BackHoe Cat

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This cutaway drawing shows many of the major components of a Caterpillar backhoe loader.

Graphic courtesy Caterpillar

Inside a Backhoe Loader

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:

An engine- In a typical backhoe loader, the tractor, loader and backhoe are all powered

by a diesel engine. The Caterpillar 80-horsepower3054 engine below has a 4-cylinder, 4-

From backyard log splitters to the huge machines you see on construction sites, hydraulic

equipment is amazing in its strength and agility! On any construction site you see hydraulically

operated machinery in the form of bulldozers,backhoes,shovels,loaders,fork lifts andcranes.

Hydraulics operate the control surfaces on any largeairplane.You see hydraulics at car service

centers lifting the cars so that mechanics can work underneath them, and many elevators are

hydraulically operated using the same technique. Even thebrakesin your car use hydraulics!

In this article, you will learn about the basic principles that hydraulic systems use to do their

work, and then we'll examine several different pieces of hydraulic machinery found on aconstruction site. You will be amazed at the power and versatility available with hydraulics

Air in the System

It is important that a hydraulic system contains no air bubbles. You may have heard about the

need to "bleed the air out of the brake lines" of you car. If there is an air bubble in the system,

then the force applied to the first piston gets used compressing the air in the bubble rather thanmoving the second piston, which has a big effect on the efficiency of the system.

The Basic Idea
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The basic idea behind any hydraulic system is very simple: Force that is applied at one point is

transmitted to another point using an incompressible fluid.The fluid is almost always an oil

of some sort. The force is almost always multiplied in the process. The picture below shows thesimplest possible hydraulic system:

A Simple hydraulic system consisting of two pistons and an oil-filled pipe connecting them.

Click on the red arrow to see the animation.

In this drawing, two pistons (red) fit into two glass cylinders filled with oil (light blue) and

connected to one another with an oil-filled pipe. If you apply a downward force to one piston(the left one in this drawing), then the force is transmitted to the second piston through the oil in

the pipe. Since oil is incompressible, the efficiency is very good -- almost all of the applied force

appears at the second piston. The great thing about hydraulic systems is that the pipe connecting

the two cylinders can be any length and shape, allowing it to snake through all sorts of thingsseparating the two pistons. The pipe can also fork, so that one master cylindercan drive more

than one slavecylinder if desired.

The neat thing about hydraulic systems is that it is very easy to add force multiplication (or

division) to the system. If you have readHow a Block and Tackle WorksorHow Gears Work,

then you know that trading force for distanceis very common in mechanical systems. In a
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hydraulic system, all you do is change the size of one piston and cylinder relative to the other, as

shown here:

Hydraulic multiplication. The piston on the right has a surface area nine times greater than thepiston on the left. When force is applied to the left piston, it will move nine units for every one

unit that the right piston moves, and the force is multiplied by nine on the right-hand piston.

Click the red arrow to see the animation.

To determine the multiplication factor, start by looking at the size of the pistons. Assume that

the piston on the left is 2 inches in diameter (1-inch radius), while the piston on the right is 6inches in diameter (3-inch radius). The area of the two pistons is Pi * r

2. The area of the left

piston is therefore 3.14, while the area of the piston on the right is 28.26. The piston on the right

is 9 times larger than the piston on the left. What that means is that any force applied to the left-

hand piston will appear 9 times greater on the right-hand piston. So if you apply a 100-pounddownward force to the left piston, a 900-pound upward force will appear on the right. The only

catch is that you will have to depress the left piston 9 inches to raise the right piston 1 inch.

Thebrakesin your car are a good example of a basic piston-driven hydraulic system. When you

depress the brake pedal in your car, it is pushing on the piston in the brake'smaster cylinder.

Four slave pistons, one at each wheel, actuate to press the brake pads against the brake rotor tostop the car. (Actually, in almost all cars on the road today two master cylinders are driving two
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slave cylinders each. That way if one of the master cylinders has a problem or springs a leak, you

can still stop the car.)

