Causes and Control of Propeller Hub Cracking
by LL Walker, Houston TX
Any cracking in the hub of a marine propeller must be regarded as serious, and any steps which can be taken to prevent such cracking are desirable. The purpose of this paper is to enumerate some of the more obvious causes of hub cracking and to suggest certain precautionary practices which can be adopted by all those who are vitally interested in the alleviation of the problem.
It is suggested that there are at least four principal causes of hub cracking. These are listed in what is perhaps the order of their importance.
1. Excessive heating at installation or removal.
2. Improper fit of key in keyways.
3. Improper fit of shaft and hub tapers.
4. Residual casting stresses.
Discussion of these suggested causes will be in the reverse order of their listing.
Residual Casting Stresses
The stresses in a casting due to shrinkage in cooling often increase, to a considerable extent, the stresses due to the operational load. The propeller manufacturer meets this problem by giving unusual care to the uniform cooling of the casting in the foundry, Commenting on this aspect of propeller manufacture Mr. A. J. Smith, Metallurgist and Assistant Foundry Superintendent, Bethlehem Steel Company, Shipbuilding Division, has said: (MODERN CASTING, Jan” 1959)
“Most propeller designs are conducive to good directional solidification as the hub section is tapered and cast with the large end up. The massive surface area of the blades … tends to dissipate heat rapidly and reduce time required for complete solidification of the casting.”
In addition to the fact that the propeller design lends itself to uniform cooling, and hence good directional solidification, the judicious use of external chills further controls the cooling. There is admittedly some possibility of hub cracking from residual casting stresses, but, because the propeller manufacturer is first and foremost a skilled foundryman, the possibility is remote and cracking from this cause must be considered the least important of the causes named.
Improper Fit of Shaft and Hub Tapers
When the fit of the tapers of a hub and shaft is once established there is little reason for this fit to be disturbed unless actual physical damage to either the hub or the shaft occurs. The fit of the hub and shaft, however, should never be taken for granted, for the possibility of physical damage to one or the other is all too great. It is to be recommended that a responsible representative of the owners or the shipyard make some positive check at each propeller change in order to assure the continued integrity of the fit.
This is particularly true if any damage has been sustained by the propeller shaft in or near the taper, or if for any reason the propeller has become loosened on the shaft; and it is equally true if, for reasons to be suggested below, the propeller hub bore has distorted. Under any of these conditions a thorough inspection of the propeller and shaft tapers should be made and remedial action should be taken before the vessel is released for return for service.
It should be pointed out that verifying the fit of the propeller and shaft is not a proper function of the propeller repair shop, but must be done at a time and place when the propeller can be actually fitted to the shaft on which it is used. Many, perhaps most, propeller repair shops do not have a full range of taper gauges, and there is little that these shops can do except to ascertain that the propeller bore is concentric at each end. Even so, any propeller repair shop can and should make a good visual inspection of the hub bore for any evidence of looseness, wear or uneven contact with the shaft. Such a visual inspection ought properly to be a part of the repair procedure, and any evidence of wear or uneven contact should be reported to the owners.
Improper Fit Of Key In Keyways
The improper fit of the key in the keyways is always a real cause of hub cracking, and for obvious reasons. If the propeller is forced over a key of improper size or one which does not fit squarely into each keyway forces of a large order will be applied to the hub bore. Even if the improper fit does not then produce hub cracking, the condition may cause a loosening of the propeller on the shaft with subsequent risk of damage to both tapers as well as excessive vibration in operation. As a result of careful investigation of numerous cracked hubs, it is the opinion of this writer that when a crack originates in the keyway it is almost certainly the result of improper key fit.
While not a part of this immediate problem, it should be noted that the use of filleted keyways can reduce the possibility of the kind of stress concentrations which have long been associated with the use of keyways with square-cut corners. This excellent practice deserves more wide-spread use.
Excessive Heating At Installation Or Removal
The most general cause of hub cracking is the application of excessive amounts of heat at the time of installation and removal of the propeller from the shaft. This stems almost entirely from the fact that the mechanics actually doing the job do not realize and have never been told how little heat is actually required to expand the propeller hub sufficiently for their purposes.
It seems likely that the actual cracking may occur in any one of three different ways. First, if excessive heat is locally applied it may cause uneven expansion and hence breakage. Second, excessive heat at installation may cause sufficient hub expansion to allow the hub to move too far forward on the taper with the result that the hub will cool at a point where shaft diameters are larger than those of the hub bore. Breakage, then, may well result. Third, the excessively hot hub may be quenched to hasten further work. This may readily cause cracking.
