U-bolts are a type of threaded fastener that is use to clamp an axle to a leaf spring on a trailer or a vehicle. The U-bolt performs this task by placing nut on the axle tube to pull a tie plate against the leaf springs. Because the U-bolt is responsible for securing the suspension components to the trailer or vehicle, the U-bolt must be tightened to a specific amount of torque.
If the bolts is not tightened to the proper amount of torque, the axle may wobble or the leaf springs may shift. Depending upon the axle to which the U-bolt will be applied, the U-bolt may have a different shape than other. For instance, square U-bolts is used on axles that have a rectangular cross-section, and the square U-bolts help to prevent the axle from rotating when the trailers brakes are applied.
How to Tighten U-Bolts on a Trailer
Round U-bolts are used on axles that have round tube, and semi-round U-bolts are used on trailers whose leaf springs can vary in height. Each of these different U-bolt shapes are manufactured to ensure that the load is evenly distributed, and to ensure that the U-bolt will not slip on the axle. When a U-bolt is tightened with a specified amount of torque, the U-bolt stretch slightly.
This initial stretch of the U-bolt is referred to as the preload. The preload creates friction between the components of the trailer suspension system, and this friction help to reduce the vibration of those components. In determining the amount of torque to which the U-bolt should be tightened, it is necessary to take into account how lubricated the threads of the U-bolt are.
If the threads of the U-bolt are lubricated, then twenty-five percent less torque will be required than if the threads were dry. Additionally, it is also necessary to take into account the grade of metal that the U-bolt is made of. For trailers whose total weight is up to seven thousand pounds, the use of Grade 5 steel U-bolts are suitable.
For heavy trailers that have dual axles or are lifted trucks, Grade 8 steel U-bolts is necessary. Finally, stainless steel U-bolts are used on trailers that have boats or drive on roads that have alot of salt. However, the torque on these bolts should be reduced to avoid the seizing of the metal components of the trailer’s suspension system.
A chart that specifies the amount of torque that each size of U-bolt should be tightened to is necessary to ensure the proper function of the trailers suspension system. The size of U-bolts range from three-eighths of an inch in diameter to one inch in diameter. Such a specification chart will provide information regarding the torque specifications for dry U-bolts and lubricated U-bolt, as well as the grade specifications of the U-bolt.
For instance, half-inch U-bolts that are used on the axles of standard trailers should be tightened to the Grade 5 torque specification. As the size of the bolt increases, the amount of torque that is required to tighten the U-bolt increases, as well. Consequently, precise tools will be required to tighten U-bolts whose sizes are larger than half inch.
In order to fully secure the leaf spring to the axle of the trailer, the nuts of the U-bolt must be tightened in a specific order and pattern. The nuts should be hand-snugged until they are even with the axle, after which the nuts should be tightened in a crisscross pattern. The nuts should not be tightened to the axle in one side of the axle prior to the other side being tightened, as this may distort the leaf springs.
The torque should be increased in stages; first to fifty percent of the specification for the U-bolt, then to seventy-five percent of the specification, and then to the remaining percentage of the specification. The use of an impact gun to apply the final torque to the nuts is not recommended. Finally, the torque of the U-bolt should be rechecked after twenty-five to one hundred miles of driving; the leaf springs may have settled into the axle, and the preload of the U-bolt may of decreased.
In applying the U-bolt to the trailer, there are a variety of mistake that may occur. One of the most common mistakes is to re-use old U-bolts. Due to the load that the U-bolt has previously carried, the metal may have permanently stretched the U-bolt.
Additionally, it is common for individuals to tighten the nuts that are attached to the U-bolt unevenly. In these instances, the metal components of the suspension system may warp. Another common mistake is to use a lubrication state for the U-bolt that does not match the torque specification for that particular size of U-bolt.
For instance, if stainless steel U-bolts are used, the nuts should be steel nuts but treated with anti-seize to prevent the metal from seizing to one another. Other mistakes include inspecting the U-bolt for cracks or rust, measuring the diameter of the U-bolt with a caliper, and using a torque wrench to recheck the torque of the U-bolt each season. Beyond trailer suspensions systems, there are a variety of other uses for U-bolts.
For instance, U-bolts are often used to clamp exhaust systems to the vehicles of which they belong. Additionally, U-bolts are often used to attach metal pipes to beams, and U-bolts are used to secure bike racks to trailers or automobiles. In each of these instances, the same logic behind the use of torque to secure the U-bolt to those components is used.
Additionally, in each of these applications, lock washers or nylon nuts are often employed as a means of securing the components of those applications against loosening from vibrations in the trailer. Finally, in these various applications, it is also recommended to use a matching tie plate to those axles or components to which the bolts are attached.
