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June, 1940

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The reciprocating steam engine is probably the most reliable of all prime movers, but, like all other machines, gives trouble at times. Some of these troubles are considered briefly in what follows.

Groaning

An engine will sometimes groan, or utter a low, continuous rumbling sound, as though it were lamenting the fact that it has to work so hard. This groaning is usually an indication of undue friction of the working parts, notably the pistons and piston rods, valves and spindles, and may be the result of unsatisfactory lubrication, or imperfect alignment of the various parts.

The lubrication may be unsatisfactory either because the parts are not receiving sufficient oil, or because the oil is of unsuitable quality. In the former case, the lubricators may not be functioning properly; or oil holes and grooves may have become more or less choked with gummy deposit and dirt. Groaning is often an indication of general neglect to keep all the lubricating arrangements, on which so much depends both as regards wear and tear and power economy, in first-class working condition. If the oil is of unsuitable quality, particularly if it has a tendency to become gummy, the groaning is likely to be especially noticeable when the engine is started up.

To obtain the best piston and cylinder lubrication, a good deal of experimenting with different cylinder oils may be necessary. It is a wise policy to avoid using cheap oils ; they are in most cases responsible for early wear of the piston rings and the cylinders, and for much waste of power in overcoming frictional resistances. Much trouble may be saved by following the advice of an oil expert as to what is the most suitable oil to meet the actual working conditions. As to the right quantity of oil to use, this can only be determined by opening out the cylinders occasionally and inspecting the bores. If there are any dry or specially bright places, insufficient oil is being supplied; if there are signs of the oil running down the surfaces, then too much. The ideal condition is a fine film of oil spread uniformly over the whole of the cylinder walls.

If the various parts are not in true alignment, excessive friction is inevitable, because misalignment results in certain moving parts bearing unevenly against the parts which support them, as for example, a piston rod against a gland, or a shaft in a bearing, and this means excessive rubbing pressure at certain places, which tends to press out the lubricant from between the surfaces, or prevent it from getting between, so that there is metal-to-metal contact, and great friction. When an engine is first installed, the greatest care is taken by the makers to get all the parts accurately lined up, but after some years working, so much wear may have taken place that the alignment is no longer true; and there is the possibility that the foundation, or a portion of it, may have settled slightly, so causing the engine to be thrown out of line.

Hot bearings

When a shaft journal runs in a bearing, there must even under the best conditions be a slight amount of friction between the journal and bearing surfaces, and to overcome this friction mechanical work has to be done, which is converted into heat. Hence, whenever a shaft is set in motion, there must be slight rise of temperature at the journals and bearings, but if the conditions of running are satisfactory, heat will be radiated from the bearing as fast as it is generated ; wherefore, after a short time, a maximum temperature will be reached, which will not be high enough to cause any practical difficulties. When the running conditions are not satisfactory, the temperature may continue to rise until the bearing becomes seriously overheated, in which case such troubles as scoring of the journal and the bearing brasses, running of the white metal, surface cracking of the journal and deep cracking of the bearing, may arise. There may also result seizure and breakdown. A hot bearing is obviously something to be carefully guarded against.

There are several causes of hot bearings, such as faulty lubrication, wrong adjustment, imperfect alignment of the shaft, too heavy a load on the bearing, and unsuitable design.

Generally, when a bearing gets hot, the best plan is to stop the engine and investigate, unless an examination under running conditions reveals that a lubricator is not operating properly or that it has become short of oil. If the lubricator is of the syphon type, it may be that the wick has not been inserted, or that it has been carelessly inserted so that the end down the oil hole is at a higher level than the end in the oil well, in which case, the syphoning action cannot take place. If the bearing is of the ring-oiled type, perhaps a ring has ceased to rotate, so that the oil is not being lifted to the shaft. When shutting down is to be avoided if at all possible, and the lubricators appear to be functioning properly, increasing the oil supply and slightly easing the bearing cap may be beneficial. Some engineers recommend castor oil for a hot bearing, and others, finely powdered sulphur, which, if added in small quantity to the oil, is said to have a pronounced effect in cooling the bearing. Under no circumstances should the bearing be cooled by throwing cold water over it, because the sudden cooling is apt to set up surface cracks in the shaft journal, and to fracture the bearing cap or brasses.

If the engine can be shut down and the bearing brasses removed, the oil passages and grooves should be cleaned out if necessary, and an examination made for any signs of grit, which is a common cause of overheating. When the brasses are replaced, and the bearing cap screwed down hard, they may bind on the shaft (which would at once account for the overheating), in which case, it will be necessary to ease them by scraping, or perhaps fit a liner at the joint.

When persistent overheating is experienced, imperfect alignment of the shaft is a likely cause, because when the shaft is not truly lined up it exerts abnormal pressure at the ends of the bearing.

A notable feature of the steam engine is that it is capable of driving a load much in excess of that for which it is designed. This accounts for the fact that steam engines frequently become overloaded as the consequence of installing more and more machinery, and this overloading commonly manifests itself by persistent overheating of the main or crankshaft bearings. Overheating of these bearings may also be caused by having the main driving belt too tight.

