ISO 14839-5:2022

Mechanical vibration - Vibration of rotating machinery equipped with active magnetic bearings - Part 5: Touch-down bearings

This document gives guidelines for identifying: a) The typical architectures of touch-down bearing systems to show which components are likely to comprise such systems and which functions these components provide; NOTE Touch-down bearings are also known as “backup bearings”, “auxiliary bearings”, “catcher bearings” or “landing bearings”. Within this document, the term “touch-down bearings” is used exclusively as defined in ISO 14839‑1. b) The functional requirements for touch-down bearing systems so that clear performance targets can be set; c) Elements to be considered in the design of the dynamic system such that rotordynamic performance can be optimized, both for touch-down bearings and active magnetic bearings (AMBs); d) The environmental factors that have significant impact on touch-down bearing system performance allowing optimization of overall machine design; e) The AMB operational conditions that can give rise to contact within the touch-down bearing system so that such events can be considered as part of an overall machine design. It also considers failure modes within the AMB system that can give rise to a contact event. This ensures that the specification of the touch-down bearings covers all operational requirements; f) The most commonly encountered touch-down bearing failure modes and typical mechanisms for managing these events; g) Typical elements of a design process for touch-down bearing systems including the specification of load requirements, the sizing process, the analytical and simulation methods employed for design validation; h) The parameters to be taken into account when designing a touch-down bearing system acceptance test programme including the test conditions to be specified and the associated instrumentation to be used to ensure successful test execution; i) The condition monitoring and inspection methods that allow the status of in-service touch-down bearings to be evaluated and when necessary identifying the corrective actions to be taken; j) The factors to be considered when designing the maintenance regime for a touch-down bearing system including the actions to be taken after specified events have occurred together with any actions to be performed on a regular basis; k) The factors to be considered regarding other life cycle topics (e.g. obsolescence management, de-commissioning and disposal).

Vibrations mécaniques — Vibrations de machines rotatives équipées de paliers magnétiques actifs — Partie 5: Paliers d'arrêt

General Information

• Status: Published
• Publication Date: 14-Aug-2022
• ICS: 17.160 - Vibrations, shock and vibration measurements
• Technical Committee: ISO/TC 108/SC 2 - Measurement and evaluation of mechanical vibration and shock as applied to machines, vehicles and structures
• Drafting Committee: ISO/TC 108/SC 2/WG 7 - Vibration of machines with active magnetic bearings
• Current Stage: 6060 - International Standard published
• Start Date: 15-Aug-2022
• Due Date: 27-Feb-2022
• Completion Date: 15-Aug-2022

Relations

• Consolidated By: ISO 25649-3:2024 - Floating leisure articles for use on and in the water - Part 3: Additional specific safety requirements and test methods for Class A devices
• Effective Date: 06-Jun-2022

Overview

ISO 14839-5:2022 provides guidelines for the specification, design, testing and life‑cycle management of touch‑down bearings (also called backup, auxiliary, catcher or landing bearings) used with active magnetic bearings (AMBs) in rotating machinery. It clarifies typical system architectures, functional targets and the operational conditions that can produce rotor‑to‑bearing contact, while addressing failure modes, environmental influences, rotordynamic design considerations, acceptance testing, condition monitoring and maintenance planning.

Key topics and technical requirements

ISO 14839-5 covers practical, engineering‑oriented topics to ensure reliable touch‑down bearing performance:

• System architecture - typical components and their functions within touch‑down bearing systems.
• Functional requirements - performance targets (clearances, design life, load capacity, life‑cycle needs) to drive specification and procurement.
• Rotordynamic design - modelling considerations for rotor and housing, clearance and soft‑mount design, and friction/contact effects to optimize dynamic performance of AMBs and touch‑down bearings.
• Trigger events and failure modes - events leading to contact (overload, AMB instability, power loss, system faults, misoperation) and common rolling element / sliding bearing failure mechanisms.
• Environmental factors - corrosion, erosion/particulate contamination, liquid contamination, temperature and cooling flow considerations that affect bearing selection and protection.
• Design verification & testing - elements of the design process, analytical and simulation methods, and parameters for acceptance test programmes including required instrumentation and test conditions.
• Condition monitoring & inspection - event detection, data capture, non‑intrusive and intrusive inspection methods, damage estimation and criteria for continued operation.
• Maintenance & life cycle - inspection plans, routine checks, refurbishment/replacement strategies, spare parts, obsolescence management, and decommissioning/disposal considerations.

