movable jaw on a pipe wrench is pivoted to permit agripping action on the work. This tool must be usedwith discretion, as the jaws are serrated and alwaysmake marks on the work unless adequate precautionsare observed. The jaws should be adjusted so the bite onthe work will be taken at about the center of the jaws.Strap WrenchThe strap wrench (fig. 1-39) is used for turning pipeor cylinders where you do not want to mar the surfaceof the work. To use this wrench, the webbed strap isplaced around the pipe and passed through the slot inthe metal body of the wrench. The strap is then pulledup tight; and as the mechanic turns the wrench in thedesired direction, the webbed strap tightens furtheraround the pipe. This gripping action causes the pipe toturn.SPANNER WRENCHESMany special nuts are made with notches cut intotheir outer edge. For these nuts a hook spanner (fig.1-40) is required. This wrench has a curved arm with alug or hook on the end. This lug fits into one of thenotches of the nut, and the handle is turned to loosen ortighten the nut. This spanner may be made for just oneparticular size of notched nut, or it may have a hingedarm to adjust it to a range of sizes.Another type of spanner is the pin spanner. Pinspanners have a pin in place of a hook. This pin fits intoa hole in the outer part of the nut.Face pin spanners are designed so that the pins fitinto holes in the face of the nut (fig. 1-40).When you use a spanner wrench, you must makesure the pins, lugs, or hooks make firm contact with thenut while the turning force is transferred from thewrench to the nut. If this is not done, damage will resultto tools or equipment or injury to personnel.SETSCREW WRENCHES (ALLEN ANDBRISTOL)In some places it is desirable to use recessed headson setscrews and capscrews. One type of screw (Allen)is used extensively on office machines and in machineshops. The other type (Bristol) is used infrequently.Recessed-head screws usually have a hex-shaped(six-sided) recess. To remove or tighten this type ofscrew requires a special wrench that will fit in therecess. This wrench is called an Allen-type wrench.Allen wrenches are made from hexagonal L-shaped1-26Figure 1-39.—Strap wrench.Figure 1-40.—General-purpose spanner wrenches.
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