Lego 8421 mobile crane, pneumatic issue

[Merluz]

hi guys, i'm new to this forum, just registered

sorry for my bad english, i'm from italy

i had just finish to build the 8421 mobile crane set, a dream that finally come true for me!

but... the pneumatic parts... when releasing the valve the boom of the crane simply crash down, it's a very bad thing to see

for now i have added some springs, from various old technich sets of motorbike, 2 little springs on the boom connected to the two pneumatic pistons and one big spings on the back of the cabin, that "stops" the fall of the boom before it crash on the top of the cabin

it's a temporary solution, but with this mod the boom can't "crash" anymore, the two little springs block it even if it don't fall over the cabin.

i would like to solve the problem in a "pneumatic way"! any suggestion for smooting the boom when fall down?

i have some extra pneumatic parts to use, one more valve, some tubes, a medium piston and a pump (the same included in the set), but i have no idea how to use them...

i've tried some solution, with the use of two valve e two pumps, but every time i release a valve the boom of the 8421 fall down like a dead arm

thanks in advance for any suggestion...

p.s. sorry again for my english

[Alasdair Ryan]

ok this might seem bad but have you tried putting the lever in the middle once boom is raised to the right position this will help

you could put a rubber band on the switch to limit the amount of air going to the ram

[DLuders]

@ Merluz: Welcome to Eurobricks! It sounds like some of your pneumatic cylinders are not working correctly. If they need to be lubricated, here is a YouTube video that shows how to do it. There is a good description of Lego Pneumatics on Wikipedia .

http://www.youtube.com/watch?v=ooB8q4DZXz4

[Siegfried]

I tend to release the air in stages to solve this issue.

(I actually expected this topic to be about the tendency of the cylinders to not be powerful enough once the beam is extended.)

[Merluz]

@Alasdair Ryan & Siegfried : thanks for the tips! if i try to move very slowly the valve i can ear some air that flows out, but nothing happen and the boom doesn't move. when the valve reach a certain position, all the air rapidly flows out and the boom falls down in an half a second

@DLuders: thanks for the useful guide, but my pistons are working properly, in fact they can raise up the boom with no problem.

problems starts when i move the valve to the "release" position, and the air came out from the pistons... the boom of the 8421 crash down at high speed

wandering if there is any method for releasing the air out of the pistons in a smooth way with the use of other lego pneumatic parts...

this 8421 set is something epic, but this falling boom is very annoyng...

here's some photos of my modification (with some lego system bricks) to the 8421

Edited April 17, 2011 by Merluz

[DLuders]

It seems that the "crashing boom" problem has been noted by others who bought the 8421 Mobile Crane. Gyl Midroni noted on this post that "releasing the pneumatic switch to lower the boom results in a crash of the entire boom to the ground. Why doesn’t it gracefully glide down? Shouldn’t the air leak out of the pneumatic cylinders more slowly? This is VERY bothersome. Is it a defective cylinder? or a defective design? I have never before seen one of those pneumatic switches where one of the two outputs is totally unused, and similarly for the cylinders - one of the inputs is unused on each pneumatic cylinder, so of course the air leaks out fast. Am I missing something here?"

There are several Brickset reviews, like this one from Bizkit: "... the pneumatic sistem for this well design crane is wrong. I don't get it why didn't they put tubing for the lowering of the boom. It would not complicated much the actual structure of the crane... and this way it simply falls down, no fun in that, I have to catch to boom every time and put a warning for others to handle with care :). The problem could be solved by putting tubes where they are missing, completing the pneumatic system this way. I think that everybody got the extra pneumatic T piece, so hold on to it and gets some tubes (I wish I have some tubes right now)."

BigMac said on this post: "I dare to say that the design of the pnematic part of ths super set was not carefull enough. To have a better control of the down movement of the crane arm, as mentioned by another review before, the air outlet/ inlet of the pneumatic pistons should all been collected, cannot have the downward outlet open. The reasons is that the crane arm is very heavy, as the down section of the piston is open, air is allowed to release through the open outlet quickly without any control. Once the air presurre in the upward section of the pistons is leased the whole crane will bang down in one tenth of a second."

