When faced with a large tree removal that requires rigging, it is likely that more than one pulley will be required to facilitate the operation. The aim of this article is to help operatives to apply equipment with a proper margin of safety and help employers comply with LOLER 98 regulations (Lifting Operations and Lifting Equipment). Applying two pulleys to a tree rigging situation normally occurs in two basic ways:

1. The lowering line will run to the top of the tree, through the highest pulley and then down and across at various angles through a lower re-directing pulley.

2. The lowering line will run part of the way up the tree, through a lower re-directing pulley and then up and across at varying angles through a higher pulley. This may also/only be to facilitate proper entry of the lowering line into the lowering device.

Which example is used depends upon the situation, but what must be understood is the anchor forces depend s upon the angle between the rope legs entering and exiting the pulley.

Example 1 would normally be used when butt hitching, tip tying or cradling branches below the re-directing pulley.

Example 2 would normally be used when snatching the tips of branches. The re-directing pulley would be positioned level or below the branch base. This helps absorb the load along the length of the branch, rather than a large force acting on the tip alone. More than one pulley may be required depending upon the branching type.

If the re-directing pulley is moved higher than the tip pulley (example 2), the lowering line pulls up the rigging line in such a way as to prevent the back cut being placed.

The highest pulley will exert a higher anchor force than the re-directing pulley because of the shallower angles created where the rope enters and exits the pulley. This can range from increasing the anchor force (reaction force) from one and a half times to almost two times the load on the line.

In contrast, the lower re-directing pulley will never have a greater angle than 90 degrees between the points of entry and exit in the pulley, and so should never exert a higher anchor force than one and a half times the load on the line.

Because of the varying angles and relative anchor forces that can be applied to the pulley, sling and branches, it is best to account for the highest likely forces when planning a system. A useful way I have found to keep a safe system, is to ensure that the highest pulley is the stronger tail hold type pulley with a rope bushing for the sling. The sling will always be at least one and a half times the tensile strength of the rope running in the pulley. This assumes that a safety factor of 10:1 is used on the rigging rope, and therefore a 7:1 safety factor is applied to the sling in the highest anchor force configuration. Both of these safety factors relate to thousands of cyles before likely failure. It is very important that these safety factors are maintained by not exceding the working load limit (WLL) of the system.

The lower re-directing pulley should only require an anchor sling the same strength as the rope running in the pulley, working on the same principles as above. The re-directing pulley normally only needs to be a rescue type. It is important that the angle between the legs of the rope where they enter and exit the pulley, is less than 90 degrees. If it isn't, the pulleys should be swapped around!

In addition, ensure that the pulleys are at least the same breaking (tensile) strength as the lowering line (hardware safety factor of 5:1) to ensure a safe system. Further clarification of this point can be found in the earlier article 'Tree Rigging Concepts'.

Don't forget to use steel karabiners when rigging. I prefer a stainless steel rescue pulley for the re-direct - alloy tends to wear quickly when clipped to steel krabs.

Finally, always ensure as much as possible that you aren't directly below a pulley, and that climbing anchor points and rigging anchor points are separate.

Stay safe and enjoy yourself!