Tolerance of Distance Between Shaft End (DBSE) 4

[Rafael Dos Santos]

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

 

[JJPellin]

It depends on the coupling design. For a disk pack coupling +/- 0.030" might be required. For a diaphragm coupling maybe +/- 0.020". For a gear coupling maybe +/- 0.060". The coupling manufacturer should provide the axial stiffness. You can use this to estimate the axial load imposed on the drive or driven machine by the variation in DBSE.

Johnny Pellin

 

[geesamand]

^ what he said. For elastomeric couplings with a hollow spider, it can be +/- 1/2", provided that the coupling halves are tightened to engage the spider appropriately.

 

[Jboggs]

That is a specific question for the coupling manufacturer, not a general online forum.

 

[Tmoose]

Hi Rafael,

Where are you in the chain of command on this project? Installation, design, drawing review , or ???

===========.

"that is a specific question for the coupling manufacturer"

And it should be included in the General arrangement drawings for the couplings, in tables in the literature provided with the coupling. One situation to be very sure of is whether the coupled equipment REQUIRES Limited End Float (LEF) couplings, or NOT.

 

[geesamand]

That is a specific question for the coupling manufacturer, not a general online forum.

I've asked my coupling suppliers this question myself. Mine would only tell me the DBSE and would not offer any tolerance. They really didn't seem to understand anything beyond their printed dogma.

Whether you're designing direct motor mounts and trying to minimize parts, or bumping in a 2 ton motor, it's essential to know what "close enough" really means.

 

[MintJulep]

Not so small that you can't fit the parts between the shafts to install them.

Not so big that you can't get enough engagement with the shafts to hold things in place and transmit your load.

 

[Rafael Dos Santos]

thank you guys for answering my question. Im just a rookie tech in rotating department. The question came up to me when we pull out pump, and we need to take DBSE before take out the pump and install it with same value of DBSE. If DBSE is same length as coupling/spacer, why bother use the same value of DBSE same like before we pull out the pump? Can we just measure the coupling and use it as DBSE when install it? or is it not the correct way? I asked my senior technicians, they want me to find it myself.

 

[byrdj]

My reading "between the lines" is the phrase I hated to hear when I was called to trouble shoot problem after maintenance performed by others. that phrase "We put it back the way we found it"

I think you are seeing it the same as I . . . How you found it NEEDS to be documented, BUT it needs to be reassembled the way it should be. Thus a drawing or at least an engineering evaluation of the assembly is required.

As per a generic answer or "rule of thumb", I can not provided. the critical couples I was once associated required an engineering evalution based upon the design data provided by the driver, the driven and the coupling
as example for those Boiler Feed Pump to Turbine couplings;dual diaphragm, 10" diameter, ~13" between couplings, the consideration for driver and driven thrust bearing clearance and thermal growth required a cold "pull",space greater than spool of about 0.10" with a 0.02" tolerance

 

[CouplingGuru]

The tolerance of distance between shaft ends of the placed equipment for most couplings isn't what is critical. It is the distance between mounting planes (hubs) that is critical. So when positioning the equipment, I would suggest using the coupling designed DBSE as the absolute minimum (otherwise the coupling won't fit), then positioning the hubs on the shaft to get as close to the desired mounting plane distance +/- .005". For most couplings axial tension or compression directly affects the usable amount of strain in the flex element. This "strain offset" if you will, de-rates the amount of angular and parallel misalignment that the coupling can withstand. Most coupling manufacturers rate the misalignment values at maximums, however a lot of people don't understand that a sum square of actual misalignment resolving back to angular misalignment determines whether or not the coupling is operating under its strain fatigue limit.

When it comes to couplings we are always here to help.

http://WWW.PSCCOUPLINGS.COM

 

[Njinear]

CouplingGuru is spot on.

Typically position the equipment a little bit more then the designed coupling BE, then slightly overhang the hub. This can be tricky if you have a taper bore and don't have that flexibility. But in scenarios with taper bores, most often the coupling manufacturer provides shims. Another reason to add some distance is if you a compensating for thermal growth. This will cause some pre-tension as CouplingGuru warned about about, so make sure your alignment is good enough to handle that pre-tension.