How Caterpillar Skid Steer Loaders &
Multi Terrain Loaders Work by Karim
Special thanks to Jeff Lind and
the rest of the Caterpillar®
team for their support in creating this
You've probably seen skid steer loaders, and maybe a
multi terrain loader, around commercial construction
sites or landscaping projects. Their small size and
maneuverability allows them to operate in tight spaces. Their
light weight allows them to be towed behind a full-size pickup
truck, and the wide array of work-tools makes them very
Photo courtesy Caterpillar The Caterpillar Skid Steer Loader (SSL)
Skid steer loaders are used to dig and move landscaping and
building materials. But the machines can also grade,
jackhammer cement and load trucks, as well as many other
tasks, which we'll see later.
When ground conditions are soft or more traction is
required, the tracks on the multi terrain loader make
it the logical choice of machinery.
The Caterpillar Multi Terrain Loader (MTL)
The Caterpillar Skid Steer Loader and Multi Terrain Loader
are neat machines. Since they are powered entirely by
hydraulic pumps, there is no mechanical
The simulator runs in a browser using Shockwave 3-D
(which you can download
for free). You need to have a decent Pentium 3 or
Pentium 4 machine for smooth animation, and a fast
Internet connection is helpful (but not necessary)
because the simulator file is about 2.5 megabytes when
it first downloads.
edition of HowStuffWorks,
we'll learn how the various systems on the Caterpillar Skid
Steer Loader and Multi Terrain Loader work, and we'll get some
insight into some of the engineering that goes into designing
and building these machines. But first, let's check out some
of the things these loaders can do and learn how to control
What They Can Do Skid steer loaders and
multi terrain loaders are versatile machines, and are fairly
easy to operate. They can turn within their own footprint,
just like a tank.
Photo courtesy Caterpillar Demonstrating the use of the grapple
There are several steps to getting started:
Use the convenient grab handles and steps to climb over
the bucket and get in. Fasten your seatbelt and lower the
Turn a key to start the engine, just like you do to
start your car
engine. If the engine is cold, you might have to wait a
few seconds for the glow plugs to warm up.
Release the parking brake to unlock all of the
The loader has a foot throttle, which, like the gas
pedal on your car, makes it go faster. It also makes the
loader arms move faster.
Operating the loader is simple. There are two
joysticks: The left-hand joystick controls direction, and
the right-hand joystick controls the loader. Each of the
joysticks controls hydraulic
valves that regulate the flow of hydraulic fluid to either
the hydraulic motors that power the wheels or the hydraulic
cylinders that power the loader.
Skid steer loaders have wheels, while multi terrain loaders
The track configuration of the Caterpillar
Multi Terrain Loader transfers machine weight to the
ground through 48 wheeled contact
The advantages of tracks include:
Increased traction (especially in slippery conditions
like mud or snow)
Low ground pressure and high flotation
Track tread design prevents damage to the surface
Reduced soil compaction
Driving a Skid Steer Loader and
Multi Terrain Loader When you push forward on the
left-hand joystick, all four wheels start to spin, or
in the case of the multi terrain loader, both tracks start to
turn. If you keep the joystick pushed forward and move it to
the left, the machine turns left. It does this by slowing down
or stopping the two left wheels or the left track. The farther
left you push the joystick, the slower the left wheels or left
track will move. The opposite is true when moving in reverse:
If you pull the stick all the way back, the machine goes
straight backwards, but if you then move the joystick to the
left, the right wheels or right track will slow down, causing
the machine to turn right. If you center the joystick and then
push it to the left, the left wheels or left track will move
backward and the right wheels or right track will move forward
-- this turns the machine around in the smallest possible
area. The animation below demonstrates how the joystick
controls the movement of the skid steer loader.
Controlling the direction of the skid steer
Operating the Loader The
right-hand joystick controls the loader arms and
bucket. Pulling the joystick back raises the arms, and pushing
it forward lowers them. Moving the joystick to the left tilts
the bucket up, and moving it to the right causes the bucket to
Controlling the loader arm and bucket of the skid
One of the cool features of the Caterpillar Skid Steer
Loader and Multi Terrain Loader is the anti-stall
device. The pumps that power the wheels or tracks and
implements can sense the load, so they won't exceed the power
capacity of the engine.
Next, let's take a look at the drive system.
Drive System As we mentioned before, the
drive system on the skid steer loader and multi terrain loader
has no transmission. Instead, it uses pumps and hydraulic
motors to provide power to the wheels or tracks (more about
the pumps later).
Photo courtesy Caterpillar The skid steer drive system, shown mounted in
the lower frame of the machine
For the skid steer loader, each side of the machine is
powered by a hydraulic motor. Each of the two motors
(one for each side) connects to a sprocket, and each sprocket
is connected by two chains to each wheel. The sprockets and
chains serve two purposes: They distribute the power from a
single hydraulic motor to both wheels, and they provide a gear
reduction to increase the torque at
The chains and sprockets are located inside
the frame, in a sealed compartment. They are immersed in an
oil bath that keeps them lubricated. Each drive
sprocket is connected to a hub via a short shaft. The shaft
passes through several seals before connecting to the hub,
which holds the wheel.
