by Chris Woodford. Last updated: October 4, 2012.
If you could have only one vehicle in the entire world, what would you pick? Many of us would opt for an amazing sports car or even a race car. Some would go for a four-wheel drive or a pickup. Given the choice of absolutely any vehicle, I think I might go for a tractor. Armed with a powerful diesel engine and huge chunky tires, I could go anywhere and do anything! Tractors are truly amazing vehicles, but have you ever stopped to consider what makes them so great? Let's take a closer look!
Photo: Farm machinery has been making light work of jobs like this since the early 18th century, when English farmer Jethro Tull (1674–1741) revolutionized agriculture by inventing the first mechanized seed drill. Photo by Keith Weller courtesy of US Department of Agriculture/Agricultural Research Service.
What is a tractor?
If you've ever been stuck behind one in slow-moving traffic, you probably think a tractor is a nuisance—but that's hardly a definition! The word "tractor" is related to words like "traction" and "tractive," from the Latin word "tractus" meaning drawing (pulling): a tractor is essentially a machine designed to pull things along, usually very slowly and surely.
Tractors have large and powerful diesel engines and, in theory, that means they should be able to go incredibly fast, just like sports cars. But in a tractor, the engine's power is designed to be used in an entirely different way: for pulling big and heavy loads. What makes this possible is the tractor's gearbox, which converts the high-speed revolutions of the mighty diesel engine into much lower-speed revolutions of the wheels, increasing the force the tractor can use for pulling things at the same time. If you know anything about gears, you'll realize that a tractor's incredible pulling power must come at the expense of speed: if you ever see a tractor going really quickly, chances are it isn't dragging a heavy load. Likewise, if you're stuck behind a really grumbling tractor inching down the road, you'll know the engine is probably working as hard as it can pulling a load.
Photo: Tractors are designed both for pulling heavy loads and powering implements attached to the back. This one (a New Holland, I think) is doing both at the same time. Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
Why tractors were invented
Tractors were originally designed to replace working animals such as oxen and horses, which people have been using to pull carts and plows since ancient times. One of the pioneers of modern tractors, American industrialist Henry Ford, got at least part of his inspiration from a simple determination to come up with something better than the horse for doing heavy farm work.
Before tractors came along, horses made life much easier for farmers, but all they could really do was pull things. Because early tractors were merely replacements for horses, pulling things was pretty much all they could do as well. Those early tractors were fueled by coal and known as steam traction engines. They looked like small steam locomotives, with large, sturdy metal wheels capable of rolling down roads, and they first appeared toward the end of the 19th century. Catching sight of one of these early coal-powered lumbering beasts was what really inspired Henry Ford to start developing tractors of his own. As he later remembered in his autobiography, My Life and Work:
"I felt perfectly certain that horses, considering all the bother of attending them and the expense of feeding, did not earn their keep. The obvious thing to do was to design and build a steam engine that would be light enough to run an ordinary wagon or to pull a plow. I thought it more important first to develop the tractor. To lift farm drudgery off flesh and blood and lay it on steel and motors has been my most constant ambition."
You can read more about Henry Ford in our article on the history of cars.
Photo: A steam traction engine similar to the one that inspired Henry Ford. You can see that it's essentially a big metal cylinder on wheels. The cylinder (colored red and gold) is where water is boiled to make steam. The wheels are made entirely out of metal, with spokes but no modern rubber tires.
Parts of a tractor
Modern tractors are much more sophisticated than traction engines and they can do all kinds of things, thanks to some really useful features.
Hauling heavy loads is still one of the most important jobs that a tractor does for a farmer. Tractors pull implements (farm machines such as plows, trailers, hay balers, manure spreaders, and so on) using a sturdy rod called a drawbar, which makes a secure but very flexible link between the tractor and whatever is following it. The drawbar can pivot so a tractor can easily pull its load around corners. Reversing is a bit more tricky!