Applications and who uses this standard

ISO 14839-5 is intended for engineers and organizations involved with AMB‑equipped rotating machines:

• OEMs and machine designers specifying touch‑down bearings and designing rotordynamics.
• Controls and AMB system integrators assessing failure modes and controller actions after contact.
• Maintenance, reliability and condition‑monitoring engineers developing inspection, testing and life‑cycle regimes.
• Test laboratories and quality assurance teams establishing acceptance tests and instrumentation.
• Asset owners and operators who need actionable guidance on failure response, repair, spare management and decommissioning.

Keywords: ISO 14839-5, touch‑down bearings, backup bearings, active magnetic bearings, AMB, rotordynamics, condition monitoring, maintenance, acceptance testing.

Related standards (if applicable)

• Other parts of the ISO 14839 series (see ISO 14839‑1 for definitions and related guidance).

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 14839-5
First edition
2022-08
Mechanical vibration — Vibration of
rotating machinery equipped with
active magnetic bearings —
Part 5:
Touch-down bearings
Vibrations mécaniques — Vibrations de machines rotatives équipées
de paliers magnétiques actifs —
Partie 5: Paliers d'arrêt
Reference number
© ISO 2022
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 2
3 Terms and definitions . 2
4 General structure and components .2
5 Functional targets . 4
5.1 General . 4
5.2 Design life . . 6
5.3 Clearance requirements . 6
5.4 Life-cycle requirements . 6
6 Touch-down bearing design considerations . 7
6.1 General . 7
6.2 Trigger events . 7
6.2.1 Overload due to abnormal process conditions . 7
6.2.2 AMB control instability . 8
6.2.3 Loss of power . 9
6.2.4 Failure in the AMB system . 10
6.2.5 Misoperation . 10
6.3 Transportation duty . 10
6.3.1 General . 10
6.3.2 AMBs without permanent magnets . . 11
6.3.3 AMBs with permanent magnets . 11
6.4 Failure modes . 12
6.4.1 General .12
6.4.2 Rolling element failure modes .12
6.4.3 Sliding bearing failure modes . 13
6.5 Environmental factors . 13
6.5.1 General .13
6.5.2 Corrosion resistance .13
6.5.3 Erosion resistance, particulate contamination . 14
6.5.4 Liquid contamination . 14
6.5.5 Operating temperature . 14
6.5.6 Available cooling flow . 14
6.6 Rotordynamic modelling considerations . 14
6.6.1 General . 14
6.6.2 Rotor and housing modelling requirements . 14
6.6.3 Touch-down bearing soft mount design considerations . . 16
6.6.4 Touch-down bearing clearance design considerations . 16
6.6.5 Friction between the rotor and touch-down bearing design considerations . 16
6.7 Contact classification/severity . 17
6.7.1 Contact duration . 17
6.7.2 Types of motion . 17
6.8 Control actions following touch-down bearing contact . 19
6.8.1 General . 19
6.8.2 AMB controller action . 19
6.8.3 Plant/variable-frequency drive control actions . 19
7 Design and design verification .19
7.1 General . 19
7.2 Design process details . 20
7.3 Documentation .30
8 Condition monitoring and damage estimation methods .30
8.1 General .30
iii
8.2 Event detection and data capture . 30
8.2.1 Contact detection . 30
8.2.2 Contact event .30
8.3 Inspection . 31
8.3.1 General . 31
8.3.2 Common techniques in non-intrusive inspection . 31
8.3.3 Intrusive inspection . 32
8.4 Damage estimation .34
8.4.1 Event data .34
8.4.2 Inspection-based estimation . 35
8.4.3 Criteria for further operation .36
9 Maintenance and life cycle factors.37
9.1 General . 37
9.2 Inspection plan . 37
9.2.1 General . 37
9.2.2 Minimum intervention cycle . 37
9.3 Interventions . 37
9.3.1 Routine checks . 37
9.3.2 Predicted excessive damage accumulation .38
9.3.3 Contamination .39
9.4 Maintenance actions . .39
9.4.1 General .39
9.4.2 Touch-down-bearing refurbishment .39
9.4.3 Touch-down bearing replacement .40
9.5 Life-cycle factors . . 41
9.5.1 General . 41
9.5.2 Spare part management . . . 41
9.5.3 Decommissioning, recycling and disposal . 42
9.5.4 Obsolescence management . 42
Bibliography .43
iv
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 108, Mechanical vibration, shock and
condition monitoring, Subcommittee SC 2, Measurement and evaluation of mechanical vibration and shock
as applied to machines, vehicles and structures.
A list of all parts in the ISO 14839 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
v
INTERNATIONAL STANDARD ISO 14839-5:2022(E)
Mechanical vibration — Vibration of rotating machinery
equipped with active magnetic bearings —
Part 5:
Touch-down bearings
1 Scope
This document gives guidelines for identifying:
a) The typical architectures of touch-down bearing systems to show which components are likely to
comprise such systems and which functions these components provide;
