Have you ever wondered what the proper way to use a torque wrench is? Do you wonder what a torque wrench is? This article describes the function of a torque wrench, when you should use one, how to use it properly, along with the do’s and don’ts related to torque wrenches.
What is Torque?
Torque is a rotational force applied at some distance from the axis of rotation. A more common definition might be “twisting force.” Any time you use a screwdriver, wrench, or knob, you are applying a twisting force, or torque, to the object.
Torque is commonly given in units of ft-lbs (“foot-pounds”), in-lbs (“inch-pounds”), or N-m (“Newton-meters”). For example, if you use a wrench to tighten a bolt, and that wrench is 1 ft long, and you apply a force of 100 lbs to the end of that wrench, you are applying 100 ft-lbs of torque.
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You can increase the amount of torque you apply in one of two ways, use more force, or increase the moment arm (distance from which the force is applied). Apply 200 lbs of force to the handle of a 1 ft wrench, and you are applying 200 ft-lbs of torque. Apply 100 lbs of force to the handle of a 2 ft wrench, you are applying 200 ft-lbs of torque.
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Why is Torque Important?
The main reason torque specifications are given in automotive or other mechanical applications is so that nuts and bolts are tightened to the correct, calibrated level of tightness. If a nut is too loose, vibration or some other force may cause it to come loose and fall off. Obviously this is not a good situation. On the other hand, if a nut is tightened too much, it can actually damage the threads, stretch the stud, or even break it off!
For nuts and bolts that are taken off and put on repeatedly (lug nuts, for example), it is critical not to over-tighten them. Why? Because each time they are over-tightened will bend the threads over a little more, so that the next time the nut is tightened, it will require even more force to achieve the desired tightness. And if the threads are deforming in one spot, this could eventually lead to a nut that won’t fit, or a broken shaft.
In the vast majority of cases most people, if they don’t have a torque wrench, will over-tighten a nut! Lug nuts that hold wheels on are a classic example. Over-tightening of lug nuts is the main cause of warped rotors – brakes that pulse and vibrate.
The Proper Way to Use a Torque Wrench
The whole point of using a torque wrench is so that the nut or bolt gets tightened to some pre-determined tightness. This value is determined by the engineers who designed the part as the specification between too tight and too loose. A calibrated toque wrench allows anyone to achieve the proper torque specification, provided it is used correctly!
These are the steps to take to use a torque wrench properly:
- Ensure the threads are clean, rust-free, dry, and undamaged.
- Tighten the nut by hand or with a wrench until it is snug.
- Set the correct torque value on the torque wrench.
- Apply the torque wrench to the nut and tighten it until you hear a click, then back off.
- Apply pressure again until you hear another click. Don’t click it more than 2 times.
- The nut is now properly tightened.
How to Set the Torque Value on a Click Torque Wrench
Setting the torque on a click-adjustable wrench is fairly straightforward but requires knowledge of using a vernier style gauge.
- Loosen the knurled locking nut at the end of the torque wrench.
- Twist the knurled handle until the top edge of it is even with the desired torque value on the scale.
- Tighten the locking nut on the end of the wrench to lock it at this setting.
- Use the torque wrench to tighten the necessary equipment.
- After using the wrench, loosen the locking nut and set the wrench setting to zero before storing.
This torque wrench manual pdf walks through the process for a click-adjustable torque wrench.
Here is an example of setting the torque value. In this case we want to set the torque wrench to 76 ft-lbs.
Step 1: Loosen the knurled locking nob
Step 2: Turn the handle until the correct torque value
For this step, we start with the torque setting on zero, since it should always be stored at zero.
Twist the handle to increase the torque. In the photo below, the torque value is 70 ft-lbs, since the top edge of the black handle is even with the line indicating 70, and the 0 on the handle is lined up with the center line.
In the photo below, the torque value is 76 ft-lbs, since the upper edge of the black handle is slightly above 70, and the 6 on the handle is (nearly) lined up with the center line.
Step 4: Use the Torque wrench
Now, use the torque wrench to tighten the lug nuts in a star pattern. Notice the proper way to hold the torque wrench: one hand should be on the head of the wrench to ensure that it is properly seated on the nut and that the rotation occurs about the axis of the nut. The other hand is on the handle of the wrench applying the force.
Step 5: After use, Reset the torque wrench to Zero
Reset to zero (or the lowest setting on the wrench) by loosening the knurled locking knob, and twisting the handle until the reading is zero again.
Torque Wrench Selection Guide
There are several different types of torque wrenches available. Here we discuss the types, the advantages and disadvantages of each, as well as the most common.
The first thing to decide is the drive size that you will need. For most applications the 3/8″ drive will be appropriate. For most automotive applications, a 1/2″ drive is the better choice since they typically have a higher maximum torque value.
Click Adjustable Torque Wrench
The click adjustable is the most commonly used torque wrench. These torque wrenches offer very high accuracy. When the dialed in torque is reached, the wrench makes a clicking sound and moves past a pivot point slightly, indicating the proper torque.
