Use of Stainless Steel as a Hanger for a Table Inside a Furnance 1

[gustavosilvano]

Good day to everyone.

My name is Gustavo and I'm an Industrial Engineer a steel mill plant here in Brazil.
We have an annealed wire furnance with two levels. One at the bottom and the other at the top os a table that is moved using the overhead crane.
The furnance reach temperatures above 750 degree celsius. To move this table, we use an external device, but this demand a time from the operator, and expose him to some risks. What we decided to do is to install a fixed hanger that is made of stainless steel. There's no welding, all fixed using screws threads, nuts and washers. We made it this way so that it would be easy to inspect it just by looking.

I want to know is will a lock washer be effective at this situation due to the high temperature reached by the furnance? Is there anything more that I can do to assure a safety in operation?

This is the hanger

This is the project, where the left picture is the bottom view and the right one, the top view.

This is the furnance, with the bottom level, the table and the upper level.

Thanks in advance.

Gustavo

 

[btrueblood]

What is it that you want the lock washer to do? (Hint: there is a FAQ on this site about lock washers, and how they don't do what you might think they do, even at room temperature)

Not sure about the wide loop on the bottom view, what stops that loop from binding into the adjacent plate? Is the loop welded to the two plates?

The decision to use stainless is to avoid oxidation corrosion? Realize that typical stainless steels will still oxidize pretty severely at 750C. From just a first glance perspective, I'd probably double the thickness of the stainless bar/loop, it just looks flimsy relative to the mass of steel it appears to be lifting, and I wonder how much margin there is for material wear from the lifting hook.

 

[IRstuff]

Look into Nordlock washers

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[arunmrao]

I agree with btrueblood's observations. Another clue look at ladle hangers using over head cranes for transport of liquid metal.

I use steel rebars of 50mm diameter for carrying liquid steel ( 10 tons weight). I change these hangers every 6 months.

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

Why would fasteners be more inspectable than welds? The critical points in your sketches are not visible.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[MintJulep]

A threaded fastener will never stay tight on this environment with this design. No locking method will help.

Fatigue cracks will start at the thread root and grow undetected until final failure.

This is not a good design. Start again.

 

[racookpe1978]

So, why not the old solution of drilling a hole through and bending a cotter pin over? Might require a larger dia rod and so larger nut.

 

[kingnero]

^ I believe that's what drawn in the sketch.

 

[gustavosilvano]

Sorry everyone. There`s some small things that are wrong on the sketch. Here is the real top and bottom view.

Here you can see the red ring wich represents the lock washer, and the blue one that is the normal washer.

The table weights something around 1.5 tons.

btrueblood

The lock washer is to prevent the nuts to get loose. But with the temperature, I belive that they`ll lose the spring characteristic.
With the new sketch, you can see that we no more have the wide bottom loop.

The stainless steel is used because it resists more to deformation at high temperature. Isn`t it?

ironic metallurgist

When we desmantle the load, the table that support the topper wires are put aside of the furnance. At this moment we can inspect the nuts and the screw thread. We're also doing a tighting routine during the inspection to ensure that we don't have any loosen nut.
Also, when welded, I think that it can create some cracks due to the high temperatures.

MintJulep

We can disassemble this device at the preventive maintenance to look for any crack. Do you thing that this is enough or effective?

racookpe1978

Don't you think that this will reduce the resistence of the material? The central beams are composed of two "C" shapped beam that are welded together. The height of it is around 200 mm.

kingnero

In fact we didn't drilled the table. We used two plates, and drilled four holes in each of them.

Thanks everyone for using your time to help me!

 

[racookpe1978]

The plate is not drilled, the rod (below the plate) is what would be drilled.

That small hole in the rod does (slightly) reduce the cross-section area of the rod, and so does (slightly) reduce the assumed strength of the rod at that temperature. So you make the rod larger to compensate. The cotter pin is physically bent to prevent it from coming out, the shear strength of the cotter pin prevents 100% the nut from rotating.

A flat washer below a locking washer prevents the lockwasher from doing its intended job (gouging into the base metal and preventing rotation) - which a lock washer does very, very poorly in any case. And even more poorly when heated.

 

[EdStainless]

Make it heavier. Assume that with time as use that the entire load will be put on two legs and not all four.
You may want to use 309 or 310 SS for this so that the properties will not degrade as much over time.
I don't like using threads either.
You would be better off using other fastening methods. Either weld or use a slot-and-tab method.

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P.E. Metallurgy, Plymouth Tube

 

[MintJulep]

We can disassemble this device at the preventive maintenance to look for any crack. Do you thing that this is enough or effective?

Visual inspection? No, you won't see any cracks.

Dye penetrant inspection, magnetic particle inspection? I'm guessing your threads are cut? No, you won't be able to identify a crack at the root of a cut thread.

Spring lock washers do not work.

Not at all.

Ever.

This thing will never stay tight because of the thermal cycling.

If you simply want the nuts to not fall off, use all-metallic prevailing torque nuts.

But...cracks that you can't find.

 

[blacksmith37]

HH 30 is pretty common furnace hardware ( cast 309 with 0.3 C) . Similar to HH 40 , HK 40, etc. It could be worth a redesign to use castings.

 

[DAVIDSTECKER]

First thing I'd do is apply anti-seize to the threads if you use stainless.
It looks like you double nutted or jamb nutted in the first photo which would be a good method I would think.
Pick punching or stacking the threads could be another method.
Castle or slotted nuts with a cotter pin yet another.
Bottom line; use anything but a "Lock Washer".
That's my opinion.
To quote the old Shell gasoline commercial "your mileage may vary".

 

[EdStainless]

I am sort of surprised that the table isn't a casting.
I have seen these in the past that were cast, and the hole in the center.
A 'spear' was used to pick them up, it was placed into the hole and then a cross bar was inserted.
Then when you set them down and removed it they were flat.

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P.E. Metallurgy, Plymouth Tube

 

[gustavosilvano]

Just a following to the improvment.

We disassembled the device. This is how on of the thread was (the left one).

The right thread was ok.

We were having a routine of tighting the nut between every production. I don't know if it's weared due to overtighting.

Now we've changed the locking system. It now has 3 nuts and a normal washer at each thread.
I think that this will improve the safety of the system.

What do you think about it now?