Coupling Parts made from Hard and Soft Materials 2

Frictional units incorporating parts made from hard and soft materials must be designed so that the rubbing surface of the harder and more wear-resistant material overlaps completely the rubbing surface of its mating member made of a softer and less wear-resistant material. Observation of this rule assures a uniform wear of the softer part. If the soft surface overlaps the harder one, a stepped worn spot appears which impairs the operation of the unit. Bellow you can find some typical applications:

Wrong design: In the taper unit in which the tappet slides in a bush, the oil-distributing recess is provided in the tappet stem.

Correct design: The more reasonable design has the recess machined in the bush, thus ensuring equal wear of the stem and bush.

The above principles remain true for fixed connections made of materials that differ in hardness. It would be wrong to fit a soft hub onto a conical portion of a steel shaft. The hub front face (the one nearest to the nut) extends beyond the shaft taper. As the hub is ground-in to the taper, and while the hole loses its roundness in running, a step is formed at part  . This step obstructs the hub assembly on the shaft in a range of repeated tightening.

In correct design the shaft taper extends beyond the hub, so that changes in the hole size when fitting and because of bearing deformations in no way interfere with the proper tightening. 

When the material of the hub isharder than that of the shaft (a fact rare in practice), then the most dangerous case is the one when the hub rear face does not reach the start of the taper, because now, in the process of grinding-in and tightening, a step is formed at length   of the shaft.

In the correct design the hub end overlaps the taper.

I

n a pinion carrier of a planetary drive, whose disk is attached to the casing by means of a satellite shaft, it is practically impossible to maintain precisely distance  in respect to all fixture points. Because of the inevitable inaccuracies in the taper diameters and axial distances between them, the lengthwise disk displacements in the course of tightening will differ from shaft to shaft. This displaces buckles and overstresses the disk. It is also impossible to ensure registration of holes in the connected parts by machining them together (as is often done with cylindrical holes). In fact, the parts cannot be accurately assembled.

The design, where the shafts are with one tapered end, is a better one because the errors are halved.

IMPORTAND

Conical fits fail to assure accurate longitudinal positioning. The mutual position of the fitted parts depends to a great extend upon the manufacturing accuracy of the tapers both on the shaft and the hub, upon the tightening force, number of reassemblies, bearing deformations and wear of the surfaces. For these reasons conical fits must not be used when accurate axial positions are required.

Such kinds of assemblies should have cylindrical fitting surfaces with the disk tightened up on stops. The distances between the fixing steps on the shafts can be held to close tolerances. Registration of the holes centers in the cover and casing is attained either by machining the holes in a jig or by through machining them together.