pnuematic guage.pptx

FLUID POWER

1. Pneumatic comparators Pneumatic comparators use air as a means of measurement. The basic principle involved is that changes in a calibrated flow respond to changes in the part feature. This is achieved using several methods and is referred to as pneumatic gauging, air gauging, or pneumatic metrology. It is possible to gauge length, diameter, squareness, parallelism, taper, concentricity, etc., using a simple set- upFor instance, if one is inspecting the bore of an engine cylinder, it is also possible to assess its size, taper, camber, and straightness in the same setting.

2. Advantages Absence of metal-to metal contact, higher amplification, and low cost. Absence of metal-to-metal contact between the gauge and the component being inspected greatly increases the accuracy of measurement. The gauge also has greater longevity because of a total absence of wearable parts. Amplification may be increased without much reduction in range, unlike mechanical or electronic instruments. Pneumatic comparators are best suited for inspecting multiple dimensions of a part in a single setting ranging from 0.5 to 1000 mm

3. Pneumatic Gauges: Based on the type of air gauge circuit, pneumatic gauges can be classified as free flow gauges and back pressure gauges .

4. Free Flow Air Gauge This uses a simple pneumatic circuit. Compressed air with a pressure in the range 1.5–2 bar is passed through a tapered glass column that contains a small metal float. The air then passes through a rubber or plastic hose and exits to the atmosphere through the orifice in the gauging head. Since the gauging head is inserted inside the work part that is being inspected, there is a small clearance between the gauging head and the work part. This restricts the flow of air, thereby changing the position of the float inside the tapered glass column. The set-up is illustrated in Figure ,Compressed air from the factory line is filtered and reduced to the required pressure. A shut-off valve is provided to ensure shut-off of air supply when not in use.

5. Flow Characteristic Curve Flow–Clearance curve Figure illustrates the relationship between the clearance and the flow rate. It is clear from the graph that the flow rate increases with the increase in the clearance. The curve has a linear portion, which is used for the purpose of measurement. This linearity in the gauging range permits dimensional variation to be accurately measured up to 1 μm . A calibrated scale enables the reading to be directly read from the scale. Amplification of up to 100,000:1 has been built into these gauges this type of pneumatic gauge is relatively free from operator error .

6. Air gaps in air gauging A typical gauge head has two orifices diametrically opposite each other If the spindle of the gauge head is moved to one side, the air flow is decreased; however, the air flow through the diametrically opposite orifice increases by an equal amount.

7. Working Principle . The float takes up a position in the tapered tube such that the air velocity through the ‘annulus’ created by the float and the tube is constant. The air then escapes through the gauging orifice. In order to use the gauge as a comparator, the user uses a master gauge of known dimension and geometric form, and sets the float to a reference value by adjusting the air flow rate. In other words, the air gauge is set to a datum rate of air flow through the system. Now, the master gauge is taken out and the gauge head is inserted into the work part being inspected. Any variation in the dimension of the work part will produce a variation in the rate of flow through the system. This is reflected in the change in height of the float in the glass column, and the difference in dimension can be directly read on the graduated scale.

8. Back pressure Gauge: Working Principle This system uses a two-orifice arrangement, While the orifice O1 is called the control orifice , the orifice O2 is referred to as the measuring orifice . The measuring head gets compressed air supply at a constant pressure P , which is called the source pressure . It passes through the control orifice into an intermediate chamber. Air exits the measuring head through the measuring orifice. While the size of the control orifice remains constant, the effective size of the measuring orifice varies because of the gap d between the measuring orifice and the work surface. Depending on the gap d , the back pressure P b changes, thereby providing a means for measuring dimension d .

9. Characteristic Curve Of A Back Pressure Gauge Assuming that the areas of control orifice and measuring orifice are A 1 and A 2 respectively, the relationship between the ratio of back pressure to source pressure and the ratio of the areas of control orifice to measuring orifice is almost linear for P b / P values from 0.5 to 0.8. This range is selected for the design of the back pressure gauge.

10. Construction: A Back Pressure Gauge Compressed air is filtered and passed through a pressure regulator. The regulator reduces the pressure to about 2 bar. The air at this reduced pressure passes through the control orifice and escapes to the atmosphere through the orifice of the measuring head.

11. Construction: Depending on the clearance between the measuring head and the work part surface, back pressure is created in the circuit, which, as already pointed out, has a direct relationship with the effective area of the measuring orifice. Various transducers are available to display the linear gap between the measuring head and the work part. In this set up back pressure is let into a bourdon tube, which undergoes deflection depending on the magnitude of air pressure. This deflection of the bourdon tube is amplified by a lever and gear arrangement, and indicated on a dial

12. Construction: Magnification of up to 7500:1 can be achievedby employing the bourdon tube principle. Readings up to 0.01 mm is common in most of the gauges. The back pressure gauge is essentially a comparator , and the initial setting is done by means of reference gauges. It is important for both the reference gauge and the workpiece being inspected to have the same geometric form. Therefore, slip gauges are used for flat workpieces and ring gauges are preferred for cylindrical workpieces .