Next, we'll take a look at the engine and pumps that
provide power to the machine.
The Powertrain In your car, the main parts
of the powertrain are the engine and
In a skid steer loader, the powertrain consists of a diesel
engine and a set of hydraulic pumps.
The Engine Why do a skid
steer loader and multi terrain loader use a diesel
engine? For the same reason that all construction, mining
and farm equipment does: Diesel engines are more efficient
than gasoline engines. A skid steer loader or multi terrain
loader may operate eight or more hours every day. Over the
course of a year, a 5 or 10 percent difference in efficiency
can make a real difference in fuel
Although the engine, cooling
system and other accessories are tightly packed into the
skid steer loader, the engine compartment is designed to make
maintenance easy. A door on the back opens wide, and the
radiator and fan tilt up to allow clear access to the engine
and all of the maintenance items (such as filters).
Photo courtesy Caterpillar The engine compartment opens wide for easy
The engines in the Caterpillar Skid Steer Loaders and Multi
Terrain Loaders range from a 49-horsepower (37-kilowatt),
naturally aspirated diesel engine to a 74-hp (55-kW), turbocharged
diesel engine. This power is transmitted to a set of hydraulic
pumps bolted directly to the output of the engine.
The Pumps There are a
total of four hydraulic pumps hooked up to the engine:
A fixed-displacement pump provides hydraulic power for
the loader arms and accessories.
A smaller fixed-displacement pump provides hydraulic
power for circulating the hydraulic fluid through filters,
and provides pressure to the pilot controls.
Photo courtesy Caterpillar The stack of four hydraulic pumps attached to
This setup allows the skid steer loader to make good
utilization of the engine's power without ever stalling it. An
engine stalls when the load on it is greater than the power it
can produce. On hydraulic machines like these, the power that
the engine can produce has to be balanced with the power that
the hydraulic system uses. The maximum amount of power that
the engine can make depends on the speed it's running at. On a
skid steer loader or multi terrain loader, an engine at full
speed can produce its top rated horsepower.
Between the pumps that power the wheels and the pump that
powers the work tools, the hydraulic system can demand more
power from the engine than the engine can generate. The system
is designed this way so the operator can apply the full power
of the engine to either the wheels or the implements at any
given time. On most skid steer loaders, it is up to the
operator to carefully modulate the controls to keep the engine
from stalling (which requires skill and practice). On
Caterpillar Skid Steer Loaders, the operator doesn't have to
worry about stalling the engine -- the machine makes sure this
The power used by a hydraulic pump is equal to its pressure
multiplied by the flow rate of its fluid. On the Caterpillar
machines, the implement pump is a fixed-displacement
pump. In this type of pump, the flow rate is determined by
the pump's speed (which is equal to the speed of the
engine) and its displacement (the volume of the
cylinders in the pump). The faster a given pump spins, the
higher the flow rate. The pressure is determined by the tasks
the operator is performing. For instance, the pressure is high
when the operator is digging a bucket of dirt out of a pile,
and it is low when he tilts the bucket to dump the load.
This pump is designed so that at its maximum pressure and
flow rate, it does not stall the engine. But, if the hydraulic
pumps that drive the wheels were to draw any power while the
implement pump is at maximum pressure and flow, the engine
could stall. This is why the pumps for the wheels are
When the operator is not using an implement, the pumps can
operate at their maximum displacement, using the full power of
the engine to drive the wheels or tracks. The speed of
the machine is determined by the flow rate of fluid from the
pumps, while the torque is determined by the pressure.
During an operation like loading a pile of dirt into a
truck, the operator uses a lot of the engine power to push the
machine into the pile. When the operator lifts a bucket load
of dirt out, it takes a lot of force to break the load out of
the pile. If the implement pump were to supply the pressure
and flow for this operation while the drive pumps were still
drawing power, the engine would stall.
When the skid steer loader or multi terrain loader
enters the pile, the wheels use more power; then, as the
operator starts to lift, the hydraulics take most of the
engine's power to break the bucket out of the
To avoid this, the Caterpillar machines automatically
reduce the displacement of the pumps. This keeps the engine
from stalling while still maintaining torque to the wheels or
tracks at a reduced speed.
Now let's take a look at the loader arms, where we'll see
some really good engineering.
The Loader The business end of the skid
steer loader is the loader arms. These arms and their
associated hydraulics are designed to hold a variety of
implements, not just buckets. The lifting capability of the
arms is carefully matched with the rest of the machine so that
the operator lifts the load, not the machine.
Most Caterpillar Skid Steer Loaders and Multi Terrain
Loaders use a lift-arm design called radial lift. These
lift arms are connected to the machine with a single pin on
each side. The pins allow the bucket to follow an arc as it
rises. As the bucket starts to rise, it first moves out, away
from the machine. After it gets higher than the height of the
mounting pin, it moves in closer to the machine.
