(i) The moving parts should be light.
(ii) The frictional forces should be minimum.
These requirements should be fulfilled in order that power required by the instrument for its operation is small. The power expenditure is proportional to the weight of the moving parts and the frictional forces opposing the movement. The moving system can be made light by using aluminium as far as possible. The frictional forces are reduced by using a spindle mounted between jewel bearings and by carefully balancing the system. Supporting the Moving Element The force or torque developed by the moving element of an electrical instrument is necessarily small in order that the power consumption of the instrument be kept low so that the introduction of the instrument into a circuit may cause the minimum change in the existing circuit conditions. Because of low power levels, the consideration of various methods of supporting the moving element becomes of vital importance. With the operating forces being small, the frictional forces must be kept to a minimum in order that the instrument reads correctly and is not erratic in action and is reliable. Types of Supports Several types of supports are used, depending upon the sensitivity required and the operating conductions to be’ met. The supports may be of the following types.
(i) Suspension
(ii) Taut suspension
(iii)Pivot and jewel hearings (i) Suspension It consists of a fine, ribbon shaped metal filament for the upper suspension and a coil of fine wire for the lower part.
The ribbon is made of a spring material like beryllium, copper or phosphor bronze. This coiling of lower part of suspension is done in order to give negligible restrain on the moving system. The type of suspension requires careful levelling of the instrument, so that the moving system hangs in a correct vertical position. This construction is, therefore, not suited to field use and is employed only in those laboratory applications in which very great sensitivity is required. In order to prevent shocks to the suspension during transit etc; a clamping arrangement is employed for supporting the moving system. (ii) Taut Suspension Suspension type of instruments can only be used in vertical position. The taut suspension has the flat ribbon suspension both above and below the moving elements, 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. Suspensions and taut suspension are customarily used in instruments of galvanometer class which require a low friction high sensitivity mechanism. But actually there is no strict line of demarcation between galvanometers and other indicating instruments. Some sensitive wattmeters and electrostatic voltmeters also used flexible suspension.
Ribbon suspensions, in addition to supporting the moving element, exert a controlling torque when twisted. Thus the use of suspension results in elimination of pivots, jewels and control springs and, therefore, pivotless instruments are free from many defects. (iii) Pivot and Jewel Bearings The moving 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.
Jewels are sometimes spring mounted to lessen the likelihood of damage because of shocks, and its bottom is also rounded.
Instrument bearings are used dry. Any lubricant, such as oil, would he of questionable value in the first place and would soon cause trouble because of gumming and by collected dust. It is true to say that good class instruments of this construction are robust and reliable in service, but it is a fact that most instrument defects are due to excessive friction caused by damaged pivots and or dirty jewel bearings, and due to damaged or distorted control springs.
These jewels, which are preferably made of sapphire form the bearings. Originally natural sapphire was used hut now synthetic sapphire is being used. The combination of steel and sapphire gives lowest friction.
It has been found that the frictional torque, for jewel bearings, is proportional to area of contract 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 cone of some what larger angle. The pivot may have a radius at tips from 0.0125mm to as high as 0.075mm depending upon the weight of the mechanism and the force it will encounter.
The radius of the pit of jewel is somewhat longer so that the contact is in the form of a circle. The contact area should not be too small otherwise the stress (which is load per unit area) will exceed the crushing strength of the material of pivot. The pivots of many modern instruments arc loaded nearly to yield point of steel.
We can have an idea of the stress produced if we consider a moving clement weighing 300 mg resting on the are of circle 0.005mm diameter. The load produces a stress of about 150 x 1066 N/m2 or of the crushing limit of the steel. Moreover, if the instruments are subjected to shocks, the dynamic effect may cause the stress to be two or three times or more the static value. It is easy to visualize that rough handling of the — deform the pivot and crack the jewel.
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