Air Friction Damping
In
this type of damping a light vane or vanes having considerable area is attached
to the moving system to develop a frictional force opposing the motion by
reason of the air they displace. Two methods of damping by air friction are
depicted in Fig.
The
arrangement shown in Fig. (a) consists of a light aluminium vane which moves in
a quadrant (sector) shaped air chamber. The chamber also carries a cover plate
at the top. The vane is mounted on the spindle of the moving system. The
aluminium vane should not touch the air-chamber walls otherwise a serious error
in the deflection of the instrument will be introduced. Now, with the motion,
the vane displaces air and thereby a damping force is created on the vane that
produces a torque (damping) on the spindle. When the movement is quicker the
damping force is greater; when the spindle is at rest, the damping force is
zero.
The
arrangement of Fig. (b) consists of a light aluminium piston which is attached
to the moving system. This piston moves in a fixed chamber which is closed at one
end. Either circular or rectangular chamber may be used. The clearance (or gap)
between the piston and chamber walls should be uniform throughout and as small as possible. When
the piston moves rapidly into the chamber the air in the closed space is compressed
and the pressure of air thus developed opposes the motion of the piston and
thereby the whole moving system. If the piston is moving out of the chamber,
rapidly, the pressure in the closed space falls and the pressure on the open
side of the piston is greater than that on the opposite side. Motion is thus
again opposed. With this damping system care must be taken to ensure that the
arm carrying the piston should not touch the sides of the chamber during its
movement. The friction which otherwise would occur may introduce a serious
error in the deflection.
The
air friction damping is very simple and cheap. But care must be taken to ensure
that the piston
is
not bent or twisted. This method is used in moving iron and hot wire
instruments.
Fluid Friction Damping
This
form is damping is similar to air friction damping. The action is the same as
in the air friction damping. Mineral oil is used in place of air and as the
viscosity of oil is greater, the damping force is also much greater. The vane attached
to the spindle is arranged to move in the damping oil. It is rarely used in
commercial type instruments.
The
oil used must fulfill the following requirements:
- It should not evaporate quickly.
- It should not have any corrosive effect on metals.
- Its viscosity should not change appreciably with
temperature.
- It should be good insulator.
In
Fig. (a) a disc attached to the moving system is immersed in the fluid (damping
oil).When the
moving system moves
the disc moves
in oil and
a frictional drag
is produced. For minimizing the surface tension affect, the suspension
stem of the disc should be cylindrical and of small diameter.
In
the arrangement of Fig. (b) a number of vanes are attached to the spindle.
These vanes are submerged in oil and moves in a vertical plane. This
arrangement provides greater damping torque.
Advantages
1.
The oil used for damping can also be used for insulation purpose in some
forms of instruments
which are submerged in oil.
2.
The clearance between the vanes and oil chamber is not as critical as
with the air friction
damping system.
3.
This method is suitable for use with instruments such as electrostatic
type where the movement
is suspended rather than pivoted.
4.
Due to the up thrust of oil, the loads on bearings or suspension system
is reduced thereby
reducing the frictional errors.
Disadvantages
1.
The instruments with this type of damping must be kept always in a
vertical position.
2.
It is difficult to keep the instrument clean due to leakage of oil.
3.
It is not suitable for portable instruments.
