Liebherr R 938 - Pneumatic Excavator

Discover the power of pneumatics through play!

Experience the fascination of a detailed hydraulic excavator with the Liebherr R 938 Pneumatic Excavator from fischertechnik.
With 689 high-quality parts, a pneumatic compressor, hand valves, and precise exhaust air throttles, you can control impressively realistic movements. Whether you're a child or an “adult,” discover the fun of designing and experience technology up close. Perfect for anyone who shares a love of detail and enthusiasm for construction projects.

The original – Liebherr R 938

Generation 8, Operating weight: 37,300–41,400 kg, Engine: 220 kW / 299 hp – Stage V, Backhoe bucket capacity: 1.00–3.00 m²

The Liebherr R 938 model from fischertechnik

The hydraulic movements are realistically simulated in the model using pneumatics and exhaust air throttles.
Pneumatics are an integral part of everyday life. You probably encounter them in various forms every day. It could start with your breakfast egg, which may have been packaged using pneumatic suction cups.

The word pneumatics comes from the Greek word “pneuma,” which means “air.” Pneumatics is primarily concerned with using air to generate motion and perform mechanical work. Almost anything can be powered by pneumatics. It can be used as an alternative to muscle power or any other form of energy, such as electricity, water, hydraulic oil, or wind power.

Today, pneumatics can still be found everywhere in everyday life. Whether it's inflating a balloon, using a jackhammer in road construction, diving with compressed air, painting, airbrush techniques, pneumatic sirens, speed measurement with ram jets in aircraft, or large industrial plants—pneumatics is used everywhere!

What are the advantages of pneumatics?

The advantages of pneumatics are that ...

Pneumatics can be stored
Pneumatics can be transported over long distances through pipes and hoses or in appropriate containers
Pneumatics is clean and does not cause contamination
Pneumatics can be used to perform movements quickly
Pneumatic cylinders can be used to perform many movements without complex mechanics
Pneumatics is explosion-proof

A little history

As early as the third century BC, the Greek mathematician and inventor Ktesibios was working on pneumatics and the exciting possibilities it offered. He developed the first machines powered by compressed air, such as a catapult that used compressed air to launch balls and spears.

A very well-known pneumatic system was developed by Heron of Alexandria, who used the altar fire to generate compressed air and thus opened the large temple doors as if by magic. The heat from the altar fire warmed the air in a pressure vessel, which was half filled with water. When air is heated, it expands and the air pressure rises. The expanding air needed more space and pushed the water out of the pressure vessel into a water tank, which lowered due to the increase in weight and thus opened the doors.

Since the beginning of the 20th century, pneumatics has been used as a drive and control technology in industry. In the construction machinery and agricultural machinery industries, pneumatics is used, for example, to drive hammers and drills. Suction and pressure pneumatics are also used in conveyor technology, for example in grain mills for sucking in grain and conveying flour. We even find pneumatics in the music industry, for example in organ building. In a pianola, a self-playing piano, the keys are controlled pneumatically. Applications for pneumatics can be found in the automotive industry, the textile and food industries, electrical engineering, even in space and in many other areas of everyday life.

A pneumatic system consists of five subsystems.

Compressed air generation
Compressed air distribution
Compressed air preparation
Motion generation by pneumatic cylinders
Motion control by valves

Compressed air can be generated by a compressor, compressor or air pump and stored in compressed air cylinders and other pressure vessels.

The diaphragm pump as a compressor

Your fischertechnik compressor is a diaphragm pump. It supplies you with the pneumatic power you need to control the individual models. In industry, this is referred to as a pneumatic power source.

How it works:

A diaphragm pump consists of two chambers separated by a diaphragm, hence the name of the component. In one chamber, the elastic diaphragm is moved up and down by a piston and an eccentric. During the downward stroke, the diaphragm is pulled back and air is sucked into the second chamber through the inlet valve. During the upward stroke of the piston, the diaphragm pushes the air out of the pump head through the outlet valve.

Note:

The overpressure generated by the compressor is approx. 0.7 to 0.8 bar. The diaphragm pump is maintenance-free. It is important that you use a 9 V alkaline battery to power the compressor.

Pneumatics

The blue hoses are the means of transport for the compressed air. They transport the compressed air to where it is needed. You can lay the air lines from the compressor to the valves and cylinders.

Pneumatics

In order for pneumatic components to function properly in industry, it is important that the compressed air is treated appropriately. To do this, the air must be filtered, cooled, dehumidified and, if present, the oil must be removed. However, this is not necessary with the Strong Pneumatics modular models.

Pneumatic cylinders

We use pneumatic cylinders to generate movement using air. A basic distinction is made between “single-acting” and “double-acting” cylinders. The Strong Pneumatics construction kit contains two pneumatic cylinders of different sizes with the same “double-acting” function.

