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australia:rigging

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rigging for camping, 4WD and recovery

see also:

  • I don't sell any of these nor do I receive any remuneration if you buy them, and I have not personally reviewed all of them, they are listed here to give you perspective
  • Newton's Second Law: Force = Mass x Acceleration
  • 1kN (kiloNewton force) = 102kg “weight” (10 Newton = 1kg mass x acceleration of gravity which is 9.8m/sec2)
  • for reference, most SUVs are around 2000kg herb weight, while a 4WD tends to be around 3000kg, then add all the extras which may add another 500kg or more and a static rope and all rigging to recover a vehicle should have a breaking strain of over 1.5 x its total weight
  • the force needed to get a vehicle moving from stationary on level ground depends upon the amount of static friction such as in the ball bearings and the tyres on the ground (this frictional force between the road and tire is what allows the tire to “push” off the road, hence flatter tires create greater friction and traction if the engine is driving the vehicle but will be harder to push from behind)
    • this can be estimated by placing the vehicle on a ramp and the angle of the ramp at which it starts to roll will allow the force to be calculated by F=mgsinθ where m = mass of vehicle and θ is the angle of the ramp
    • if the vehicle's wheel is in a hole and a ramp has been placed in front of it then an additional force will be needed depending upon the angle of that ramp and any additional frictional or sucking forces of any mud in the hole.
    • the force needed to push a car of mass m sideways depends upon the static friction coefficient (μ) of the tire on ground, and for rubber on concrete μ is very close to 1 when both materials are dry, thus the force to start pushing the car sideways (static frictional force = μxmg) is very close to the force needed to lift the car of the ground (F = mg)
  • the force to keep a car moving at constant speed is the force needed to overcome the amount of dynamic friction (which is usually less than the static friction) and any force in excess of this will result in acceleration = mass / additional force

Introduction

  • an understanding of how to use pulleys and levers to create mechanical advantage may help you get out of a range of situations
  • the amount of work done is NOT REDUCED by using pulleys, levers, gearing or hydraulics BUT they can REDUCE the FORCE needed
    • law of conservation of energy - work done = force x distance and this stays CONSTANT - you have to work just as hard but at least you may have the capacity to generate the force needed to achieve this
    • as is outlined below for pulleys, levers and gears, the mechanical advantage REDUCES the force needed in PROPORTION to the GREATER the distance you need to apply the force for hence WORK is unchanged, in fact, using pulleys or gears results in MORE WORK DONE as you also have to overcome FRICTION of the pulleys or gears (and if lifting, you will also need to lift a pulley which adds to the work done)
    • Work DONE in Joules = Force in N x distance in metres

