by Curtis Von Fange
When working with fasteners a torque wrench can play an important role in the correct assembly of critical equipment components. Lets examine in detail some of the reasons for using a torque wrench along with how to correctly use the variety of wrenches available.
Applying the proper torque to a fastener accomplishes a number of tasks. First of all correct torque will hold the parts together. This may seem like a trite statement but think about it for a second. Engineers have designed the fastener used in a particular application to accomplish a certain task, that being, to hold a part together under normal and, often times, abnormal use. Field use of equipment compounded by jarring and pounding by surprises under the field grass can stretch fasteners to the limit. Proper torque ensures that one is getting maximum safe usage from a capscrew or bolt and that the equipment will not loosen up and fall apart in the field.
Another purpose of proper torque on a fastener is to secure the mating parts in such a fashion as to keep warping or distortions from occurring. Internal engine components not brought to proper torque can ruin an engine in very short order. Consider a cylinder head. The mating surfaces are milled to extreme tolerances; much finer than the common eyeball can observe. When installing the head to the block assembly the correct pattern of tightening and torquing the related parts will prevent leakage of compression gases and warpage of the cast iron components due to heat and related stress. Another example pertains to the crankshaft areas. When connecting rods are mated to the journal caps the correct bolt torque will keep the parts from flying apart at the high rotation speeds while still providing the necessary oil clearances so the spinning parts dont seize.
Torque also provides for a proper crush on a gasket to provide for the most optimum sealing. A gasket too loose will leak and drip; one that is too tight will be excessively crushed causing leakage at a later time. The correct torque is matched to the relative gasket crush so it will still be flexible and withstand leakage when the matched parts heat up and expand. Always check when installing a part or engine component for the correct torque setting.
Torque wrench construction is based on a principal of physics found described in Hookes Law. By bending a steel beam, the relationship between the pull, or torque, on the handle and the amount of beam deflection can be readily determined. In the case of our wrench, if the handle is one foot from the head that covers the fastener and that handle is pulled thereby deflecting the beam, a fixed rod attached to the wrench head pointing to a scale will correctly indicate the amount of beam deflection registered in units called foot-pounds. A ten-pound pull on the handle will deflect the beam a given distance registered by the fixed pointer and scale as ten foot-pounds.
A common torque wrench found in many shops is the round beam type. It looks like a long fixed breaker bar that has a free-floating pointer and a numbered scale attached to the handle. The head has the male counterpart to receiving a socket that attaches to the fastener. Pulling on the handle will cause the fixed pointer to register the sought after torque.
An additional type of torque wrench commonly used is the clicking type. It looks like a large diameter socket ratchet with a release on the end. When the release is unclipped the end of the shaft can rotate. The rotation applies pressure to an internal spring that will show up on a calibrated scale embossed on the handle that determines the torque you are looking for. When the fastener reaches the appropriate torque the wrench will have a distinctive click to it. This wrench is great to use as one can watch the work being tightened as opposed to watching a rod and scale. It is very accurate and easy to use.
When using a torque wrench there are some basic preparatory steps to consider. First, make sure the threads of the fastener and the mating hole are clean and free of rust, dirt, carbon and goop. Debris in the hole or on the threads will cause galling of the threads and give an inaccurate torque reading. Secondly, if no special sealers or instructions are given for torque applications are given, use a small dab of high temperature grease on the threads and where the fastener head contacts the part. Use of this lubricant will keep the fastener from sticking and reduce the possibility of galling and stripping. In addition the grease will assure the proper tension of the threads to the mating parts when the correct torque is applied. It will also make the fastener easier to remove in the future. A third consideration is to use the appropriate locking device in conjunction with the application. Other details are obvious. Make sure the hole is deep enough to accommodate the capscrew length. Dont use nuts or bolts that have damaged threads. Put the correct fastener in the correct hole. Some bolts might have drilled passages in them for oil lubrication. Others might require a bolt shorter than another. Some might be a different thread pitch. Watch what goes where and in what hole!
When tightening down a part in preparation for final torque make sure to use the recommended sequence. A cylinder head, for example, has a number of capscrews to tighten. The proper order of tightening will keep the head from warping and give correct sealing the gasket. A rule of thumb is to start from the middle of the cylinder head and work outwards. Screw all the fasteners down finger tight and then begin with a low torque setting that is one third of the final setting. Tighten all the capscrews in proper sequence then return to the start again and retorque at a higher setting that is two-thirds the finished torque. The third time is at the recommended torque and same sequence for the project. A step often overlooked is to check the final torque a fourth time. It is amazing how often one of those twenty or so capscrews will get overlooked. The double check also allows for some thread expansion time at higher torques between the first and last fastener. Dont leave out this important double check step.
Torque readings for various applications are found in the manufacturers assembly manuals for what is being worked on. If no specific manual is available some of the generic brands like Chiltons, Petersens, and others all carry charts for engine internal and external parts. If those are not helpful then look for the torque charts for bolts and fasteners buried in the back of the books. Some technical manuals might have them; some auto/farm suppliers might have them available. The charts are basically a listing of bolt/capscrew sizes, threads per inch, hardness, etc. and their recommended torque values for safe and maximum usage.
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