Gin Pole Design (Lifting Shears)

I need to construct a 'lifting shear' (a variation of a gin pole lift/crane) for the posts and beams in a round timber build.  I would like the apex to be twenty feet high.  I will be lifting round timber spruce/fir beams up to 20' lengths & up to 12" diameters, as well as planting posts that are up to 20' tall & up to 12" diameters (incl. post holes of about 4').  Question #1: spruce or fir for the two legs of the shear?  Question 2: how thin can the round timber shear legs be?  The load logs are still somewhat wet, so I am supposing their weight to be up to 1000 lbs.   I am hoping that 4" diameter spruce or fir legs would be fine.  I found a photo that shows pretty much what I want.

If you use a pair of spruce or fir poles lifting a thousand pound load, I would feel safe with minimum 5" diameter at the small end (bark off). If you have any good hardwoods, especially oak or hickory, I expect 4" minimum would be plenty strong enough.

Between spruce and fir, I think it is more up to the quality of the individual poles. A common lumber grade is SPF, spruce/pine/fir, which indicates that those will have similar capacities at the same size.


Grade is the wrong word - SPF is a species designation, but the idea stands.