Bent Gate Stem 3

[JA]

Hi there,

I have a question about a bent gate stem used in the operation of a dam gate structure. The stem is threaded stainless steel and is raised a lowered via a geared system. One of the stems is out-of-plumb by approximately 1/16 of an inch. My question is will this bend affect the torque required to operate the gate? We are concerned that this bend will make operation a challenge, however do not have anything quantifiable to confirm that. Does anyone have any experience with this kind of situation and can offer even an explanation of whether a bent stem like this is a concern?

Thanks

 

[LittleInch]

A drawing would help a lot.

If the rod is in compression at any time then yes, especially if fairly slender, the propensity to buckle increases even with very small initial deflections.

A lot depends on length, diameter, other items holding it out of position etc, none of which we know other than it is 1/16" out of plumb.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[cvg]

is the stem actually bent or is the guide mis-aligned? how long is it? what size gate and stem? it doesnt sound terrible, but sure that it will increase the required torque. the operating force is usually limited to 40 lbs so not much room for error. it sounds like this is new so why not ask the valve rep to check it during commissioning?

 

[JA]

Thank you both for the prompt responses. The stem is 2.5 inches in diameter and is 164 inches long. The gate is about 20' wide by 6' high and is raised via a tandem stem method. One of the stems is straight with the other one exhibiting the bend as discussed.

The stem is actually bent. The stem is an old stem that the owner is considering trying to salvage. The stems raise into aluminum stem covers.

 

[racookpe1978]

But you didn't answer the needed question. Is this a tension application, or a compression (buckling) application?

Are the fixed threaded supports (for the threaded rod) themselves fixed, or do they pivot as the gate moves?

 

[cvg]

I assume a sluice gate something like the image. to raise or lower the gate requires torque on the stem and a bent stem will definitely require more torque to operate. It will put stress on the guides also. A gate of this size is a custom design and the increased torque due to a bent stem may be difficult to estimate. USACE allows up to 1/16 tolerance in gates and frames and half that for alignment of the operating machinery. So you would have trouble meeting that requirement. I would hesitate to install a bent stem just because the cost and down time to remove and replace with a new one would be prohibitive if it does not work, not to mention the engineering cost to analyze the torque requirements and stress on the guides. I would be inclined to either try and have it straightened or get a new one.

3.1.4 Operating Machinery
Operating machinery for the gate assembly and supporting components,
including [bonnet,] [bonnet cover,] [pedestal,] [and] [base plate], shall
be positioned and aligned to the installed location of the gate frame and
guides and anchored in place. The location of the slide gate stem shall be
projected to and scribed on the sill of the installed gate frame to serve
as a reference point for the alignment of operating machinery and
supporting components. Operating machinery and components shall be aligned
to within 0.762 mm 0.030 inch of the reference point. Prior to being
embedded in concrete, an alignment template shall be bolted to the
[bonnet,] [bonnet cover,] marked, and drilled to match the exact center
point of the gate stem.

 

[LittleInch]

Still needed a drawing to understand where this bend is. I'm astonished you can see or measure 1/16" in 164 inches.

Is it a gentle bend? What does the grated operator look like? I'm finding it difficult to see how there isn't 1/16"of give or flexibility in this system. [pre][/pre]

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JA]

cvg - thank you for the information. It would appear then that we are pushing the boundaries of acceptable tolerance. The image you have include is very similar to what we have, except we have a tandem stem system with a cross shaft like the picture below.

I believe we will suggest to have it straightened if possible.


Thank you all for your responses.

 

[Nescius]

I'm astonished you can see or measure 1/16" in 164 inches.

Yeah, my brain went in that direction, too.

Note that I have zero professional experience in this area. However, I do not believe that the .030" spec noted above can be applied to the straightness of the screw, it just doesn't make sense. That's not how the spec reads, either, in my opinion. For example, a quick peek at McMaster-Carr shows that some off-the-shelf, ground, linear bearing shafts have a straightness tolerance of .002 per foot.

164 inches ==> 13.7 feet ==> .0273 straightness error (and this is precision ground shafting, not a rough service screw that spends decades in the weather doing a crude task)

"Alignment of operating machinery and supporting components" is not the same as straightness of the screw. In this arena, I'm a nobody, but I have strong doubts about the numbers being thrown around here.

 

[IFRs]

1/16" in 164" seems like a "who cares" to me. I'd bet that the rod would bend that far just from being in direct hot sun on one side and shade on the other. I wonder if the threads and gears are that tight that they would care.

 

[moon161]

Lacking relevant experience, I'll say that it looks like a 'who cares' when the door is lowered, that will grow in significance as the door is raised. If that guide is at say 10' and the door has 8' travel then the question is will it bind, bend or be stressed when that 1/16 is over 2'? If the rod is significantly stressed in that last position, low cycle fatigue could be a worry.

 

[racookpe1978]

No, if the "bent" screw thread is pulling the gate up, then the tension will straighten the screw thread. Shutting requires compression force to overcome friction, but the weight of the gate remains and acts to straighten the screw thread. I'd be suspicious of being able to "re-bend" a bent threaded rod: the effort of clamping it and forcing it over a focus point to re-bend it will more likely damage the threads.

 

[JA]

Thank you all for your input. The measurement was taken via a string line where it was observed that the stem is bent 1/16" so it was able to be notice. The other stem used in the gate was checked as well and it exhibited no bed at all, so the bend is noticeable.

 

[cvg]

in addition to binding of threads, there is also binding in the slides on the gate.

I agree, it seems like a small bend and probably not an issue but, in my opinion hardly worth trying.

this is a dam and the gate is on the water side, generally below the ordinary high water line. installing or removing a gate would be done by divers or by lowering the reservoir and possibly working from a barge. If it binds during operation it may possibly jeopardize normal operation during a flood stage. If this is a gate on a penstock than dewatering the penstock would be difficult. all good reasons to make sure it works the first time.

 

[LittleInch]

Cvg, look at the photo.

My guess I'd that at some point this gate got jammed with some gunk and the rod went into compression.

If this gate arrangement is fixed to a value of less than an inch I would be astonished. A system like this that doesn't allow for some lateral movement of the gate is going to jam very easily. I'm really struggling with this one being an actual problem.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Nescius]

We can see some small amount of straightness error in one of the screws. We then wonder if the slight bend in the screw could cause a problem, a perfectly logical question.

I think it's also important to know if the slight bend really tells the whole story. The screw could be damaged more than it shows, because it's already binding and the structure of the thing is fighting the screw and winning, springing the screw back to a straight-ish condition.

In other words, how bent would the screw be if it was out of service, just sitting on a workbench?

 

[JA]

Nescius, we actually measured the bend when it was out of service and the stem was removed from the gate structure.

Thanks again to you all!