On Mon, 19 May 2008 13:29:47 -0500, Bill wrote:

The old man wants to put a 'dumbwaiter' into the barn so he can get
'stuff' up to the second story easier, he's getting old (79) and walking
up stairs is getting harder so I've been kicking plans around and want
to see if anyone could help with a detail I haven't worked out yet. I'd
like to make it that the 'dumbwaiter' gets locked in place when on the
second floor, was thinking of something like pawls on the guide tracks
that the carriage would click by so if the 'dumbwaiters' hydraulics
would fail then there would be no way for it to fall. This way if you
are loading a couple of hundred pounds on it it can't suddenly drop
taking your arms, legs or feet with it, although since I'm planning on
hydraulics it would take a really big failure of the cylinder to get
that fast a drop. What I'd like to do is make it that the 'dumbwaiter'
would have to be raised a few inches before going down to disengage the
pawls but I'm not sure how to go about designing it to do this. I could
do it with a solenoid but I think that adds another failure point and
would rather have it be all mechanical in nature. I'm not worried about
a failure while ascending or decending since the failure mode would be a
slow descent as the fluids bleed out, so I don't need pawls all the way
up the guide rails just at the 2nd floor. Anyone ever seen something
like this and could point me in the right direction??

Just remember this is a for a 'dumbwaiter' not an elevator, I don't want
the nanny's out there panicing about giving advice or suggestions for an
elevator and I'm not looking for designs just an idea how I might go
about making a pawl that can be disengaged by raising a platform higher.
I just want to add as many layers of protection as I can.

I've got some general designs on paper and as soon as I flesh it out a
bit more I'll start crunching numbers for loading and stress for beem
sizing and such. I plan on over building the 'dumbwaiter' but not
stupidly overbuilding it

Bill

Take a look at the pawls on an extension ladder -- it seems to me that
such a design (or a pair of them ripped off of an old ladder) would work
just fine.

Having said that, I think I'd take the suggestions to use a cable hoist.
Why do something fancy when something simple will do just fine?

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html