Pump Hold Down FastenersBy PumpWorks Engineering Team / April 14, 2021
How many times are pump operators forced to modify fasteners that are used to secure motors and/or pumps to baseplates? A warehouse spare is ready to be installed and nothing fits, i.e alignment is not successful, major piping does not bolt up cleanly, and operations are anxiously awaiting their new equipment.
Chain falls and come-alongs are rounded up and fasteners are reduced in effective diameter to allow for the equipment to be installed. There is a high probability of premature failure. The maintenance department will be revisiting this again soon.
What can be done to mitigate these reliability killers? Expensive plant projects that involve major piping revision, baseplate modification that is implemented only during expensive turnarounds is the best solution – but probably not realistic.
A reasonable solution is to provide some additional flexibility in how these pumps (and motors) are installed.
During shop maintenance performed—probably at a well qualified OEM or independent repair facility—the solution would be to enlarge the through holes for hold down bolts. While not a perfect solution to frustrating installation issues, my experience has been that this simple modification can often provide almost instant relief.
Things To Consider
There are several very important points to consider when making this modification:
- Top of all through holes should be spot faced or re-spot faced.
- Verify the flatness of the bottom of each foot and coplanar. Follow API 610 guidelines, even for motors – or IEEE841. Older NEMA frame motors are particularly bad in this area.
- Include special heavy duty washers in the repair specification, NOT the standard mill supply F436 washers. Washers should be at least ¼” thick manufactured of 400 series SS. The OD of washers should be smaller than the ID of the spot face in order to move around inside the spot face. Use ID larger than the bolt size by – 1 to 2 – ⅛” increments. Attention to detail is critical here.
- Utilize fasteners with heavy hex head shapes, not hex head. The heavy hex head size is required for proper contact with the new special washers.
- Size increases in the new through hole – at least ⅛” – Assess all the components. Writing a general specification for this modification will most likely result in poor results. Involvement with each piece of equipment is critical.
- Know your equipment and discuss this with the craftsmen who install the equipment.
Another option for mounting holes that should be considered in some unusual cases is to install a second set of through holes in each mounting foot. I have done this several times, particularly on large induction motors above NEMA frame size or 541 motors with great success. With a second through hole, severe misalignment problems can be immediately solved by abandoning the first hole and moving to the second. This will of course require field drilling new threaded connections in the baseplates.
Be careful of the long term implications of this modification. If you make this modification to one motor serial number of a particular frame/horsepower size, you must be prepared to make this modification to every other motor of the same family.
Documentation is critical here for long term equipment reliability. Don’t leave a trap for the next machinery professional.
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