Mechanical Engineering Consulting Evaluation Rules Out Excessive Weight
The axle manufacturer rated the original axle at 7,000 lbs. After the original failure event, the trailer manufacturer replaced the axles with new 8,000 lbs. axles made by the same axle manufacturer. Photographs of the tires after both incidences on the CargoMate™ showed excessive wear on the inside of the tires, resulting in a smooth surface on one side.
The manual indicated that this type of failure was due to loss of camber or overloading. A mechanical engineering consulting investigation was able to rule out overloading. The user provided copious weigh slips that demonstrated that the trailer was not overweight. The loss of camber is consistent with torsion-axle failures. This would generally be true regardless of the brand of torsion axle.
De-Rating of Multi Torsion Axles
Torsion axles must be de-rated when used in multi-axles configurations. Another manufacturer learned this simple concept through hard experience. According to the information provided within the Tie Down Engineering technical service bulletin,
“Tie Down recommends the down rating of each tri axle by a factor of 20% from the designated axle model rating.” (Emphasis added.)
Using this de-rating recommendation, the following formula was used to determine the GVWR (gross vehicle weight rating) of a tri-axle trailer:
- GVWR3= number of axles * de rating factor * axle rating = 3*(100%-20%) / 100%*W1 = 2.4 * W1
- GVWR3= 2.4 * W1 where,
- GVWR3: gross vehicle weight rating of a triaxle trailer, lb
- W1: Single axle rating, lb
The first column of Table 1, labeled W1 (lbs), shows the load rating for a single axle in a single axle configuration. The second column of Table 1, labeled GVWR3(lbs), shows the load rating for a total of three axles in a tri-axle configuration.
Table 1 De-Rating of Tri-Axles
Litigation Support Analysis and Mechanical Design Conclusion Regarding Tri-Axle De-rating
Since the client needed to haul 21,000 pounds of supplies, which the manufacturer claimed the trailer could reliably carry. Our mechanical engineering consulting efforts showed in the litigation support analysis that manufacturer should have equipped the trailer with three 9,000 pound rated axles, not three 7,000 pound axles. Tri-axles need to be de-rated by 20% to accommodate for single tire excursions. Or in other words, the sum is less than the parts!
The manufacture made an error in mechanical design by not de-rating the axles in the tri-axle configuration. The specification and recommendation by the trailer manufacturer resulted in the axle failure and excessive tire wear.