Coupling Balancing Options - What You Should Know

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.

For further questions on flexible coupling balancing, please see The Coupling Handbook, Part VII, the standards referenced and linked above, or contact Lovejoy Technical Support.