  The precessing jet-nozzle scenario has been applied to interpret the
VLBI-kinematics of twenty-seven superluminal components in blazar 3C345
measured during an about 38-yr period. The superluminal knots could be ejected
from a double jet-nozzle system forming a double-jet structure.Both nozzles
could precess with a period of about 7.30yr and in the same direction.For both
jets a steady precessing common trajectory could exist, along which different
knots moved according to their precession phases. Most knots were
model-simulated to be accelerated with their bulk Lorentz factor in the range
of about 4 to 30 and their Lorentz/Doppler factor profiles were derived. The
radio light-curves of knot C9 were well coincident with its Doppler boosting
profile. The double precessing nozzle scenario has been applied to interpret
the VLBI-kinematics for four blazars (3C279, OJ287, 3C454.3 and 3C345). The
charateristic parameters of the four putative supermassive black hole binaries
(including hole masses, orbital separation and gravitational radiation
lifetime, etc.) were derived and compared, showing that they are in physically
reasonable ranges, and well consistent with some theoretical arguments for
close black hole binaries.
