Tillering trees?

[AH]

So today, I was working on my yew war bow, had it at about 80 lbs at 20" on the tiller, weight increasing about 4-5 pounds every inch, and I decided to take it to 21" after tons of exercising. I pulled it to 21" and then heard an earsplitting BANG!!! 
But it wasn't the bow. The bow was intact, and so was the dacron string.
Turns out my tillering tree blew up. 
So how do you guys set up your tillering trees? I had a pulley attached to a wooden block bolted to the wall, and a little grooved ledge on the tip of the tree for the string to fit in while the pulley yanked the bow downwards (I tiller upside down now so that if a bow breaks the pieces go down and not flying up all over). both the ledge and the little wooden block that held the pulley broke.
any input is appreciated,
thanks

[Dances with squirrels]

Mine is made the other way... I pull the string down. But regardless, the parts of the tillering tree are obviously under a lot of strain, so you need to be mindful of the type of wood you use as well as the grain and growth ring orientation within each piece.

[Hrothgar]

I don't make heavy war bows and don't use a pulley--just my hands, but have found it useful sometimes to temporarily lash the bow to the tree when I'm exercising it. String, rope, binder twine, belt, about anything should keep it from rolling when pulled.