Hydrostatic Transmissions Require Advanced Fluids.

Professionals have zero tolerance for zero-turn-mower inefficiency.

Mike Nelson | Senior Product Development Engineer

Landscape professionals rely on their zero-turn mowers to get work done on time and up to standards. When a crew member jumps on a mower and pushes the handles forward, he expects instant and predictable mower response. Anything else wastes time and reduces efficiency.

AMSOIL 20W-50 Synthetic Hydrostatic Transmission Fluid - Quart Bottle

AMSOIL Synthetic Hydrostatic Transmission Fluid (AHF) helps deliver smooth, responsive operation. To understand how, let’s look at how a zero-turn-mower hydrostatic transmission works.

In most systems, hydraulic lines connect a variable-displacement pump to a fixed-displacement motor that powers one of the rear wheels. Moving the mower handle forward increases pump displacement and turns the motor that turns the wheel. The system offers infinite variability, meaning the mower’s engine can be running at 3,600 rpm, but wheel speed depends on how far forward the operator pushes the handle.

A typical hydrostatic transmission also uses a charge pump, which cycles new fluid into the hydrostatic loop to replace fluid that leaks through the pump cases. This provides the added benefit of cooling the pump and motor as new fluid enters the loop.

Variable-displacement piston pumps are the mainstay in zero-turns. They use a ring of pistons that ingest and compress fluid based on the angle of a swash plate connected to the control handle.

Some zero-turn mowers use what’s called a gerotor motor. They use a valve body to pressurize fluid between a rotor and stator, which causes the rotor to orbit. As it orbits clockwise, it rotates counterclockwise at a much lower speed. The rotor is splined to the output shaft to turn the mower wheels.

Hydrostatic drives place specific demands on fluid. The gerotor motor in particular generates increased sliding forces on the splines. This requires anti-wear additives like zinc and phosphorus to form a wear-resistant barrier on parts to prevent scuffing and wear. Because these transmissions operate with extremely tight clearances under high pressure, they mechanically shear lesser-quality oils, reducing viscosity and changing the behavior of the machine.

To deliver maximum performance, AMSOIL Synthetic Hydrostatic Transmission Fluid is formulated with high-quality base oils that resist shear. In severe service, they maintain viscosity better than fluids that use viscosity modifiers to compensate for lower-quality base oils. They also resist oxidation in extreme heat better due to their uniform molecular structure. This provides more predictable performance while fighting harmful varnish and deposits inside the drive.

When developing our hydrostatic fluid, we tested various formulations in our mechanical lab. Our testing is designed to punish lubricants beyond conditions seen in the real world. We push them to the limit for up to 1,000 hours to see how the oil formulations and hydrostatic drives survive. Some of the wear-prone areas we see in these units are in the motors that drive the output shaft. As you can see in the image, it’s evident when a test formulation fails to protect metal in this critical area. Wear like this leads to inefficiency and increased replacement costs.

Zero-turn-mower hydrostatic drives are ingenious systems that help workers cover a lot of ground quickly – if the fluid can protect against wear and deposits. Thanks to extensive testing, AMSOIL Hydrostatic Transmission Fluid delivers. It helps mowers and other equipment provide the smooth, responsive operation needed to get jobs done on time and up to standards.

AMSOIL Synthetic Zero-Turn Hydrostatic Transmission Fluid is for use in hydrostatic transmissions that require 15W-50 or 20W-50 oil recommended for API SM, SL, SJ, SH or SG, including (not limited to) those found in zero-turn, riding and stand-on mowers made by SCAG*, Toro*, Ferris*, Snapper Pro*, Gravely*, Ariens*, Exmark*, Walker*, Hustler*, Wright*, Dixie Chopper*, John Deere*, Husqvarna*, EverRide*, Bad Boy* and Cub Cadet*.
Not for use in engines.

For more information on AMSOIL Synthetic Zero-Turn Hydraulic Transmission Fluid or any AMSOIL Product contact SLS Associates.

*All trademarked names and images are the property of their respective owners and may be registered marks in some countries. No affiliation or endorsement claim, express or implied, is made by their use. All products advertised here are developed by AMSOIL for the use in the application shown.

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