Brands |
N/A Bird Precision |
Suggested Pressures |
N/A 0.060 O.D. - Low Pressure (Under 150 psi) 0.118 O.D. - Medium Pressure (5000 psi or lower) 0.087 O.D. - High Pressure (Above 5000 psi) |
Material |
N/A Niva Alloy |
Diameter |
N/A 0.0397 in0.0403 in |
Length |
N/A 0.656 in |
Angle |
N/A 50 º |
Radius of Point |
N/A 0.0008 in0.0011 in |
Design Considerations for Sapphire and Glass Vee Jewel Systems |
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Sapphire and glass vee jewels are commonly mounted in fine pitch screws.
The generally accepted adjustment is 0.003/0.004". The adjustment must be done cautiously, never locking up the points; the screw is turned down until only the slightest hesitation is sensed. Then the screw is backed off. Locking up the shaft and vee will damage the points, so a very light touch is needed. Some instrument manufacturers set the moving element in motion, when they see a hesitation then they back off. So if you are using a 100-100 thread this is approximately 1/4 turn of the screw. For small radius points (0.0008/0.002"), a weight of 50 grams or more can potentially put a flat on your pivot point, so if weights exceeding 50 grams are needed then the radius points of the pivot must be increased. (perhaps 0.005/0.007" typical) depending on weight The best frictional result is when the ratio of vee radius and shaft radius is 3:1. Example if the sapphire vee radius is 0.006/0.009 the shaft radius should be approx. 0.002/0.003. Sapphire and glass vee jewels can be either spring loaded or cushion loaded. That is the vee jewel can be made to support a certain weight before it begins to move. Spring loading, or cushion loading one or more of the jewel screws can help protect the points in shock or vibration environments. The spring load is set to support the moving element weight so movement only results under some type of g force. The pivots are fashioned with shoulders; the shoulder distance is preset to avoid the spring bottoming during shock. Some amp and volt meters are designed to survive being dropped in this manner. Most instruments where the movement is vertical will have a spring loaded vee on the bottom position and a fixed vee on the top. Horizontal movements can have one or both ends spring loaded. Bird Precision will be happy to recommend a set of bearings to fit your particular application needs. With over 100 years of design experience, Bird has a huge supply of precision screws, springs, vee jewels and matching pivot choices to facilitate custom design needs. So a turnkey bearing system may be possible, or ask a Bird design associate for custom options as well. For your design consideration we include the following listings.
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Choosing Pivots |
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Choosing the correct pivot points to match the sapphire vee jewel is critical. Below is a list of stock pivots . Please pay particular attention to the radius of the points. As noted the correct ration between the sapphire cone radius and the pivot point radius is 3:1. Modern Instruments generally use Niva Alloy material. This is a chrome, cobalt and vanadium alloy, comparable to Elgiloy N-100. This premium material is non-magnetic, corrosion resistant, RC 62 or greater hardness. This material exhibits low frictional qualities with both glass and sapphire vee bearings. To a lesser degree other materials in use are tungsten carbide, 440 C stainless, 420 f stainless and European alloys such as Sandvik 20 AP, or 20WIV. Hardness is very important, since the point can mushroom or wear down if they are soft. Generally hardness should be above RC45 for best performance. |
Additional Information |
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Glass vee bearings have slightly lower friction because they have fire polished surfaces. Fire polishing minimizes the interlocking asperities common to lapped surfaces. This factor is especially evident when the vee bearing is used in the horizontal position. In this position the mating conical shaft will try to roll up the surface of the vee bearing radius. But with the fire polished glass the interlocking of the pivot and glass surface is almost non-existent so friction and drag are optimized. Vee bearings in the vertical position have the lowest friction due to the point contact. Another nice feature of vee systems is that the conical entry will afford a small amount of misalignment, without binding. Vee jewels are commonly employed in moving coil and moving magnet meter movements, compasses, encoders, turbine flow devices, galvanometers, wind speed indicators, dip needles, balances and various timing mechanisms. Because of the low elastic deformation, hardness and chemical resistant properties of the sapphires materials used, friction and torque do not increase appreciably over the life of the bearing, wear is usually found in the pivoting member or shaft rather than the bearing. This may be a function of load and speed and the choice of shaft materials. Choice of the size of the Vee bearing radius and the pivot radius have a lot to do with the load bearing capabilities and the friction induced. Typically the conical vee angle is 85 degrees and the conical pivot angle is 55 degrees thereby allowing a little angular misalignment. Glass vee bearings with small radii will support up to 200 milligrams. Larger radius points will tolerate much higher loads. Sapphire vee bearings will support much higher loads, it is usually the pivot point that is the weaker member. For instance a pivot point of less than 0.002” may be damaged with a load as little as 50 grams. When under shock or vibration whereas larger pivot radii may support 500 grams or more, vertically. There are tremendous pressures exerted at the point contact area of the vee and shaft. According to the Hertz formula, used by V. Stott in his study, The Use of Pivots and Jewels in Instruments and Meters. For a 40 gram load on a 60 degree hardened steel pivot point, with a radius point of 0.002” the expectant force would be 1,237 KG per mm²!. But if the pivot radius is increased to 0.0032” the force is reduced dramatically down to only 131 KG mm². Friction also increases in a vee system as the pivot radius increases, so the designer must determine the optimum point were performance and durability meet. Choice of pivot material is also extremely important to good design. Generally pivot points should be hardened to Rc 45 or better, and have a point and cone finish of 4 micro-inch. Corrosion resistance and non-magnetic properties are also nice features depending on the application. |
