Instead of pairs of slide gates, some elevators
use BASKET VALVES to select one of two ducts at a
diversion point. A Basket valve is a simple
mechanism in which a blocking plate hangs on a
shaft which rotates to move the plate from one
duct to the other. The VRML model to the right
shows the Ship Gate and Return Gate replaced by a
single Basket Valve. The controls are simpler,
because only one motor needs to be started,
stopped, and reversed. Only two limit switches are
needed, one at the limit of valve travel in each
direction, compared to four for the pair of slide
gates. Also, only two indicators are needed, one
for each limit switch. This model is also fully
operational and correctly wired. By looking at the
model from the side, you can see that the limit
switches are not inside the duct. Instead, they
are mounted on the outside of the front cover of
the ductwork, which is transparent in this model.
Because of its simplicity, a basket valve is
easy to wire correctly. In the model shown, each
indicator is wired to the Normally Open (NO)
contact of its limit switch, so it comes ON when
its duct is CLOSED. When the Basket Valve is
between the two ducts, grain can flow in both
directions and both indicators are OFF. It would
also be proper for each indicator to be wired to
the Normally Closed (NC) contact of its limit
switch, so it would come on as soon as its duct
opens by just a crack. Then when the Basket Valve
is in motion, both indicators would be ON.
As with a Slide Gate, the placement of the
limit switches on a Basket Valve is critical. Each
limit switch must stop the valve just as it seats
over the entrance to a duct. Otherwise, the valve
can leak grain into the wrong duct, or the motor
could try to drive the valve through the wall of
the duct.
This model also has a monitor screen which
moves down to cover the indicator llights when you
click on it. The icons on this monitor are
identical to the icons for a pair of slide gates.
This is the same practice followed in real
elevators. The icons convey the same information
for a basket valve as they do for slide gates
(whether a duct is open or closed off), so no new
software is written just to make the icon look
like a basket valve.
NOTE: If the model
does not appear below you do not have a VRML player
installed.