Pulley systems

  • when a pulley is attached to an anchor, the tension in the in rope is equal to the tension in the out rope (less the efficiency factor of the pulley) while the tension on the anchor is the SUM of these
  • use this fact to COUNT your mechanical advantage in any given system - start at the haul line.
  • you then need to factor in loss of efficiency due to friction, a high quality new rope used with the following may have these values:
    • a high efficiency ball bearing pulley may be 90% efficient
    • an older bushing type pulley may be only 72% efficient
    • an oval carabiner may only be 45% efficient
    • a less rounded edge carabiner may only have 25-30% efficiency
    • efficiency may worsen as load increases on the device
    • MEASURE EFFICIENCY by checking weight required on the haul line to balance the load on a single 1:1 anchored pulley system
  • NB. changing the angle of haul pull on a simple directional pulley reduces the forces on the anchor depending upon the angle of deflection, whilst the haul force needed remains constant - see https://www.ropebook.com/information/angular-vector-forces/
  • adding extra carabiners to the system to theoretically increase MA from 3:1 to 5:1 may MAKE IT WORSE as the frictional losses may result in reducing it from an actual 1.6:1 to only 1.1:1 !!
  • USE HIGH EFFICIENCY PULLEYS where possible as your 5:1 theoretical may at least be 4.2:1
  • in general, you want your most efficient pulley closest to the haul end so you don't lose tension at the start
  • adding extra slings to haul the load which have an angle to each other results in increasing loads being placed upon each sling as that angle increases
    • if two lines pull up a 100kg load vertically, each line will have 50kg tension
    • if those two lines are angle 45deg apart, each line will have 54kg of tension
    • if those two lines are angle 90deg apart, each line will have 71kg of tension
    • if those two lines are angle 120deg apart, each line will have 100kg of tension!
    • the loads on each line grow rapidly as the angle increases beyond 120deg and hits infinity at 180deg at which point the two haul lines are only pulling against each other and not on the load at all.
    • Force in each line = (load x 0.5) / cos(alpha x 0.5) where alpha is the angle between the two haul lines1)
    • thus, don't spread the pulleys out too far or else the load on each component will be increased!
  • if you want to use compound or complex systems to gain higher mechanical advantage, you need:
    • anchor to be the strongest as it will have at least twice the tension of the haul line and perhaps 12x the tension if using an 11:1 system eg. 9-13mm Dyneema (although you could reduce this by using multiple anchor lines to spread the load, or use steel components instead of line to secure the pulleys to the anchor such as via a rigging plate)
    • load to be as strong as anchor eg. 9-13mm Dyneema (although you could reduce this by using multiple lines to spread the load)
    • one or two prusik loops to be adequate strength as they may have 2-4x the tension of the haul line, but needs to be thinner than the haul line eg. 6mm
      • climbers need a heat resistant material for fast rapelling hence avoid Dyneema with its lower melting point and prefer 5.5mm kernmantle plusiks (Aramid core) as more shock resistant than Aramid and smaller than 5.5mm gets difficult to work with
    • haul line to have adequate strength and length eg. 8-9mm Dyneema (remember the length the haul line needs to be moved is the MA x the distance the load needs to be moved so if you don't have enough haul line length, you will need to be resetting it whilst ensuring the load does not fall back)
    • pulleys to have adequate strength and with least friction possible and sized to match the haul line (eg. up to 13mm line capacity)
    • a ratchet, also called a progress capture device or PCD, is a device that, when attached to an anchor, will hold the rope so that:
      • the load will not lower back down when the pulling force is released. This acts as a safety so the load will not fall back down if the haul team lets go of the rope.
      • allows you to reset the mechanical advantage pulleys so they can haul the load a further distance.
    • a compound system will usually require many more resets - see https://blog.rocorescue.com/roco-rescue-blog/calculating-compound-m-a

Z-Drag 3:1 System

  • designed to free a boat stuck against logs in a current
  • can be used to lift a person up a cliff face
  • multiplies the pulling force on the hauling rope by a factor of three while the brake Prusik maintains the tension between hauls
    • ignoring friction, if the tension in the haul line = 1, the tension on the line going to and from the anchor will each be = 1 (tension on the anchor is thus = 2) while the tension on the load will be 1 from the anchor and 2 from the 1st carabiner on the haul line and the travelling prusik to be a total of 3
      • if one uses high efficiency pulleys of 90% instead of carabiners you should get 2.7:1
      • if one uses a carabiners with 45% efficiency and cam belay device on the anchor with 28% efficiency, you should get 1.6:1
      • the actual advantage will be slightly less than this as the prusiks also add friction
      • see https://www.youtube.com/watch?v=x1FSdt3_Nlc
  • you need:
    • 30m of heavy duty hauling rope
    • 2×1.5m loops of 5-10mm rope to act as Prusik loops
    • 1 anchor sling to go around a tree trunk anchor and attach to the anchor carabiner
    • 2 very strong carabiners to act as pulleys - preferably locking style (screw-down)
  • see https://www.netknots.com/rope_knots/z-drag-system

www.netknots.com_application_files_4315_5076_9563_z-drag.jpg

climbers rigging systems

  • these tend to have a breaking strain of around 36kN (3600kg) which may just be enough to recover a vehicle if you need to resort to manual methods
  • rigging plate
    • these allow addition of multiple pulley lines
  • pulleys
    • a collection of one or more wheels over which you loop a rope to make it easier to lift or haul things
    • one pulley allows you to change the direction of pull
    • adding extra pulleys creates a mechanical advantage so you don't have to pull as hard but you will have to pull FURTHER
    • a two pulley system gives a mechanical advantage of 2 so you only have to pull half as hard but you have to pull it twice as far to achieve the same distance (eg. 2m to move load by 1m)
  • how to count mechanical advantage and the tension on your anchors:
  • how to convert a simple 5:1 MA system to a compound 15:1 system:

block and tackle

  • a pulley based lifting system
  • the wheels and their mounts are the blocks and the rope that loop around them are the tackle
  • one block is usually in a fixed position whilst the other moves
  • they are either a pulley block which has the sides fixed and you must feed the line through, or a snatch block which has rotating sides so you can slide the line over
  • removing tree stumps with a small tractor
    • a 4:1 mechanical advantage 5500kg rated system with 3 heavy duty snatch blocks (1 attached to the base of a much larger tree and two attached to the tree stump, one end of the line is anchored on the base of the large tree and this goes to the 1t block on the stump then back to a block attached to the tree trunk, then back to a 2nd block on the stump and thence to the tractor which is ~30deg angle with the main tree trunk so all 4 lines coming from the stump are in a similar direction of pull) - see Youtube example