In most other hydraulic systems, hydraulic cylinders and pistons are connected through valves toa pump supplying high-pressure oil. You'll learn about these systems in the following sections.

The major components of a log splitter are shown here.


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How Log Splitters Work

The simplest hydraulic device that you find in common use today is the log splitter. It containsall of the basic components of a hydraulic machine:

An engine, normally a smallfour-stroke gasoline engine,provides the power for the

system. The engine is attached to a hydraulic oil pump.

The hydraulic oil pumpcreates a stream of high-pressure oil, which runs to a valve.

The valvelets the operator actuate the hydraulic cylinderto split a log. There is also a tankto hold the hydraulic oil that feeds the pump and usually a filterto

keep the oil clean.

In cross section, the splitter's important hydraulic parts look like this:

High-pressure oil from the pump is shown in light blue, and low-pressure oil returning to the

tank is shown in yellow. Click the button to activate the piston!
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In the figure above you can see how the valve can apply both forward and backward pressure to

the piston. The valve used here, by the way, is referred to as a "spool valve" because of its

resemblance to a spool from a spool of thread.

Let's look at some of the specifics of these components to see how a real hydraulic system

works. If you take a trip down to your local building supply center or a place likeNorthern Tool

and Equipmentand look at the log splitters, you will find that a typical backyard log splitter has:

A 5-horsepower gasoline engine

A two-stage hydraulic oil pump rated at a maximum of 11 gallons per minute (3 gpm at2,500 psi)

A 4-inch-diameter, 24-inch-long hydraulic cylinder

A rated splitting force of 20 tons

A 3.5-gallon hydraulic oil tank

A two-stage pumpis an ingenious time-saver. The pump actually contains two pumping

sections and an internal pressure-sensing valve that cuts over between the two. One section of thepump generates the maximum gpm flow rate at a lower pressure. It is used, for example, to draw

the piston back out of a log after the log has been split. Drawing the piston back into the cylinder

takes very little force and you want it to happen quickly, so you want the highest possible flowrate at low pressure. When pushing the piston into a log, however, you want the highest possible
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pressure in order to generate the maximum splitting force. The flow rate isn't a big concern, so

the pump switches to its "high pressure, lower volume" stage to split the log.

We'll look at hydraulic pumps in the next section

Hydraulic Pumps

One thing you can see is that the advertised "20-ton splitting force" is generous. A 4-inch piston

has an area of 12.56 square inches. If the pump generates a maximum pressure of 3,000 poundsper square inch (psi), the total pressure available is 37,680 pounds, or about 2,320 pounds shy of

20 tons. Oh well...

Another thing you can determine is the cycle time of the piston. To move a 4-inch-diameterpiston 24 inches, you need 3.14 * 2

2* 24 = 301 cubic inches of oil. A gallon of oil is about 231

cubic inches, so you have to pump almost 1.5 gallons of oil to move the piston 24 inches in onedirection. That's a fair amount of oil to pump -- think about that the next time you watch how

quickly a hydraulicbackhoeorskid/loaderis able to move! In our log splitter, the maximum

flow rate is 11 gallons per minute. That means that it will take 10 or so seconds to draw thepiston back after the log is split, and it may take almost 30 seconds to push the piston through a

tough log (because the flow rate is lower at high pressures).
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From this discussion you can see that just to fill the cylinder with oil, you need at least 1.5

gallons of hydraulic oil in the system. You can also see that one side of the cylinder has a larger

capacity than the other side, because one side has the piston shaft taking up space and the otherdoesn't. Therefore, big hydraulic machines usually have:

Large appetites for hydraulic oil(100 gallons is not uncommon if there are six or eight

large hydraulic cylinders used to operate the machine.)

