Over the years I have read many comments on 'tension' in logs. Unfortunately the comments are often incomplete or (sometimes) inaccurate. For those who are new to milling or still confused by 'tension', the following may help.
All trees develop tension, though depending upon verticality of trunk, site, species, age and size the amount of tension will vary.
So what is tension? Tension is a growth response to stresses on the trunk of a tree. Trees respond to these stresses by laying down cells which are 'tensioned' in one of two ways - they are either tensioned like a stretched rubber band (ie want to contract (positive tension)) or tensioned like a spring (ie want to expand (negative tension)).
Where are these different types of tensioned cells in the trunk of a tree? In a 'normal' tree with a vertical trunk, the outside (sapwood/ outer heartwood) cells are tensioned like a stretched rubber band whilst those in the centre are tensioned like a compressed spring. In each case the greatest amount of tension is at the outside and centre respectively and there is a transition from maximum positive tension to zero tension to maximum negative tension as you move from the outside to the centre. This change in the amount of positive to amount of negative tension occurs over small or large distances from the outside to the centre of the log depending upon log size. The rate at which this change occurs (change in absolute tension over distance from pith (centre) gives rise to a stress gradient. Let us consider two logs, one large (wide) and one small (thin) which both have the same absolute value of positive and negative tension. It is clear that the change in tension from positive to negative occurs over a greater distance in the large log and therefore the stress gradient is shallow (little change in amount of tension per unit of distance moved from outside to centre). Conversely, The same magnitude of change i tension occurs over much shorter distances in the small log and the stress gradient is steeper.
What does all this mean to a sawmiller? It is the stress gradient which determines how much movement there is 'off the saw'. High stress gradient equals high movement and low equals low movement. NOTE that it is not the amount of tension that matters but rather the rate at which it changes (or stress gradient). cont'd