By Josh Cosford, Contributing Editor
Hydraulic machines hate the cold just as much as the average retired snowbird does. However, a ski slope groomer doesn’t have the luxury of living from November to April in Florida or Arizona to avoid shoveling, brushing off, or walking in snow. Quite the opposite, actually, since no ski resort could manage without the nightly prowling of a snowcat to smooth over the slopes after a day of rut and track creation.
The hydraulic designer must create a package that, first of all, allows the machine to simply start and run. That part can be easy, for the most part, but the trick is to avoid detrimental side effects at operating temperatures or for machines designed for all-season operation. Simply using AW22 mineral oil could help the machine start and run when the mercury is low, but such an oil could run dangerously thin when the machine is up to operating temperature or not changed for the summer months.
It might be more reasonable to use an “arctic oil” manufactured with a synthetic base stock and designed with an exceptional viscosity index. Viscosity index describes the capacity of hydraulic oil over a wider temperature range rather than just at 40°C (104°F). A decent hydraulic oil has a VI over 100, but the very best arctic oils will see that increase to 200 or more, giving you the best of both worlds, ranging from cold startup to operational temperatures.
Still, even something as low as ISO grade 15 can be thick when exposed to severe cold down to -40°C (where Fahrenheit is also -40°), so to help with the first minutes of startup, various heating methods should be used to speed up the process. Electric tank heaters can be installed in the reservoir and thermostatically controlled to maintain operating temperature. As the oil circulates, it will mix with the peripheral oil and cool somewhat, but at least you can jump in and go once the engine is warmed up as well.
Because internal combustion engines heat up so quickly, you can take advantage of that excess thermal energy by circulating coolant through a heat exchanger, and then warm-up bypass circuits can pump oil through valves, filters, and the reservoir until a good portion of the oil is at operating temperature.
It helps to select pumps, valves, and actuators that are efficient even with high-viscosity oil, such as axial piston pumps. Selecting only premium-quality components will reduce the internal leakage risk of using thin hydraulic oil, but also keep in mind how you route critical plumbing, such as the pump’s case drain line, out of sympathy for its backpressure sensitivity. Although there is valid discussion around using generous clearances for things like valves to prevent high viscosity instability during startup, it’s my opinion that an effective startup procedure is a minor inconvenience compared to living with poorly performing valves subjected to high leakage, especially for all-season machinery.
Secondary only to thermostatic control is moisture control — all bridges must be crossed to prevent water and humidity from entering cold-weather hydraulic machinery. Sealed reservoirs that mimic aspiration using bladders or accumulators can prevent the introduction of moisture, which occurs most frequently through breather caps, melting snow, or machine washdown. Mild pressurization of 5-10 psi stabilizes pump inlet pressure despite changes in oil volume from differential cylinders, while ensuring the pump resists cavitation in the face of low viscosity.
Of course, preventing water ingestion is one thing, but the oil filled into the machine must be dry. Dehydrated oil transferred while cold is ideal, since any free water that appears during cooling can be waylaid before transferring. However, this practice must be maintained throughout the machine’s life, so it’s just as much a service concern as a design concern.
Because as oil temperature drops, hydraulic oil loses its capacity to hold water as humidity, and can “rain” out of emulsion the colder you get. Localized and frozen water can damage internal components as it expands during phase change, so eliminating every opportunity for it to form is a wise design decision for reliable, cold-weather machinery.
Finally, the designer needs to select appropriate sealing materials, such as HNBR, FKM, and certain polyurethanes. Not only should the polymer have a wide usable temperature range, but it must also be cold-weather stable to prevent cracks or extrusion during inevitable low-temperature operation. Despite strong wording in operating manuals, we all know how machine operators just like to “get in and go.”
Designing for winter is not an easy task, and we can expect that the provisions added to ensure arctic survivability will add a considerable investment cost to any machine. Aside from suitably selected and designed hydraulic systems, the extra investment also factors in cab comfort and engine/transmission upgrades. However, if you’ve been lucky enough to drive a snow groomer, you know how far we’ve come in the design and execution of cold-weather hydraulic machinery.
