In most situations, a designer can get the optimum total bearing solution in his new-equipment design. However, a user replacing a bearing or redesigning present equipment may be constrained to existing envelopes. Load and speed information are necessary when selecting the bearing type; but loading and mounting conditions are most important in selecting a housing style.

Most makers of mounted rolling-element bearing units provide a variety of housing styles. Primary styles include 2 and 4-bolt base pillow blocks, flanges, piloted flanges or flange cartridges, hangers, take-ups, and cylindrical units. The style you should select depends on:

• The most appropriate mounting configuration.
• The loading characteristics.
• Whether misalignment or expansion capabilities, or both, are needed.

Pillow block housings

Pillow block housings can be 1 or 2- piece split types held together by two or four cap bolts. Most bases provide two or four mounting bolts. Most blocks are of Class 30 gray cast iron. For more strength and ductility, usually they can be furnished in ductile iron or cast steel. In choosing the attributes of a pillow-block housing, you should know the magnitude and direction of the radial load, Figure 1, and if present, the thrust load the unit must support.

If the pillow-block load is purely radial and directed toward a fully supported base (0 deg, Figure 1), then the load is limited only by bearing capacity. The base is in compression and its compressive strength is high.

If the load is purely radial but its direction is other than 0 deg, the housing strength and the clamping force of the base mounting bolts — cap assembly bolts for a split housing — must be considered when choosing the housing style. See Table 1.

When the load is at an angle into the base half of the pillow block or into the split of a 2-piece housing (0 to 90 deg or 0 to 270 deg, Figure 1), the mounting-bolt clamping force must be enough to prevent housing slippage. The housing will slip if the load overcomes the friction force which is a function of mounting-bolt clamping force and coefficient of friction at the mounting surface. Typical values for coefficient of static friction for metal on metal range from 0.15 to 0.40. A typical equation for friction force is:

Friction force = (Coefficient of static friction) x

(Clamping load) x (Number of bolts)

Clamping force is a function of bolt grade, applied torque, and nominal bolt diameter. A typical equation:

Clamping force = (Torque applied to the bolt) ÷ (0.2 3 Nominal bolt diameter)

Table 1 shows different grades of bolts and their clamp loads and dry torque requirements. The higher the grade number, the more torque that can be applied, resulting in a larger clamping load. This is due to the increase in the boltmaterial tensile strength with increasing grade.

A few of many Type USAF split, cast, 4-bolt-cap, 2-bolt-base, cast-closed-end, Nyloncoated pillow blocks on a belt filter press. At the Western Carolina Regional Sewer Authority plant, Greenville, S.C., the press processes 1,050 dry lb/hr, running 7 hr/day.

The load component parallel to the mounting surface, Figure 2, must be compared with the product of clamping load, coefficient of static friction, and a suitable safety factor, usually 2. This comparison determines whether to use a 2 or 4-bolt base pillow block. If the 4-bolt base unit is not enough to overcome the applied load, you may have to provide shear bars, Figure 2, to block the housing and oppose potential housing slippage.

Note that you do not count on resistance of the bolts in shear to keep the unit in place.

When the load is directed into the top half or cap of the pillow block (90 to 270 deg, Figure 1) you must consider other parameters. You must analyze the load component parallel to the housing base as in the previous case. Moreover, the 180-deg component, Figure 1, must satisfy three primary conditions:

• The load that would cause fracture of the housing, either of the cap or the base feet, must not be reached. The housing manufacturer can give you such data. It usually carries a large safety factor due to casting-process variables.
• For split housings, this load component must not exceed the bolt clamping force holding cap and base together. Include a minimum safety factor of 2.
• The load component must not exceed the bolt clamping force holding the pillow block to its mounting surface. Include a minimum safety factor of 2.

If the load is a thrust or has a thrust component, the housing must meet another set of conditions. Bolts securing the housing to the mounting surface must prevent slippage as before. Do the same analysis regarding bolt clamping load and coefficient of friction as you did for radial loading. Use shear bars if the load would cause housing slippage.

An overturning moment due to thrust load on the pillow block is the product of thrust load and distance from the base of the housing to the shaft centerline — the moment arm in Figure 3. This overturning moment is opposed by a resisting moment made up of the clamping force and a distance from the bolt centers to a pivot or tipping point of the housing. The overturning moment determines whether a 2 or 4-bolt base is needed and the bolt grade. This force analysis is a function of housing geometry. The pillow-block manufacturer can provide it.

Thrust can also induce a separating force on the housing if the insert’s surface is spherical. The force acts to split the housing perpendicular to the thrust, Figure 4. Casting strength is at issue here. In split housings, cap strength and cap bolts oppose this force. The manufacturer can quantify the force required to fracture the housing or overcome the clamping force of the bolts that assemble the two halves of a split housing.

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