[MOC] 8868B revisited

[pleegwat]

The 8868 B model has always been a favourite of mine because of the fully automated pneumatic loop included in it. Some time last year I realised that the new pneumatic valves introduced with 42080 might allow a more elegant way to implement this same mechanism, and I decided to try for a 'modern-style' build, in studless style.

My first concepts were on power functions, but nothing quite worked for me until I decided to switch to PU and put the battery below the cargo rack. Several months of off-and-on working later this is the result

It doesn't work quite as well as I'd like to. Particularly the small cylinders I used on the claw have too much internal resistance (or the arm is not heavy enough) leading it to not always be closed correctly.

The front valve is not hooked up to the arm directly but has a gearing mechanism, since the axle holes don't line up. You can also see the rear valve, which similarly required some fiddling to line up.

The 'dumb' technic hub would have worked better here.

Working steering wheel

Detail of the compressor

Chassis shot.

Unfortunately, while the PU program is trivial, it is still a PU program and I can't run it and record video on the same smartphone at the same time. I may record one when I get a guest over to assist.

[gyenesvi]

As a kid, I remember being blown away by that looping mechanism that was involved in this fantastic B model, one of my all-time favorites. What makes it simpler with the new pneumatic valve? Is it that you can put an axle through it for switching? Curious about the video :) Nice work by the way!

[pleegwat]

Exactly - the original required a more space-consuming lever mechanism. Between that and the steering axle sitting below the racks instead of above I've freed up quite some space, though most of it ends up sitting unused below the compressor unit.

[howitzer]

I've been thinking taking apart my 8868 for a long time in order to build the B-model and actually examine the pneumatics in it... I only had it built once as a kid, and while it was as cool as any big Technic set, I couldn't perhaps appreciate it properly back then. Nice to see a modernized version of this one too!

[pleegwat]

The mechanism has two pneumatic valves, both of which are actuated automatically.

The forward valve is connected to the main arm (in the original using a lever mechanism, in mine using a gear) and controls the pneumatic cylinder in the rear of the cab.. As the arm tips over, this valve switches to the other position. Importantly, this valve is in neutral approximately when the arm is at its topmost position.

The pneumatic cylinder in the rear has as its only function to toggle the rear pneumatic valve. This rear pneumatic valve controls all other cylinders - the two cylinders raising the main arm, and the cylinder (one large on in the original, two small ones in my version) which close the jaw.

The cycle starts when the arm is in the forward position. Pressure is applied to close the claw and raise the boom. On the original, the claw tends to close first, as it has the lowest load. In my version, this does not work very reliably, as the internal resistance of the two small cylinders is too high. The big cylinders at the sides start raising the boom.

Once the boom crosses its topmost position, the forward valve switches. This reduces the pressure on the forward system, as part of the pressure is diverted to the rear cylinder. However, the main boom tips over and falls to its rearmost position before the rear cylinder has much of a chance to move.

The rear cylinder takes a second or so to fully switch the rear valve, inverting the direction on the front system

This causes the front system to work in reverse. Again, the claw tends to move first (even in my version, if there is a load in the claw the claw will open before the boom starts moving). The boom pulls back forward, and as it crosses the topmost position the forward valve is switched again.

After the forward valve switches, the boom falls back into its full forward position, and the rear cylinder moves inverting the front system back to its starting position. The loop repeats.