U-Bolt

Type of U-Bolts

Three basic types of bends are used on U-bolts depending on the suspension design and the shape of the mating parts:

1. Round Bend
2. Semi-Round Bend
3. Square Bend

Additionally, each of these bend types may use forged material. This is used primarily where additional clearance is required between the U-bolts and, for example, the frame.

           
           
Regardless of the bend type, all U-bolts are measured in the same manner as shown here. The U-bolt size is given as Diameter (A) x Distance Between Legs (B) x Leg Length (C). Additionally the bend type must also be specified; either round, semi-round or square.

What does a U-Bolt do?

A U-bolt serves in four critical and related roles:
  • Primarily, the U-bolt provides 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-bolt. 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 U-bolts provides the desired spring stiffness and contributes to maintaining the vehicle ride height and handling characteristics as originally specified for the vehicle.

Service Recommendations

By following the service tips outlined below, many of the common problems associated with U-bolts can be avoided.

*DO NOT REUSE 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 retorqued, 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.

As previously explained, the primary function of the U-bolt is to maintain the clamping force on the spring assembly and related parts. Since clamping force is a function of the condition of the U-bolt, the damaged threads of the used U-bolt will make reaching this force very difficult. Considering that reusing U-bolts can lead to premature spring failure and other suspension damage it is clear that U-bolts should never be reused.

Inspect for signs of loose U-bolts

Virtually all leaf spring failures through the center hole are caused by inadequate U-bolt clamping. Also look for signs of movement within the area between the U-bolts. Worn or polished surfaces on axle seats or top plates are sure indicators that unwanted movement has occurred. Closely inspect and if necessary replace each damaged component.

Retorque the U-bolt

Retighten U-bolts after the first 500 to 1,000 miles. As new or repaired springs wear in some settling of the spring stack will occur. Even a minor amount of this settling can cause a dramatic reduction in U-bolt clamping force. If possible, retorquing of U-bolts should be done with the vehicle under load.

Torque U-bolts Evenly

     
Follow these guidelines when installing U-Bolts:
  • Lubricate U-Bolts and washers with oil or anti-seize compound to reduce nut friction.
  • Tighten all U-Bolts until they are snug only.
  • Tighten in the sequence shown to approximately 1/3 of recommended torque.
  • Repeat, using the same sequence, gradually increasing the torque through a second and third stage until the recommended final torque is attained.

Suggested Torque values for U-bolts

Grade 5
 
Grade 8
 

Size

(Diameter x Thread)

 

Torque

(Ft. lbs.)

 

Size

(Diameter x Thread)

 

Torque

(Ft. lbs.)

 
3/8-24
 
30
 
5/8-18
 
170
 
7/16-20
 
45
 
3/4-16
 
300
 
1/2-20
 
65
 
7/8-14
 
480
 
9/16-18
 
90
 
1-14
 
740
 
5/8-18
 
120
 
11/8-14
 
1300
 
3/4-16
 
215
 
11/4-14
 
1780
 
7/8-14
 
345
         

Custom U-Bolt

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