Edited by Kenneth J. Korane
"Design bearings that don't seize,"
When equipment relies on sleeve or journal bearings, the lower the operating temperature, the better. Cooler running extends oil life, reduces differential thermal expansion of the journal, and causes fewer alignment problems.
So taking steps to bring down the operating temperature is sometimes necessary and often desirable for oilfilm bearings. Engineers have a number of means at their disposal to cool bearings and reduce friction, resulting in lower temperatures. But not all are equally effective.
Here’s a look at several options for cooler-running bearings and the benefits each method brings. Specifically, we test how each technique relates to temperature rise in 4 and 8-in.-diameter bearings. Results show that in general, the two sizes have similar performance except at the highest temperatures with the larger bearing. That’s because the oil film traverses a longer circumferential path in converging to the high-load zone at the minimum film thickness, and this tends to generate more heat.
The simplest way to lower bearing temperature is using lower-viscosity oil. The Viscosity and Oil Performance table illustrates typical effects when ISO viscosity grades for industrial mineral oils drop from VG 100 to VG 68, 46, and 32.
And the viscosity-versus-temperature graphic illustrates the values in the table for two bearing temperatures. First, the maximum, or peak, temperature in the zone of minimum oil-film thickness. And second, overall mean-film temperature as reflected by the journal temperature, commonly monitored by the bearing-oil drain temperature. Technicians can usually monitor sleevebearing temperature with a thermocouple embedded in the babbitt liner about 30° beyond the vertical load line.
The cooling effects of lowering viscosity are somewhat limited. As overall temperature drops, corresponding viscosity in the oil film itself rises. For instance, data for the 8-in. bearing demonstrate that changing from VG 100 to VG 32 representing more than a threefold change in the oil’s 40°C viscosity lowers the maximum bearing temperature from 199 to 171°F. That change is only 35% less than the rise from ambient with VG 100 oil.
Nevertheless, low-viscosity-grade VG 32 oil is usually advantageous in high-speed machinery running at and above 1,800 rpm. In addition to somewhat lower bearing temperatures, it can also considerably cut power losses in filters, oil coolers, and piping throughout the lubrication system.
The Effects of Oil Feed Temperature table shows that lowering the inlet-oil temperature is of marginal value in reducing maximum bearing temperature. While dropping the feed oil from 120 to 80°F does lower mean bearing temperatures 18°F for both the 4 and 8-in. bearings, their maximum temperatures only fall 4 and 5°F, respectively. The drop in mean bearing temperature is usually about half that of the oil-inlet temperature reduction. The other half of the potential drop in bearing temperature is negated by higher oil viscosity at the lower temperature.
The effect of excess oil flow through the bearing is similar to that of lowering oil-feed temperature. After initially reducing oil temperature in the bearing feed grooves, most of the additional oil leaks from the sides of the bearing when the oil converges into the minimum- film-thickness zone. Leakage generally increases power loss in the bearing and keeps bearing temperature from dropping despite the cooler oil film.
Radial bearing clearance
Operating clearance can have a pronounced influence on bearing performance. The Radial Clearance Effects table and the accompanying Clearance-versus-temperature graphic illustrate the cooling effect as radial clearance increases. Doubling the clearance cuts peak and mean temperatures by 28°F for the 8-in. bearing. Tighter clearance leads to higher peak and mean temperatures.
If nominal clearance is too small, the journal can expand as it warms and become constrained in its cooler housing. At the extreme, this sometimes leads to seizure. The effect is especially pronounced at start-up because the shaft expands rapidly before the bearing housing reaches thermal equilibrium.
On the other hand, too much clearance can lead to unbalance and other rotor-vibration instabilities. To minimize temperature rise while avoiding undue vibration, experts often recommend diametral clearance on the order of 0.0015 to 0.002 in./in. of diameter. These numerical values are the same as the C/R ratios of radial clearance to bearing bore radius given in the illustrations.
Changing the bearing’s internal geometry can also lower operating temperature. The accompanying Bore Modifications graphic shows two such shapes. The first variation eliminates much of the unloaded bearing surface by recessing an overshot circumferential groove in the upper half of the bearing. This feature essentially eliminates any oil-film power loss in the upper region, while feed oil entering from the rising-side oil-inlet groove cools the upper shaft surface.
At low speeds where oil flow is laminar, circumferential grooves offer minor benefits because the upper half of a conventional split-sleeve bearing normally experiences little power loss. With oil-film turbulence at high surface speeds, however, as encountered with 3,600-rpm bearings above about 16-in. diameter, power loss is essentially proportional to the total oil-film area. An overshot groove half the bearing length cuts the oil film area 25% with a corresponding reduction in power loss and temperature rise.
The second bearing modification involves machining a slightly enlarged bore in a split bearing. This generates an elliptical, or lemon-shaped, bearing and typically gives an internal diametral clearance ratio C/R of 0.0025 to 0.0030 in./in. of diameter. The bore is machined after placing separating shims in the splits at both sides of the bearing. For an 8-in.-diameter bearing, for instance, technicians might use 0.010-in. shims between the bearing halves while machining an 8.024-in. bore. Removing the shims after final machining leaves a horizontal diametral clearance of 0.024 in. (C/R = 0.003) for an 8-in. shaft and a vertical clearance of 0.010 in. (C/R = 0.00125).
The resulting C/R of 0.003 for curvature in the loaded lower half of the bearing feeds more oil to the bearing’s loaded zone, resulting in a desirably smaller temperature rise as indicated in the Clearance-versus-temperature graph. At the same time, tighter clearance in the unloaded upper half of the bearing provides greater vibration stability associated with low internal clearance with little additional self-loading or temperature rise in the lower bearing half.
The final table, Effects of Bearing Modifications, summarizes the general magnitude of cooling that engineers can expect for bearings in industrial equipment running at 3,600 rpm and operating under the typical conditions listed in the Viscosity and Oil Performance table.
These comparisons indicate the bearing-temperature can be cut somewhat by switching to lower-viscosity oil, reducing oil-feed temperature, or doubling oil-feed rate. Larger bearing-bore clearance can produce a significant temperature drop. And shifting to an elliptical bore generates a further and more reliable drop.