|The focal track of this anode looks a bit
like my garden path. Repeated heating and cooling has roughened up the
surface, opening up small cracks, which have spread over the whole of
the area under bombardment. The end result resembles a stretch of
microscopic crazy paving.
To make matters worse, the bearings have developed a fault that causes the anode to fail to start up on some occasions, so that there are a number of areas where the tungsten has melted. If a droplet of molten tungsten were to fall on the glass envelope, it could melt its way through, leaving a small hole through which oil will flow, drawn in by the vacuum. On striking the hot anode, the oil would be vaporized, the rapid expansion would then burst the tube.
The rotation of the anode produces a characteristic sound that is different for different makes and models of tube, but the overall sound should follow the same sequence. The anode should be heard to accelerate to its normal running speed in about 1 second. At the end of the exposure it should undergo deceleration in about the same time, and then come to a halt. If fluctuations in speed can be heard during the time that the anode is running, this could indicate that the bearings are worn, and could fail completely without warning. If speed variations are heard, the tube should be inspected by a competent engineer. If there is any suspicion that the bearing are beginning to fail the tube should be replaced, since the consequences of a tube bursting would be much more expensive than simply replacing the tube. X-ray tube are high cost items, high power tubes costing tens of thousands of pounds. Low power tubes will of course be correspondingly cheaper.