Ordinarily, you’d choose the housing in conjunction with selecting the bearing itself. There are plenty of guides for bearing selection, but how do you choose a housing?
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.
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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A sample pillow-block pick
Consider this example. Assume that we have determined the loads on a bearing that will support a 2 15/16-in. horizontal shaft turning at 65 rpm. The pillow block is to mount on a flat horizontal surface.
The first consideration: Should it be a 1 or a 2-piece pillow block? Most ball bearings come in 1-piece housings. Tapered and spherical roller bearings come in either version. Tapered roller bearings in 1-piece housings do not allow expansion or self-alignment capability. If these are important capabilities, use a 2-piece housing. Two-piece housings primarily provide for easy bearing insert replacement. One-piece housings usually need complete pillow-block replacement upon failure.
The bearing must resist a radial load of 8,000 lb at 330 deg, Figure 1. You should verify acceptable L10 life before pursuing housing style. We won’t cover that here. The 8,000-lb radial load at 330 deg resolves into a horizontal component of
FH = 8,000(cos 60) = 4,000 lb and a vertical component of FV = 8,000(sin 60) = 6,928 lb
The horizontal force must be resisted by an opposite force that can always balance it to prevent sliding. Assuming a static friction coefficient of 0.3 and a safety factor of 2, and remembering that the friction coefficient is the quotient of the force normal to the surfaces divided by the force parallel:
(2)3(0.3)3(4,000) < (0.3) x (Clamping force) x (Number of bolts)
which reduces to the statement: The product of bolt clamping force and number of bolts must be greater than 8,000 lb.
Thus, you can use either a 2-bolt base block with bolts that each have a clamp load at full torque of more than 4,000 lb, or a 4-bolt base block with bolts that each have a clamp load at full torque of more than 2,000 lb. Table 1 shows that, for a 2- bolt base, you would need at least 9/16-18 grade 2 bolts, 1/2-13 grade 5 bolts, or 7/16- 14 grade 8 bolts. A 4-bolt base requires much smaller bolts as a minimum.
The vertical component is directed into the base of the housing. Due to the high compressive strength of cast iron, it need not be considered.
Flange housings are usually provided in mounting configurations that include from two to six mounting bolts. A flange housing mounts a bearing onto a surface that is perpendicular to the shaft axis. Some types of flange housings have pilot surfaces, Figure 5. A pilot is a cylindrical extension on one side of the housing. It is concentric with the bearing bore, and it locates the flange more accurately. It also supports the load directly into the frame or mounting structure rather than relying solely on the friction force induced by bolt clamping forces. In choosing a flange mounting, it is important to know the magnitude and direction of forces to be supported.
For flange bearings, support of the radial component of the force is the function of the mounting bolts or the pilot. In the standard flange, radial load is transmitted through the bearing and into the flange housing. It is supported by the friction force induced by the mounting bolts clamping the mounting surfaces. Thus, the number of mounting bolts needed depends on the size of the radial load. That load must not exceed the product of number of bolts, clamping force of each bolt, and static friction coefficient between the mounting face of the flange and the mounting surface. If the radial load exceeds this product, you need more or stronger bolts, or shear bars.
Shear bars block the flange from slipping on the mounting surface. They can be welded to the surface at right angles to the radial force vector after the flange is mounted.
For a piloted flange (flange cartridge), radial load transmits from bearing to flange housing and directly into the mounting structure through the pilot, Figure 5. The pilot material is in compression and is very strong. The piloted flange housing also offers easier installation, and control over potential misalignment.
Support of the thrust component of the load applied to a bearing mounted in a flange or piloted flange housing is primarily a function of the mounting bolts and flange strength. The ideal configuration for the thrust-carrying bearing is for it to mount so that the thrust pushes the housing against the mounting surface. Here, you must consider whether the bearing can withstand the thrust and, for a split housing, whether the separating forces would prevail. Ask these questions of the housing manufacturer.
If the ideal configuration is impossible, the thrust load will act to force the housing away from the mounting surface. The bolts will oppose the action. Success is a function of their collective clamping force. If it is large enough, you may risk fracture of the housing material. In most situations, housings supported by grade 2 bolts can support the forces that the bolts can support. If you determine that stronger bolts are needed to resist the thrust load, consult with the housing manufacturer about housing strength.
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Cylindrical units, inserts
A cylindrical unit is just a bearing mounted within a housing that has a cylindrical OD. Ball bearings and tapered roller bearings are available in this configuration. These housings require a way to mount the unit to the mounting structure and provide for radial and thrust load. Additional provision for non-expansion and expansion capability as well as anti-rotation, if needed, must be provided by the user. In most applications, these housing styles provide for the most economical use of space. If you have questions on details of mounting or precautions for handling loads, direct them to the manufacturer of the units.
A typical cylindrical unit is mounted by boring a hole into the mounting structure to accept the cylindrical OD of the housing, and providing either a shoulder on one side and a plate on the other or snap rings on both sides to block up the housing and position the unit axially. Shoulders, snap rings, or both if used, must be strong enough to support the thrust load. The mounting surface providing the seat for the cylindrical OD must be able to support the radial loads that the bearing transmits to it. This is similar to the piloted flange mounting system. The expansion bearing must mount in a way that provides for axial float.
You can purchase tapered roller bearing inserts separately for replacement in 2-piece housings or in user-manufactured housings. In a 2-piece tapered roller bearing housing, the bearing is pressed into an intermediate housing, clearances are factory preset, the bearing is sealed, and then factory lubricated. The OD of the insert is machined with a spherical surface to provide misalignment and expansion capability within the matching outer 2-piece housing. The cylindrical tapered roller bearing unit is a variety of this type of insert. If these units are to be used outside the 2-piece housing that the manufacturer provides, you should contact the manufacturer for mounting details and precautions.
In all cases, you must use quality fasteners of sufficient strength and torqued to the proper specifications. Equally important: mountings must be of quality material and construction for proper operation. All the housings discussed are readily available with ball, tapered roller, and spherical roller bearings.