Special thanks to Caterpillar for all of the help and information they have provided in creating this article!
The Caterpillar® equipment that you see working on a construction site or in a farmer's field is always amazing! There is the size of it, the brute power, and the ability it has to literally "move mountains" -- everything about it is larger-than-life.

Caterpillar is the world leader in a variety of industries. Caterpillar's product line ranges from the company's new line of compact construction equipment to hydraulic excavators, backhoe loaders, track-type tractors, forest products, off-highway trucks, ag equipment, diesel and natural gas engines, and industrial gas turbines.

Have you ever stopped to think about how they make equipment this big? Where do they start? What does the factory look like? How do the people who build the equipment move the huge pieces? How do they test it when it is done?

If you have ever wondered where Caterpillar machines come from, then this edition of Stuff.dewsoftoverseas.com will be fascinating! We will travel to a Caterpillar factory in DeKalb, IL, where Caterpillar Agricultural Products Inc. assembles gigantic Challenger agricultural tractors that weigh up to 40,000 pounds. These tractors are used by innovative farmers around the world, as well as on some construction sites.

The Challenger 95E tractor from Caterpillar, weighing about 40,000 pounds, ready for shipment!

These tractors, known as large Challenger tractors, have incredible pulling power, but on this factory tour you will see that each one starts with a single strip of steel. It is then assembled piece-by-piece by skilled people until the finished tractor finally rolls off the end of the line and is ready to get to work!

Inside the Challenger Tractor

Special thanks to Caterpillar for all of the help and information they have provided in creating this article!
Before we start talking about the assembly line for the Challenger tractor, it is helpful to learn a little bit about the tractor itself as well as the parts that go into it. That way, you'll understand how everything fits together as you learn about the assembly process.

The Challenger from Caterpillar is one of the world's most powerful agricultural tractors that you can find both on farms and on some construction sites.

The most obvious thing that distinguishes the Challenger tractor from a wheel tractor is its tracks. The advantage of tracks is that wide rubber belts spread out the tractor's weight over a large area:

  • Tracks give the Challenger tractor better traction. The amount of contact area that the four wheels of a wheeled tractor might have with the ground is just two or three square feet per tire at the most. The contact area of a Challenger tractor approaches 90 square feet.
  • Tracks reduce soil compaction. If you have ever walked in a freshly tilled field you know that your feet compress the soil and leave footprints. That's compaction. If you were to wear snow shoes you would compress the soil less by spreading out the load. Large tractors weigh tens of thousands of pounds, so spreading out the load to reduce compaction is important.
  • Tracks also give better flotation. If a wheel tractor tried to drive through a muddy field, it would likely get stuck. Tracks, however, allow a Challenger tractor to work right through the wet field.
The fact that the belts are made of rubber is also unique. Most Caterpillar bulldozers use steel tracks instead of rubber because steel tracks last a very long time and have great traction. The problem with steel tracks is that you cannot take them on the road because they chew up the asphalt. Rubber belts let farmers move quickly between fields and paved roads, at speeds up to 20 MPH.


The following short videos show you different features of the Challenger tractor:
Large Challenger tractors are huge, standing 12 feet tall, 21 feet long and weighing up to 40,000 pounds. They come in four models:

  • 65E - 310 gross engine horsepower
  • 75E - 340 gross engine horsepower
  • 85E - 375 gross engine horsepower
  • 95E - 410 gross engine horsepower
This article talks about large Challenger tractors. But Challenger tractors also come in a smaller "row-crop" version. Challenger row-crop tractors stand 10 1/2 feet tall, are nearly 18 feet long, and weigh up to about 22,000 pounds. The space between the belts can be adjusted for the tractor to work between rows of corn or other crops. These tractors can also be used for some soil tillage work and many other jobs around the farm.

Row-crop Challenger tractors come in three models:

  • 35 - 175 PTO horsepower
  • 45 - 200 PTO horsepower
  • 55 - 225 PTO horsepower

The tractors also come with many different options. The customer can order special hitch and PTO packages, different tread widths, regular or deluxe cabs, etc. As each tractor moves down the assembly line, a machine order tells the people working on the line exactly what equipment will go on each tractor.

