wrote in message
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On Thursday, 5 December 2019 18:14:20 UTC, wrote:
On 05/12/2019 16:11, newshound wrote:
On 05/12/2019 14:19, wrote:
My house has a very large and boarded attic and I want to make access
easier by replacing a flimsy metal loft ladder that someone fitted in
place of an original wooden ladder. The existing trapdoor is quite
large (690x1450) so I'm tempted to make my own replacement ladder to
avoid having to put filler pieces in the ceiling - either a 3-section
z-fold or a 2-section slider. Making the ladder would be
straightforward but it's going to be heavy (especially when connected
to the trapdoor, which is timber framed and solid) and I can't see how
to counterbalance it safely. I could set-up 2 wires to pulleys on a
purlin and use weights, but the load will change as the ladder hinges
down. Springs would change their rate and probably do the job, but I
can't see how to design the mechanism and trial'n'error would be a
pain.
I may give-in and buy an off-the-peg loft ladder, but it would be
useful to hear whether anyone else has tackled the same problem.

It's potentially quite a complicated design problem, a lot depends on
the clearance that you have above the hatch. A common way to cope with
the fact that load varies with geometry is to have friction somewhere
in
the system. It's the sort of challenge that I quite relish, but then I
have been analysing mechanisms a bit like this for the best part of 50
years (usually when they are not working properly because of friction).
Even so, my starting point now would be to look at adapting a
commercial
product.
Clearance isn't an issue; the hatch isn't at the centre of the span but
there's no shortage of space above it.
I've been looking at commercial mechanisms and spare parts, but mine
will be heavier (mostly because of the door and because I want to make
the steps as wide as possible). There seem to be lots of spring design
websites but I've no idea how to estimate the required spring force.
The answer might be to use off-the-peg bits and to keep adding springs
until the weight is roughly balanced, but the loads on the attachments
might get worrisome.
With all the other jobs in progress I'm not exactly wanting another, but
it's a problem that needs solving and it's useful to gather info.

If you're not short of headroom just use a 1 piece ladder with weight on a
pulley.

When using multisections the pulley mech doesn't need to balance it, just
needs to keep it within workable limits.

Havent seen too many roofs with that much room.

He did say its not at the peak of the roof.