A landscaping contractor brought in a large-frame skid-steer loader with a frustrating complaint: the left side of the machine was noticeably slower than the right side, but only when pushing into a pile of dirt. On flat ground, it drove perfectly straight.
Because it was a hydrostatic machine, the natural assumption is that the left drive pump is weak, or the left drive motor is bypassing internally. We put flow meters on both drive lines and ran the machine up a ramp. To our surprise, both pumps were putting out identical, perfectly healthy flow rates. The hydraulic system was flawless.
The problem was purely mechanical, hidden inside the chain case. In a skid-steer, the hydraulic motor connects to a sprocket, which turns a heavy roller chain that spins the front and rear axles. That chain is kept tight by a spring-loaded tensioner. When we opened the chain case cover, we found the drive chain was completely loose, dangling against the bottom of the case. The tensioner spring had broken.
Here is the tricky part that catches a lot of mechanics off guard: when you push into a dirt pile, the loader's tracks get a sudden surge of resistance. On the side with the broken tensioner, the slack in the chain allowed the sprocket to "jump" or skip a tooth under the sudden load. Every time it skipped, the machine lurched and lost speed. On flat ground, there wasn't enough resistance to make the chain skip, so it drove fine. But the real disaster was the chain itself. Because the chain had been slapping violently against the metal case for weeks, the side plates of the chain links had been work-hardened and polished to a mirror shine-we call it a "glass belt." The chain looked physically intact, but it was completely brittle and stretched. If we had just replaced the tensioner spring and kept driving, the chain would have shattered into shrapnel, destroying the chain case and the axle housings. We replaced the tensioner, the hardened chain, and the sprockets as a matched set. Always open the chain case if a skid-steer has a mysterious drive power loss.
