hydraulic valves and other articles

8/2/2019 Hydraulic Valves and Other Articles

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Hydraulic ValvesWith our very simple hydraulic machine, we pushed down on some oil with one piston andthat oil pushed up a larger piston, thereby multiplying the force of our effort. This sort ofhydraulic mechanism is great for systems where you need to apply a force very briefly, everyonce 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. 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).

Photo courtesyCaterpillarBackhoes pump oil through a complex system of hoses and

valves.(Click on each picture for a larger image.)

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 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:

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
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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. In

this animation, 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 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 differenthose, back to the oil tank.


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Photo courtesyCaterpillar

This control stick uses hydraulics to operate the spoolvalve.

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.

What is a Backhoe Loader?Backhoe loaders have a very unique appearance -- they have components sticking out every which way. It's

obvious what a dumptruck does just by looking at it; but what are the different appendages of a backhoe usedfor?
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Click the icon to move around the backhoe loader.

Photo courtesyCaterpillarA D-Series Caterpillar backhoe loader

A backhoe loader is an interesting invention because it is actually three pieces of construction equipment

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 operatorusually uses all three components to get the job done.

The TractorThe 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 (a steering wheel, brakes, etc.). Backhoe cabs

are either completely enclosed or have an open canopy structure to give the operator protection.

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

different functions.
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The loader is used primarily to carry dirt, gravel and othermaterial.

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 BackhoeThe 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 asewerbox. It can lift this material and drop it in a pile to the side of the hole.
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The backhoe is a powerful digging tool.

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


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The Stabilizer LegsThe other appendages you'll typically notice on a backhoe loader are thetwostabilizer legs just behind the rear wheels. These legs are crucial tobackhoe operation because they take the brunt of the weight when abackhoe 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 stabilizerskeep the tractor steady, minimizing the jostling effect of digging with thebackhoe. They also secure the tractor so that it won't slip into the ditch orhole.

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.

Hydraulic PowerIf you've ever watched a backhoe at work, you know that it is

an extraordinarily powerful tool. An experienced operator candig a 5-foot-deep, 10-foot-long ditch in less than 15 minutes. Just think how long it wouldtake you to do that with only a shovel! Amazingly, all of this work is done 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

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 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

Walk-around

Photo courtesyCaterpillarClick hereto view an interactivewalk-around of a Caterpillarbackhoe loader. This will let youmove around a construction siteto see the backhoe from allangles. This file is 800K, so it willtake a while to load over a dial-up connection. To send usfeedback on this walk-

around,click here.
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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. Sincethe 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 theleft-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.

Photo courtesyCaterpillarHydraulic multiplication enables backhoes to dig with

tremendous force.

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!


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

Photo courtesyCaterpillarHydraulic hoses snake around the boom arm to the backhoe's

hydraulic rams.

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.

Hydraulic rams are the muscle in a backhoe arm.In this animation, you can see how the piston on one of the cylinders moves.All of the hydraulic cylinders in the backhoe-loader work in this same way.
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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 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.

Photo courtesyCaterpillarThe boom on this European Caterpillar backhoe can shift from

side to side.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 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 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 forresidential constructionapplications -- 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.
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Photo courtesyCaterpillarThe backhoe has many applications.

(Click on each picture for a larger image.)

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.

Hydraulics in the LoaderWe've mostly focused on the backhoe here, but the loader is also driven by hydraulics. Itshydraulic rams are configured in a slightly different way -- they work as pairs. The rams liftthe bucket in exactly the same way you would lift a heavy box -- you grab both sides and liftwith both arms. The valve system pumps the same amount of oil to each ram in the pair sothat 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 theykeep the bucket level with the ground. Without parallel lift,

Morphing Backhoe

Photo courtesyCaterpillarClick hereto see a "morph" of aCaterpillar backhoe loader. Thebackhoe morphs from aCaterpillar B-series to a C-series,and then to a cutaway. This file is600K, so it will take a while toload over a dial-upconnection.Click hereto send us

feedback on this animation.
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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.

Photo courtesyCaterpillarThe loader can do all sorts of jobs.

(Click on each picture for a larger image.)

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.

Click hereto see a demonstration of how ride controlworks. To send us feedback on this video,click here.

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 the hose carrying the oil from the lift ram to the ride control accumulator. With eachbounce of the loader bucket, oil is squeezed through this small opening. The energy
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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.

The Hydraulic PumpAll of the hydraulic systems on a backhoe get their hydraulic pressure from a hydraulic pump. There are twotypes of pumps in common use:

Gear pumps Variable-displacement pumps

In a gear pump, a pair of inter-meshinggearspressurizes the hydraulic oil. The disadvantage of gear pumps isthat pressure rises and falls with engine speed, and the only way to get high pressure is to run the engine at fullpower.

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 platepulls 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-

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 pump simply 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 less available power.

Click hereto watch a video demonstrating how the hydraulic pumpworks.

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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.

Photo courtesyCaterpillarThe hydraulic pump from a Caterpillar backhoe

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.
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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.

MEDIUM STRENGTH LOW CARBON .05-.15%

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