How common is common sense

[murphy4trees]

How common is common sense...
An old climber buddy stopped by the house today and I set a rope in the oak out back to show him some of the new gear...
Anyhow he was looking through the new Sherrill catalogue and we got to talking about blocks and force multipliers...
He just couldn't quite get that the overhead anchor has to hold the weight of both ends of the line... kept saying 500 lbs is 500 lbs... This guy has a good head for MA etc.. but he just couldn't get it...

 

[Gord]

i think that's not really common sense- everyone that first hears that feels that it is counterintuitive.

 

[Bradley]

If you understand MA though then what's not to understand about O/H anchor loading; the two are one and the same.

 

[Tom Dunlap]

A friend of mine defines common sense as the practical application of everything you know.

Tom

 

[Kneejerk Bombas]

I agree with your freind, it's 500 pounds.

 

[NeTree]

500# of what?

 

[NeTree]

An Obituary


Today we mourn the passing of a beloved old friend by the name of
Common Sense who has been with us for many years.
No one knows for sure how old he was since his birth records were long
ago lost in bureaucratic red tape.
He will be remembered as having cultivated such value lessons as
knowing when to come in out of the rain, why the early bird gets the worm
and that life isn't always fair.
Common Sense lived by simple, sound financial policies (don't spend
more than you earn) and reliable parenting strategies (adults, not kids,
are in charge).
His health began to rapidly deteriorate when well intentioned but
overbearing regulations were set in place.
Reports of a six-year-old boy charged with sexual harassment for
kissing a classmate; teens suspended from school for using mouthwash after
lunch; and a teacher fired for reprimanding an unruly student, only
worsened his condition.
It declined even further when schools were required to get parental
consent to administer aspirin to a student; but, could not inform the
parents when a student became pregnant and wanted to have an abortion.
Finally, Common Sense lost the will to live as the Ten Commandments
became contraband; churches became businesses; and criminals received
better treatment than their victims.
Common Sense finally gave up the ghost after a woman failed to realize
that a steaming cup of coffee was hot, she spilled a bit in her lap, and
was awarded a huge settlement.
Common Sense was preceded in death by his parents, Truth and Trust,
his wife, Discretion; his daughter, Responsibility; and his son, Reason.
He is survived by two stepbrothers; My Rights and Ima Whiner.
Not many attended his funeral because so few realized he was gone. If
you still know him pass this on; if not, join the majority and do nothing.

 

[TheTreeSpyder]

Counterintuitive?

Originally posted by Bradley
If you understand MA though then what's not to understand about O/H anchor loading; the two are one and the same.

The Porty is the anchor, the support pulley is the load for the 2:1; and the 500# is the pulling power. Same V pattern; just different perspective.

Each man is his own savant.



Or something like that
:alien:

 

[Kneejerk Bombas]

I've been to several climbing/rigging demonstrations where this concept is brought up. There's alway one or two guys that that don't understand. I remember the first time I heard this idea, it was hard to understand at first.
To show it's true, use the tools on your belt. Have your freind hold a false crotch and lift a chainsaw. He'll see it's twice the weight.

 

[murphy4trees]

Have your freind hold a false crotch and lift a chainsaw. He'll see it's twice the weight.

Mike,
Brilliant and so simple... why waste time explaining whena simple demo will speak soooooooo much louder than words...

 

[Stumper]

OR make it simple math.Forget lifting. Think balance. Of course 500lbs is 500lbs. How much does it take to perfectly balance 500lbs? 500lbs! What is 500 lbs added to 500lbs? 1000 lbs! You win the prize!!!!! On the teeter-totter the big kid can sit closer to the fulcrum than the little kid and still balance. With rope and pulley we keep the fulcrum but throw the lever away-sooo....... in and out forces are pretty much equal (angle of departure for the line does change things a little but lets keep it simple.)

 

[ROLLACOSTA]

sorry to be such a bore but im not certain what the heck you guys are on about..could somebody elaborate a little


but anyway heres what i think your on about please correct me if im wrong ..do you mean if your lowering 100 lbs you need 100lbs on the other end to lower/hold the load in place ie the blocks holding up 200 lbs or could that be 100lbs ermm got the old grey matter thinking ...please somebody put me straight thanks ...im off to check my old college notes

 

[murphy4trees]

I used the 2:1 MA lowering system on pg 44 to show that the overhead anchor bears the weight of the sum of the lines it supports which is reduced by 25% in the 2:1 system...
He kinda got that but still hada hard time with the force multiplier idea..

