Blakbird

[MOC REVIEW] Grove GMK6400 Mobile Crane

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In another active topic on Eurobricks, modifications to the LEGO 42009 Mobile Crane MKII have been discussed. Many of these modifications have been rolled into a single "ultimate" model which has been under discussion for several months with a full instruction project well underway. I felt that it was time that this magnificent model had its own topic. In this topic I will present the history of the model, detail how all the individual functions work one by one, then go through my review of the build process and thoughts on each function. I expect this writing process will take at least several days, so watch this topic for updates, and please feel free to add to the discussion. By the time the review is done, I'm hoping the "final" instructions will also be available for download. Drafts are already available.

It all started with Technic set 42009, "Mobile Crane MKII", by far the biggest crane LEGO has ever released and also the largest Technic set ever by part count. You might think this would be enough to satisfy anyone, but you would be wrong. From nearly the day that the set was released, fans around the world were already modifying the set to make it even bigger and better.

The first major modification came from Jurgen Krooshoop. His "Ultimate 42009" looks nearly identical to the official set, but instead of having a single motor and a gearbox to switch between them, Jurgen's model featured 8 motors to control all the original motorized functions as well as driving and steering. Any of these could be operated simultaneously. Jurgen was kind enough to produce a full set of professional instructions for his model.

The original model features a 3 stage telescoping boom, but the full extension of this boom is not very impressive because there is so much overlap of each section at the stops. Cristophe Moittie solved this problem by creating an extended boom which was nearly the same length as the original when retracted, but was much longer when extended. Instructions for the extended boom were then created by afol1969 here at Eurobricks.

Finally, Eurobricks member Gerger decided to extend the cranes 5 axles to 6. While he was at it, he added full independent suspension to every axle. At this time the model no longer resembled 42009 very closely, so Gerger changed the appearance to loosely match the Grove GMK6400 mobile crane. He was kind enough to provide an LDD file of the completed model, and from there (and a couple of design iterations later) various EB members such as myself took over to make an LDraw file, a parts list, and full instructions.

Finally, we arrived at this brilliant model:

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From the outside, you might be tempted to miss how complex this thing is, but from inside you can see the 9 motors and copious mechanical functions.

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In this color coded image I've produced, you can see the interweaving of all the functions:

  • Red = Drive: 2 XL motors, all 6 axles driven
  • Blue = Steering: L motor, 5 axles steered
  • Yellow = Suspension: all 12 wheels independently suspended
  • Teal = Outrigger Extension: L motor
  • Brown = Outrigger Lift: L motor
  • Pink = Slew: M motor, 360 degree rotation
  • Tan = Boom Luff: L motor, parallel linear actuators
  • White = Boom Telescope: L motor, 3 stage telescope using cable return
  • Green = Winch: M motor

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At current count, this model features 3504 parts and 199 pages of instructions.

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OMG, this is simply awesome. Now if only TLG would produce something like this...

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Let's start by looking at the drive system. This model drives all 6 axles (12 wheels) via a pair of XL motors. By way of comparison, the real Grove GMK6400 is a bit different. Only the first 2 axles are driven by the engine through a conventional transmission, axles 3 and 6 are not driven at all, and axles 4 and 5 are hydrostatically driven only at low speed. So our model is superior to the real thing in some respects!

The image below shows the drive system. We start with a pair of XL motors in parallel at the front of the vehicle under the cab. The first thing that might surprise you is that these two motors are torque summed through a bevel gearbox. Bevel gears are known for their weakness, so you might expect them to break. However, the bracket used to support them seems to solve this problem by supporting them securely enough to avoid skipping teeth. After a single stage 12:20 gear reduction, a longitudinal axle then runs the entire length of the vehicle taking the torque to each axle. The next thing that might surprise you is that each axle is powered by an 8 tooth gear. These gears are also known for being weak, so make sure to use the new reinforced style gears. Each 8 tooth gear drives the ring gear of a differential. You'll notice that the diffs are not oriented laterally like a typical vehicle because there was no room. Gerger has provided a clever solution to orient the diffs longitudinally. The output of the diff is a pair of 20 tooth double bevel gears which drive a 16 tooth gear on an axle on either side. Each drive axle has a gearbox which can pivot on the suspension axis.

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Here is a close-up view of a single driven axle mechanism. You can see the 8 tooth gear driving the diff and the tan output gears in the front and back. CV joints are used as output to the hubs which are from 42000. Steered axles use one hub (10ea) and non-steered axles use the other (2ea).

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The XL motors provide adequate power to drive the vehicle. It is not fast, but a crane like this should be slow. The bevel gears and the 8 tooth gears survive the torque well, even when negotiating small obstacles (like the end of my finger). There is one problem, however. As you can see in the above picture, the final tan 12 tooth bevel gear is supported in its gearbox by a red 2L axle. This red axle then rests inside a CV joint. The problem is that the recess inside the CV joint is 1.5L deep. This means that the red axle can back into the recess allowing the tan bevel gear to fall out. This has happened to me repeatedly. As the crane drives along, it occasionally deposits a bevel gear on the ground behind it. Thankfully, it keeps right on driving and even more thankfully, it is not hard to replace the gear by removing the wheel. Unfortunately, this cannot be solved by using a longer axle because 3L is too long. We really need a 2.5L axle.

