Plain bearings

Very small fans may have the simplest of bearings consisting of a plain sleeve in which the shaft rotates. The sleeve material may be sintered brass or phosphor bronze impregnated with a lubricant. If oil is the lubricant, a felt pad may be incorporated as an oil reservoir. Plastics materials may be used where the pres­ence of oil is prohibited but these may not be suitable for high speed. PTFE impregnated bearings are also used on small fans and provide good performance over a wide operating range. Graphite sleeves can be used in locations where other materials are sometimes not suitable.

The shaft and bearings need to be manufactured to tight toler­ances for optimum performance, the shaft usually being hard­ened and polished. The bearing sleeve may have a spherical seating to overcome misalignment and a flange to accommo­date limited axially loading.

Sleeve bearings

For other than the smallest of fans the above arrangement is not an acceptable system and rolling bearings are universally used on most other small and medium size fans. On the largest fans and some ultra-quiet fans, sleeve bearings with a lubrica­tion system may be favoured particularly as the life can be su­perior to that of rolling element bearings. The complexity of sleeve bearings and sometimes the need for a separate cooling system make the cost greater than that of rolling element bear­ings. Sleeve bearings of this type are generally only suitable for horizontal running.

In the case of the disc, a lip ensures that oil is picked up and contained within the outer part of the disc by centrifugal force action and then a scoop extracts oil from the lip region to top up the oil chamber above the bearing. The oil reservoir can have sufficient surface area to ensure the oil temperature is kept within limits and large bearings will usually have this outer sur­face provided with cooling fins. In the case of large, high-speed fans (approximately 2000 kW and above) a separate cooling fan driven off the main fan shaft and blowing air over the reser­voir may be required. Alternatively, the oil is pumped through a separate cooler, or cooling water pipes are incorporated in the reservoir. On high pressure, high speed fans, even at only mod­erate power the bearings may be forced lubricated from a separate oil lubrication system with its own pump.

For the bearing to operate, the oil must form a wedge between the journal and the sleeve. This oil wedge is not present imme­diately after start-up and so rubbing between the journal and sleeve surfaces will occur until sufficient speed is reached. At start, the shaft journal will tend to climb up the side of the sleeve and draw oil in to form the wedge. At very low speeds some wear will take place, but normally a transition speed is quickly reached with partly metal-to-metal contact and some oil film present before a full, load-bearing, oil wedge is established. The wedge is formed because the journal is running eccentric with respect to the sleeve and so the shaft centreline position can vary between stationary, start-up and running conditions. The journal-to-sleeve clearance (normally referred to as “bear­ing clearance”) is small and the different shaft positions can be accommodated by the shaft system and coupling.

Plain sleeve bearings can exhibit a whirling action within the bearing whereby the journal, in addition to the normal rotation, rotates about a centre offset slightly from the geometric centre. It arises because the journal may try to roll around the inside of the sleeve. This is often at half the shaft rotational speed, and is known as “half-speed whirl”. It is particularly evident if the jour­nal bearing is lightly loaded, as may be the case with a verti — cal-shaft fan — using plain sleeve bearings — this is one reason why such bearings are rarely used on vertical motors. It may also occur with narrow high speed centrifugal blowing fans. In some cases shaft whirling may give rise to unacceptable vibrations.

Whirling can be overcome by using non-circular sleeves, either in the form of lobes or wedge shapes as shown by the examples in Figure 10.6 These shapes may be confined to a limited axial length at the centre of the bearing, essentially forming shallow pockets and leading to the name “pocket bearings”. Where wedge shapes are used only one direction of rotation is possible.

 

1.

Block

Z

Cap

3.

End Covers

4.

Sphere

5.

Liner

6.

Thrust Washer

7.

Oil Inlet

8.

Oil Outlet

 

Figure 10.4 Ring-oiled sleeve bearing

 

1. Uadi 7

T Cm /

X MComti

Plain bearings

3 4 1

Figure 10.5 Water cooled sleeve bearing

 

Because of their special nature, bearings of this type are often designed and manufactured by the fan company itself. How­ever, some transmission suppliers have also entered the field, and typical ring-oiled sleeve bearing plummer blocks are shown in Figures 10.3, 10.4 and 10.5.

A table of typical applications of sleeve bearings for large fans is shown in Table 10.1.

