by Chris Woodford. Last updated: September 13, 2021.
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: The first tractors were little more than replacements for farm animals: all they could do was pull things. Modern tractors can do much more because they have a power takeoff (the rotating power axle at the back) and front and rear hitches (the hydraulic lifts on the front and back). This John Deere is cutting a hedge in Staffordshire, England with the yellow implement attached to the back (a Shelbourne Reynolds Powerblade).
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.
Photo: Tractors are designed both for pulling heavy loads and powering implements (plows, trailors, and other kinds of machines) attached to the back. This John Deere tractor is doing both at the same time: the tractor powers the machinery inside the hay baler it's pulling behind it.
Tractors have large and powerful diesel engines, just like
bulldozers, cranes, railroad trains, ships, and other giant machines. 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.
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.
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.
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 what they were mostly used for 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.
Photos: Blasts from the past: 1) An old, almost
hand-made looking tractor. This is so primitive that you can almost see how it all works. Look at the steering wheel and its linkage, for example, or the parts of the engine. Notice how the rear wheels are made with spokes and little wedges around the rim to dig and grip the soft ground. 2) A tricycle-type John Deere tractor. Some tractors were built this way, with front wheels close together, to allow them to be driven through rows of growing crops without squashing them. They were extremely unstable, however, and that design is no longer popular today.
Both photographs courtesy of the Carol M. Highsmith Archive, Library of Congress, Prints and Photographs Division.
Pulling and powering
Modern tractors are much more sophisticated than traction engines and they can do all kinds of things, although their main job is still to pull things. Tractors can pull or power implements (farm machines such as plows, trailers, hay balers, manure spreaders, and so on) in three different ways: using a simple drawbar, with a much more sophisticated three-way attachment called a hitch, or (if the implement needs power) with a mechanized driveshaft called a power takeoff.
The simplest way to pull things like trailers or wagons that don't need any kind of lifting is with a sturdy
steel rod called a drawbar, which makes a secure but flexible link between the tractor and whatever is following it. Something like a trailer can be attached to the drawbar with a thick metal rod
called a drawbar pin, held in place by a metal security clip. The trailor then pivots on the drawbar so a tractor can easily pull it around corners, although reversing is a bit more tricky! The drawbar can be pulled out or pushed in, fixed in place (stationary) or allowed to pivot so it swings from side to side, allowing trailors and things turn behind it more easily. A drawbar can be slid out of the way or removed altogether if the farmer wants to use a more elaborate and secure form of attachment called a hitch.
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. Modern plows, like the one being towed behind this Massey-Ferguson tractor, were
invented about a century later by Illinois blacksmith John Deere (1804–1886).
This plow is being lifted, raised, and dragged through the soil by a lifting system called a hydraulic hitch, which Massey-Ferguson originally pioneered.
All a horse can really do is drag something behind it with a drawbar, which is a problem because implements have to be lifted up and moved from one field to another, sometimes by driving them down public roads. Modern tractors get around this using a three-way pulling and lifting system at the back, known as a hitch.
Photo: Look around the back of a tractor and you'll see a complicated, hydraulic lever mechanism called a three-point hitch. Implements like plows are attached here.
A hitch has many advantages over a simple drawbar. It's usually called a three-point hitch (or three-point linkage) because it attaches an implement, very securely, in three places: with a central link at the top and
with two hydraulically powered lift arms, one on either side. The hydraulic arms make it easy for a tractor to lower something like a plow when it is working on a field, and then raise it up again to drive it somewhere else—but that's not all a hitch does. It can raise and lower implements off the ground with a flick of a switch, but it also automatically keeps the tractor pulling effectively as the ground conditions change and give more resistance. Being attached in three places at once keeps implements level and stops them wobbling from side to side. It also transfers an implement's weight (and the load of pulling it) to the back wheels of the tractor, giving it more grip against the ground. The mechanical design of the hitch keeps the whole tractor safe and stable and stops it flipping backward if the implement it's pulling suddenly snags in the ground. Very usefully, it allows one tractor to work with many different implements.
Almost all tractors use similar hitches, so virtually any implement will work with any make of tractor. Harry Ferguson, an Irish-born British tractor pioneer, popularized the modern three-point hitch system in the 1940s, though it was largely developed by his engineer and mechanic, Willie Sands. The hitch made Ferguson rich and famous, partly because of a short-lived partnership with Henry Ford; today, Ferguson is best remembered as one of the founders of the Massey-Ferguson tractor company.
Photo: Rear view of a tractor. The hydraulic hitch is a lever mechanism that raises and lowers implements, powered by two lift arms (hydraulic rams), one on either side. You can also see a drawbar and the power takeoff just above it. At the top, there are hydraulic connections to power implements and 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. As its name suggests, the PTO's job is to power an implement, not pull it. That means, even if you're using an implement with a PTO, you need to hook it up to either the drawbar or the hitch as well.
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 500–1000 rpm 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 mower is attached securely to the
Other parts of a tractor
Although most tractors attach implements to the rear, some have front hitches as well, which makes it possible to use two
different implements at once in a single drive through a field. Some tractors also have a front PTO.
Photo: Front hitch: a close-up of the John Deere in the top photo, showing the front hitch
The most noticeable thing about a tractor are its giant wheels and tires. Big 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. The more the tires spread the load, the less "compaction" damage (squashing) the tractor does to the soil it's driving over.
Photo: Tractor tires are bigger and chunkier than car tires and often tubeless (without separate inner tubes, like the ones in bicycle tires). That doesn't mean they're solid lumps of rubber! It means the air fills up the entire space between the tire and the wheel rim. Some tractors use what are called flotation tires (extra-wide tires that spread the load and sink less on soft or sandy terrain). Others have "dual wheels" and tires (four wheels and four tires at the back and sometimes four at the front as well). This gives extra pulling power, better flotation, and more grip on boggy ground.
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!
Sometimes tractor tires are partly filled with calcium chloride solution to give them extra weight, which is called ballast. This improves the tires' grip and helps to lower the tractor's center of gravity (so the overall weight is lower down, making the tractor less likely to tip over). "Partly filled" usually means a maximum of between a half and three quarters full. Why not just use plain old water? Because, in cold countries, it would simply freeze into a solid lump in winter. The calcium chloride lowers the freezing point (much like antifreeze) and keeps the solution in a liquid form.
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. A few experimental vehicles (like the one in the photo here) are powered by
hydrogen fuel cells. They make electic power from a tank of hydrogen, like a cross between
diesel engines and batteries.
Photo: This New Holland tractor is powered not by an engine but by a hydrogen fuel cell.
Photo by Keith Wipke courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL) (NREL photo id#33995).
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 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 rollover bars. Older models without cabs just have a simple rollover bar behind the driver's seat. Although tractors could never be described as luxurious, most now have heated cabs, some have air conditioning, and quite a few are fitted with GPS satellite navigation (sat nav) to help farmers plan how they work their fields with military precision.
Some tractors even have small fridges for keeping your lunch cool.
It's all a far cry from working your fields with a horse and cart!