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Introduction

The 12.12 sale (popular among countries with a strong Chinese influence I believe) gave me an opportunity to add to my growing inventory. Over the sale, I picked up 2 new sets, the Technic Compact Crawler Crane (42097), and the Technic Heavy Duty Forklift (42079).

One topic I’d like to cover in my Getting Started with LEGO MOCs series would be purchasing LEGO parts for your MOCs, and perhaps this would be a good example of leveraging sale prices to beef up and diversify your inventory. Costing a total of SGD171.99 including shipping, the 20% saved over the Recommended Retail Price made me feel better about spending this much on toys.

However, this won’t be the aim of this post. Rather, I’d like to show you the sets through the eyes of a Technic Builder (mine, in particular) to expose the technicalities that allows the user to build comfortably, and at the end of the build, play with the toys without the fear of it falling apart or failing over time and misuse.

The Technic Compact Crawler Crane

Apart from EV3 Core and Expansion sets, and a smattering of parts from past competitions, the only Technic Sets that are in my inventory would be 2 Technic BMW R 1200 GS Adventure Sets purchased for their enormous, yet svelte wheels used in our hyp3rcar and competition builds by our kids.

The Technic Compact Crawlier Crane appealed to me as it added a bunch of parts which would allow us to build more compact linear motion mechanisms. In particular, it included a Linear Actuator, as well as 4 Gear Racks with their Housings. While the EV3 set does allow one to assemble a Rack and Pinion mechanism, it is not uncommon to see gear slippage unless the mechanism is well-braced, which would necessitate an increase in footprint.

Using the Right Parts

Ok, this may seem self-evident but my experience tells me otherwise.

LEGO has made a ton of parts. 57,741 according to Bricklink. While the Technic build system uses the same spacing across most of its parts, at 8mm per space, this isn’t universally true. Unfortunately, there are bound to be parts which can be physically put together, but do not work as well as intended when used outside the configurations prescribed.

This is especially true for gears. Where available, using configuration specific housings will save the user a lot of trouble with bracing power-trains which will be subjected to large loads. LEGO axles are soft and warp under torsion and bending forces. If these are not kept in check by transferring loads to the supporting structures, gear slippage will occur, even resulting in wear and tear and breakage in the long run. For builders new to Technic, checking out official LEGO sets gives one a good starting point to determine optimal gear meshing that they were built for.

One simple and often underutilized technique is to trap Studded Axles between beams and cross bracing those beams. The most basic technique for preventing movement of axles or parts connected to axles would be to use stoppers/bushings. However, this relies on friction to keep parts where they are supposed to be. Personally, I am of the opinion that stoppers/bushings should only be used as spacers. This is especially true if you have an older inventory with parts that no longer fit as snugly. Furthermore, relying on friction often results in inconsistent results. With this simple technique, shifting of axles and driven elements will be reduced, if not eliminated.

In the same vein, regular parts under high loads can also be trapped! This technique is utilized in this build, where a single stud lift-arm traps in place this small turn table which is used to hold a quarter of the weight of the whole build when under load. Without this small but significant piece, the turntable could overcome the friction of the pegs holding it in place when loaded at certain angles. This is a good alternative to cross bracing and can easily be implemented when using these Technic frames.

Finally, one often overlooked aspect of increasing structural rigidity involves building into the third dimension. Oftentimes it is tempting to do everything within the same plane as it makes thing less complex and uses fewer parts (definitely a boon for competitions that require you to build on the spot). However, one has to consider the trade-off in rigidity. The chassis of this crane, and that of any robot serves to hold the individual components together. A lot of calculations that we do when programing robots are also based on the Rigid Body Assumption. That means that parts within the robot is always assumed to have a fixed displacement with each other. One particularly important case would be the displacement of drive wheels with respect to each other. Slight movements will result in incorrect calculations in the Differential Drive model, with these errors stacking up across duration of the run. With Technic, it is easy to use these I-shaped components and frames to build into the third dimension simply and with few parts. The resulting increase in structural rigidity when done correctly is definitely worth the extra components!

