The head is cut open so you can watch the pawl. Drag the handle back and forth, or use the slider: the pull locks and turns the bolt, the swing back clicks over the teeth. Drag anywhere else to turn the view, scroll or pinch to zoom, and click a part to use it.
Try itDrag the handle, or press Swing. On the pull the pawl locks into the gear; on the way back it clicks over the teeth.
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LeverCut
This browser can’t show the 3D model, so here is a still picture.
Scroll to take it apart
What’s under the cover
Keep scrolling: the ratchet comes off its socket, the cover and the lever lift away, then the parts of the one-way lock come out.
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Step 1Off the socketA ball in the side of the square drive keeps the socket on until you press the release button.
Step 2Lift the coverA cover plate and its screws close the head. The lever sits on top and turns the selector inside.
Step 3Lift out the worksThe gear, the pawl and the selector with its spring and plunger: the whole one-way lock.
Meet the parts
Eight parts, one click
Here it is taken apart: the tool lifted off its socket, and the gear, pawl, selector and cover lifted out of the head. Tap any part of the picture or pick a name below.
Hover or tap the picture. Gold rings mark every part.
Parts diagram
Follow the force
The head cut open and seen from above at an angle. Your pull goes from the handle through the head and the pivot pin into the pawl, which pushes on one gear tooth; the gear turns the socket and the bolt. Numbers 1 to 7 follow that path, 8 to 11 make it one-way, and 12 lets go of the socket.
How it works, step by step
Thirteen steps, one way only
The same steps as the video. Each picture is a moment from it; press Show me in 3D to set the model up for that step.
Step 1 of 13
Swing it, and it only turns one way
Demo · swing room
Find your swing
Deep in an engine bay there may be room for only a short swing. On every swing back the pawl has to get past the next tooth. If it can’t, it drops back into the same gap and the bolt never moves.
The teeth set the smallest swing that works: a little more than 360° divided by the number of teeth, so the pawl can drop past the tip of the next one. Set the room you have and the tooth count, then drag the handle or press Swing. Every click is the pawl dropping into the next gap.
24 teeth: a click every 15°. This ratchet.
72 teeth: a click every 5°, so it needs a third of the room.
Teeth
15°between clicks
–bolt turn per swing
–swing that does nothing
–swings for one turn
Demo · torque
Tighten it right
The handle turns your pull into torque: force times the distance from the bolt. This bolt runs down easily until its washer meets the bracket. After that, every degree takes more torque. Its spec is 22 N·m.
Handle
Lever
0.0torque on the bolt, N·m
0°bolt turned past snug
0.0clamping force, kN
0clicks
Demo · the strut
Why the pull locks
On a pull, the gear tooth pushes back on the pawl’s tooth. Which way that push points decides everything.
The push acts square to the tooth face. If its line passes between the pivot and the gear, it turns the pawl into the gear, so the harder you pull, the harder the pawl is pressed in. Tilt the faces too far and the line passes the other side of the pivot: the push levers the pawl out and the ratchet slips.
Locks: the push turns the pawl into the gear.
Slips: the push turns it out, and the gear jumps a tooth.
–push on the pawl tooth, N
–its lever arm about the pivot, mm
–turning the pawl, N·m
–result
More to know
Kinds, history and care
Six ways to turn back and forth into round and round
Pawl and gearThe one on this page: a 24-tooth gear and one rocking pawl. Simple, strong and easy to rebuild.
Fine tooth72 to 90 teeth, so a swing of 4 or 5 degrees is enough. Smaller teeth carry less, so these heads often share the load between pawls.
Twin offset pawlsTwo pawls that sit half a tooth apart. One of them is always close to a gap, so a 72-tooth gear catches every 2.5 degrees.
Flex headA hinge between the head and the handle lets you swing round a corner or reach a bolt at an angle.
GearlessRollers or small wedges grip a smooth drum. They catch at any angle and make no clicks, but the head is bigger.
Not only spannersBicycle hubs, winches, ratchet straps, cable ties and clock winders all use teeth and a pawl to let things move one way only.
From winches to quick release
Winches and windlasses use a toothed wheel and a pawl, so a load can be wound up without running back.
J. J. Richardson of Vermont patents a ratchet wrench.
Snap-on is founded on one idea: a few handles that fit many sockets. “Five do the work of fifty.”
Peter Roberts, who designed it as an 18-year-old store clerk, files a patent on the quick-release button. Sears bought it for $10,000; after a 20-year court fight he won millions from them.
Fine-tooth heads with 72 to 90 teeth or more, and twin pawls, cut the swing to a few degrees.
Keeping one clicking
Break tight bolts loose with a breaker bar, then spin them out with the ratchet. One pawl tooth carries all the force.
Never slip a pipe over the handle for more torque. The pawl or the gear teeth give way first.
Push the lever all the way over. Half way, the pawl can sit on the tips of the teeth and slip.
Keep grit out. A dirty head lets the pawl stick and slip. A few drops of light oil keep it clicking.
If it slips under load, rebuild it. Kits with a new pawl and spring cost little.
Pull toward you, hand open, so a slip doesn’t bang your knuckles.
Things worth knowing
One click per toothA 24-tooth ratchet clicks 24 times for every turn of the bolt; a 72-tooth one, 72 times.
Half a tonne on one toothAt 50 N·m on the bolt, the gear tooth pushes on the pawl with about 5 kN, the weight of half a tonne.
Square sizesDrives come in 1/4, 3/8, 1/2, 3/4 and 1 inch. This one is 3/8 inch, or 9.5 mm across.
Bicycles click tooA freewheel hub has pawls and teeth like these. The buzz when you coast is the pawls clicking over the teeth.
Winding a clockThe clicks you hear as you wind a clock are a pawl stopping the spring from unwinding.
Feynman’s ratchetThe physicist Richard Feynman showed that a tiny ratchet can’t turn the random jiggling of molecules into work when everything is at one temperature.
Every Mechanism
