The bearing assembly supports the ROTATE motor drive shaft, which in turn transmits drilling thrust and rotational power to the drill bit. Drilling parameters, such as weight on bit (WOB), circulation rate, and bit pressure drop, directly affect the bearing assembly.
The ROTATE bearing assemblies is an open bearing, mud-lubricated design that utilizes two types of bearings: thrust and radial.
ROTATE uses a stacked multiple ball-and-race design for the thrust bearings.
The thrust bearings support the downward force resulting from the WOB and the loads from the combination of hydraulic thrust and weight loads from internal components.
ROTATE provides different bearing assemblies, designed to meet the diverse requirements of the drilling industry.
1) Bit nozzle pressure loss is 7.0 Mpa, ROTATE uses a stacked multiple balls design for the thrust bearings. The thrust bearings support the downward force resulting from the WOB and the loads from the combination of hydraulic thrust and weight loads from internal components.
Each bearing section utilizes high capacity roller thrust bearings. For all but the 3-3/4" motors, the thrust bearings are stackable in either a 4:2 or 3:3 on bottom/off bottom arrangements to optimize load capacities to expected drilling conditions.
Each thrust bearing is backed with a “Belleville style" spring washer which serves to evenly distribute the load over multiple bearings and provide shock load cushioning created while drilling.
All bearing components are held in compression within the assembly to prevent rotation. Rotating and stationary sleeves are pre-machined to length to provide proper Belleville spring preloading.
This stackable bearing arrangement serves to minimize diametric step changes along the shaft length thus reducing stress concentrations produced by shoulders and grooves. Each bearing section utilizes high capacity roller radial bearings.
Radial bearing sets are spaced apart with sleeves to provide higher radial load carrying capacities.
Bearing sets, top and bottom provide radial support to the drive shaft as well as the rotary seal preventing wear and extending seal life.
However, bearing component fatigue causes life to decrease linearly as speed increases; fatigue makes ball bearings unsuitable for high rpm motors. In addition to life, friction and horse-power losses are often of interest in drilling. Ball bearings are referred to as frictionless bearings due to the rolling nature of the elements; however in practice frictional losses exist due to rolling resistance and sliding.
An order of magnitude approximation of the coefficition of friction (CoF) for an angular contact bearing operation in ideal non-abrasive lubricant conditions is 0.0032. In drilling mud, the CoF can be assumed to be significantly higher.
2) Bit nozzle pressure loss is 14.0 Mpa, ROTATE uses PDC thrust bearings. It has longer life and higher bearing loading capacity.
Sliding Bearings (PDC Bearings)
Polycrystalline Diamond Compact (PDC) bearings have historically been utilized in high speed motors as they are not subjected to the same fatigue mechanism experienced by rolling element bearings. These bearings operate in a sliding manner and relative moving parts to transmit load. Common PDC bearings designs use an array of round PDC pads mounted to a ring.
In the case of PDC the value of coefficient of friction (CoF) can be estimated by 0.05 to 0.08.