Straps and Shackles

goonarmy said:

How does a weigh bridge measure the weight of a pull, friction coefficient etc?

Click to expand...

300bhp/ton said:

Do you have a reading problem?

Click to expand...

miktdish said:

Try it, next time you are on a weigh bridge tie yourself to something off the bridge and pull.
You don't need to know the coefficient of friction you just need to know approximately how much wieght (of pull) is needed to start to lift your landy, with the brakes on, up over a brick (or a slab, or a bit of wood, or a small dead animal, or anything).
If the direction of pull is above your centre of gravity (ie above the bonnet) it will be less, if you are on pea gravel, on an iced over lake, in a soap factory on a rainy day it will be less too.

Click to expand...

goonarmy said:

Why are
You assuming there is a lift? That's where the friction comes in.

Click to expand...

miktdish said:

I know i'm around 3 ton

Click to expand...

miktdish said:

There is a whole raft of stuff on the mechanical advantages of using blocks but in simple terms it's like this ...

Lets say the load is 6 tonne.
The landrover has a winch
Winch to load = 1 line
To move the load the winch produces 6000 kgs which is transfered to the load and the winch.
In this scenario the landrover would get dragged towards the load.

Next we take the winch rope to the load and come back, through a pulley, towards the landrover and attach the rope to the landrovers recovery point.
Winch to load = 2 lines. The same 6000 kgs of pull is applied.
We still don't move the load as we are still sharing the load between us but now we move together at half the speed. The load is still on only 1 rope as the line from the winch to the first pulley is simply changing the direction of pull.

Instead of attaching the 2nd line back to the recovey point on the same landrover with the winch, we attach a pulley to an anchor (another landrover for example) and pass the rope through that and go back to the load.
Now the load has 3 lines attached to it. 1 (from the winch) and two supporting the load to two anchors.
We produce 3000 kgs of 'pull' from the winch and the 6000 kg load moves.

If we added another pulley to the load and took that line back to another anchor (a tree this time) we would only need 2 ton on the winch to move the 6 ton load.
Add another pulley at the tree and go back to the load and now you need only 1.5 ton on the winch.

Another pulley - 1.2 ton
Another - 1 ton

Haltrac Hoists typically have 8 pullys wheels (or sheaves) and the load is therefore moved with a 8:1 ratio.
1 ton lifts 8 ton
125kgs lifts 1000 kgs, you can easily lift a landrover engine out of the car using a Haltrac Hoist but just pulling on the 'tail'.
Haltrac (and other engine) hoists are typically made up of 2 multi sheave pully frames which have 3 connection points.
#1 is you pulling on the chain/rope/tail
#2 is the frame
#3 is the engine (or load).
The total weight of the load is supported by the frame (or anchor point) and your 'pull' is amplified 8 times so lifting the engine out is easy.
Having more than 1 anchor point splits the load (equally) between them.

So: (Fag Packet = simple division) Load Wieght/number of pulleys = pull on landrover required to move the load.
6000/3 = 2000
6000/6 = 1 ton

Do I need to "beef up" my D90 recovery points = No
The various recovery points/bumper/chassis legs/dumbirons etc will take around 9000kgs before they deform. My winch and line is rated @ 5000kgs (with margin).
My D90 weighs approx 2500kgs, in gear with the brakes on and a brick/chock at each wheel it needs around 3000kgs to move it (might be a but less but hey ho, safety) so I know when I winch 'in' and the landy tries to climb over the bricks i've reacherd about 3 ton.
2 pulleys I move 6 ton
4 pulleys I move 12 ton
If I tie the back of the landy to something I can pull circa 20 ton - all within a safety margin.




They are not rated.
They are not designed for 'lifting'.
I have some 50mm sheaves on a 2T WWL shackle and a 90mm sheave on a 3.25T shackle.
The shackles are 5:1 rated.
The sheaves likely to be no more than 5 tonne - i've not broken one yet . If it does collapse i'm not overly concerned as the pin of the shackle will 'save' the day.

Click to expand...

marjon said:

What I was asking you seemed to have totally ignored, I still see that you went over the SWL of your equipment. As I said testing rating is for brand new equipment and is not something you should rely on which the way I see it you did.
Or am I missing something?
If I had calculated that in real life and done it, my career would have been very short
J

Click to expand...