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FIG. 11

To determine whether the clearance is right, one of the 2-inch plugs is taken out and some marking material, such as red lead or anything that would be used on a surface plate or bearing to mark the high spots is rubbed on the taper gage, and it is pushed into the gap between the buckets and intermediates as far as it will go, and then pulled out, the marking on the gage showing just how far in it went, and the nearest mark giving in thousandths of an inch the clearance. This is noted, the marking spread again, and the gage tried on the other side, the difference on the gage showing whether the wheel is high or low. Whichever may be the case the hight is corrected by the step-bearing screw. The wheels should be placed as nearly in the middle of the clearance space as possible. By some operators the clearance is adjusted while running, in the following manner: With the machine running at full speed the step-bearing screw is turned until the wheels are felt or heard to rub lightly. The screw is marked and then turned in the opposite direction until the wheel rubs again. Another mark is made on the screw and it is then turned back midway between the two marks. Either method is safe if practiced by a skilful engineer. In measuring the clearance by the first method, the gage should be used with care, as it is possible by using too much pressure to swing the buckets and get readings which could be misleading. To an inexperienced man the taper gages would seem preferable. In the hands of a man who knows what he is doing and how to do it, a tapered pine stick will give as satisfactory results as the most elaborate set of hardened and ground clearance gages.

Referring back to Fig. 11, at A is shown one of the peep-holes opposite the intermediate in the third stage wheel for the inspection of clearance. The taper clearance gage is inserted through this hole both above and below the intermediate, and the distance which it enters registers the clearance on that side. This sketch also shows plainly how the shrouding on the buckets and the intermediates extends beyond the sharp edges of the buckets, protecting them from damage in case of slight rubbing. In a very few cases wheels have been known to warp to such an extent from causes that were not discovered until too late, that adjustment would not stop the rubbing. In such cases the shrouding has been turned or faced off by a cutting-off tool used through the peep-hole.

Carbon Packing Used

Where the shaft passes through the upper head of the wheel-case some provision must be made to prevent steam from the first stage escaping. This is provided for by carbon packing (Fig. 12), which consists of blocks of carbon in sets in a packing case bolted to the top head of the wheel-case. There are three sets of these blocks, and each set is made of two rings of three segments each. One ring of segments breaks joints with its mate in the case, and each set is separated from the others by a flange in the case in which it is held. In some cases the packing is kept from turning by means of a link, one end of which is fastened to the case and the other to the packing holder. Sometimes light springs are used to hold the packing against the shaft and in some the pressure of steam in the case does this. There is a pipe, also shown in Fig. 12, leading from the main line to the packing case, the pressure in the pipe being reduced. The space between the two upper sets of rings is drained to the third stage by means of a three-way cock, which keeps the balance between the atmosphere and packing-case pressure. The carbon rings are fitted to the shaft with a slight clearance to start with, and very soon get a smooth finish, which is not only practically steam-tight but frictionless.

The carbon ring shown in Fig. 12 is the older design. The segments are held against the flat bearing surface of the case by spiral springs set in brass ferrules. The circle is held together by a bronze strap screwed and drawn together at the ends by springs. Still other springs press the straps against the surface upon which the carbon bears, cutting off leaks through joints and across horizontal surfaces of the carbon. The whole ring is prevented from turning by a connecting-rod which engages a pin in the hole, like those provided for the springs.

The Safety-stop

There are several designs of safety-stop or speed-limit devices used with these turbines, the simplest being of the ring type shown in Fig. 13. This consists of a flat ring placed around the shaft between the turbine and generator. The ring-type emergencies are now all adjusted so that they normally run concentric with the shaft, but weighted so that the center of gravity is slightly displaced from the center. The centrifugal strain due to this is balanced by helical springs. But when the speed increases the centrifugal force moves the ring into an eccentric position, when it strikes a trigger and releases a weight which, falling, closes the throttle and shuts off the steam supply. The basic principle upon which all these stops are designed is the same—the centrifugal force of a weight balanced by a spring at normal speed. Figs. 14, 15, and 16 show three other types.