Scenario: Ken, a millwright, suffered an eye injury, minor burns, bruises, and abrasions as a result of an accident he suffered while testing a hydraulic motor. Surprisingly, when the accident occurred he was following the test procedures that were included in the troubleshooting section of the machine manufacturer’s service manual.

Ken had received a report that a conveyor stopped unexpectedly and would not respond to the start/stop control switch on the operator’s console. During his initial examination of the equipment, he noticed that the operating temperature of the oil had risen from a normal of 110° F to approximately 170° F. He also noticed that oil was leaking from the shaft seal in the motor.

He then referred to the troubleshooting section in the machine manufacturer’s service manual for guidance. It stated that shaft-seal leakage is a symptom of excessive internal leakage, and gave general instructions on how to test for such leakage. Here are the recommendations given by the manufacturer to perform the test:

1. Disconnect the case drain line from the port at the motor housing.

2. Hold the open end of the drain line in a receptacle.

3. While holding the line, operate the machine through a load cycle.

4. If a small amount of oil exhausts from the line, the internal leakage is satisfactory, so only the shaft seal needs to be replaced.

5. If a large volume of oil exhausts from the line, leakage is excessive. The motor should be replaced.

Next, Ken locked out the machine in accordance with his company’s lockout policy and proceeded with the test. He first disconnected the hydraulic line that was connected to a port in the motor housing, but the case drain line was approximately 75 feet long, and not readily accessible, so he fabricated a (temporary) shorter length of hose and connected it to the motor. He placed the open end of the hose in an empty 5-gal oil container.

Now he was ready to perform the test, so he removed his lock from the breaker. Holding the hose in the neck of the container, he called the operator on his 2-way radio and asked him to run the machine through a load cycle. When the test commenced, the conveyor was unloaded.

As the motor began to turn, the millwright pulled the hose away from the neck of the container so he could see any leakage, Figure 1. A stream of oil began to exhaust from the hose, into the container. At first the leakage appeared to be minor. Then, without warning, the oil began to spray out of the hose with such intensity that it jerked the hose violently, tearing it out of his hand. It jerked back and forth rapidly; the oil struck him in the face, knocking his safety glasses off as the hose pulled away. As it twisted back the other way, it struck him against the chest, knocking him backwards and over the edge of the platform on which he was standing. He fell approximately 3 feet to the ground. The operator shut the machine off when he noticed the low-oil-level warning light flashing on the instrumentation console.

Ken was taken to a hospital and treated for minor burns and abrasions and had to have hydraulic fluid flushed from his eyes. He was thankful that he was wearing his safety glasses, because the oil exhausting from the hose hit his face with such brutal intensity that the rims of his safety glasses bruised the perimeter of his eye sockets.

What caused the flow to increase so dramatically?
First, you must remember two fundamental principles applicable to hydraulics. One, pressure is a function of resistance to flow. Two, the greater the pressure difference across an opening, the greater the flow rate.

When the test commenced, the conveyor was running unloaded, so the system pressure was relatively low. Thus, even if a motor was worn or damaged, the leakage could be minor. However, when the load was placed on the conveyor, the pressure immediately increased to meet the load demand. High-pressure differential can force a large volume of oil through excessive clearances caused by wear or damage.

This is precisely why testing any hydraulic component to atmosphere can cause severe injury, death, or substantial property damage — it should never be done!