Valve stem buckling

[theminimalist00]

Dear All,
I am trying to analyse the buckling of a knife gate valve stem . Is it logical to consider that the stem is fixed on its lower end .

 

[gerhardl]

Analyse possibility for buckling or a buckling already happened?

To your question: the best knife-gate construction will not have the spindle mechanically stiff fixed to the blade by one or several bolts. If this is done the blade and spindle will work as one elongated item and all imprecision and skew loads up and down will be reinforced and give extra wear both at gate and spindle sealing, threading and gear.

The knife is instead often fastened by a through-going hole in the blade and a bolt and fork lifting arrangement, or other 'not stiff' arrangements.

But both arrangements exist.

A buckling of a knife-gate stem is known problem, and always caused by very high overloading of the spindle by brutal mechanical force. This will be avoided by correctly adjusted torque switches if motorized and limitations on mechanical force if manually operated. End-switches alone is not enough as firm obstacles can be caught in gate with knifeblade closing. (Valves has been tried closed by levers pushed by a truck!)

If you are trying to calculate or dimension a knife-gate valve-stem, however, you will probably be OK by assuming the stem to be fixed between driving gear and knife-gate top, and pushed down from gear.

 

[theminimalist00]

Thank you for your response.

Actually I am trying to model the stem operation using a FEA software and I was wondering what boundary conditions should I put.



PS: Sorry for my terrible english,It is not my native language.

 

[gerhardl]

Neither is it mine! No excuses necessary!

Buckling of stems of knife-gate valves are a very, very seldom occurring problem, and only as stated above given by use of brutal force.

By stem extensions, however, which I have seen applied at lengths to more than 4 meters, buckling is more likely to occur. The normal solution is either pipe in pipe, or solid rod, alternatively single rod or pipe with mechanical buckling-prevention devices (outer piping sections).

Calculation? As my old professor said after an accurate strength calculation on the blackboard: '... and then we usually multiply by a safety factor of 5!'

Good luck!