Choosing the Right Tractor
Transmission and horsepower
A mechanical transmission has long been the norm, but has recently given way to a hydrostatic model. A mechanical transmission is very similar to the selective gear transmission in a car or truck. The operator uses a shift lever to select a specific gear, which tells the tractor to maintain a constant ground speed for a constant engine speed.
Most transmission models have two gear selector levers. One allows for on-the-fly shifting between a few gears, meaning the gear can be changed while the tractor is moving. The second lever chooses the gear range, but can only be shifted when the tractor is at a complete stop.
A hydrostatic transmission does not have a lever to select specific gear ranges. The advantage is that any tractor ground speed can be chosen between zero and the maximum speed. This feature would be useful in light mowing operations where the implement power is low and the need for operating efficiency is very high. The major drawback to a hydrostatic transmission is that it is much less efficient than a mechanical transmission, which results in a significant increase in power loss.
When studying a tractor's horsepower, you should carefully consider the type of horsepower measurement being stated by the manufacturer or dealer. Two ratings are used to define tractor power: Brake (or engine) horsepower and PTO horsepower (the most common).
"The brake horsepower is what is put out by the engine without loss caused by the transmission and other components," explains Nowatzki. "The PTO horsepower is the power that is available to operate equipment from the PTO shaft."
Different implements require certain PTO horsepower, so knowing the tractor's PTO rating is important when you are sizing an implement to a tractor.
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