I once had a customer call to request I visit them at their own customer’s site to help with a “breakdown.” I’m always down to help a customer, so I gladly paid a visit. Arriving on-site, many looming pieces of machinery greeted me in a heavy equipment yard. An excavator sat atop a float trailer itself resting on the ground. This float trailer disconnects at the fifth wheel to allow the front end to lower under gravity. These trailers load from the front and then reconnect to the fifth wheel assembly, where a hydraulic package lifts the trailer once again, ready for transportation.
The technician complained that the trailer’s standard hydraulic package was not strong enough to lift this massive machine, or the components were now tired and worn. The technician working on the job was a heavy truck mechanic, a sort of jack-of-all-trades fellow who could get many machines running, but this one confounded him. One of the two eight-inch bore cylinders may be bypassing oil at the piston, but I understood the trailer worked fine before the overweight lift attempt.
Regardless, when I came on the scene to investigate, the technician described his attempt to modify the hydraulics to provide the “power” needed to lift the load. He went to a local tractor supply outlet and purchased a significantly larger pump, couplers and bell housing and then installed the new pump. They tossed the old pump out at the tractor supply outlet under the suspicion of failure, but I asked what pump it was. He had no part number, but I could deduce it was akin to the Haldex GC-Series. The GC-Series is a tiny pump, even in its largest displacement.
The replacement pump the technician chose was more like an off-brand, Parker Commercial aluminum pump meant for dump trucks. Maybe I’m exaggerating, but it looked comically large for a 5-hp gas engine. I inquired about the pump choice under my inference there was simply none other in stock. But he responded that his choice was intentional, and in fact, many more sizes were available.
His solution was to install a larger pump, because, of course, why wouldn’t a larger pump provide more power? The logical fallacy that bigger is better persists through society and is not limited to fluid power. The technician connected like with like, the same way most feel a larger baseball bat hits a baseball farther. Most forget the baseball bat doesn’t do the hitting; the athlete does. A larger bat does nothing when the person swinging remains the same.
This fallacy of the larger pump is much more common than I’d like to believe because you can’t get something for nothing. The technician wanted more force to lift the trailer and thought the larger pump provided more force. A larger pump provides increased flow by employing larger gears, vanes or pistons. However, force is limited to the system’s designed maximum pressure combined with the actuators’ size.
In fact, if the technician were trained in fluid power, he’d first try to get away with adjusting the relief valve to raise the maximum pressure. A small increase in pressure to the very large cylinders installed on this trailer would yield a significant force advantage. Of course, we make two assumptions to win at this hack.
First, we must assume the relief valve is not already set to its maximum value or is even adjustable at all. With most hydraulic relief valves, the manufacturers offer a few spring ranges to suit a broad range of pressure requirements. Depending on the valve’s design, a 3,000 psi spring will not adjust efficiently to 300 psi, for example. Conversely, a relief valve with a maximum spring value of 1,500 psi will cause the spring to compress solid when adjusted much higher.
Equipment manufacturers sometimes install non-adjustable relief valves with fixed spring settings. I’m a fan of these for two reasons; they’re less expensive and prevent end-user tampering. Regardless, this particular trailer had a simple inline relief valve plumbed in just after the pump, and it was adjustable.
Secondly, we must assume the prime mover itself has the extra power capacity. You see, just as the baseball player swings the bat to create homerun power, so too does the gas engine on this hydraulic power unit generate the power to eventually lift the load. Energy only travels from an area of higher potential to lower potential, and never with 100% efficiency.
In this case, a 5-hp gas engine provided the motivation to ultimately lift a 50,000 lb excavator. Internal combustion engines are highly inefficient, and you’re lucky if half its rated power is available to do useful work. Gas engines used to be rated using antiquated formulas that estimated the chemical energy in the fuel they used rather than the output at its shaft. Recent changes in legislation prevent this, clearly as a result of industry-wide complaints. Most small engine manufacturers no longer publish or state their engine’s power, now only publishing its displacement.
Anyway, it goes without saying raising the pressure was not an option in this case, especially since our larger pump was already eating up our excess power. In fact, the larger pump was actually the cause of the stalled motor. Starting the engine did allow the pump to pump, but straight through the open center valve. Although the extra flow created some backpressure, it was not enough to overcome the engine.
However, pulling the lever to lift the trailer, the pump (and engine) were exposed to full system pressure, instantly stalling the engine. The larger pump, in this case, was literally accelerating the problem. Whether intuition leads one to choose the larger pump to move the larger load or not, the technician is attempting to skirt around the laws of thermodynamics.
Hydraulic power factors using pressure and flow, so increasing one must result in a decrease of the other for any given input of power. The technician should have actually downsized his pump to create less flow and then turned the relief valve setting up to make the extra force.
To be honest, the technician’s customer was trying to use the trailer to transport a load outside the realm of its design limitations. Rather than modify the trailer, they should have phoned in for a larger one. Designers chose the upper pressure limit of the hydraulic system to prevent an unreasonable and unsafe load from barreling down the highway.
In the end, my recommendation included advice to remain within the limits of the hydraulic system components, and of course, that he should never expect to get something for nothing.