BigMac suggested these improvements: "To improve the pneumatics control, I added two air tanks to it. You may have to follow the suggested steps below, otherwise all the air stored in the tanks will released by just one downward movement. After the alteration, now, I can have crane arm ( 2/3 extended), at least six up and seven down movements with two full tanks of compressed air.

Step 1. Plug the only open outlet on the air switch, ie fully air tight, no air can go through. To avoid the air leaking from the tanks when switching to lower the crame arms.

Step 2. Connect the air tanks and fixed at the left and right side of the crane, ie. move together with the crane platform is being turned. Comnect the hose to the pump and tanks. If you have the patience to read up to here, you should know how to connect air tanks.

Step 3. Connect the TWO open ends of the pistons ( ie one on each piston) by ONE single hose. pierce the hose (perferrably about one cm from one end, so that it can be reused in future by renmove this end) with a thin pin, just pierce once, you don't have to see the hole, actually cannot be seen by naked eye. This is to controll the air relaesed from the pistons and hence the downward movement of the crane arm.

After the alteration, I still have to controll the switch very carefully and slowly, but its all under control. The above works quite well for my model. I don't think this alteration will damage the compressor or the pistons."

[5150 Lego]

BigMac suggested these improvements: "To improve the pneumatics control, I added two air tanks to it. You may have to follow the suggested steps below, otherwise all the air stored in the tanks will released by just one downward movement. After the alteration, now, I can have crane arm ( 2/3 extended), at least six up and seven down movements with two full tanks of compressed air.

Step 1. Plug the only open outlet on the air switch, ie fully air tight, no air can go through. To avoid the air leaking from the tanks when switching to lower the crame arms.

Step 2. Connect the air tanks and fixed at the left and right side of the crane, ie. move together with the crane platform is being turned. Comnect the hose to the pump and tanks. If you have the patience to read up to here, you should know how to connect air tanks.

Step 3. Connect the TWO open ends of the pistons ( ie one on each piston) by ONE single hose. pierce the hose (perferrably about one cm from one end, so that it can be reused in future by renmove this end) with a thin pin, just pierce once, you don't have to see the hole, actually cannot be seen by naked eye. This is to controll the air relaesed from the pistons and hence the downward movement of the crane arm.

After the alteration, I still have to controll the switch very carefully and slowly, but its all under control. The above works quite well for my model. I don't think this alteration will damage the compressor or the pistons."

This system will work, but if you don't already have two airtanks already lieing around, this fix can become very expensive. The eaiest fix, is to simply use to complete the pnuematic system by using pnuematic hose to conect the unused "nipples" on the pistons to the unused ones on the control valve. This will slow down the the air coming out of the pistons giving you a little bit more control.

For mine, i found the most cost effective way to cure this is to add two more pistons to handle the extra weight. Not only will this help raise the boom, but it will increase the load lifting, as well as the extra friction will slow down the "crashing" of the boom conciderably.

Edited April 20, 2011 by 5150 Lego

[davidmull]

This is a major problem with pneumatics and why there not very persise,I hope they have some new parts to cure this problem on the 8110 unimog.

[5150 Lego]

This is a major problem with pneumatics and why there not very persise,I hope they have some new parts to cure this problem on the 8110 unimog.

I don't see this as beeing a major problem in general with pneumatics, unless...

1.- there not set up correctly such as 8421..

2.- there no strong enough to support the load there handling..

3.- your trying to lower the arm with a heavy load

I don't see them doing anything different with the new Unimog other than whats already been done.

[grindinggears]

This is a major problem with pneumatics and why there not very persise,I hope they have some new parts to cure this problem on the 8110 unimog.

The 'new' part you need is water. It is not compressable so the piston stays in place when the switch is closed and makes the piston move with the same (slow) velocity no matter which load is applied.

<offtopic>I will finish a MOC with working hydraulics in the next weeks.</offtopic>

[allanp]

Just use a tiny bit of skill moving the valve to lower the boom at a graceful speed. Works for me anyway