Like any large piece of equipment, Challenger tractors contain thousands of parts. The following diagrams help you to understand how all the parts fit together.

A cutaway drawing of the Challenger tractor

In the drawing above you can see, from front to back, the radiator and other cooling modules, the engine, the transmission, the differential steering system and the final drive, as well as the undercarriage (with front axle, tensioning cylinder and bogies), the cab and the fuel tank in the rear.

There are separate cooling modules for the engine, the differential oil, hydraulic oil, and the turbo-charged air cooler

The Challenger tractor's engine -- The tractor features an inline 6-cylinder diesel displacing 10.3 or 12.0 liters and generating up to 410 horsepower

The differential steering system is truly unique --
Learn more in this video

The single reduction final drive, which contains the largest bull gear found in any piece of Caterpillar equipment, connects the tracks to the differential steering

The front axle and its tensioning cylinder. The cylinder contains a nitrogen charge and an extremely large spring and generates 17,000 pounds of tension force on each track!

The bogie axles and mid-wheels help distribute the load while keeping the maximum amount of belt in contact with the ground at all times.

The Challenger tractor's cab

How the Factory Works

Special thanks to Caterpillar for all of the help and information they have provided in creating this article!
When you walk around the factory that builds Challenger tractors, you immediately notice two things:

  1. The factory covers several acres! It is so large, in fact, that there are bicycles located throughout the factory floor and people use them to ride from one end of the factory to the other!
  2. The parts that the workers must move around to assemble these tractors are huge. The engine weighs over a thousand pounds. So does the transmission. So does the cab. Because the tractor is so large, many pieces require overhead cranes or special lifting devices to move them.
This factory contains a number of specialized tools that help make assembly easier and safer for the employees who work in the factory. Here's a list that describes the most common: All of these tools together allow the workers in the factory to assemble these huge machines safely and efficiently.

The First Piece of Steel

Special thanks to Caterpillar for all of the help and information they have provided in creating this article!
Whenever you build something, there has to be a starting point. In the case of the Challenger tractor, the starting point is the frame rails, which you can see highlighted in this figure:

One of the main frame rails in a Challenger tractor, indicated by the red arrow. Note also the large triangular bracket below the rail just to the right of the arrow. You will see this bracket welded in place toward the end of this page.

These frame rails are about 13-1/2 feet long and run the entire length of the vehicle. They are important for three reasons:

The completed frame rails look like this:

Completed frame rails

The frame rails, and therefore an entire Challenger tractor, start their life on a robotic welder in the far back corner of the factory. This area is known as the Rail Weld Station:

For size comparison, this is the author alongside a single strip of steel -- one of the first pieces in the life of a Challenger tractor!

The piece of metal that you see on the welder is a steel strip that is 0.39 inches (10 mm) thick, 12.75 inches (32.7 cm) wide and 13-1/2 feet (4.1 meters) long.

This strip is welded to a C-channel to form what is known as a box section rail. Before that happens, several stiffeners are welded inside the C-channel to strengthen it at major stress points.

A template is used to align the gussets inside the C-channel.

A supervisor shows what C-channels look like after stiffeners are welded in place.

The robotic welder attaches the C-channel to the wrapper plate to create the completed box-section rail. During this operation, it is important that no warping occurs in the rail. Two things about the robotic welder eliminate warping:

  • The welder's rigid frame clamps the piece firmly using a number of hydraulic cylinders and holds it firmly.
  • Both sides of the rail get welded at the same time, so the expansion and contraction caused by heating moves uniformly down the piece.

An overhead crane and a magnetic clamp are used to move the C-channel onto the welder.

The robotic welder ready to begin welding.

The welds created by the robotic welder are extremely even and clean.

Once the frame rails are complete, they move to the Rail Assembly Station, where an extremely strong track tensioning mounting bracket and two differential mounting plates get welded on. The triangular mounting bracket will eventually hold one end of the massive pressure cylinder that provides the 17,000 pounds of force to tension the rubber belts, so it is pretty important in the grand scheme of things! In the first photo below you can see where two of the stiffeners welded inside the C-channel give the frame extra support around this bracket.

