Agricultural tractors are complex, highly-engineered vehicles. They are capable of operating at much higher speeds than most off-highway equipment, yet they must also demonstrate reliable performance at low speeds for tilling and harvesting applications. Tractors have to operate across a vast spectrum of temperatures, from the very coldest to searing hot. They work in hostile environments where mud, dust, ice and water are constant hazards. In addition, they have to perform with total reliability at the turn of the key, as any in-service failure could have major financial consequences far beyond the cost of the repair itself.
It is no wonder, then, that tractors and their subsystems are considered to be one of the toughest applications in the engineering world. This also applies to their drivetrain fluids, especially the hydraulic fluid that operates almost all of the systems on the vehicle and which is key to its round-the-clock productivity.
The hydraulic system of an agricultural tractor is uniquely exposed to every challenge known to the fluid engineer. The fluid has to perform perfectly when starting up in extreme cold, yet has to retain that performance in extreme heat, too. It needs to tolerate the ingress of sand, dust, air and water as the tractor’s implements are changed in the field, and its friction characteristics must be consistent under all conditions so that wet brakes do not grab dangerously, transmissions engage smoothly and attached equipment operates with the required precision.
In order to provide optimal performance across the spread of operating conditions, the fluid must contain the appropriate additives; one of the most important is the viscosity modifier (VM). VMs are molecules which are small when cold but expand as the temperature rises. When the fluid cools down again, the molecules shrink back down to their original size, allowing the oil to flow freely. The influence of VMs allows the best of both worlds: the fluid retains its thickness for protection at high temperatures, but also flows easily at lower temperatures to ensure efficient lubrication and easy circulation immediately after a cold-weather start-up.
Fluids containing VMs are labeled multigrades. They are vastly different from monograde oils, which in earlier eras were the universal choice. With monogrades, many operators had the tiresome task of draining and refilling their oil with a thicker grade to cope with summer heat and reversing the process in the autumn, when the approaching winter demanded a thinner oil. Except in certain special applications, multigrades are now the preferred choice.
Universal Tractor Transmission Oil
A typical Universal Tractor Transmission Oil (UTTO) has to perform a multifunctional role. The UTTO must act as a lubricant for the gears and other components in the hydraulic system, support the wet braking system typically used in modern tractors, provide corrosion inhibition, be water tolerant and act as a medium for generating and transmitting huge hydraulic pressures of over 5,000 psi to operate ancillary equipment.
The UTTO also has to maintain stable, high dynamic friction for power transfer and consistent clutch operation. This is particularly important to the smooth functioning of the latest generation of powershift transmissions, especially under high torque conditions.
Why viscosity modifier design is crucial
We have already seen that the VM is an oil-soluble structure, with molecules that decrease in volume when cold and increase in volume when hot. Unfortunately these complex structures can also be damaged by the very equipment they are trying to protect. “Shear,” where VMs are literally chopped up by the action of gears meshing together, is something VMs must survive if they are to remain effective in their role. Shear stability is inversely related to the molecular weight of the VM polymer. As a rule, high molecular weight polymers offer high viscosity but lower shear stability, while polymers with low molecular weights provide lower viscosity in tandem with higher shear stability.
Achieving a level of shear stability appropriate to the task at hand is largely a function of performance and cost. Fluids must be of high quality but offer good value, too, so the shear stability of a good quality UTTO falls somewhere between that of an engine oil and an EP gear oil.
A good multigrade fluid should have a relatively high viscosity index (VI), the numerical measure of viscosity change with temperature. Fluids must flow smoothly at low temperatures, while retaining viscosity as they heat up. This is especially important in a UTTO, as changes in viscosity can lead to variable performance of the hydraulics and the wet braking system. Essentially, VMs allow the hydraulic fluid to provide a similar performance across the operating temperature range. Where equipment is being used in extremely cold climates, it is essential that UTTOs possess good low-temperature properties, especially during winter. Poor pumpability due to low-quality VMs and lower-grade base oils can cause severe damage to tractor components under these conditions.
Dirt and water ingress is a particular problem in UTTOs, because hoses are regularly detached and re-attached as different types of equipment are used. The strategy is to homogenize the water with the fluid rather than reject it, and a good UTTO will have a water tolerance of up to 1 percent.
Quality matters
High-quality formulations are essential. Cheap hydraulic fluids simply cannot match the performance of higher-quality fluids when it comes to shear stability and low temperature performance. The use of cheap fluids in some markets causes much higher wear rates, resulting in damage to equipment that far exceeds the extra cost of using a premium quality fluid.
Research and development
Lubrizol invests considerable effort in researching and testing new formulations to create additives that enable its customers’ UTTOs to have excellent shear stability. Particular attention is given to excellent low-temperature performance to protect the equipment during start-up in cold conditions.
The starting point for developing a new VM is to examine the viscometrics, the low and high temperature performance and the response of the fluid in the operating temperature range.
The next stage is to conduct hardware tests with the full additive package to ensure component compatibility, and to establish whether the combination of components in the system or subsystem works together smoothly in providing wear protection and friction performance. For example, “chatter” tests are performed to check for noise or vibration during braking.
Many of these hardware test rigs are built using partial or full-sized pieces of equipment from tractors in the field. This is to ensure good correlation with the real world, so that once a UTTO has passed all of the required hardware tests, extensive field tests can be initiated prior to UTTO commercialization.
With these systematic development and testing processes, Lubrizol strives to provide the best performing, most sophisticated and most cost-effective additives for its customers’ UTTOs.
