The Importance of Viscosity Modifiers in Tractor Hydraulic Fluids

In the field of drivetrain fluids, one of the toughest applications is the hydraulic system of an agricultural tractor because 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; and
  • Act as a medium for generating large hydraulic pressures of over 5,000 psi to operate ancillary equipment.

And through all of this, it also has to maintain stable 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.

In order to perform optimally, the fluid must contain the appropriate additives including viscosity modifiers (VMs). VMs allow the fluid to remain thicker to protect the equipment at high temperatures and loads, and flow easily at cold temperatures, to ensure efficient operation and protection immediately after start-up.

What is a viscosity modifier?

A VM is an oil-soluble polymer, with large liquid molecules that expand and contract with temperature helping boost viscosity when hot and minimize viscosity when cold. Yet, due to their large size, 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— must be avoided to maintain efficacy. A fluid capable of resisting shear is said to have shear stability. Good shear stability is crucial for maintaining the equipment as it creates a sufficient, thicker film lubrication between metal-to-metal contact when operating under high loads and high temperatures.

Achieving the required shear stability is largely a balancing act between 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 a gear oil.

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 inconsistent performance of the hydraulics and the wet braking system. Essentially, VMs allow the hydraulic fluid to provide a similar performance across the full operating temperature range of the equipment.  Where equipment is being used in northern climates, it is essential that UTTOs possess good low temperature properties.. 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 contamination 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 containing VMs will have a water tolerance of up to 1 percent.

Quality matters

High-quality formulations are essential. Bargain-priced tractor hydraulic fluids simply cannot match the performance of higher-quality fluids when it comes to shear stability and low temperature performance. The use of these low quality fluids can cause damage to equipment that far exceeds the cost of using a premium quality fluid.

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