What Is Breakaway Torque?

Emma G.
Emma G.
Breakaway torque can refer to the force needed to turn a faster like a nut.
Breakaway torque can refer to the force needed to turn a faster like a nut.

In physics and mechanics, torque is rotational force, or the force necessary to cause an object to rotate around an axis. In most cases, more torque is needed to start the rotational motion than is needed to keep it going once it has begun. This initial force is called breakaway torque.

The amount of breakaway torque needed to move something is determined in part by static friction. Static friction is the force that exists between two physical bodies to keep them from moving. For example, a wing-nut may have high breakaway torque because there is a lot of static friction between the nut and the bolt. If, however, the bolt is greased, the torque will be lower because the static friction has been decreased.

Torque is needed to turn the crankshaft in an engine.
Torque is needed to turn the crankshaft in an engine.

Breakaway torque is almost always discussed in one of two contexts, either the power of an engine or the force needed to turn a threaded fastener such as a nut. If a mechanic wants to remove a nut from a bolt, he or she must apply torque to the nut using a wrench. As anyone who has ever done this knows, turning the wrench takes a lot of force at first, but usually becomes easier after a few turns.

Mechanics and engineers often measure the breakaway torque of threaded fasteners in part to ensure product integrity as part of a process called torque auditing. If the breakaway force of a nut is too low, the vibration of the equipment may cause the nut to loosen; if it's too high, the threads may strip and the bolt may be impossible to remove. Part of testing the fastener includes measuring the torque at the moment the fastener starts moving, after the torque has been overcome. Auditing can either be done using sensors to measure the torque or by a trained operator applying torque by hand.

Another area where breakaway torque is important is in cylinder engines. As with threaded fasteners, the breakaway torque of an engine is greater than the running torque. In an engine, torque is used to spin a crankshaft. The crankshaft in turn causes the pistons to move up and down.

An engine that is designed with only enough power to keep the engine running will fail. The engine must also have enough power to get the movement started in the first place. Yet it is a delicate balance. If the engine is allowed to continue running with enough torque to cause breakaway, it may overheat. Once the breakaway has been achieved, the engine torque should be reduced to a normal operating speed.

How To Calculate Breakaway Torque

Calculating breakaway torque correctly depends on each different situation. First, there are different factors to consider for actions that a human produces versus the ones that a machine does. There are also many similarities. In general, figuring out breakaway torque relies on figuring out all the different forces at play on an object to change it from being at rest to mobile. These forces include both the force the person or machine exerts and the friction forces that work against it.

The General Formulas

The simplest version of the equation that determines force and friction is F=ma where F stands for force, m stands for mass, and a stands for acceleration. When multiple different forces are involved, such as in breakaway torque situations, this formula becomes more complicated. One needs to figure out each different force that exists on an object at any one time and then apply a force greater than the sum of all those forces together to break the object away and cause it to rotate.

Torque is a special kind of rotational force that requires specific modifications to this equation. To calculate torque forces, use the equation T= rFsinθ, where T stands for torque, r stands for radius (usually the length of the lever used), F stands for force, and θ stands for the angle between the force being applied and the radius or lever length. Generally, torque is measured in N-m or newton-meters.

Ways To Lower the Required Initial Torque Necessary

Once a person figures out the minimum amount of force needed to cause rotational movement in a static object, that person has discovered the breakaway torque. Some additional factors can then help to achieve breakaway torque. Lengthening the leveraging object can help achieve a greater downward force to encourage an object to start moving. For a person, this leverage might be a wrench, while for a machine, it is usually the crankshaft and pistons in the engine.

One can also increase the height at which the lever starts, thereby increasing the angle and the force applied to help create greater overall torque. As the above equation for calculating torque indicates, increasing this angle multiplies its effects into the equation and increases the ending amount of torque. In breakaway torque, this helps apply more torque force without having to increase the weight, distance, or mass. Basically, think about this as a form of increasing the acceleration from the first formula, F=ma.

