5 : Leverage and bending moment
A good understanding of the impact of leverage is critical when assessing tree structure for rescue anchors .
When force is applied to a lever arm , the component of force which is perpendicular to the lever is multiplied by the length of the lever .
Simple common sense will indicate that one of the anchor points show on the left is probably acceptable for small loads ( green pulley ) and the other is positioned very badly ( red pulley ).
The difference is leverage – the multiplication of force by the use of a lever arm . The process would be familiar to anyone who has used a shovel , crowbar , high-lift jack or similar – or to any tree faller who has used a pulling rope to influence the direction of fall of a tree or tree part .
The images on the left show how force , lever arm and bending moment are related .
fulcrum
fulcrum
A force , in this case applied by hanging a weight from a beam , applies a bending moment to the fulcrum or axis at the end of the beam .
As the weight is moved out along the beam , the length of the lever arm increases , and the bending moment applied at the fulcrum increases proportionately .
The force applied at the fulcrum is called either torque or bending moment and is measured in Newton metres ( Nm ).
One Newton metre is the amount of bending moment that would be applied at a fulcrum by 1 Newton of force acting perpendicular to a lever arm of 1 metre .
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