Lego Construction Site - Ideas

Home ] JCB Excavator ] All Terrain Crane ] Dump Truck ] Compact Excavator ] Skid Steer Loader ] Mobile Crane ] Instructions & Merchandise ]
Extras ] [ Ideas ] Links ] Contact ] FAQ ] About Jennifer Clark ]

This page contains sketchy documentation on various mechanisms I've been experimenting with, and as such the design of both the mechanisms and page will be a little rough. This is intentional, however, as spending too much time on these mechanisms detracts from the point of the exercise - rapid experimentation and prototyping. For more polished models, please see my main page.

Pneumatic and Electric Controller Unit  

I designed this combined electric and pneumatic hand controller unit with on board compressor as a direct replacement for the one featured on the skid steer loader pages, but with the notion of it being a "designed for manufacture" production type unit rather than a prototype for my own use. In order to achieve this the unit had to be:


Cheap: The original used a large battery box and polarity switch for the pressure limit switch, box of which are expensive components.


Easy to use: Should fit ergonomically into the hand. The battery must also be changeable without dismantling the unit in any way.


Robust and low maintenance: No parts falling off during normal use, no long term loosening of parts.


Power: Should be comparable to the original in terms of airflow and pressure.


Good Appearance: I wanted to achieve a look similar to that of the controller for a remote control car or airplane.


New motor: The original used the old style 9V motor to drive the compressor; since these are no longer available and are less efficient, the new 9V gear motor should be used instead.


Health and Safety: No external moving parts or gears - the 40 tooth gear on the original drew some suspicious stares!

Anyway, enough specifications... here are the results.

As you would (roughly) see it in use. The compressor can be switched off by removing the cable attached between the two green studs. When the cable is attached, it works in "on demand" mode.

The pneumatic cylinder for the limit switch is on the bottom right. The two yellow switches provide electrical power to the model via cables (not shown for clarity); the two grey switches supply compressed air to the pneumatic sections of the model.

Some time ago I received an email from Klaas Meijaard telling me he had built an air compressor using the grey switch on the small black 9V battery box as part of the pressure limit switch mechanism. I thought this was a great idea, giving pneumatic power on demand without having to use one of the expensive polarity switches found in most compressors, including my own. The idea was especially appealing since I had accumulated many of these black battery boxes through purchases of the 8266 kit in sales.

From this, I had the idea of powering not only the compressor from the battery box, but also the electric parts of the model in mind, my skid steer loader. Of course, if you use the switch on the battery box as part of the limit switch to the compressor motor, then you will only be able to supply electric power to the model when the compressor is running. This is obviously undesirable, so I powered up a soldering iron and made a few modifications to the battery box...

Since the power to the compressor motor comes from the terminals on the top of the box, and polarity switches were in use for driving the model in any case, a permanent power supply to the latter was obtained by soldering the ends of a standard cable directly to the battery terminals inside the box. Believe it or not, this was surprisingly easy. While making this modification I also bored a couple of Technic pin sized holes in either side of the box to allow it to be mounted on the controller without having to brace it from underneath, thus allowing access to the battery without dismantling the controller.

The above picture shows the underside of the controller with a clear view of most of the components. At the bottom left is the compressor motor, in the middle are the compressor pumps, the battery box is to the right and the limit switch is at the top right. Initially the compressor was only a single pump unit with a higher gearing, but this did not provide adequate pressure or airflow so 2 pumps acting 180 degrees out of phase were employed driven directly from the motor.  The limit switch, however, is perhaps the most interesting part of this device...

Closed - power flows to the compressor motor

Open - the red arrow indicates the battery box switch.

The pressure limit switch mechanism is very simple, with elastic bands providing the opposing force to the air pressure. When the air pressure is low, the bands pull the lever made from two 1x4 liftarms down which depresses the power switch on the battery box and switches on the compressor motor. When air pressure is high enough to overcome the elastic bands, the lever rises and the switch (which is spring loaded to return to the off position) rises, cutting off power to the compressor. 

The lever does not have to rise to the height it is at in the above right picture to switch off the motor - in actuality the movement required is slight, and I simply moved it this much to emphasise the workings. A good side effect of this slight movement, as opposed to the large movement required in limit switches using the polarity switch is the the on/off and off/on cycles are quicker and more symmetrical, exhibiting less hysteresis.

I have created a DAT file for this controller in MLCAD which can be downloaded by clicking here. Unfortunately the version of MLCAD I have does not include the pneumatic cylinder and compressor pumps so those have been omitted, but it should be obvious from the photos on this web page where they go. Similarly, I have no idea how to create pneumatic tubing in MLCAD, so that has also been left out. 


3 Section Telescopic Crane Boom - 3/12/00

Click to enlarge
Fully closed (please click on pictures to zoom in)

Click to enlarge
Half open

Click to enlarge
Fully open

Creating a three section telescopic crane boom from Technic Lego is something that occurs to many people, however, actually making one is far from straightforward. Part of the attraction of building this mechanism is that all the official Lego crane models only have two section booms. While it is certainly possible to achieve this with a rack and pinion on each moving section, this entails having a motor on both the bottom boom and the middle boom - not an ideal solution.

How real telescopic cranes achieved this was a mystery to me until recently, when I came across a picture showing the workings of a boom in this Acrobat document from the Liebherr web site. Basically, only the second section is moved by a hydraulic ram attached to the first section, with all the other sections being moved simultaneously by a complex system of pulleys. It is actually very simple when you think about it, but to come up with this idea in the first place must have required sheer genius. Here is a diagram showing how it works, in the simpler case of a three section boom; it is possible to totally conceal the pulley system within the booms, but showing it this way facilitates understanding.

The red line denotes the cable, red crosses show where it is attached to the boom, and the red circles represent pulleys. It can be seen from this diagram that holding one section still and moving any other section relative to it will produce an equivalent movement in the remaining section. So, to move the third section in sync with the second, all you need to do is move the second section relative to the first and it will happen automatically.

My Lego implementation uses a rack and pinion system powered by a single motor to move the second section relative to the first. What may not always be obvious at first glance is that the pulley system will actively move the third section when the boom is telescoping both upwards and downwards - gravity is not required to retract the boom.


Driven and Steered Vehicle Axle Using Znap Axles - 10/10/00

While the above has been achieved with the 8880 Supercar including independent suspension, building such a mechanism without the specialised parts contained in that kit (which only work with the 8880 wheels in any case) is a very difficult proposition. The difficulty is basically that standard Technic Lego is just not really setup in terms of strength and scale to do this, particularly if your vehicle is smaller than Supercar scale.

As a somewhat sneaky workaround, I had the idea of using the flexible axles provided in a Znap kit to power the wheels, which can be fed directly through the standard steering components, whereas a universal joint cannot. This is a very unrealistic method, not at all like a real vehicle, but if it could be made to work I thought there may be some mileage in it. It actually worked rather well, and while power transmission is not as theoretically efficient as a standard driven axle system, it seemed to make very little difference in practice.

The rack and pinion steering is a little too stiff for manual control, but an electric motor moves it perfectly well. This whole idea is geared towards a vehicle driven by motors, as opposed to a motor driven by the vehicle.

The top differential is the one relevant to the front axle; the bottom one was intended to be the centre differential in a four wheel drive setup.

The Znap axles are rubbing against the chassis, but this could be avoided with better design