Air ratchets, also known as pneumatic ratchet torque wrenches, are popular and useful tools for tightening and loosening bolts. Used in the heavy manufacturing, heavy construction, automotive and aerospace industries, air ratchets harness force from high-pressurised gas (between 75 psi and 120 psi) and convert it into powerful yet controlled torque through a series of turbines, gearboxes and ratchets.
The air pressure from compressed gas is converted into mechanical energy through a small pneumatic turbine located in the lower handle of the tool. Similar to a windmill, the high-pressure air pushes against sturdy blades (known as "vanes") that run parallel to the turbine's axis (Figure 1) to create torque.
When the user pushes down on the trigger, pressurised air is released into an inlet opening in the wall of the chamber. Meanwhile, an outlet opening leading to the exhaust line is on the opposite side of the chamber. In an attempt to equalise pressure (the compressed gas pressure is dozens of times higher that the atmosphere's pressure), the inlet air will rush toward the outlet opening, pushing the vanes in the process.
Because the pressures of the atmosphere and compressed air are constant, this powerful rush of air is constant, leading to a constant RPM for the vane-lined turbine.
The turbine's axle is attached to a single gear that fits into a device called a planetary gear set (Figure 2). In a planetary gear set, a central gear (sun gear) has its teeth interlinked with those of four surrounding gears (planet carrier gears). Meanwhile, an outer ring lined with teeth (the ring gear) surrounds the planet carrier gears and allows them to rotate freely.
When the sun gear rotates, the four-piece carrier gear assembly rotates in the opposite direction. As a result, the carrier gear assembly rotates more slowly than the sun gear but with greater torque. To see an animation of a planetary gear assembly in action, see References below.
The carrier gear assembly is attached to a freely rotating, needle-bearing-stabilised rod with a round knob called a drive bushing at the end. As the assembly goes, so does the rod--with one exception: The drive bushing is offset such that it revolves around the rod's/assembly's axle (rather than rotating on the axle itself).
This revolving drive bushing fits into a sturdy, U-shaped groove at the base of the ratchet head; as the drive bushing revolves, it pushes the ratchet casing back and forth (Figure 3).
The ratchet casing is surrounded by a special row of gear teeth interspersed with four spring-loaded fingers called pawls (Figure 4). When the ratchet casing swings to the right, the gear teeth slide effortlessly past the pawls. But when it swings to the left, the teeth catch the pawls firmly. Because the pawls are attached to the ratchet head, the left swinging casing forcibly rotates the pawls/ratchet head clockwise. Due to the speed of the turbine, these small, clockwise turns happen dozens of times per second, creating the illusion of continuous movement.