Fluid dynamics: Understanding working fluids.

Fluid dynamics: Understanding working fluids.

June 29, 2020 0 By Ray Bohacz

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

Answer: The truck with an automatic transmission.

The definition of a working fluid is one that operates a device, machine, or apparatus.

In an automatic transmission, the fluid is employed to transfer power through the torque converter, apply clutches during gear changes, and 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 farm equipment with a gear drive transmission will use oil, also known as tractor hydraulic fluid in the drive train and for the loader or three-point hitch operation.

Thus, the location determines if it is being employed as a working fluid. It needs to be understood that a working fluid can be used for a lesser task, but a non-working fluid cannot be called upon to do work.

In Europe during the late 1970s, there was offered a unique oil that was both a lubricant and a working fluid under the title, Super Tractor Universal Oils (STUO).

These were considered by many to be the pinnacle of oils. The same product was used in the engine and all other systems (transmission, brakes, and hydraulics). This concept allowed the farmer to purchase only one fluid. The theory never took hold in the United States, where specifications were stricter as engines, drivelines, and hydraulic components became more advanced and emission strategies employed.

More than meets the eye

For a hydraulic tractor fluid to be a viable product, it needs to be created from good quality base stocks. And include inhibitors and detergents for stability, anti-corrosion 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 braking system’s proper friction requirements. These are generic attributes of an adequate tractor hydraulic fluid, but there is more to consider.

Each equipment manufacturer has their own requirements for the fluid used in their machines.

This is 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, no industry standard needs to be met, as is with motor oil or brake fluid.

For this reason, each equipment manufacturer offers their own brand of hydraulic tractor fluid that meets the standards of their system, with indifference to the requirements of other brands.

Most farmers harbor disdain for that fact since they believe that all tractor hydraulic fluid is the same. 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 tractor fluid is manufactured and the methods used.

As consumers, we are not privy to that, so we need to attach some qualifiers to discern the quality of the fluid we are pouring into our farm machines.

My logic is I use the original equipment fluid from the dealer, and then I do not need to give it any more thought.

If you run multiple color machines, this may be a little burdensome. Still, since the fluid is not a consumable like diesel fuel, the extra effort is more of a psychological issue than anything.

It is essential to recognize that the in-service fluid’s color does not 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 fluid’s ability 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 save a costly repair.