If you ever cared to crack open your owner's manual, you likely have ran across several instances where the manufacturer specifies a torque value for screws and bolts. In fact, most modern dirt bike manuals actually have a torque chart listing nearly every fastener on the bike. Why is this? What difference does it make? Plenty of folks work on their bikes and just tighten screws by feel and call it good. Granted, an experienced mechanic may be able to pull this off, but there is a good chance most of the time tightening is incorrect, technically. Have you ever had problems with screws falling out, gaskets leaking, screws breaking or threads stripping? How about parts coming loose? Sometimes, these problems are a result of improperly tightened screws.
Consider the following excerpt from one of my other articles About threaded fasteners":
A bolt or screw, when properly tightened, should act as a retaining spring so to speak. When tightened to the specified torque value, a bolt (or screw) should have a resulting tensile force. The resulting tensile force (force in tension), ideally would stress the bolt material enough that it would be in its elastic range. This means that if the bolt tension is released, the bolt would spring back to its normal relaxed state. If the tightness of a bolt exceeds the elastic characteristics, the material is then stressed so much that it yields. The stress region where yielding occurs is called the plastic region. It is in the plastic region that, when released, the bolt cannot return to its normal geometry. Stripping of the threads could occur before this permanent stretch , but in either case, this would indicate a failed fastener. To summarize a bit, it is the spring action of tightened bolt that holds a joint together properly. The amount of tension required is by design which is why torque values are specified by manufacturers.
The above should now give you a clue as to why there are grades of bolts. It is not only about the strength of a fastener but also the application. If for example you were to use a very high strength screw to hold a piece of plastic on an aluminum frame, it would be impractical to tighten the screw to its design torque. The plastic material in this example may continue to crush and end up being damaged while screw torque would never practically be reached. In an application such as this, a fastener of the appropriate strength could be used having a torque specification that is in line with the properties of the plastic being mounted. Now you know why there are insert bushings in the screw holes of dirt bike plastics: to provide for proper tightening while protecting the plastic from damage.
There is another way to look at torque however. Assuming we have a fastener that is stronger than necessary, a torque could be specified such that the elasticity of the parts being fastened provide the spring force needed to exhibit a properly bolted (or screwed) joint. Of course I am speaking here from the perspective of engineering. The important take away here is that torque is specified for a reason and you, the mechanic or service technician, should realize as such for the benefit of the machinery you work on. Torquing a fastener is simple, but wrench designs do vary. How you set a wrench and how you read the torque depends on the type of torque wrench being used.
A beam torque wrench has no setting. It has a numeric scale in which a pointing needle attached to a deflecting beam rides across. As the applied torque increases, the beam deflects more. The scale is observed during tightening. To achieve accurate torque values with this type of torque wrench, the user must hold it properly. The handle of a deflecting beam torque wrench is design to facilitate this by employing a pivot pin in the handle. Correct use is to apply force on the handle such that the handle remains balanced on the pivot pin.
A click style torque wrench needs to be set to the desired torque. This is done by rotating a barrel adjuster (which is also the handle) to the setting. Typically, the top of the barrel lines up to scale markings embossed into the wrench body. A bolt is tightened until a distinctive click occurs. It is important to realize that a bolt can be over tightened if the click is ignored, as the click does nothing to limit tightening.
There are other types of torque wrenches as well such as dial types that use a dial type scale. Digital types are now prevalent from ones with digital readouts to models that are fully programmable with the capability of storing measured values. Whatever type is used, there are some fundamentals of fastener torque setting that should be adhered to. A slow, smooth tightening action, correct placement of the hand on the wrench handle and having fasteners and/or fastener threads that are free from defect are important for accurate results. For example, if a bolt is bent or has an egg shaped diameter, there is no way the resulting torque will be correct. Finally, here is an excellent video by Summit Racing discussing several different types of torque wrenches:
Types of Torque Wrenches - Summit Racing Quick Flicks . 2:46 minutes.
Torque wrenches are available in different drive sizes and torque ranges. From the smallest screws to the largest bolts, there is a torque wrench suitable.
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