Stem Slenderness Ratio

[Sandro_S]

Hi,

We manufacture slide gate valves which are operated by a stainless steel (304) round bar stem. Since I have started this job, I have been calculating the slenderness ration by taking the round bar diameter and dividing it by 4, then multiplying this number by the specified maximum slenderness ratio (usually 200). This gives me the maximum spacing for guide bracket supports. This has been the calculation described to me by both my boss and the job specification itself.

We just got returned drawings to us with comments about our guide spacing being outside of the slenderness ratio. They are telling us that my calculation should be dividing the RADIUS by 4. This means that I have to half my spacings, therefore increasing the number of brackets and therefore cost. Have I been calculating this wrong all this time and it just slipped past the reviewers? Can a spec simply state this is how they want it calculated even if its wrong?

I have looked all over the internet and have not been able to find a simplified calculation page for all structural shapes let alone round bar. Any help would be appreciated!

Thanks
Sandro

 

[Sandro_S]

Hi Again,

We needed this answer ASAP so we contacted our engineer and he confirmed that we had been calculating this right all along. Ill leave this post up in case anybody else ever wonders the same thing.

 

[MGZmechanical]

I think you're right. Slenderness = 4·L/d <= 200 is the usual value (some discuss may come here about what d to use nominal, root or some intermediate but that's another issue)
Just remember:
slenderness = L/Giration radius
Giraation radius = sqrt(Inertia/Area)

 

[Sandro_S]

Hi again,

I have a follow up question. Our gate valves normally operate with the stem being under compression forces to seat the door.In some cases, we build the valves like weir gates (open downward). This means the weight of the door will allow the door to freely open and when closing, the stem will actually be under tension to close the gate.

How does this affect the slenderness ratio? Does the slenderness ratio even apply at all anymore? The specification says "The L/r ratio of the unsupported stem shall not exceed 200." Technically the stem will be unsupported for a certain length but why would this matter when we could simply use high strength cables to pull the door closed?

Any help would be great!

Thanks
Sandro

 

[MGZmechanical]

Even for parts under tension this limit is usually keep because it protects you from secondary forces you have not taken into account and that could bend your stem (this is something wouldnt happen with a cable)