sets in and stroke quality drops. The sweet spot for a 1000 mm handle and a 10 mm rack pitch sits right around the 5 tonne, 250 N point.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Fhand | Hand force required at the handle tip | N | lbf |
| W | Load lifted by the rack | N | lbf |
| p | Rack tooth pitch (rise per stroke) | m | in |
| L | Effective handle length from pawl pivot to grip | m | in |
| η | Mechanical efficiency (typically 0.65-0.80 for a clean, well-lubricated jack) | dimensionless | dimensionless |
Worked Example: Skip Jack in a county bridge-deck repair crew
Your county bridge maintenance crew in Vermont is lifting the south end of a 6 m steel I-beam off its bearing pad on a back-road timber-deck bridge so the abutment cap can be repointed. The beam end carries 4500 kg of dead load. You're using a 7.5 tonne rated Tangye-pattern Skip Jack with a 1000 mm handle, 10 mm rack pitch, and an estimated efficiency of 0.72 from clean teeth and fresh grease. You need to know whether one operator can pump it, or whether you need to swap operators every few strokes.
Given
- W = 4500 × 9.81 = 44,145 N
- p = 0.010 m
- L = 1.000 m
- η = 0.72 dimensionless
Solution
Step 1 — at nominal load of 4500 kg, plug the numbers in:
Step 2 — compute the nominal hand force:
That's about 31 kgf at the grip — firm two-handed pumping, sustainable for 20-30 strokes before a fresh operator should take over. Each stroke lifts the beam 10 mm, so a 100 mm jacking lift takes 10 strokes.
Step 3 — at the low end of the typical bridge-crew range, say a 1500 kg lift to free a stuck bearing plate:
That's barely 10 kgf — light enough to pump one-handed while the other hand steadies the load. The jack feels almost too easy and operators sometimes over-pump and over-lift, which is its own hazard.
Step 4 — at the high end of rated capacity, 7500 kg:
That's 52 kgf — at or beyond what one operator can sustain. Above this point you'll see stroke length drop off because the operator can't complete a full pump, the pawls don't fully seat, and you start risking a pawl skip. This is why 7.5 tonne rated jacks are sized so the rated lift sits at a hand force a real human can actually deliver.
Result
At the nominal 4500 kg lift the operator delivers about 307 N at the grip — firm but sustainable two-handed pumping, with each stroke raising the beam 10 mm. The full operating range tells the story: at 1500 kg the jack pumps almost too easily at 102 N, the 4500 kg working point sits in the comfortable zone, and at the rated 7500 kg the 511 N hand force is right at single-operator limits and demands a fresh crew member every 5-10 strokes. If you measure significantly more hand force than predicted — say 450 N at the 4500 kg point — the most likely causes are a dry or grit-packed rack dropping efficiency below 0.5, a worn drive-pawl pivot bushing letting the pawl rock and only partially seat, or a bent rack rubbing the body casting and adding parasitic friction. Check the rack teeth for shiny burnish only on the tip third — that's the tell-tale of partial pawl engagement and you should pull the jack from service before the next lift.
When to Use a Skip Jack and When Not To
Skip Jacks compete with bottle jacks, screw jacks, and small hydraulic rams in the 1-10 tonne hand-lift bracket. Each has a clear application window — the Skip Jack wins on stroke length and side-load tolerance, loses on raw capacity per kilogram of jack.
| Property | Skip Jack (Ratchet-and-Pawl) | Hydraulic Bottle Jack | Screw Jack |
|---|---|---|---|
| Typical load capacity | 2-10 tonne | 2-50 tonne | 1-20 tonne |
| Maximum stroke length | 400-1200 mm (long rack) | 150-250 mm typical | 150-400 mm typical |
| Lift speed (mm per stroke) | 8-12 mm per handle stroke | 1-3 mm per handle stroke | 0.5-2 mm per handle turn |
| Side-load tolerance | Tolerates moderate eccentric load via toe lift | Poor — seal blow-out risk under side load | Poor — bends lead screw |
| Self-locking under load | Yes — holding pawl locks every tooth | Yes — until seal fails | Yes — non-back-driving thread |
| Cost (typical 5-tonne unit) | £200-500 / $250-650 | £60-150 / $80-200 | £100-300 / $130-400 |
| Failure mode under abuse | Pawl skip — sudden but recoverable if release works | Seal blow-out — total instant load drop | Thread strip — gradual then total |
| Best application fit | Rail, recovery, bridge erection — long lifts in dirt | Workshop axle lifts, clean environments | Trailer landing legs, sustained holding |
Frequently Asked Questions About Skip Jack
That's almost always the holding pawl skipping a tooth, and the rack teeth aren't usually where you'll see it — look at the pawl tip and the pawl pivot bushing. A worn pivot bushing lets the pawl cock at an angle, so it engages only the top corner of the tooth instead of the full flank. Under load that corner shears or the pawl rotates out, and the rack drops one or two teeth before the drive pawl catches it.