Large external reservoirsto hold the difference in the volume of oil displaced by thetwo sides of any cylinder

Now that you understand the basics of a simple hydraulic system, we can look at some really

interesting equipment


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Large Hydraulic Machines

One of the best places to get up close and personal with large hydraulic machines is at aconstruction site. The thing that is most amazing about these machines is their sheer size. For

example, here is a medium-size shovel/excavator:


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This machine weighs just over 28 tons, but it can be quite swift in its actions. The bucket canscoop out more than a cubic meter of dirt, which weighs in at approximately 1 to 1.5 tons, and

move it around with no difficulty at all. Moving a person around is trivial.

To have this sort of agility, this particular shovel uses an 8.3-literdiesel enginecapable of

generating 340horsepower.The engine is connected to a pair of pumps that can generate 140gallons per minute at 4,500 psi. The hydraulic pistons have 5.5-inch-diameter faces with 4-inch-
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diameter shafts. In addition, there is one hydraulic motor for each track and a hydraulic motor

that can spin the cab and arm at 11 rpm.

You can see from the picture that the arm has a pair of pistons working in unison at the"shoulder" -- one at the "elbow" and then one to rotate the bucket. These pistons, along with the

two track motors and the rotating motor, are all controlled by two joy sticks and four pedals in

the cab.

These controls send electrical signals to an electrically-operated valve block located next to the

pump.

From the valve block, high-pressure hydraulic lines make their way to the cylinders.


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Tracks

The tracks are interesting. If you look at the tracks on any piece of large machinery, you will find

that there is a hydraulic motorat one end, a free-spinning toothed wheelat the other, and then

a set of rollersfor the track to move over.

Shovel


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Here are the specifications for the Halla HE280LC shovel:

General

Weight: 28.2 tons

Length: 10.6 meters

Width: 3.2 meters

Height: 3.3 meters

Ground clearance: 0.5 meters

Bucket width: 1.3 meters

Bucket capacity:1.1 cubic meters

Working range

Digging depth: 7.5 meters

Vertical digging depth: 5.7 meters

Reach: 10 meters

Driving

Maximum travel speed: 4.9 km/hr


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Maximum traction force: 25,143 kgf

Swing speed: 11 rpm

Maximum digging force: Bucket: 18 tons; Stick: 14 tons Ground pressure: 7.7 psi

Engine

Cummins 6CT 8.3-C

8,270 cubic centimeters

340horsepowerat 1,900 rpm

Pump

Maximum pressure: 5,000 psi (4,500 psi typical) Oil flow: 2x270 liters per minute

Capacities

Fuel: 530 liters

Engine oil: 22 liters

Hydraulic oil: 320 liters
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Skid/Loaders

Another common piece of equipment at any construction site is theskid/loader(also known

generically as a "Bobcat" because that was the name given by the manufacturer that firstproduced them):

Skid/loaders have three pairs of pistons:

The first pair raises and lowers the bucket.

The second pair rotates the bucket to dump its contents.

The third pair splits the bucket so you can use it to grab and pick up things (such as logs).
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There are also hydraulic motors on the four wheels

Dump Trucks

A dump truck isn't much more complicated than the log splitter we saw previously. Dump trucks

typically have one cylinder or a pair of cylinders that lift the bed. The only unique thing aboutthese cylinders is that they often telescope, giving them a very long range of motion.

Caterpillar Backhoe Loaders Work

Introduction to How Caterpillar Backhoe Loaders Work 2

What is a Backhoe Loader?

3

Why Are Backhoe Loaders So Popular?

4

Hydraulic Power
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A D-Series Caterpillar backhoe loader

Photo courtesyCaterpillar
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If you were to ask a large group of people what they could tell you about backhoe loaders, a lot

of them wouldn't know what you were talking about. But if you showed them a picture of one,

almost everybody would understand what you meant. We've all seen backhoe loaders, commonlycalled backhoes. They are used for a number of different jobs and are often the only piece of

heavy equipment at a construction site. We pass them on the side of the road all the time. Buteven somebody who has passed by dozens of backhoes may not know that much about them.

