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Understanding Working Fluids

Pop quiz: You have two pickup trucks. One has a manual transmission and the other has an automatic transmission. Both units call for ATF (automatic transmission fluid). In which application is the ATF a working fluid?

Answer: The truck with the automatic transmission.

A working fluid is a fluid that operates a device, machine, or apparatus. 

In an automatic transmission, the fluid is employed to transfer power through the torque converter, to apply clutches during gear changes, and to operate circuits in the valve body. 

In a manual transmission (most automotive-style gearboxes use ATF today), the fluid is a lubricant and cooling agent for the gear train. In like fashion, most equipment with a gear-drive transmission will use oil (also known as tractor hydraulic fluid) in the drivetrain and for the loader or three-point hitch operation. Thus, the location determines if it is being employed as a working fluid.  

In Europe during the late 1970s, there was offered a unique oil that was both a lubricant and a working fluid. It was called super tractor universal oil (STUO). These oils were considered by many to be the pinnacle of oils since the same product was used in the engine and all other systems (transmission, brakes, and hydraulics). This concept allowed farmers to purchase only one fluid. The theory never took hold in the U.S., where specifications were more strict. 

For a hydraulic fluid to be a viable product, it needs to be created from good-quality base stocks and include inhibitors and detergents for stability, anticorrosion properties, and the ability to promote cleanliness in the system it is used in. 

The fluid must also have compatibility with the seals used in the machine and, if required, the proper friction requirements for the braking system. 

These are generic attributes of a proper hydraulic fluid, but there is more to consider.

Each manufacturer has its own requirements. These are based on the seals employed, the design of the pump(s), the fluid flow path, the system’s maximum operating temperature, and the propensity for the pump to cavitate. 

If used in a transmission with a clutch system, the fluid must be compatible with the friction material and not work against it. To further complicate the purchase decision, there is no industry standard that needs to be met as there is with motor oil or brake fluid. For this reason, each maker offers its own brand of hydraulic fluid that meets the standards of the system. 

Most farmers harbor disdain for this fact since they believe all hydraulic fluid is the same and the machine maker is just trying to sell them their own fluid. They ignore the application-specific additive package that may be required. 

Another obstacle is how the hydraulic fluid is manufactured and the methods used. As consumers, we are not privy to that, so we need to attach some qualifier to discern the quality of the fluid we are pouring into our machines. 

It’s important to recognize that the color of the in-service fluid doesn’t tell the true story of its health. Once the color degrades, the fluid is way beyond being spent and is hurting your machine. 

Thermal cycling dissipates additives and introduces moisture. This then creates corrosion, acids, sludge, and the ability for the fluid to cavitate in the pump and foam. 

When any working fluid system experiences an alteration in performance, sound, or responsiveness, the first thing to do is to change the fluid and filter. Often, that is all that is required to restore like-new performance and to save a costly repair.

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