June 24, 2016
Coupling Tolerances vs. Shaft Alignment Tolerances, What’s The Difference?
By Brad Case
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
Grid Style Coupling
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.
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!