The Case of the Leaky Lift Truck

DN Staff

September 26, 2005

4 Min Read
The Case of the Leaky Lift Truck

Solving an engineering mystery can improve a product or maintain a company's good reputation. These challenging cases provide relief from often gruesome personal injury cases.

Scene of the Crime

A manager of a pharmaceutical warehouse seethed because lift truck oil leaks made his sparkling floors streaked and slippery. The truck dealer's technician discovered hydraulic fluid leaking from a recently-installed rebuilt motor and new power steering pump. Oil again dripped from another newly-rebuilt motor and new pump. Another new pump and freshly-rebuilt motor were installed and soon leaked. The warehouse manager threatened to find another dealer and an alternative brand of lift trucks.

The lift truck dealer brought the lift truck into his shop and called the lift truck manufacturer's tech rep who suggested that the rebuilt motors had faulty seals. The truck dealer then called the motor rebuilder who also manufactured and supplied new motors to both the pump manufacturer and the lift truck manufacturer. All three suppliers said that the motor and pump were correct, properly made units.

The Investigation

With no solution in sight and repeated pleadings from the truck dealer, the motor manufacturer asked me to determine the cause and find a fix for the leaky motors. At the motor factory, I looked at several leaky motors from different customer locations, including the motor from the warehouse noted above. I also viewed new motors being manufactured in an area separate from the rebuild department. The manufacturers of the pump, lift truck, and motor manufacturers were cooperative, and they all furnished product specifications and drawings I used for my inspection of both rebuilt and new motors.

The motor from the complaining truck dealer was cleaned and rebuilt. I determined that the shaft seal type, shaft diameter, and surface finish matched new motor specs. This motor was overnighted to the service technician who reported almost instant leakage. Believing the motor to be OK, I now suspected the pump. Engineers from both the lift truck and the pump manufacturer said that the pump did not have a shaft seal in order to allow a small amount of hydraulic fluid to weep past the pump bearings into the cavity between the pump drive input and the motor output shafts. Aided by this oiling method, the simple slot and tang drive system had delivered reliable operation for years with no problem. I also learned that other competitive lift truck manufacturers using pump/motors with a dry coupling system were known to have short coupling life.

On a sample pump, I noted a small passage from the cavity where the pump shaft extended back to the pump's inlet port. I surmised that with the motor/pump not running, this passage would allow oil from the reservoir to flood the coupling cavity, and when running, oil seepage past the pump bearings would be drawn into the pump inlet port. Lift truck and pump manufacturers' engineers concurred with me, but the leaky mystery remained.

The Smoking Gun

From motor/pump speed, shaft diameter, and seal type, I calculated a shaft seal pressure capability of 10 psi. From the technician's measurements of the truck's hydraulic system, I calculated a static pressure of about 0.20 psi at the pump inlet port. The technician fitted a pressure/vacuum gauge at the pump inlet, ran the motor, and obtained less than 0.15 inch/Hg vacuum. The motor seal should not have leaked! At my request, the technician drilled, tapped, and installed a pressure gauge into the pump body to sense any pressure in the shaft cavity. Running the motor and turning the steering wheel to pressurize the system, the pressure gauge shot up over 20 psi!

By phone, the tech and I compared his pump and my sample. On his pump, the passage from the shaft cavity passed only a small wire, apparently due to the pump manufacturer's drill point barely breaking through to the pump inlet port. The tech opened the hole with a drill, cleaned and re-installed the pump, connected the pressure gauge, and ran the power steering system. Result: No oil leak and virtually zero pressure in the shaft cavity! Following my report, the lift truck manufacturer checked pumps in stock; the pump manufacturer checked pumps on-hand and in the manufacturing stream, and checked the drilling process; while the motor manufacturer alerted customers reporting leaky motors to check and change out pumps as needed.

With cooperation, the mystery was solved and there were smiling faces all around!

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