The Importance of Viscosity Modifiers in Tractor Hydraulic Fluids

The Importance of Viscosity Modifiers in Tractor Hydraulic Fluids

Feb 11, 2016

In the field of drivetrain fluids, one of the toughest applications is the hydraulic system of an agricultural tractor. In order to provide optimal performance, the fluid must contain the appropriate additives and include a viscosity modifier (VM). VMs allow the fluid to remain thicker at high temperatures and flow easily at cold temperatures to ensure efficient operation and protection immediately after start-up.

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 huge hydraulic pressures of over 5,000 psi to operate ancillary equipment. It 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.

What is a viscosity modifier?
A VM is an oil-soluble structure, with molecules that decrease in volume when cold and increase in volume when hot. Yet, they can also be damaged by the very equipment they are trying to protect. “Shear,” where VMs are literally disintegrated by the action of gears meshing together, is something VMs must survive if they are to remain effective. A fluid capable of resisting shear is said to have 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 northern 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 components are synergistic 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, expensive field tests can be initiated prior to UTTO commercialization.

With these thorough, systematic development and testing processes, Lubrizol strives to provide the best performing, most sophisticated and cost-effective additives for its customers’ UTTOs sold on the market today.


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