OLDS-Overhead Lowering...

When I came upon this thread I was very interested in the idea that the lobed bollard could provide extra holding force compared to a round one because the rope is forced to bend back and forth several times as it traverses the bollard. In the case of the round bollard there is only one back and forth in the rope path. Though BMS doesn't tell us, it would be very interesting to know how large this effect is.

Both Tom D. and BMS have made claims for the physics of the device that bear closer examination. This gets at the heart of it:
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(Sean, post # 247003)...If the flats are 1 or 100" long, the rope still turns the corner in the same radius, just that each corner is extended away from the next. The rope runs straight between each corner. It would seem that if you took the straight sections our all together, you would have the same rope bend radius as if they were 100" apart. It would change your friction generating surface area, but not the bends.

Am I thinking about this wrongly?

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This is a very insightful observation, and I hate to complain about it in any way, but it contains one assumption that I would dispute. This is the idea that long flats give you more friction-generating surface. Because they are flat they do not generate any friction at all! This is an essential ingredient in the famous capstan equation that describes the behavior of a tensioned rope wrapped around a bollard. So if the flats do nothing but interrupt the curves, then the behavior of the BMS spool is, except for the back-and-forth effect, no different than a round bollard made of the same steel.
 
From the BMS Web site we have this:

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The BMS Belay-Spool works essentially on this principal with the additional advantage of the internal friction of the rope fibres created by the bending and straightening of the rope as it moves over it’s “lobed” surface under load.

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Very nice. But then we have this:

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This same principal makes the brake bar rack an effective device given its relatively small contact surface with the rope...Rope moves relatively easily through the Belay-Spool when not under load by virtue of the rope’s tendency to form a loose coil around the drum. This coil generally only makes contact with the top of the lobes so friction is low...

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I can fix the first sentence by substituting "angle" for "surface", but the second sentence, to be true, requires a rewrite of the physics books. Even though the BMS Belay-Spool is lobed and not round, it still obeys the capstan equation.

From The Mechanics of Friction in Rope Rescue: http://www.jrre.org/att_frict.pdf

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Figure 4 summarizes the capstan equation for friction over a drum. Using this simple friction law leads us
to conclude that the frictional force for a rope depends only on three things:
• the tension in the rope
• the coefficient of friction
• the total angle of contact

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Only three things. No mention of surface area, no mention of size, and no mention of shape.

This is very non-intuitive, I know, but numerous experiments bear it out. Note that in the case of the BMS spool, there is an "angle of contact" only where the rope is in contact with the lobes. This is where all the friction is generated. The fact that the rope doesn't even contact the flats when yarding in the unloaded rope does not mean less than normal friction. It does mean the rope is fairly stiff and not doing as much back and forth bending as it travels from lobe to lobe, but it would still be more than in the case of a perfectly round bollard, where there is no such motion except at the beginning and end. It has zero friction advantage over the round bollard in this scenario.

I did the following simple experiment about a year ago:

I took a 3-foot length of debarked maple about 5 inches in diameter and carefully sanded the surface. I then rigged up a rigid horizontal mount for it so I could test the capstan equation. With my climbing rope draped over the bollard, I could measure the forces involved in raising and lowering various known weights. The results were very consistent--I could control a descending weight (force) using about half as much force.

What would happen to this 2:1 ratio if I greatly reduced the area of contact between my rope and the maple? This was easy to test. Just make several flats on the maple (just like the BMS device) with a plane, round over any sharp edges, and repeat the testing. The reworked maple had only 40% as much contact area as before. The friction ratio was still 2:1.
 
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...Rope moves relatively easily through the Belay-Spool when not under load by virtue of the rope’s tendency to form a loose coil around the drum. This coil generally only makes contact with the top of the lobes so friction is low...

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There is no better teacher than a little public humiliation. I was wrong about this. Had I read this more carefully, I would not have assumed "top of the lobes" meant the whole top of the lobes. There is a cool thing going on here because of the stiffness of the rope.

The actual contact angle for a round bollard, regardless of rope stiffness, will always be the full angle of the wraps. There is no way the rope is going to stand out from the bollard. With the BMS device, rope stiffness allows the rope to do what they claim--stand out away from much of the lobe so the actual contact angle is much reduced. This does not violate the capstan equation. The equation applies wherever the curved rope contacts the metal. It is the sum of all the contact angles that matter. For a stiff rope and a low load this could be very small, giving the almost pulley-like ease described by Tom.

Strictly the capstan equation isn't supposed to apply to a stiff rope. This very interesting situation is a great example of how it can break down, tripping up the recklessly unwary like myself.
 
As I work in a wet environment, my ropes get dirtier than when dry. Some muck, or wet soild/ mud build-up, occurred in places where I figured it would be wiped off by the rope. It seems that even when under load, the rope stands off the flats to some extent.
 
ok so does someone want to invent a port-o-wrap that is designed to be hung in the tree
grin.gif
 
I'm sure the spool works fine but there's nothing wrong with the small portawrap or even having a full size in the tree for bigger rigs. The only advantage I see is the spool's weight and to me that's not a big deal. I bet the portawrap is easier to work with and lock off which is what's needed for top friction devicing.
 
The Spool captures the rope so there is no chance of having a turn of rope jumping off.

I've used a bollard in a tree...it is a pain in the doopa to use compared to the Spool.
 
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I'm sure the spool works fine but there's nothing wrong with the small portawrap or even having a full size in the tree for bigger rigs. The only advantage I see is the spool's weight and to me that's not a big deal. I bet the portawrap is easier to work with and lock off which is what's needed for top friction devicing.

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The spool does work fine AND there IS nothing wrong with a small POW, topside. In fact, the spool is slightly heavier than a small porty. They both have their advantages and disadvantages. Wouldn't use the spool for "bigger rigs". But ...

Some of the 'significant' advantages of the spool for lots of 'smaller rigs' are:

It can be 'run' remotely from <u>anywhere</u> below the rigging TIP. That is, set the spool high, once, and rig-down all the lower limbs without ever having to climb back up as you would to re-setup a porty for the next rig-down.

With the spool the standing and the running parts can be reversed. Every time a piece is lowered, a new working end is being raised up to the work. So, there's no time wasted waiting for un-tieing the load or yarding rope back up to a porty.

For 'lock-off' ... often the spool does not need a lock-off. But, if needed, it too, can be locked off remotely without climbing back to the rigging point. Lock-off can be efficiently accomplished with a sling to a nearby limb/stem and friction hitch on the standing part. Thus, virtually any load (up to the limit) can be easily controlled without adding or removing wraps. This actually works exceptionally well. You can even pretension the line this way.

Again, they both have some advantages but significant time &amp; effort can be saved when lowering a lot of limbs with a spool.
 
Love the OLDS.
It is used in this video where I had one employee to pull the limbs away from the garage and utilities.
Usually you stand helpless in tree waiting for the limbs to
be layed down, now you run the ropes and become more productive and frees the groundie up to focus on bucking it up or running other ropes and stuff.

http://www.youtube.com/user/Ropearmour#p/u/8/P59L8lOxklY

Keep it with in reach and I have also used for spar rigging successfully(nice).
One of those tools I wish I knew about along time ago.
Saving and money maker!
Tough tool and kinda replaces knots, pulley and lowering device all in one.
Works well with 11mm haven't tried it with anything larger.
Keep it in site with routine climbin and riggin tools cause it comes out to play quite often.
1ST STRING TOOL WITH BIG TIME SCORING POTENTIAL.
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