On Tue, 18 Sep 2007 03:01:34 -0700, jkn wrote:

Hi Cic,

That one looks quite good, although the return springs (bungee cords)
might not give a smooth return with the possibility of the top plate
tilting and jamming. Nonetheless, it's a clever and simple way of doing
the job - possibly a bit dangerous as those cords will need to be under
considerable tension to retract the column of a bottle jack.

The smoothness of the return doesn't bother me too much. When you say ...
'retract the column'... I guess you're simply referring to the bungees
having to lift the combined weight of the Jack + platen? Yes, this is
something I wondered about.

Is it possible to use a bottle jack upside down? if so, you could fix it
to the top of the frame, pointing downwards, and the bungees would only
have to lift the platen, and the bit of the bottle jack that has just been
'released' ... whatever that's called.

J

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I don't think it's just a case of lifting the jack and platen. The bungee
cords will have to force the jack's column back down into the housing.
This requires considerable effort, which is usually applied by the weight
of a vehicle or other load. I think you would have to experiment to see if
the bungees were strong enough to do this or decide to do it by hand. That
shouldn't be a real problem unless you go in for mass production with a
time and motion programme in force.

I don't believe the average jack will work upside down. Whilst making a
small press recently to remove car stub axles I came across the
information that most of the common car type hydraulic jacks will only
work on their side if the handle side of the jack is facing downwards.
This restriction would seem to suggest that they won't work upside down
but it's worth testing individual jacks - under load - to see if they will
do so.

Cic.
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