The mounting bracket in its fixture before welding.
The fixture ensures that the bracket is located in its exact position on the rail.

The mounting bracket after welding, along with a supply of completed rails.

Watch a frame rail being assembled in this video clip.

Once the frame rails are finished, they move down the welding line to form a complete frame...

Welding the Frame

Special thanks to Caterpillar for all of the help and information they have provided in creating this article!
The next step in the assembly process is to create the frame itself, starting with two of the frame rails seen in the
previous article. To make a complete frame, you start with two frame rails:

You add a very large steel casting called the differential case. It will hold the differential steering system and final drive gear box in the completed tractor:

You also add a large saddle to hold the front idler:

Along with the front nose plate:

All of these components are mounted together and held in place on a massive roll-over fixture, like this:

The frame is welded together on the roll-over fixture.

The same frame, shown at a later stage

Roll-over fixtures are extremely important to the welding process. They can roll the frame into any position so that the welder can work on the piece with minimum fatigue and maximum visibility. With a roll-over fixture, all welds are applied in the horizontal plane -- this produces the best weld possible.

Once the frame assembly is complete, the weld splatter is taken off with a rotary grinder. Removal of weld splatter is important because it reduces chafing on wire, cable, and hoses, and eliminates small pockets that can trap moisture and dirt.

The frame moves from station to station down the line using the overhead cranes. As it proceeds, dozens of brackets, bosses and pads are attached. These different pieces each have a specific role in the finished machine. Some of them act as mounting points for things like the engine, transmission and cab. Others act as tie-down points for cables and hoses. Their positioning must be precise, so special fixtures place them at exact locations and the welding process is controlled so that distortion and warping will not occur.

Dozens of brackets, bosses and pads adorn the finished frame

Once the frame is finished, an overhead crane lifts it onto a transporter that allows the frame to move easily across the factory. The frame is washed with a phosphorus solution and then moved to the paint booth, where a coat of prime paint is applied electrostatically. The paint is a two-part polyurethane, and it is positively charged while the frame is negatively charged. The paint is physically attracted to the frame by the opposing charges. This means there is virtually no overspray and that the paint moves into hidden areas even if the spray does not hit them directly.

The painted frame on its dolly ready to move to the assembly line.

Next the frame will move to the beginning of the assembly line...

Starting the Assembly Line

Special thanks to Caterpillar for all of the help and information they have provided in creating this article!
The welding line seen in the previous two sections is a fabrication process. Fabrication in this case involves the creation of something new (a frame) by welding together different pieces of steel. Once completed, the frame is a single, inseparable entity.

The rest of the Challenger tractor line in this factory is an assembly process, and is therefore referred to as an assembly line. Assembly involves bolting components onto the frame to create the final tractor. One difference between assembly and fabrication is that you can reverse the assembly process (by unbolting everything) and get back to the original frame. There is no easy way to undo the frame into its components -- it is a single piece. The advantage of assembly is that you can take things apart to fix them. The advantage of fabrication is strength. Each has its place in any factory, and the engineers who design anything carefully choose what will be assembled vs. what will be fabricated.

In this Challenger tractor factory, the frames move to the beginning of the assembly area to receive several different parts:

A collection of frames at the beginning of the assembly line

The large yellow disk is the parking brake

The installation of the wiring harness is one of the more involved parts of the process.

The wiring harness before installation

Uncoiled, the wiring harness is a 20-foot long branching vine containing dozens of individual wires and connectors. One interesting thing about the Cat wiring scheme is that every wire gets identified with number and color codes, which you can see in the following photo:

Each wire is coded with a unique number and color

For example, the number 200- means that the wire is a ground connection. A number like C3 or C19 identifies the wire uniquely, and its purpose in life can be decoded using the electrical schematics.

