We’ve all been there. You’re working on that important project, and your biggest holdup is that stubborn nut that just won’t budge. It may be rusted out or in such tight quarters that you can’t get the leverage you need. But you don’t care what the reason is —this nut has to go.
You need more force than your socket wrench can apply, so you reach for your impact wrench. A few pulls on the trigger and within seconds that stubborn nut is loose. But how did it do that? The answer: torque. Inquiring minds want to know, but even the less inquisitive need enough working knowledge of this rotational force to select the right tool for the job. So here’s your user’s guide to torque.
What is Torque?
Simply put, torque is the amount of work required to make an object rotate. Formally, torque can be calculated by the equation T = F x d sin ꝋ, where F represents force, d stands for distance (which in tool terms would be the length of the lever arm), and ꝋis the angle between the force and lever.
Sounds complicated at first, but since force is applied perpendicularly to the shaft, torque is typically just the product of force and shaft length
Units of Torque
In SI units, torque is measured in Newton-meters, or N∙m. In English units, torque is either measured as inch-pounds (in∙lbs) or foot-pounds (ft∙lbs), depending on the magnitude.
A quick glance at the units, and some will notice that they are the same as those used to denote work. This has caused people who are less familiar with torque to confuse it with work, but they are very different from each other.
Work is the total amount of energy required to exert a force through a given distance, whereas torque is simply the amount of rotational force generated by applying a linear force at a perpendicular lever arm. Especially important for tightening or loosening, torque also has a direction (clockwise or counterclockwise), while work does not.
All the physics and math aside, torque tells you how much force you’ll take to that stuck bolt.
Torque Terminology
With such a clear physical definition, you would think that tool manufacturers would have a uniform standard for conveying the torque output their tools supply —but this sadly isn’t the case. There is no set industry standard for stating how much torque a tool can deliver, leaving suppliers room to frame their tools’ capability on their own terms. This causes confusion, as customers aren’t sure how to compare products to find the best tool.
To help you compare apples to apples, here is some torque terminology you’ll need to know.
●Time interval. As you’ll see later, impact wrenches apply force over a certain amount of time. This is called the time interval. The longer the time interval, the greater the maximum torque your tool will have —and the longer itwill take to do the job.
●Working torque. Typically considered the average torque exerted on most jobs, working torque is defined as the amount of forward torque applied over a time interval of 15 seconds to fasten your nuts.
●Maximum torque. Just like working torque, max torque is calculated over a 15-second time interval and is the greatest recommended torque before stepping up to a higher performance tool. This means that we don’t try to deceive you with greater numbers on our maximum torque by applying it over a longer time interval; the value you see is what you get.
●Breakaway torque. The amount of torque it takes to get a stuck nut to begin to move is the breakaway torque and is where static torque becomes dynamic.
●Nut busting torque. As with breakaway torque, this is how much torque it takes to start the motion of a stubborn nut. It’s measured by using a torque wrench with a torque multiplier until reaching the maximum nut busting torque. Then it’s loosened with an impact wrench within 30 seconds.
Note that these are just general terms, and some manufacturers may define their torque outputs differently. So, to truly compare which impact wrench is best, you’ll need to know how long they used it to tighten their bolts, and what their testing process is.
Types of Impact Wrenches
All those theories and terms are great, but you have a job to do. When your project is stuck because of your nuts, what type of impact wrench do you reach for? There are two basic kinds.
Electric
Whether corded or battery-powered, these impact wrenches use an electric motor with a spring attached to the input shaft, which is connected to a hammer. As the shaft rotates, the anvil makes contact with the hammer, pressing it back against the spring. When contact is released, the spring thrusts the hammer forward, making impact with the output shaft, and driving the nut into place as it rotates.
Some advantages of electric impact wrenches are:
●Reliability
●Portability (if battery-powered)
●Fewer vibrations
●More precise control over the amount torque applied
A few downsides are:
●Higher cost
●Less durable
●Increased risk of overheating
●Corded versions are not portable, and battery-powered ones must be charged
Pneumatic
Instead of relying on an electric motor, pneumatic impact wrenches flow air through a nozzle that makes contact with a rotating cylinder called a rotor. The rotor has fan blades attached to it which catch the airflow and cause the rotor to spin, creating rotational energy. The rotor is connected to the hammer by a spline, so as the fan rotates, the hammer spins with it and strikes the output shaft —thereby generating torque.
Some of the biggest advantages of air-powered impact wrenches are:
●Less overheating, because the incoming air cools the system
●Less expensive
●More durable
As for the drawbacks:
●More vibrations can be uncomfortable for projects involving a large number of nuts
●They’re dependent on an air compressor for a constant air supply
Although some control can be maintained by adjusting the airflow, air-powered impact wrenches do offer less variability in the amount of torque they offer —and as with all pneumatics, don’t forget to check your air lines if you experience a problem.
Torque and Impact Wrench Performance
Whether electric or pneumatic, there’s a wide variety of impact wrenches on the market today. So how do you know which ones are right for you? Here are a few things to keep in mind:
●Output. Some impact wrenches can provide hundreds of foot-pounds of nut-wrenching force, and our 1/2″ Twin Power impact wrench delivers 1,000 ft∙lbs of torque at 8,300 rpm. That means that however much torque you need, you’re likely to find an impact wrench for the job.
●More isn’t always better. Once you’ve overcome the breakaway torque, your nut is likely to spin much more freely. That means you only need a large amount of torque for the time it takes to get your nut moving, and after that, a lower torque value will suffice. Consider the application, and don’t go overboard by assuming more torque is better.
●Driver vs. Wrench. Many impact drivers provide enough torque for light-duty, but there are differences between the two. Impact drivers can provide more precision and control, while impact wrenches are better for larger fasteners. A good rule of thumb: if your project requires a socket adapter, go for the impact wrench instead.
With so many different types of impact wrenches to choose from, consider the application and output you think you’ll need, and remember that these tools are best where stripping or damaging your surface is unlikely.
Torque Meters
You deserve a tool that makes good on its promises, but how do you know your impact wrench is delivering? Thankfully, even though those testing standards can be ambiguous, torque meters can show you how much rotational force you’re really getting.
Types of Torque Meters
There are two types of torque meters: rotary torque sensors and reaction torque sensors.
The latter measures stationary or static torque —the rotational force applied before the object begins to move —while the former measures dynamic torque, or the force applied as the object is moving.
While both use strain gauges or sensors as a transducer to convert rotational force into an electrical signal that can be easily read, the difference between rotary and reaction torque sensors lies in where the strain gauge is located. Rotary sensors have their gauges placed between the motor (or rotor) and the output shaft to detect how much force it is subjected to. They are typically fitted onto the shaft by slip rings or bearings. Common applications include transmissions, turbines, and generators —and of course, impact wrenches.
Since reaction sensors measure torque that isn’t enough to get the nut rolling, it wouldn’t help much to mount them on a shaft. Instead, reaction sensors are placed in between the motor and a fixed ground point. The shear forces between the ground and cell can be measured to calculate the amount of torque placed on them by the load.
So how do you read a torque meter? That’s even easier: just connect the sensor to your load, and you’ll receive a reading of how much torque is being applied. Just be sure the output matches your application so that you get a reading that matches your setup.
Turn The Wheels With Torque
Mechanics like you have no time to waste on stuck nuts. You need tools that can break them loose, so you can get on with your project.
Whether it’s a pneumatic impact wrench or a meter that proves you’re getting the force you paid for, Capri Tools has what you need to get your project going. Check out our impact wrenches, and never get stuck on a stubborn nut again