In the second part of this series, Dave Vickers looks at the forces involved and required safety margins when combining wire and rope based offset systems.
Last month we focused on assisted felling and the forces involved when using an offset pull. We introduced the formula “pull = 2t(cos x)” where t is the force in tonnes exerted by the winch and x is half of the angle the wire/rope passes through the pulley. I concluded by suggesting a ready reckoner chart could be useful when selecting equipment. We need to consider two things when dealing with offset systems combining rope and wire; firstly the potential offset loads, and secondly the strength of the rope.
Diagram 1
Calculating offset loads
Table 1 provides the information required to assess peak loading when using an offset pull. Estimate the offset pulling angle (see diagram 1) then read off the multiplier in the right hand column of the table.
Using the formula, if you are using a winch with a maximum load capability of 500kg, with a 90° offset, the maximum loading will be 500 x 2 x (cos 45) = 707kg. Note that an offset pulling angle of 0 means you have doubled the winch cable back on itself.
Joining wire and rope
We need to be particularly careful of the potential forces involved in assisted felling when combining a winch (wire-based) system with a rope. I commonly use a Tirfor style 1.6t winch when carrying out assisted felling operations on training courses, but what effect does this have on my choice of rope?
The weakest point in the system must be the winch, in this case the shear pin fitted to the winch, designed to fail at 1.6t. To my knowledge these are not CE marked and tested, so I’ll have to take the manufacturers’ word that the pin will fail at 1.6t. The rope must therefore have a Working Load Limit (WLL) of at least 1.6t.
According to HSE, 2001, Industrial rope access – Investigation into items of personal protective equipment, Contract Research Report 364/2001, using a double figure-of-eight knot retains between 66% and 77% of the rope’s strength; a bowline retains between 55% and 74%.
Taking into account the knots used to attach the rope to the tree and the winch cable, it’s not unreasonable to assume we will retain around 65% of the rope’s strength. So, for 1.6t to make 65% of our WLL, we’re going to need a rope with a WLL of around 1.54 x 1.6t, making 2.5t (1.6 is 65% of 2.5).
Diagram 2
The European Commission’s Guide to application of the Machinery Directive 2006/42/EC 2nd ed. 2010, states on p.298: “the working coefficient for textile ropes or slings… is as a general rule, equal to 7.”
So the Minimum Breaking Strength (MBS) will need to be at least 7 x our WLL of 2.5t, making 17.5t.
The Teufelberger Sirius Bull 20mm rigging line has an MBS of 8,980kg, well below the17,500kg (7.5t) that we need (8,980kg equates to a WLL of 1,283kg).
Hopefully you can begin to see the issues with combining rope systems and wire-based winch systems!
Let’s summarise the problem we face in this example. We have a winch shear pin that should fail at 1.6t (this must be the weakest point in the system). Our 20mm rigging line has a working load limit of just 1.3t. To keep within our safety margins, we have two choices:
1) Remove the rope from the system and reset the winch position.
2) Use a lower rated winch.
In this case, I would change the winch from a 1.6t Tirfor to a Model 4000-20SH Lug-All rated at 0.7t. This means that the weakest point in the system is once again the winch.
How has this been achieved? The weakest point is the winch failsafe at 0.7t. Using a knot to tie the rope to the winch cable means we only retain 65% of the rope’s strength, so to find out the required rope WLL we must multiply 0.7t by
100/65 or 1.54.
The required rope WLL is therefore 0.7t x 1.54 = 1.08t (0.7t is 65% of 1.08t).
It’s time to go back to the rope manufacturer’s datasheet to find the most suitable rope.
The Sirius Bull 18mm line has an MBS of 7,700daN (roughly equivalent to 7.7t), giving us a WLL of 1.1t. This is cutting the safety margin a little fine. Alternatively, we could use the Sirius Bull 20mm line to stay well within the safety limits.
Drivelink Training provides City & Guilds NPTC forestry and arboriculture short courses run by David Vickers, a City & Guilds NPTC approved trainer and assessor, qualified teacher with a BA (Hons) Ed. and QTLS. Drivelink Training provides training from basic maintenance to dealing with windblown trees, from basic tree climbing to rigging / dismantling. Visit www.drivelinktraining.co.uk to find out more.
