By Myron J. Boyajian, P.E.
Mix engineering responsibilities defined by labels with a lack of labeling on wiring and you have a recipe for calamity.
Decades ago, when the national economy was on a downward slide, a major Midwestern beer brewer sidelined several electric forklifts not needed due to reduced sales and production levels. Normally, the brewer ran three shifts working 24/7, with virtually no resting time for the forklifts. The brewer’s maintenance department decided this slow period was a good time to perform badly needed maintenance.
A dozen or so forklifts were transformed with new parts and paint. Traction motors got new brushes, while lift and power steering motors were removed for complete servicing. The brewer’s management decided to rotate the rebuilt trucks into the active fleet and gradually perform similar work on the balance of the fleet of more than 100 forklifts.
As the refurbished trucks were put back to work, complaints trickled in that centered on problems with the power steering system. The brewer wanted a quick solution and requested help from the forklift manufacturer. As the manufacturer’s liaison engineer between engineering, marketing and service, I was sent to the plant to solve this conundrum.
I drove several of the forklifts. Both with and without a load, each forklift seemed to have perfectly functioning power steering. I asked the maintenance boss, “So what’s the problem?”
He seemed as puzzled as I was, noting that each truck had a service request tag, saying “steering faulty.”
A forklift operator was asked to describe the problem. He said his job included stacking pallets of beer, and as he slowly inched forward and backward, steering in the narrow aisle between stacks of pallets, the power steering failed when he lifted a pallet. Without hydraulic assistance, the stiff steering resulted in the forklift hitting the stacked pallets.
The first step in finding a resolution was to install a pressure gauge between the power steering pump and steering control valve. Testing showed normal pressures, but dropped to zero psi when the lift function operated. Somehow the lift system was starving the power steering pump. I knew that the steer pump was fed directly from the reservoir, so I put this idea on the back burner. Because the motors had been removed, serviced and re-installed, I suspected a wiring problem.
A voltmeter connected to the steer motor terminals showed full battery voltage until lifting, when the voltage dropped almost to zero. Up on the shop hoist, inspection revealed incorrect power steering motor connections. The steering motor’s negative terminal was connected to the positive terminal of the lift motor rather than the lift motor negative terminal, which provided both motors with a return to battery negative.
With this being the case, I wondered how the power steering motor could operate at all. I concluded that the steering motor’s current easily passed through the low resistance of the lift motor armature and field windings, then out of the lift motor negative terminal to battery negative. But when the lift motor operated, its spinning armature generated a counter-electromotive force, and that voltage was high enough to reduce the voltage available to operate the steer motor.
I asked to have the motor wiring corrected, but this was put on hold until an electrician could be found. I asked why the forklift mechanic couldn’t make the change, but was told that three different craft unions were involved in maintenance and repair operations. A pipe fitter disconnected and reconnected hoses and fittings, an electrician disconnected and reconnected wiring and cabling, and a mechanical technician unbolted the motors and other mechanical parts.
In time, an electrician came and corrected the wiring, and testing showed perfect power steering under all conditions. Orders were cut to have all forklifts corrected in the same way.
So what was the real problem? I concluded that it was a serious lack of communication during the refurbishment program. The person that disconnected the cabling didn’t tag the wires he unhooked, and a different electrician reconnected the wires through guesswork. It appeared that all these trades were doing what one person could do - but without proper communication between roles - and this procedure became an invitation for future calamities.