NOTE Touch-down bearings are also known as “backup bearings”, “auxiliary bearings”, “catcher
bearings” or “landing bearings”. Within this document, the term “touch-down bearings” is used exclusively
as defined in ISO 14839-1.
b) The functional requirements for touch-down bearing systems so that clear performance targets
can be set;
c) Elements to be considered in the design of the dynamic system such that rotordynamic performance
can be optimized, both for touch-down bearings and active magnetic bearings (AMBs);
d) The environmental factors that have significant impact on touch-down bearing system performance
allowing optimization of overall machine design;
e) The AMB operational conditions that can give rise to contact within the touch-down bearing
system so that such events can be considered as part of an overall machine design. It also considers
failure modes within the AMB system that can give rise to a contact event. This ensures that the
specification of the touch-down bearings covers all operational requirements;
f) The most commonly encountered touch-down bearing failure modes and typical mechanisms for
managing these events;
g) Typical elements of a design process for touch-down bearing systems including the specification of
load requirements, the sizing process, the analytical and simulation methods employed for design
validation;
h) The parameters to be taken into account when designing a touch-down bearing system acceptance
test programme including the test conditions to be specified and the associated instrumentation to
be used to ensure successful test execution;
i) The condition monitoring and inspection methods that allow the status of in-service touch-down
bearings to be evaluated and when necessary identifying the corrective actions to be taken;
j) The factors to be considered when designing the maintenance regime for a touch-down bearing
system including the actions to be taken after specified events have occurred together with any
actions to be performed on a regular basis;
k) The factors to be considered regarding other life cycle topics (e.g. obsolescence management, de-
commissioning and disposal).
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 2041, Mechanical vibration, shock and condition monitoring — Vocabulary
ISO 14839-1, Mechanical vibration — Vibration of rotating machinery equipped with active magnetic
bearings — Part 1: Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 2041 and ISO 14839-1 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 General structure and components
Rotating machinery equipped with AMBs is typically also equipped with touch-down bearings. These
touch-down bearings are intended to support the rotor when the AMB system is not activated or during
a failure or overload of the AMB system. In these instances, the touch-down bearings are required to
support the rotor until either levitation is recovered or the rotor is brought to zero rotational speed
without damaging to other parts of the machine.
During normal operation of the machinery, the touch-down bearings have a clearance with the rotor
and consequently do not apply force. The clearance at the touch-down bearings is typically the closest
clearance within the rotating machine. This ensures in the event of a problem with the AMB, when the
rotor moves away from its normal “centred” operating position, the first item to make contact between
the rotor and stator is the touch-down bearing. Such an event occurring during rotation is referred to
as a “touch-down event”, “landing event”, “contact event” or “drop event”. Such events have historically
been categorized by some vendors as either “hard” landings, where a full de-levitation from high speed
occurs or “soft” landings where either a partial de-levitation or a momentary contact occurs.
Touch-down bearings are required to constrain rotor movement in the degrees of freedom normally
constrained by the AMB system. In the case of a rotor with two radial AMBs and one axial AMB, the
touch-down bearings are required to constrain the associated five axes of movement. This is typically
achieved by using:
a) two radial touch-down bearings with a separate axial touch-down bearing;
b) two radial touch-down bearings, each with a single acting thrust face; or
c) one radial touch-down bearing with a combined radial/axial touch-down bearing.
Touch-down bearings use a range of technologies, such as:
d) stator mounted rolling element bearings;
e) rotor mounted rolling element bearings;
f) dry lubricated plain bushings;
g) dry lubricated pad construction;
h) foil bearings;
i) aero-static bearings;
j) fluid-film bearings; and
k) hybrids of technologies d) to j).
In most instances on large machines the touch-down bearing comprises of a rotor part (commonly
