Mud-lubricated bearings

Mud-lubricated tools for drilling have their own issues and challenges, for which QA Bearing™ Technologies Ltd. has developed some innovative solutions.

Issue #1
The traditional thrust bearing used in mud-lubricated downhole motors is a four-point angular contact ball-bearing stack, with a full complement of loose balls. This bearing is made up of a series of separable inner and outer races that need to remain in matched sets, with respect to both their position in the stack and their orientation. QA Bearing™ Technologies Ltd. places identification grooves on the ID and OD of its bearing races to aid this matching and orientation requirement when manufacturing this design.

However, the loose balls needed to make up an assembly must come from the same manufacturing batch, or there is a risk that a few oversized balls might carry the entire load. The handling of so many loose balls and races from different motor rebuilds often causes what is called "infant mortality," in which a rebuilt motor fails after only a few hours of operation.

Also, the typical contact angle for traditional four point-bearing stacks is 45°. This is because all four raceway shoulders must be high enough for ball contact. While the four points of contact provide a dual-acting bearing, with the same capacity in both on- and off-bottom load directions, the stacks are limited in their thrust capacity per row of balls.

To determine whether a bearing should be reused with existing balls, or whether a set of races should be reused with a new set of balls, the bearing must be stacked with the correct matching and orientation of all the races. The endplay of the assembly is checked to determine if it is within a set of predetermined limits. If the endplay exceeds these limits, a new set of balls should be used or the races scrapped and the new balls kept for another bearing. This takes time, effort, and often causes maintenance problems.

Solution: In order to address the maintenance and loose-ball concerns of many motor rebuilders, QA Bearing™ Technologies Ltd. has developed a unitized mud-lubricated angular contact ball bearing.

Issue #2
Recent innovations in downhole drilling tools demand that both radial and thrust bearings accept misalignment during operation. This misalignment is extremely hard on oil seals, particularly if the mandrel and the bearing housing are misaligned during steering operations.

There has been a resulting trend back to simpler mud-lubricated tools. However, the cages traditionally used in catalogued, self-aligning bearings don't perform well in abrasive lubricant.

Solution: To address this issue, QA Bearing™ Technologies Ltd. has developed a self-aligning bearing stack for mud-lubricated service.

Click Image to enlarge
4-Point angular contact mud-lubricated ball bearing stack	Unitized full complement angular contact ball bearing
Dual directional angular contact ball bearing stack	Dual directional thrust bearing stack with clamping across outer races in assembly
Dual directional thrust bearing stack with clamping across the inner races