The Case of the Breakaway Bucket

A useful construction machine is the excavator with a dipper arm and a bucket that seems to face the "wrong" way. The dipper arm has heavy cross pins to hook up various buckets and attachments. Some excavators, like the subject machine, have a hydraulic-controlled quick-release attachment to allow rapid tool changes.

Scene of the Crime

At a Maryland hospital, an excavator was being used to cut a trench. Stopping the excavation, the project foreman climbed into the trench to check measurements. A trench box built of side plates and cross braces was used to prevent cave-ins. Satisfied with the measurements, the foreman signaled to have the excavator pull the trench box ahead. As the excavator arm swung to the open trench, the bucket slipped from the quick-release fixture and dropped into the ditch. The 4,000-lb bucket would have crushed the foreman, except for the bucket's ripper teeth catching on the trench box. The foreman sustained several broken bones and a crushed hand. Through his attorney, the foreman sued the quick-release device manufacturer.

The Investigation

By the time I was retained to determine the cause of the mishap, the defendant's expert had inspected the machine and opined that operator error caused the bucket to drop. I inspected and observed the machine in action. Service records showed that, after the accident, the dealer made repairs including change-out of the quick-change release cylinder. Workers said the bucket had fallen off once prior to the accident, but not after the repairs. I inspected the operator's compartment and positions of control levers, the quick-release attachment, and hooked procedures. Changing a bucket required deliberate actions of curling the bucket toward the dipper arm and operating a release control to send hydraulic pressure to the release cylinder. Attachment and release controls seemed well-located to prevent inadvertent operation. From these observations, I thought operator error seemed too simple an answer to this mystery.

Literature searches revealed that makers of other quick-release devices used a manually set safety lynch-pin to secure the bucket or other tool after connection to the quick-release device. Even more revealing was the patent held by the quick-release attachment manufacturer: Their patent included a manually set safety lynch-pin that was not used in the production version.

This part of the mystery seemed to be solved; that is, a safety pin device would have prevented loss of the bucket. But why did the bucket fall off in the first place?

There were no broken or deformed parts of the bucket, quick-release attachment, or dipper arm to account for bucket loss. Except for the previously mentioned cylinder, there was no record of any repairs or replacements made to the hydraulic system. After reviewing hydraulic schematics of the excavator, I concluded that sufficient fluid loss from the release cylinder could cause the bucket loss.

Fingering the Culprit

Recalling that the release cylinder was replaced after the accident, I compared part numbers and descriptions in the service and parts manuals against the service invoices. I found that the replacement cylinder had a different part number and was equipped with remotely piloted check valves. This now made sense—the valves prevented any fluid flow in or out of either side of the release cylinder unless the opposite cylinder port was intentionally pressurized to hook up or release a bucket. These valves locked fluid in the cylinder assuring positional stability. Operating the bucket produced pressure spikes in the release cylinder and, without check valves, fluid loss was likely due to leakage back through the release control valve. Part two of the mystery was solved!

The attachment manufacturer only carried the newer cylinder as a regular service item but, even in light of previous accidents, never had a recall or service campaign to replace the older style cylinder. I testified in both deposition and trial that had this machine been equipped with a safety lynch pin and the cylinder with check valves, the accident would not have happened. The defendant settled before jury deliberations.