Hydrostatic Transmission and Luggage Troubles

     The following story begins just like most of the stories from the battlefield section - with a "mysterious failure". One of our regular clients - an airport baggage handling company - contacted us for a technical assistance to one of his luggage conveyors. For those of you who don't know - a luggage conveyor is a small self propelled vehicle, equipped with an elevating transport belt, which is used to load them bags to an airplane (Google the term "luggage conveyor" for a picture). The truck was equipped with a closed loop transmission, which consisted of a Linde BPV pump with an automotive control and a couple of radial piston motors connected in parallel, one for each side of the rear axis - a standard layout for a medium size/speed service vehicle.

     The operators were complaining that they couldn't effectively control slow speed of the vehicle, and the machine would start and stop quite abruptly, which was extremely dangerous since these vehicles need to have precise slow speed control to be able to approach an aircraft with caution. Instead of slowly and gradually following the engine's acceleration, the closed loop pump would kick in suddenly at high rpm, and would stop just as abruptly as soon as the accelerator pedal was lifted, causing jerky starts and abrupt stops. The medium and high speed control was good. However it was another "symptom" that left the maintenance personnel puzzled, and that was also very cool to observe - as the vehicle was stopping with the rear wheels squealing and smoking - instead of, as you naturally would expect from an abruptly zeroed closed loop transmission, locking the rear wheels - one of the wheels was not locking at all, while the other was not only locking-up, but also turning in the opposite direction! The site was quite spectacular if you observed the machine from the "correct" side - slowing down, slowing down, slowing down - and then - Bang! - the rear wheel locks up and then starts spinning the other way around! When it comes to abrupt stops - it doesn't get any cooler than this!!!

    Despite seemingly mystical - the malfunction, as well as the mysterious "going backwards" symptom had a very simple explanation, and all that was needed to troubleshoot correctly the stoppy behavior of the rebel truck was the basic knowledge of how closed loop transmissions with automotive controls work, and a keen eye to spot the one peculiarity that was making that particular vehicle different from the "normal" four wheelers.

     The abrupt stops were, naturally, being caused by the pump, that was going to zero displacement while the machine was still moving. There are several designs of mechanic automotive controls for closed loop pumps, but the basic idea is always the same - you choose forward/reverse with a solenoid valve, and then control the displacement of the pump and the maximum pressure with the servo pressure, which, in its place, depends on the input shaft rpm. Often (but not always), the engine throttle pedal is equipped with a micro-switch, which only allows the electric current to reach the direction controlling valve when the accelerator pedal is pressed - a safety feature assuring that the vehicle remains stationary when nobody is pressing on the throttle pedal, even if the forward/reverse switch is engaged. It turned out that the micro-switch got loose and was engaging too late, and disengaging too soon, and what happens when a closed loop pump goes to zero displacement and the vehicle still has tons of inertia? That's right - the wheels lock up and smoke!

   But why on that particular machine the rear wheels were behaving in such a strange manner? The short answer would be - weight distribution? Say what? Yes, weight distribution! Since the main purpose of the vehicle was to carry a long luggage transporting conveyor on one of the sides - the engine, the pumps and the hydraulic tank were all placed on the other side, which meant that the wheel on that heavy side had much more traction! When the pump was going abruptly to zero displacement, the heavy side motor, driven now by the inertia of the vehicle, was becoming a pump that was turning the light side (low traction side) motor in the opposite direction. In fact, a classic mechanical differential on the rear axis would probably perform in the same way under these circumstances.

   As you can see - there is always an explanation for everything. And please, agree with me that watching a vehicle stop with a rear axis wheel turning backwards while squeaking and smoking is a cool thing to look at!