differential pulley

  • similar to a block and tackle but usually uses a loop of chain to drive three teethed pulleys of different sizes (one of the pulleys is smaller but shares the same axis of the top larger pulley)
  • user then pulls on the chain, the teeth hold the position and the load is lifted with very high degrees of mechanical advantage
  • mechanical advantage = 2 x radius of large pulley / (radius of large pulley - radius of small same axis pulley)
  • re-arranging gives mechanical advantage = 2 / (1 - radius of small same axis pulley/radius of large pulley)
  • when these pulleys are the same size, MA becomes infinite but moving the chain no longer lifts a load
  • when the smaller pulley is tiny, the MA approaches 2 and the system acts like a simple gun tackle
  • a simpler method of calculating mechanical advantage can be accomplished by simply counting and comparing the chain link pockets in the two differentially-sized sprockets (P1 and smaller P2):
    • MA = 2P1/(P1-P2)
    • eg. a 1-ton differential chain fall might have a 15-pocket and a 14-pocket sprocket set. This would provide a total of 2 X 15/(15-14), or 30:1 mechanical advantage.

levers

  • levers can create a similar mechanical advantage as with pulleys
  • by having the load closer to the axis of rotation than the action to pull down the lever, you need less force BUT you have to move it further - just as with a two pulley system
  • an example of use of levers for vehicle recovery is the flip-flop winch which uses two 2m logs and rope to pull a vehicle out of a bog

gears

  • gears are essentially levers in the shape of wheels
  • the ratio of the number of teeth of one gear wheel to the number of teeth of another gear wheel gives the mechanical advantage
  • on a bike, you can select a gear to allow you to use less force so it FEELS easier but you have to pedal MORE to ride the same distance
  • in a car, when stationary, it needs a large amount of force to get it moving so we start in a LOW gear with lots of mechanical advantage to reduce the force needed
  • gears can thus be designed to reduce the force needed, or in a reverse fashion, to increase the speed

hydraulics

  • hydraulics use water as it is NOT compressible
  • Pascal's principle:
    • because the liquid in a pipe is incompressible, the pressure must stay constant all the way through it, even when you're pushing it hard at one end or the other.
    • pressure = force x area
    • HENCE, pressing on a piston in a large diameter pipe filled with water will move a piston at the other end and if that other end is a smaller diameter pipe, then that piston will move a greater distance (ie. with more speed) but at less force (conservation of energy)
    • a hydraulic ram works in the reverse manner to create a greater force by pushing liquid from the smaller piston into a larger one, the liquid is pushed back into the other end via a hose pipe to create a closed system

abseiling

  • basic equipment needed for controlled descent:
    • climbing harness
    • gloves for longer descents
    • helmet
    • kernmantle rope - usually 8mm and needs to be LONGER than the descent and with a control knot at the end so you don't slip off if you miscalculated
    • something to create friction on the rope to control your descent such as:
      • descender or a GriGri
        • allows a very smooth lower and a fail-safe lock
      • or, belay devices
        • are NOT as good as there is no fail-safe lock so if you let go of the rope, you fall
        • the same piece of rock climbing gear you use to belay your partner in rock climbing
    • carabiners
    • a suitable “unquestionably sound” anchor point at the top

crevasse rescue

  • see also:
  • 4 person team rescue
    • on ice or snow:
      • team members anchor the rope initially with their heels, ice picks and by lying down until the front team member can create an anchor point then moves towards the lip and places an ice pick under the rope to prevent the rope further cutting into the lip
      • then the use a prusik loop using 5-6mm accessory cord while the pull the rope up by walking backwards
  • self-rescue
    • requires two Prusik loops of 180–250cm, one HMS carabiner and two identical snap carabiners
    • use of two long prusiks loops (longer one for foot and one for body harness) used continually sequentially to work your way up as you push up the Prusik knots on the rope once your weight is moved to the other Prusik loop
    • may need a self-pulley system using a Garda hitch with two snap carabiners in parallel and the HMS carabiner as the upper pulley to get over the crevasse lip
      • a Micro Traxion (a rope clamp with integrated pulley) is a better alternative than a Garda hitch if available
1)
https://www.ropebook.com/information/vector-forces/
australia/rigging.txt · Last modified: 2023/01/14 21:21 by gary1
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