Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)
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Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

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Rolling Element Bearings and Lubrications 535 Table 20-2 Influence of Lubrication On Service Life (Source: FAG Bearing Corporation) Oil Oil Grease Dry lubricant Rolling Rolling bearing with Rolling Rolling bearing gearwheels and other bearing bearing alone wearing parts alone alone Circulation with Circulation with Automatic filter, automatic filter feed oiler Oil-air Oil-mist Circulation without filter* Sump, regular renewal Sump, occasional renewal Oil-air Oil-mist Circulation without filter* Surnp, regular renewal* Rolling bearing (a) in oil vapour (b) in sump (c) oil circulation Sump, occasional renewal Rolling bearing (a) in oil vapour (b) in sump (c) oil circulation Regular regreasing of cleaned bearing Regular grease replenishment Occasional renewal Occasional replenishment Lubrication for-Sife Regular renewal Lubrication for-life * By feed cones, bevel wheels, asymmetric rolling bearings. ** Condition: Lubricant service life < Fatigue life. However, at least one large petrochemical company in the United States has expressed satisfaction with sealed ball bearings in small centrifugal pumps as long as bearing operating temperatures remained below 150°C (302 °F) and speed factors DN (mm bearing bore times revolutions per

536 Centrifugal Pumps: Design and Application minute) did not exceed 300,000 [15]. Studies showed, on the other hand, that close-fitting seals can cause high frictional heat and that loose fitting seals cannot effectively exclude atmospheric air and moisture that will cause grease deterioration. It should be assumed that these facts preclude the use of lubed-for-life bearings in installations that expect "life" to last more than three years in the typical petrochemical plant environment. Moreover, we believe this to be the reason why some bearing manufacturers advise against the use of sealed bearings at DN values in excess of 108,000. At least one bearing manufacturer considers the DN range between 80,000 and 108,000 the "gray'* area, where either sealed or open (shielded) bearing would be acceptable. The DN range below 80,000 is generally considered safe for sealed bearings. This would generally exclude sealed bearings from all but the smallest centrifugal pumps. Oil Viscosity Selection The Oil Viscosity Selection Chart (Figure 20-2) may be used as a guide for the selection of the proper viscosity oil. The chart may be used for applications where loads are light to moderate with moderately high conditions of speed and temperature. Applications of Liquid Lubricants in Pumps The amount of oil needed to maintain a satisfactory lubricant film in a rolling element bearing is extremely small. The minimum quantity required is a film averaging only a few micro-inches in thickness. Once this small amount has been supplied, make-up is required only to replace the losses due to vaporization, atomization, and seepage from the bearing surfaces [12]. How small a quantity of oil is required can be realized when we consider that 1/1000 of a drop of oil, having a viscosity of 300 SUS at 100°F (38°C) can lubricate a 50 mm bore bearing running at 3,600 RPM for one hour. Although this small amount of oil can adequately lubricate a bearing, much more oil is needed to dissipate heat generated in high speed, heavily loaded bearings. Oil may be supplied to rolling element bearings in a number of ways. These include bath oiling, oil mist from an external supply, wick feed, drip feed, circulating system, oil jet, and splash or spray from a slinger or nearby machine parts. One of the simplest methods of oil lubrication is to provide a bath of oil through which the rolling element will pass during a portion of each revolution. It can be shown that only a few drops of oil per hour would satisfy the lubrication requirements of a typical rolling element bearing in

Rolling Element Bearings <strong>and</strong> Lubrications 535<br />

Table 20-2<br />

Influence of Lubrication On Service Life<br />

(Source: FAG Bearing Corporation)<br />

Oil Oil Grease Dry<br />

lubricant<br />

Rolling Rolling bearing with Rolling Rolling<br />

bearing gearwheels <strong>and</strong> other bearing bearing<br />

alone wearing parts alone alone<br />

Circulation with Circulation with Automatic<br />

filter, automatic filter fe<strong>ed</strong><br />

oiler<br />

Oil-air<br />

Oil-mist<br />

Circulation<br />

without filter*<br />

Sump, regular<br />

renewal<br />

Sump, occasional<br />

renewal<br />

Oil-air<br />

Oil-mist<br />

Circulation<br />

without filter*<br />

Surnp, regular<br />

renewal*<br />

Rolling bearing<br />

(a) in oil vapour<br />

(b) in sump<br />

(c) oil circulation<br />

Sump, occasional<br />

renewal<br />

Rolling bearing<br />

(a) in oil vapour<br />

(b) in sump<br />

(c) oil circulation<br />

Regular<br />

regreasing of<br />

clean<strong>ed</strong> bearing<br />

Regular<br />

grease<br />

replenishment<br />

Occasional<br />

renewal<br />

Occasional<br />

replenishment<br />

Lubrication<br />

for-Sife<br />

Regular<br />

renewal<br />

Lubrication<br />

for-life<br />

* By fe<strong>ed</strong> cones, bevel wheels, asymmetric rolling bearings.<br />

** Condition: Lubricant service life < Fatigue life.<br />

However, at least one large petrochemical company in the Unit<strong>ed</strong> States<br />

has express<strong>ed</strong> satisfaction with seal<strong>ed</strong> ball bearings in small centrifugal<br />

pumps as long as bearing operating temperatures remain<strong>ed</strong> below 150°C<br />

(302 °F) <strong>and</strong> spe<strong>ed</strong> factors DN (mm bearing bore times revolutions per

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