[TC29] Mini Excavator

[2GodBDGlory]

After deciding to enter this contest, I went through a few different model ideas before settling on some kind of excavator. Now, there's been a fair bit of discussion lately about how the latest Volvo excavator set has failed to advance on the legacy of 8043, so this seemed like a good opportunity to see how that classic set could be improved, while retaining its basic spirit. Coincidentally, the contest motor limit of 4 corresponds with the motor count of the original set, so a similar gearbox approach will be needed to create an RC model!

Now, in order for my model to actually advance on 8043, it can't just have one motor for shifting between two modes, for six motorized functions. No! It must have at least three different modes, for nine motorized functions! However, that means that I need more things to actually motorize, which brought me to the Mini Excavator topic. My model is certainly not going to be "mini," but it will be a model of a real-world "Mini Excavator" I've worked around these cool little machines on a few occasions, and have always thought they'd make a good Technic model, on account of their extra functions, which typically include an adjustable blade on the base, a claw for the bucket, and a rotatable arm. I don't have enough motorized functions to do the rotatable arm, so I'll gladly leave it out to avoid the absolute nightmare of trying to route all the driveshafts for the arm through both vertical and transverse pivots! So, the functions will hopefully be as follows:

Motorized:

1x Servo motor for switching between three modes.

3x other motors for running three functions per mode

Functions:

Mode 1: Drive track 1; Drive track 2; rotate superstructure

Mode 2: Lift arm; bend arm; operate bucket

Mode 3: Operate blade; operate claw; tool change/PTO

To expand on a couple of those. Operating the blade is a challenge, because I've already got two functions going through the turntable for drive. What I'm hoping to do to get around that is to use vertical motion of the inner drive axle (going through the turntable) to operate the blade. It's got a sliding 8T gear on the bottom, and will be able to slide through a gear on the top as well, which should allow some other method pushing down on the axle to push down the blade (A rubber band will have to work to lift it up). It sounds complicated, but should be interesting to try to make work.

I'd like to have a tool change function to match the Volvo excavator set, and I think having a PTO for some other attachments would be nice. To fit that within my function limit, I'm hoping that I can use one motor for both, that will run the PTO while slipping a clutch on the tool change function. We'll see if that works or not.

Anyways, most of that is far in the future, but this is what I've got so far!

It's always fun when a 40T gear turns out to be needed!

Note the small linear actuators being used for manual track tension adjustment. It seemed like a fun function to include, and I had room for it!

I realize that this is an ambitious model, and I'm in very early stages, but hopefully it comes together according to plan!

I just finished college for the year, and I have yet to find summer work, so for the next while, I do have a disturbing amount of time on my hands to build Lego!

[Jurss]

Since when this is mini?

[Aurorasaurus]

27 minutes ago, Jurss said:

Since when this is mini?

7 hours ago, 2GodBDGlory said:

My model is certainly not going to be "mini," but it will be a model of a real-world "Mini Excavator"

[2GodBDGlory]

Well, I have a bit more progress to show! The only catch is that this is where the progress was at as of maybe three weeks ago, and I haven't worked on it since, as my summer schedule is a bit busier than my college one, and other hobbies have been taking up my time lately. And I now want to build a Crawler MOC for our Toronto truck meetup later this summer. With all that going on, I have given up any intention of finishing this model in time for the end of the contest. However, I do still think it's an interesting model, and I do hope to finish it at some point, even if it's not until the winter!

Anyways, this is the progress that I have:

I finalized a lot of the base of the model, and added the scraper blade. As explained earlier, this blade is the third function below the turntable, and so is activated by pressing down on the LBG axle going through the center of the turntable, like this:

After that, I started working on the distribution gearbox, but didn't get very far at all:

Anyways, I probably won't be competing in this contest after all, but I just thought I might as well get my progress to date put up here!

[Teo LEGO Technic]

Loving it! Very ambitious project indeed, but even if it isn't done in time for TC29 it definitely seems worth pursuing. How are you planning to use the servo to operate the three "modes"? If it's the PF servo, will you set it to 90 degrees left for mode 1, center for mode 2, and 90 degrees right for mode 3? If so, how do you maintain it in modes 1 and 3 without allowing it to return to center?

On 5/23/2025 at 5:42 PM, 2GodBDGlory said:

Note the small linear actuators being used for manual track tension adjustment. It seemed like a fun function to include, and I had room for it!

Great idea with the actuator to adjust the tension, ingenious!

[2GodBDGlory]

10 hours ago, Teo LEGO Technic said:

Loving it! Very ambitious project indeed, but even if it isn't done in time for TC29 it definitely seems worth pursuing. How are you planning to use the servo to operate the three "modes"? If it's the PF servo, will you set it to 90 degrees left for mode 1, center for mode 2, and 90 degrees right for mode 3? If so, how do you maintain it in modes 1 and 3 without allowing it to return to center?

Great idea with the actuator to adjust the tension, ingenious!

Thanks! You're right, the plan is to use the -90, 0, and 90 positions for the three modes. That could be done by just holding a stick in position for long periods of time, but another possibility would be to use the PF train remote, since it can hold a position

[Paul B Technic]

Great to see the progress on this, even if you don’t make the competition cut off, it would still be worth finishing it when you can.