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Click Adjustable 1/2″ drive Torque Wrench – Buy on Amazon – (FYI, this is the wrench I have and recommend)
Beam Torque Wrench
The beam torque wrench is the old-school style. These tend to be low-cost and relatively easy to use. These are becoming much less common than they used to be. The beam torque wrench relies on the bending strength of the wrench arm. A needle points to a placard with calibrated torque values. Tighten until the needle is pointing at the correct specification.
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Digital Electronic Torque Wrench
The digital electronic torque wrench is the easiest to read, since the torque value is displayed on a digital screen. These are more expensive than the other types of torque wrenches, but do not require setting the desired torque value ahead of time.
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Other Considerations When Using a Torque Wrench
In addition to using the torque wrench properly, there are often other considerations to keep in mind when using a torque wrench. The following are some of the more common.
Tighten in Proper Sequence
In addition to getting the torque correct, it can be important to tighten nuts or bolts in the proper sequence. For example, it is common to tighten 5-bolt lug nuts in a ‘Star’ pattern in order for the wheel to be properly seated when tight.
Other applications would include cylinder head bolts for avoiding a blown head gasket. In cases such as this, not only are the bolts tightened in a certain sequence, but also in multiple stages. For example, 3-rounds of tightening may be used in order to assure that the head gasket is compressed evenly all around.
Torque Conversion Factors
It is important to pay attention to the units used in the torque specifications you are working with. For example, ft-lbs and in-lbs differ by a factor of 12 (12 inches in a foot). It is also important to note the units on your torque wrench. Many wrenches have multiple scales, so make sure you are using the correct scale. Below are some common conversion factors between units of torque.
- 1 ft-lb = 12.0 in-lbs
- 1 ft-lb = 1.36 N-m
- 1 N-m = 0.74 ft-lbs
- 1 N-m = 8.85 in-lbs
- 1 in-lb = 0.083 ft-lbs
- 1 in-lbs = 0.113 N-m
Torque Extender (Multiplier)
There are cases where the torque specification required is greater than the maximum your torque wrench can provide. In this case, a torque extender can be used. A torque extender is essentially an arm of known length that increases the torque applied by increasing the moment arm of the applied force. In this way, the torque applied on the torque wrench is applied to the end of the torque extender, which in turn applies that force at an additional distance from the axis of rotation.
When using a torque extender, you are actually applying a greater torque to the nut or bolt than what is read on your torque wrench. In order to apply the proper torque, use the following formula to determine what setting you should use on the torque wrench.
Wrench-Setting = Desired-Torque × (Length-of-Wrench / Length-of-Wrench + Extender)
For example, if your desired torque is 200 ft-lbs, the length of the wrench is 1 ft and the length of the extender is 0.5 ft, then the wrench setting should be:
Wrench Setting = 200 × ( 1 / 1.5 ) = 133 ft-lbs
When measuring the length of your wrench, measure from the center of the axis of rotation to the center of the handle grip.
You can make your own torque extender fairly easily. Here is an example of a home-made torque extender made out of a piece of pipe by a friend of mine.
First he welded a good-sized nut to one end of a pipe, and drilled a hole near the other end.
When he’s going to use it, he puts the appropriate-sized socket onto the welded-on nut.
Then attaches the torque wrench to the socket.
And puts another socket into the end of the pipe. The hole in the pipe is there because a pin can then be put through that hole that aligns with a corresponding hole in the second socket wrench. The reason for this is so that the precise length of the extension is known.
Here is an example of this torque extension being used to tighten the axle nut on a Toyota Sienna.
The use of socket extensions with a torque wrench is perfectly fine. However, it can make it slightly more difficult to make sure that the torque is being applied squarely. Make sure the extension is not angled, even slightly, in such a way that additional torque is being applied.
Do’s and Don’ts
The following are some Do’s and Don’ts related to the use of a torque wrench:
- Reset the torque wrench to ‘zero’ when done using it and for storage.
- Look up the proper torque specifications in the owner’s manual or an official website. Don’t guess!
- Handle with care. This is a calibrated instrument and should not be jumbled around with other tools. Keep it in its case if it has one.
- Apply force to the handle only. The torque is calculated based on force being applied to the handle. Applying force on other parts of the wrench will result in a different torque value being applied.
- Make sure the wrench is on the nut squarely. The force should be applied at a 90° angle to the axis of rotation at the handle. The load should be balance over the pivot point.
- Don’t continue forcing the torque wrench after the click. This negates the whole purpose of using a torque wrench!
- Don’t use a torque wrench as a breaker bar. It can be tempting, since it is typically a long-handled wrench, but using it as a breaker bar will cause the mechanism to go out of calibration.
- Don’t lubricate the threads upon which you are tightening (unless specified by the manufacturer). In general, the threads should be clean and dry.
- Don’t drop a torque wrench. This can cause it to go out of calibration significantly.
Proper use of a torque wrench for tightening nuts and bolts to specification is not only safe, but will extend the life of your mechanical parts for years to come. Thanks for reading, and check out these helpful vehicle maintenance articles.
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