Fig. 127. Spring pulley
for windows.
The simplest ones are a set of shelves counterbalanced by weights. When the elevator is raised, the weights drop down, and when it is lowered, the weights rise.
Window weights hung over a pulley in the top of the window sash work on the same principle as dumbwaiters—the weights help in raising the window. The only care needed is to replace the rope when worn.
Another window pulley is made of metal like that in a clock spring (Fig. 127). The spring is drawn out when the window is lowered, and the weight of the window is just enough to hold it, so very little force is needed to raise the window, as the spring is pulling on it, too.
229. Check Valves. Check valves are made to prevent doors from slamming. They are used in offices and public buildings, and, occasionally, in homes (Fig. 128). One kind contains glycerine and castor oil, which move from one compartment to another as the door is opened and slowly flow back as a spring pulls the door shut.
Fig. 128. Check valve.
The other kind is operated by compressed air and a spring. The air causes the steady action of the door stop. Another type of pneumatic hinge is attached to a door which is hung so that it would naturally swing shut. When the door is opened, the air is exhausted from part of the hinge. After it has been opened, the slow equalization of the air inside the door stop and outside allows the door to close slowly without slamming.
Fig. 129. Door
holder.