 

Fan application

Lubrication/

Cooling

Bearing

Diameter

Fan speed rev/min

Radial load N

Thrust load N

CFB

Fluidising air

Oil circulation

90

3565

4000

1000

Steelworks

B. O.S

Oil circulation

125

1445

22000

3000

Boiler

Forced draught

Ring oiled Water cooled

125

1485

7000

12000

Boiler primary air

Ring oiled Water cooled

140

1490

20000

14000

Boiler

Gas recirculation

Ring oiled Water cooled

180

743

37000

3000

Boiler

Forced draught

Ring oiled Water cooled

180

743

68000

4000

Boiler

Induced draught

Ring oiled Water cooled

200

990

54000

4000

Steelworks sinter waste gas

Oil circulation

250

1000

112600

5000

Boiler

Induced draught

Oil circulation

300

740

178000

15000

Table 10.1 Typical applications of sleeve bearings for large fans

Courtesy of Howden Group

 

Figure 10.6 Examples of non-circular sleeve shapes

 

Figure 10.7 shows a schematic diagram of a plain sleeve jour­nal bearing lubricated by means of a single ring in an oil reser­voir.

The bearing sleeve is shown as fitting into a spherical seating which is the usual practice on large bearings of this type. At ei­ther end of the bearing enclosures, seals — often labyrinth seals — are embodied. The shaft can slide axially within the bearing and this end float is typically ±5 mm.

 

Plain bearings Plain bearings Plain bearings Plain bearings

The manner in which persistent, positive and indestructible pressure-oil-films are produced and maintained between the bearing surfaces is clearly shown in Figures 10.8 and 10.9. Fig­ure 10.8 illustrates the action in a Michell thrust block and Fig­ure 10.9 shows a similar process taking place in a Michell journal bearing.

Figure 10.7 A schematic diagram of a plain sleeve journal bearing

подпись: 
figure 10.7 a schematic diagram of a plain sleeve journal bearing
It will be observed that the tapered pressure-oil-film or wedge of lubricant is self-generated by the mere motion of the shaft or collar and is not dependent on any extraneous pressure from an oil pump.

All Michell bearing pads, whether for thrusts or journals, are so designed and proportioned that they tilt and float the load on their own oil films. The stream-like photograph in Figure 10.10 shows how some of the lubricant escapes at the sides of a Michell thrust pad leaving the remainder to feed the trailing edge.

MOVING COLLAR

Figure 10.8 Michell thrust pad Courtesy of Michell Bearings

подпись: moving collar
 
figure 10.8 michell thrust pad courtesy of michell bearings
Plain bearings

HOUSING

подпись: housing

Figure 10.9 Michell journal pad Courtesy of Michell Bearings

подпись: figure 10.9 michell journal pad courtesy of michell bearings

Tilting pad bearings

The ultimate extension of film lubrication my be seen in the tilt­ing pad bearing, first introduced by the British engineer A. G.M. Michell, FRS, when working in Melbourne, Australia.

General principles

When a well-lubricated journal bearing runs with normal clear­ance between shaft and bush, a tapered oil film is naturally formed, the thinnest portion of it being that under the load. As the shaft turns, oil is drawn in to feed this wedge (some, of course, being squeezed out at the sides) and an internal oil pressure is set up in the film exactly balancing the bearing load. The faster the shaft revolves, the more oil is drawn in and the thicker and stronger the film becomes.

Moreover the internal film pressure builds up from zero to a maximum just where it is wanted at the point where the load is greatest. But only about one-third of a journal half-brass is re­ally effective. Obviously therefore if each redundant side can be cut out and replaced by a pad which can help to share the total load, the bearing will be much more efficient and this is what is done in the Michell Bearing.

Plain bearings

Figure 10.10 Stream-lines of oil flow in tilting thrust pad Courtesy of Michell Bearings

It is natural to suppose that, as there is no metallic contact, it is unnecessary to white-metal the faces of Michell thrust and jour­nal pads. The reasons for so doing are because white metal is the least liable to damage from minute particles of grit and for­eign matterwhich occasionally find theirway into the lubricating oils of even the best kept systems; and also during the bound­ary conditions (or partial lubrication) when starting and stopping.

Tilting pad thrust bearings

The thrust bearing functions on the lines just described. Natu­rally, flat thrust surfaces cannot adapt themselves (as does a journal bearing) to create any form of tapered oil film, so Michell conceived the idea of dividing the thrust carrying surface into a number of pads, each pad being supported — not by a flat abut­ment — but by a pivot or step which allows it to tilt slightly. As the thrust collar revolves in its oil bath, the oil adhering to its surface is carried round and lifts every pad at its leading edge to admit the tapered oil film. Thus each of the pads round the thrust col­lar generates a tapered pressure oil film of a thickness appro­priate to the load, the speed, and the viscosity of the lubricating medium.

