Impact wrench
An impact wrench 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. In operation, a rotating mass is accelerated by the motor, storing energy, then suddenly connected to the output shaft, creating a high-torque impact.
Compressed air is the most common power source, although electric or hydraulic power is also used, with cordless electric devices becoming increasingly popular since the mid-2000s.
Impact wrenches are widely used in many industries, such as automotive repair, heavy equipment maintenance, product assembly, major construction projects, and any other instance where a high torque output is needed. For product assembly, a pulse tool is commonly used, as it features a reactionless tightening while reducing the noise levels the regular impacts suffer from. Pulse tools use oil as a medium to transfer the kinetic energy from the hammer into the anvil. This gives a smoother impulse, a slightly lower torque to weight ratio and a possibility to design a shut off mechanism that shuts the tool down when achieving the correct torque. Pulse tools are not referred to as "impact wrenches" as the performance and technology are not the same.
Impact wrenches are available in every standard socket wrench drive size, from small drive tools for small assembly and disassembly, up to and larger square drives for major construction.
History
The impact wrench was invented by Robert H. Pott of Evansville, Indiana.Mechanism
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, with some designs delivering faster, weaker blows twice per revolution, or slower, more powerful ones only once per revolution.The 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. 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.
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 design encases the pounding mechanism in hydraulic fluid to reduce the amount of metal to metal contact, greatly reducing noise and vibration.
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. 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-Wihelm 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. 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.
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, 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.