An impact wrench (also known as an air wrench, air gun, rattle gun, torque gun) is a socket wrench power tool designed to deliver high torque output with minimal exertion by the user, by storing energy in a rotating mass, then delivering it suddenly to the output shaft.
Impact wrenches are widely used in many industries, such as automotive repair, heavy equipment maintenance, product assembly (often called "pulse tools" and designed for precise torque output), major construction projects, and any other instance where a high torque output is needed.
Impact wrenches are available in every standard socket wrench drive size, from small 1/4" drive tools for small assembly and disassembly, up to 3.5" and larger square drives for major construction. Impact wrenches are one of the most commonly used air tools, and are found in virtually every mechanic's shop.
In operation, a rotating mass (the hammer) is accelerated by the motor, storing energy, then suddenly connected to the output shaft (the anvil), creating a high-torque impact. The hammer mechanism is designed such that after delivering the impact, the hammer is again allowed to spin freely, and does not stay locked. With this design, the only reaction force applied to the body of the tool is the motor accelerating the hammer, and thus the operator feels very little torque, even though a very high peak torque is delivered to the socket. This is similar to a conventional hammer, where the user applies a small, constant force to swing the hammer, which generates a very large impulse when the hammer strikes an object. Energy is stored over time, allowing a very strong, but short output impulse to be generated from a relatively weak, but constant input force. The hammer design requires a certain minimum torque before the hammer is allowed to spin separately from the anvil, causing the tool to stop hammering and instead smoothly drive the fastener if only low torque is needed, rapidly installing/removing the fastener.
Impact wrenches are available in all sizes and in several styles, depending on the application. 1/4" drive wrenches are commonly available in both inline (the user holds the tool like a screwdriver, with the output on the end) and pistol grip (the user holds a handle which is at right angles to the output) forms, and less commonly in an angle drive, which is similar to an inline tool but with a set of bevel gears to rotate the output 90 degrees. 3/8" impacts are most commonly available in pistol grip form and a special inline form known as a "butterfly" wrench, which has a large, flat throttle paddle on the side of the tool which may be tilted to one side or the other to control the direction of rotation, rather than using a separate reversing control, and shaped to allow access into tight areas. Regular inline and angle 3/8" drive impact wrenches are uncommon, but available. 1/2" drive units are virtually only available in pistol grip form, with any inline type being virtually impossible to obtain, due to the increased torque transmitted back to the user and the greater weight of the tool requiring the larger handle. 3/4" drive impact wrenches are again essentially only available in pistol grip form. 1" drive tools are available in both pistol grip and "D handle" inline, where the back of the tool has an enclosed handle for the user to hold. Both forms often also incorporate a side handle, allowing both hands to hold the tool at once. 1.25" and larger wrenches are usually available in "T handle" form, with two large handles on either side of the tool body, allowing for maximum torque to be applied to the user, and giving the best control of the tool. Very large impact wrenches (up to several hundred thousand foot-pounds of torque) usually incorporate eyelets in their design, allowing them to be suspended from a crane, lift, or other device, since their weight is often more than a person can move. A recent design combines an impact wrench and an air ratchet, often called a "reactionless air ratchet" by the manufacturers, incorporating an impact assembly before the ratchet assembly. Such a design allows very high output torques with minimal effort on the operator, and prevents the common injury of slamming one's knuckles into some part of the equipment when the fastener tightens down and the torque suddenly increases. Specialty designs are available for certain applications, such as removing crankshaft pullies without removing the radiator in a vehicle.
Various methods are used to attach the socket or accessory to the anvil, such as a spring-loaded pin that snaps into a matching hole in the socket, preventing the socket being removed until an object is used to depress the pin, a hog ring which holds the socket by friction or by snapping into indents machined into the socket, and a through-hole, where a pin is inserted completely through the socket and anvil, locking the socket on. Hog rings are used on most smaller tools, with though-hole used only on larger impact wrenches, typically 3/4" drive or greater. Pin retainers used to be more common, but seem to be being replaced by hog rings on most tools, despite the lack of a positive lock. 1/4" female hex drive is becoming increasingly popular for small impact wrenches, especially cordless electric versions, allowing them to fit standard screwdriver tips rather than sockets.
