Impact wrenches or impact guns are one of the most powerful handheld power tools. It is widely used by car mechanics to undo and tighten lug nuts of tires.
What is an Impact Wrench?
An impact wrench is a power tool used for loosening or tightening lug nuts, large bolts, and frozen or rusted fasteners. They provide a very high rotational torque that a regular power driver cannot provide. Impact wrenches achieve this by an internal hammering mechanism that transfers kinetic energy onto the output shaft.
I will explain the working principle of an impact wrench in the next section. They work similar to an impact driver. You could call an impact wrench as a bigger brother of the impact driver. However, there are some differences which I have written in a previous article. You can read impact driver vs impact wrench for a more detailed explanation.
Uses of Impact Wrench
Impact wrenches are widely used in automotive industries. The most common use that we all come across is the removal lug nuts when we change the car tires. It is a must have power tool for all the vehicle repair shops and automotive body shops. Other uses of impact wrench include assembly of automobiles, heavy equipment assembly, and maintenance, manufacturing of machine tools, large construction projects etc.
Impact wrenches have square shaped output shaft. They are available in different sizes; the most widely used is the ½-inch square ones.
How do Impact Wrenches Work?
Imagine you want to loosen a frozen bolt or stubborn nut with a hand wrench. With the normal force applied by hand, the nut may not break loose. But what if you hit the wrench with a hammer? With enough number of hammer blows the rusted nut will break loose. In simple terms, this is what happens inside an impact wrench.
Impact wrenches are powered by electric motor or compressed air. Since the air impact wrenches are most popular, let’s take a look at its working principle. Shall we?
The power source will drive the rotor and the rotor in-turn will drive the hammer and the hammer cage. During the operation when there is no resistance on the anvil (output shaft), the driving motor power is transferred to the anvil. But what makes impact tools special is the hammer mechanism inside that actuates when the force is not sufficient.
So when the anvil faces resistance, the hammer slips from the engaging pin. This causes it to rotate at a higher velocity and hence higher energy gets stored. And then as it rotates, it is suddenly connected to the anvil. This sudden impact of the rotating hammer transfers the kinetic energy stored inside the hammer to the anvil. The result is a very high twisting force (torque) on the output shaft.
Once the sudden blow of energy is consumed, the output shaft may face resistance again. In that case, the hammer slips again and starts rotating and the cycle repeats itself. These cyclic blows are what make the impact wrench so powerful.
Video: What is inside a double hammer impact wrench?
When you take a closer look at the movement, you will find that the rotation of the output shaft is not continuous. Rather a small burst of movements; that is an intermittent rotation.
Types of Impact Wrenches
Impact wrenches can be classified according to the power source used. Electricity and compressed air are the most commonly used source to power impact guns. Hence we have corded electric, cordless impact wrenches and air impact wrenches in the market.
Corded Impact Wrenches
These are electric impact wrenches that you need to plug into the electricity socket to power them up. An electric motor fitted in the rear end of the tool drives the hammer units which transfers its motion to the output shaft.
Pros:
- Better speed and torque control
- Easy to carry around since you don’t need an air compressor.
Cons:
- Expensive when compared to air tools.
- Overheating due to moving parts.
Cordless Impact Wrench
A battery powered impact wrench has one unique advantage over the other types. It is highly portable. You can throw-in one in your car trunk (boot if you are from the UK) and use it anytime you want to change the tires. The cordless design also enables it to be used in tight work-spaces where a corded or air pipe can be a hindrance.
However, the rechargeable batteries in a cordless impact wrench can get drained pretty quickly especially if you use it continuously on stubborn nuts and bolts. 
Pros:
- Highly portable.
- Ability to work in tight spaces
Cons:
- Most expensive option.
- Continuous use will drain the battery quickly.
- Heavy compared to corded and air versions.
If you decide to buy a cordless impact wrench, then get the one with a brushless motor. They are more efficient and produce less heat during operation. Also, get extra Li-ion rechargeable batteries.
Pneumatic Impact Wrench
These are compressed air powered impact wrenches. Pneumatic or air impact wrenches are widely used in automobile repair shops, automotive assembly lines, heavy equipment manufacturing etc. They have some distinct advantages when compared to electric wrenches.
