Steam Boilers and Stay Bolts

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Here is shown how steam pressure tends to bulge boiler head.
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Hollow stay bolt installed in water leg of boiler.
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This drawing shows how a pressure is equal all around the circular surfaces and requires no bracing or staying.

3196 Macarthur Road, Decatur, Illinois 62526.

Pressure within an approximately cylindrical vessel is to force
the shell into a truly cylindrical shape. Thus any steam boiler
surface under pressure and which is not cylindrical or
hemispherical in shape has forces on it which tend to cause the
surface to bend out in a curve. On the other hand a flat boiler
head is subjected to a high internal pressure and has a tendency to
bulge as shown by the drawing. To prevent the plate from assuming
the bulged shape and possibly exploding or cracking, stays or
braces must be used to hold it. Stay bolts in a steam boiler should
support the plate completely. They should allow free water
circulation and they should allow room for boiler inspection. The
principal types are the solid screw stay bolt and the hollow bolt.
The solid screw stay bolt is completely threaded and is screwed
into the boiler plates so both ends extend inch. All stays in the
water legs must be hollow or have holes drilled in the ends
3/16′ in dia. and ‘ deep. If the bolt breaks, steam or
water will rush out and give warning of the break. The hollow stay
bolt is threaded only on the ends as the middle is smooth and
resists corrosion. The hole is rolled during manufacture. All stay
bolts have 12 threads to the inch and are usually the U. S.
standard thread. If either end of the bolt projects over ‘ it
should be neatly cut off. Both ends are then hammered over to form
a head. This is called upsetting the stay bolt and prevents any
pressure leak. The tubes of boilers on steam traction engines have
another purpose other than helping raise steam pressure. This is
that they have a staying effect and help hold both ends of the
boiler in the desired flat shape and help to prevent bulging. This
is true only when they have been properly expanded, rolled and
beaded over.

I have tried to make this article as brief as possible and still
give the basic information. I hope you can use this sometime and
perhaps I could submit other articles as I believe this is one area
where your fine Album could be improved. All information submitted
has been carefully authenticated.

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