Calculation Formula: Pcr=fm*d*d*d*d/L*L    ensure    Pcr>W*SF (SF=4)
Pcr = Critical load (N)
d = Thread root diameter (mm)
fm = Support factor
L = Distance between load supporting point and mounting point as drawing (mm)
W = Equivalent load of single jack (N)
SF = Safety factor (generally SF=4)
Verify the screw stability when the axial compression load exists. If the load is greater than the critical load, increase the sizes before calculation.
fm = 2.5 x 10000, screw jack base fixed, screw end free rotation. 
fm = 1*100000, screw jack base and screw end supports (double clevis mounting). 
fm = 2*100000, screw jack base fixed, screw end fixed or supports.