How Caterpillar Backhoe Loaders Work

Backhoe loaders have a very unique appearance -- they have components sticking out every which way. It's obvious what a dump truck does just by looking at it; but what are the different appendages of a backhoe used for?

A backhoe loader is an interesting invention because it is actually three pieces of construction equipment combined into one unit. A backhoe loader is:

Each piece of equipment is suited to a particular sort of work. On a typical construction site, the backhoe operator usually uses all three components to get the job done.

The Tractor

The core structure of a backhoe loader is the tractor. Just like the tractors that farmers use in their fields, the backhoe tractor is designed to move easily over all kinds of rough terrain. It has a powerful, turbocharged diesel engine, large, rugged tires and a cab with basic steering controls (a steering wheel, brakes, etc.). Backhoe cabs are either completely enclosed or have an open canopy structure to give the operator protection.

The Loader

The loader is attached in the front and the backhoe is attached in the back. These two components serve very different functions.

The loader can do several different things. In many applications, you use it like a big, powerful dustpan or coffee scoop. You usually don't dig with it; you mostly use it to pick up and carry large amounts of loose material. It's also used to smooth things over like a butter knife, or to push dirt like a plow. The operator controls the loader while driving the tractor.

The Backhoe

The backhoe is the main tool of the backhoe loader. It's used to dig up hard, compact material, usually earth, or to lift heavy loads, such as a sewer box. It can lift this material and drop it in a pile to the side of the hole.

Basically, the backhoe is a big, extremely powerful version of your arm or finger. It has three segments:

This arrangement is very similar to your arm. Your arm has three segments -- your upper arm, forearm and hand.

The backhoe segments are connected by three joints, comparable to your wrist, elbow and shoulder. The backhoe moves in pretty much the same way as your arm. In a Caterpillar® backhoe, the boom is bent upward to make it easier to dig with obstacles in the way. This design also provides extra space for the bucket when the operator curls it in with a full load.

The backhoe can dig all sorts of holes, but is especially suited for digging ditches. To use the backhoe, the operator has to park the tractor and turn the seat around.

So what do the tractor, loader and backhoe have to do with each other? The tractor component is for moving the other two components from place to place, and the operator also maneuvers it when using the loader. The loader and backhoe components are a natural combination for all sorts of jobs. When you dig up a lot of dirt to make a ditch or any other sort of hole, you generally need a loader to either move the dirt out of the area or to fill the dirt back in once you've got the pipes, power lines, etc. in position. The most common application for a backhoe loader is this basic job -- digging a trench with the backhoe and then back-filling it with the loader.

The Stabilizer Legs

The other appendages you'll typically notice on a backhoe loader are the two stabilizer legs just behind the rear wheels. These legs are crucial to backhoe operation because they take the brunt of the weight when a backhoe is digging. Without the stabilizer legs, the weight of a heavy load or the downward force of digging into the ground would strain the wheels and tires, and the whole tractor would bounce constantly. The stabilizers keep the tractor steady, minimizing the jostling effect of digging with the backhoe. They also secure the tractor so that it won't slip into the ditch or hole.

The stabilizer legs have two types of "shoes," so that they can be planted securely on both dirt and pavement. The grouser shoe side digs into the dirt for a better grip, but would tear up the pavement if you were to use it on a road. For a good grip on asphalt, the operator simply flips the rubber-padded shoe into position.

Backhoes have been around more than 40 years, and they've gotten even more popular in the last decade. Caterpillar has sold more than 100,­000 backhoes since 1985. The main reason we see backhoes at work all the time is that digging and moving dirt is a big part of a lot of different projects. For example, you need to dig ditches to lay pipes and underground cable, set up foundations for buildings and create drainage systems. There are a number of tools that do this sort of work, often more efficiently than a backhoe, but many construction crews use a backhoe instead because of a number of factors.

For one thing, backhoes are remarkably compact when compared to large, specialized equipment such as excavators. They can move around all sorts of construction sites and you can take them on the road. Mini-loaders and backhoe units are actually smaller than a typical backhoe loader, but if a contractor needs to dig and load, it's usually better to have both units in one. It saves a lot of time because the operator doesn't have to switch between two different pieces of equipment.

The backhoe is also popular simply because of its amazing capacities. The Caterpillar backhoe loader in the picture above has a huge amount of power. Its backhoe can dig with 15,200 pounds (67.6 kN) of force and can reach more than 25.9 feet (7.9 meters) away. The loader can lift loads up to 8,760 pounds (3,970 kg) and can hold 1.75 cubic yards (1.3 cubic meters) of dirt in its standard bucket. The backhoe and loader components don't have quite as much power as larger equipment, but they do very well, even with fairly difficult jobs.

