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U-bolts Torque Requirements

A U-bolt serves in four critical related roles:

• Primarily, the truck u-bolts provide the force required to clamp the leaf spring and  related components firmly together. In addition to the leaf spring, these  components include the top plate, axle seat, axle and bottom plate.

• The properly installed u-bolt eliminates any flexing of the leaf spring in the area between the u bolts. This is particularly critical since the hole for the centerbolt  in each leaf acts as a stress concentration which would lead to rapid leaf  breakage if spring flexing was not totally eliminated by the U bolt clamping force.

• By firmly clamping the spring to the axle seat the horizontal forces acting on the centerbolt are greatly reduced which in turn prevents shearing of the centerbolt.

• Proper clamping of the spring by the truck u-bolts provides the desired spring stiffness  and contributes to maintaining the vehicle ride height and handling characteristics as originally specified for the vehicle.

 Types of Leaf Spring U bolts

 1.     Round Bend                         Each of these bend types may use forged material when clearance     

 2.    Semi-Round Bend                is required between the frame and the U-bolts.                      

 3.     Square Bend                             

         Do not reuse auto or truck Leaf Spring U bolts

  • Used U-bolts will have rusted and damaged threads from the previous installation.

  • A previously torqued U-bolt will suffer from distorted threads from the engagement of the deep nut.  Deep nuts should be tightened once and re-torqued, never loosened and retightened.

   • A used u bolt may have suffered from fatigue as well excessive stress since achieving accurate torque with commonly used impact wrenches is very difficult.     

       Rod/U-bolt Diameter Determination

       Torque Requirements

 U-bolts must be tightened and torqued usng a cross pattern sequence

      -Tighten #1 then #4 to partial torque then partial torque #2 and #3

        -Using the same sequence, fully torque U-bolt Nuts

U-bolts that have been torqued to a set preload are recommended to be re-torqued after a short time to take-up the stretch in the set of the material.

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