designing suitable stud to operate under vibration
designing suitable stud to operate under vibration
(OP)
i would like assistance on how to design a governor valve positioner link/stud suitable to withstand the up & down system vibrations of 19.1mm/s as shown in the attachment. the positioner link stud shown in the attachment has fractured 3 times in the past year. when it fractured the second time we replaced the mild steel stud with a high tensile stud which snapped within 6days most probably due to its its brittle nature, it could not withstand the bending caused by the vibrations. we replaced the 6mm HT stud with an 8mm mild steel stud. i need assistance on how to design a stud to withstand the vibrations whilst awaiting the outage to investigate the root cause of vibrations. i would also want assistance on the vibration allowance limits on the governor valve.
THANK YOU...
THANK YOU...





RE: designing suitable stud to operate under vibration
RE: designing suitable stud to operate under vibration
RE: designing suitable stud to operate under vibration
Ted
RE: designing suitable stud to operate under vibration
@mintjulep, i think u lost me there
RE: designing suitable stud to operate under vibration
Move it to the best possible location so that you don't need the long cantilever at the upper connection.
RE: designing suitable stud to operate under vibration
RE: designing suitable stud to operate under vibration
I would try a rod that is threaded only at the ends, use a torque nut on the rod to apply a preload and if possible have a counter sink machined into the larger structure (or weld on a fat sleeve) that the smaller part can recess into. This should remove or at least lesson the tranverse loading of the connecting stud.
RE: designing suitable stud to operate under vibration
RE: designing suitable stud to operate under vibration
Make a triangular steel plate stiffener/brace, which is coped to clear the head casting yoke. This should clamp over the large center nut or yoke casting, and it should run out almost to the tip of the positioning rod/link cantilever where it is clamped to the positioning link. This will settle down the vibration at the tip of the link. You might also clamp the link to the stiffener plate near its mid-length or back near the stud.
RE: designing suitable stud to operate under vibration
RE: designing suitable stud to operate under vibration
the basic problem is typical mistake in linkage design. "Do not transfer force accross a thread". as suggested by several others above, you need to have a solid fit of the rod into the yoke. I would hope the yoke was spot faced so that the rod face was in full face contact. By boring this spot face deeper by the diameter of the rod with a slight interferance fit would be worth a try. I see a possible concern can become the thickness of the yoke and the depth of the thread does not allow for too much engagement before the actuator stem is contacted.
Is that a washer under the center nut?. If so, a better design might be to make an arm to replace the washer. this arm would need to have an indexing dowel into the yoke to prevent it from rotating when tightening the nut and hold alignment to the LVDT.
Another suggesting worth repeating would be move the LVDT to be mounted near one of the lift rods on the side, thus reducing the cantilever. the spring in the long cantilever can contribute to the control system noise and actually make the actuator hunting worse.
I would need to see more and possible a actuator/valve drawing, but it might even be simplier to move the LVDT off to the side and down lower beside the piston.
good luck
RE: designing suitable stud to operate under vibration
one of my wisdoms I state with problems related to steam turbine control instrumentaion is that;
1) the mounting of the attachment must be capable for a mechanic to use to pull himself up and stand on. &
2) the device must be robust enough that when removed, the mechanic can use it as a hammer.
(I don't think your design meets #1)
besides the modulation vibration, the design must be able to withstand the impact of tripping.
RE: designing suitable stud to operate under vibration
Attached is a sketch showing my suggestion for modifications to your link installation that could be incorporated fairly easily. Based on your description it seems like the current arrangement creates a large stress concentration right at the point where the stud threads enter the yoke. The dynamic vibration environment likely makes the problem worse by producing a high-cycle reverse bending load on the stud.
CoryPad's suggestion for a lengthened and preloaded stud is a great start. In my sketch I also suggest adding an enlarged base to the fixed end of the link, which will increase its moment capability. Tapering the link's cross section along its length should create a more even distribution of bending stresses.
Hope that helps.
Terry
RE: designing suitable stud to operate under vibration
It could be from the control system driving the actuator.
It could be resonance from the excessive turbine rotor vibration.
it could be resonance due to improper support, driven by acceptable turbine vibration
or it could be flow induce by the valve disk and seat.
for flow induced, is this a new turbine/design valve? if so, the OEM needs to get involved. If this valve has operated satesfactroy in the past (or on its sister unit) then there could be a problem with either the balance chamber, pilot valve or stem to bushing clearances
RE: designing suitable stud to operate under vibration
RE: designing suitable stud to operate under vibration
RE: designing suitable stud to operate under vibration