A construction crew that does all sorts of different work will usually buy a backhoe rather than more efficient specialized equipment because the backhoe performs well in a wide variety of situations. For small to medium digging jobs, a backhoe is certainly sufficient. And as we'll see later on, some backhoe models do a lot more than just digging and loading.

If you've ever watched a backhoe at work, you know that it is an extraordinarily powerful tool. An experienced operator can dig a 5-foot-deep, 10-foot-long ditch in less than 15 minutes. Just think how long it would take you to do that with only a shovel! Amazingly, all of this work is done with hydraulics -- pumping liquid to move pistons.

The concept of hydraulic machinery may seem pretty bizarre -- how can pumping liquid give you such power? -- but it's actually very simple. First, let's look at the basic idea of a hydraulic system, and then we'll see how a backhoe uses these systems to dig and load such huge amounts of dirt.

Hydraulic systems simply transmit forces from point to point through fluid. Most systems use an incompressible fluid, a fluid that is as dense as it can get. This sort of fluid transmits nearly all of the original force instead of absorbing some of it. The most commonly used incompressible fluid in hydraulic machinery is oil.

In a very simple hydraulic machine, the operator pushes on the oil with one piston so that the oil pushes on another piston, raising it up.

Hydraulic multiplication

Because the second piston has a larger diameter than the first piston, the second piston moves a shorter distance but pushes up with greater force.

The basic concept at work is a trade between distance and force. The work you do in pressing down on the piston on the left has two components -- the amount of force you apply and how far you push the piston. This pushes down a certain amount of fluid. Since the fluid is incompressible, it can't absorb the force you apply, so it pushes up on the piston on the right. The fluid has the same pressure (pounds per square inch) at every point in the system. Since the pressure at the piston on the right is working on a larger area, that piston pushes upward with a greater force.

It's pretty easy to figure out the exact multiplication factor. Assume that the piston on the left has a 2-inch diameter (1-inch radius), while the piston on the right has a 6-inch diameter (3-inch radius). The area of each piston is Pi * r². The area of the left piston is therefore 3.14 (3.14 * 1²), while the area of the piston on the right is 28.26 (3.14 x 3²). The piston on the right is nine times larger than the piston on the left. This means that any force applied to the left-hand piston will be nine times greater on the right-hand piston. So, in the illustration above, the 100-pound downward force applied to the left piston creates a 900-pound upward force on the right piston. But, in keeping with the force-distance trade-off, you've moved the left-hand piston 9 inches and raised the right-hand piston only 1 inch.

In the backhoe loader shown above, the hydraulic system pumps oil at up to 3,300 pounds per square inch, and the cylinder pistons in the backhoe arm have a diameter of up to 5.25 inches. This gives each cylinder piston a force of 70,000 pounds!

With our very simple hydraulic machine, we pushed down on some oil with one piston and that oil pushed up a larger piston, thereby multiplying the force of our effort. This sort of hydraulic mechanism is great for systems where you need to apply a force very briefly, every once in a while -- a brake system, for example. But in a piece of equipment such as a backhoe, you're always moving pistons, so you need constant oil pressure.

In a backhoe, this pressure comes from an oil pump that is powered by a diesel engine. The pump does the same sort of thing as the narrow piston we saw in the earlier example. It applies a lesser force to the oil at a high rate of speed, generating enough pressure to move another piston more slowly but with greater force. The pump keeps a steady supply of high-pressure oil flowing to a valve block system, which directs the pressure's force (later on, we'll see exactly how this works).

So, the powerful pistons in a backhoe are actually moved by the same forces that we saw working in the simple hydraulic design. There are some significant differences in how the two systems operate, however. The simple piston we looked at could only apply multiplied force in one direction. If you pushed down on the narrower piston, the wider piston moved up with greater force. But for a backhoe to dig, its arms have to be able to move in different directions. To move this way, the pistons must be able to push and pull with full force, which requires a more complex system.

If you were to cut open one of the piston cylinders from a backhoe, you would see something like this:

You can see that the piston rod that extends outside the cylinder is actually moved by a piston head inside the cylinder. There is fluid on both sides of this piston head, fed by two different hoses. If the force is greater on the blue side, the piston will move to the left, and if it is greater on the orange side, the piston will move to the right. So all you have to do to change the direction of force is stop pumping oil to one side and start pumping it to the other. This sort of piston cylinder is commonly called a hydraulic ram.