The blue piston rod is movable and the cylinder is sealed. If air is blown into the cylinder through one of the two hose connections, the piston rod moves. If air is blown in on the opposite side, the piston moves back. The piston can therefore actively work in both directions of movement.

The connection through which the piston rod is extended is called connection A, and the connection for retracting is called connection B. Since the piston rod of the cylinder can be extended and retracted with air, the cylinder is called a “double-acting cylinder.” To see this in practice, perform an experiment.

Try this:

Attach a piece of black hose to connection A of a cylinder and connect it to the hose connection of the compressor, which is already connected to the battery holder. When you switch on the compressor, the piston rod extends. Since this is a double-acting cylinder, the piston returns when you connect the hose to connection B and supply pneumatic pressure to the compressor again.

As already mentioned, there are also “single-acting cylinders.” With these cylinders, the piston rod can only move in one direction. A spring is often used to move it in the other direction.

To demonstrate that air can be compressed, perform another experiment.

Experiment:

Now extend the piston of the cylinder again by reconnecting the black hose connected to the compressor to connection A and supplying pneumatic pressure. Once the piston rod is extended, switch the hose connection to B, keeping connection A closed with your finger.

Observation:

The piston rod can only be pushed in a little bit. Do you know why?

Explanation:

Since you are holding the air connection A closed with your finger, the air in the cylinder cannot escape. But air can be compressed. This is why the piston rod has been pushed in a little. The more air is compressed, the greater the pressure in the cylinder. This pressure can be measured with a pressure gauge. The unit of pressure is “bar” or “Pascal.” The amount of pressure can also be calculated. The formula for calculating the amount of pressure is:

Pressure = force/area or, in short, p = F/A

Using this formula, you can see that the amount of pressure depends on how much force is exerted on the round surface in the cylinder.

Valves

In pneumatics, a valve controls the air flow to the pneumatic cylinder so that the cylinder either extends or retracts. A valve can be operated mechanically, electrically, pneumatically, or manually.

The kit includes manual valves. These valves each have four connections:

The pneumatic pressure is supplied from the air reservoir via the middle connection P. The left or right connection (A or B) controls the pneumatic pressure to connection A or connection B of the cylinder. The connection R on the lower side of the valve serves as a vent. The air returning from the cylinder escapes through this connection. To test the function of the valve, carry out the following test.

Experiment:

Connect the compressor, which is already connected to the battery holder, to one of your valves. To do this, take a piece of the black hose and attach it to the hose connection of the compressor and to connection P of the valve. Leave the other connections free. Set the blue switch on the hand valve to the middle position and switch on the compressor.

Observation:

Nothing happens.

Explanation:

When you set the switch on the hand valve to the middle position, the connections are closed and no air can escape.

Next experiment:

Now turn the valve switch to the right and switch on the compressor again. While doing so, tap the open connectors A and B with your finger. Do the same when you have turned the valve switch to the left.

Observation:

The air always flows through connection A when you turn the blue valve switch to the right and through connection B when you turn the switch to the left.

Explanation:

The illustration helps you understand how the air flows through the valve when you turn the switches in different directions. The blue line represents the pneumatic air flowing through the valve. The black lines show how the air flows back from the cylinder.

The valve therefore has four connections and three switch positions (center – left – right). For this reason, the valve is referred to as a 4/3-way valve in pneumatics.

Exhaust air damper

Have you ever stood on a garden hose or kinked it? If so, you will have noticed that suddenly less water comes out of the hose. But why is that? The kink in the hose means that the water has less space to flow, so it is restricted and therefore slows down.

Exactly the same thing happens in pneumatics when the air in the transport medium, in our case the black hose, is constricted and has less space to pass through.

You may be wondering why we would want to do this in the first place.

By throttling the air, the various movements can be performed more slowly and in a more controlled manner.

Pneumatics used in industry are usually operated at a pressure of between 6 and 8 bar. This allows pneumatic cylinders to extend very powerfully and, above all, very quickly when required. However, a powerful but slow and controlled movement is often required. A movement that is too fast could endanger machine parts, workpieces being handled or even people.

To move cylinders slowly, the pneumatic pressure is throttled. We allow less air to flow through a hose or pipe by simply narrowing the cross-section of the pipe.

Experiment:

Perform the following experiments with the setup shown below. All you need to do is connect a hose to the valve at P and the compressor with the battery holder:

Do not throttle the exhaust air by turning the blue screw into the housing only far enough so that the hose is not compressed. Observe how quickly the cylinder extends or retracts.
Now restrict the exhaust air more and more by gradually turning the blue screw further in. Observe the cylinder again. What do you notice?

You can see that the exhaust air restrictor helps you to adjust the speed at which the cylinder extends or retracts.