Making lawn mowing fun

Zero-turn-radius lawnmowers have been available since the mid 1960s. While they used to be used by professional lawn maintenance services, over the last four decades they have become increasingly popular with individual consumers.

The zero-turn-radius mower’s manoeuverability in small spaces and around obstacles allows operators to pivot 360 degrees in one spot for precision cutting. This ability - along with ground speeds up to 15 mph- reduces mowing time compared to the traditional lawn tractor allowing lawn maintenance services more profitability and consumers more leisure time.

Left and right steering levers directly linked to the mower’s transaxles provide the control of the mower’s speed and pivoting actions.

In the past, this control was provided by friction plates, but as mobile hydraulics have become more cost effective, manufacturers have adopted hydrostatic transmissions to operate and control the drive wheels.

One such company - Magic Circle Corporation - makers of the Dixie Chopper lawnmower, adopted hydrostatic transmissions for their new Silver Eagle mowers — a line of medium-duty, rear-engine, mid-deck mowers designed for the consumer and entry-level lawn maintenance markets. These hydraulic transmissions consist of two pump/motor combinations — one for each drive wheel.

Traditionally, these transmissions were made up of pumps from one manufacturer and motors from another. However, for this application, Magic Circle sought to find one vendor to supply a total package.

White Hydraulicswas able to satisfy this design criterion with their “Total Drive System (TDS).” Besides reducing administrative, purchasing and logistical costs, the Total Drive System enabled Magic Circle to match pump and motor displacements to meet the machine speed and weight, engine horsepower and RPM, wheel diametre and grade climb design criteria.

For this application, Magic Circle wanted to achieve a ground speed of about 8 mph with a 30-degree grade climb capacity. To meet these design criteria White Hydraulics combined their KP series pump RE series motor into a complete drive system.

The steering levers control each hydraulic pump’s (gray) displacement volume and flow direction causing the motors (red) to turn the wheels.

The KP series pump itself is a direct-control, variable displacement, axial-piston design. It can generate pressures up to 3,500 psi with maximum continuous pressures at 1,425 psi.

In the design, the steering levers control the pumps’ flow direction (forward/reverse) and displacement volume. Having both levers in the neutral position stops the mower. Pushing on them moves the mower forward. Pulling on them moves the mower backward. Pushing one while pulling one turns or pivots the mower. The degree to which they are pushed or pulled controls the speed of the mower and what turning radius the operator will have.

Most zero-turn-radius mowers are designed to operate at an oil temperature of 200 to 220OF, however, if the mowers are run full tilt on a hot day for an extended period of time this temperature can come close to 250 OF — the point which oil begins breaking down. The oil breakdown includes viscosity changes and loses of lubricity and anti-wear characteristics and oil film. The oil film is needed to lubricate and cool the thrust bearing, pistons and the wear plate in the pump along with the gerotor set in the motor. When the oil breaks down, it accelerates the point of a failure, such as galling (internal welding) and lock-up of the pump, motor or both.

This simplified schematic shows the hydraulic transmission circuit for a zero-turn-radius lawnmower. Each pump/motor combination drives a wheel.

The KP series pump has increased case flow due to a larger charge pump and optimized orifice sizing, doubling the amount of oil going to the heat exchanger and reservoir, which helps to keep the oil temperature as cool as possible. An added benefit of this combination is its ability to make up for system loses during dynamic braking on downhill grades while keeping oil pressures low.

Keeping system pressure low also helps to keep motor efficiency high and extend service life essential to achieving the nominal zero-turn-radius mower’s 500 to 700 operational hours each mowing season. To that extent, the RE series motor uses a 1.5” diameter driveshaft and dual driveshaft bearings, which are matched to the application.

While being made of a lightweight die-cast aluminium housing, to reduce machine weight, the KP series pumps are ruggedly constructed helping the mower provide a long maintenance free life. This construction includes a steel wear plate with a bronze overlay, a steel swash plate, steel pistons, a bronze bushing and a cylindrical roller bearing.

With each passing day, noise generated from outdoor power equipment, such as zero-turn-radius lawnmowers, is becoming a large issue for homeowners and municipalities.

Municipalities are passing stricter and stricter ordinances governing how much noise will be tolerated and when. In Europe, ISO 11094 stipulates that the sound power generated by a large lawnmower cannot exceed 105 dB and this will be reduced to 103 dB in 2006.

While Magic Circle has not conducted noise testing on the Silver Eagle line, it believes that the KP series pump is noticeably quieter than other brands, helping with noise abatement.