How Caterpillar Skid Steer Loaders & Multi Terrain Loaders Work

The Caterpillar Skid Steer Loader (SSL).
The Caterpillar Skid Steer Loader (SSL).
Photo courtesy Caterpillar

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 flexible.

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.

The Caterpillar Multi Terrain Loader (MTL)

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 Skid Steer Loader and Multi Terrain Loader are neat machines. Since they are powered entirely by hydraulic pumps, there is no mechanical transmission.

Photo courtesy Caterpillar

In this 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 one.

What They Can Do

Demonstrating the use of the grapple bucket
Demonstrating the use of the grapple bucket
Photo courtesy Caterpillar

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.

There are several steps to getting started:

  1. Use the convenient grab handles and steps to climb over the bucket and get in. Fasten your seatbelt and lower the wrap-around armrest.
  2. 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.
  3. Release the parking brake to unlock all of the hydraulics.

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 have tracks.

The track configuration of the Caterpillar Multi Terrain Loader transfers machine weight to the ground through 48 wheeled contact points.

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.

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Controlling the direction of the skid steer loader

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 dump.

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Controlling the loader arm and bucket of the skid steer loader

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

The skid steer drive system, shown mounted in the lower frame of the machine
The skid steer drive system, shown mounted in the lower frame of the machine
Photo courtesy Caterpillar

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).

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 wheels.

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

The engine compartment opens wide for easy maintenance access.
The engine compartment opens wide for easy maintenance access.
Photo courtesy Caterpillar

In your car, the main parts of the powertrain are the engine and transmission. 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 costs.

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).

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:

  • Two variable-displacement pumps located in a single housing provide hydraulic power for the two hydraulic drive motors.
  • 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.
The stack of four hydraulic pumps attached to the engine
Photo courtesy Caterpillar

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 doesn't happen.

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 variable-displacement pumps.

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.

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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 pile.

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

Two different linkages and the arcs they follow
Two different linkages and the arcs they follow
Photo courtesy Caterpillar

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 (325 cm).

The new vertical-lift linkage uses two extra links on each side. The animation below shows how the vertical-lift linkage works.

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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.

Work Tools

Photo courtesy Caterpillar

The thing that the makes a skid steer loader or multi terrain loader so useful is the wide variety of work tools that are available.

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.

Here are some of the work tools offered:

Dirt Bucket
Utility Bucket
Multi Purpose Bucket
Pallet Forks
Utility Grapple Bucket
Light Material Bucket
Utility Fork
Industrial Grapple Bucket
Industrial Grapple Fork
Angle Blade
Cold Planer
Hydraulic Hammer
Landscape Rake
Landscape Tillers
Material Handling Arm
Stump Grinder
Vibratory Compactor

Building the Skid Steer Loader and Multi Terrain Loader

Photo courtesy Caterpillar

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 customers:

  • 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.

Corrosion-Resistant 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 assembly line.

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.

Photo courtesy Caterpillar

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.

In-Process 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.

A robotic operator exercises the hydraulics during a thermal cycle test.
Photo courtesy Caterpillar

If a problem is found at this point, it's much easier to fix than after the machine is fully assembled.

Electronic Torque 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.

An electronically controlled tool for tightening bolts
Photo courtesy Caterpillar

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.


Caterpillar Multi Terrain Loader

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