Threaded Fasteners 00.04 General Information nut. See Fig. 5 . Class 8 hexnuts are always used with class 8.8 bolts; class 10 hexnuts with class 10.9 bolts. stretched slightly. This stretching (tensioning) results in a preload that reduces fatigue of the fasteners. The torque values given in the tables in Specifica- tions, 400 have been calculated to provide enough clamping force on the parts being fastened, and the correct tensioning of the bolt to maintain the clamp- ing force. Use of a torque wrench to tighten fasteners will help prevent overtensioning them. Overtensioning causes permanent stretching of the fasteners, which can re- sult in breakage of the parts or fasteners.
1 2 When torquing a fastener, typically 80 to 90 percent of the turning force is used to overcome friction; only 10 to 20 percent is used to stretch the capscrew or bolt. About 40 to 50 percent of the turning force is needed to overcome the friction between the under- side of the capscrew head or nut and the washer. Another 30 to 40 percent is needed to overcome the friction between the threads of the capscrew and the threaded hole, or the friction between the threads of the nut and bolt. 10/19/93 f310224a 3 4 NOTE: Grade 2 bolts have no grade marking; grade 2 bolts are rarely used by Freightliner.These grade mark- ings are used on plain hex-type and flanged bolts (capscrews).
In addition to the grade markings, the bolt head must also carry the manufacturer’s trade- mark or identification. 1. Grade 2 2. Grade 5 3. Grade 8 4. Grade 8.2 The amount of torque required to tighten a fastener is reduced when the amount of friction is reduced. If a fastener is dry (unlubricated) and plain (unplated), the amount of friction is high. If a fastener is wax- coated or oiled, or has a zinc phosphate coating or cadmium plating, the amount of friction is reduced. Each of these coatings and combinations of coatings has a different effect. Using zinc-plated hardened flatwashers under the bolt (capscrew) head and nut reduces the amount of friction.
Dirt or other foreign material on the threads or clamping surfaces of the fastener or clamped part also changes the amount of friction. Fig. 2, Bolt Grades Frame Fasteners The standard fasteners used to assemble the vehicle frame and to attach most components to the vehicle frame are threaded lockbolts (Spin Hucks). These fasteners are covered in Section 31.00 . For some other components attached to the frame, grade 8 and 8.2 phosphate-and oil-coated hexhead bolts and grade C cadmium-plated and wax-coated prevailing torque locknuts are used. The prevailing torque locknuts have distorted sections of threads to provide torque retention.
For attachments where clearance is minimal, low-profile hexhead bolts and grade C prevailing torque locknuts are used. See Fig. 6 . Even though each different condition affects the amount of friction, a different torque value cannot be given for each different condition. To ensure they are always torqued accurately, Freightliner recommends that all fasteners be lubricated with oil (unless spe- cifically instructed to install them dry), then torqued to the values for lubricated- and plated-thread fasten- ers. When locking compound or anti-seize compound is recommended for a fastener, the compound acts as a lubricant, and oil is not needed.
Tightening Fasteners When a capscrew is tightened to its torque value in a threaded hole, or a nut is tightened to its torque value on a bolt, the shank of the capscrew or bolt is Business Class M2 Workshop Manual, Supplement 0, January 2002 050/2