Info

Breakaway

in (mm)

Magnets/Row Overall Gap in (mm)

Torque ft-lb (N • m)

4.3 (109)

12 0.240 (6.10)

30 (40)

6.0 (152)

18 0.260 (6.60)

60 (80)

TABLE 3 Coefficients of thermal expansion (in/in/°F X 10-6)/(cm/cm/°C X 10~6)

Magnet material

Parallel to Axis

Perpendicular to Axis

NdFeB

5 (9)

9 (16.2)

SmCo

20 (36)

16 (28.8)

• Block width is two to three times the thickness.

• Block length is three to five times the thickness.

• Doubling the block thickness will increase the strength by approximately 20%.

For reference: 1.0 in3 (16.387 cm3) of NdFeB per assembly at a mean diameter of 4.0 in (101.6 mm) with a 0.25 in (6.35 mm) overall gap produces approximately 7 ft-lb (9.5 N • m) of torque. A 3% reduction in flux density is equal to a 6% reduction in torque.

Characteristics of magnet material:

Density: 0.273 lb/in3 (7.56 g/cm3) Tensile: 12 X 103 lb/in2 (844 kg/cm2) Compression: 110 X 103 lb/in2 (7.7 X 103 kg/cm2) Flex stress: 36 X 103 lb/in2 (2.53 X 103 kg/cm2)

Coefficients of thermal expansion are shown in Table 3.

Radial and Axial Magnet Forces The inner and outer carriers have to be restrained radially by bearings from contacting each other. In the example of "Torque Capability," a single row of 18 magnets with a 6 in (152 mm) mean diameter, the radial force with magnets concentric is 40 lb (18 kgf). When the magnets are offset by .005 in (0.127 mm), the radial force is 55 lb (25 kgf) (Figure 9). When the magnets are against each other (no gap), the radial force is 80 lb (36 kgf).

In the previous example, it takes 60 lb (27 kgf) in an axial direction to separate a single row of concentric magnets and 180 lb (82 kgf) for three rows. It is strongly advisable that provisions be made for personnel to address these loads during assembly and disassembly of the carriers.

Encapsulation of Inner Carrier Magnetics Encapsulation can be accomplished with either metallic or polymer materials. The encapsulation of the inner magnet and conducting ring is probably the most expensive and extensive process in a magnetic drive sealless pump. After encapsulation, the carrier should be nondestructively tested to confirm 100% effectiveness.

The pros and cons of polymer encapsulation (Figure 10a) are as follows:

2. SmCo magnets are used because of the high exposure temperature when applying polymer over the magnets.

3. The overall gap is increased because of the required thickness of the polymer.

4. Magnets must be restrained mechanically on the conducting ring by adhesives or high-strength polymers around the magnets.

5. Polymers like PFA/PTFE or PEEK are more corrosive-resistant than most metals.

6. Production polymer construction is much less expensive than metallic construction.

7. Polymer tooling is expensive.

The pros and cons of metallic encapsulation (Figure 10b) are as follows:

2. Thickness of the encapsulation material over the magnets can be 0.030 in (0.76 mm).

3. Welding of the components can be conventional, electron beam, or laser. However, care must be taken with conventional welding to prevent the arc from jumping toward the magnet flux.

4. If castings are used for the inner carrier, porosity can be a problem.

5. Gassing of polymers from welding heat coming out of the seams can be a problem.

6. Adhesives are not required to keep the magnets in place at 3600 rpm with metallic encapsulation; the outer shield performs this function.

Encapsulation of Outer Carrier Magnets The magnets for the outer carriers do not have to be encapsulated (Figure 11). However, material like NdFeB has an infinity for water absorption that results in rusting and swelling. The magnets can then break loose and move in position relative to one another. It is highly recommended that they be encapsulated with an epoxy or metal sheathing for atmospheric protection and handling.

Construction The outer carrier can be a casting or fabrication. The carrier is attached to the power end shaft in the bearing housing (Figure 3) or directly to a motor shaft, which is then called close coupled construction. When attached to the bearing housing shaft, there is very little axial or radial load applied to the bearings. This lightly loaded condition can result in internal skidding of the rolling element bearings within their races, resulting in premature bearing failure. Therefore, it is best to preload the bearings with a spring to prevent skidding. This can be accomplished outside of the bearing by a spring-loading feature (Figure 11).

Containment Shell The containment shell shape and thickness depends on working pressure, material, and temperature. The shell thickness is usually uniform. To keep the

FIGURE 11 Inner and outer carrier
FIGURE 12 Shell ratio L/r for pressure
TABLE 4 Pressure capabilities of shells with various end shapes for the same thickness and material

Shape

L/r

Allowable pressure—lb/in2 (kPa)

Ratio of allowable pressure to that for a flat plate

Flat

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