Unsuitable design of the bearing may account for persistent overheating, in which case, the only remedy will probably be to replace the bearing by one of more suitable design.

In the great majority of cases, the trouble in question may be traced to one or other of the causes above mentioned. Very occasionally, the greatest difficulty is experienced in accounting for the trouble. Thus, in one instance, a big-end bearing gave trouble from the first, and for a long time, all efforts to cure it proved unavailing. Finally, it was suspected that the cause was unsatisfactory balancing of the engine. A balance weight was accordingly fitted to the flywheel, and this effectively overcame the trouble.

Pounding and knocking

This is one of the most serious running troubles, and will almost certainly lead to early breakdown if it is not quickly prevented.

The usual cause of pounding, thumping, or knocking, is looseness of some part, or wear and consequent lost motion. The working loose, for example, of a piston nut, will allow the piston to become slack, so that it may bang violently on the rod at the end of the stroke, when reversal of the piston motion takes place, so imposing excessive stresses both on itself and on the rod; also, wear in a big-end bearing will cause violent banging because the connecting-rod end brasses keep moving away slightly from the crank pin and then coming into heavy contact again.

In many instances, the cause of knocking is quite obvious, but in others it is far from being so, and much difficulty may be experienced in locating the knock because the noise sometimes manifests itself at a place far removed from the source of the trouble. Thus, looseness of the flywheel keys may cause knocking which appears to be due to something wrong in the neighbourhood of the crosshead and piston rod. Again, the wearing slightly oval of an eccentric may give rise to a pounding which travels along the eccentric rod to the cylinder. A simple form of stethoscope consisting merely of a hard wooden rod may be of great value in locating the source of elusive knocking of this kind. Thus, it may be difficult to determine whether or not the source of a knock is inside a cylinder, but if one end of the rod be placed against the cylinder, and the other held firmly between the teeth, and if the ears be then closed by the fingers, the noise can be heard much more distinctly, and therefore may be more easily located. During recent years much attention has been given to this question of locating internal and external sounds about machinery, and special sound-detecting instruments have been devised, which may be purchased for a few pounds.

A possible cause of knocking which should always be borne in mind is misalignment of the parts, which, as We have seen, is also a cause of hot bearings.

Not infrequently, knocking occurs after an engine has been overhauled or repaired. Thus, the taking-up of the connecting-rod end brasses with the object of preventing knocking due to slackness or wear of the brasses may be followed by knocking in the cylinder. The probable explanation is that the piston has worn the cylinder until ridges have formed at the ends of the stroke, and the adjustment of the connecting rod end brasses has slightly altered the centres or effective length of the connecting rod, so that the piston strikes against one of the ridges every revolution of the engine. If the clearance between the piston, when at the end of its stroke, and the cylinder end, were very small, it is possible that after adjustment of the brasses, the piston might strike against the cylinder end. This would cause very heavy banging, and perhaps breakage of the cylinder end and the piston. Similarly, with slide-valve engines, adjustment of the valve gear may result in the valve striking against one end of the valve chest.

In a few instances, where the cylinders have been fitted with liners, the slight shifting of a liner has been responsible for heavy knocking which has proved most difficult to locate, and occasionally a considerable amount of dismantling and much unnecessary repair have been carried out before the true cause has been discovered.

An indication of the engine will sometimes furnish a clue as to the cause of mysterious knocking. For example, the indicator diagram will show if the valve setting is correct, and whether there is a suitable amount of steam compression at the ends of the stroke. When there is little or no compression, the moving parts are not brought smoothly to rest at the ends of the stroke, as they should be, and if there is any slackness or wear of the parts, there will be a much greater tendency to knocking than would otherwise be the case. On the other hand, excessive compression might itself be a cause of knocking by lifting a slide valve repeatedly from its seating at the ends of the stroke.

Water in the cylinders

This is a common cause of knocking, and also of engine breakdowns. The presence of water in the cylinders may be accounted for by priming in the boilers, and by excessive condensation of steam in the steam pipes and the engine cylinders. Opening the drain cocks will generally stop knocking caused by water in the cylinders, but this is only a temporary measure. The great thing is to avoid priming in the boilers by working with a reasonably low water level, using good water, firing without forcing, opening the stop valve always very gradually, and to avoid excessive condensation by keeping steam pipes and engine cylinders efficiently covered by good non-conducting composition. A good separator connected with the main steam pipe near the engine will arrest much of the water which comes over with the steam. Any water which enters the cylinders is of course swished about by the rapidly moving pistons, but if sufficient water enters, the clearance space between the piston and the cylinder ends will be completely filled, and the piston will then strike against an incompressible mass of water, so that a disastrous breakdown is almost inevitable. Superheating the steam eliminates the danger of water in the cylinders, and since it prevents initial condensation, which is one of the principal sources of loss in the steam engine, it also effects considerable economy of steam and coal.