Lubricating the object is another way to help achieve the breakaway torque required. Lubrication causes the frictional forces to decrease. Lowering the forces of friction in turn lowers the amount of force needed to initiate the rotation of an object. Some of the more common lubrication substances are oil and grease.

Another way to affect the friction forces is to use a rough cloth or pad to grip the object to help move it. The uneven surfaces on the object apply additional friction to the object and can help even out or lower the effects of the frictional forces present in the stationary object. This method is especially useful for human-applied torque but less practical for machine-applied torque.

What Is a Breakaway Torque Converter?

A breakaway torque converter is a specially designed part in a machine engine that helps create the needed breakaway torque forces. A torque converter is an additional way to help apply the necessary force to move a stationary object. This device is generally a donut-shaped fluid coupling in combustible engines that helps transfer the rotational forces or power from the engine to the load it is trying to move. One common example is found in automobiles. Automobile torque converters exist between the engine and the transmission specifically in automatic transmission vehicles.

Discussion Comments

anon997613

In response to post 5,

The reason your breakaway torque doesn't match your applied torque is due to friction and directions of forces.

When you apply torque to a bolt, you first have to apply enough torque to overcome the friction forces between them, then after that, the applied torque is going into building up a static force between the mating forces of the two sets of threads that holds the bolt down at a particular set force due to that friction you built up.

As you loosen the bolt, you still have to first overcome that set friction, but now the static force is being released in the direction that it wants to go, and is slightly assisting you in loosening.

anon289473

@everetra: Torque wrenches are tools used to precisely apply a specific torque to a fastener such as a nut or bolt. It is usually in the form of a socket wrench with special internal mechanisms.

anon277127

Why doesn't breakaway torque match torque? I was torquing a bolt to 46 ft lbs with a calibrated torque wrench. I waited several minutes and performed a breakaway torque using the same wrench and the results were roughly in half @25ft lbs. Any idea why?

nony

@hamje32 - Yeah, you can buy different tools with torque calibration and they all work in a similar manner.

For example, I used a torque screwdriver to remove a shower head from my bathtub shower. The reason I used that tool was that an ordinary screwdriver wasn’t giving me enough force and it kept slipping from the thread.

I even tried to get a screwdriver with a longer handle. Then a friend suggested a torque screwdriver – he actually lent me his and showed me how it worked.

I adjusted the settings until the screw came out cleanly and I was able to change my shower head. Work is so much easier when you have leverage, and that’s what the torque tools give you. They’re not just for mechanics. You can use them in everyday household projects.

hamje32

@everetra - One thing I’d like to point out is that your torque wrench calibration accuracy will vary depending on how you handle the tool.

For example, you can set it to 80 lbs, but if you hold the wrench at a right angle you will get one measurement, whereas if you hold it another way you’ll get a different measurement.

You will more or less still be around 80 lbs, minus a pound or two here or there. It’s just something to keep in mind.

Engineers need precision when they use these tools, but if you’re just doing stuff around the house then the approximations will probably be good enough for your purposes.

miriam98

@everetra - What you normally call a socket wrench would classify as a torque wrench in most cases; the principle is the same, the wrench uses a rotational motion to give you added force (or torque) which you apply against the object.

One of the things you need to pay attention to however is the quantity of the force that you want to apply. They measure these quantities in foot-pounds, so for example you can buy a torque wrench that has a very long handle and delivers anywhere from five to eighty foot pounds.

You can adjust it for the different forces you want. For your project, you should try to get an estimate of how much torque you need and then buy a wrench that will give you that amount of force.

I don’t think that ordinary socket wrenches can be calibrated for all of these situations, but it would probably depend on what kind you have.

everetra

Does anyone know what is a torque wrench? I was told that I need to get one for a construction project that I am working on.

I have a regular wrench and even a socket wrench. I am assuming that a torque wrench is similar to a socket wrench but I’m not sure.

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    • Breakaway torque can refer to the force needed to turn a faster like a nut.
      By: brankatekic
      Breakaway torque can refer to the force needed to turn a faster like a nut.
    • Torque is needed to turn the crankshaft in an engine.
      By: dany
      Torque is needed to turn the crankshaft in an engine.