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Found 1 result

  1. I saw this posted online and wanted to share. (Credit: J Frost) Types of Staybolts Rigid bolts Rigid bolts are rigidly connected to the inside and outside sheets that form the 2 shells that make up the firebox. The bolts are either threaded (and then hammered over to expand the threads in the hole to create a seal. The end of the bolt in the firebox can then be “seal welded” which will mechanically join the bolt to the inside sheet of the firebox to allow the heat from the fire to pass from the end of the bolt to the sheet so the end of the bolt doesn’t become overheated) or full penetration welded to the sheets. On the small, keyhole shaped boilers of the 1800’s, rigid bolts were the type of bolts that were used. With the smaller fireboxes, the inside and outside shells didn’t expand and contract that differently from each other. As the fireboxes got larger and the sheets got bigger, the sheets would expand and contract differently enough in certain areas that it would cause the bolts to bend and form an “S” shape. This would stress the bolts and they would break. Traction engine boilers and boilers used with live steam engine use rigid bolts as the boilers are small enough that rigid bolts are used without a problem. Flexible bolts With the larger, more rectangular boxes it was noticed that certain areas of the box would break the rigid bolts. These areas are known as, “breaking zones,” and were around the front and rear corners of the firebox and where the side sheets transitioned into the crown sheet. The flexible bolt was invented to give more flexibility in the bolt without having it break. The bolt has a ball end that rides in a sleeve on the outside of the boiler and the bolt is rigidly attached (threaded or welded) to the inside firebox sheet. The bolt is inserted through the sleeve and the sleeve is sealed with a gasket and a cap. The ball and socket joint (similar to hip and shoulder joints) will allow the bolt to rock while still bearing its load. As the inside sheet moves relative to the outside sheet, the bolt will rock on the ball and socket joint and will form a gentle curve instead of trying to make an “S” curve like the rigid bolts. Expansion stays The firebox sheets are anchored to the mud right. As the inside of the firebox heats up, the sheets will expand upward. This can put a heavy strain on the sheets, stays and knuckle joint where the crown sheet joints the top of the rear flue sheet as the rear flue sheet will also be expanding upwards. Expansion stays were invented to allow the crown sheet to pick up as it is being heated up by the fire, before it has steam pressure, and then settle back down to its normal position once there is steam pressure. The expansion stays are rigidly attached (threaded or welded) to the outside wrapper sheet and to the crown sheet. The top part of the expansion stay forms a yoke near the crown sheet. The yoke has a hole through it to allow the crown sheet part to slip through and be threaded into the crown sheet. The crown sheet part of the stay has a head on it that will bear against the bottom part of the yoke. When the crown sheet expands upward, the crown sheet part of the stay will slip up in the yoke. As the steam pressure builds up and pushes the crown sheet back down, the head of the crown part will bear against the bottom of the yoke and the expansion stay will support the crown sheet just like a rigid bolt. Expansion stays were generally installed in the first few rows of bolts in the crown sheet. Compromised bolts If the holes in the firebox sheets become too large (due to removal of old bolts and installing of new bolts), the new, too large bolt, will be too rigid and won’t bend enough and can cause the sheets to bend and crack instead as the sheets expand and contract. The ends of the bolt are made the correct size to fit the oversized hole and then the middle of the bolt is turned down to the desired diameter to give the flexibility desired, while not going too small of an area to properly support the sheet. Tell-tale holes The hole in bolts is called a, "tell-tale hole." With rigid bolts (threaded/welded into both sheets as in these pictures) the hole is 1-1/4" deep (so it will extend pass the firebox sheet as the bolts break near the sheets) or drilled all the way through the bolt. The holes are 3/16" in diameter and if the bolt is cracked or broken, water will weep or squirt out the tell-tale hole showing that the bolt is broken. For a compromised bolt, the hole needs to extend into the necked down diameter. For flexible bolts, if they have tell-tale holes, the holes are drilled through the entire body and drilled at least into 1/3rd the diameter of the head. That way if the bolt is broken/cracked any place along its length, the leak will show through the tell-tale hole. If flexible bolts are drilled hollow, the caps do not have to be removed and heads hammered to make sure the head is still attached to the bolt. Threads With threaded bolts, the USA standard is that all stays have 12 threads per inch (12 tpi). When a bolt is removed and holes are cleaned up, if the threads are damaged, a new larger tap is run through the hole to enlarge the threads to the next size. Stay bolt taps are a special type of long tap that has a reamer on the end to expand the old hole out to the correct size for the next size larger bolt. The tap is run through bot sheets and the threads in both sheets are in time with each other.
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