13. Construction: The master gauge is used and the instrument is set to a reference value by varying the input pressure of air as well as by means of variable bleed to the atmosphere. This can be done by operating the pressure regulator. Air pressure is adjusted so that the instrument is set to some datum value on the scale. Now, the reference gauge is taken out and the workpiece is introduced with the measuring gauge. The deviation in dimension can be directly read on the scale.

14. Solex Pneumatic Gauge This air gauge has been developed and marketed by Solex Air Gauges Ltd, USA, and is one of the most popular pneumatic comparators in the industry Compressed air is drawn from the factory air supply line, filtered, and regulated to 2 bar

15. Extra air, by virtue of a slightly higher supply air pressure, will leak out of the water tank in the form of air bubbles and escape into the atmosphere. This ensures that the air moving towards the control orifice will be at a desired constant pressure. The air at a reduced pressure then passes through the control orifice and escapes from the measuring orifice in the measuring head. Based on the clearance between the work part and the measuring orifice, a back pressure is created, which results in the head of water being displaced in the manometer tube. The Solex comparator has a high degree of resolution, and variation in dimension up to a micrometre can be determined easily. Amplification of up to 50,000 is obtainable in this gauge. Solex Pneumatic Gauge

16. Applications of Pneumatic Comparators

17. Applications of multiple-orifice gauging heads

18. P is the source pressure while p is the back pressure or pressure between the control and measuring orifice. The relation between the p and P depends on the relative sizes of the two orifices Oc and Om. p=P when Om is blocked and p= 0 when Om is increased indefinitely. Let C is orifice area of Oc and M is of Om

19. Characteristic Curve

20. WE are interested in the linear form of the curve. Between the values of 0.6 to 0.8 the curve approximates to linear curve. The equation of which may be written as , Value of A= 1.1 for series of characteristics curves. Slope b however is not constant , b=0.6 when P= 0.13 kg/cm2 b= 0.4 when P= 5 kg/cm2

21. Area of the escape orifice: When the clearance between surface and nozzle face is zero, no air escapes from the nozzle and area of the escape orifice is zero. When the clearance between surface and nozzle face is large, no air escapes from the nozzle and area of the escape orifice is Between these two extreme values when clearance is small and gauging is applied area of the escape orifice is , πDL .

22. Range of the linear measurement: As we know that, 0.6<p/P<0.8, the characyteristic curve is linear within 1%. Mn = Minimum value of M for p/P of 0.8 and Mx = Maximum Value of M for p/P of 0.6 Then if we take the ratio,

23. Again we know that M= Lx- Ln =

24. Pneumatic Sensitivity: Differentiating the above equation w r t M Again, for p/P of 0.7 ..M has average value Ma becomes, so that

25. Overall sensitivity: The overall magnification of the apparatus is the ratio of the linear movement of the pointer or index of the pressure measuring instrument to the change in the dimension which produces it. It actuall depends on Pneumatic sensitivity The way in which area M change changes as L chnges The sensitivity of the pressure measuring instrument

26. Overall sensitivity: Overall sensitivity = M= Now if λ be the length of the scale of pressure measuring device corresponding to the pressure change from 0 to P

27. Overall sensitivity: =

28. Numerical The operation of the pneumatic comparator is represented by equation, for 0.6>>0.8 Where , P= supply pressure p= pressure between measuring and control orifices b= constant =0.5 for P=2 kg/cm2 M= πDL L= separation between nozzle surface and the surface to be gauged D= measurement orifice diameter d= control orifice diameter= d2

29. The control orifice is 0.5 mm diameter and measuring orifice is 1 mm diameter hole. Calculate : a) The range of linear measurement b) measuring head , pneumatic gauge and overall sensitivities. The back pressure gauge has a deflection of 20 mm for 100 kg/cm2 pressure change. Air supply pressure is constant at 2 kg/cm2

30. corrosponding to maximum separation , where Lx is the maximum separation. After solving for d=0.5 , D= 1 mm , we will get, Lx= 0.062 mm corrosponding to minimum separation , where Ln is the minimum separation.

31. After solving for d=0.5, D= 1 mm , we will get, Ln = 0.037 mm. Therefore , Linear Range = Lx- Ln = 0.062-0.037=0.025 mm b) Measuring Head sensitivity = = πD =3.142 x 1 mm=3.14 mm Pnuematic sensitivity= Ma= πDLa =π . 0.05 mm2

32. c) gauge sensitivity d) Overall sensitivity = 20 x 509 x 3.14=32000