Table of Contents

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

  • 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

www.netknots.com_application_files_4315_5076_9563_z-drag.jpg

climbers rigging systems

block and tackle

differential pulley

levers

gears

hydraulics

abseiling

crevasse rescue

1)
https://www.ropebook.com/information/vector-forces/