Undercarriage Inspection Protocol: Measuring Remaining Life on Used 3-5 Ton Excavators

Elongation is when you hear that rhythm dipping or syncopated, when there’s double-tap in the beat, or whenever that rhythm stumbles. A chain is not in good alignment with the sprocket anymore. Pins and bushings have worn to the point that the pitch (Center to Center distance between the links) is out of factory tolerances.

A sprocket that has less than a 30% wear rating will have a rounded tip that resembles the edge of a butter knife and feel smooth. At 50% wear, the tooth has a slight hook shape and a slight ridge will be felt at the limit of the hardened surface layer where it is beginning to delaminate. When above 70%, the tooth gets a “pointer”.

The geometry of the sprocket tooth has a particular pressure angle (usually about 30 °) to spread the pull of the chain over the face of the sprocket. Contact patch decreases in size.

The flange is a raised lip along either-side of the roller that helps prevent the track from sliding off to the side. The flange is sharp and square when it’s out of the factory. If it still is square on the edge, then my nail will stop dead.

The outside of the flange begins to “mushroom,” thereby increasing the surface touching the track link, causing still more heat to be generated, causing the steel to become even softer. It is a “thermal runaway” scenario and you can check to see if it is already in progress, simply by looking at your fingernail.

This type of play explains why the pin can “elongate” faster, as it is not in one rotation center. On every load application it is pounding at a different point on the inside of the bushing, ovaling out the bore that much more.

Using a silver Sharpie, I put a dot on the 1st pin center. Then, I count up 21 links and make a mark in the centre of the 21st pin. Then I take out a steel tape measure and measure the actual distance between the two markings.

If you are within this range of 2,835mm – 2,892mm then you should be fine. When you cross 2,900mm you’re in the replacement zone, and it is mandatory. When operating under heavy-tractive loads, the chain will begin to skip teeth across the sprocket and if it does so during the dig cycle, it will cause a drastic deceleration that may again result in a crack in the track frame or the final drive mounting bolts shearing.

I put the depth gauge base flat on the unworn part of the body of the roller (on the inside about 10mm inboard from the flange) and then push the probe down into the groove until it begins to touch the depth gauge. For a healthy roller, it should be less than 0.5mm.

When new it’s a blunt, round-edged tooth. It’s similar to a crayon tip. At 20-30% wear, there is a slight flattening of the tip and incisions are noted on the sides in vertical score marks from the movement of the chain bushings.

The spot on the end is flat and is 50% of the way down into a score mark that is now made up of grooves. This is now a chisel-like tooth. Use the flashlight and notice how the tooth faces have little shadows in the grooves if you shine the light at an angle across the tooth face.

There will be a seller that says that the sprockets are at 50% now but still reversible, and that’s correct, but they have already rotated them once. You are on wear cycle #2. The required geometry of the tooth may be 50%, but 75% of the tooth material is used up. Check the back, or trailing edge of the tooth not contacting the chain. If it also features wear grooves it means that someone already played the flip card.

What ‘under 50’ really means when you shine a flashlight on it; not only is there a seamless transition but there’s no contrast in colors and when you pull a white rag across the tooth it doesn’t snag.

Fractures on track frames (hairline). Take a strong magnet (such as ones used to pick up dropped sockets), and wrap it in a clean white rag. Slide it gently along the track frame welds, particularly, the roller mounting bosses. This crack is opening and closing while loading, oxidizing and grinding the edges to powder.

Data sources for the table above:https://www.heavyequipmentforums.com/threads/d5h-front-track-roller-frame-broke.16768/

There’s just the bit of rust on the front near the idler mount.