An air impact wrench is simple in engineering design and hence they are relatively inexpensive. Since there are no electronic or electric parts, the chances of a malfunction are very rare. But the biggest advantage is that unlike its counterpart the air impact wrench won’t get heated. Any heat generated by the rotating elements inside the impact gun is cooled by the circulating compressed air. This makes the air impact wrench ideal for assembly lines since it can be used for long working hours continuously without worrying about overheating.
The overheat protection by the air flow and relatively simpler design makes air impact wrenches more durable. Additionally, they are easy to repair and maintain.
The obvious downside is that air impact wrenches need a continuous supply of compressed air. Most auto body shops and manufacturing units will have large air compressor units and to provide the adequate supply of compressed air at constant pressure. However, the small portable compressor found at the average DIY enthusiast’s garage may not be sufficient for heavy jobs. It will be frustrating to start working with your pneumatic impact wrench and then wait for minutes before the compressed air reaches the adequate pressure to let you continue with your work.
In addition to the standard ½ inch square shank, pneumatic impact wrenches also come with 3/8 inch, ¾ inch, and 1-inch shanks.
Pros
- Inexpensive
- The air cools the moving parts
- Longer tool life
Cons
- Requires uninterrupted supply of compressed air.
- Limited control over speed and torque.
Lack of speed control is something that you should consider. The pneumatic impact wrench I use has just two controls.
One is the on-off trigger. The second one is a forward-reverse switch which also works an air-flow regulator. When less air is allowed to go inside the wrench in effect it changes the output torque. However, this is not an accurate control like the electronically controlled speed regulators found in corded and cordless impact wrenches.
Cordless vs Air Impact Wrench
You already know the main difference between the electric impact wrench and the pneumatic one is the power source. But how do they affect the end user? Here is a quick comparison chart of the different types of impact wrenches.
| Cordless Impact Wrench | Air Impact Wrench | |
| Power Source | Battery | Compressed Air |
| Pros | Highly portable. Not obstructed by electric cable or air pipe. Better controls. | Smaller & Lighter. More powerful. Affordable. No overheating. |
| Cons | Expensive. Not suitable for continuous use due to heat generated by moving parts. | Require air compressor. Less mobility. |
| Working Principle | Impact force in rotational and spring force in the axial direction. | Hammer impacts only in rotational direction. |
| Controls | Accurate speed control | Minimal speed control by adjusting air flow |
| Uses | DIY, Car tire changing, Home improvement, Construction | Auto repair, Assembly lines, maintenance of heavy equipment |
Note: The air impact wrenches are smaller compared to m cordless impact wrenches. Hence it can go into a tight workspace. However, this advantage to be able to work in tight spaces should be taken only after considering the disadvantage of air connection required.
Safety
Compared to power drills and saws, power wrenches are relatively safer to use. However, there are few precautions you must take as a part of the safe working procedure.
Electric Shock
Beware of electric shock hazard while using a corded impact wrench. The cordless version also poses risk arising from damaged batteries and short-circuits. However, they are not life-threatening like cored wrenches.
Use Impact Sockets
If the nut or bolt is too stubborn the wrench socket will get stressed. Always use the correct impact sockets. These sockets have thicker walls and are hardened and tempered to slightly lower hardness to avoid shattering. You should also wear goggles to protect your eyes.
Noise Pollution
The average impact wrench produces approximately 90 – 100dB noise pollution. According to the white paper published by Hytorc on the effects of vibration on workers, If you use an impact gun with 100dB noise level it takes only 15 minutes of usage to reach the maximum allowable limit. In contrast you can work nearly 8 hrs with an impact wrench that produce 85dB(A) noise.
The hammer inside the impact wrench will produce loud noise. I strongly recommend you to wear ear protection equipment such as safety ear muffs or earplugs. While you may not see the mechanic at the car changing shop wearing it, hearing protection is very important for anyone using impact wrenches for more than a couple of minutes.
Summary: Who should get an Impact Wrench?
The impact wrenches are highly useful for removing rusted bolts, over-torques nuts and for driving fasteners that require very high torque force.
Impact wrench: Buy an impact wrench if you are doing auto repair, maintenance of heavy machinery and equipment, manufacturing and assembly units and large construction projects.
Impact Driver: Not everyone needs the power of an impact wrench. DIY enthusiasts, handymen, and hobbyists will find an impact driver or even a cordless drill/driver more useful.
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