An important part of air suspension maintenance routine is a regular pivot bush inspection.
The TRI-FUNCTIONAL Bush (TFB) is a key factor in both ride quality and roll stability of Hendrickson suspensions. The bush voids allow articulation that provides exceptional control during turning and roll events. The design of the bush and the void contours enhance the axle’s ability to function as a torsion bar resisting trailer roll forces.
Periodic inspections are an important part of your air suspension maintenance routine. Depending on the age of the suspension, the used bush may experience various states of fatigue that could include surface cracks or cracks forming between voids. However, the pivot bush may still have many years of service life remaining.
NOTE: Under no circumstances should a shaker table or extended iron pry bar be used as a method to determine the functionality or serviceability of a TRI-FUNCTIONAL Bush. A Shaker Table merely demonstrates the ability of the TFB to absorb the primary road forces, whilst an iron bar compresses the TFB at the void area.
Pivot Bush Assembly
Hendrickson does not recommend disassembling the pivot connection to inspect the pivot bush. The recommended procedure is to make measurements in the relationship between the beam tube and the frame bracket.
On an unloaded trailer, measure from the bottom of the beam assembly to the bottom of the frame bracket as shown in the images.
- If the measurement at A is less than or equal to the 19 mm, then the bush is OK.
- If A is greater than 19 mm, then the pivot connection must be disassembled, and beam assembly lowered to enable closer inspection.
It is important to take the measurement at the correct position, to get an accurate reading. Place a straight edge or steel ruler across the frame bracket, underneath the pivot bolt, just past where the beam assembly is welded onto the bush tube as shown in the images. A measurement should then be taken between the straight edge and beam assembly.
The pivot bush can be inspected from underneath the trailer without disassembling the pivot connection. With the trailer wheels chocked and the trailer properly supported, look up at the bush tube and inspect the side of the tube that offers more access, or in other words, has the larger gap between the bush tube and the frame bracket. Use a screwdriver to push the bush tube spacer against the frame bracket and out of the way so a portion of the pivot bush can be seen. Use a torch to illuminate and inspect the end of the pivot bush.
During this inspection, look specifically at the bush voids. In most cases, it will not be possible to clearly see both top and bottom voids, but enough of the bush can be seen to make an evaluation. By design, the bush voids will be at the twelve o’clock and six o’clock positions (±five degrees) when the suspension is at the designed ride height.
Minor superficial cracks will appear over time that have no detrimental effect on bush performance and therefore no action is required. However, cracks in the rubber extending between the void and the bush’s inner metal or an excessive amount of vertical movement can indicate that the bush may need to be replaced.
The appearance of smeared blackened rubber or hanging strands of rubber around the bush tube edges or bush tube spacers is a sign the bush is heating up and melting. The source is likely to be continuous rapid vibration induced into the bush through the beam.
It is usually caused by an imbalanced wheel-end on the same side as the affected bush. A wheel-end can be out of balance due to several reasons that will require further inspection for correct diagnosis. These reasons may include issues with the tyre(s), improper assembly, dropped or out-of-round drum, mud or debris collected on the rim and non-functioning shock absorber.