Quick diagnostic: with the jack unloaded, wiggle the pawl side-to-side at the tip. More than about 1 mm of lateral play at the tip means the pivot bore is past tolerance and the jack should be taken out of service until the pin and bushing are replaced.
No — and this is where people break jacks and get hurt. The rated capacity isn't set by hand force, it's set by the strength of the rack teeth, the pawl, and the body casting. A 1000 mm handle on a 7.5 tonne jack is sized so that 500 N of hand effort gives you the rated lift. Slip a 1500 mm pipe over the handle and you can apply 750 N, which translates to an 11 tonne load on parts designed for 7.5 tonne.
What fails first is usually the drive-pawl tooth or the rack tooth corner — both shear suddenly with no warning, and the rack runs free. If you need more capacity, get a bigger jack. The handle length is part of the engineered force balance, not a starting point for upgrades.
Two 5-tonne jacks per end, every time. Re-railing applies eccentric load — the wagon's centre of gravity isn't directly over the lift point, and the jack takes a bending moment as well as compression. A single 10-tonne jack under one corner concentrates that bending into one body casting and one rack, which is exactly what cracks rack-mounting feet on older Tangye and Duff-Norton jacks.
Two jacks spread the load, give you redundancy if one pawl skips, and let you correct lateral lean by pumping one side ahead of the other. Standard Network Rail and heritage-railway practice is paired jacks for any re-railing operation above about 3 tonne per end.
That's the drive pawl partially engaging. Two common causes: weak drive-pawl spring, or grit packed into the tooth roots so the pawl tip can't seat to the full depth. When the pawl only sits on the upper third of the tooth, it slips off after a 4-5 mm rise instead of carrying the rack the full 10 mm pitch.
Pull the jack apart, clean the rack with a wire brush and degreaser — not just a wipe-down — and check the pawl spring force with a small fish scale. It should pull about 2-2.5 kg to fully compress on a typical 7.5 tonne jack. Anything below 1.5 kg and the spring needs replacing. After cleaning, regrease lightly with an open-gear lubricant; heavy grease just collects more grit.
Because the formula assumes 100% efficiency, and a real Skip Jack runs at 65-80% depending on condition. Pawl friction, pivot bushing friction, and rack-to-body sliding contact all eat mechanical advantage. A theoretical 30:1 ratio with η = 0.72 gives you an effective ratio of about 21.6:1, which is what your hands actually feel.
If you've checked the calculation and the felt force is still much higher than predicted with reasonable efficiency, the jack is binding somewhere — usually a bent rack rubbing the body, or a pawl pivot that's seized partially and dragging on every stroke. A jack in good order should feel noticeably easier on the first stroke after fresh grease, and that difference is your efficiency margin.
Three conditions favour the bottle jack: short lift height, clean environment, and pure vertical load. If you're lifting a car axle 100 mm in a workshop, a bottle jack is faster, cheaper, and lighter. The Skip Jack wins when any of those conditions break down — long lifts (300 mm+), dirty environments where seals would fail, or eccentric loads where you're lifting at a toe rather than directly under the load.
Rail and bridge work hits all three Skip Jack conditions: lifts of 400-800 mm, ballast and weather everywhere, and side toes under rail flanges or beam edges. That's why the pattern has survived for 130+ years in those industries despite hydraulic jacks being available the whole time.
References & Further Reading
- Wikipedia contributors. Jack (device). Wikipedia
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