In order to prevent shocks to the suspension during transit etc. Taut suspension:. Suspension type of measuring instruments can only be used in a vertical position. The taut suspension has a flat ribbon suspension both above and below the moving element, with suspension kept under tension by a spring arrangement. The advantage of this suspension is that exact levelling is not required if the moving element is properly balanced. Taut suspension.
Suspensions and taut suspensions are customarily used in instruments of galvanometer class which require a low friction and high sensitivity mechanism. Some sensitive watt meters and electrostatic voltmeters also use flexible suspension. Ribbon suspensions, in addition to supporting the moving element, exert a controlling torque when twisted.
Thus the use of suspension results in the elimination of pivots, jewels and control springs and therefore, pivotless measuring instruments are free from many defects. Must Read: Deflecting system in indicating measuring instruments.
Pivot and jewel bearings:. The deflecting system is mounted on a spindle made of hardened steel. The two ends of the spindle are made conical and then polished to form pivots. These ends fit conical holes in jewels located in the fixed parts of instruments as shown below.
These jewels, which are preferably made of sapphire from the bearings. Originally natural sapphire was used but now synthetic sapphire is being used. The combination of gives lowest friction. Jewel bearing. It has been found that the frictional torque, for jewel bearings, is proportional to the area of tact between the pivot and jewel.
Thus the contact area between pivot and jewel should be small. The pivot is ground to a cone and its tip is rounded to a hemispherical surface of the small area. In the arrangement shown in fig a , a light aluminum 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 favored these days and the one shown in fig. In this method, one or two light aluminum vanes are attached to the same spindle that carries the pointer.
As the pointer moves, the vanes swing and compress the air. The pressure of compressed air on the vanes provides the necessary damping force to reduce the tendency of the pointer to oscillate. In this method, discs or vanes attached to the spindle of the moving system are kept immersed in a pot containing oil of high viscosity. As the pointer moves, the friction between the oil and vanes opposes the motion of the pointer and thus necessary damping is provided.
The fluid friction damping method is not suitable for portable instruments because of the oil contained in the instrument. In general, fluid friction damping is not employed in indicating instrument, although one can find its use in Kelvin electrostatic voltmeter.
Two methods of eddy current damping are generally used. In the first method, as shown in the figure, a thin aluminum or copper disc is attached to the moving system is allowed to pass between the poles of a permanent magnet. As the pointer moves, the disc cuts across the magnetic field and eddy currents are induced in the disc. In this way, eddy current damping torque reduces the oscillations of the pointer.
The degree of damping decides the behaviour of the moving system. If the instrument is under-damped, the pointer will oscillate about the final position for some time before coming to rest. On the other hand, if the instrument is over-damped, the pointer will become slow and lethargic.
However, if the degree of damping is adjusted to such a value that the pointer comes up to the correct reading quickly without passing beyond it or oscillating about it, the instrument.
The damping torque in indicating instruments can be provided by. Two arrangements of air friction damping. In the arrangement shown in Fig 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 preferred. 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.
In this method, discs or vanes attached to the spindle of the moving system are kept immersed in a pot containing oil of high viscosity. As the pointer moves, the friction between the oil and vanes opposes the motion of the pointer and thus necessary damping is provided. This method is not used because of several disadvantages such as objectionable creeping of oil, the necessity of using the instrument always in the vertical position and its unsuitability for portable instruments.
In general, fluid friction damping is not employed in indicating instruments, although one can find its use in Kelvin electrostatic voltmeter.
Eddy current damping is the most efficient form of damping. Two methods of eddy current damping are shown in Fig: A thin aluminium or copper disc attached to the moving system is allowed to pass between the poles of a permanent magnet. Spring controlled instruments have the following advantages over the gravity controlled instruments.
This force is necessary to bring the pointer to rest in its final deflected position quickly. Without such damping, the combination of deflecting torque and controlling torque makes the pointer to oscillate about its final deflected position for some time. It resulting in a waste of time while taking the reading.
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