You can also see the V-8 engine in the first image which is belt driven by a shaft connected to the ring gear of the first axle differential. This shaft has the same drive speed as the main longitudinal drive axle. The belt reduces the speed a further ~3:1 which means that the motor turns very slowly. Ideally, the upper and lower pulleys would be reversed which would speed up the engine by 9x which would be perfect! Unfortunately, there is no room for the larger pulley on the lower axle. Alternately, the engine could use a small pulley (same size and the lower pulley) which would increase the speed by 3x.

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The amount of modifications are amazing. How does the boom lifting perform? I hope I can build this model some day

Edited by Rishab N

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OMG, this is simply awesome. Now if only TLG would produce something like this...

It is unlikely LEGO is ever going to produce a $1000 Technic model, but we can dream. I think this would fall into the same bucket as the Land Rover Defender 110 Cuusoo project. It would have to be so simplified to meet their quality and marketing standards that it would bear no resemblance to this model. In fact, this has already happened and it is called 42009!

The amount of modifications are amazing. Have you built it Blakbird? If so how does it perform. I hope I can build this model some day

Yes, I have built it. I'll be providing my impression of each function as I go along in the review. I'll also provide some build photos.

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Now I'm sad that today I got the last parts to build Jurgen's Ultimate 42009...

But not too sad. I've now got quite a few more parts than I did even three months ago, and that's not bad.

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I'm already collecting the parts to build this beast, only a few parts to buy (I've 98% of the parts)

As you can see in the above picture, the final tan 12 tooth bevel gear is supported in its gearbox by a red 2L axle. This red axle then rests inside a CV joint. The problem is that the recess inside the CV joint is 1.5L deep. This means that the red axle can back into the recess allowing the tan bevel gear to fall out. This has happened to me repeatedly. As the crane drives along, it occasionally deposits a bevel gear on the ground behind it. Thankfully, it keeps right on driving and even more thankfully, it is not hard to replace the gear by removing the wheel. Unfortunately, this cannot be solved by using a longer axle because 3L is too long. We really need a 2.5L axle.

I've enough 3L axles to cut some of them to 2.5L. I've already used this method in Markus53s' Unimog.

I know about the weakness of the 8tooth bevel gears, does it help to place a 4th gear in the differential, to have more stability? This was already discussed in another topic.....

Greetings

Alex

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Nicely written and I love the mechanical functions rendering very much! Good job, Blakbird! I hope your build go smooth as I haven't reassemble everything while I was stepping the LDraw files.

I will keep pressing F5 everyday to follow your reviews.

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Nicely done, Alex. Is that something you're going to do the entire instruction set?

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Nicely done, Alex. Is that something you're going to do the entire instruction set?

Yes, I can do it like this. While I write this post, I've already done axle2. I think first I do each model separate, later I'll see how to combine the models to get the whole construction. Gerger has made a good stepping, and in combination with blakbirds' drafts it helps me a lot to review the instructions for perfect presentation. But I've also to build the crane before definitively share the final instructions.

Another suggestion: The turntable should contain a second battery box or other weight parts to have an good counterweight for the boom. Once the instructions ready, I search for a solution to implant the modification.

Greetings

Alex

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Well, the set for Axle 1 looks great. This project represents such a wonderful collaboration among the experts on this site. All the cooperation is sure impressive to this new member.

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I know about the weakness of the 8tooth bevel gears, does it help to place a 4th gear in the differential, to have more stability? This was already discussed in another topic.....

I assume you mean 12 tooth bevel gears. The gears in the differential are not a problem. They cannot fall out because the 4L axles with middle stop are used, and they are not breaking. Adding an extra spider gear to the differential does nothing structurally unless it is guided by a pin on the housing.

Do you have any idea when the instructions will be complete?

When will there be instructions?

I suspect that "final" instructions will still take a while, but my draft 3 instructions are already available and are sufficient to build the model, however they don't include any of the improvements we are making to solve specific issues.

I've made an example for reviewing the instructions, by changing some part order and using Callouts.

Suggestions are welcome.

I like the submodel callouts you used. I had actually thought of those exact same things but decided to skip them in favor of getting usable instructions as quickly as possible. I'm glad to see that we are thinking of the same types of instruction improvements!

Two things: you need to use a PLI substitution to get a complete shock absorber in the parts list instead of a separate cylinder and piston. The belt is also missing from the parts list.

Yes, I can do it like this. While I write this post, I've already done axle2. I think first I do each model separate, later I'll see how to combine the models to get the whole construction. Gerger has made a good stepping, and in combination with blakbirds' drafts it helps me a lot to review the instructions for perfect presentation. But I've also to build the crane before definitively share the final instructions.