Cracking from excessive local application of heat is readily recognizable from the marked discoloration of the metal where the heat has been concentrated. In some extreme cases small areas will actually begin to melt. Cracking due to cooling and contraction of the hub after being allowed to move too far on the taper is much more difficult to recognize, and the location and nature of the crack may cause it to be confused with a crack caused by poor taper fits or even improper key fit.
Heating of the hub for removal, particularly of larger propellers, may be regarded as having some justification. The only real problem is that the heat be regulated and that persons entrusted with this task be instructed as to the amount of heat required to do the job. In this case the least amount of heat is usually the best amount. Heating the hub for installation, on the other hand, must be regarded as wholly unnecessary. There is nothing to justify this practice. A properly machined shaft and hub, when clean and free of any burrs, can be tightened to any acceptable standard by use of the shaft nut alone. When the hub is heated and shrunk on the shaft tremendous hoop stress results so that the hub is operating under a stressed condition that is wholly unwarranted and for which it was never designed. No propeller manufacturer can or will assume any liability under such circumstances and the vessel owner should realize that he indulges in the practice at his own risk.
Heating in shipyard is usually with the oxy-acetylene torch. The oxy-acetylene flame has a temperature of very nearly 6000oF. Propeller metals melt at temperatures far below this. For example, manganese bronze melts at about 1800oF., while a typical austenitic stainless steel melts at slightly more than 2600oF. The amount of heat available in the oxy-acetylene flame is, therefore, far in excess of the needs and the greatest of care should therefore be exercised in its application. The use of a standard welding or cutting torch for heating should be avoided. Because of the nature of the welding or cutting tip the flame is tightly concentrated, and the result may be a localized application of an amount of heat far beyond the requirements for loosening the propeller on the shaft.
That extremely high temperatures are not required for loosening a propeller on the shaft is made evident by an examination of the expansion rates for various alloys. It should first be borne in mind that alloys expand relatively more in the temperature range up to 212oF. than they do at higher temperatures. The chart below indicates the relative expansion of a medium carbon low alloy steel, an austenitic stainless steel, a manganese bronze and a nickel-aluminum bronze.
Since the tolerance for the small end diameter of propeller hubs, as prescribed by the SAE standards, is only plus or minus one-thousandth of an inch, it can be seen that the hub need not be expanded more than a few thousandths of an inch to cause it to become loosened from the shaft. Because this paper has been intended to be nontechnical and non-mathematical no calculations of any kind are being submitted, but anyone desiring to do so may calculate the mechanical effects of heating by making use of the appropriate tables appearing in any of a great number of handbooks.
The above chart indicates that some alloys expand far more than others at the same temperature. This does not mean that the steel, for example, must be brought to a still higher temperature, for it can be readily shown that sufficient expansion of a propeller hub can be obtained within this temperature range. What it does mean is that mangenese bronze may require less heat to accomplish the desired expansion.
Recommendations and Conclusions
We know that whatever the desired temperature may be it cannot be produced uniformly over the entire surface of the hub merely by the application of heat from a torch. However, it is necessary to apply the heat as uniformly as circumstances permit, not allowing the torch to dwell over-long in any one place but being certain that it is passed from end to end of the hub and from one portion of the hub to another in succession. The surface temperature should not exceed about 200oF. for any propeller alloy.
No mention has been made of the possibility of hub cracking from impact. The nature of the propeller blade is such that it tends to absorb the greater part of the impact sustained by the propeller, and there is nothing in the literature of propeller design to indicate that the propeller hub is ever considered vulnerable to damage from impact alone.
There seems to be little value in increasing the hub diameter beyond some optimum which is generally considered to be about 15%. of the propeller diameter. Most propeller manufacturers do not recommend that the shaft diameter exceed 60%. of the hub diameter, although one nationally known manufacturer has used smaller hubs for years with excellent results. Not only are exceptionally large propeller hubs detrimental to performance in most cases, but it is a fact that the mechanical properties of most castings decrease as the section size increases. It must be concluded, therefore, that hub size has relatively little to do with hub cracking.
