What is often called “axial alignment” of couplings is really not shaft alignment at all. Proper shaft alignment consists of offset (parallel) alignment and angular alignment, which is really the coupling gap difference (top to bottom and side to side). These measurements are vertical and horizontal. Axial spacing allows for sufficient space between the coupling hubs (and shaft ends) for proper coupling contact ( jaw to elastomer, “gear tooth” contact, etc.) while minimizing thrust load on the bearings and allowing for any slight axial changes in operating condition, such as thermal growth, or magnetic center “float.”
On some couplings, axial spacing is not critical (or even specified), while other types may have reasonably tight axial spacing tolerances. Let’s look into some of the more common types of couplings.
Spider Couplings
Image by LoveJoy
These couplings do not have critical axial spacing requirements but should be installed with a slight degree of axial movement once the alignment is completed. Many of these couplings have “spacer dots” molded into the insert.
These dots serve to set the proper axial spacing. When installing, the coupling hubs should be slipped toward each other until these dots make slight contact with the hubs. The hubs should then be locked into place using the set screws. These set screws should be torqued to the proper value. Consult the manufacturer’s installation instructions to confirm proper torque values.
Grid Couplings
Image by TB Woods
These coupling types have a more precise gap requirement. Generally, there should be a sufficient gap between the hubs to allow for axial thermal growth and end play. The teeth on the hubs should make good contact with the steel ribbon insert but not spaced so far that the “teeth” on the hubs contact the radius “turns” in the insert.
Common axial spacing requirements can vary depending on the manufacturer from 1/32 to ⅛” per inch of motor shaft diameter. Consult the coupling manufacturer’s installation instructions to be sure.
Omega© Elastomeric Couplings
Image by Rexnord
Axial spacing on the shafts is dependent on the mounting method. Depending on the mounting requirements, the hubs can be positioned inside, outside, or in a combination. However, axial spacing on the element is determined by the proper alignment of the bolt holes. The bolt holes in the element should align centered with the threaded holes in the hubs.
Consult the manufacturer’s instructions to be sure. Improper alignment can lead to uneven load distribution and premature wear. Ensuring correct spacing helps maintain optimal performance and extends equipment life.
Disc Pack Couplings (shim pack)
Image by LoveJoy
These couplings use laminated steel discs as the flexible element. Axial spacing is critical to minimize the machine bearings’ thrust load and prevent disc delamination, warping, and cracking.
Axial spacing tolerances can be (+/-) a few thousandths of an inch. Using a caliper, inside micrometer, or other method is recommended to check this dimension and control any changes during installation or shaft alignment.
Numerous other types of couplings are not listed here, but the moral of the story is this: follow the manufacturer’s recommendations whenever installing, replacing, or inspecting couplings. Alignment, both vertically and horizontally, is essential. But the third dimension – axial – can also be critical.