What exactly do they do? Why are they used for so many different sorts of construction projects?

How can they dig such big holes in such a short amount of time? How strong are they?

In this article, we'll look at what backhoes can do, examine the machinery that makes this work

possible and show you how workers actually control a backhoe. The next time you pass abackhoe loader working alongside the freeway, you'll know exactly what it's doing


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A backhoe loader combines a backhoe, a loader and a tractor into one piece of equipment.


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What is a Backhoe Loader?

Backhoe loaders have a very unique appearance -- they have components sticking out every

which way. It's obvious what adump truckdoes just by looking at it; but what are the differentappendages of a backhoe used for?

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

A tractor

A loader A backhoe

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
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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,turbochargeddiesel engine,large, ruggedtiresand a cab with basic steering controls (asteering wheel, brakes, etc.). Backhoe cabs are either completely enclosed or have an open

canopy structure to give the operator protection.

The Loader

The loaderis 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 topush dirt like a plow. The operator controls the loader while driving the tractor.

The Backhoe

The backhoeis 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 asewerbox. It can lift this material and drop it in a

pile to the side of the hole.
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Basically, the backhoe is a big, extremely powerful version of your arm or finger. It has three

segments:

The boom

The stick

The bucket

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 thebackhoe, 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 isfor 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


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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 backhoeloader is this basic job -- digging a trench with the backhoe and then back-fillingit with the

loader.

The Stabilizer Legs

The other appendages you'll typically notice on a backhoe loader are the two stabilizer legsjust

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 loador 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 intothe 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 shoeside digs into the dirt for a better grip, but would tear up thepavement if you were to use it on a road. For a good grip on asphalt, the operator simply flips the

rubber-padded shoeinto position


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You see backhoes doing all sorts of jobs.


Why Are Backhoe Loaders So Popular?

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
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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 backhoeinstead because of a number of factors.

For one thing, backhoes are remarkably compactwhen 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 typicalbackhoe 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 largerequipment, 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 thanmore efficient specialized equipment because the backhoe performs well in a wide variety of


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Hydraulic multiplication enables backhoes to dig with tremendous force.


Hydraulic Power

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 isdone with hydraulics-- pumping liquid to move pistons.

The concept ofhydraulic machinerymay 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 hydraulicsystem, and then we'll see how a backhoe uses these systems to dig and load such huge amounts

of dirt.

Hydraulic systems simply transmitforcesfrom point to point through fluid. Most systems use an

incompressiblefluid, a fluid that is as dense as it can get. This sort of fluid transmits nearly allof the original force instead of absorbing some of it. The most commonly used incompressible

fluid in hydraulic machinery is oil.
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In the very simple hydraulic machine shown below, 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 diameterthan 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 forceyou apply and how

faryou push the piston. This pushes down a certain amount of fluid. Since the fluid isincompressible, 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 agreater force.

It's pretty easy to figure out the exact multiplication factor. Assume that the piston on the lefthas 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 * 1

2),

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


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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 andraised 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

Hydraulic Valves

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 hydraulicmechanism is great for systems where you need to apply a force very briefly, every once in a

while -- abrakesystem, 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 adiesel engine.Thepump 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
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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 withgreater 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 andpull 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:


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You can see that the piston rod that extends outside the cylinder is actually moved by a pistonhead 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 greateron 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 pistoncylinder is commonly called a hydraulic ram.

A backhoe loader uses something called a spool valveto direct oil to either side of a ram. In theanimation below, you can see the basic design of this sort of hydraulic system. 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 spoolvalve changes its

configuration so that the high-pressure oil goes to the other side of the ram. As the high-pressureoil 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 withjoysticksin the backhoe cab. In some backhoes,

control sticks are directly attached to different spool valves, acting as a leverto move the spool

directly.


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In other backhoes, the joysticks operate hydraulic pistonsthat 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 particularhydraulic 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 theirapplied forces translate into fluid movement in the backhoe and loader


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The backhoe has many applications.



