The Front and Rear Idlers on an elevated sprocket machine are effectively the spreaders for the track chain. The idlers keep the chain reasonably tensioned and guides the chain as it rotates. The track chain is essentially a rail that is laid down and picked up again in a continuous movement that allows the machine to move over just about any area.

Damage to the idler shafts can be fatal to the chain especially when the idler shafts collapse.

The typical progression of the idler shaft collapse is as follows:

Step 1. The chain is over-tightened by the track adjuster.

When the track adjuster exerts too much force on the chain, it increases the load on the power train and the spreaders on the ends i.e. the idlers. The rear idler will be hardest hit as the machine extends power through the rear idler to the final drive in the forward direction while pushing dirt or ripping. See the following image showing taught track over the rear idler and final drive as the machine pushes dirt.

Track Tension is measured as Track Sag at every inspection. It is singularly the most important measurement to be taken at an inspection.

Step 2. Excessive heat build-up due to friction between the Idler shaft and bush bearing.  

This can be detected by infrared thermo-graphic imaging. In such cases, the plugs on the end of the shaft will pop out due to the build-up of heat and pressure – detected by observing missing shaft end plugs and leaked oil.

Step 3. The excessive heat build-up can cause a few types of failures at this point.

  1. The bushes can seize on the shaft and start to spin in the idler bore. This is due to forces so strong under tension that the lubricating oil between the bronze bush bearing and idler shaft is driven out resulting in direct contact between the shaft and bearing. Note the scouring of the material in the image below.



Galling and Metal transfer between Bush and Shaft

Scouring of Idler Bore Wall due to Bush seizing on shaft

2. The shaft can snap in extreme circumstances, causing damage to the idler bore and other components.

Snapped Idler Shafts

Snapped Idler shafts can be detected by observing that the idler is no longer in position and grinding against the inside surfaces of the frame assembly. As the shaft snaps, the idler is no longer aligned, and the chain is forced against the flange of the track roller thus causing the heavy wear pattern on the flanges of the closest rollers to the idler. These end track rollers will now be performing the job of the failed idler in guiding the chain, which they are not designed to do resulting in additional wear to the track link assembly.

Undercarriage Inspectors should keep a lookout for Idler Failure signs when performing the undercarriage inspections with specific attention to: –

  1. Too much track sag or excessive track tension
  2. Infrared images of front, rear idlers and final drive noting any abnormal operating temperatures
  3. Missing idler shaft end caps and leaking oil from idler collars and retainers
  4. Excessive abnormal wear on track roller flanges
  5. Side thrust wear indicated on successive chain links
  6. Abnormal noises emanating from the undercarriage while machine is moving