Electrical Engineering ⇒ Topic : Air Friction Damping

Air friction damping. Two arrangements of air friction damping are shown in Fig. (a) and Fig. (b). In the an-angement shown in Fig. (a), a light aluminium piston is attached to the spindle  that carries the pointer and moves with a very little clearance in a rectangular or circular air chamber closed at one end. The cushioning action of the air on the piston damps out any tendency of the pointer to oscillate about the final deflected position. This method is not favoured these days and the one shown in Fig. b (i) is preferred.

The arrangement shown in Fig. b (ii) is widely used. In this method, one or two light aluminium vanes are attached to the same spindle that carries the pointer. The vanes are permitted to swing in a sector-shaped closed box that is just large enough to accommodate the vanes. As the pointer moves, the vanes swing in the box, compressing the air in front of them. The pressure of compressed air in the vanes provides the necessary damping force to reduce the tendency of the pointer to oscillate.

                                   (a)                                                                                                                (b)

                                                                                      figure

Air Friction Damping

Figure (a) shows air friction damping. There is a light aluminium piston inside the air chamber and it is attached to the moving system at one end. The clearance between the chamber walls and piston is uniform throughout. Due to the oscillations of the moving system, the piston moves into and out of the chamber. When the piston moves into the chamber, the air inside the chamber is compressed and opposes the motion of the piston. When the piston comes out, the pressure in Figure 14.6 Air friction damping. the closed space falls. In this case, the pressure on the open side of the piston is greater than that of the other side. Once again there is an opposition to motion. This type of damping is very cheap. Attention must be paid that the piston should not be twisted or bent, otherwise solid friction will come into play if it touches the chamber walls.

                                                      figure (a)