The bucket sits close to the machine when it is in the down
position to make the machine more stable and compact when
moving loads around. As the bucket is raised, it moves away
from the machine and then straight up. This gives the machine
extra reach, making it easier to dump a load into the middle
of a truck or place a pallet into a deep shelf, which is why
Caterpillar recently released a skid steer loader with a new
vertical-lift linkage. On a vertical-lift machine, the
bucket starts in close -- the same as it does on a radial-lift
machine. But by the time the bucket gets to about the
operator's eye level, it has moved away from the machine by
about 2 feet (0.6 m). From there, the bucket goes almost
straight up until it reaches its maximum height of 128 inches
Photo courtesy Caterpillar Two different linkages and the arcs they
The new vertical-lift linkage uses two extra links on each
side. The animation below shows how the vertical-lift linkage
The vertical lift linkage moves the loader arms
forward as they rise.
Next, let's take a look at some of the tools that these
loader arms can hold.
thing that the makes a skid steer loader or multi terrain
loader so useful is the wide variety of work tools that are
Caterpillar Work Tools are designed with an interface that
matches the machine's universal quick coupler system,
which allows for fast, secure work tool changes. With the
quick coupler, the operator can switch work tools without even
leaving the cab.
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Building the Skid Steer Loader and Multi Terrain
Loader Caterpillar takes every bit as much care in
the building of loaders as it does in the building of the much
larger, more expensive machines it manufactures. Great
measures are taken to ensure that the loaders leaving the
assembly line meet the high expectations of Caterpillar's
Several layers of corrosion-resistant coatings are
applied to most metal components.
Quality validation takes place throughout the process,
not just afterward.
Electronically controlled torque tools for tightening
bolts are utilized.
Coatings One of the most impressive things about the
Caterpillar Skid Steer Loader and Multi Terrain Loader
assembly plant is the coating and painting operation.
Caterpillar has contracted with a company called
Metokote to build an electrocoating
and painting facility that is in the same building as the
All of the metal parts, including the frame, roll cage,
loader arms, buckets and most brackets on the machine, are
sent though this process, which starts with a thorough
cleaning. The parts are then coated with zinc
phosphate and sealed with a chrome sealant. This
gives them a base layer of protection in case the paint and
electrocoat are scratched.
After the zinc phosphate and sealant are applied, the parts
are electrocoated. They pass through a tank of water
filled with paint particles. The tank and particles are
electrically charged to 200 volts, and the hangers that hold
the parts are grounded. This attracts the charged paint
particles to the grounded parts, coating the parts uniformly.
After the parts come out of the tank, they are cured in an
oven until the coating hardens.
After the electrocoat is cured, any of the parts that are
going to be exposed to sunlight are painted in a spray booth.
This multi-layer protection makes the parts rust resistant --
so resistant that if a scratch were to penetrate all of the
coatings down to the bare metal, the zinc phosphate would keep
any rust from spreading.
Validation Caterpillar tests the loaders during the
manufacturing process. For instance, they test for leaks in
the hydraulic systems before the body is attached to the frame
of the machine.
Photo courtesy Caterpillar A robotic operator exercises the hydraulics
during a thermal cycle test.
If a problem is found at this point, it's much easier to
fix than after the machine is fully assembled.
Control A lot of construction machinery is assembled
using air-powered impact wrenches (like the ones you see your
mechanic using). These wrenches are notoriously inaccurate
(plus or minus 10 percent, at best) at controlling the torque
on a bolt or nut.
Getting the correct torque on the nuts and bolts is
important. The torque is used as an indicator for how much the
bolt stretches. For a good joint, the bolt has to stretch a
little (this is what gives it its clamping force). A bolt with
too little stretch won't hold the parts on tightly, and could
even work loose over time. A bolt with too much stretch could
break during installation, or be weakened during installation
and break later.
The Caterpillar assembly line uses all electronically
controlled wrenches. These wrenches can control the torque
on the bolts to within 2 percent, ensuring a more accurate
amount of bolt-stretch.
Photo courtesy Caterpillar An electronically controlled tool for
Some joints are so critical to the machine that an even
more advanced technique is used. For instance, on the skid
steer loader, the lug nuts that hold the wheels on are
tightened by a special, electronically controlled tool that
uses the torque-turn method. This tool tightens the
nuts until a threshold torque is reached, and then it turns
them a fixed amount -- maybe one more revolution. This enables
even more accurate control of the bolt stretch.
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Complete Skid Steer
There is now a complete 3-D Skid
steer simulator available. You can drive the skid-steer
loader with the keyboard, one joystick or two joysticks!
It runs in Shockwave 3-D (which you can download
for free). You need to have a decent Pentium 3 or Pentium
4 machine for smooth animation, and a fast Internet connection
is helpful (but not necessary) because the simulator file is
about 2.5 megabytes when it first downloads.
For the following
3-D model you'll need to download:
Once you've downloaded the plug-in, use
version if you have a slower computer and the large
version if you have a fast
Photo courtesy Caterpillar 3-D model of Caterpillar Skid Steer Loader -
You can rotate it and zoom in and out! Use the small
version if you have a slower computer. Use the large
version if you have a fast computer.
here to see Caterpillar Skid Steer Loaders in
action. (6 MB)
Watch the Caterpillar
Multi Terrain Loader in action! Just click on your