Bill
I would agree with some of what you have said but not with all of it. The calculations are done on the basis of solid wood not jointed wood. They are also done to the Britsh Standard that takes into account the strength loss due to knots up to a point, so assuming that the wood fits within the visual characteristics of C16 as dictated by the standard then there should be adequate strength especially as the code includes a factor of safety.
I would also agree that avoiding knots around the critical support points is of fundamental importance.
What I can also tell you is that I safely lifted a Triumph Spitfire Engine using a home made gantry built out of the steel frames of two old desks, which consisted basically of four uprights with horizontal side braces and diagonal corner braces on the front and back and a central lift beam along the middle running fore and aft. It worked just fine, was wider than the car and was made from about 50mm x 25mm x 1.5mm steel.
If one is going to go to the complexity of making up a compound wood and steel section, then one might as well sandwich the 8x4 between two lengths of steel U section with the flanges point upwards on top and downwards on the bottom with bolts through both the web of the U section and the wood. That way the overall Z value would be massively increased and the wood would be simply acting as the web for the compound section and woudl take very little of the actual bending stress.
As for the deflection of a 8x4 over a 3m span with a 138kg plus load on it:
Elastic Modulus for Scots Pine: E = 10 GPa = 10E09 N/m^2 = 1000 N/mm2
Moment of Interia: I = 666.7E03 x (200 / 2) = 666.7 E05 mm^4
where 200/2 is the distance from neutral axis to further most fibre
For a simply supported beam with a point load in the middle:
maximum deflection = load x span^3 / (48 x E x I)
Assuming our load is 138kg x 2 to account for dynamic amplification etc:
maximum deflection = 138 x 2 x (3000)^3 / (48 x 1000 x 666.7e05) = 2.33mm
As for the overall safety of an 8x4 beam, the allowable loading according to the Britsh Standard is 3.5 times the proposed load and that is excluding the safety factor already built into the code. A typical lift of a several thousand tonne offshore oil platform being lifted by a floating crane that is continuously moving would not typically require such a high safety factor as that.
Whilst agreeing that a hydraulic engine crane might be easier and handy, if one doesn’t already have one they are expensive, even to hire, for a one off job. Where as if one has the wood already available and suitable block and tackle or similar then there is no real reason why it can not be safely utilised. In fact I think it shows great sense coming on here in the first place to check out the minimum beam size.
Andy