DBSE (Distance Between Shaft Ends) 1

[svkd]

For new lube oil pump ,We have faced issue of not able to achieve the required DBSE. The deviation between the design and actual DBSE is 3mm.The motor bolts are bolt bound and there is no margin available to shift motor to achieve the required DBSE.I know there is one way which to enlarge bolt hole diameter in motor with to achieve DBSE but Guys ,Is there any other reliable way to solve this problem that we can do it on base frame pad rather touching the motor.

 

[JJPellin]

If the coupling hub fits are cylindrical, it may be possible to move the hubs up or down the shaft to achieve the DBSE. You did not mention the type of coupling. Some couplings could be modified by machining. You really need to provided more details about the coupling to fully understand the options.

Johnny Pellin

 

[Tmoose]

DBSE = Distance Between Shaft Ends I presume.

"Is there any other reliable way to solve this problem that we can do it on base frame pad."
Depends what the drawings and pictures of the base frame show.

If you cannot access the holes in the base to enlarge them Another option is using bolts with strategically reduced body diameter.
Much discussion amongst various parties will likely required before such a modification is permitted.
Reduced diameter bolts are Commercially available.
Here is one source - reduced diameter bolts

Whenever bolt holes are enlarged nice thick hardened washers are needed to keep from "coning" and forcing the motor etc to move back toward center.
Actually, NTH washers are good to have in every installation.
And no lock washers.

 

[maxdistortion]

Is the coupling 3mm long or 3mm short? and what coupling gap do you need to achieve? is it a horizontal pump or a vertical pump? and is it a rigid coupling or a flexible coupling.

It would help to know the manufacturer of the pump.

 

[svkd]

maxdistortion , DBSE(Distance Between Shaft Ends) is 3mm more than the required value.The motor bolts are bolt bound and there is no margin available to shift motor by 3mm towards pump to achieve the required DBSE. It is Lube oil screw pump horizontal type.The coupling is flexible type TB woods sureflex coupling.

 

[tbuelna]

It would be best to do whatever is necessary to obtain the correct alignment/positioning specified by the coupling vendor. Correcting the issue now will save you many times the cost of fixing the problem later on. That sureflex coupling toothed rubber insert looks like it requires precise axial positioning of the mating flanges.

 

[svkd]

maxdistortion , I am looking for proven and reliable way to achieve this DBSE , I appreciate if you share your exp on this topic.

 

[maxdistortion]

svkd, sometimes we slide the hubs down the shaft just a bit, a guy could make up the difference if there were enough room on the shaft, and move the pump hub a bit and the motor hub a bit.

The way we do this is fab up a rigid plate you can bolt on the hub, and make a spacer of the desired adjustment, 60 thou or whatever you want. then you start to apply torque to the plate and the hub starts moving. Sometimes it doesn't move and you can apply some heat to the hub to loosen it up. It depends on the pump.

This fix has to be agreed upon by the owner of course. Also, vendors don't always give you couplings that fit, and when you call them out on it they will tell you to move the hub. Additionally, some flexible couplings are pretty forgiving. Check the coupling against the data sheet to make sure it matches, and see if you can find a tolerance for axial distortion for the coupling.

Let me know if something here doesn't make sense.

In my experience, as long as most of your hub is on the shaft you are ok. This is subject to owner's approval and they will decide if they take it or not.

 

[JJPellin]

You have still not confirmed that the hub fits are straight. Overhang both hubs by 1-1/2 mm. This is better than slotting the motor bolt holes or undercutting the bolts. If this is not acceptable, drill out the motor hold-down bolt holes oversized. Plug the holes. Drill and re-tap at the correct location.

Johnny Pellin

 

[svkd]

maxdistortion & JJPellin ,As per Engineering Standards How much is the tolerance which we can overhang the coupling hub on the shaft to get desired DBSE? I don't have vendor recommendation regarding the coupling hub area that should cover the shaft.

 

[Tmoose]

" As per Engineering Standards ... "
My experience was the coupling manufacturer's engineering support will be ready, willing, and able to answer that.
Before making the call Be sure to have the info on hand that would be required to select the coupling initially.
An Example is Page 268 / T-10 here -
file:///C:/Users/Robyn%20Timberlake/Desktop/cd%20chnger%20repair/Torsional2010.pdf

I'd request a formal response via e-mail to keep in the job records, just in case.