referred to as a landing sleeve) together with a stator part. The landing sleeve is intended to ensure that
no damage to the core shaft occurs on touch-down and typically is a replaceable item. An alternative
to the landing sleeve is to land directly on the shaft surface, which has a wear resistant coating or
treatment.
The stator part typically comprises a low-friction element, which contacts the landing surface and is
supported by a compliant element. The compliant element has an associated stiffness and damping
which is intended to improve vibration response during a touch-down event.
The compliant element can have these characteristics:
l) preload;
m) stiffness;
n) damping;
o) touch-down bearing hard-stop clearance.
When considering the minimum design clearance at any axial location, the total rotor motion at the
touch-down bearings, which includes the clearance and the touch-down bearing hard-stop clearance,
shall be considered together with other system stiffnesses and tolerances/concentricities. This is
discussed in Clause 6.
Schematic drawings of typical configurations are shown in Figure 1 and Figure 2.
Key
1 core shaft 6 preload spring
2 touch-down bearing housing 7 radial clearance
3 rolling element bearing 8 axial clearance
4 compliant element 9 landing surfaces (or sleeve)
5 hard stop clearance
Figure 1 — A typical configuration for a rolling element touch-down bearing installation
For dry bush type radial touch-down bearings as depicted in Figure 2, the landing surface may be
either a cylindrical bush (which moves as a single entity) or by articulated pads which are able to
move independently of each other. In both cases the moving element(s) are supported by one or more
compliant elements which provide both stiffness and damping within the hard stop clearance. In the
case of the pad arrangement, the compliant elements also provide a defined pre-load force. In both
cases the material of the landing surface may be optionally bonded to a backing material with suitable
mechanical properties.
The condition of the touch-down bearings can be of utmost importance in case of an AMB failure. The
touch-down bearings shall be able to safely bear the rotor during an event such as momentary contact
or a full rundown to standstill. The stringent operational demands, such as high acceleration rates and
high forces, lead to a very limited number of such events being allowed, thus the touch-down bearings
are considered consumable parts. However, replacing touch-down bearings which have not yet reached
the end of their lifetime should be avoided. Therefore, condition monitoring of touch-down bearings is
essential.
5 Functional targets
5.1 General
This clause introduces the functional requirements for a touch-down bearing system that would
typically be communicated by a customer of such systems to the vendor. These requirements comprise
the conditions under which the touch-down bearing system shall operate, the limits to rotor excursion
that the touch-down bearing shall protect, and the lifetime that the touch-down bearing shall sustain.
a)  Radial bushing
b)  Radial articulated pads c) Axial fixed pads
Key
1 core shaft 10 articulated pad (landing surface)
2 landing sleeve (optional) 11 rotor landing sleeve
3 air gap 12 pad stop
4 bush landing surface 13 compliant element with preload
5 bush backing material (optional) 14 rotor stator clearance
6 compliant element 15 fixed pad
7 touch-down bearing housing 16 pad backing material
8 hard stop clearance 17 pad landing surface
9 articulated pad (backing material) 18 fixing screw
Figure 2 — Typical configurations for dry bush touch-down bearings
5.2 Design life
Touch-down bearing design life requirements shall be agreed between the customer and vendor prior
to project execution and specific requirements can vary depending on the application. The touch-down
bearing system shall be designed to handle a minimum number of contact events without requiring
replacement or refurbishment. The agreed upon minimum number of contact events shall consider
both transient contact events (momentary contacts) and sustained contact events (rundowns) and the
maximum time period over which this capacity shall be sustained (effective service life of the touch-
down bearing).
Additional factors that can affect touch-down bearing design life requirements include, but are not
limited to:
a) the likelihood of abnormal process conditions;
b) hazardous environmental risks;
c) availability of a braking system (for sustained shutdown events);
d) power grid reliability;
e) the availability of uninterruptible power supplies (UPS) for backup power;
f) machine availability requirements (see 6.2).
It is important to recognize that the viability of the touch-down bearing can degrade over time due to
