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EVERYTHING YOU EVER WANTED TO KNOW ABOUT U-BOLT’S AND MORE

LEAF SPRING SUSPENSION CLAMP DOWN

    U-bolts are designed to clamp the axle housing to the spring packs with sufficient force to prevent any movement between them.  U-bolts should not be subject to shear loads; in other words, a loose U-bolt, or one that has insufficient clamping force, will be forced to withstand a loading force perpendicular to the bolt rod.  Threaded fasteners do not serve well when laterally loaded.

    U-bots also prevent any flexing of the leaf spring in the spring-perch (mounting pad) area. The area of the spring where the center bolt hole is punched is a high-stress area, and if leaf-spring flexing occurs at or near the bolt hole, leaf-spring breakage can occur.  Properly tightened U-bolts will help prevent this problem. 

   Clamping force is largely determined by the tightening torque used on the U-bolt assembly.  For this reason, it is best to properly torque U-bolts for the safest and most reliable axle housing-to-leaf-pack assembly.

   Drawn steel rods are typically used to manufacture U-bolts.  Once drawn to the correct bolt diameter, the rods are usually chamfered on the end, threaded, and bent to the final U shape.  U-bolts may also be heat-treated and coated with a corrosion-resistant coating, depending on the application.  U-bolts may be fine or course thread.

   U-bolt threads are formed by one of two methods.  On rolled-thread bolts, the un-threaded bar diameter is slightly smaller than the threaded portion of the bar.  The rolling equipment used to manufacture these bolts actually reforms the raw bar so that the area to be threaded increases in diameter to the final thread size.  Cut-thread bolts have a final thread size that is the same diameter or slightly smaller than the size of the unthreaded portion of the bar.

   Most U-bolts are manufactured with smooth-rolled threads, whereas the mating nuts are made with sharp-cut threads.  When the U-bolt is tightened, the bolt stretches, and the threads are slightly deformed.  Removing the nuts actually damages the threads by cross-threading.  Once this occurs, the nuts can no longer be properly retorqued, and final assembly clamping force is reduced.

    U-bolts should be torqued to the manufacture’s recommended specifications.  This practice properly stretches the bolt and correctly preloads the U-bolt.  An impact wrench should never be used to tighten U-bolts as this will usually cause over tightening of the bolt.  U-bolts come in a variety of sizes and should always be retorqued after approximately 100 miles to ensure that the u-bolts have fully seated and the entire axle assembly stays well secured.

   U-bolts are manufactured in steel grades running from Grade 2 through Grade 8 and higher for certain applications.  The higher the U-bolt grade, the greater its rated yield and tensile strength and the lower the U-bolt will stretch for a given torque value.  Typically, Grade-5 and Grade-8 U-bolts are used in light-truck applications, with the Grade-8 bolts providing about a 40 percent increase in strength over equivalent Grade-5 bolts.  With the higher strength provided by the higher-grade bolts, torque specs can be increased and clamping force at the axle assembly maximized.

 U-bolts should be initially tightened and then retorqued after a short Seating period.  U-bolts should not be loosened and then retightened.  Ideally, an antseize lubricant should be used on U-bolt threads to obtain accurate torque readings, and U-bolts should be tightened gradually in a cross pattern to tension the U-bolts in an even manner.  If proper U-bolt tightening is not achieved, the clamping force of the U-bolt will be greatly reduced and may lead to spring fatigue and failure.  Retorquing U-bolts should be done with the vehicle in a loaded condition (at its ride height) and not while lifted off the ground.