[python] Re: Pythonjetrike
- From: Patrick van Gompel <patrick_van_gompel@xxxxxxxxxxx>
- To: <python@xxxxxxxxxxxxx>
- Date: Tue, 20 Sep 2011 16:36:49 +0200
Thank you very much for your concerns Jaculus.
I just did a quick test: the rope around the unthreaded M6 bolt and a spring
balance attached to each end. And yes, I was indeed completely wrong. With a
gentle pull on only one spring balance, I could pull 10kg, while the other was
at 5kg. Pulling harder would make the rope move. Seemed to be the same for a
lower force; 6 to 3 kg. There was hardly any difference between the 3mm or the
7mm rope, having the 3mm only slightly less friction (10 to 6kg). So that is
quite some difference for only a gentle pull. I couldn't really check, but a
bigger load or a quicker pull might even give the full load to one end. So you
are right, a double 150kg line around a bolt doesn't add up to 300kg at all.
I will reconsider things again, but so far, wear didn't seem to be the problem.
Unless I can't see wear on the outside of a rope, I feel safe enough by
checking it from time to time. Of course, I need a better safety line
attachement.
DirkS, I had both: a line around a bolt (back and forth) and a seperate (but
slack) safety line.
Pfffttt, you need to know so much about so many things, and I know only a
little about so few things...
Thanks guys,
Patrick
> Subject: [python] Re: (No From: Jaculus <jaculusbent@xxxxxxxxx>
> To: python@xxxxxxxxxxxxx
> Date: Tue, 20 Sep 2011 08:10:13 -0400
> From: python-bounce@xxxxxxxxxxxxx
>
> Dirk understood me correctly. But don't underestimate friction.
>
> Friction of a rope around a bolt, rod, or whatever depends on three
> factors (which are all absent in a perfect pulley -obiously those
> don't exist-):
>
> -Sharpnes of the bend
> Bending a rope takes force, mostly not that much when there is no
> load, but in your set up there is load on the rope. I don't have
> proper calculations but judging from experience it is safe to say that
> if the (smooth surfaced) rod you pull a (loaded) rope over is not at
> least double the diameter of the rope friction is well over 25% if not
> over 50%. A 3 mm line over a 6mm bolt might still work a bit, but a 7
> mm line over that same bolt should be assumed to be fixed, because
> friction is so much, the last bits of movement are safer to just
> forget.
>
> -Surface of both the rope and the bolt
> A bolt has lots of sharp edges which could trap strands of the rope
> and increase friction drastically.
>
> -Load
> The more load there is the higher the percentage of friction will be,
> because load reduces the freedom of movement within the rope.
>
> Example which I come across often:
> Hoisting 30 KG with a 10 mm rope over a malfunctioning pulley of 30 mm
> diameter or more takes about 90 kg (to be more correct 900 N) on the
> other end of the rope.
>
> If you twist a 10 mm climbing rope 3 times around a smooth 10 mm rod,
> you can hold the weight of a human single handed for a really long
> time. DON'T TRY, FOR CLIMBING THERE ARE WAY MORE TRUSTWORTHY WAYS TO
> SECURE SOMEONE !!!
>
> But there is another reason why I would not have the rope just lose
> around a bolt. This causes a lot of wear (and I am used to trust the
> rope with my life, so I am really afraid of wear). If an occasional
> failure is acceptable you could choose not to worry about wear.
>
> If you really want to depend on strange rope set ups please be advised
> by someone who actually knows how to calculate on these things.
> Because the set ups You want to use are way different from anything
> considered safe in climbing. (Which does only mean that I can't really
> predict everything.)
>
> Jaculus
>
>
> 2011/9/20 dirk@xxxxxxxxxx <dirk@xxxxxxxxxx>:
> > Hi,
> >
> >
> > Patrick van Gompel <patrick_van_gompel@xxxxxxxxxxx> hat am 19. September
> > 2011 um
> > 21:56 geschrieben:
> >
> >>
> >> Thanks guys for the tips and help!
> >> Jaculus, can you please clearify so I am sure I understand it correctly.
> >> Are
> >> you saying that when I put a line around a bolt whitout a knot or anything
> >> and
> >> I hold both ends, that I can't multiply the line strenght by two? Of course
> >> the bolt does give some friction, but aren't the forces on both ends
> >> roughly
> >> the same? Isn't that how pulleys work? If you pull one end, the other end
> >> will
> >> move and have the same force to it, or am I completely wrong here?
> >> Is a M6 bolt to sharp a bend for a 3 or 7mm thick rope? Please advice.
> >>
> >
> >
> > i understood as well, that two lines at individually fixed positions are
> > never
> > evenly loaded. However, a pivot, bolt or pulley design distributes the load
> > evenly on both lines, which doubles the overall force, while the other
> > approach
> > doubles the safety (one line loaded, another as an unloaded spare)
> >
> >
> > greetings,
> > DirkS
> > ============================================================
> >
> > This is the Python Mailinglist
> >
> > //www.freelists.org/list/python
> >
> > Listmaster: Jurgen Mages jmages@xxxxxx
> >
> > To unsubscribe send an empty mail to
> > python-request@xxxxxxxxxxxxx
> > with 'unsubscribe' in the subject field.
> >
> > ============================================================
> >
> >
> ============================================================
>
> This is the Python Mailinglist
>
> //www.freelists.org/list/python
>
> Listmaster: Jurgen Mages jmages@xxxxxx
>
> To unsubscribe send an empty mail to
> python-request@xxxxxxxxxxxxx
> with 'unsubscribe' in the subject field.
>
> ============================================================
>
Other related posts:
