EC750E brain dump

I'm still waiting for parts, but some have come in. The double rims with studs as a sprocket does fit well in size, but does not drive the track at all. I'll either have to use spur gears elsewhere to drive the track or use something else for a spur gear.

While I've been waiting for parts for the shoes I thought about tackling some of the other problems.

One of them is the drive system. The Volvo EC750E has a two speed automatic gearbox per shoe. This permits both high and low speed. It would be nice to have this in the model so what are the parts required. Two motive sources(maybe XL motors), two transmissions(one per shoe), one battery box, one receiver. Due to the rotation of the body I'm expecting to need to locate all of the parts below the body such as in the X-frame and shoes. I find the automatic transmissions done in Lego to be a bit clunky, but they work. I've also looked into Lego CVT transmissions, but they have a big problem in that if rotation is entirely stopped on the output shaft the internal gears battle amongst each other to counter-rotate and torque to the output shaft is lost. This will not do. I'll have to revisit the CVT concept as I would love to have a smooth final drive ratio like say 1:20 to 1:4 depending on the friction of the environment driven on.

The large technic turntable is roughly 7 studs wide, but I need something closer to 11 or 12 studs wide for the model. I've been playing around with all sorts of ideas about how to accomplish this like support rollers front and back, but none have stood up to tests very well. I'm not leaning towards a hybrid technic/system brick solution wherein the force on the turntable will be distributed across a 12L brick built ring and an internal turntable. First try at this resulted in parts pulling themselves apart because of an inability to anchor the weight of the body to the X frame. What I've come to as a solution in to have two stacked central large turntables so that the one on the bottom can pull up on the X frame from underneath and the one on top can push down while sharing the load with the brick built 12L ring that acts like a fixed washer/bearing. There is enough space to have an 8 tooth spur gear driving a type 2 large turntable on top. A type 3 turntable can be used on the bottom for added rigidity over the type 2. 

Then there is the problem of the boom drive and possibly the cylinders tearing themselves apart under too high a load. Just for fun I put a 3 pound jar (roughly 1.3Kg) on the front boom of my BWE standard model and supported the central frame with one hand while operating the gear to actuate the cylinders with the other. They held on just fine so I'll move ahead with my own paired design cautiously and do another weight test after the current body revision is built.

Lots more learning about the model and possible Lego implementation has been going on, but I'm a bit stalled while trying to solve too many problems at the same time. Maybe I'll tackle the CVT first.

EC750E boom drive part 3

Fitting the new narrower cylinder drive still has overlap problems. At 12 stud width it is still too wide to fit in the maximum width that the model can handle before being way out of spec.
Looking at this photo it is clear to see how the turntable overlaps the space for the cab.

Time to rebuild the boom drive yet again using the half width gears, giving up a little on final drive ratio, and using an even thinner support frame to try and achieve a 9 stud width so that it fits within the space that the model requires on the frame.

In the photo above the new build is on the right side. It is one full stud width narrower. Note the single half beam between the tan coloured 12 tooth gears. This is the same gearing principles as the prior builds, but lower gear ratio to permit a tighter transfer box. This worked out better for framing strength. Bracing is added in the middle of the gears to always have a support on each side of the gears even in the middle of the transfer box. The space for the boom is now 6 studs, not the 7 that I had originally built so the boom will require a redesign. I may also reconsider the support structure for this boom drive as it would work better structurally if it more closely mimicked the real Volvo body frame:

EC750E boom drive part 2

I created a very basic frame for sizing the width and length of the excavator body based on the calculated Lego dimensions. Comparing the frame to the current boom drive the current width of 13 studs is clearly not going to work. 27(26.3 rounded up) studs is the max width of the cab so ideally the cylinders are 9 to 11 studs wide. Here is the same drive principle, but only 11 studs wide.