Also on the second half of page 44 I noticed an old porty that appears to be improperly wrapped as discussed in an earlier thread and some physical impossibilities... anyone see what I Am talking about???

 

[TheTreeSpyder]

Originally posted by ROLLACOSTA
do you mean if your lowering 100 lbs you need 100lbs on the other end to lower/hold the load in place ie the blocks holding up 200 lbs

Yes, it takes 100# equal and opposite to hold 100#. The pulley has no hold power (like being tied off to support would have); so that 100# holding power to match the load has to come from the ground. Placing 2 - 100# loaded (legs of) lines on the pulley.

If the line was thrown over the branch, it would go from a non-frictional redirect tot he ground (pulley on support) to a frictional redirect of support to the ground. The 100# load would still have to be matched to suspend it. Whatever the friction of the limb didn't hold the line to the ground once again would have to; but it would be less pull on line to ground (control leg); therefore less pull on the support, because the friction is helping to hold load, along with your efforts.

So that increasing friction, reduces the control leg pressure needed to hold, in turn reduces the support load. When friction increase on support so much that the load is tied off to the support, (no pressure needed on control leg); then the support load equals the load itself.

If you set up a 2/1 to pull with it would be the same pattern. Here the log load cut is the power input into the system, and it exerts it's 2:1 pull on the support pulley position, because the control leg is tied down, or held with matching force. If the load is allowed to run, then it isn't totally supported, and the support pulley position's load gets reduced by 2x the slip. Also, installing a pulley in this position places more stretchable line area for shock load dampening down the whole length, rather than localizing to a small area of line.





We have all learned a lot from the cartoons/Sherrill tips, nice catch on gibbs; the fiddle block set is pictured with a prusik grab line i think.

 

[ROLLACOSTA]

thanks tree spyder ...shame we dont have sherrills overhere ...if we where [uk arborists] too order online all the gear would have to pay import tax..so hence i have no catalogue

 

[RescueMan]

Originally posted by RockyJSquirrel
But when was the last time you had an ascender that was rated to hold a 1000 lb load?

Rocky,

The CMI Ropewalker (in the new Sherrill catalog p. 37) can take up to 5/8" rope and has a cam strength of 7700 lbs (though I haven't seen any numbers on the other components). I've stripped the sheath off an old 1/2" kernmantle rope with one at 1800 lbs, so I'm sure a good 5/8" rope can take a lot more load.

The Gibbs 4s can take 3/4" rope and has a breaking strength of 7100 lbs., has a working load of 1000 lbs and may cause rope damage above 3000 pounds lbs. according to the manufacturer.

- Robert

 

[Lumberjack]

Originally posted by RockyJSquirrel
The only way the setup on page 44 would be possible is if the upper part of the fiddle block set were attached to the lowering line with an ascender. But when was the last time you had an ascender that was rated to hold a 1000 lb load? The fiddle block set is a good idea but I still cannot picture in my mind how to effectively (time and cost) harness it's potential on a jobsite.

I use the 3 gen porty and my MA (Using the double sheeved blocks on that page) like that quite often. However I use a 8-10 coil prusick. It comes in handy when you have a tree that is split into 2 codominate leaders and you want to lower one side in a single shot. With some will power you can push up or lower the prusick with a polesaw. However a gibs might be what I need.

It is great to have when you need it, it offers more rigging capabilities (that can save time if properly applied) than the standard porty.

Or you could just get a GRCS and be done with it.

 

[John Paul Sanborn]

When i run into people having troubles getting the concept I start throwing out words like counterbalance, load force/holding force.

then you go into the anchor force, which equals the sum of the two opposing forces.

It's not the listeners lack of comprehending, but the instructors inability to find the right language.

You redefine the argument to try to nudge the listeners mind to click.

 

[Tree Trimmer]

MA 2:1

Show him (or draw) a picture like the one attached.

Ask him what the conditions are for the system to balance. If you say m1 is 100, he'll naturally say m2 is 100. Then ask him how much of an effort is needed to hold the pulley up with this load (both m1 & m2). JPS and MM are right, sometimes a demonstration or picture can say much more than words. People know or can figure out a lot more than they seem to know. It's kind of like those times you get confused about which is right or left...

 

[murphy4trees]

There are several problems with the physics on pg 44...
It would take more than 200 lbs to lift 1000 at 5:1 with zero friction... you could balance 1000 but not lift...
Then there is the friction and the fact that the 200 lbs is not pulling straight down, loosing some pulling power with the angle..