Bolting up hot brass parts with cap head screws

[dr pepper]

A machine I'm repairing has hot brass heating irons that run at 250 degrees C, the bolts holding them in sometimes get so tight they break on removal, they are not put in that tight I think theres some reaction thing or thermal thing thats making them grab.
I have tried steel, stainless steel and iron screws, and in steel I've tried plain and zinc passivated, still no luck, I'm resisting using anti seize as the machine can run material that handles foodstuffs.
Any metallurgists here that know what to do?

[J Tiers]

There is anti-sieze which is approved for food handling machines

https://www.mcmaster.com/products/lubricants/food-industry-antiseize-lubricants-6/

https://www.mcmaster.com/products/lubricants/metal-free-food-industry-antiseize-lubricants/

[mikey553]

A machine I'm repairing has hot brass heating irons that run at 250 degrees C, the bolts holding them in sometimes get so tight they break on removal, they are not put in that tight I think theres some reaction thing or thermal thing thats making them grab.
I have tried steel, stainless steel and iron screws, and in steel I've tried plain and zinc passivated, still no luck, I'm resisting using anti seize as the machine can run material that handles foodstuffs.
Any metallurgists here that know what to do?

What is the female thread material? Anti seize may still be a good idea if used sparingly. 250 deg. C would not degrade the steel screw material, but it may cook whatever lubricant was left in the female threads. What kind of screws are they - hex head, socket head, etc.?

[Georgineer]

Is there a reason for not using brass screws?

George

[oxford]

I would try coating the hardware with milk of magnesia.

[Toolguy]

I would try coating the hardware with milk of magnesia.

Does that make it poop the screws back out on demand?

[tom_d]

I would try coating the hardware with milk of magnesia.

Does that make it poop the screws back out on demand?

Never tried it on brass before. Go into any aircraft maintenance shop and check the seasoned A&P's tool box. There will be a bottle of Milk of Magnesia. It's used as an anti-seize lube when installing the steel spark plugs in aluminum cylinder heads.

[J Tiers]

The anti-sieze I linked at McMaster has a listed temp rating of 2300 F. Should be good enough for a brass heater............

[Mcostello]

The Milk of Magnesia may help Him to not get an upset stomach.

[ChazzC]

There is anti-sieze which is approved for food handling machines

https://www.mcmaster.com/products/lubricants/food-industry-antiseize-lubricants-6/

https://www.mcmaster.com/products/lu...ze-lubricants/

It’s important to keep in mind that “Food Safe” and “Food Grade” don’t mean that a product can be used in all circumstances: these terms apply to “incidental contact” resulting in less than the possibility of less than 10 ppm in food.

[Rich Carlstedt]

A machine I'm repairing has hot brass heating irons that run at 250 degrees C, the bolts holding them in sometimes get so tight they break on removal, they are not put in that tight I think theres some reaction thing or thermal thing thats making them grab.
I have tried steel, stainless steel and iron screws, and in steel I've tried plain and zinc passivated, still no luck, I'm resisting using anti seize as the machine can run material that handles foodstuffs.
Any metallurgists here that know what to do?

We had Dies heated to 500 F which is close to your application. I am surprised your heaters do not use a steel strip to hold them , but for direct mounting through a bronze/brass heater, we always had springs or belllville washers to compensate for the thermal expansion , which is 50 % greater than steel. the washer made sure the heater had 100 % contact , other wise you could
have a runaway or burnout from a too hot heater
Rich

[Ian B]

I saw the "milk of magnesia" trick mentioned in relation to bolts in furnaces decades ago. i always wondered if it really worked. Still do, very occasionally...

Ian

[J Tiers]

The Belleville washers will work for the tension on the cap screws. They will not do anything for corrosion etc which may act to seize the threads in the mounting holes.

The description does not really explain whether in actual fact the cap screws are getting tighter against the brass heaters, vs the threads becoming seized. Without knowing that, the proper solution is impossible to determine.

Brass has almost twice the thermal expansion rate of steel, and is softer. If anything, one would expect the brass to expand, and possibly deform so that it is looser, if brass is through-bolted to a steel surface with a steel bolt.

If the screws are into the brass, as it may be, then the brass would expand away from the steel bolt, possibly allowing other materials to be drawn into the space created. That could be corrosive materials, which would tend to corrode and seize the brass or the screw, or it might be relatively inert material that simply leaves behind a residue that mechanically jams the screw in the hole.

In either of those cases, some form of anti-seize material would be helpful.

I cannot see any mechanism that could cause the screws or bolts to tighten up against the brass, but that does not mean there isn't one that I have not thought of.

[Noitoen]

Could try this:

https://jax.com/product_directory/food-grade-anti-seize/

[Doozer]

Magnesia, Greece is where magnetic loadstones were first discovered.
As for the heaters attachment, I was also going to suggest silicone washers.

-D