Part of the frame's welding process involved the installation of a number of bosses, pads and brackets, as seen here:

Many brackets, pads and bosses get welded to the frame

At the beginning of the assembly line, you immediately begin to see what they all do for a living:

Each bracket and boss holds something down in the finished tractor

Each one has a specific purpose. The wiring harness, for example, is attached to the frame at numerous points to prevent rubbing, movement, and flex that would shorten its life. Hoses also get attached in this way.

Probably the most important component installed in this part of the factory is the differential steering system. It drops into the differential case at the back of the frame. The case will eventually hold 59 gallons of oil to lubricate and cool the differential.

Once all of the components are installed and checked out in this area, the frame moves by overhead crane and gets married to the axles and mid-beam.

Marrying the Frame

Special thanks to Caterpillar for all of the help and information they have provided in creating this article!
The next step in the assembly process is the installation of the front axle and mid-beam. The front "axle" holds the front idler wheels, and the mid-level holds the four bogie wheels that support the middle of the track. The third axle -- called the final drive hub -- actually drives the tracks and will take its power directly from the differential. We will see it get installed in the next section of the factory. You can see all three of the axles in this cutaway:

The front axle and mid-beam are massive pieces of steel, as you can see in this photo:

The front axles

The front axle weighs more than 500 pounds, while the mid-beam weighs almost 400. The front hubs get attached to the front axle. Although it is called a front "axle," it is more like a steel beam. It does not rotate with the idler wheels. Among other things, it connects the front hubs to the tractor frame in a way that allows them to pivot/rotate for a smooth ride in varying ground conditions.

The front hubs ready for installation

Front axle, ready to accept the frame

The overhead crane lifts the frame, which now weighs over a ton, and carries it to the waiting axle and mid-beam. You can see the frame move into place in this short video.

The overhead crane positions the frame onto the front axles

The axle and mid-beam are attached to the frame

The axle and mid-beam are attached to the frame

It is at this point that the Challenger tractor officially arrives at the assembly line. In this factory, the assembly line is a railroad track several hundred feet long. On this track run small pneumatically powered "transporters," two for each tractor moving down the line. You can see the transporter in this photo and in this short video:

One of the pneumatically powered transporters that will carry the frame down the assembly line

Each frame sits on one powered transporter and a non-powered trailer, like this:

A powered transporter and trailer ready to accept a frame at the beginning of the assembly line

Track tension cylinders are installed on a frame at the beginning of the assembly line

Final Drive

Special thanks to Caterpillar for all of the help and information they have provided in creating this article!
At this point in the assembly process, the major components of the drive train get installed, starting at the rear of the tractor. The first component is called the final drive. The final drive contains the rear axle and will ultimately drive the Challenger tractor's rubber tracks. It takes power from the differential steering system and reduces it one last time using a massive pair of bull gears.

The final drive mates to the frame using more than two dozen bolts on a wide mounting flange. To prevent leaks, the flange is first coated with an anaerobic sealant that acts as a gasket:

Anaerobic sealant is applied to the final drive's mounting point

There are two final drive units on a Challenger tractor, one on each side. They provide a 5.08-to-1 reduction ratio between the differential steering system and the final drive axle that turns the tracks. Each final drive contains a massive, heat-treated bull gear that is about two feet in diameter and four inches thick -- it is the largest gear currently used in any piece of Caterpillar equipment.

The final drive weighs about 1,000 pounds.

An overhead crane is positioned to move the final drive.

The final drive is moved into position with the crane.

The transmission is the next drive-train component to be installed. It sits between the engine and the differential steering system. Challenger tractors use an automatic power-shift transmission with 10 forward and two reverse speeds. As you can see in the following photo, the transmission is almost as big as a washing machine!

A Challenger tractor assembler with a Caterpillar transmission.

This nice cutaway of the transmission lets you see inside.

The transmission has several roles in a Challenger tractor. Its main job is to connect directly to the engine and transmit power in the form of speed and torque through the driveshaft to the differential steering unit. There are also three different hydraulic pumps that attach to the transmission and get their power from it:

The transmission is installed. You can see the large hydraulic pump that attaches directly to the transmission in the foreground.

