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Is a constant-force belay device possible ? (engineering question)


Original Post
Serge Smirnov · · Unknown Hometown · Joined Oct 2015 · Points: 242

In an unrelated thread, it was pointed out that you would fall less far (for a given peak force) if the force were relatively constant over the course of the fall.

I.e., if we plot force (y axis) vs displacement (x axis), then energy absorbed (area under the curve), for a given peak force, is maximlized when the curve is flat.

Dynamic ropes (or anything resembling a spring) fall well short of this ideal behavior - they waste a lot of fall distance letting the climber fall with little resistance.

Screamers have the problem that a screamer long enough to absorb the energy of a large fall wouldn't fit in the system length-wise.

An ideal fall catching system, then, would be a static rope running through a near-constant-force belay device.  Is such a device possible ?

Bill Shubert · · Lexington, MA · Joined Jul 2012 · Points: 55

The trublue auto belays are sort of like this. My understanding is that they have a magnetic clutch that provides more resistance as you fall faster, effectively limiting the maximum speed you can drop. I've used them, it works well. But they are also very large and heavy, nothing you could use as a belay device at a crag.

Jim Titt · · Germany · Joined Nov 2009 · Points: 490
Serge Smirnov wrote:

In an unrelated thread, it was pointed out that you would fall less far (for a given peak force) if the force were relatively constant over the course of the fall.

I.e., if we plot force (y axis) vs displacement (x axis), then energy absorbed (area under the curve), for a given peak force, is maximlized when the curve is flat.

Dynamic ropes (or anything resembling a spring) fall well short of this ideal behavior - they waste a lot of fall distance letting the climber fall with little resistance.

Screamers have the problem that a screamer long enough to absorb the energy of a large fall wouldn't fit in the system length-wise.

An ideal fall catching system, then, would be a static rope running through a near-constant-force belay device.  Is such a device possible ?

Sure, in fact I know of a couple which are used to give constant hand force when testing belay devices. A while back there was a Japanese guy doing research into this for a climbing application but there is one huge drawback in real life (well actually two but they are related). The first is if something goes wrong with the device you are falling onto a static rope, the second is if there is a lot of friction between the belay device and the faller they are again falling on a static rope.

ColinW · · San Diego, CA · Joined Sep 2014 · Points: 70

I'm no physicist, but I'm guessing a damn good belayer who knows how to let rope out gently during a fall to lessen the falling forces on the climber & equipment would be considered a constant and reducing force belay device.    With the climbing gym craze going on, it sure seems like a lot of people are so focused on the training for optimum climbing performance that they fail to focus on the #1 skill that's most important for safety. Belaying. 


amarius · · Nowhere, OK · Joined Feb 2012 · Points: 20

Of course it is possible. As someone mentioned above - eddy current based braking device is already available and being used. Would be a fun engineering project to see how small it can be made using rare earth (neodymium) magnets and planetary gear sets. I don't think it would be any larger than Revo.

Franck Vee · · Unknown Hometown · Joined Apr 2017 · Points: 30
ColinW wrote:

I'm no physicist, but I'm guessing a damn good belayer who knows how to let rope out gently during a fall to lessen the falling forces on the climber & equipment would be considered a constant and reducing force belay device.    With the climbing gym craze going on, it sure seems like a lot of people are so focused on the training for optimum climbing performance that they fail to focus on the #1 skill that's most important for safety. Belaying. 


My physics is a bit rusty, but thinking out loud...

The graphs of the force applied as a function of the distance fallen would more or less look like a inverted U. You can't, as a belayer, do much about the first part (the ramp up in force) - some sort of maximal force will be applied and it has to go from zero to there somehow. Therefore flattening the graphs means lessening the peak force - which is what you are doing with 1 - dynamic ropes and 2- dynamic belaying. So in a sense you are correct in saying a good belalyer does that. What a well-time jump does is in fact give more time to friction to do its work and dissipate energy and it also allows the belayer being lifted to take on some of that energy (e.g. you need energy to lift someone from the ground, and any energy needed comes from that of the fall and a bit from the impulsion the belayer gives himself if any).

However, there's a limit to how much you can do that - this comes at the cost of a longer fall for the leader. For the first couple draws ou can't maximize the dynamism you put in the belay. If there's a ledge/weird topography you may not be able either. Then we'd have to calculate, but I wonder how much of a dynamic belay you would need to completely minimize the force and flatten as much as possible that curve - the result may end up being an incomfortably long fall (even if safe in the context) for the leader...

There's a related video here - not quite on what we're discussing but relevant: https://www.youtube.com/watch?v=_0GGsBgPic4&t=628s

amarius · · Nowhere, OK · Joined Feb 2012 · Points: 20
Serge Smirnov · · Unknown Hometown · Joined Oct 2015 · Points: 242
amarius wrote:

Of course it is possible. As someone mentioned above - eddy current based braking device is already available and being used.

It's actually not obvious that those devices would give constant force over the range of speeds encountered in lead falls (5-15 m/s).

(I am, however, satisfied with Jim's explanation that in practice a constant-force belay device could create dangerously high forces on the top piece / climber due to rope drag)

Guideline #1: Don't be a jerk.

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