# Example: Active Magnetic Bearing Selection for High-Speed Compressor (Mechanical Engineering)

**Status:** Draft
**Phase:** The Bedrock Phase

## What This Example Demonstrates

Context record structure (OE-0003) applied to a rotating machinery bearing selection decision, illustrating how competing failure mechanisms — contamination, fatigue, and rotordynamic stability — are weighed against each other and how the chosen solution eliminates a category of failure entirely rather than mitigating it.

## The Observation

Rotordynamic analysis of a centrifugal compressor rotor on conventional fluid-film bearings predicted two rigid-body critical speeds within the operating range of 15,000 to 25,000 RPM, forcing the machine to traverse two resonance zones during every startup and shutdown sequence. Process gas analysis revealed that the compressed hydrogen-rich synthesis gas is incompatible with conventional lubricating oils — a comparable facility using oil-lubricated bearings measured lubricant carryover into the process stream at 2 to 5 parts per million, exceeding the 0.5 ppm product purity specification by a factor of four to ten. Meanwhile, magnetic bearing vendor test data from an analogous 18,000 RPM compressor demonstrated rotor orbit amplitudes below 15 micrometers at all operating speeds with no mechanical contact during controlled coast-down, suggesting that a contact-free bearing system could simultaneously solve the contamination problem and the critical-speed traverse problem.

## Engineering Translation

Bearing selection for high-speed rotating machinery is governed by the interplay between the bearing's load-support mechanism and the consequences that mechanism introduces for the broader system. A fluid-film bearing supports the rotor through a pressurized lubricant film, which introduces the lubricant as a contaminant risk if the process gas cannot tolerate it. A rolling-element bearing supports the rotor through mechanical contact, which introduces fatigue as the dominant life-limiting mechanism at high speeds and temperatures. An active magnetic bearing supports the rotor through controlled electromagnetic forces with no physical contact, which eliminates both the contamination pathway and the mechanical wear pathway — but introduces a new dependency on the control system's reliability and power continuity. The engineering insight is that in this specific application, the dominant constraint is process gas purity (a hard specification limit at 0.5 ppm), not bearing life or rotordynamic complexity. This inverts the typical decision hierarchy: instead of selecting the simplest bearing that meets the load and speed requirements and then managing the consequences, the bearing must first satisfy the contamination constraint and only then be evaluated for the other performance requirements. This reasoning hierarchy must be preserved because a future engineer evaluating a bearing change for this machine — perhaps to reduce cost or simplify the control system — must understand that contamination elimination was the primary selection driver, not rotordynamic performance or maintenance interval (OE-0007).

## Context Record

| Field | Content |
|---|---|
| **Decision** | Select active magnetic bearings (AMBs) over hydrodynamic fluid-film bearings for a high-speed centrifugal compressor operating at 25,000 RPM. |
| **Observation** | (1) Rotordynamic analysis of the compressor rotor on fluid-film bearings predicted the first rigid-body critical speed at 8,200 RPM and a second at 19,500 RPM, both within the operating speed range (15,000–25,000 RPM), requiring careful speed transition management through two critical zones. (2) Process gas analysis showed the compressed gas (hydrogen-rich synthesis gas) is incompatible with conventional lubricating oils — contamination of the process stream by bearing lubricant carryover was measured at 2–5 ppm in a comparable facility using oil-lubricated bearings, exceeding the 0.5 ppm product specification. (3) Magnetic bearing vendor test data from an analogous 18,000 RPM compressor showed rotor orbit amplitudes below 15 µm at all operating speeds, with no mechanical contact during planned coast-down. |
| **Alternatives** | (A) Hydrodynamic fluid-film bearings with oil lubrication — rejected: two critical speeds within the operating range create operational constraints (prolonged speed ramps, vibration monitoring), and oil contamination of the process gas at 2–5 ppm exceeds the 0.5 ppm product specification. Gas-lubricated bearings were considered but could not support the required rotor weight at low speeds during startup. (B) Rolling-element bearings with dry-film lubrication — rejected: calculated L10 bearing life at 25,000 RPM with the applied radial and axial loads was 8,400 hours, below the 40,000-hour minimum maintenance interval. Dry-film lubrication degradation at the operating temperature (180°C) further reduced the projected life. (C) Active magnetic bearings — selected. |
| **Constraints** | Must operate continuously at 25,000 RPM. No process gas contamination above 0.5 ppm. Minimum maintenance interval of 40,000 hours (approximately 4.6 years). Maximum allowable vibration amplitude at the bearing locations is 25 µm peak-to-peak. Rotor mass is 340 kg. Available backup bearing clearance must accommodate a 15-minute coast-down from full speed without bearing damage. |
| **Reasoning** | Magnetic bearings eliminate the fundamental conflict between lubrication and process gas purity — there is no physical contact and therefore no lubricant to contaminate the process stream. The non-contact nature also eliminates wear, making the 40,000-hour maintenance interval achievable (magnetic bearings have demonstrated 100,000+ hour life in comparable applications). The active control system can modify bearing stiffness and damping in real time, allowing the rotor to pass through what would be critical speed zones on passive bearings without vibration amplification. The rotor orbit data from the vendor's analogous machine (below 15 µm) provides confidence that the 25 µm vibration constraint will be met. |
| **Verification** | (1) Factory acceptance test: full-speed operation at 25,000 RPM for 72 hours showed vibration amplitude of 11 µm at the magnetic bearing locations (below the 25 µm limit). Process gas analysis at the compressor discharge showed oil-free operation — total hydrocarbon content below 0.05 ppm. (2) Controlled coast-down test from 25,000 RPM: rotor landed on backup bearings at 2,500 RPM and decelerated to rest in 12 minutes (within the 15-minute constraint). Post-coast-down inspection of backup bearings showed no visible wear. (3) Rotordynamic sweep: vibration amplitude remained below 18 µm through the entire speed range (0–25,000 RPM) with no critical speed amplification peaks. |
| **Lineage** | Builds on rotordynamic analysis CR-MECH-2023-009 (bearing type screening for three candidates). Inherits the process gas contamination measurements from CR-MECH-2023-004 (comparable facility assessment). |
| **Assumptions** | The active magnetic bearing control system maintains stability through all transient conditions including sudden load changes and power supply disturbances (the backup power system provides 30 seconds of ride-through, verified during factory testing). The electromagnetic properties of the bearing laminations do not degrade over the 40,000-hour maintenance interval at the operating temperature of 180°C. The backup bearings (rolling-element, designed for coast-down only) are not subjected to frequent unplanned trips that would accumulate wear. |
| **Open Questions** | What is the failure mode of the active control system, and can the backup bearing system be designed to handle a full-speed drop without damage? How does the power consumption of the magnetic bearing system compare to the parasitic losses of fluid-film bearings at this speed and load? |

## Self-Fading Assessment

This example builds a bridge from the context record structure (OE-0003) to mechanical bearing selection, where the enduring lesson is that the decision hierarchy is determined by which constraint is hardest to satisfy — not by which constraint is most familiar. The reader has crossed this bridge when they can examine any bearing selection problem and immediately identify whether the primary selection driver is load capacity, speed, contamination tolerance, maintenance interval, or rotordynamic stability, and then structure the context record's Reasoning field to reflect that hierarchy. The deeper insight — that a contact-free bearing eliminates an entire category of failure rather than mitigating it — is the conceptual payload that makes this example self-fading: once the reader internalizes the distinction between mitigation and elimination as a selection strategy, the specific compressor application is no longer needed (OE-0007).