All a horse can really do is drag something behind it, which is a problem because implements often have to be moved from one field to another, sometimes by driving them down public roads. Modern tractors get around this using a hydraulically powered lifting system at the back, known as a hydraulic hitch. This can raise and lower implements off the ground with a flick of a switch. The hitch makes it easy for a tractor to lower a plow when it is working on a field, and then raise it up again to drive it somewhere else.
Photo: Rear view of a tractor. The hydraulic hitch is a lever mechanism that raises and lowers implements, powered by two hydraulic rams either side. You can also see the tow bar and the power takeoff just above it. At the top, there are electrical sockets that can be used to power things like brake lights on implements when they're being towed on the road.
Early traction engines could be used to power harvesters, elevators, and other kinds of equipment by parking them, disengaging their driving wheels, and then transmitting their power to another machine. Typically, this was done by looping a long rubber belt over the spinning wheel on top of the traction engine so it passed over a similar wheel on the machine that needed to be driven. Power was carried between the machines in much the same way as a bicycle chain takes power from the pedals to the back wheel (only with a rubber belt instead of a metal chain).
Virtually all modern tractors can power implements or machines using what's known as the power takeoff (PTO). It's a rotating shaft, usually at the back of a tractor, from which power can be taken from the tractor's engine. To use the power takeoff, you need to hook up a special spinning rod (with clever, flexible connections called universal joints) between the tractor and the implement. A machine like a hay baler has spinning rakes, wheels, and gears inside it. When it's hooked to the back of a tractor, it's connected to the power takeoff so the tractor's engine powers the machinery inside the bailer as well as driving its own wheels. That's why tractors pulling powered machinery have to drive relatively slowly: a fair bit of their engine power is being diverted to the equipment behind them.
Look closely at a tractor working in a field and you can often see the power takeoff rod spinning between the tractor and whatever it's pulling. But never get close to one: they spin at about 500rpm and can be extremely dangerous.
Photo: The yellow bar coming off the back of this John Deere tractor is the spinning power takeoff—one of the most dangerous parts of a tractor. Beneath the power takeoff, the drawbar is what attaches the mower securely to the tractor.
The most noticeable thing about a tractor are its giant wheels and tires. Large pneumatic (air filled) tires spread the weight of the tractor over a larger area and deep treads give excellent grip. By reducing the pressure on the ground, the tires stop it from sinking in to soil and mud that would quickly bog down a conventional car.
Most tractors have two-wheel drive, with the large rear wheels driven from the engine and the small front wheels used only for steering. Since a tractor is usually pulling things, that's fine: the heavy weight behind it pushes the rear wheels down, increasing their grip, so there's no particular advantage in having powered front wheels as well. (Four-wheel drive tractors are also less common because they're more complex and expensive.) Some tractors have extremely wide, multiple wheels and tires for negotiating particularly soft or difficult terrain; for obvious reasons, you won't see those on the roads!
Photo: This John Deere tractor has an extra set of tires and wheels to give it super-good grip. Photo by Scott Bauer courtesy of US Department of Agriculture/Agricultural Research Service.
Tractors are generally powered by large diesel engines, which are particularly good at providing high pulling power at very low speeds (that's why they're used in trucks and buses). Smaller tractors may have gasoline engines and some are powered by LPG (liquified petroleum gas), usually to make them more economical or environmentally friendly.
Photo: Another John Deere tractor, this time with a more environmentally friendly LPG propane engine. Photo by Jerry Hymer courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
Driving a tractor might look easy—the engine is doing most of the work!—but it needs a great deal of skill. Power-assisted steering and braking are essential to help tractor drivers keep heavy loads safely under control. Since tractors are heavy and often have to work on steep slopes and soggy, unstable ground, there's always a risk they might tip over so modern tractors generally have reinforced cabs fitted with anti-roll bars. Although tractors could never be described as luxurious, most now have heated cabs, some have air conditioning, and a few are fitted with GPS satellite navigation (sat nav) to help farmers plan how they work their fields with military precision. It's all a far cry from working your fields with a horse and cart!