Looks good, but there is a problem here. The base of the cylinders is the small turntable and the gear teeth stick out of the 3x3 space into a 4x4 space making the 11 width design take 12 stud width space at best and 13 stud width if counting full width beams. Now we need an even more narrow design, but will it be possible to drive the turntables without resorting to twisty axles?

EC750E boom drive cylinder

Driving the cylinders that lift the main boom requires taking many parameters into consideration. The bucket, arm(3.55), and boom(7.7) fully extended have a reach of 92 studs. If the longest arm were used it would be 100 studs. The pivot point used by the cylinder is 6 studs away from the base of the boom. The cylinder is clearly at a mechanical disadvantage. If the pivot were at 46 studs then the weight of the bucket/arm/boom would be the only force down on the cylinder, but it is at 6. This is a mechanical disadvantage of 7.666. Given that the bucket/arm/boom may weigh up to 1Kg fully laden, the cylinders have a lot of compression force to deal with. Any give in the design of the cylinder drive is going to show quickly.

This joint fails because the 13L dark grey liftarms that sandwich the light grey 3L cross block used for the drive shaft gears is not locked in place. As the torque goes up the beams pull apart and the whole mechanism fails.

The chassis needs to be able to handle the weight down and the gear boxing needs to arrest the dimensional forces of each gear. More on the chassis later. Two gears in the same plane acting on one another will try to push the other away in the one plane. When two gears are in perpendicular planes the forces push the gears apart in all three dimensions. The simplest way to handle perpendicular gears is to use a box on each perpendicular connection. Having more that one perpendicular connection on a single gear can be tricky as two boxes need to interact together adding to the forces that are trying to pull them apart. Ideally the two perpendicular gear connections are split into two gear sets connected by a driveshaft. This permits more room for structural components even if complexity goes up a bit.

This setup does the above.

EC750E shoe free spinning wheel

A different track related question is which wheel looks better for the front free spinning wheel/sprocket?
I'm inclining towards the wheel in the middle as it is 3 studs wide whereas the tire is 4 and the wheel on the right is 5.

EC750E track drive update

The first track drive design had some problems. Notably the carrier for the planetary gear set was locked to the ring gear instead of the sun gear. Even better would be if it could counter rotate at a fixed rate to increase the output gear ratio. This is done below. The "drive sprocket" is now a paired ring wheel which also solves the problem of how to take power off the drive wheel to drive a proper sprocket since the drive wheel is not a proper sprocket.

In the picture above, the pink half of a turntable will be removed to permit a more narrow build. Power comes in from the red axle. The blue carriage on the turntable side will spin based on the planetary gear set of the turntable. This will increase the final drive ratio of the ring wheel driving the track by counter rotating the carriage of the drive wheel gear set. Finally the paired drive wheel will continue to transfer power along the green drive axle and power a sprocket somewhere along the track. At least I hope so. Once the parts that I have ordered through bricklink come in, I'll make this for real and test it out.

EC750E track drive sprocket

The dimensions in studs for each shoe of the excavator is 46 studs long by 8.3(maybe 8 or 9) studs wide by 10.5 studs tall. That means the total shoe structure should be 45 studs long and 10 studs tall with the track making up the remaining space above, below, front, and back.

From the discussion on the Eurobricks forum, there is interest in what sprockets will be used. I have not been able to find dimensions on the diameter of the main drive and idler sprockets at the front and back of the tracks. From various diagrams and photos I'm guessing these to be about 8 studs. The stock sprockets are roughly 3 and 5 studs in diameter; clearly too small. I'm currently considering the wheel from the old mining vehicle for its ability to be used as a planetary gear set and the large turntable for its almost 8 stud size. Probably something like this:

The only problem here is that while the teeth on the type 3 large turntable work on the track because of the larger spacing between teeth(vs type 2 pictured which doesn't), it doesn't drive the track acceptably well when I try it in practice. Probably the driven turntable will in turn drive some other gear mechanism going to a small 3 stud sprocket to drive the track at the same rate. At least that is what I'm hoping since I like the size of the large turntable.