There are two basic types of torque wrenches on the market, what are called “beam” wrenches and “micrometer” wrenches. Beam wrenches count on the built-in flexibility of any material. As torque is applied, the wrench flexes, while another rod, with a pointer at the end, indicates torque being applied on a scale.
Micrometer wrenches are pre-set to the required torque on a vernier scale. An internal spring loaded mechanism measures the torque. When the bolt is tightened to the desired torque it “clicks.”
Of the two types of torque wrenches, the micrometer wrench is definitely the more accurate. Typically, they are rated as being accurate to four percent of torque set. However, this accuracy can easily be lost if the wrench is not reset to zero before storage. Beam wrenches don't have this possibility of losing their accuracy, but are less accurate right out of the box. The accuracy of these beam wrenches is typically 12 percent, but can be less on higher torque settings, especially if the operator's hand is shaking from the pressure they are applying.
If you are buying a torque wrench for your car, you should buy a 1/2 inch square drive tool, as 3/8 in drive tools only work on lower torque settings (typically below manufacturer's specifications). Adapters are available to reduce the 1/2 inch square drive to 3/8 inch square to fit the more common socket size that most people have.
When tightening a bolt to the proper torque, it is essential that your hand which is pulling the wrench is cantered on the handle of the wrench. Having it too close or far from the socket actually affects the ultimate torque value you are tightening to. Moving your hand closer to the socket reduces the total “foot-pounds” of torque you are applying, while moving it away, closer to the end of the wrench increases the “foot pounds of torque you are applying.”
Most manufacturers of torque wrenches supply them with a plastic storage box to protect them. They also often come with the 1/2 inch to 3/8 inch square drive adapter; some even come with a drive extension.
Torque wrenches are precision tools and should be treated as such. Although most micrometer wrenches have a reversible ratchet head, they really should not be used as a ratchet, and especially not as a breaker bar. Most people who break their torque wrenches do so by trying to loosen an overly tight bolt with it. This overly stresses the internal mechanism, causing it to break, especially if the micrometer isn't set to a higher torque value than the pressure that is being applied.
A properly cared for torque wrench will provide years of practical, quality use to the tradesman.
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