A backhoe loader uses something called a spool valve to direct oil to either side of a ram. The spool valve system lets the backhoe move pistons in two directions.

The pump takes oil from a tank and pumps it through a hose to the spool valve. When the operator moves the controls to change the direction of the backhoe, the spool valve changes its configuration so that the high-pressure oil goes to the other side of the ram. As the high-pressure oil pushes on one side, the low-pressure oil is forced through a different hose, back to the oil tank.

The operator manipulates this valve block with joysticks in the backhoe cab. In some backhoes, control sticks are directly attached to different spool valves, acting as a lever to move the spool directly.

In other backhoes, the joysticks operate hydraulic pistons that control the movement of the spool valves. When you move the joystick in a certain direction, it presses down on a particular piston. This piston pushes oil through a hose to move the spool valve controlling a particular hydraulic ram. By moving different spools, you extend or retract different hydraulic pistons. In the next couple of sections, we'll look at the arrangement of these pistons, and see how their applied forces translate into fluid movement in the backhoe and loader.

Now we've seen how the backhoe's valve system can move hydraulic pistons in two directions with great force. But how do equipment designers use this technology to create such powerful digging machines?

Let's go back to the idea of a backhoe being a huge, powerful version of a human arm. We compared the steel segments -- the boom, the stick and the bucket -- to three pieces of your arm, similarly connected by three joints. It's obvious that your arm wouldn't be quite as useful without muscles -- your muscles provide the force that actually pulls the various segments of your arm toward and away from each other. The cylinders in a backhoe serve the same function. All of the segments are hinged together and each cylinder can either pull a connected segment closer or push it away.

Each cylinder piston is controlled by its own spool valve. When you dig with a backhoe, you're actually controlling at least four individual spools (which move four different pistons). In the animation below, you can see how an operator activates some of these different pistons together to dig with the backhoe.

The backhoe also has two hydraulic pistons near the base of the boom arm. The boom arm is connected to the tractor with a swing casting so that these pistons can swing the backhoe arm from side to side. They are synchronized so that when you push with one, the other pulls. In many European backhoes, the boom is attached to a side-shift mechanism, a bracket that can move the entire backhoe arm horizontally on the tractor. This lets the operator dig in spaces where it would be very difficult to maneuver the entire tractor into a good working position.

One of the most significant variables in backhoe performance is dig depth. This is simply a rating of how deep the backhoe arm can dig. Typically, dig depth is somewhere between 12 and 16 feet (3 to 5 m). Many backhoes have an extendible stick that lets them increase this dig depth a few feet when needed. Most backhoe jobs don't require operators to dig ditches and holes more than 10-feet deep, but the dig depth is still a useful measure because it also indicates how far out the backhoe can reach.

Another important rating is horsepower. If you've read How Horsepower Works, then you know that horsepower is a measure of how much work something can do in a certain amount of time. A backhoe horsepower rating tells you how much power the engine provides for all of the systems in the backhoe, which gives you an idea of what the backhoe is capable of.

Backhoe models with greater dig depth usually have more horsepower. Increasing both of these factors expands the backhoe's abilities. Backhoes designed for residential construction applications -- such as digging foundations, grading, and digging ditches for sewer and utility lines -- generally have a 14- to 16-foot dig depth and 70 to 85 horsepower. Backhoes designed for heavier industrial and commercial applications -- such as road and bridge maintenance or large-scale construction -- have a dig depth greater than 17 feet (5 m) and at least 100 horsepower.

Backhoes also have breakout force ratings. Breakout force describes the maximum force that the arm can apply on a load. It's measured by how hard the end of the bucket can push, but all of the hydraulic rams on the arm contribute to the total force. Backhoes also have stick lift and boom lift ratings, which tell you the maximum weight the stick and the boom can lift individually when the hydraulic rams are pushing with full force. This is another measure of a backhoe's general capacity, and is especially useful for contractors who plan to use the backhoe as a sort of crane for lifting heavy loads. The backhoe in the pictures above has a 14,712-lb (65.4-kN) breakout force, a 6,250-lb (2,830-kg) stick lift capacity and a 3,940-lb (1,787-kg) boom lift capacity.

We've mostly focused on the backhoe here, but the loader is also driven by hydraulics. Its hydraulic rams are configured in a slightly different way -- they work as pairs. The rams lift the bucket in exactly the same way you would lift a heavy box -- you grab both sides and lift with both arms. The valve system pumps the same amount of oil to each ram in the pair so that they move in unison. This stabilizes the loader bucket.

