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In lugnet.technic, Paul Davidson writes:
> Hi, I'm considering an Adventurers-themed Lego project that involves a
> structure that, when triggered, can sink into the "ground" (as on The
> Mummy). My general idea is that at four locations (two on each of opposite
> sides), the part of the structure that must sink will have gears that can
> roll up and down along long, vertically-oriented toothed "racks" which are
> part of the stationary base (the ground).
>
> Without any further mechanical apparatus, this will plunge downward at
> whatever acceleration gravity exerts, coming to a final crash at the bottom.
> Is there any kind of gearing I can add (without using motors) that can slow
> this motion? I.e., is there a way to have the structure *slowly* sink
> rather than suddenly? I've thought about adding resistance through a rubber
> band that is pulled taught as the structure sinks, but I'm not convinced
> this will work well. Perhaps some kind of hydraulics system?
Well, I know you don't want to use motors....but.... :)
I just picked up a 3038 "Spider Slayer" set, and the motor therein is actually
quite weak - put a wheel on the axle and it spins real easy.
If you built your gear mechanism then hooked the motor to it (without
_powering_ the motor) you could then controly the descent speed by adding
different resistances to the motor leads - higher resistance in ohms would mean
higher resistance in force, thus a slower descent. This, I believe, is called
dynamic braking. (no doubt there's an electronics engineer out there who can
supply the true name for this particular usage of a motor/generator) It's used
on diesel-electric locomotives, and I know that my R/C racecars use it to great
effect.
I know it's not what you asked for but I thought I'd throw it out there in case
you'd not thought of using a motor in that direction.
Other options? Hm.
Use lots of gears and their inherent friction might supply plenty of resistance
- the only problem there is that it's difficult to control and likely
inconsistent.
Or gear it way up, so a slow movement generates a high rpm, and add a fan blade
to the high rpm axle - use wind resistance as the restrictor.
Use some pneumatic cylinders, but clamp off the line out so that they only let
air out at a very slow rate when you click the valve to the "lower" position. A
potential problem here is again that precise rate control may be a problem.
However, you can then click the valve to the "raise" position and raise your
building if you wish by pumping it back up - or powering it with an air supply.
The only problem here might be travel - cylinders are short. I think a series
of levers might help there though - a scissors action like that used in many of
the Cyberstrikers sets (3038 and 8257 for example). I think a similar mechanism
is used in a jack in one of the earlier sets from the supercar series.
As I write I'm starting to think that this might be your best option. If you've
got plenty of technic and pneumatic parts.
Well, it's time for bed for me. But ideas seem to still be slowly trickling
into my head so if any more 'gel' I'll post again to let you know!
Have fun, let us know what you come up with! (especially if it's one of my
ideas!ha!)
K.M.
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