This week we are going to take an in-depth look at a seriously damaged final drive sent in by a customer — and how to keep the same thing from happening to you!

Badly Damaged Final Drive

This final drive was sent in for clogged case-drain related problems, as we have discussed in previous blog posts. This one, however, is especially dramatic. The failure the motor portion of the drive is quite obvious when we look at the images below.

Extent of Damage

In this picture, you can see that four of the piston shoes were blown off, and those that managed to remain attached are severely scarred up — this is beyond simple scoring. You can imagine the level of pressure it takes to blow piston shoes off the piston cylinders.

The image below shows where some of those piston shoes (and parts of them) ended up. One of the last things you want moving around within the motor side of your final drive is piston shoes and broken, jagged pieces of metal. The amount of damage this can do is unbelievable — including contaminating the hydraulic system with this amount of debris.

In this last image, you can see where one of the loose piston shoes is wedged, and how it was deformed and twisted. You can imagine the amount of force required to do this type of damage to a bronze piston shoe. This is, needless to say, not a good thing for your final drive. You can also see the extent of damage tot he surfaces inside the final drive.

Possible Causes

Here’s something you may not know: this type of catastrophic failure due to a case drain can actually come about in one of two ways. Most of us know this can happen when a case drain filter is clogged, causing a restriction or back pressure in the case drain line.

The other reason, which is not quite so well known, occurs when the case drain line and the two-speed line are reversed. They are the two smallest lines on a hydraulic motor and easily mixed up because they are just about the same size. You can see below where the two-speed line is marked with a green arrow and the letter S, while the case drain line is marked with a C and a red arrow.

Once they are hooked up incorrectly, your machine will be damaged as soon as the 2-speed line is pressurized (usually when you got into “rabbit” gear). You will see damage very similar to what was shown here.

Preventing This Type of Damage

To prevent this type of damage to your final drive, make sure that for a two-speed drive you hook up the case drain line and two-speed lines correctly (referring to manufacturer’s instructions or contacting an expert if you are unsure). Whether you have a two-speed final drive or not, make sure that you check and change the case drain filter on a regular basis.