Bearing housings and arrangements

Bearing arrangements for fans may be designed in a variety of ways dependent on the size, operating conditions and rota­tional speed. Cost also is a consideration together with the ex­pected life. The comments and selections which follow are to a certain extent in ascending order of price and reliability.

Light duty pillow blocks

These are normally recommended for light duty fans having a shaft diameter of 50 mm or less. Such bearings have a zinc-coated bore and an extended inner ring with eccentric locking collar.

In the arrangement shown in Figure 10.29 the fan impeller is supported by Y-bearings mounted in cast iron housings. As both Y-bearings are located, the sheet steel sideplates of the fan must accommodate possible thermal elongation of the shaft.

As the bearing bore tolerances are to plus limits to permit mounting on drawn steel shafts (say tolerance h9/IT5) a clear­ance fit is obtained. This leads to a slightly eccentric operation with resulting vibration, therefore the use of Y-bearings should be confined to low or medium speed operation. Relubrication is not normally required as the bearings are supplied lubricated for life. However, if necessary, Y-bearings fitted in cast iron housings can be relubricated.

Figure 10.29 Light duty double inlet, double width (DIDW) centrifugal fan fitted with pillow blocks and ball bearings Courtesy of SKF (UK) Ltd

Plummer block bearings

Where silent running is stipulated with relatively high speeds, self-aligning ball bearings mounted on adapter sleeves are rec­ommended for light and medium duty fans with shaft diameters up to and including 110 mm. For heavier duty fans spherical roller bearings mounted on adapter sleeves, may be neces­sary. Normally the bearing is mounted in a cast steel plummer block housing. Various types of seal are available. Relubrication can be arranged if there is a suitable grease es­cape arrangement for use with the seal.

Figure 10.25 in Section 10.4.9 shows an arrangement using a self-aligning bail bearing mounted in an SNA plummer block housing with grease escape valve, type TAV. The efficiency of relubrication has been much improved by mounting an extra V-ring inboard of the V-ring seal washer at the side where grease is supplied, so that grease can only leave the housing at the opposite side after passing through the bearing. It should be noted that grease is usually supplied to these housings on the side away from the lock nut.

Tolerances

Shaft h9/IT5

Housing Standard plummer block H8

Plummer block with

Grease escape valve H7

Lubrication

A high quality lithium base grease is normally recom­mended.

10.9.1 Plummer block bearings for oil lubrication

Spherical roller bearings with cylindrical bore and also with ta­pered bore plus the relevant adapter sleeve, are recommended for the larger heavy-duty fans. Appropriate housings will be found for both cylindrical and taper bores, in most bearing man­ufacturers’ catalogues.

Where long relubrication intervals are desirable oil lubrication is recommended and specially designed plummer block housings can be used. These have an adequate space for an oil reservoir and have been developed mainly for high speed fans. They are equipped with effective labyrinth seals to eliminate oil losses.

For applications where low vibration and silent operation are re­quired, preference is given to the use of spherical roller bear­ings with cylindrical bore mounted in series, see Figure 10.30.

Spherical roller bearings with tapered bore mounted on adapter sleeves are frequently used where easy mounting is required.

Figure 10.32 Cartridge assembly with single row deep groove ball bearings Courtesy of SKF (UK) Ltd

Figure 10.30 Heavy duty fan with oil lubricated plummer blocks Courtesy of SKF (UK) Ltd

Figure 10.33 Hot gas fan fitted with cooling disc, heat shield and grease lubri­cated bearings Courtesy of SKF (UK) Ltd

In this case a different design of housing is available in three variants:

• Shaft end, non-locating bearing — suffix AL

• Through shaft, non-locating bearing — suffix BL

• Through shaft, locating bearing — suffix BF Tolerances

Shaft

Cylindrical seatings —

Direct mounting m6

Cylindrical seatings —

Figure 10.34 High pressure fan fitted with angular contact ball bearings and roller bearing to take vee belt drive loading Courtesy of SKF (UK) Ltd

Mounting on sleeves h9/IT5

Housing F6

Lubrication

Oil lubrication is used. To keep the bearing temperature as low as possible with the minimum amount of oil in the bear­ing, the oil is lifted from the reservoirto a collecting trough, as the shaft rotates, by a pick-up ring which hangs loosely on a sleeve on the shaft and dips into the oil in the lower half of the housing. The oil then passes through the bear­ing on its way back to the reservoir.

IWV

Bearing arrangements using long housing cartridge assemblies

Deep groove ball bearings, paired angular contact ball bearings and cylindrical roller bearings have all been used in various combinations in two bearing cartridge housing assemblies. Such housings are available from the bearing manufacturers complete with their shafts, but are also manufactured by the larger fan manufacturers with special features to suit the application.

Figure 10.31 Two bearing cartridge assembly fitted with ball and roller bear­ings for grease or oil lubrication Courtesy of SKF (UK) Ltd

Perhaps the most common combination of races within a long housing is for a deep groove ball bearing at the impeller end and a cylindrical roller bearing at the drive end. (Figure 10.31.) The ball race “looks after” the end thrust whilst the cylindrical roller can take the radial load imposed by a vee belt drive. It twill

Figure 10.35 Cartridge assembly for heave radial loads (roller bearings) and ball race for location Courtesy of SKF (UK) Ltd

Be seen that grease or oil lubrication are both possible. However, many other combinations are available as shown in Figures 10.32 to 10.35.

10.9.2 Spherical roller thrust bearings

Spherical roller thrust bearings may be used in conjunction with deep groove all bearings, cylindrical roller bearings and spheri­cal roller bearings. When high axial forces have to be accom-

AJ

T — ^ — H — r —

«a

10.37 Spherical roller thrust bearing used for centrifugal fan with vertical shafts Courtesy of SKF (UK) Ltd

Shaft

10.36 Spherical roller thrust bearing for horizontal shaft fan Courtesy of SKF (UK) Ltd

Modated, it is sometimes necessary to use a thrust bearing for the support.

Figures 10.36 and 10.37 show respectively a horizontal and a vertical fan, each fitted with a spherical roller thrust bearing. In each case, the spherical roller thrust bearing is radially free and therefore only axially loaded; the housing washer is loaded by using several springs, equally spaced around the periphery, to prevent the bearing from separating when the fan is started or the thrust load reversed.

Tolerances

Deep groove ball bearings
D : 100 mm	K5
D > 100 mm	K6
Cylindrical roller bearings
D ‘ 140 mm	M5
D > 140 mm	N6
Spherical roller bearings
D : 140 mm	M6
D > 140 mm	N6
Spherical roller thrust bearings
All diameters	J6
Deep groove ball bearings
(with O-ring to prevent creeping)	H7
Cylindrical roller bearings	M7
Spherical roller bearings
(with O-ring to prevent creeping)	H7
Spherical roller thrust bearings	Clearance

Housing

Lubrication

Circulating oil lubrication is used for the bearings in the horizontal fan. Oil bath lubrication is preferred for the bear­ings in the vertical fan. The pumping action of the spheri­cal roller thrust bearing is utilised to ensure lubrication of both bearings in this arrangement.

Posted in Fans Ventilation A Practical Guide