It is common practice in gearboxes to configure the coupling sleeves and the coupling rings of the disengagable gearwheels with coupling teeth which are tapered so that mutually contacting longitudinal ends of the teeth on the coupling sleeves and the coupling rings are V-shaped, i.e., the teeth are wider at their ends than they are at a distance from said ends. It is customary to say that the flanks of such coupling teeth are configured with a “back rake”. The rotational force upon mutually contacting surfaces with back rake will consequently produce a resultant force which acts in the direction of engagement of the coupling sleeve in order to prevent an engaged gear from being unintentionally disengaged.
In the manufacture of the aforementioned coupling elements, efforts have previously been made to configure the back-raking toothed portion with flanks having the same pressure angle as the original tooth flank. In this way, the least possible material quantity is pressed aside or removed from the original flank.
Following manufacture of a coupling element having constant cross-sectional profile of the coupling teeth, various machining methods have been utilized in order thereafter to form toothed portions having back-raking flanks. The two most common methods are press-rolling and machine-cutting. Both have advantages and disadvantages.
In press-rolling, the material is pressed aside from the portion with back rake. This method has the advantage that the tooth can be configured with back rake right down to the tooth root without the need for undercutting, which otherwise weakens the tooth. The disadvantage is that the wear on the tool is very high, resulting in relatively short service life, so that the tool costs become relatively high. Pressed-aside material is left behind as a bead, which can also be disadvantageous.
In the other method, material is removed by machine-cutting using a cutting tool to which motion along a cycloid curve is imparted. The advantage is longer service life of the tool but, in return, it is not possible to form, for example, internal teeth on a coupling sleeve with back rake right down to the tooth root without any undercutting in the root of the profile of the original tooth or, in the absence of undercutting, without a sharp shelf being formed in the tooth at a distance from the bottom of the tooth space of the original tooth. In both cases, this can mean that the strength is reduced in the coupling teeth with back rake. Undercutting can mean, moreover, that the strength is reduced in the profile of the original tooth, which can be a major disadvantage should the continuation thereof, substantially axially in connection with the back-raking coupling teeth, be used, for example, in a splined joint or as a rolling tooth against an opposing gearwheel.