Caterpillar has two types of loaders on its backhoes -- a single tilt (yellow) and a parallel lift (black). Both types use a piston pair to lift the loader arms. This piston pair is attached to the tractor and the arms holding the bucket. The pistons extend to raise the arms and retract to lower them. Parallel-lift loaders use a second pair of rams attached to the loader arms and the bucket itself. These rams extend to dump the bucket and retract to tilt it back up. Single-tilt loaders do this with only one central ram.

Parallel-lift loaders have an eight-bar-linkage design that improves loading performance. In this system, different sets of bars in the loader are connected in such a way that the bucket doesn't tip as it rises. Basically, the two main sets of parallel bars that hold the bucket move together so that they keep the bucket level with the ground. Without parallel lift, the loader would be something like a seesaw with a crate nailed to one end. If you filled the crate with oranges when the see-saw was level, a lot of them would fall out when you tilted the seesaw up. A parallel-lift system allows for more efficient loading because it keeps more of the material in the bucket as it lifts.

Another cool function in some backhoe loaders is a technology called ride control. Carrying a full load with a backhoe loader makes for a fairly bumpy ride because the wheel base is so small compared to the total inertia of the equipment and the load -- the weight on one end rocks the whole structure back and forth. To make the ride a bit smoother, backhoes with ride control use the loader lift hydraulics as a shock-absorber system. Basically, as the bucket bounces, it pushes down on the oil in the hydraulic cylinders. The oil flows to another piston cylinder, the accumulator, which has compressed nitrogen gas on the other side. Unlike oil, this nitrogen gas can be compressed, so it acts like a spring -- when the incompressible oil from the loader rams pushes down on one side of the piston, the gas compresses a little before pushing back up on the piston.

With just this mechanism, the oil would simply be pushed back and forth, so the bucket would keep bouncing. To create a smooth ride, the ride control system has to absorb some of that energy as the oil flows. The damping mechanism that accomplishes this is a small orifice in the hose carrying the oil from the lift ram to the ride control accumulator. With each bounce of the loader bucket, oil is squeezed through this small opening. The energy expended to force the oil through the opening is converted into heat. This energy loss essentially absorbs the bouncing energy, making for a smoother ride.

Like backhoe arms, loaders are rated by their breakout force. This rating tells you the maximum force the loader's hydraulic rams can apply to the front bucket, which gives you an idea of how well a loader will be able to push and lift a load.

When you stop and think about all the different moving parts in a backhoe loader, it seems unbelievable that you need only one person at the controls. As we saw in the last two sections, the backhoe arm swivels on four different hinges (some bucket designs have five) and the loader moves on two to three hinges. Additionally, the operator controls the stabilizer arms and moves the tractor around while loading. How does one person do all of this?

The main controls for a Caterpillar backhoe are two computer-style joysticks. Here are the functions of the joysticks:

Digging effectively with a backhoe requires practice, like learning to drive a car. The hardest part of learning to drive is usually paying attention to all of the different things going on. It takes a lot of practice to keep all of the various controls in your mind at once. Learning how to operate a backhoe is the same way. Picking up something with your arm is incredibly easy because you move every muscle automatically. But imagine how hard it would be if you had to stop and think about every muscle you were moving in that one simple motion.

An experienced driver doesn't even think about most of the things he or she is doing while driving. Backhoe operators reach this same level. With enough practice, the controls become second nature. But in addition to learning the controls, the operator must also learn to position the arm so that it will dig efficiently. That means knowing the best angle for the bucket as it sinks into the dirt, knowing when to move the boom and when to move the stick and getting a sense of what arm positions provide the best leverage.

Operating the loader is relatively simple because it only dumps, raises and lowers. The main loader control is a joystick on the right-hand side of the operator. If you pull the joystick back toward you, the first set of hydraulic rams push out to lift the arms up. When you push it away from you, the arms lower. To dump out the bucket, you move the joystick to the right. To scoop the bucket in, you move the joystick to the left.

So the loader is pretty easy to learn compared to the backhoe. To get much use out of it, however, you have to be able to operate it while moving the tractor around the site. The tractor basically handles like a car, with a steering wheel, accelerator, brake pedal and gear shift. The loader and the tractor are powered by the same engine, which has a variable speed control. For an extra boost in loader force, the operator can put the tractor in neutral so that most of the engine's power goes directly to the hydraulic system.

Experienced backhoe operators use the backhoe loader in much the same way you would use a shovel or wheelbarrow at home -- they know exactly how to move the controls to dig and load quickly and effectively. And they're always thinking ahead to their next few moves, planning their strategy. This is also something like driving a car: When you see a traffic jam ahead of you on the road, you're already deciding how you're going to navigate it. Just as with driving, learning how to operate the backhoe is only the first step -- the real skill is in knowing how to use the backhoe to accomplish different tasks.

