Another Type of Gate: Basket Valve

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 closed (NC) contact of its limit switch, so it comes ON as soon as its duct OPENS, even by just a crack. When the basket valve is between the two ducts, grain can flow in both directions and both indicators are ON. It would also be proper for each indicator to be wired to the normally open (NO) contact of its limit switch, so it would come on when its duct CLOSES. Then when the basket valve is in motion, both indicators would be OFF.

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.

A pair of independently-controlled slide gates can be made to block both outlets at the same time, stopping the flow of grain. A basket valve cannot do this. If an icon for a basket valve ever shows both outlets closed at the same time, either it has been wired incorrectly or the gate is not a basket valve.

The icon for this model is identical to the icon for a pair of slide gates. This is the same practice followed in real elevators. The icon conveys 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.

In some elevators, a pair of slide gates is geared so that when one slide opens, the other one closes. With this arrangement, only one motor is needed to drive both gates. Just as with a basket valve, only two limit switches are required: a fully-closed limit switch on the return duct and a fully-closed limit switch on the shipping duct.

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