Reducing Power-train Loads

When building with EV3, especially for competitions, most rotating elements do not involve heavy loads (wheels, and picking up of relatively light LEGO elements over short distances). One exception would have been the WRO 2015 Senior High Mission, where teams were required to place a block on top of a tall structure, far from the accessible area of the level playing field. As such, most teams decided that the best approach would be to use some sort of linkage to enable to long reach. Unfortunately, most approaches I have seen loads the axle and motors directly. Such prolonged strain would not only result in warped axles as discussed previously but too, wobble and inconsistency in a mechanism with such a long reach as a small angular error caused by wobbling of the drive axle results in a large error in translation at the end of the mechanism. The easy solution to this problem would be to use turntables to transfer loads from the axle to the chassis of the robot. With a direct connection that is spaced out over 3 holes, it results in a much more stable mechanism capable of carrying larger loads. In the case of this crane, both the supporting arms and the main boom utilize turntables, all of which use an appropriate size for the load it is carrying.

While this may not exactly reduce load on the power train, having exposed adjustment knobs is something commonly seen in official Technic sets, but not enough in competition robots. With these knobs geared down for torque, it is much easier for competition teams to ensure that their mechanisms are starting out in the right position. The friction from these gears, while normally unwelcome, may also act as a damping force when the mechanism is under the load of gravity.

One easy way to reduce loads on power trains would be to ensure that gears are braced. The rationale for doing this is similar to why one would build a wheel brace. With large loads, cantilevered elements used for power transfer is not optimal. Apart from axle flex, there is also some tolerance between the axle and the hole it is passing through. With a brace on the opposite side of the axle, a sandwich is created, supporting the gear from both sides and preventing a small angle error from accumulating displacement along the length of the axle. This reduces gear slippage and wear and tear, also providing a more consistent motion as gears do not go out of mesh before rotating their followers when under load.

One common mistake made by those new to building using the Technic System would be stacking beams to bridge spaces in 2D. Unlike regular LEGO Bricks of the stud variety, cross bracing is easy with the Technic system. By wrapping the object to be cross braced with a plane-changing part, one can build a rigid connection across a great length. The individual connections between the plane-changing parts and the beam do not need to be particularly irremovable. This build actually uses axles, which may look suspect to the unacquainted. However, the magic is in the beam used for cross bracing. Since beams are one of the hardest elements, with each space mostly consistent even after years of use, they keep different parts of the build at a fixed distance from each other. In other words, it brings us closer to the Rigid Body Assumption. One other advantage would be that this creates a much lighter and sparser build which is not weak in the stacking axis (when stacking beams to bridge spaces) since it is not held only by the friction of individual pegs.

With long power trains spanning large distances, there is always the temptation to use long axles. After all, they seem to be the quickest way to bridge between 2 points. However, this is often inadequate as longer axles are softer and warp more easily under torsional load. For loads that are not extreme, such as in this crane build, the distance is bridged by multiple shorter axles with axle connectors between them. With larger loads, axles used should be even shorter (I often use a 2 length axle), and massive loads should not rely on axles at all (something I might cover in future posts).

When there is a need for mechanisms that can be back driven to hold large loads for sustained periods, it is advisable to implement mechanical solutions rather than to rely on software control. While it is possible to implement a controller (PID or otherwise) to hold the motor’s position, doing so at the limits of the motors performance will result in large current draws, possibly burning the motor out if precautions are not taken in software.

In this scenario, this can be seen with the hoist mechanism. While this model does not come with a particularly heavy load, it is possible that the designer of the set had in mind that the payload could descend due to gravity without user input. This would reduce the playability of the set as the user would then have to keep one hand on the knob at all times.

As can be seen, a Ratchet and Pawl mechanism was implemented so that the mechanism can only be turned counter-clockwise, lifting the load up and preventing it from slipping back down due to the influence of gravity. This same mechanism can be implemented when driving large, sustained loads with motors, prolonging their lifespans and increasing battery life.

Conclusion

The LEGO Technic Crane was a fun build which allowed me to point out features that are helpful for anyone from the MOC enthusiast looking to design a build that not only is aesthetically pleasing but also does not fall apart during play, but also competitive roboticists looking to squeeze every ounce of performance from their builds.

As someone who never had access to these sets, and developed my techniques independently, it was great checking out how professionals integrate similar techniques into their builds, underpinned by the same concepts.

The next set I will be deconstructing while constructing would be the Heavy Duty Forklift. Do let me know if you would like your opinions on any other sets!

Special Note

While this set was bought in December 2019 and built around the time, this article was half written and lying dormant in my drafts. However, with the COVID-19 situation in Singapore, I am currently staying home in my little effort to stop the spread. This gave me the motivation to finish this piece in hopes that I am able to relief a little of your boredom.

Stay home, stay strong, and build on.