Categories: Shaft Alignment,Other Topics

Coupling Tolerances vs. Shaft Alignment Tolerances, What’s The Difference?

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By on June 24, 2016

Plenty!

This question still comes up fairly often in our training classes. If you look any of the coupling manufactures installation instructions there is a chart for the maximum allowable misalignment based on the coupling size. These tolerances are for the coupling, what about the machines the coupling connects?

Shaft alignment tolerances are more about increasing machinery health were as coupling tolerances are more about how much misalignment a coupling can handle before it fails. However, coupling manufactures do recognize the importance of good alignment as there is typically a note in the installation instructions stating “Proper alignment yields the longest service life”.

That being said, the coupling maximum allowable misalignment tolerances are too “loose” when compared to shaft alignment tolerances and are also based on the coupling size. Let’s say a facility has 20 different size machines all operating at 1800 RPM and the 20 machines all have different size couplings. That’s 20 different coupling tolerances the mechanics will need to remember. If said facility aligns to shaft alignment tolerances, which are based on RPM, they have one tolerance for all 1800 RPM machines.

The following coupling tolerances are taken from the installation instructions for an elastomeric insert style coupling and a grid style coupling. Different coupling manufactures tolerances may vary. Also the angular coupling tolerance listed on the instructions were given as a gap or face value at the coupling O.D. which  I  have converted to mils/1″ (1.0 mil = .001″).

Elastomeric Insert Coupling

Elastomeric Tol.

Grid Style Coupling

Grid Tol.

VibrAlign’s recommended shaft alignment tolerances are based on “acceptable” tolerances used by industry and should be used in place of coupling tolerances when no alignment tolerance is specified by a company’s engineering or reliability department.

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Examining the 1800 RPM “Shaft Alignment” tolerances in the above table and comparing to the tolerances for the 3 coupling sizes above; the “Acceptable” Angular Misalignment is 0.7mils/1”. That is 25 times better than the elastomeric insert coupling tolerance and 1.7 to 2.4 times better than the grid style coupling tolerance.

The “Acceptable” Offset shaft alignment tolerance for 1800 RPM is 4.0 mils. That is 3.75 to 8 times better than the elastomeric coupling tolerance and 1.3 to 3 times better than the grid style coupling tolerance.

The goal of precision shaft alignment is to make the rotational centerlines of the machines collinear when in operation. Doing so minimizes forces being transmitted across the coupling, even with elastomeric style couplings, which increases rotating equipment reliability as well coupling life. Our goal is to “Save the Machine”.

Long Live the Machine!

 

 

 

 

About the Author

Brad Case has been associated with VibrAlign since 1990, first as a manufacturer’s representative, than joining the company as a direct employee in 2005. He has over 35 years experience in aligning industrial machinery. Brad attended Texas Tech University, in Lubbock, TX.

Brad began his career in the automotive industry providing technical training, sales, and service for Murray Goldseal an aftermarket air conditioning component manufacturer. His background includes 25 plus years experience in sales, service and training, of Centralized Lubrication Systems, Couplings, Gearing, and Gear Reducers.

Brad and his wife currently live in Clifton, TX.

8 responses to “Coupling Tolerances vs. Shaft Alignment Tolerances, What’s The Difference?”

  1. Kyle Harper says:

    What about a turbine turning 12k with an 8″ diameter? The coupling is a flex coupling with shim packs.

  2. Brad Case says:

    Kyle, I would contact the turbine OEM for their recommendation. I would think the OEM or your engineering department would have a recommended tolerance due to the high RPM.

  3. Stu says:

    What are the industry standards for shaft alignment tolerances? API 686 only gives a recommended angular tolerance and nothing for offset alignment. I can’t see anything in API 610 or API 617 for pump or compressor shaft alignment tolerances.

  4. Brad Case says:

    Stu, thanks for your comment. I found an old version of API 686 (first edition 1996) that in Chapter 7, Section 5.4.6.2 states;

    “When using rim and face alignment or alignment computers that resolve misalignment into angularity, the alignment tolerance is 0.03 degrees. This angle must be determined at each hub on spacer couplings. When using rim and face alignment methods to align machinery trains with elastomeric coupling or close coupled machines, the angularity shall not be greater than 0.03 degree and the offset at the center of the coupling shall not exceed 0.02 millimeter (1 mil).”

    Fixturlaser Alignment Tools display angular misalignment as a slope (rise over run) in mils/1″. The 0.03 degrees angularity in API 686 converts to 0.5mils/1″.

    You can set up this API 686 tolerance in your Fixturlaser Alignment System as 0.5/1″ angularity and 1.0 mil offset.

  5. Rafael Dos Santos says:

    what is the tolerance of Distance Between Shaft End (DBSE) when installing coupling. Let say the length of the coupling is 120mm.

  6. Brad Case says:

    Rafael,

    The allowable Min-Max coupling DBSE should be listed in the installation instructions for the coupling. The allowable DBSE tolerance will vary dependent upon coupling type. For instance a disc pack or shim pack coupling will have a tighter DBSE than say a coupling with a elastomer insert. It’s best to consult with the coupling manufacture for additional information.

    Also, there may be a DBSE requirement for the shafts of the driver and driven machines to allow for thermal expansion of the shafts axially in high temperature applications. If so, the coupling DBSE will need to set accordingly.

  7. Rafael Dos Santos says:

    Thank you for your reply brad. In my manual, IDP HDX cetrifugal pumps by Flowserve, I cannot find the value that I needed to install coupling. If this comments section allow me to send some pictures, I’d be happy to share with you brad

  8. Brad Case says:

    Rafael I have emailed you directly for the pictures. Thanks