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May 27, 2021


Experiencing Spring Fatigue and Failure with Your 3-IN-1s? We Know Why.

Proper air support and electrical lines are vital to the operation of a tractor-trailer unit. Unsupported cables can become chaffed and cracked from rubbing or dragging on the deck plate. Not only is this unsafe, but it is also a DOT violation. In addition to safety and citation concerns, cables that continually require maintenance to rectify this damage create unnecessary downtime and unwarranted additional costs. However, when cables are correctly installed with the correct spring(s) for their application, these concerns are virtually eliminated. 

Cable support on the back of the cab comes in two forms – pogo sticks or tender springs. Tender springs, being the more popular choice, are available in different lengths and tensile strengths. Different lengths accommodate for back-of-cab connections and cables. Different tensile strength accommodates for the different cable weights.

Spring Tensile Strength

Extra heavy-duty springs are required for heavier cable applications such as combination assemblies or straight cables. Standard heavy-duty springs are used for coiled cables. So, if standard springs designed for lighter assemblies are used for heavier assemblies, they can become overstretched and lose their effectiveness, answering our question.

Springs supporting a 3-IN-1 are experiencing fatigue or even failure likely because the springs being used are designed to support the lighter weight of coiled assemblies versus the heavier weight of a combination assembly.

Spring Lengths

Different spring lengths were designed in combination with the back-of-cab connections and cable lengths in mind. Typically, 20” to 25” spring lengths are the standard seen on the back of a tractor. Although, some applications will use shorter springs based on the use case. However, Phillips has found that in jack-knife situations with 3-IN-1 assemblies installed, 25” springs work best, allowing the working spring to do its job and stretch far enough to extend the cables to their full working length.

3-IN-1 Spring Installation

Cables between the tractor and the trailer need to expand and contract as the vehicle pivots and turns. When using a combination assembly or straight cables, the springs do all this work instead of their coiled counterparts, where the coils do all the work. 

3-IN-1 combination assemblies are typically installed using two extra heavy-duty springs, one of which acts as the support spring and the other as the working spring. With a two-spring set-up, the combination assembly forms to create a large loop that expands and contracts as the truck moves, similar to how the coils in a coiled cable work. The support spring should be installed left of the working spring so it can support the cable leads. The working spring, installed to the right, stretches to allow the loop to expand and contract to utilize the full working length of the cable.

By selecting and installing the correct springs for your 3-IN-1 application, spring fatigue and failure can be avoided, keeping a vehicle on the road longer.