8421 Mobile Crane

[j3tang]

edit: crap, didn't realize you couldn't edit the title, as it's kinda not very descriptive at the moment.

I searched and there weren’t really that many topics focused on discussing the 8421 Mobile Crane Truck, mostly just a handful of threads asking specifically about something. So I decided to start one up to see if someone could help me out with an issue of mine.

I recently just started playing with it again and I found that I could not extend the boom. The way it works is the motor basically has a forward and reverse motion, and depending on what position one of the levers is in, it would be controlling the extension/retraction of the hook’s line or the boom’s length.

I have no problems extending/retracting the hook’s line, but when I put the lever in the other position to control the boom, it will not extend properly. I have to give it a hand by gently pulling on it as I use the motor to extend it. I can retract it fine when it is extended. It almost feels like the weight of the boom itself combined with some kind of gear or motor ‘slip’ (???) is preventing the boom from extending. But as soon as I gently pull/guide it along, it works … well … ok. If I don't help it along during the extension, it just ends up extending the hook's line instead.

Also, I find that I have to constantly go back and forth between boom and the line control with the lever to extend the boom such that there is enough line to the hook (I’m not sure how to explain this part correctly).

Anyways, I’ve taken the boom section right off the truck and disassembled a good portion of it, but I don’t see anything wrong with the gearing and nothing seems to be out of place or misaligned. What am I missing? Was there something wrong with my build? I’m tempted to take this boom section apart and build it from scratch again. I’ll be getting my money’s worth I guess by building it a second time, but it’s still kinda annoying by the same token.

Anyone have any ideas?

Edited January 20, 2010 by j3tang

[CP5670]

The boom extension works fine for me. It sounds like something is not built quite right, or it's possible that you have a clutch gear that slips too easily. You could try replacing the clutch on top (easily accessible) with a standard gear and see if that improves anything, although I didn't need to do this.

Also, I find that I have to constantly go back and forth between boom and the line control with the lever to extend the boom such that there is enough line to the hook (I’m not sure how to explain this part correctly).

I think I see what you're saying here. The model is designed so that the line gets unrolled at the same time as the boom extends, but it doesn't happen at the same rate. If you want to extend the boom all the way, you will need to lower the hook a lot first, or it will eventually rise up to the end of the boom and get stuck.

[Sergiu]

I have no problems extending/retracting the hook’s line, but when I put the lever in the other position to control the boom, it will not extend properly. I have to give it a hand by gently pulling on it as I use the motor to extend it. I can retract it fine when it is extended. It almost feels like the weight of the boom itself combined with some kind of gear or motor ‘slip’ (???) is preventing the boom from extending. But as soon as I gently pull/guide it along, it works … well … ok. If I don't help it along during the extension, it just ends up extending the hook's line instead.

As CP5670 wrote, the problem with the extending boom is the clutch, which keeps slipping. I also had this problem and I replaced the clutch with the regular 24 teeth gear and now it works very well.

For the line problem, I don't think there is a simple solution so you will have to live with the problem or you could try to modify the boom, but I don't know how because I was never bothered by this problem.

I hope you manage to solve the crane's issues.

[j3tang]

thanks guys for the comments! I'm not too concerned about the line problem as i've heard a few other people mention this issue too, but I'm going to make an attempt with the boom extension problem.

Just to be clear, what's referred to as the clutch is the white 24-T gear that I've circled in the pic below right? The one to replace that's in the vehicle is the one that you can see looking at the boom from the top (not the one from behind closer to the motor)? And the regular 24-T gear is the one I boxed in the pic below (which I do not have a spare of).

If what I'm understanding is correct, can someone explain the reasoning behind it? is it because the white clutch has some kind of a bushing in the middle for the axle to go through, which is what is causing the "slipping"? whereas the other one does not have a bushing ...

Edited January 21, 2010 by j3tang

[Sergiu]

Like you said, the white gear is the clutch and the gray one is the regular gear.

The clutch you need to replace is the one located in the boom and not the one behind the motor, it should be pretty easy to do. Also the principle of the clutch is quite simple and it is made out of 2 parts. It slips when the torque applied to clutch exceeds a certain limit. Here you can see how the clutch looks inside.

I hope this helped you.

[j3tang]

I took apart another technic model and took the 24T gear and swapped the 24T clutch gear out with it.

It actually works slightly better, and I can see how the clutch has it's use here.

As I'm extending the boom, it feels like that gear I swapped in is really grinding its way in order to get the boom to extend. The fact that there's no bushing inside forces the gear to drive the smaller spur gear. I hear clicks as the spur gear slips from engagement with that larger 24T gear.

I'm now hesitating on whether I should replace it (for the safety of the model) :)

[Sergiu]

If the gears in the boom slip a lot, then you should probably put the clutch back, although this problem is quite strange because my crane works very well with the regular gear. Maybe you have done some mistakes during the build of the boom or maybe the load you are trying to lift is too heavy.

Good luck solving the problem.

[j3tang]

actually, there is little to no load on the hook.

It seems to do an alright job while extending forward (i.e. not in a raised position), but it has a lot of trouble extending if it's pointing up due to it's own weight.

I might just take the lift apart and rebuild it (the entire book #2), but thanks for the insight.

I sure hope it fixes the problem :p

[Frequency]

I found the same problem as well when assembling my 8421 yesterday, with the clutch gear slipping even trying to extend the boom with no elevation. Fortunately i did remember reading this thread a while ago and swapped it out for a spare bley 24 gear i had and problem solved.

as an aside i had a play with the clutch gear side by side with one from my brand new 8258 and the older one is slipping with far far less torque applied to it (with my torque ratchet and a calculator it is about 1.8Ncm) so i'm just putting the problem down to the age of the gear itself as opposed to with the design of the crane boom

[Jetro]

Check the colour of the centre of the gear. There are at least three versions of the clutch gear and the oldest one, with a light grey centre has a much higher torque. You can see the difference here:

full info: http://sariel.pl/2009/09/gears-tutorial/

[DLuders]

"awendelgass" just posted this YouTube video of the venerable 8421 Mobile Crane in action. Although the text is "auf Deutsch" (in German), one can easily see all of the fantastic functions on this 2005 set. I've added the sharp render from Blakbird's Brickshelf gallery.

http://www.youtube.com/watch?v=LHEpttIk3Hw

[jantjeuh]

Blakbird really is the king of renders

Love the looks of the 8421, but sadly it's insanely expensive (at least € 375). Will probably build it one day when I have enough parts, perhaps upgrading it to a four section boom and fully motorizing it with PF.