[Teo LEGO Technic]

On 6/21/2025 at 10:41 AM, 2GodBDGlory said:

Thanks! You're right, the plan is to use the -90, 0, and 90 positions for the three modes. That could be done by just holding a stick in position for long periods of time, but another possibility would be to use the PF train remote, since it can hold a position

Sounds good! Yeah, I like the train idea - it's more elegant than having to hold a stick in a position. Alternatively, maybe it can be done programatically, like how the BuWizz app interface lets you create sliders without return-to-center. 

[2GodBDGlory]

10 hours ago, Teo LEGO Technic said:

Sounds good! Yeah, I like the train idea - it's more elegant than having to hold a stick in a position. Alternatively, maybe it can be done programatically, like how the BuWizz app interface lets you create sliders without return-to-center. 

Yeah, programming could be a nice solution, but it would most likely require using a Buwizz, which I don't have (or using PU, which is possible but rather bulky). I think I've seen some programs that can control PF from a smart device, so there's probably a theoretical way, but not an easy one

[Teo LEGO Technic]

3 hours ago, 2GodBDGlory said:

Yeah, programming could be a nice solution, but it would most likely require using a Buwizz, which I don't have (or using PU, which is possible but rather bulky). I think I've seen some programs that can control PF from a smart device, so there's probably a theoretical way, but not an easy one

Yeah, that's a good point. Alternatively, since I think you will have a physical remote, maybe you can come up with a mechanical solution to lock the lever or dial into place? 

[2GodBDGlory]

1 minute ago, Teo LEGO Technic said:

Yeah, that's a good point. Alternatively, since I think you will have a physical remote, maybe you can come up with a mechanical solution to lock the lever or dial into place? 

Yeah, I definitely could lock the lever into place, but the train remote is already a pretty ideal solution, since the dial will stay where it is and the position will be held by default. The only downside is that one of the other motors has to be run off that controller.
I could use a locking mechanism on a normal remote's lever, but then it's burning batteries all the time

[Teo LEGO Technic]

1 minute ago, 2GodBDGlory said:

Yeah, I definitely could lock the lever into place, but the train remote is already a pretty ideal solution, since the dial will stay where it is and the position will be held by default. The only downside is that one of the other motors has to be run off that controller.
I could use a locking mechanism on a normal remote's lever, but then it's burning batteries all the time

Good point, in that case, the train remote is best, you're right. 

[2GodBDGlory]

It has been a LONG time since I had an update on this project, and it's gone for months at a time without progress, but I do have some more to share now.

So, one of the first problems was figuring out how I'm going to route four functions through the arm, without making it absolutely massive. Universal joints seem like the perfect compact solution to it, but the issue is that they can only rotate about 45° upwards before starting to lock up. However, I couldn't think of anything better, so I ended up going with four parallel doubled U-joints, allowing for about 90° upward motion! Finding a pivot point for the arm that aligned with the virtual pivot of the U-joints was fairly hard, and required half-stud offsets in two dimensions, but it seems to work well

I then spent a while building up the area above the turntable, which was pretty challenging. I essentially needed to route 8 separate axles (the four from the arm, both track drives, turntable rotation, and blade actuation) from existing locations to reasonable locations with longitudinal axles, for the gearbox.

Also note the work on the mechanism for activating blade motion through the turntable (described further in previous updates). I've now got a worm gear mechanism rotating an arm that pushes down on the LBG 2L axle extender on the 2L transmission driving ring in this picture:

The TDR is only really being used to create a 16T gear that can have a sliding axle go through it, like those red 8T gears.

As mentioned before, that axle is being used both for driving one track, but also for pushing the blade down through axial motion.

 

After that I moved on to the gearbox, which is really complex! It's a 3-input, 9-output distribution gearbox, with six driving rings involved. These are split into three sets of two rings, with three functions from each set. Interestingly, there's only two wave selectors being used, which are geared together for synchronized motion. In order to keep it compact, two selectors are actually driving the same ring from opposite sides, just because those are the locations they needed to be placed in! 

I'm still busy with other hobbies, and will probably not be near my Lego over my Christmas break, so I don't know when the next update will come, but I think most of the most complex parts have been finished now!

I guess the next issue will be figuring out where to install motors, and then working on installing the arm

[Paul B Technic]

That is coming along really well. You have clearly put a huge amount of time and effort in to it.

[2GodBDGlory]

After TC30 got me back in the Technic habit, I've been plugging away some more at this model. I've now got three L motors installed to drive the functions, and a servo motor to shift between different modes. So far all the installed functions (drive/steering, turntable rotation, front blade, and first boom segment) are working satisfactorily, so hopefully they all keep working under the model's full weight! There's a lot of complicated gearing going on, so there are a lot of potential failure points. I ended up having to use chain for a both the drivetrain from the servo to the shifter (probably a good solution), as well as from one of the L motors to the gearbox (a little scarier to have transmitting that torque), so hopefully that keeps holding up.

I've also pivoted from the original contest-legal solution of using a 8878 rechargeable battery and two IR receivers, to using the much nicer MouldKing 6.0 battery.

Furthermore, the more I look at the model, the more I feel like its proportions match a NORMAL excavator, rather than the MINI excavator I was planning on building. To do a proper mini, I'd have to push the cabin further back into the section occupied by the mechanicals, but everything should work fairly nicely with a normal excavator, so I think I'm going to move to that. It'll still have the front blade and the claw/thumb by the bucket, which aren't nearly as typical on full-size excavators, but probably exist somewhere?

It feels like most of the challenging parts have come together, so hopefully I can have a finished model to present before too long!