Buying a Used 1.8–3.5t Compact Excavator in the UK: Undercarriage and Hydraulic Checks Before You Commit

The knocks occurred once a revolution on the left track. It was one (very quiet) thud, about where the chain broke off the idler and began to run back on the bottom rollers. No softness between edges of steel–flat, dead.

I had been driving that 2.8-tonne machine only a few metres on a hardened concrete dealership makesheet in the Midlands, in dribble that wasn’t even the thickness of a paint coloured by a drizzle. I heard it four times and then I got off the walk.

Next, I got down on my knees and did what I usually do when I need to do this – I ran my thumbnails along the outside of a bushing.

Not the fingertip. Slowness in drawing lengthways the thumbnail. An healthy bushing is smooth and slightly convex and the nail glides with a dull pull. This one had a step you could feel before you could see and on tilting my head into the light had a bright polished band on the pin end where the hardened portion of the bushing had wore away and was rubbing against something it should NOT have been touching.

If I’m being honest, this one feeling is the answer to saving me more money than ALL of the inspection reports I have ever paid for.

After 4 hours I was there on the same hardstanding with a pot of cold tea on the counter inside with “no” for an answer still. Was there less than two thousand recorded on the hour meter? It’s apparent the undercarriage had been doing a lot of work vs the engine.

An eight-hour slot of machine trenching counts towards hours in the same manner as an eight-hour use of a breaker for hammering the same square metre of concrete. Same clock. Very different bill.

The more difficult aspect is that having a low count on a compact will sometimes be just the opposite of what the buyer thinks. A 1.8t 900 hour unit has been trailered on and off 30 times in a hire fleet on a Friday afternoon, when there was no shortage of drivers. Hours is a low quality indicator of care and is an even poorer indicator of the condition of the undercarriage.

I confused my measure to begin with and I will start there. I placed a steel rule between the two pin centres and read a number, but it was meaningless since I did the measurement over a length of chain which is not mentioned by the manual, and it was a curve of the chain. Tension is set as written and pitch is measured over a predetermined number of links (usually 4 or 5) and measured on a straight run. Anywhere else and you’re creating a”big number”for the machine.

Data sources for the table above:https://www.cat.com/en_US/articles/for-owners/small-dozers/small-dozer-track-pins-and-bushings.html

Objective part is percentage elongation. Most OEM manuals will list a percentage of“new pitch”as the wearout limit (usually in the low, single digits) and that is when the chain is“scrap”(or close to it).

Increase in pitch occurs over time (Pitch Growth). Each joint containing a pin and a bushing wears slightly, and the result is the chain“grows”since the sprockets are still the same size. That causes the chain to be raised above the level they were designed to be on.

You can see that. Get low and sight along the sprocket that the chain is installed, and see how much tooth is showing over the bushes. Fresh assembly: The assembly condition of the tooth is good. Worn: when the chain rides up on each other and resting on the tips rather than on the flanks.

Then feel the tip. The grinding is controlled resulting in a new tooth being blunt and slightly rounded. A worn is sharp, and sharp in a hooked direction with the sharp part pointing the direction of the chain. The ends of your thumb it catches on that hook, the sprocket is finished and I don’t care what the rest of your machine looks like.

In this class you will find rubber tracks; they have to be in their own paragraph as perhaps the UK market is full of them, and the checks are different. You are not measuring pitch! You’ll notice embedded steel cords, anywhere there is any cord showing then it’s out of your safety line, and you’ll notice the drive lugs on the inside face have rounded off and are cracked where the sprocket was slipping. A lug in poor condition and with a rounded hump may shear up under load and an increased chirp signal will be heard on start up when operating on wet ground.

Order is important and many times people have the reverse of it.

Begin with the cold hand and make note of the functions of the hand before making any judgments. Then operate the machine under load for a sufficient period to heat the machine up—remember it was not under load when it was being idle, even a compact in clay will reach the “genuine hot” state sooner than a compact which is lifting air. Hydraulic note change to listen as the oil thins. Somewhat more hardened, somewhat more articulate. That is the machine which you are really purchasing.

Each function until end of stroke, is held. Monitor the rods with a creeping watch. If a rod drifts under hold, in the valve block it is bypassing somewhere and this shows you which area of the valve block to suspect.

Then the sticks. Relax, press down gently from neutral and experience the dead-band – first centimetre or two which does nothing. If the dead band is very broad and so not narrowly specified, it indicates that the pump is healthy, but that the pilot pressure is not correct or that the spool is worn. You do it while resting your hand flat on an armrest: a rumbling when you compensate late radiates from the structure long before your eyes catch up on some gauge.

If the shoes are as fresh as can be, and the shoes are on a hooked sprocket, you should come to a screeching halt. The two wear together with a rate of wear that approximately follows that of the other. If they used new shoes on a worn out sprocket, then that means one of two things and both following is bad: They replaced the lesser part of the system for less money so that the whole machine would look good, or they stretched the chain out, and wanted the whole assembly to appear younger than it really is.

Clean on undercarriage, with fresh paint. Don’t paint on any face you don’t need to, such as paint on the face of the paint roller and inside the idler bore; nobody can see it so there’s not much point. If there is paint present, someone covered an area of rust and a rust-covered seal under paint on a roller has already failed. Head also to see the track frame overspray line. It generally is positioned about 3 inft. too low.

No documentation on filter for new hydraulic oil. Oil Change, no filter and no records is NOT maintenance – it is evidence removal. There is no information on the oil now. The filter will have informed you of all of that and it had to go in the skip.

Honest low-hour machine is very very dull. Unwashed. Original paint rubbed off around the areas where the boots and hoses rub. Filters were changed on time and the old ones stored in a box in the shop room as the owner was one of those who keeps things. Heavily rotating rollers. A chain that is resting in its place, in the valley. There’s no pictures to take, there’s no secrets to keep.

Hence, the following sequence is short. Never shop from pictures! Shop by evidence. Check undercarriage records and filter the compact crawler inventory before picking up the phone—the reason is that a unit that has undercarriage record means someone was paying attention to it so the majority of units on that list will have been eliminated for that Saturday.

Take the thermal-load test along. Bring the magnet. Bring a steel rule.

When your thumb catches, walk away! Always, always there is another machine!