environmental factors even if no hard or soft landings occur.
A means shall be provided to estimate the status of the in-service touch-down bearing system in relation
to the design life requirements, without requiring a shutdown to perform the evaluation, see 8.3.
5.3 Clearance requirements
Permitted maximum motions within a machine depend on other close clearances within it (e.g. those
in labyrinth seals). The touch-down bearings are required to limit the motions to ensure there is no
unintended contact when running on the touch-down bearings.
Clearance control within the touch-down bearing will yield a maximum clearance requirement for it.
This ensures that there will be no unintended contact with other parts of the machine when operating
on the touch-down bearings. Clearance will depend on the assembly and manufacturing tolerances
within the complete rotor and stator assembly, differential thermal effects, load cases, and other
application specific forces and shall take account of the rotordynamic response when running on the
touch-down bearings and during transient events.
Permitted minimum clearances in the touch-down bearing will ensure no un-intended contact with
the touch-down bearing occur when the AMB is operating under specified conditions. This depends on
the manufacturing concentricity tolerances of the rotor within the machine together with the nominal
clearances at other locations within it. Lifetime factors such as contamination, differential thermal
effects and other application specific forces need to be considered and taken into account.
Where lifetime factors are known to the machine designer, they shall be communicated to the AMB/
touch-down bearing vendor as part of the AMB functional requirements.
5.4 Life-cycle requirements
The touch-down bearing system shall require inspection or replacement if the accumulated damage
approaches or exceeds the design life requirements. The requirement for inspection or replacement shall
be evaluated if the touch-down bearing system is operated outside its intended design requirements,
especially for an extended period of time.
Following inspection, the touch-down bearing system can require maintenance, which can include
refurbishment or replacement depending on the priorities at hand and spare parts will typically be
available in advance to minimize machine downtime. The vendor shall maintain a source of touch-down
bearing spare parts for an agreed period.
6 Touch-down bearing design considerations
6.1 General
This clause gives an overview of those items to be considered during the design of the touch-down
bearing system by the AMB/touch-down bearing vendor and is intended to support the transformation
of functional requirements into design requirements. Many requirements can be derived directly
from the functional requirements where such information is known by the machine designer, but in
many instances they need either to be synthesized by the vendor from the functional requirements
or where this information is not known by the machine designer, estimated by the vendor based on
prior experience. Touch-down bearings are included in AMB systems to provide backup or auxiliary
rotor support in the rare situations where the AMBs cannot completely control the rotor. These rare
situations are referred to as trigger events and are described in 6.2.
6.2 Trigger events
6.2.1 Overload due to abnormal process conditions
6.2.1.1 General
AMBs have a limited peak load capability characterized by saturation in their ferromagnetic pole
pieces. When loaded beyond saturation, the rotor falls out of support and needs to be retained by the
touch-down bearings to prevent damage to the machine. Overload can be due to an abnormal process
condition occurring or unexpected external loading source. In many cases, design margins can be
included in the AMB sizing to provide extra capacity for such events; however, it is important to avoid
providing substantial unneeded capacity in the AMB system. Oversized magnetic bearings can lead
to poor actuator bandwidth, undesirable rotordynamic characteristics, and less robust control. Some
common sources of overloading force are mentioned in 6.2.1.2 to 6.2.1.8.
6.2.1.2 Compressor surge
Although compressor surge is reasonably well understood, when it occurs, the affect on the AMB control
system of the amplitude of the imposed load, its excitation frequency and frequency of occurrence is
difficult to predict and depends on factors that are not always in the machine designer’s control. In
some cases, surge results in short-term or intermittent contact with the touch-down bearings (also
called touch-and-go) followed by recovery to continuous operation on the AMBs.
6.2.1.3 Shock from seismic events, explosions or external impacts