 

[maxdistortion]

If your hubs are flush now I wouldn't loose sleep over a 60 thou overhang. This is a vendor question. Ask the vendor if they would like the pump back for so they can properly align it as they should have done correctly in the first place. If you bought one of those final sale lube oil packages and the vendor won't cooperate with you then overhang the hubs and move on.

As far as engineering standards on overhang, depends on torque, area of shaft to hub, if you really want to guess then take the limiting agent there, probably the shear key and do some math voodoo.

 

[svkd]

Guys ,
One more help , see attachment- my coupling drawing shows DBSE (minimum-127mm , maximum-141.14mm and actual-127mm ), so which value(Actual/Minimum/Maximum) is to considered while setting DBSE in field?

Also usually how much tolerances allowed in DBSE and spacer length? For example If spacer length is 126mm then what should be DBSE(+/-) for Sureflex TB woods coupling for a Lube oil pump?

 

[JJPellin]

This coupling has a range of acceptable spacing that is allowed by the design. You are within that range. Technically, no adjustment is needed. I would rather not be at the extreme end of this range. I would overhang the hubs to get the ideal spacing. The coupling would have been selected with a service factor. We typically require 1.5 as a minimum. All elements of this coupling are over-designed by at least 50 percent. This includes the coupling hub to shaft engagement (which is probably not the limiting factor). An overhang of 1.5 mm reduces the engaged area by a very small percentage.

Ideally, you would get the coupling manufacturer to review this and give you written confirmation that it is appropriate. But, I know some manufacturers do not respond well to these sorts of requests.

Johnny Pellin

 

[svkd]

JJPellin,

I have below queries:
1.What do mean by ideal spacing?Is it the Actual DBSE-127mm?
2. How much hub overhang is allowed are you aware of any engineering standard reference?
3. How much tolerances allowed in DBSE and spacer length? For example If spacer length is 126mm then what should be DBSE(+/-) for Sureflex TB woods coupling for a Lube oil pump?

 

[JJPellin]

I guess I don't understand. The drawing that you provided lists the acceptable range for DBSE. I was thinking that you had provided the measured value. But, in looking back over the chain, I see that you did not. What is the measured DBSE? It is within the range on the drawing? If it is, overhang the hubs as needed to set the coupling spacing (shown as 1.0 inch on the drawing).

If I am given an acceptable range for something like DBSE, I would prefer to be right in the center of that range to allow for necessary adjustments in the future. The range on the drawing shows that you can overhang the hubs by as much as 0.28 inches (one half of the difference between minimum and maximum DBSE). The drawing actually shows the hubs overhanging.

The correct setting for this coupling is not defined by DBSE. The drawing clearly shows the required spacing between the inner faces of the coupling hubs at 1.0 inch. That is what it should be set to. It is not shown, but I assume that this includes an axial clearance between the rubber spacer and the inside of the hubs. We have an internal specification that defines this internal axial clearance for a Woods coupling of this type as 1/16" as shown in our drawing (attached). The couplings we use for our governors are much smaller than the one you are working on, so the required axial clearance may be greater. But, ultimately, if you set the hubs at a 1 inch spacing, you will be set as the manufacturer requires.

Johnny Pellin

 

[svkd]

JJPelin,Really appreciate your inputs.

The measured DBSE is 130.14mm and it well within the range specified in drawing(Maximum-141.14mm,Minimum-127mm)

If you don't mind ,just for my knowledge I want to understand the importance of G (Coupling spacing shown in drawing)? Is there any tolerance allowed in this value G?

Also what is significance of internal axial clearance for a Woods coupling of this type?

 

[JJPellin]

I would set the G dimension within 0.030 inch (+/-). The axial space is needed to avoid imposing a thrust load on the pump or driver.

Johnny Pellin

 

[tbuelna]

The installation tolerances seem to allow for some adjustment in the axial location of the coupling. Since this coupling connects an oil pump to an electric motor, after aligning the pump and motor shafts as accurately as possible, I would then position the shaft ends at the low end of the axial offset tolerance, and then I would shift the coupling as close to the oil pump as tolerances permit. The reason being to minimize the radial loads on the outboard pump bearing due to shaft misalignment/runout at the coupling. Motor shaft bearings are usually better able to handle radial loads than oil pump shaft bearings.