Richard L. Parish | 3/9/2006 10:51:15 PM
Shopping for a riding mower or mowing tractor requires several choices – including the transmission, according to an engineer with the LSU AgCenter’s Hammond Research Station.
Riding mowers and the various classes of mowing tractors use three primary transmission systems, says Dick Parish.
"Mower decks are designed to operate at a constant speed – typically full engine speed," Parish says. "This means that any adjustments in vehicle speed have to come from the transmission, not by varying the engine speed."
Parish says belt transmissions are the simplest and cheapest and are found on some of the smaller riding mowers and tractors. In some cases, a fixed-speed belt drives a 3-, 4- or 5-speed gear transmission. In other cases, a variable-speed belt drive is used. The variable-speed belt drive may be used in combination with a gear transmission or may be the only transmission.
"Belts have the advantage of not needing a separate clutch," he says. "The belt can be slackened to provide clutching."
Belt drives can work quite well on lower-horsepower tractors and provide an inexpensive way to vary speed on-the-go, the engineer adds. Variable-speed belt transmissions are normally controlled by a lever and should be changed only on-the-go.
Parish says gear transmissions are similar to manual transmissions on cars. A gear transmission on a small tractor generally provides three to five forward speeds plus reverse. A gear transmission may be connected to the engine by a mechanical clutch or by a belt drive, which can be slackened to provide declutching.
On most small tractors, the gear transmission is integrated with the axle and differential into something called a "transaxle," Parish says. Mechanical transaxles tend to be durable and trouble-free. Unless coupled with a variable-speed belt, they do not offer incremental speeds between the gear selections.
"With a gear transmission, you get low cost and high reliability but give up infinite speed control," Parish says. "When a gear transmission is combined with a variable-speed belt drive, you get a variable speed drive combined with low cost. Gear transmissions are controlled by a lever, and a clutch pedal must be depressed before shifting."
Parish says hydrostatic transmissions offer the most flexibility in speed control, but they come with a price.
"Hydrostatic transmissions are more expensive that other types and have lower efficiency," he says. "You’ll burn more fuel doing the same mowing job and will probably also need a more powerful engine for the same size mower.
"Hydrostatic transmissions also require more careful maintenance," Parish adds. "Hydraulic oil and components must be kept squeaky-clean."
The engineer explains that hydrostatic transmissions consist of a hydraulic pump driven by the engine to supply oil under pressure to a hydraulic motor that drives the wheels. The displacement of the pump can be increased, decreased to zero or reversed to provide a range of forward, neutral and reverse speeds. It is also possible to reduce the displacement of the hydraulic motor to provide the equivalent of a "high gear," but this is seldom done.
"Hydrostatic transmissions offer the ultimate in tractor control but are expensive to buy and operate," Parish says.
Hydrostatic transmissions make turning and maneuvering a mower much easier. Hydrostatic transmissions are usually offered on lawn, lawn & garden, garden and compact tractors and may be standard with some brands and models. Hydrostatic transmissions can be controlled by a hand lever or by one or two foot pedals. An increasingly common design uses one foot pedal for forward and a second pedal for reverse. A "cruise control" is sometimes provided with a hydrostatic transmission so the operator can remove his foot from the pedal while mowing.