Hello everyone,
I have an application with a belt driven lift fan on a hovercraft
which requires a variable ratio in relation to the input motor speed.
The lift fan needs to operate at a fairly regular speed of around
3,500 to 4,000 rpm. It consumes approximately 8 to 10 hp. The input
motor is far greater horse power than this as it also drives the
propulsion fan or propellor of the hovercraft. The input motor is
around the 50 hp mark and its rev range varies between 1200 idle and
7500 rpm at full throttle. The propellor is normally fully engaged at
3,500 engine rpm although for this exercise we are not concerned with
hp sharing or anything in particular outside the set parameters.
The lift fan is required to operate at full capacity when the
engine is anywhere between 2000 rpm and 7500 rpm.
It is preferable that the chosen transmission to achieve this is
simple and does the variable ratio in an automatic fashion following
either the lift fan rpm rate or the engine rpm rate or both. Main
thing is that it is automatic in operation as the engine rpm is likely
to change often and there is plenty for the operator to do without
concentrating on manually adjusting lift fan rpm.
The type of principal I am looking for is similar to a cvt belt
drive torque convertor where two pulleys change in diameter according
to torque and rotation speed to achieve a variable ratio.
I am not looking for a hydraulic solution, the hydraulic equipment
is available to achieve this scenario and is well documented, and has
a weight penalty in this application.
So, in short form, the parameters are, achieve 3,500 to 4000 rpm
fairly constantly from a varying speed engine during the input range
of 2000 rpm to 7500 rpm. At engine idle speed, 1200 rpm, the lift fan
may either be at less than 3,500 rpm or undriven/neutral.
It requires a starting ratio of ~1:1.8 (2000/3500) a middle band
ratio of 1:1 (3500/3500)~(4000/4000) and an ending ratio of ~1.9:1
(7500/4000). All these figures are similar to belt cvt transmissions
except that belt cvt's have their starting and ending ratios at the
opposite ends of what is required here.
Does anyone know of a transmission that does this already apart
from hydraulics and preferably using belts, not electric VFD ? or, how
to achieve this scenario using a cvt or parts that are reasonably
available. ?

Alan
hoverlover