Liebherr Litronic 912 Backhoe Excavator

Michael Adler

Introduction 

The prototype Liebherr Excavator is a very large hydraulic actuated machine which has an operating weight of over 20 tons.  It is a marvel of strength and versatility for earth moving, and has an interesting range of related activities. 

The bucket capacity is 1,40 cubic metres, and the four cylinder direct injection turbo charged diesel engine has an output of 130 hp.  The machine can lift a load of 5 metric ton at minimum radius.  The diesel engine and many functions are under electronic control, which minimises energy loss and maximises power and efficiency.  It drives a variable displacement axial piston double pump and is fitted with an electro-hydraulic regulator which react in seconds to changing requirements of power and speed. 

The machine consists of a rugged under-carriage with tracks and hydraulic drive motors.  The low machine centre of gravity provide exceptional stability.  The final drive to each track and also the swing is an axial piston motor and planetary reduction gear. 

The upper-carriage includes engine and oil pump, electronic control, drivers cab and controls, which include joysticks on the arms of the chair, foot pedals for swing control, and digital instruments to monitor oil temperature and pressure and engine RPM.  The drivers cab is roomy and has oversize windows for all round view of the jobsite. 

The gooseneck boom is operated by dual hydraulic rams, and the stick and bucket each have separate hydraulic rams.  Several attachments and add-on kits are available for the machine, including longer extended reach goosenecks, clamshells, grapples, hammers, shears and augers.   

In following the original design, one becomes aware of interesting features that are not at first obvious.  One of these is certainly a low centre of gravity which decreases the possibility if tipping over while lifting a heavy load, perhaps while itself standing on uneven ground.  To provide for this, the tracks are set well apart, and the machinery is as close to them as possible. 

An obvious requirement is that the roller bearing should have great stability and rigidity while allowing easy rotation at fast speed.  Notice how the boom is set directly over the roller bearing on an 'A' frame, so that the weight is taken vertically downwards.  But there are also large tilting offset loads generated when the machine is working, and the central bearing must be able to withstand these. 

One cannot but be extremely impressed by the shear power of hydraulic actuation.  Not only must the Boom, Stick and Bucket be moved from positions which seems to have a poor mechanical advantages, but they must do a day's job of work moving hundreds of tons of material.  The working life of such a machine might be as much as seven to ten years, which implies a high degree of reliability and construction from good quality materials.  

In years gone by, a spade was required to dig and move earth and materials.  Now, one person has his hands on levers which move tons with the greatest of ease, precision and speed, and he can do it consistently and for prolongued periods of time.  

The Meccano Excavator   

If one could build a model of this machine, in a scale which allowed for sufficient room for motors, actuators and controls, perhaps one could share in the power and dexterity which the driver must surely feel when he takes charge.  This is especially so, if one can arrange the levers as in the prototype. 

It is not presently possible to use miniature hydraulics in a model of this type.  In order therefore to build a model which will work smoothly and with the correct speed for each movement, simulation of the motions are required.  There are various methods of achieving this, including coaxial cables, lead screws with miniature motors and sliding mechanisms controlled by cables running from motor driven winches.   It is this latter method which has been found to be highly reliable after extensive experimentation, and gives a smooth and powerful linear drive. 

It is not at all clear in the photographs how the three movements work which are actuated by hydraulic rams.   The mechanical motion is hidden.  All three employ wire actuated sliders running on guide rails.  The wires are moved by winches which are geared to electric motors.  

The Boom is moved by a slider in the chassis. A drag rod links the slider to a large crank extending below the bearing point of the Boom.  One of the design features is to make this crank as long as possible, in order to increase its mechanical advantage.  

In the case of the Stick and the Bucket, although sliders are once more employed, they are in the Boom and the Stick respectively, and must be linked by long wires to their actuating motors.  

A rod projects backwards from the back of each of the two pistons within the hydraulic cylinders.  This requires that there is sufficient room proximally behind the attachment point at the base of each cylinder for the rod to move to its full extent. This space had to be carefully designed into the machine without making it obvious.  The slider is attached in each case to the end of this rod by a drag rod. 

As building progressed, it became apparent that the model would have a great weight, and this itself influenced design and construction.  It required constant testing, building and rebuilding to achieve the desired results.  Another consideration was to have reliable mechanisms, which would be able to perform repeated operations without undue maintenance. 