The position of the pivot, which is the edge of a radial step on the back of the pad, is of some importance. For maximum effi­ciency — in other words minimum friction — the pivot is beyond the centre of the circumferential width of the pad measured from its leading edge, and these pads are termed “off-set”, being right or left-handed to suit the direction of rotation.

The Michell thrust bearing is a simple single-collar unit capable of carrying at least 20 times the load per unit area of a flat multi-collar thrust bearing, with only about one twentieth of the frictional loss. No subsequent adjustment is required when once the thrust bearing is installed and the entire absence of

Wear at all speeds, even when overloaded, makes it one of the most reliable pieces of machinery.

Tilting pad journal bearings

It is clear that when effective films are induced at other parts of the circumference than that just underthe load, the carrying ca­pacity of a journal bearing is correspondingly increased.

As in tilting pad bearings, the same principle of segmental pads is adopted in Michell journal bearings. The usual pair of solid brasses gives place to a series of pads, generally six in number, surrounding the shaft journal. Each pad is free to tilt slightly in its cylindrical housing and is prevented from cross-winding by suit­able flanges engaging the machined ends of the housing. Oil is automatically introduced between each pair of pads from an an — nulus in the housing and any surplus that is not carried all the way across escapes naturally at the ends of each pad. As the shaft revolves, all the pads tilt to admit oil along their leading edges, and each one thus creates its own characteristic tapered oil film.

At speed, the shaft thus becomes surrounded by a close-fitting oil garter, constantly renewing and maintaining itself, which un­der the severest conditions of load and shock, has never been known to fail. Loads up to and exceeding 360 kgf/cm2 of pro­jected surface have been registered experimentally, and pads, after many years of hard service, have shown no signs of wear for the very good reason that metallic rubbing contact has never occurred.

The load carrying capacity of such bearings is enormously greater and the friction much less than the best solid brass types, and they can be made much shorter in consequence. This is often a matter of supreme importance where space and weight are restricted.

For ordinary conditions of bath lubrication, journal bearings are provided with a light collar secured to the shaft in halves and dipping into an oil well below. Oil is lifted over the top centre by this revolving collar and the resulting spate of oil guided to the top of the bearing and into the oil annulus feeding the pads. No packed end glands are necessary, any surplus oil being pre­vented from creeping out along the shaft by special oil deflec­tors fitted at the ends of the bearing. These bearings are en­tirely self-lubricating and self-contained and can be adapted for certain duties where automatic functioning for prolonged periods without attention is a requirement.

Load carrying capacity of tilting pad bearings

The load that can be safely carried on the oil films of a tilting pad bearing depend on its diameter, length, peripheral speed and oil viscosity. The load carrying capacity also increases with the revolutions, and loads exceeding 400 kgf/cm2 have been sus­tained on prolonged tests. These bearings are in successful op­eration at all speeds ranging from five revolutions per minute, up to the highest speeds encountered in modern fan technology.

Friction losses

In the foregoing it has been impossible to ignore friction entirely — there must be friction in every type of bearing. Tilting pad bearings howeverare unique in that whatever friction there may be, it is never metallic friction but simply oil friction. In other words, the only resistance to relative motion between shaft and bearing pads is that required to shear the intervening layers of oil comprising the film. This resistance is a measurable quantity and can be calculated from the rotational speed, pressure and oil viscosity. Certain experiments with a bearing loaded to 40 kgf/cm2 gave a coefficient of friction (n) of 0.0020 against a cal­culated figure of 0.0022 — near enough for all practical pur­poses. The coefficient of friction of a good ordinary bearing is

0. 036 — about eighteen times as much. The coefficient of friction in tilting pad bearings ranges from.001 to.005 and varies with the factors mentioned above. When starting under load, the friction is naturally considerably greater for the first half revolu­tion, by which time the oil film is generated.

The heat generated in a tilting pad bearing is affected more by speed than load and there are three methods of dissipating the heat.

1. Air cooling by natural radiation. This covers the major­ity of applications of moderate speed.

2. Water cooling, which becomes necessary at higher speeds.

3. Circulated oil, which is required for the highest speeds.

In the first case air cooling is obtained by means of suitable ex­ternal ribs on the bearing casing.

In the second case the self-contained oil in the bearing casing is kept cool by means of a water jacket incorporated in the hous­ing or by water passing through solid drawn coils or tubes in the oil well.

In the third case the oil is pumped through an external cooler in the oil circuit.

It should be noted that when circulated oil is used it is not neces­sary to have a high oil pressure at the pump. All that is required is sufficient to ensure a free flow through the circuit of the amount required for cooling. Forced lubrication, as usually un­derstood, is not necessary, the oil pressure in the films being generated by the action of the tilting pads.

Posted in Fans Ventilation A Practical Guide


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