by William Hewitson, Vice President of Manufacturing and Engineering, Ruland Manufacturing Co., Inc
The keys to avoiding coupling failure are correct coupling selection using all application design criteria, proper installation, and periodic system maintenance.
Mechanical couplings typically connect rotating shafts to facilitate the transfer of rotary motion and torque. As with all mechanical devices, a coupling must match its intended purpose and application parameters, including different performance, environmental, use, and service factors. When selected with these parameters in mind, and when installed and maintained correctly, a coupling should have no failure issues over its lifetime. However, when all design parameters are not met a coupling can prematurely fail, resulting in either a small inconvenience or possibly financial loss or personal injury.
The most common coupling specification mistakes
As with all mechanical devices, a coupling must match its intended purpose and application parameters, including different performance factors. Downtime can seriously affect many processes. So it’s important to look beyond performance criteria and equally address issues such as the application environment, serviceability, maintenance and speed of replacement.
A common issue involves deciphering what manufacturers’ product specifications actually mean. For instance, when a manufacturer specifies axial load data, is that information stated as ‘failure mode?’ If so, under what conditions were the measurements determined? It’s important to fully understand the specifications as well as the design criteria for the system or application under review.
Another challenge involves the types and degree of any misalignment in the system or application under review. Is the misalignment angular or parallel? Is there axial motion? Do all three conditions exist … and to what degree for each? Proper selection of a coupling cannot be correctly made without a complete understanding of the misalignment being addressed in the system.
Problems that arise when the wrong coupling is chosen
A mismatched coupling for a given application can have a number of consequences. When one or more of the design criteria and overall system attributes in coupling selection is not met a coupling can fail, resulting in either a small inconvenience or something more serious, such as financial loss or human injury.
Two of the most important factors to address are torque and shaft misalignment. Failure to accurately consider torque, including starting and stopping torque as in servomotor applications, could result in the coupling failing in the application.
When shaft misalignment is a design factor a beam coupling is a good performance and economic choice. However, beam couplings are best in handling angular misalignment and axial motion. Beam coupling failure may occur in applications with parallel misalignment because the single beam must bend in two different directions simultaneously. These forces could cause premature coupling failure. When excessive parallel misalignment exists in the application other types of couplings should also be considered, such as bellows and Oldham, subject to the system design criteria.
How to choose the best coupling
Selecting a coupling for an application can be a complex process, but need not be excessively time consuming. Carefully consider all system design criteria, which includes: torque, shaft misalignment, stiffness, speed, inertia, space requirements, shaft mounting, and others. In addressing all of these, the coupling ultimately selected will perform as desired in the application.
Choosing the correct coupling is not the end of the job, though. It is equally important to install the coupling properly, verifying that design considerations were correct (for example: is there a greater degree of misalignment than originally specified?). Then, be sure to regularly maintain the application assembly to assure that design parameters have been consistently maintained and that no system component or coupling wear, contamination or other detrimental factors have been introduced.
To minimize the possibility of coupling failure – ask the following questions:
- Does the application require high torsional stiffness?
- What are the accuracy requirements?
- Does the application require dampening or shock absorption?
- How much misalignment is present in the design? Is it angular? Parallel? Axial? Complex?
- Does the coupling need to be the break-first point in the system? Does it need to be fail-safe?
- Is electrical isolation a requirement?
- What is the maximum torque applied to the coupling?
- At what speed or speeds will the coupling be operating?
- In what temperature will the coupling operate?
- Are there other environmental factors for the application (e.g. chemicals, wash down, vacuum)?
Consider all of the application requirements early in a design as this will reduce the risk of selecting the wrong type of coupling. Install the coupling properly, verifying that design considerations were correct. Last, regularly maintain the system to ensure that design parameters have been consistently maintained and that no wear, contamination or other detrimental factors have been introduced to any system components.
Ruland Manufacturing Co., Inc.