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The article authored by Andy Pugh in i Saluti, “Threaded Fastener Facts”, has prompted me to share some little information regarding torque wrenches.
The torque wrench we know today was first developed mid 1930s through corroboration between Micromatic Hone and the Chrysler Corporation. After the initial designs, Cedar Rapids Engineering Company manufactured and sold the first wrenches commercially. The very first individual to sell the wrenches on commercial basis was P.A Sturtevant.
Presently, there are several types of torque wrenches available in the market, however only three are commonly used and will most likely be available in your nearest auto shop, namely:
- the incased beam dial-indicating torque wrench,
- the rigid frame toggle action wrench and
- the direct-reading bending beam torque wrench.
I am only going to elaborate more about these three in this article.
Direct-reading Bending Beam Torque Wrench
Featuring a rugged construction, the direct-reading bending beam is not affected by fatigue and wear like the others. It will therefore remain accurate whenever the pointer is set to zero and if it does not break. I have one that I use regularly and it has lasted over 30 years now. This bending beam configuration has a few disadvantages though:
- The measuring scale should always be visible when using the wrench
- Sometimes it gets difficult to pull the exact values
- You will need to apply extra force through the handle pivot as because you cannot use handle lengtheners.
Operating the direct-reading bending beam wrench is however simple and easy. When you apply force on the fastener, the beam will bend to a certain degree with the pointer remaining straight and indicating on the scale the amount of torque that has been applied on the fastener.
This torque wrench has a handle that is mounted on the pivot to concentrate the applied force on a particular place on the beam ensuring great accuracy–see Fig. 1.
Toggle Action Torque Wrench
The second wrench, which is the toggle action torque wrench, is more dominant in auto shops than the other wrenches mainly because it is inexpensive and easy to use. To operate this wrench, you will need to set the pointer to a predetermined position first. When using it, you will hear a click once the set value has been reached.
Built with a rigid housing, the toggle action wrench uses a coil spring with a measuring element. The adjustment screw is set on different positions to impose different spring pressures on the toggling mechanism. The spring pressure holds the load lever, toggle and toggle slide bar end surfaces tightly together. As you apply force to the handle, the fastener will turn until the turning force overcomes resistance from the pressure on the spring element. When this resistance is overcome, the end surfaces that were being held tightly together will break apart. The wrench will now become rigid because the load lever will be resting against the inner surface of the wrench. A click will be heard as result of the toggle action breaking the end surfaces apart and by the load lever striking the surface of the frame. Any more force applied on the handle after the click will only apply excess torque to the fastener than the one that has been set already by the adjustment screw.
The toggle wrench has some advantages over the other wrenches including:
- One is able to repeat applying torque
- You don’t have to see the measuring scale when using the wrench
- Its drive end can be fitted with a ratchet
- It is relatively cheaper when compared to the other wrenches
Although the toggle action wrench is vulnerable to wear, you can repeat torque application pretty consistently during a session. However, the set accuracy may be degraded if the wrench is not properly cared for. The action of this wrench is as shown in Fig. 2.
Incased Beam Dial Indicator Torque Wrench
The last wrench that I will address in this article is the incased beam dial indicator equipped torque wrench. This one is designed with a stout beam as the measuring element. It has a rotating drive head which attaches the beam that rests on stops on the other end. Any torque applied on the fastener causes the beam to deflect by a small value. However, this value of deflection is mechanically magnified and will read torques in the dial indicator as Newton-Meters or Pound-feet–see Fig. 3.
There is a different type of a dial indicator equipped torque wrench which uses a torsion bar as the measuring element. However, because hand held torque wrenches have size limitations, the torsion bar is very short. This therefore result in angular movement or twist that is limited as well. Because this wrench uses a mechanism that is rarely used, I did not include an illustration for this wrench.
The incased beam and torsion bar tools that I have discussed above have delicate and complex internal mechanisms which like many other tools are subject to friction losses and wear. They don’t do well in greasy and dirt environments, require constant calibration and are a little bit expensive. Also, only high-end auto shops are likely to stock the dial indicting wrenches.
Both indicating and adjustable torque wrenches should be able to deliver accuracy that is within 10% of readings that are between 30% and 100% of the full scale. Nonetheless, you should avoid extreme ends of the scale just like for other measuring instruments.
Torque Wrench Adapters
If you need to torque a tubing fitting or you are using a fastener which is not accessible directly on its axis, then you can use an adapter with your wrench to get the job done. An example of an adapter is a crow’s foot attached to a torque wrench. You can fabricate adapters from your old tools or get one cheaply from your neighborhood hardware stores.
They are very useful in extending the centerline of the wrench or extending the drive end out at 90 degrees. Moreover, it is possible to extend the drive end out at any angle within the range of 0 to 90 degrees. However, for angles beside 90 degrees, you will need to do some math and also mind the dimensions of some critical parts of the wrench. Adapters that extending from the drive ends at 90 degrees do not have scale reading error as shown in figure 4.
Ratcheting type torque wrenches are an exception when it comes to use of adapters. This is because distance between the torque centers changes as wrench ratchets around the drive head. The only exception is when the ratio of the torque distance (A) to the torque arm length (L) is less than 0.10. The crows-foot that is used for torqueing a tubing fitting seldom exceeds this A/L ratio.
Handy Tips That Should Be Obvious
Here are a few tips that you will handy when dealing with torque wrenches:
- You should always buy a high quality fastener, also when applying torque, ensure the wrench is perpendicular to the fastener’s axis.
- Clean threads will ensure efficient application of the wrench. Lubrication of the threads should be done giving regard to the particular design.
- Before buying a torque wrench, you should be sure it is the correct one for your application range
- Like any other tools, keep your wrenches cleaned and organized in a box or carefully organized in your work area.
- When storing the toggle type wrenches, the dial setting should be within the lower 25% of the scale range.
- Torque wrenches should only be used for their intended purposes.
For more tips on proper handling and storing torque wrenches, go here.
While working on this article, I did question why a handle extension cannot be used on a rigid-frame wrench. It was clear to me that rigid-frame wrenches can be grasped anywhere along their length. Why would this then not extend beyond their lengths? Craftman’s Micro Tork comes with use and care instructions which warn against using an extender. In fact, they state in bold that; “Their use will result in erroneous torque readings, and may damage the grip or adjusting screw.”
Personally, I did come up with a contrived device that could extend the handle while not causing damage on the grip and the adjusting dial in my garage. The rudimentary tests that I carried out did not prove that an error was introduced by the handle extender. Nevertheless, I would still follow the recommendations from the manufacturer. Although I’m not a physicist, I have done extensive research and therefore highly suggest that you get one of the best torque wrenches I have recommended because of their ease of use and numerous other features.
The torque values listed above are approximate and as such should not be taken as accurate limits. There are several indeterminant factors such as type of plating, lubrication and surface finish in very specific applications that prelude publication of values that are accurate and that can be used universally. Different manufacturers depending on the type of equipment they manufacture will usually provide very specific instructions that should be followed when tightening. Furthermore, the above values should not be used for joints of soft materials and for gasketed joints.