Many users choose to equip their air-powered impact wrenches with a short length of air hose rather than attaching an air fitting directly to the tool. Such a hose greatly aids in fitting the wrench into tight areas, by not having the complete coupler assembly sticking out the back of the tool, as well as making it easier for the user to position the tool. An additional benefit is greatly reduced wear on the coupler, by isolating it from the vibration of the tool. A short length of hose also prevents the air fitting from being broken off in the base of the tool if the user loses their grip and the tool is allowed to spin.
The hammer mechanism in an impact wrench needs to allow the hammer to spin freely, impact the anvil, then release and spin freely again. Many designs are used to accomplish this task, all with some drawbacks. Depending on the design, the hammer may drive the anvil either once or twice per revolution (where a revolution is the difference between the hammer and the anvil), with some designs delivering faster, weaker blows twice per revolution, or slower, more powerful ones only once per revolution.
A common hammer design has the hammer able to slide and rotate on a shaft, with a spring holding it in the downwards position. Between the hammer and the driving shaft is a steel ball on a ramp, such that if the input shaft rotates ahead of the hammer with enough torque, the spring is compressed and the hammer is slid backwards. On the bottom of the hammer, and the top of the anvil, are dog teeth, designed for high impacts. When the tool is used, the hammer rotates until its dog teeth contact the teeth on the anvil, stopping the hammer from rotating. The input shaft continues to turn, causing the ramp to lift the steel ball, lifting the hammer assembly until the dog teeth no longer engage the anvil, and the hammer is free to spin again. The hammer then springs forward to the bottom of the ball ramp, and is accelerated by the input shaft, until the dog teeth contact the anvil again, delivering the impact. The process then repeats, delivering blows every time the teeth meet, almost always twice per revolution. If the output has little load on it, such as when spinning a loose nut on a bolt, the torque will never be high enough to cause the ball to compress the spring, and the input will smoothly drive the output. This design has the advantage of small size and simplicity, but energy is wasted moving the entire hammer back and forth, and delivering multiple blows per revolution gives less time for the hammer to accelerate. This design if often seen after a gear reduction, compensating for the lack of acceleration time by delivering more torque at a lower speed.
Another common design uses a hammer fixed directly onto the input shaft, with a pair of pins acting as clutches. When the hammer rotates past the anvil, a ball ramp pushes the pins outwards against a spring, extending them to where they will hit the anvil and deliver the impact, then release and spring back into the hammer, usually by having the balls "fall off" the other side of the ramp at the instant the hammer hits. Since the ramp need only have one peak around the shaft, and the engagement of the hammer with the anvil is not based on a number of teeth between them, this design allows the hammer to accelerate for a full revolution before contacting the anvil, giving it more time to accelerate and delivering a stronger impact. The disadvantages are that the sliding pins must handle very high impacts, and often cause the early failure of tool.
Yet another design uses a rocking weight inside the hammer, and a single, long protrusion on the side of the anvil's shaft. When the hammer spins, the rocking weight first contacts the anvil on the opposite side than used to drive the anvil, nudging the weight into position for the impact. As the hammer spins further, the weight hits the side of the anvil, transferring the hammer's and its own energy to the output, then rocks back to the other side. This design also has the advantage of hammering only once per revolution, as well as its simplicity, but has the disadvantage of making the tool vibrate as the rocking weight acts as an eccentric, and can be less tolerant of running the tool with low input power. To help combat the vibration and uneven drive, sometimes two of these hammers are placed in line with each other, at 180 degree offsets, both striking at the same time.
Many other designs are used, but all of them accomplish the same goal of allowing the hammer to spin freely of the anvil, allowing it to be accelerated and store energy, then delivering that energy suddenly to the anvil, before allowing the process to repeat.