All of the hydraulic lines in the tractor are connected to the transmission in one way or another, and many of them are installed at this point in the assembly process.

A typical rear-drawbar package on a Challenger tractor. The customer can
choose from a variety of configurations for the three-point hitch and drawbar.


Special thanks to Caterpillar for all of the help and information they have provided in creating this article!
As you might expect, the engine that powers a 40,000 pound tractor is big. In the case of a large Challenger tractor, the engine is quite substantial! Here are some of the more interesting specifications for the engine that goes into a Challenger 95E:

You can see just how big the engine is in this photo:

Final assembly of engine components

Caterpillar is the largest engine manufacturer in the world. The company makes engines for everything from small bulldozers to huge cargo ships, railroad locomotives, and power plants.

The Challenger tractor's engine arrives from another Caterpillar factory in a fully assembled state. The Challenger tractor's crew adds things like the starter, alternator, compressor, wiring harness and drive shaft to the engine, and then it is ready for installation in the tractor.

In this subassembly area, components like the starter and alternator are installed

The engine is moved into place with an overhead crane

The engine is bolted in place

Once the engine is in place, a number of hoses and wiring harnesses get connected

You can see the engine being installed in this short video.

One of the more interesting features on this engine is the Electronic Control Module (ECM), which you can see in the following photo:

The ECM mounts along the side of the engine

The ECM is a specialized microprocessor that controls things like the fuel ratio, injection timing and turbo boost to provide maximum performance. The ECM also monitors the engine and records a log that can be helpful during maintenance. The ECM can also display engine performance data to the operator in the cab.

Since the ECM is mounted directly to the engine, heat could be a problem. An unusual cooling system eliminates the heat. A Challenger tractor burns between nine and 18 gallons of fuel per hour depending on the model. As the fuel flows to the injectors, it passes through a channel surrounding the computer to cool it!

This cutaway shows how diesel fuel flows around the ECM (in the yellow channel) to cool it.

Diesel engines work more efficiently with high compression ratios, so turbocharging is a natural way to boost performance. To further increase performance, Challenger tractors use an air-to-air aftercooler (also known as an intercooler). When the turbo charger compresses air, the air naturally heats up, and this reduces its density. The aftercooler cools it back down to increase the density and get the maximum benefit from the turbo.

The turbocharger

This photo shows the inlet pipe for the aftercooler before installation.
The pocket knife in the foreground is a standard four-inch-long Swiss Army knife to illustrate the size of the component.
Air from the turbo flows through this pipe into the aftercooler, then exits
the intercooler on the other side through a similar pipe and straight into
the intake manifold.

The turbocharger connected to the aftercooler

In terms of the assembly process, the aftercooler actually gets installed with the radiator package in the next section of the assembly line...


Special thanks to Caterpillar for all of the help and information they have provided in creating this article!
At this point in the assembly process, the engine, transmission, differential and final drives are all in place. Two other major components get installed, and then the tractor enters the mid-line test area. The two major components are the radiator package and the fuel tank.

A Challenger tractor has five different cooling cores:

All five cooling cores are mounted together in a subassembly area and then hoisted into place with an overhead crane.

The location on the frame for the radiator package

A radiator package ready for installation

The bogies are installed here:

Bogies ready for installation

Bogies installed

At this point the drivetrain installation is complete, and the tractor is ready for the fuel tank.

The fuel tank holds 310 total gallons and 280 usable gallons of diesel fuel. It is injection-molded by a supplier and has its fiberglass trim and lights installed here at the Challenger tractor factory.

A fuel tank ready for installation

The fuel tank is installed.

In the above photo, you can see that the assembly line next passes through a large yellow door. Behind this door is the mid-line test area. In this area several different things happen:

  • Antifreeze, oil, hydraulic fluid, etc. get added
  • A computerized cab simulator is attached to the frame
  • Sensors are mounted at several points
  • A temporary muffler and exhaust system (which vents out through the roof) is installed
  • The engine is started for the first time since being installed on the tractor
With the engine running, the computer operator can "drive" the tractor and perform a number of tests on the engine, the transmission, the drivetrain, the cooling system and the hydraulics. These tests verify that every major system of the tractor is performing as expected.