Alex, I highly recommend that you wait a bit before doing any more work because you are working with an old version of the file. The version I am using is different and incorporates some changes I made while building the model to solve problems. I'll give you a copy of the file as soon as I am finished with the changes.

Another suggestion: The turntable should contain a second battery box or other weight parts to have an good counterweight for the boom. Once the instructions ready, I search for a solution to implant the modification.

This is one of the things I intend to bring up in the review. The superstructure needs more counterweight.

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Now we'll look at the steering. This model steers 5 axles (1,2,4,5,6) making the model rotate roughly around the 3rd axle. By way of comparison, the rear Grove GMK6400 steers all 6 axles, although axles 3 and 4 disconnect at high speed. The real crane also has an additional crab steer mode.

Looking at the picture below, you can see that the steering is driven by an L motor. As I was building I was thinking that an L was overkill, but having finished the model and experienced the weight, I can see that it was a good choice. The L motor drives through a bevel gearbox into a worm gear giving 8:1 reduction. The use of a worm gear also means that steering backdriving loads won't make it to the motor. Like the drive system, the steering uses a longitudinal axle which runs the length of the carrier. This single axle is important to keep the steering in sync. It is also important while building to make sure that the steering is centered while joining each module. You can see that axles 1 and 6 use a 12 tooth gear on the steering rack, and axles 2, 4, and 5 use an 8 tooth gear. This results in two different steering angle ratios. Ideally, every axle would have a different steering ratio based on its distance from axle 3 (the pivot point), but in practice this is difficult to achieve. The result is that there is considerable strain on the tires and suspension when driving with the steering turned.

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The steering power is adequate, but the gearing results in steering which is a bit fast making it tricky to find the straight ahead position. An additional complication to driving straight ahead is the fact that the steering links are installed at a considerable angle. This results in quite a bit of free-play or wobble meaning that the wheels on a given axle are not necessarily parallel depending on the driving forces being applied to them. This is especially apparent on carpet when you can see quite a bit of toe out on several axles while driving forward. Still, the steering works very well given the stiffness constraints of the parts we are working with and the tremendous weight of the model.

I am using all V1 receivers on my model and I find that the drive motors slow considerably when I am steering at the same time. I'm not sure whether this is a limitation of the receiver or the current limit of the battery box, but it would be a good experiment to try a V2 receiver at least at this position. I think all the others could be V1. Alternatively, the steering and drive could be on separate receivers as long as you set up your transmitters to accommodate it.

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This is an awesome model. Thanks to all the people working on instructions for this beauty: you guys rock!

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Blakbird... You have brought this project to such a professional level with your renderings and instructions, and creating this forum topic. Thank you for the huge effort. I know you are enjoying this time spent, and we are fortunate that you share your expertise with us. Again, thank you.

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Next come the outriggers. The following image shows the two outrigger functions integrated together (and they do overlap considerably). Outrigger deployment is shown in teal and lowering is shown in brown. The general mechanical arrangement is nearly identical to that in the official 42009 set. The difference is that instead of being driven by a motor in the superstructure through the turntable, these outriggers are driven by a pair of vertically mounted L motors.

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We'll look at the deployment function first. The L motor uses some bevel gears in a vertically mounted gearbox to drive a longitudinal axle which keeps the front and rear outriggers in sync. There is only a single stage of 20:12 reduction before a set of 12 tooth gears drive the racks which deploy the outriggers. This means that, unlike 42009 which deployed the outriggers painfully slowly, these go out really fast. It could be argued that they are actually too fast for scale, but it is not a problem. This function requires very little power and therefore probably could have used an M motor, but the L motor is much easier to attach to a studless chassis, especially in this orientation.

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Now we'll look at the outrigger lowering. Again, the L motor uses some bevel gears in a vertically mounted gearbox to drive a longitudinal axle which keeps the front and rear outriggers in sync. After a 12:20 reduction and a bevel gearbox to convert the rotation to a perpendicular direction, a long 16L axle goes to each outrigger. Like 42009, the key to this whole thing is the red 8 tooth gear through which the 16L axle can slide freely. This allows the system to drive at any outrigger position, including stowed. As the final drive axles turn, each drives a small linear actuator at each corner. Whether or not the actuators have the power to lift the model is a trivial technicality, because they bottom out internally before they extend far enough to do so. Still, they provide enough support to stabilize the suspension while the boom is slewing to the side of the model. Lowering speed is acceptable and reasonably scale.

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All in all, the outriggers work very well and I have no complaints. The only minor thing to watch out for is to make sure the vertical linear actuators are stopped before they hit their end stops. If you drive them all the way to the stops, it adds a lot of strain the gear train which in turn makes it difficult for the other motor to slide the axles through the red gears. If you run into the stops, just back off for a fraction of a second before stowing and everything will move much more smoothly.

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