This story begins like many other stories from the battlefield collection - with a "mysterious failure"...
An airport baggage-handling company contacted us for technical assistance to one of their mobile luggage conveyors. For those of you who don't know - a mobile conveyor is a small self-propelled vehicle, equipped with an elevating transport belt, used to load bags into airplanes. The rig 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-sized service vehicle.
The operators were complaining that they couldn't effectively control the vehicle at low speed. Instead of slowly and gradually following the engine's acceleration, the closed-loop pump would suddenly kick in when the engine rpm was way above idle and then would stop just as abruptly as soon as the accelerator pedal was halfway up, causing jerky starts and violent stops, which was extremely dangerous since these vehicles must have precise slow-speed control to be able to approach an aircraft. The medium and high-speed control was good.
To top it off, there was another "symptom" that left the maintenance personnel puzzled, and even caused them to question the quality of the recent pump overhaul. As the vehicle was stopping, instead of locking the rear wheels - something that one would naturally expect from an abruptly zeroed closed-loop transmission - 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 sight was quite spectacular if you observed the machine from the "right angle" - slowing down, slowing down, slowing down - and then - Bang! - the rear wheel locks up and then starts spinning the other way around, squealing and smoking! When it comes to abrupt stops - it can't possibly get any cooler than that!
As it turned out, the malfunction, as well as the mysterious "single wheel turning backward" symptom had a simple explanation, and all that was needed to troubleshoot the "edgy" behavior of the rebel rig was the basic knowledge of how automotive controls work, and a keen eye to spot the one peculiarity that made that particular vehicle different from "normal" four-wheelers.
There are many designs of mechanic automotive controls for closed-loop pumps, but the basic idea is always the same - there's a system modulating the servo pressure in relation to the shaft rpm (usually devised around an orifice placed in the discharge path of the fixed displacement charge pump), and a forward/reverse selector valve (basically a DCV that directs the modulated servo pressure to the desired side of the servo-piston).
The abrupt stops were, naturally, being caused by the pump going to zero displacement while the machine was still moving, and the jerky starts by the pump engaging at a higher than normal speed, which pointed to an electrical issue, i.e. the pump DCV engaging too late, and disengaging too soon. It was found that the throttle pedal was equipped with a micro-switch, which prevented the electric current from reaching the forward/reverse selector valve when the accelerator pedal was up - a safety feature to assure that the vehicle remained stationary even when the forward/reverse switch was not in neutral. The micro-switch got loose and instead of activating when the pedal was in the top position, it was engaging halfway down. An easy fix. I wish they all were like that!
But what about the "wheel spinning the other way around" business?
The short answer would be - uneven weight distribution! The main purpose of the vehicle was to haul around a long luggage-transporting conveyor belt, which occupied the complete length of the rig on one side. And so, the cabin, the engine, the pumps, and the hydraulic tank had to be crammed into the other side of the frame, making it a lot heavier, which also meant that the wheel on the heavy side had much more traction than the other one. The two hydraulic motors were connected in parallel, and when the pump was doing the "unceremonious zero" thing, the heavy side motor, driven by the inertia of the vehicle, was becoming a pump that was turning the 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 wheel turning backward squeaking and smoking is the coolest thing to behold!