Before the tractor exits the mid-line test station, it is thoroughly washed and dried before it and enters a paint booth. Several parts are masked off, and then the entire thing gets another coat of black paint:

The entire drive train gets a coat of black paint.

This short video shows you the painting process.

In the next section of the assembly line, the tractor will actually start to look like a tractor...

Cab and Hood

Special thanks to Caterpillar for all of the help and information they have provided in creating this article!
With the drivetrain complete and tested, it is time to add two pieces that will be extremely important to the owner: the cab and the hood! The cabs arrive at the factory completely fabricated and partially assembled to Caterpillar's design specifications.Things like the seat, floor mats, lights and air conditioning system are bolted on by workers here in the factory because some of these features can be customized at the time of purchase.

Cabs ready for sub-assembly and installation

Once the cab is ready for installation, an overhead crane hoists it onto the frame, and it is bolted to the rubber mounts that isolate it from frame vibration. This short video shows you the cab installation process.

A cab is bolted in place

The driver's seat and trainer seat

The dash board

The side console

The intake/exhaust system is assembled from a dozen components in a subassembly area. This assembly, along with the hood, go on next.

The author, illustrating the size of the air filter, muffler and exhaust system.

Hoods are unloaded as they arrive at the factory.

This short video shows you the hood installation process. All that's left is the tracks...


Special thanks to Caterpillar for all of the help and information they have provided in creating this article!
Before the tractor can go anywhere, it needs its belts. Challenger tractors use steel-reinforced tensioned rubber belts that are supported by a front idler wheel, rear-drive wheels, and four mid-wheels, as shown in this cutaway diagram:

The first step is to install the mid-wheels and the large front idler and rear-drive wheels :

Mid-wheels and idler wheels are bolted into place.

A drive wheel is prepared for installation.

Finally, the belts can be mounted. The customer can choose from several belt widths and tread bar configurations at the time of purchase. A typical belt weighs 1,500 pounds, but goes on at the factory in only about five minutes.

One of the rubber belts is installed.

This short video shows you the belt installation process.

Once the belts are in place, the huge tensioning cylinder applies 17,000 pounds of force per side to the front axle. Tensioning the belts gives better weight distribution across the weight-bearing surface of the belt and also holds the belts firmly in place.

At this point the tractor is complete! All that is left is calibration and testing...

The End of the Line

Special thanks to Caterpillar for all of the help and information they have provided in creating this article!
The tractor is now complete! Starting from a
single strip of steel, an entire 40,000 pound Challenger tractor has been assembled from thousands of components.

The last step on the assembly line is final testing, calibration and inspection. Several sensors and a computer monitor are attached to the tractor, the engine is started and the computer operator can run the tractor through a series of calibration steps. All of the computer tests run at the mid-line inspection are repeated to verify that all systems are operating correctly. Steering and brakes are also tested.

Calibrating the drive train

The only thing left to do is lower the tractor to the ground, drive it off the end of the assembly line and take it out onto the test track:

A Challenger tractor goes on the test track.

On the test track, an inspector runs the tractor through a complete series of tests from a seven-page long checklist. Every gear is used for five minutes, the brakes, steering and other systems are all tested, and the tractor's ride and handling are certified. Any problems are remedied in the factory's final test and adjust area, and the tractor is ready for shipment.

This short video shows you some of the maneuvers during the testing process.

Once they leave the factory, Challenger tractors are most often found pulling huge implements on large farms around the world. Some are also used on construction sites.

A Challenger tractor pulling a large soil tillage tool.

A Challenger tractor pulling a large air-seeder at planting time.

A Challenger tractor at work on a construction site.

I'd like to take a moment to give special thanks to the people at Caterpillar for all of the time, information and assistance they have provided to make this behind-the-scenes view possible, as well as to everyone at the factory who took time to explain their part of the assembly line to me and make things clear!