Compressed air is the most common power source for impact wrenches,^{[citation needed]} providing a low-cost design with the best power-to-weight ratio. A normal vane motor is almost always used, usually with four to seven vanes, and various lubrication systems, the most common of which uses oiled air, while others may include special oil passages routed to the parts that need it and a separate, sealed oil system for the hammer assembly. Most impact wrenches drive the hammer directly from the motor, giving it fast action when the fastener requires only low torque. Other designs use a gear reduction system before the hammer mechanism, most often a single-stage planetary gearset usually with a heavier hammer, delivering a more constant speed and higher "spin" torque. Electric impact wrenches are available, either mains powered, or for automotive use, 12-volt, 18-volt or 24-volt DC-powered. Recently, cordless electric impact wrenches have become common, although while their power outputs were initially significantly lower than corded electric or air-powered impact wrenches, the introduction of brushless DC motors and lithium-ion batteries have made cordless impact tools as or even more powerful than air or corded impact wrenches of 1/4" to 1" drive sizes, with cordless impacts rarely produced above 1" drive sizes. A ratcheting impact wrench is also a type of impact wrench with a locking mechanism that locks in place when it's turned the required number of revolutions. The locking mechanism prevents the tool from being turned by hand. A ratcheting has a range of 200-300 lbs. Some industrial tools are hydraulically powered, using high-speed hydraulic motors, and are used in some heavy equipment repair shops, large construction sites, and other areas where a suitable hydraulic supply is available. Hydraulic impact wrenches have the advantage of high power-to-weight ratio.

Impact wrenches are available in all sizes and in several styles, depending on the application. ¼" 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°. ⅜" 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 ⅜" drive impact wrenches are uncommon, but available. ½" 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. ¾" 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. 1¼" 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 pulleys 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 through-hole used only on larger impact wrenches, typically ¾" drive or greater. Pin retainers used to be more common, but seem to be being replaced by hog rings on most tools,^{[citation needed]} despite the lack of a positive lock. ¼" female hex drive is becoming increasingly popular for small impact wrenches,^{[citation needed]} especially cordless electric versions, allowing them to fit standard screwdriver tips rather than sockets. Adapter attachments can be purchased that allow hex drive tools to couple to square drive sockets, and vice-versa.

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.^{[citation needed]}

As the output of an impact wrench, when hammering, is a very short impact force, the actual effective torque is difficult to measure, with several different ratings in use. As the tool delivers a fixed amount of energy with each blow, rather than a fixed torque, the actual output torque changes with the duration of the output pulse. If the output is springy or capable of absorbing energy, the impulse will simply be absorbed, and virtually no torque will ever be applied, and somewhat counter-intuitively, if the object is very springy, the wrench may actually turn backwards as the energy is delivered back to the anvil, while it is not connected to the hammer and able to spin freely. ^{[citation needed]} A wrench that is capable of freeing a rusted nut on a very large bolt may be incapable of turning a small screw mounted on a spring. "Maximum torque" is the number most often given by manufacturers, which is the instantaneous peak torque delivered if the anvil is locked into a perfectly solid object. "Working torque" is a more realistic number for continually driving a very stiff fastener, and is typically measured by driving on a Skidmore-Wiheim torque tension test device for 5-10 seconds. "Nut-busting torque" is often quoted in impact tool specifications, with the usual definition being that the wrench can loosen a nut tightened with the specified amount of torque in some specified time period. Accurately controlling the output torque of an impact wrench is very difficult, and even an experienced operator will have a hard time making sure a fastener is not under-tightened or over-tightened using an impact wrench. Special socket extensions are available, commonly marketed as torque sticks or torque extensions which take advantage of the inability of an impact wrench to work against a spring, to precisely limit the output torque. Designed with spring steel, they act as large torsion springs, flexing at their torque rating, and preventing any further torque from being applied to the fastener. Some impact wrenches designed for product assembly have a built-in torque control system, such as a built-in torsion spring and a mechanism that shuts the tool down when the given torque is exceeded. When very precise torque is required, an impact wrench is only used to snug down the fastener, with a torque wrench used for the final tightening. Due to the lack of consistent standards when measuring torque outputs, some manufacturers are believed to inflate their ratings, or to use measurements with little bearing on how the tool will perform in actual use.^{[citation needed]} Many air impact wrenches incorporate a flow regulator into their design, either as a separate control or part of the reversing valve, allowing torque to be roughly limited in one or both directions, while electric tools may use a variable speed trigger for the same effect.

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 called a ball and cam mechanism 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, although impact tools with three impact dogs have been produced. 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 in the same manner as a drill or powered screwdriver. The ball and cam impact mechanism is most commonly seen in electric impact tools, as it allows the power source to be continuously rotating even while impacting, which prevents the motor from stalling. 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, making this design less efficient than the pin clutch or counterweight mechanisms seen in pneumatic impact tools. This design is often seen after a gear reduction, which reduces the speed and increases the torque of the motor driving the impact mechanism, and compensates 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° offsets, both striking at the same time.

A new (as of 2016) design encases the pounding mechanism in hydraulic fluid to reduce the amount of metal to metal contact, greatly reducing noise and vibration.^[3]

Sockets and extensions for impact wrenches are made of high tensile metal, as any spring effect will greatly reduce the torque available at the fastener. Even so, the use of multiple extensions, universal joints, and so forth will weaken the impacts, and the operator needs to minimize their use. Using non-impact sockets or accessories with an impact wrench will often result in bending, fracturing, or otherwise damaging the accessory, as most are not capable of withstanding the sudden high torque of an impact tool, and can result in stripping the head on the fastener. Non-impact sockets and accessories are made of a harder more brittle metal. Safety glasses should always be worn when working with impact tools, as the strong impacts can generate high-speed shrapnel if a socket, accessory, or fastener fails.

Pneumatic impact wrench brands that are commonly used in shop and construction application include Ingersoll Rand, Snap-On, MAC Tools, Matco, and Chicago Pneumatic. Cordless impact wrenches that are commonly used are made by Milwaukee, DeWalt, Makita, and Bosch. For specialized applications such as pitstops in motorsport, where a high amount of torque must be applied to a wheel nut in a short amout of time. Modified Ingersoll Rand impact wrenches were commonplace in NASCAR competition until the start of the 2022 season,^[4] when center-lock wheels were introduced and the old Ingersoll Rand wrenches were replaced by Paoli impact wrenches, which are used extensively in Formula One, Indycar, and the World Endurance Championship.^[5]

An impact wrench typically delivers more torque and accepts larger tool bits than an impact driver. This makes the impact wrench more suitable for large bolts and nuts in heavy mechanical settings (like for instance lug nuts), while the impact driver with its lesser torque and smaller tool bit is more suited towards driving smaller screws, like for instance in construction work.

• Socket wrench
• Torque multiplier
• Pneumatic torque wrench
• Impact driver
• Hydraulic torque wrench
• Air tool

• Basic Automotive Service & Maintenance
• Outdoor Power Equipment
• Slow Motion - How an Impact Wrench Works