Throughout the range of tractor sizes, you have a choice in transmissions. Three primary transmission systems are used in riding mowers and the various classes of mowing tractors.
Mower decks are designed to operate at a constant speed - typically full engine speed. This means adjustments in vehicle speed have to come from the transmission, not by varying the engine speed.
Belt transmissions are the simplest and cheapest, and they 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 it may be the only transmission.
Belts have the advantage of not needing a separate clutch; the belt can be slackened to provide clutching. Variable-speed belt drives are obtained by moving one side of a sheave in or out, thus changing the pitch diameter of the sheave. The second sheave is normally spring loaded, so it responds in a direction opposite to the control sheave. Thus, as the operator moves the speed control lever, one sheave is, in effect, made smaller while the other sheave is made larger. This causes the transmission ratio to change and slows or speeds the tractor.
Belt drives can work quite well on lower-horsepower tractors and provide an inexpensive way to vary speed on the go. Variable-speed belt transmissions are normally controlled by a lever and should be changed only on the go.
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 (Figure 1). 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."
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. 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.
Hydrostatic transmissions offer the most flexibility in speed control, but they come with a price (Figure 2). Hydrostatic transmissions are more expensive than other types and have lower efficiency, meaning you will burn more gallons of 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. Hydraulic oil and components must be kept squeaky-clean! Hydrostatic transmissions consist of a hydraulic pump, driven by the engine, supplying oil under pressure to a hydraulic motor that drives the wheels. The displacement of the pump can be increased, decreased to zero or reversed, thus providing a range of forward, neutral and reverse speeds. It is also possible to reduce the displacement of the 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. Hydrostatic transmissions make turning and maneuvering a mower much easier. Hydrostatic transmissions are usually offered on lawn, lawn and garden, garden and compact tractors; they 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 or her foot from the pedal while mowing.