Precision maintenance personnel ensure that rotating machinery is aligned well beyond excellent shaft alignment tolerances, but overlook other crucial elements that affect the operation of the machines.
One of these elements is the coupling. The coupling is where the “handshake” between the driver and driven machine takes place. The coupling assures consistent power transfer and allows for flexibility in both the axial and radial directions. Proper application, installation and maintenance of the coupling are topics that appear to not get the attention that they deserve.
Application
Many alignments are completed without question as to whether the coupling in use is indeed the correct one for the application. The questions that should be answered are:
- Is this the original coupling from the manufacturer or did it get changed due to lack of replacement parts for the original?
- Was this the correct choice in the first place?
- Has the process changed such as increased load, increased starts & stops?
We as mechanics regularly change out a motor or pump then reinstall the coupling hub off of the removed equipment. This could actually be the cause of future machine or coupling failures.
There are resources available at your fingertips that require the input of minimal information so the coupling selection can be verified or corrected. One app is from LoveJoy Couplings. You enter the Horsepower, RPM’s and Service Factor. Then choose which coupling type and it will give the specs of size, torque and max bore sizes. This app is available from the iTunes and Google Play stores. Another source is the application guide included with a coupling.
A perfect example of “the wrong coupling for the application” presented itself when a recent training class aligned a pump/motor set with a Grid Flex coupling. Before preforming the precision shaft alignment the crew checked the directions that came with the coupling and determined that a gap of 125 mils (1.0 mil = 0.001”) was required between the hubs for proper installation.
When they corrected the misalignment to meet vertical and horizontal tolerances the gap between the coupling hubs was almost immeasurable. When they would set the coupling gap first and maintain it, they could not get a good alignment. The coupling hubs were of a design that did not allow for repositioning on the shafts.
It was determined that this was not the proper coupling choice for this application. At the conclusion, the crew was utilizing manufacturer sites to determine the appropriate replacement.
Installation
Proper installation has many facets. Too many to cover for every coupling type. In my training classes I ask the mechanics if they have seen the piece of paper that comes in the box with coupling hubs or inserts. I ask them if they know what the paper is for. It’s good for a laugh and leads to a discussion about how most of us never look at the instructions that are included with the couplings. The instructions includes application, installation and maintenance specifications. If the installation guide is followed, many future problems would just disappear. If you don’t receive an installation guide, it is well worth the time to research the proper specifications and procedures for proper installation.
Maintenance
What maintenance is required? I will answer this one with these questions to consider.
- How much grease do I put in? What kind? How often?
- What is the useable life of the elastomer or rubber insert’s compound?
- What is the shelf life?
- Do I need to torque or re-torque the bolts?
- Can I reuse the bolts or must they be replaced?
As stated before, the answers are in the instructions that come with the coupling parts! The moral of this story is that all of the information that we need is right under our noses. Read the installation and maintenance guide to get maximum life and performance from your couplings.
Below are just a few links to some coupling manufacturers for reference.
http://www.rexnord.com/Products-Services/Process-Motion-Control/Couplings
http://downloadthefilenow.com/manuals/falk-grid-coupling-manual.html
3 Comments
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Wondering if thier is an allowable tolerance for coupling gap? Lets say Lenny is aligning a motor with a new coupling and a .188″ recommended coupling gap.. but once the motor is alligned to the mills spec the coupling gap reads .240″ being bolt-bound and un-able to achive both alignment and coupling gap.
In this scenario, Lenny needs to reposition the pump to correct for the bolt-bound situation. Then the coupling gap needs to be set to the proper gap. As stated in the original post, the gap spec is there for the proper operation for the specific operational parameters of that coupling.
The other option, to allow for a different gap, is change to another type of coupling that meets the process needs and clearance. Either way, the misalignment must first be corrected.