How large should such a model be?  A great deal depends on the space for machinery, including size of motors, reduction gearing and levers.  The present scale of approximately 1:9 has been chosen to allow space for all the necessary mechanisms needed and to assist in building a scale model.  It was thought essential to have each movement working smoothly and at the correct speed.  It was not one of the requirements to have the machine do a job of work, but to provide a challenge for model building and control, and a taste for the driving experience. 

The tracks must be able to take the machine load, and provide smooth motion.  In a search for reliability, it became apparent that a non standard method of locating and driving the tracks was required.   Eventually a simple design for the drive sprocket and idler wheel using Teflon was found. 

The roller bearing has to provide great stability to the superimposed very heavy rotating super-structure where the weight distribution is not symmetrically distributed.   The solution was found in hook rollers, which provide a smooth and positive action, and great stability.  The chassis of the super-structure is built directly over the roller bearing, and the four hook rollers gain purchase directly from the underside of the chassis.  Above the chassis, the 'A' frame is also directly built on the chassis, so that all the forces are concentrated together. 

Modular Construction. 

The model was designed to be built in sections, each of which must be fully completed before final assembly.   Great attention should be paid to pre-fitting, in order to make final assembly easy.   Once the model is completed, the cables running in the boom and stick prevent their easy disassembly.  The model can however be very easy dis-assembled into two halves between roller bearing and sub-chassis.  The completed model is extremely heavy.  The weight is more than 16 kilos or over 35 lbs.   

Method of Control 

Control of the model was an important consideration.  As mentioned, it was a requirement of the model that it could be driven in a way that simulated the driver's own experience.  This means joystick control.   In the prototype, the driver has a joystick in each hand, both of which have x and y motion.  Using these, and the foot pedals, he is able to easily control the speed of six motors in both directions which result in quite complex digging and movement operations. 

It is intended that pulse width modulation control for the six main motors can be achieved, known as 'armchair control', using centre off joysticks, which automatically control motor speed and direction. This provides smooth power through the entire range of motion. If the joysticks are arranged as for the driver, a good simulation can be obtained. An alternative  and simpler way however can be achieved through direct current control using switches and potentiometers.    

Great attention was given to the use of parts to provide a strong structure and an attractive appearance.    It is hope that you will enjoy building and operating this model, and find it a pleasing challenge. 

One day, it will be possible to actually power the model with a type of hydraulic actuator.  Experiments are continuing to realise this possibility.  It has been found that if two exactly similar double acting cylinders are connected by hydraulic tubing, the displacement of fluid from one cylinder to the other, if one piston is mechanically driven, will match the return flow from the driven cylinder.   The actuation is smooth and extremely powerful.  This raises exciting possibilities, for the basis of many modern machines is hydraulics, and Meccano has lacked the ability to faithfully reproduce these mechanisms.  We have been told that the scale of Meccano prevents the employment of actual hydraulics, which includes oil pumps, spool valves with electrical/electronic control, oil reservoirs, and reliable leak proof hydraulic cylinders, pistons, connections and glands.  So perhaps the cylinder to cylinder method will be the best way to go in the future.

Acknowledgements

The inspiration for this model came from admiration of these marvelous machines and the desire to build and operate one like the real thing.  It is extremely difficult to model one of these machines in Meccano because of the need to move very heavy parts with little mechanical advantage and with realistic prototype speed.   As there is no genuine hydraulic system which makes the real machine operate so powerfully and seemingly effortlessly, it is therefore necessary to simulate the various movements and there are a number of ways of doing so.    One can use a screwed rod turning at high speed.  One can also use a slider with a system of pulleys with cable working from a winch.   It is this latter idea which was adopted in this model, and my grateful thanks are extended to Tony Rednall and also Mervyn Wood who built excavators using this system and proved that it could be done.  The simulated hydraulic rams look like the real thing, but the rods which actuate the boom, stick and bucket work through them.  The mechanisms of this machine made from Liverpool Meccano are my own work.   The ideas are entirely theirs.    

My thanks also to Maurice Luck, Robin Davidson and Philip Edwards for help with their designs for the electronic controller used on the prototype.   

Michael Adler 

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