While the subject has already been covered near exhaustively in Part VII of The Coupling Handbook, this post was written to cover a much higher level 30,000 foot perspective on the topic. If the discussion below leave you wanting more, please make sure you check out the rich and deep dive provided in the Coupling Handbook.
Please be aware that each level of
balancing requires additional manufacturing processes that will impact the
price of the final product and the lead time associated with the additional
manufacturing steps.
When coupling balancing is required,
normal balance options offered by Lovejoy and many other coupling manufacturers
generally conform to the specifications spelled out in the relevant
AGMA or ISO standard. The three primary types of
balance operation are generically described below.
Component Balancing
When
performing component balancing, the coupling components are balanced separately
from each other prior to assembling the complete coupling. This general
practice is normally accepted by most balanced coupling requirements.
Hardware used for coupling assembly is usually weigh balanced to prevent issues
caused by variances in the weight of bolts or other hardware. Weigh
balancing is defined with the Key Balance Definitions below.
The manufacturing processes for most
gear and disc couplings routinely produce components which possess some default
level of inherent balance usually defined in the manufacturer’s catalog under
allowable speed “unbalanced”. Any balance requirements for speeds higher
than this “unbalanced” value will cost extra and require extensions to lead
times. These catalogs typically provide an additional ‘balanced’ speed value
showing maximum speeds supported when additional balance operations are added
to the manufacturing process. Note that elastomeric type couplings
typically specify only a single speed capacity since the elastomeric couplings
have a limited level of balance that can be achieved. This is due to the
inability or difficulty of balancing an elastomeric insert or hubs with
clearance fit bores.
Determining the component’s inherent
balance as manufactured requires checking the components to the level covering
the published maximum speed. When specifying that a component is sold as
‘balanced’, it does not always mean that components were manufactured any
differently than other standard parts. Off-the-shelf components may already
satisfy some level of component balance. For example, Lovejoy disc
coupling components, as manufactured, have an inherent balance that conforms to
an AGMA Class 9 balance; Lovejoy gear coupling products, as manufactured, have
an inherent balance that conforms to an AGMA Class 8 balance. To meet
additional balance requirements, steel hubs machined with an interference fit
bore, as well as some spacer components, can be balanced to levels greater than
the inherent balance from the manufacturing processes.
Whenever customers specify some level
of balance possibly related to speeds greater than the published catalog
“unbalanced“ speed, or the maximum speed for elastomeric couplings,
manufacturers generally charge for this extra service. This is also true
if the customer requests the balance be checked, or confirmed for speeds equal
to or less than the published catalog “unbalanced” limit. Under this
circumstance, the extra charge is designed to cover steps necessary to verify
that the actual coupling balance meets the required balance criteria. Manufacturers will generally perform the hub balance verification after the
bore is made, but prior to machining the keyway.
With elastomeric couplings, the hubs
are typically machined with a clearance fit bore which is not ideal for
balancing. Non-metal components, hardware, and elastomers (spiders,
sleeves, donuts, etc.) typically cannot be balanced to the tighter levels
specified by AGMA or ISO, thus specific “balanced” speeds are not published in
the manufacturers’ catalogs. Even if the hubs are steel and manufactured
with interference fit bores, the addition of the elastomer de-rates the
benefits of component balancing.
Component
Plus Assembly (Hybrid)
This is a special requirement where
two balancing steps are required. This procedure is necessary for units that
will be operating at speeds in excess of the catalog “unbalance” rating for the
standard gear and disc coupling. In addition, the customer may require
balance confirmation beyond normal component balancing.
First, the components that meet a certain balance criteria, usually already
component balanced, are selectively assembled and machine checked to determine
the ‘assembly residual balance’.
Then the coupling will be checked with dummy half keys. Under this requirement,
no additional attempt will be made to change the balance, except for
adjustments to the selected components’ fit.
Full
Assembly Balance
Some
exceptional cases exist where extreme balance specifications, beyond the
inherent manufacturing level, must be met. These cases may require
couplings to be fully assembled prior to balancing, or what is referred to as
‘assembly balance’ (see Full Assembly Balance definition below).
The components in a coupling that is assembly balanced must be specially marked
to ensure the coupling can be reassembled with the exact same relationship or
orientation of one component related to the mating component should the
coupling ever be disassembled for service or installation. This type of
balance will obviously cost more and require a significantly longer lead
time. Prior to getting a quotation for a high-speed coupling, there
should be extensive communications between the customers’ and manufacturers’
engineering teams, and final balance specifications should be approved by both
teams.
Since assembly balancing is a special requirement, it is generally only
used for high-speed equipment. For example, this type of balance is often
used on high speed disc style couplings that need to meet API requirements, or
on high speed Lovejoy Sier-Bath gear couplings.
Full assembly balancing involves special consideration which often requires
specially designed parts, weigh balanced hardware, and extremely tight
machining tolerances. For gear couplings it also generally means assembly
balancing with slight interference fits at the gear tooth major diameters
(between hubs and sleeves), and subsequent re-machining for actual operation in
the final equipment.
Special consideration must be taken when servicing this type of balanced
coupling. Replacement parts cannot be installed in this type of coupling
unless the new component meets the original component balance criteria.
When the coupling is reassembled, the final assembly must be re-balanced and
components “match marked”.
Broadly speaking, assembly balanced coupling units are assembled with
balanced component parts, balance corrections are made on this assembly to
bring the unit into final compliance, and hub keyways (if required) are added
to the bore after final balance.
Key
Balance Definitions (relevant when talking with a coupling manufacturer)
- Inherent Balance: The balance of the part as designed and
manufactured without additional balance operations or correction.
- Residual Balance: The balance as the part is on the balancing equipment
after the balancing operations, or corrections are completed. It is the
value specified to the balancing equipment operator.
- Potential Balance: The part’s balance state before installation in the
customer’s equipment. It is what the customer originally specified.
- Weigh Balance: This is the process where coupling hardware is
weighed to create a complete set of bolts (and other hardware) where the
hardware items all have identical weights.
API
671 & API 610 Coupling Requirements
The American Petroleum Institute has
specific standards that specify unique application requirements that can be met
using one of Lovejoy's disc couplings. Blog articles have already been
published on this website related to the API 610 requirements and API 671 standards.
This post will not cover
these specific subjects other than to remind readers that the API standards
typically define specific requirements regarding coupling materials, spacer
lengths, anti-flail, metal flex member, and special documentation that are
required in addition to special balance requirements. API 610 and API 671
generally require a disc style coupling, such as the Lovejoy DI series, necessary to meet the intent
of the API standards.
