Fig. 237 shows the method of holding tubes in a shell boiler. The ends of an ordinary tube are expanded to fit tightly against the sides of the holes in the tube sheets. The stay tubes, heavier than ordinary tubes, are screwed into the tube sheets. Stay tubes are spaced at intervals among the ordinary tubes to brace the tube sheets rigidly.

It is frequently the practice to flare or bead over the tube ends to insure tight joints.

Tubes of water-tube boilers are usually expanded into their headers. Very few water-tube boilers have screw tubes.

Fig. 237. - Method of Fastening Boiler Tubes.

Fig. 238 shows a tube expander for expanding tube ends. This consists of a sleeve S carrying three hard-steel rollers R in loose bearings, a cap C, and a tapered-steel pin P. The sleeve is placed in the end of the tube far enough to bring the edge of the cap against the tube sheet. The steel pin is driven in fairly tight, and is revolved by a small steel rod placed through one of the holes at the end. The pin presses against the inner edges of the rollers R, and as the pin revolves it turns the rollers around against the inside of the tube, expanding the end tightly against the hole in the sheet.

Fig. 238. - Tube Expander.

Beading is done by a beading tool, shown in Fig. 239. This tool may be struck by a hammer or operated by a pneumatic holder such as is used in chipping or riveting.

Fig. 239. - Beading Tool.