Gate valve stem galling

[MoReSo]

Hello,

I want to know what is the reason of galling (Photo) on the gate valve stem. the stem is 410 and the yoke nut is A439 D2. as you can see from the photo there is a very bad galling occurred.
please advise on following items:
1. why it happened? how I can calculate the force(thrust) they applied on this stem to torque the stem and probably it was too high.
2. how can I remove this galling?
3. can I use a hardened nut with higher hardness to remove the deformed threads and make it smooth? please advise


Materials and Welding Eng.

 

[gerhardl]

1. To high force, combined with out of alignmet nut, misfitting nut, smudged nut, lacking greasing, nisfitting parts because of too high or low temperature, forces applied skew and a lot of other possible things.
2. Completely new part, anything else increase chances of breakdown and damage of counterpart.
3. No.

( A cleaned, damaged part without any pictures of the real situation and other parts gives not enough information to give any estimates on real cause)

 

[MoReSo]

Thank you Gerhardl

Is it helpful to know how much torque applied to prevent similar situations later? And if the answer is yes how can I know how much is the limit torque or thrust?

Materials and Welding Eng.

 

[gerhardl]

Using too high torque is a common problem when valves are stuck. Stuck valves have commonly other causes than thread problems. Thread problems indicates strongly misfits (wrong threads) or mis -alignment, or pushing-pulling forces (for instance strong forces without limit control).

You will have to find the real cause of the thread problems before you can find the cure.

Regarding forces the valve supplier or an actuator supplier for a normal actuator could give you an indication of max torque. Under normal conditions, such a valve should be a able to be operated with a single handweel (or combination gear reduction plus handweel) giving directly a max force. Alternatively as a rough estimation: anything above an imaginative lever of (say) 0.5 - 1 meter operated by a max pulling by the operator (say 20 - 50 kilos) would probably be the normal maximum allowed force. (Without responsibility!)