All of the hydraulic systems on a backhoe get their hydraulic pressure from a hydraulic pump. There are two types of pumps in common use:

In a gear pump, a pair of inter-meshing gears pressurizes the hydraulic oil. The disadvantage of gear pumps is that pressure rises and falls with engine speed, and the only way to get high pressure is to run the engine at full power.

A variable-displacement pump is more sophisticated. It has a series of piston cylinders fixed in a ring inside a barrel. The engine spins the barrel around so that the cylinders revolve. The cylinder pistons extend out the back of the barrel, where they are attached to an angled swash plate. As the barrel spins around, the angle of the swash plate pushes the pistons in and then pulls them out. You can see in the diagram that as the swash plate pulls the piston out, the cylinder sucks in oil from the tank. As the plate pushes the piston in, the cylinder pumps oil out into the hydraulic system. Just before a cylinder rotates from the intake side to the discharge side, it's holding the maximum amount of oil. As it rotates from the hydraulic-system side to the intake side, it's holding the minimum amount of oil. This pressurizes the oil so that it is pumped out with great force.

The heart of the load-sensing hydraulic system is the variable-displacement pump.

This pump is especially cool because you can very easily adjust how much oil it pumps. All you have to do is change the angle of the swash plate. When the swash plate is pressed closer to the barrel, there isn't as great a difference between the size of a cylinder's fluid compartment on the left side and the size of the compartment on the right side. Consequently, the pump doesn't pump as much oil. When the swash plate is pressed all the way up against the barrel -- so that it isn't at an angle at all -- the system doesn't pump any oil.

The angle of the swash plate is determined by the needs of the hydraulic system. Special circuits monitor the pressure on the various hydraulic rams and adjust the flow rate to the necessary level. This load-sensing hydraulic system has a couple of significant advantages over a system using a fixed-displacement pump.

First of all, the variable-displacement pump is more efficient because it only pumps the amount of oil that the hydraulic system needs. When none of the hydraulic rams are operating, the pump simply stops pumping oil. This reduces the fuel consumption of the backhoe a good deal.

Secondly, this sort of system makes the best use of available engine power. Most backhoes have several different engine-speed options. When the engine is at maximum speed, the backhoe has the most power to work with. When the engine is at a reduced speed, the backhoe has less available power.

If the pump tries to draw more power than the engine can produce (at a particular speed), the engine will stall. So, to provide maximum pressure to the hydraulics at all times, the system has to make intelligent use of the available power.

In a backhoe, power is just flow rate multiplied by hydraulic pressure. The pressure is determined by the operation being performed -- lifting heavy objects or busting through hard ground requires higher pressure than does moving an empty bucket. Relief valves determine the maximum pressure in the hydraulic system.

On backhoes with fixed-displacement pumps, the flow rate is constant at any particular engine speed. Since the flow rate multiplied by the maximum pressure can't exceed the available engine power, the system always pumps the amount of oil needed for maximum pressure. Some oil is used by the hydraulics and the rest goes to the tank. This means that if you are not demanding full pressure, you're wasting available engine power and wearing out the system for no reason.

Backhoes with variable-displacement pumps don't have this problem. The system monitors the pressure of all the hydraulic rams and controls the angle of the swash plate to meet the demands of the ram that has the highest pressure level. If you are not demanding full pressure, the pump will increase its displacement (which increases flow rate), making the tools move faster. When the system demands full pressure, the pump will decrease its displacement so that it can provide the pressure without exceeding the engine's available power.

Some backhoe-loaders let you connect a wide variety of tools to either the backhoe stick or the loader in place of the standard buckets. Caterpillar backhoe-loaders have an integrated toolcarrier (IT) that hooks up very easily to a number of compatible components. Different tools include specialized buckets, street sweepers and pallet forks.

The backhoe arm also connects very easily to different tools. As you can see in the video below, the operator must secure the tool to the backhoe stick with a connecting pin. All sorts of tools are available, including:

The ability to attach different tools expands the backhoe's versatility a great deal. The different tool attachments let the backhoe loader do a number of different things on the job site.

As you've seen, backhoe loaders are filled with hydraulic valves and cylinders. In addition, backhoe loaders have all sorts of things you would find in any tractor, car or truck. In this section, we'll look at some of the components that make a backhoe work.

All backhoe loaders have a set of standard components. In any backhoe, you'll find:

These components, as well as all the other backhoe parts we've looked at in this article, are all assembled on a sturdy steel frame.