These events are difficult to predict and can vary widely in magnitude and bandwidth. An AMB
system can be designed to meet specific seismic requirements, but seismic events beyond the design
requirements usually result in touch-down bearing impact. Occasionally, AMB systems are subjected
to large shocks (e.g. resulting from explosions or external impacts), which are generally expected to be
absorbed by the touch-down bearings.
If a shock load event is expected during the system lifetime, its amplitude and duration shall be defined
in order that it can properly be taken account of in the AMB design analysis.
6.2.1.4 Sudden rotor unbalance due to the loss of solids built-up during process flows
Some turbomachinery processes result in a build-up of solid matter on rotor surfaces. Portions of this
build-up can flake off during operation resulting in a sudden unbalance. Often the resulting unbalance
is small enough to be acceptable for steady-state operation of the AMB, but the impulse created can
produce a short-term or intermittent contact on the touch-down bearings.
6.2.1.5 Sudden rotor unbalance from loss of a turbine blade or other partial failure
A failure of this type usually results in an unbalance load that is well beyond the capability of the AMB
system to tolerate and thus results in a substantial initial impact load being applied to the touch-down
bearings, followed by a full-speed spin down onto the touch-down bearings. For some types of machine,
a blade out failure is a design load requirement for the touch-down bearings.
6.2.1.6 Abnormal motor loads – phase unbalance
For machines driven by electric motors, a phase unbalance can result in a radial load on the rotor that
does not exist in normal operation. Sudden loss of a phase during operation can result in an impulse
load that overloads the AMBs, resulting in touch-down bearing contact.
6.2.1.7 Rub at machine close clearance
In a machine with an AMB control system, the touch-down bearing clearance shall be set such that any
excursion of the rotor from its rotational axis centre shall result in touch-down bearing contact before
touching any other stator element not intended to wear. Efficiency requirements in turbomachinery
encourage seal clearances to be set at minimal levels in many machine designs. However, unintended
rubbing contact with a seal can occur when the AMB system design has not allowed for adequate
clearance margin or seal concentricity relative to the touch-down bearing. Abradable seals are designed
for such contact and their use is permitted.
6.2.1.8 Liquid slugging
In some processes, a slug of liquid can be introduced in the machine causing a shock or impulse load
that results in touch-down bearing contact.
6.2.2 AMB control instability
6.2.2.1 General
The nature of AMB compensator design is such that there often are frequency bands or operating
scenarios where the AMB forces produce negative damping for one or more natural vibration modes of
the rotor/AMB/housing system. AMB design to avoid instabilities is covered in ISO 14839-3.
6.2.2.2 Inadequate control robustness to allow for process variation
Inadequate AMB control robustness can arise from a range of process variations that generate
destabilizing forces, such as:
a) fluid dynamic forces in compressors, turbines and labyrinth seals can be destabilizing under
certain conditions (often characterized by cross-coupled stiffness);
b) forces that are not adequately specified for the defined control scheme could not have the necessary
stability margin to keep one or more modes stable under all operating conditions;
c) process fluid density much higher than predicted can result in higher destabilizing forces;
d) variation in suction pressure results in higher destabilizing forces.
6.2.2.3 Lack of slew rate margin to control the dynamic loads
To respond to dynamic loads an AMB has to produce a certain control current at a required frequency.
As the frequency increases, the required voltage to push the desired current through, the control
increases. Since power amplifiers are sized with some specific overhead (or bus) voltage, the voltage
demand of a particular load can exceed the available overhead. In this case the current is limited by the
maximum dI/dt or current slew rate. This situation almost always leads to touch-down bearing contact.
6.2.2.4 Lack of power supply capacity to control the axial dynamic loads
AMBs generally impose very low real power requirements compared to other types of bearings.
Additionally, in most cases it is straightforward to provide an adequately sized power supply. However,
if process conditions impose unexpected axial dynamic loads, the AMB power requirements can exceed
the design case due to the difficulty in predicting eddy current losses (which use real power) in the
