By Josh Cosford, contributing editor
Let’s face it, the world can be an extreme place. In the past decade, it appears the world is becoming exponentially more extreme, especially environmentally. The cold seems colder, the hot seems hotter and the wet seems wetter. I’m lucky enough to live in a part of the world experiencing the least extreme weather and climate events, but I’m a minority. Billions live where it’s hot and getting hotter, cold and getting colder, and wet and getting wetter. Regardless of climate, work still needs to get done, and mobile hydraulic machinery exists to get that work done, no matter how extreme.
Hydraulic machinery trumps nearly every other mechanical form of power transmission when it comes to working in extreme environments; it’s compact, powerful, reliable, controllable and can be unstoppable. Although straight off the factory floor not every piece of mobile machinery is manufactured to operate in both desert and arctic climes, they can be designed or optioned out to work very well in either. On top of that, pretty much only hydraulic machinery can manhandle an underwater task with both power and reliability.
Warming up to the cold
You’d be surprised at how much work is done in arctic climates. Anchorage and Whitehorse residents don’t hibernate, and construction, maintenance and recreation don’t stop, not matter how cold. Construction season used to be April until November in Northern states and provinces, but if you live in any large city, you know it’s now a year-round annoyance. The same goes for habitats situated upon permafrost … do you think that patch of runway asphalt at ANC airport can wait until July to be patched? Not likely.
So why is fluid power so good in cold weather? Consider a hydraulic system to be like the internal workings of your body. If your blood was near freezing, you wouldn’t achieve much in a work day, to say the least. Although hydraulic oil won’t freeze like your blood could, it doesn’t move well when it’s cold. But just like your mammalian nature, so too is a hydraulic machine warm blooded.
Humans are capable of extraordinary feats of endurance in even the coldest of conditions, party because of our internal furnaces, and party because of appropriate equipment. With the help of insulated outerwear, we can survive and thrive in extreme cold. A hydraulic machine appropriately outfitted for cold weather operation can also thrive.
The first step is choosing appropriate “blood” for low-mercury conditions. Standard, single-grade hydraulic oil wouldn’t stand a chance in zero-degree weather. It would move as well as maple syrup, and would waste huge energy just to convey itself around the circuit. Even a typical, high-quality synthetic fluid with excellent viscosity index (the ability to maintain its viscosity over a wide temperature range) just won’t do in severe cold. No, you need arctic grade oil.
Arctic oil starts with low viscosity synthetic base, typically 15-20 cSt, which itself is good for very cold temperatures. However, because of the nature of hydraulic machinery, oil warms itself up simply by operation. Heat is created within the oil from the pressure drop of moving the fluid and the internal leakage of pumps, motors and valves. This warm blooded nature of hydraulics is why it does so well in the cold, but under heavy workloads, and even in the cold, hydraulic oil can get too hot for 15 weight oil.
To combat a loss of lubrication related to low viscosity, arctic oils are heavily modified with viscosity index improvers. It’s not uncommon to see in the range of 200-300 VI for arctic oils. You read that correctly; 200+ viscosity index. This prevents the hydraulic oil from getting too thin under operating conditions, especially on those balmy 40° F degree summer days in the Arctic Circle.
I should mention, of course, mobile machinery even with arctic oil will still be sluggish upon start-up. Just like a hibernating Arctic ground squirrel, it takes time for the lifeblood of the machine to come up to operating temperature upon start-up. Response time can be improved with hydraulic oil heaters submersed in the reservoir, which are common for cold-weather machinery.
Keeping their cool
Diametrically opposed to cold weather operation, is the extreme heat. More mobile machinery has to endure sweltering climates than they do sub-zero, and there are really only a few things a machine designer can do to tackle the problem of heat, but limiting the contribution to viscosity breakdown through heat is the primary goal.
I’ve said this in other articles, but efficiency is the most important goal of a hot-weather machine engineer. Any fluid lost at pressure without doing useful work is converted to pure heat. Making the most of your input energy to ensure little energy is lost to poor circuit design or high-leakage components, ensures little internally generated heat, which itself can be much higher than ambient temperatures.
It should go without saying, any mobile machinery worth its weight in scrap steel should start off with a load sensing hydraulic circuit coming from an efficient variable piston pump. Load sensing works by sensing pressure downstream of every metered valve (often a proportional directional valve, but could just be a feathered lever valve), and then comparing downstream pressure to upstream pressure. The load sense network could be hydraulic pilot circuits or pressure transducers, but both will work well.
The point of load sensing is to provide only enough pressure and flow demanded by the load at each actuator. Typically some energy is wasted through pressure drop, but it’s negligible, and more or less just what’s required to move fluid through the system. Electronic load sensing is even more efficient, and can operate with less pressure drop than is required with hydraulic control.
Just as important to choosing an efficient circuit design is choosing efficient components. All hydraulic components require at least a little bit of internal leakage to lubricate moving parts, but some are inherently more conservative with that leakage. You will win no friends in the desert if your wheel drive motors are the gerotor type, even if it’s the more efficient disc valve design. Gerotors leak like selfies from an iCloud account, and should be avoided at all costs. Only use piston pumps and motors for your mobile machinery, which are efficient in the range of 90% plus. Gerotor motors are lucky to get 60% efficiency, meaning they convert 40% of their input energy to pure heat.
Heat in a mobile hydraulic system is as sure as the sunrise, and mobile machinery tends to run hot anyway. An operating temperature of 140° isn’t uncommon, and if heat is becoming too much of a problem, upgraded cooling can benefit. Large heat exchangers with hydraulically driven fan motors can remove massive amounts of heat, even when ambient temperatures are above three digits. To cope with a naturally hot fluid, mobile machines can run thicker viscosity multi-weight oils, which as opposed to arctic oils, are designed to maintain their viscosity well into the high mercury.
Power density underwater
After the extremes of cold and hot comes the extreme of water. Mechanical systems work well in water, but don’t have the controllability of hydraulics. Electrical actuation is possible under water, but power density is extremely limited with electric machinery. Hydraulics work well in water because of their inherent water tightness. Keeping water out isn’t much different than keeping oil in, although seals may need to be upgraded if water submersion is deep enough to witness high ambient pressure. Other than a prime mover’s requirement for air, if it’s an internal combustion engine, hydraulics don’t care if they’re surrounded by water or air.
I once worked with a company offering underwater inspection and maintenance services. They had many electric ROVs for inspection work, but had nothing for getting work done. They were developing a hydraulically operated, stainless steel skid-steer robot small enough to fit into a manhole, but was powerful enough to move through and vacuum sludge. It used two small hydraulic motors as drive wheels, and included a hydraulically driven auger and pump. As small as it was, it still weighed over 500 lb, making electric actuation difficult.
The robot was controlled electronically, and had a single integrated hydraulic circuit with four proportional directional valves. It was fed by a remote power unit, through a custom 500 ft umbilical incorporating 3/8-in. pressure and tank lines, in addition to all electrical wires. The pump supplied just 1.5 gpm, but because there was a 1000 ft of hose to move fluid through, pressure drop was over 600 psi.
The robot was capable of vacuuming up toxic sludge in up to 100 ft deep water when its water hose was connected. It was essentially impervious to its environment, and had the power density to chew up solids with its auger, something just not possible with electric motors in such a small package.
Only fluid power can operate in such diverse extremes so reliably. Although few hydraulic machines can be pulled from one extreme and plopped into the other, there isn’t an extreme environment on Earth where fluid power isn’t the first choice.