[TC29] Tracked Bucket Truck

[lmdesigner42]

Hi everyone, my idea for the TC29 contest is a tracked bucket truck similar to the one below. The plan is to have six motorized functions (2x driven tracks, outriggers, 3x boom movements) using a distribution gearbox à la 8043. White would be fine for a colour scheme, but it's a tad boring, maybe purple and yellow will work?

So far, I have a rudimentary tracked chassis with 3-bogie suspension. The 3x19 frames are placeholders, but the width is correct.

I have a good start on the distribution gearbox, the orange shaft drives a worm gear to move the changeover catches back and forth. It's going to be fun routing the coaxial driveshafts to go under the turntable :).

[Paul B Technic]

Nice work so far and this is a great idea.

[lmdesigner42]

I made some more progress on my entry, the distribution gearbox is now finished. The left and right outputs on the motor side (LBG 16t gears) drive the left and right tracks through the bevel gears and brackets at the other end of the assembly. The central output on the motor side will drive the outriggers via the red axle connectors. On the non-motor side of the gearbox, the central and right outputs are transferred concentrically up through the turntable to operate the boom. Finally, the last output, the isolated red clutch gear on the left, will drive the turntable. This setup allows the turntable and boom bending to be operated together when all three driving rings are on one side of the gearbox, with the tracks and outriggers on the other side.

Now the next step is to put a sturdy frame around all of this to mount the tracks and turntable.

I also have a rough draft of the boom done. It would be nice to give it a boxy cross-section like the real machine, but it's hard to do at this scale due to the size of the large linear actuators. At least the range of motion is pretty good.

[Paul B Technic]

That is coming along well.

 

[Teo LEGO Technic]

Looking great! The solution with the red clutch and clutch gears to transfer two functions through the turntable reminds me of the LEGO 8043 excavator, still one of my favourite sets ;)

[lmdesigner42]

I finally have a bit more progress to report. I added a bit more structure to support the turntable, and a rotation mechanism using a worm/8z gear combo. More reinforcement is needed, though, the tracks are only attached by pins. There's very little space to add a proper chassis around the gearbox, but on the plus side, the narrow width means that the bending stresses are already lower.

The arm has also been lengthened and updated with a more efficient actuator placement and a 4-bar linkage for a wider range of movement. It reaches pretty high, even without the bucket.

Finally, a simple video to demonstrate the motorized functions. I'm happy to report that everything spins the right way!

 

[lmdesigner42]

The outriggers are all done. They're not strong enough to actually lift the vehicle due to the linkage geometry, but the folding/unfolding range is pretty good. I also added a few more parts to finish the bucket arm and start on the bucket itself. If I hurry, I think I can get the rest finished in time.

[2GodBDGlory]

Looks like some good progress!

[lmdesigner42]

Finished just in time! It's not the prettiest, but all 7 motorized functions I wanted are included. The colour scheme ended up being like Liebherr, with dark grey, yellow, and white. And yes, the railings are all made with DBG axle joiners.

The most important, and also the weakest feature, was the second joint in the arm. As seen in the video below, the L-Motor is barely strong enough to lift it. This is due to a combination of factors. 1) the L-Motor is only geared down 5:3 before the actuator, 2) there is extra friction from the long gear train and shafts, 3) the 4-bar linkage at the second joint increases the mechanical effort, 4) the arm tends to twist and bend under torque and load (see below picture), and 5) the length of the arm greatly increases the effective load from the bucket.

The last factor is the most significant. The first version of the bucket was made from stacked beams, and proved way too heavy to lift, so it was replaced with a more lightweight version. It may be possible to increase the stiffness of the arm and the gear reduction from the L-Motor, but that is difficult without increasing the weight and bulk too much. On the plus side, the total reach is pretty high.

The 7 motorized features work as follows. There are 3 L-Motors coupled to a distribution gearbox à la 8043, shifted by an M-Motor. One side of the gearbox controls the left and right tracks and the outriggers, the other side controls the arm joints and rotation. The 7th function is a motorized winch at the front, operated by the M-Motor that shifts the gearbox. A white clutch gear allows the motor output to slip when the gearbox shifter locks, while continuing to rotate and operate the winch. Thus, 7 RC functions are accomplished with only 4 motors. The battery box is easily removable from the underside with 2 red pins. There is quite a bit of empty space under the grey 'engine cover,' more functions probably could have been added with a more compact battery box/control setup.

 

A few small details were also added, such as a cabin with seat, lights, control sticks, opening door; as well as railings and supports for the arm and bucket.

Finally, a little video showing all of the features:

More pictures are on Bricksafe: https://www.bricksafe.com/pages/lmdesigner42/tracked-bucket-truck

[Jundis]

Really cool model and clean built :-)

[Paul B Technic]

That is very cool and I love how high it goes.

[Bartybum]

Looks awesome, love the functions. Quite a spindly arm though, it almost keels over in the video . Would any reinforcement near the bottom half of the lower boom work to stiffen it? I don't imagine that'd require much additional leverage for the actuator to lift

[lmdesigner42]

Thanks for the kind comments, all. @Bartybum It might. I thought about replacing the liftarms with studded technic bricks since they are stiffer, but I don't have very many in yellow. The main issue is that the boom's cross-section (-----) is terrible for resisting the torque of the driveshaft running through it. A circular or box cross-section like real bucket trucks have is much better.

[Bartybum]

12 hours ago, lmdesigner42 said:

The main issue is that the boom's cross-section (-----) is terrible for resisting the torque of the driveshaft running through it. A circular or box cross-section like real bucket trucks have is much better

I think that reinforcing the lower half of the bottom boom will help with torsional rigidity quite a bit, since that's where the shaft is located. Alternatively, is there a way to speed up the shaft and then slow it down at the LA? That would reduce the torque along its length

[lmdesigner42]

@Bartybum I implemented your suggestion, reinforcing the lower boom with 3x11 panels. The twisting and bending is greatly reduced now, but the motor still struggles to lift the upper boom. Maybe I can fit some gear reduction into the superstructure.

I don't think increasing the shaft speed along the length of the boom would help, since the full torque of the actuator would still need to be resisted by the lower boom at the end.