thrust bearing.
6.2.2.5 Operation of a machine outside of its design case speed range
In highly gyroscopic machines, such as those with single overhung impellers, the AMB control may be
gain scheduled. In this case the AMB control is adjusted based on spin speed. If the machine is operated
above the design speed one or more rotor vibration modes can become unstable, resulting in touch-
down bearing contact.
6.2.2.6 Unexpected machine acceleration/deceleration profile
During excessive acceleration/deceleration, fluid dynamic forces can be much larger than designed for.
6.2.3 Loss of power
6.2.3.1 General
AMB systems require an electrical power source to operate. Loss of electrical power results in
deactivation of the amplifiers and shutdown of the control system. If this happens, the rotor drops onto
the touch-down bearings. Generally, some type of backup electrical power is included as part of the
AMB control system design, so that the AMBs will operate to allow safe spin down when external power
is lost.
6.2.3.2 AMB systems with no backup power source
If no backup electrical power source is provided, a loss of power results in a rotor drop onto the touch-
down bearings at speed.
6.2.3.3 System with an uninterruptable power supply (UPS)
Many AMB systems have a UPS sized to allow spin down of the AMBs in the event of power loss. In these
systems, power loss should not be an issue unless there is a defect or failure of the UPS.
6.2.3.4 Systems that incorporate a motor/generator having a regenerative backup system
Such a system can generate enough electrical power to supply the AMB in the event of power loss;
however, the electrical power generator can drop out below a certain speed, often 20 % to 25 % of
maximum speed. Such systems generally have a relatively benign low-speed drop (e.g. without incurring
measurable system damage) as part of a system power loss event.
6.2.4 Failure in the AMB system
6.2.4.1 General
Failure in some part of the AMB system generally results in a drop and spin down on the touch-down
bearings. Depending on the action taken by the controller, this can be a drop of one or more axes up to
and including a full machine drop followed by a spin down request from the magnetic bearing controller
(MBC).
6.2.4.2 MBC component fault
Failure of the MBC power supply or a component or sub-system failure that stops the control program
always results in a five-axis drop. Other component failures generally result in loss of control on one or
more axes, followed by either a full or partial drop – depending on the control action taken.
6.2.4.3 Actuator or actuator cable failure
An actuator short or open circuit results in the inability to apply a reaction force to the rotor. This
results in a loss of control and is detected as an excess displacement, excess control current, or low
power supply voltage and results in a drop.
6.2.4.4 Transducer or transducer cable failure
A transducer failure can result in an undesirable control action that applies the full force of the AMB to
push the rotor onto the touch-down bearings, adding an additional impact and static load to the touch-
down bearing loading.
6.2.5 Misoperation
Misoperation of the system occurs when:
a) the process flow spins the rotor while the AMB is shut down, which results in the rotor running on
the touch-down bearings, possibly for some time;
b) there are operator or maintenance errors, such as:
i) input power turned off;
ii) cables become disconnected during operation;
iii) accidental cutting of cables during operation;
iv) incorrect installation of the touch-down bearings.
c) the system is sabotaged.
6.3 Transportation duty
6.3.1 General
The touch-down bearings may or may not provide adequate support to the rotor during transportation.
Where the rotor is not locked by other means this shall be addressed.
When a machine has its AMB control system turned off, the rotor is typically free to move throughout
the clearance defined by the touch-down bearings. This behaviour depends on whether the AMBs
incorporate permanent magnets.
Standards for testing response to mechanical shock, and vibration due to transport can be found in
a) ASTM D6344,
b) ISO 2244, and
c) ISO 13355.
Importantly, these standards include power spectral density data for typical shipping scenarios, so
they provide a basis for the evaluation of possible damage to the rotor or touch-down bearings during
transport.
In addition to mechanical shock considerations, transport can expose AMB equipment to corrosive or
abrasive environments unless they are sealed adequately.
6.3.2 AMBs without permanent magnets
In AMBs without permanent magnets, the rotor is free to move t
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