Hydrostatic drive systems have been a staple of riding mowers and other large equipment for decades, but they’ve only recently reached walk-behind equipment including Billy Goat’s aerators and brushcutters. What are the advantages? Do they require any maintenance? What kinds of problems can they have? Here’s what you need to know if you’re considering or already own equipment with one of these transmissions.
How Does It Work?
A hydrostatic drive uses two main components: a pump and a motor.
The pump has a circular cylinder block, a set of pistons and a wash plate. The pistons mount to the wash plate using ball joints. This plate sits at an angle. As the plate and cylinder block spin, the pistons move up and down inside the block, pumping the hydraulic fluid.
The motor uses the same parts as the pump, but the angle of the wash plate can be changed to control the motion and speed of the output shaft. Tilt it one direction, and the shaft spins clockwise. Tilt it the other way, and it will spin counter-clockwise. Change the amount of tilt, and the pistons will move more or less with each rotation, trading motor speed for torque or vice versa.
Billy Goat uses “close coupled” hydrostatic transmissions. This design puts the pump and motor into a single unit that shares the same valve body. This makes the system compact and eliminates high-pressure hoses, the most common source of problems in hydrostatic drive systems. These units also have a built-in differential, making them transaxles. The wheels bolt up directly to the axles, and the engine powers the pump using a belt drive connected to a pulley on top of the transaxle case.
Why Use a Hydrostatic Transmission?
Speed is controlled entirely by the position of the hydraulic motor’s wash plate. This lets the drive speed be completely independent of the engine speed so the tool can be driven at full speed no matter how fast the equipment is moving. It also allows for infinite speed adjustment while in motion, letting the operator match speed to the terrain without having to stop and change gears.
Efficiency has improved tremendously in the past decade, making this lawn mower staple practical in smaller machines. While older designs greatly increased weight and power demands where they were used, today there’s little in the way of a weight or power penalty when choosing a hydrostatic transmission over a geared transmission
Which Models Use Hydrostatic Drive?
Currently, Billy Goat makes 6 pieces of equipment with a hydrostatic transaxle:
– OS901 Hydrostatic Overseeder
– AE1300H Hydro Aerator
– PL2500 PLUGR Hydro-Drive Aerator
– Next Gen 18” Hydro Drive Sod Cutter
– 18” Hydro-Drive Sod Cutter for Golf Applications
– BC26HHEU Outback Brushcutter
How Do I Maintain My Transaxle?
The Tuff Torq transaxles Billy Goat use have an aluminum case to dissipate heat. If this becomes caked in dirt or mud, the pump and motor can overheat. Check the case if your equipment is coated in mud, and make cleaning a regular part of maintenance.
A failing transmission can slip if damaged, but this symptom is usually caused by problems with the belt between the transmission and the engine. As with any belt drive, the belt should be replaced if it shows signs of cracking or tearing, or it has stretched to the point that the idler pulley can’t take up the slack.
The transaxle should last as long as the equipment without needing a fluid change. If you do need to change the fluid due to contamination or a leaking seal, it’s highly recommended that you use Tuff Torq’s own fluid, as it has the additives needed to deliver this long fluid life. For most models, the company recommends using 10W30 diesel motor oil as an alternative fluid, not hydraulic fluid. They specify Class CC or CD oil, which is a very old API service classification. Any off-the-shelf CJ-4, CI-4 or CH-4 oil will meet these service requirements.
How Do I Identify the Transaxle?
The serial number is printed on a flat metal surface either on the top or front of the case.
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