Thread: Winching Formulas

Originally Posted by Blknight.aus

same applies on all equipment depending on how its rigged...

lets look at a drag line VS as lift line... lets pretend that we know that we need exactly 1000kg of pull to make it move along the ground and due to the magic of the hypothetical situation it needs exactly 1000kg to lift it up...

most dragline applications only require a 2x safety factor so while in theory I would get away with a 1000kg winch and line to pull it the OH+S side of it requires 2000kg of pull so everything must be able to deal with 2t (assuming a continuous rate single line pull ala capstan winch)

Now for lift your safety factor becomes 5 so to lift it you now need 5000kg of pull.

in the real world this is done by taking the nominal breaking strain of an item and deviding it by its nominated safety factor which for chains, wire ropes and shackles is 5 because you can guarentee that some idiot is goint to try an use one to lift what he should be towing..

due to the decreasing cost of manufacturing gear and the increasing costs of ticketing inspecting and stamping to 2 differing standards 90% of gear comes marked with a single SWL or WLL which is what it can lift, safely.

If you ever get chance to see a set of 10% test to destruction tests being done I highly reccomend you take it... hearing and seeing a bit of 2 inch diameter SWR playing itself like a guitar string is an amazing thing.

Dave, I used to work at Cockatoo Island Dockyard in Sydney and I used to build my own slings for specailised lifting projects, like lifting Gun Turretts of Destroyers, or power generator turbines, biggest cable was a 4" dia 60' long slings for the floating crane Titan for lifting large Ball and Rod mills, was good fun testing the swages on the test bed, never had one go pop though, thank christ, Regards Frank.

Originally Posted by Hymie

Has anybody read Australian Four Wheel Drive Monthly, particularly Pat Callinans article on how to winch and calculate how much force is needed to extricate a bogged Forby?
What a load of rubbish!!
Read Pats "formulas" for calculating required pulls then sit back and consider a few thing I'm going to teach y'all.
Pat is right on a few things, there are 4 types of resistance to take into consideration.
1. Rolling Resistance, this being the resistance to motion given the surface the vehicle is on, ie, Bitumen, Grass, Sand or Mud
2. Gradient Resistance, the effort required to overcome gravitys pull on an incline
3. Damage Factor, did you damage anything that might increase resistance tomotion, rip a tyre off a rim, bend a steering arm etc.
4. Add a safety margin.

To work out rolling resistance, take the weight of your vehicle and divide it by a. 25 for bitumen or concrete
b. 10 for gravel
c. 7 for grass
d. 5 for sand
e 2 for mud

To work out Gradient resistance,
a. calculate the slope
b. Factor 1/60th of a tonne for every degree of slope

To work out a Damage factor,
throw in 25% of the vehicles mass for every damaged wheel.

Safety Factor is 25% of the total of all of the above.

This will give you a Total Pull Required. (TPR)

Sooooooo.
Using the example in the magazine of a 3.3 tonne GU Towning a 1 Tonne Camper trailer Bogged to the chassis.

RR= 4.3/2= 2.15 Tonne
GR=0
DF=0
Total =2.15 Tonne
Safety Factor 2.15/4=.53 Tonne

TPR=2.68 Tonnes

A whole lot less than the 12.9 Tonne quoted!

Lets do another,

Blknight.aus has a fully loaded Big Red of around 3 Tonne stuck on a muddy slope of around 40 degrees. ( sorry Dave, hope you don't mind)

RR= 3/2 = 1.5
GR= 3000kg/60*30=1.5 (1500Kg)
DF=0
Total = 3.00
Safety Factor = 3.00/4=.75
TPR=3.75 Tonnes

Now you know why Tirfors are so much fun

Having tested recovery loads using a load cell those numbers are about what I measured, except that I made rolling resistance 2.5% on flat bitumen at road pressures -- it reads like 25 above. My figure for de-bogging a GU with camper trailer and not driving (we actually did it, not just theory) was equal to the weight. However we did not dig it out at all, if you do that loads are dramatically reduced. There will never be an exact figure but 12 tonnes to remove that GU/camper....it'd need to be sucked down into the most glutinous mud.

So if I understand this, Robert measured the recovery force of a GU and camper trailer bogged to the chassis as approximately equal to the weight.

So about 4.3 tonnes

Hymie calculated the recovery force as 2.15 tonne

So the required recovery force was double the theoretical value from using Hymie's method.

by the time you've calculated the load required to remove the bogged vehicle, you could have grabbed a shovel, removed some of the material bogging the vehicle and placed it in feed bags, free with horse food so not free (it costs your soul, an arm and a leg) place the bags in front of the bogged or floating with no traction wheels and drive out.