Mechanical Engineering Expert says Lift Truck Malfunction Caused Driver’s Injuries


Lift Truck MalfunctionIn this case, the plaintiff was working in a distribution center in Wyoming and driving a lift truck. As he attempted to make a right turn into a side aisle, he attempted to plug break so that he could slow and make the turn. The plaintiff claims that he started plugging by tapping the multi-function handle and that the truck started to slow on the first two taps. By the third tap, however, the truck no longer responded. He said the steering malfunctioned, preventing his ability to steer. He then applied the service brake, but there was not enough time or space for the truck to stop. The truck subsequently struck the end-cap, crushing his leg, which had flung outside the operator compartment during the attempted turn. The truck underwent two post-accident inspections, neither of which revealed a mechanical issue.

Question(s) For Expert Witness

  • 1.) Is testing of your theory required in this case?
  • 2.) How did the plaintiff’s foot escape the compartment?
  • 3.) How do you explain the lack of any mechanical defects being found?

Expert Witness Response

The lift truck in question is a piece of mobile, electro-hydraulic lifting equipment. I have designed and/or managed the manufacture of mobile electro-hydraulic lifting equipment for over 10 years. Defense counsel has asserted that I performed no testing on alternative designs. Testing for the sake of testing is a waste of both effort and money, particularly when proposed alternative designs have already been tested and proved on competitive models in the same industry. The right foot brake pedal exists and has existed for many years on at least two other major manufacturers’ stand up, narrow aisle rider lift trucks, including those predating the manufacture of the subject machine. Additionally, my design background and engineering experience permit opinions on this particular issue without testing, since it is intuitively obvious to me that a right foot actuated brake pedal is entirely feasible and compatible with the existing design since it only actuates a simple switch to operate the electric brake, and this can be accomplished just as easily with the opposite foot. Since both the right and left pedals simply engage on/off switches, swapping their functions would have no impact on the operational component of the machine whatsoever, and the feasibility of using a right pedal to actuate a switch is already proved in this case on the very machine that is the subject of this litigation.

Testing to compare operator stability on right foot pedal actuated, side stance machines with 3 sides and an opening on the left side is similarly unnecessary. I have personally conducted numerous stability studies on a variety of equipment to determine stability, both during my stint in industry and in my present role as a consultant. This includes stability of humans on machines. Fundamental to the physics of stability is the location of the center of gravity, be that of a person or a machine. When the center of gravity moves outside the footprint of the machine or person, imbalance is the result. It is intuitively obvious due to my education, training and experience that a person who raises his left foot off the ground will fall to the left in the absence of a counteracting movement to shift the person’s center of gravity back over the location of the right foot, since the center of gravity of a typical person is symmetrical and located essentially in the middle of the torso in the coronal (frontal) plane. In a panic situation, when deceleration is unanticipated and movement of the operator is to the left on the subject truck, there is nothing to stop him from falling toward the opening. In a similar circumstance with the right foot being raised, the natural movement of the body is to the right, which would counteract deceleration in the opposite direction toward the opening. Additionally, the close proximity of the operator to the enclosed right side of the operator’s compartment would prevent or minimize movement in this direction for opposite direction braking.

Nothing has been produced by the defendant that details how or if the power steering remains enabled after the pedal switches are disengaged. Plaintiff testified that the steering locked up during the accident sequence in conjunction with a loss of plug braking. Additionally, there are at least 25 instances of reported steering failures in accident reports produced by defendant on stand-up rider machines, so this is not an isolated event. However, irrespective of how the power steering functions, its contribution to this accident is incidental. Plaintiff would have still lost his balance and come out of the machine as a result of this design, and changing the pedal functions as noted herein would have no operational effect on the steering or other truck operation.

As with many infrequent occurrences of problems associated with electrical controls, machines will often function as designed after an unexpected event and do not show a trouble code. They are therefore, deemed “fine” by mechanics who investigate them afterward. For instance, a stuck or out-of-adjustment switch or contactor will often be corrected as a result of an impact; thus nothing appears to be malfunctioning. I have personally investigated numerous accidents on forklifts and other equipment where this was precisely the case. As noted earlier, there are at least 25 other instances of defendant’s stand-up riders failing to plug brake or losing steering and causing an accident. In several of these cases where after accident investigations were performed, the machine is listed as operating properly. Intermittent faults are just that, they show themselves occasionally. There is no documentation that lists prior instances of power steering failures occurring on this particular machine. However, that does not mean a fault could not have occurred prior to this accident, or that prior incidents of this type were not reported. And contrary to the representations made by defense council, I did not opine that the cause of the accident was the loss of power steering. My opinion was that this loss of steering started the sequence that led to the accident. The accident was caused by plaintiff unintentionally falling from operator’s compartment and being struck by the machine. The location of plaintiff’s foot below the plane of the floorboard when it was crushed permits only one sound conclusion regarding how his foot arrived in this location, irrespective of what the defendant’s experts opine. To say that no deceleration value will cause bodily instability is ludicrous, as one only has to lift up one foot and they will fall in that direction in the absence of any other horizontal force, unless there is a shift of the upper portion of the body to counteract the now unstable position of the center of gravity of the body, as noted earlier. How quickly one reacts to this unstable condition determines: a) whether they will fall, or b) how far out they must place the elevated foot to prevent the fall. Given how close the left foot is to the outside edge of the operator’s compartment during normal operation of the subject truck (a few inches to virtually no distance at all), it is entirely plausible that operator reaction could cause his leg to leave the safety of the operator’s compartment, and that with no surface to stop motion the leg would move outward and down, allowing it to be crushed between the skirt of the machine and the curb where the Plaintiff’s leg was crushed.

My opinions on how the leg escaped from the operator’s compartment are supported by the evidence; the defense experts’ conclusions are not. Outfitting a body with an accelerometer and performing “testing” by trying to simulate how a person reacts in a panic situation in this type of scenario is completely irrelevant and cannot be done reliably. I can opine with a reasonable degree of engineering certainty that there are numerous body orientations where an operator’s leg can be thrown out of the operator’s compartment during deceleration provided by the machine, even at the rates cited by defense experts, based on the aforementioned facts, and my education, training and experience.

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