There are many aspects of babbitted bearings that need to be understood for their proper specification, inspection, installation, and troubleshooting. This article will attempt to cover some of these areas for babbitted journal and thrust bearings.
The simplest type is the “2-axial groove” sleeve journal bearing, commonly used today. These bearings are split into upper and lower halves and feature axial grooves to distribute oil evenly. Oil supply holes or slots feed these grooves. Bleed-off grooves ensure proper oil flow and cleanliness. Journal bearings have critical dimensions, including the inner diameter (ID) and outer diameter (OD). The ID, or bore, of the bearing, needs to be larger than the shaft diameter to have clearance. Normal design clearance is 1.5 mils per inch of diameter. The OD of the bearing is designed to match or be slightly larger than the housing it goes into. This size difference is called “crush,” which is essential for bearing operation. Even spherical OD bearings require some crush. Oil Ring Bearings is an alternative design to the 2-axial groove bearing is the oil ring bearing, where a rotating ring picks up oil and delivers it to the journal, eliminating the need for bleed-off grooves. Elliptical bore bearings are also used to ensure sufficient lubrication.
Pressure dam bearing
The pressure dam bearing is another common type of sleeve bearing used today, often employed in high-speed or low-load applications to address stability issues like oil whirl or shaft “whip.” This design incorporates a carefully machined “pocket” in the upper half of the bearing, which terminates abruptly at the “dam.” The purpose of this pocket is to create pressure that forces the shaft downward within the bearing. This is achieved as the oil dragged into the pocket is squeezed out under pressure. Pressure dam bearings are designed with a specific pocket depth, typically ranging from 10 to 20 mils deep. The abrupt termination of the pocket is crucial to create the desired pressure. This design aims to stabilize the shaft by preventing issues like oil whirl and shaft whip in high-speed or low-load applications. It’s important to verify the depth of the pocket during installation and maintenance. Additionally, inspect the dam for any sharp edges. While a sharp edge is not always necessary, it’s crucial that the pocket is not feathered out, and there should be no bleed-off grooves connected to it. In some pressure dam bearing designs, a relief track is machined into the lower half of the bearing. This track typically runs from one axial groove to the other. Its purpose is to disrupt the oil wedge formed between the journal and the bearing, further encouraging the shaft to ride lower within the bearing. The depth of this pocket is generally not as critical as that of the pressure dam pocket, as long as it exceeds 1/32″ deep. While specific details about pressure dam and relief track design are not covered in this context, it’s essential to understand that these designs are not arbitrary. Careful consideration should be given to any redesign, as they play a critical role in the stability and performance of the bearing in the given application.
Another widely used bearing is the elliptical (or lemon bore) bearing. This bearing is similar to the 2-axial groove design except the bore is machined into the bearing with precision shims at the split line. When these shims are removed and the bearing reassembled the resulting bore is larger horizontally then vertically. In this case, the vertical clearance may be somewhat smaller (to 1. 3 mils/inch for instance), while the horizontal clearance is often twice the vertical. The ends of the bearing are usually left round at the smaller clearance to help seal in the oil.
Other sleeve bearing designs
Other designs include 3 and 4 axial groove designs; 3 and 4 lobe bearings which take the elliptical bore bearing a step or two further. Multi-pocket bearings, tapered land or canted lobe bearings which further complicate the design by tilting these lobes for optimum performance, and offset half bearings which are bored with the upper half purposely offset from the lower half. All of these designs are relatively rarely used and if designs this complicated are required a tilting pad bearing is usually considered.
Tilting pad journal bearings
Most of this paper is about journal bearings but a brief discussion on thrust bearings is in order for completeness. Taper Land Thrust Bearing: Shown in the top drawing of Figure 1, this type belongs to the fixed geometry family of thrust bearings. It features babbitted segments that are tapered to create a diverging oil wedge. These bearings are rotation-sensitive, with each segment having a tapered section for load capacity during operation and a flat land section acting as a bumper thrust during low-speed/low-load intervals. Tolerance for flatness and parallelism between the back and babbitt face is crucial, with variations of 1/2 to 1 mil requiring repair before installation. The taper is optimized for specific load and speed conditions.
Tilt Pad Thrust Bearing: Shown in the center drawings of Figure 1, this type includes pads that are free to tilt. The ability to tilt in any direction allows these pads to align themselves optimally throughout the load and speed range. Similar to fixed geometry bearings, it’s essential to ensure flatness and parallelism between the retainer back and front (where the pads ride). Additionally, the pads should have a consistent thickness within 0.0005 mils and be lapped flat. Figure 14 depicts a non-equalized tilt pad thrust bearing with an upgrade featuring copper alloy pads for improved heat conductivity.
Fully Equalized Thrust Bearing: Shown at the bottom of Figure 1, this bearing design features offset pivots that enhance load capacity. Like the taper land bearing, it is also rotation-sensitive. The offset pivots contribute to improved load-carrying capability, making this bearing suitable for specific applications where high load capacity is required.
To conclude, this paper has attempted to cover aspects of babbitted bearings that can be directly applied in the field. lt is not meant to be an instruction manual and should not be used as such.
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Author: John K. Whalen, PE, former Manager of Engineering TCE (now Miba Industrial Bearings)