Interference fits are quite frequently utilised to assemble other components into zinc castings, particularly bearings, be they rolling element or sintered bronze or iron. It is usual for these bearings to be press fitted into as cast holes or perhaps cast holes that have been sized by pushing a tungsten carbide ball through them.
It must be appreciated that the interference fit will normally produce a high initial stress in the casting, which will relax quite rapidly to a lower “final” level. The magnitude of this “final” stress depends upon temperature and the alloy chosen. The load on the fitted part will depend upon the stress in the surrounding casting and its section. Hence a heavier wall section surrounding the fitted component will achieve a higher retaining load.
It is a common engineering practice to specify the retention capability by a “press-off load,” which is the axial force required to separate the members. Note that this force cannot be predicted from accepted textbook formulae for the interface pressure between cylindrical members, for the following reasons:
Formulae for interface pressures apply only to cylinders, whereas the actual components usually incorporate irregular features.
There are insufficient data for the coefficient of friction of zinc alloys against other metals at the pressures developed by interference fits.
Zinc alloys will relax, causing the interference forces to diminish with time.
Under conditions of high interference, a steel member will remove metal from a zinc alloy member during assembly, altering the dimensions of the zinc alloy component.
Casting Loaded in Compression
The following procedure is recommended for interference fits with zinc castings where the zinc alloy is in compression (eg. A bearing pressed on to a diecast hub) or rolling element bearing is pressed in to a diecasting. Retention may be aided by supplementary means such as adhesives, staking, and crimping. An interference of 0.0005 to 0.0010 mm/mm diameter is recommended as a beginning point, subject to verification by test and development. The designer should investigate, by test and development, the effects of increasing the dimensional interference to increase retention. In the actual case, some of the zinc alloy may be removed during the assembly operation so that stresses will not reach the levels indicated by calculations.
Casting Loaded in Tension
In the case of interference fitted bearings it is necessary to consider any affects that variations in section of the perimeter of the hole to receive it might have on the roundness and straightness and parallelism of the hole. Also the effects of these same section variations on the relative stiffness of the structure in different planes should be estimated. These will bring about a distortion of the bearing after fitting, which may or may not be permissible.
The relative thermal expansion coefficients of the zinc diecasting and the fitted component may also be a factor for elevated temperature applications and especially for those subject to temperature fluctuations. Usually the component fitted into a zinc diecasting has a lower coefficient of expansion. The effect of this is to reduce the stress in the casting when the temperature rises, with consequent reduction in the retaining load. Whilst not desirable from a functional viewpoint this does tend to counteract to some extent the increased relaxation rate that results from the higher temperature.
Fitting Sintered Bearings (also solid inserts)
For sintered bearings a different technique is appropriate which is far less demanding of the tolerances of the casting and bearing yet still gives highly consistent results in terms of retention properties and the internal diameter of the assembled bearing. (Note: This approach is only appropriate when the bearing is pushed into largely tubular section of the casting that has a fairly consistent wall section. Although it is still applicable when the tubular section is buttressed by ribs and passes through a wall section). The amount of interference with the casting under all combinations of bearing and casting dimension in their tolerance ranges, should in most cases be set to cause the casting to exceed its “elastic limit” when the bearing is pushed in. The diametrical expansion caused by pushing the bearing in should exceed 0.25%. In principle this will achieve a consistent internal diameter of the bearingprovided the bearing is fitted in the usual way with a sizing pin in place. It will be found that the ID of the bearing will grow in the first few minutes after fitting but thereafter will remain practically constant. Because it matters little by how much the elastic limit of the casting is exceeded, provided that the casting is not broken, the tolerance on the bore into which the bearing is fitted can be reasonably generous if the wall section around it is fairly consistent. It is important here to recognise that a wall section through which bearing bosses commonly pass constitute a short but very thick wall section of the boss. In consequence this section of the bore will behave in a different manner from the rest. If its internal diameter were identical to the rest of the boss then it would tend to cause a local constriction of the fitted bearing. Fortunately in most cases the ID of the boss where it passes through the wall section tends to be slightly larger than the rest, because of contraction effects during cooling in the die. As a result constriction of the bearing where it passes through the wall section is not normally a problem.
Fitting Rolling Element Bearings
For rolling element bearings the above approach might produce excessive stress in the bearing and hence potential damage to the races and rollers or balls. One way of avoiding this is to use crush ribs, this allows the yield stress of the alloy to be locally exceeded without developing excessive load on the bearing. This then allows tolerance on the hole diameter to be widened considerably, eliminating the necessity for secondary operations prior to assembly. This approach can also be used for sintered bearings if required. Note however that the retention forces achieved are lower than by a complete diametrical fit. The retention of the bearing, sintered or rolling, can be improved if required by crimping over a raised lip on the casting.
Crush ribs may be used to develop much of the retention force of a press fit without requiring the very close dimensional tolerances associated with interference fits. The ribs should be small, typically 0.4 mm. A slight relief at the root may enhance plastic flow of the rib.