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Hydraulics in the Backhoe

Now we've seen how the backhoe's valve system can movehydraulicpistons in two directionswith 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 withoutmuscles-- your muscles provide the force that actually pulls the various segments of your arm

toward and away from each other. The cylindersin 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 theanimation below, you can see how an operator activates some of these different pistons together

to dig with the backhoe.
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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 castingso 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. Inmany European backhoes, the boom is attached to a side-shiftmechanism, a bracket that can

move the entire backhoe arm horizontally on the tractor. This lets the operator dig in spaceswhere 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 arating 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 morethan 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 readHow 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 thesystems 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 thesefactors expands the backhoe's abilities. Backhoes designed forresidential construction
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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 orlarge-scale construction -- have a dig depth greater than 17 feet (5 m) and at least 100

horsepower.

Backhoes also have breakout forceratings. 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 ofthe hydraulic rams on the arm contribute to the total force. Backhoes also have stick liftand

boom liftratings, 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 abackhoe'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


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The loader can do all sorts of jobs.(Click on each picture for a larger image.)


Hydraulics in the Loader


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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 withboth 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 thetractor 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 dothis with only one central ram.

Parallel-lift loaders have an eight-bar-linkagedesign that improves loading performance. In thissystem, 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 somethinglike 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.


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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 thewhole 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 pushesdown 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 ramspushes down on one side of the piston, the gas compresses a little before pushing back up on the

piston.

Click hereto see a demonstration of how ride control works.
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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 dampingmechanism that accomplishes this is a small orifice in thehose 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 oilthrough the opening is converted into heat. This energy lossessentially 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.


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This is a standard mechanical linkage control system from a backhoe. When you move the stickon the far left, it operates the spool valves that move the boom from side to side and forward and

backward. The stick on the far right operates the spool valves that move the stick and the bucket.

The two center controls move the spool valves that extend and retract the stabilizer legs.


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Operating the Backhoe Loader

When you stop and think about all the different moving parts in a backhoe loader, it seemsunbelievable 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 thefunctions of the joysticks:

Thejoystickon the left moves the boom and swings the entire backhoe from side to side.

The joystick on the right moves the stick and the bucket. Pulling the joystick toward you moves the boom or the stick closer to you, and pushing

the joystick away moves the boom or stick farther out.

Pushing the left-hand joystick to the left swings the entire backhoe to the left, andpushing the joystick to the right swings the arm to the right.

Pushing the right-hand joystick to the left scoops the bucket in, and pushing it to the right

dumps the bucket out.
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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 operatea 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 thinkabout 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 whiledriving. 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 itsinks 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 ofhydraulicrams push out to lift the arms up. When you push it awayfrom 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.
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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 asteeringwheel, accelerator, brake pedal and gear shift. Theloader and the tractor are powered by the same engine, which has a variable speed control. For

an extra boostin loader force, the operator can put the tractor in neutral so that most of theengine's power goes directly to the hydraulic system.

Experienced backhoe operators use the backhoe loader in much the same way you would use ashovel 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 youon 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.
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This is a standard mechanical linkage control system from a backhoe. When you move the stickon the far left, it operates the spool valves that move the boom from side to side and forward and

backward. The stick on the far right operates the spool valves that move the stick and the bucket.

The two center controls move the spool valves that extend and retract the stabilizer legs.


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The Hydraulic Pump

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:

Gear pumps

Variable-displacement pumps

In a gear pump, a pair of inter-meshinggearspressurizes the hydraulic oil. The disadvantage ofgear 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 pumpis more sophisticated. It has a series of piston cylinders fixed ina 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 outinto 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
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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 thebarrel, 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 atan 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 necessarylevel. This load-sensinghydraulic 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 pumpsimply stops pumping oil. This reduces the fuel consumption of the backhoe a good deal.


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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 lessavailable 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

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