While some hub cracking can be sufficiently repaired as to give acceptable service life, the practice of hub repairing by any method now known to this writer is not to be recommended. The factors which argue against the practice are: residual welding stresses which cannot be properly relieved; distortion of the bore either from its opening up as a result of the crack, or because of warpage related to the welding. process; and, finally, the unfavorable ratio of repair cost to guaranteed service life. Hub repairs can be and are being made, and propellers with repaired hubs have been known to last indefinitely, but the fact still remains that such successful repairs are more often the result of a happy conjunction of events than the predictable skill of the operator making the repairs.
Conclusions and recommendations
From the foregoing discussion it can be seen that the principal causes of propeller hub cracking are excessive heating at installation and removal of the propeller, improper fit of the key in the keyways, improper fit of shaft and hub tapers and residual casting stresses. It has been shown that of these causes the most general and the most serious is that of excessive heating. It must be concluded that cracking for this reason continues only because mechanics doing the work do not know and have not been told how little heat is required. From these conclusions certain recommendations can be made which would divide the responsibility among the propeller manufacturer, the propeller repair shop, the shipyard and the propeller owner.
1. A propeller should never be heated for the purpose of installation. The propeller hub is not designed to be shrunk on the shaft and such practice is wholly unnecessary if the tapers are clean and free of burrs or irregularities.
2. The propeller key should never be allowed to bottom in the propeller keyway. It is a good rule to allow from .020″ to .030″ clearance between the key and the bottom of the propeller keyway. Keys should be made accordingly and owners should arrange for proper inspection of propeller keys and appropriate re-work when necessary.
3. The use of a propeller puller should become standard practice in every shipyard. Every propeller having a hub small end diameter of sufficient size to accommodate such bolts should be drilled and tapped for two puller bolt holes. Owners of propellers now in service should have those propellers drilled and tapped. Propeller manufacturers would be well advised to include such holes in new propellers, and to adopt standards for or at least recognize the relationship of bolt hole and propeller hub or shaft diameters.
4. Shipyards should thoroughly acquaint themselves with the implications of this problem and take such precautionary steps as may be necessary in the individual yard. The first step is to investigate current propeller removal practices in the light of this new knowledge of the problem and to provide such instruction as may be necessary to inform employees and to remedy any present unfavorable practices. The second step is to be certain that proper heating equipment – which would be available in almost every shipyard – is on the job site and in use for propeller removal. A third step is to encourage owners to have their propellers drilled for puller bolts and to show them that this small one-time cost will have long-range benefits. Finally, the shipyards should realize that the money spent for a strong hydraulic or mechanical universal puller will be the means of saving costly man-hours, and that the time and material involved in applying excessive amounts of heat can be saved.
5. Propeller repair shops should incorporate in their repair procedures a systematic visual inspection of the hub and bore of every propeller for any evidence of uneven contact or looseness on the shaft, for any evidence of bore distortion or lack of concentricity, and for evidence of physical damage to the bore or keyway which might prevent proper fit on the shaft at the next installation. Such shops should also inform owners of the benefits of using the bolt-type puller and encourage them to have their propellers drilled and tapped accordingly.
6. Owners should come to the realization that the fit of the propeller and shaft cannot be taken for granted. Regardless of the reliance of the owner on the judgment of either the shipyard or propeller repair shop, the fact remains that the ultimate responsibility rests with the owner or his representative to require that the necessary inspections be made and to require of the shipyard or propeller shop a report of their findings. When propeller shaft damage has been sustained the owner should require a precautionary re-fit of the shaft and propeller for his own protection – and he should be prepared to pay for the cost of such additional service. If the owner hauls his vessel at various yards and is not sure that proper puller equipment is available he should provide his own and carry it aboard.
It was the announced purpose of this paper to enumerate some of the more obvious causes of propeller hub cracking and to suggest certain precautionary practices which can be adopted by all concerned. If the paper has at all accomplished these ends then it can be counted as being of some slight value to the industry.
This paper has been prepared by L. L. WALKER, JR.
Houston, Texas, as a service to the marine industry LL Walker, Jr. had one of the first, if not the first, propeller shop in Houston, TX. In early days he worked on airplane propellers. He was well respected in the marine business. He desired to contribute to the professionalism of the shipyards by offering advice on good installation practice. I found this paper in some old files, and it is as true today as it was when it was written. I do not know when that was, but my guess was about 1970. Mr LL Walker was also a member of The Rotary Club of Harrisburg and that club continues to meet every Tuesday. The name Harrisburg refers to the early city of Harrisburg TX. It is part of Houston TX now.