A ratchet is a tool that allows sockets to be used easily and without pulling them on and off the nuts or bolts every time they have been turned to their accessibility limit. Ratchets get their name from the one-way ratcheting mechanism that is often reversible with a switch. The ratchet allows the part of the tool that the socket attaches to via a square drive to rotate with the tool in one direction and move independently in the other direction.
Pivoting Head Ratchets
A pivoting head ratchet allows more movement freedom by allowing the handle to pivot out of the way of potential obstacles, allowing more range of rotation.
Roto-Head Ratchets
A roto-head ratchet improves upon the pivot head design by moving the pivot point inline with the center of rotation. This allows for using the ratchet like a screwdriver after breaking the bolt loose, greatly speeding up the removal of some fasteners.
Shown at the right is a pair of roto-head ratchets (Gearwrench 81223) that have the roto-head design.
Gearwrench Roto-Head Ratchet Review by Cars Simplified
High Tooth Count Ratchets
The limiting factor behind how little a ratchet has to turn before catching and being able to be moved again is typically tooth count, so high tooth count ratchets allow for using the most movement for loosening a fastener in a location where there isn't much room to swing the ratchet handle.
In addition to the tight space benefit, some people say these ratchets "feel better" because they always grab quickly (compared to their low-tooth-count counterparts) after the back-swing.
High tooth count ratchets typically have a trade-off in either durability, size, and/or price, since the higher tooth count is more complicated to produce and leaves the tool with smaller teeth (which have to withstand the forces put on them through the handle to the fastener) if the gear itself isn't made larger than the lower tooth count peer that it is being compared to.
Stubby Ratchets
Stubby ratchets fill two roles! Sometimes the are needed to fit in a tight space where a long swinging handle would get in the way. They can also prevent over-torqing because they are too short to get too much leverage.
Bent-Handle Ratchets
Bent-handle ratchets are designed to keep the hand and knuckles out of the way of components that would otherwise be in-line with the socket.
This type of handle is commonly used when changing spark plugs because they tend to have a lot of components in the way, especially on V engine layouts where the spark plug access isn't right at the top of the engine.
Gimbal Ratchets
Gimbal ratchets have no handle and are meant to be held by their outside ring. The ring pivots on another ring, and together those pivot points allow the hand to rotate the ratchet in many comfortable positions. The fastener may need to be broken loose with a stronger tool first, but the gimbal ratchet can make removal of long fasteners more comfortable, especially compared to spinning just the socket by hand.
Pass-Through Ratchets
A pass-through ratchet uses different sockets that allow long threaded rods or other components to be passed through the center of the ratcheting mechanism. A niche tool for some, but a vital one in certain scenarios.
Drop-In Ratchet Units
Some brands produce drop-in replacements or upgrades for their tools. The upgrades don't always fit in everything, but present the opportunity for custom tool combinations that can make for an ideal working experience.
If you intend to upgrade a ratchet you own with a drop-in upgrade, make sure you do your research to ensure they are compatible!
Ratcheting Mechanisms in Other Tools
Ratcheting mechanisms have found their way into other tools, such as the box end of wrenches, torque wrenches, and screwdrivers. Instead of being called ratchets, they are called "ratcheting [type of tool]" to make the difference clear.