Screw-type extruding machine having a screw element defining a groove with an expansion region at each end thereof

A screw-type extruding machine includes a housing; a driveable shaft disposed inside the housing and including a rotational moment transfer element; and at least one screw element. The screw element has two ends and defines a longitudinal groove therein for receiving the rotational moment transfer element of the shaft, the shaft thereby transferring its rotational moment to the screw element. The groove is configured such that it defines two expansion regions, each expansion region having an axial extent, the groove having side surfaces which diverge with respect to a longitudinal axis of the groove along the axial extent of each expansion region in a direction toward a corresponding one of the two ends of the screw elements, a width of the groove thereby increasing in a direction toward its two ends along each expansion region.

FIELD OF THE INVENTION 
The invention relates to a screw-type extruding machine for processing 
materials, in particular for the processing of plastics, having at least 
one drivable shaft that is arranged inside a housing, onto which 
wear-resistant screw-type elements can be inserted in an optional 
sequence. 
BACKGROUND OF THE INVENTION 
Nowadays, screw shafts for screw-type extruding machines are designed in 
accordance with the prior art in a modular fashion. For that, screw-type 
and kneading elements for the most varied functions such as conveying, 
plastifying, mixing and cutting, homogenizing, degasing and building up 
pressure are fitted in a freely selectable sequence onto a shaft. An 
interlocking connection secures the screw elements against twisting and 
permits the transfer of the rotational moment from the rotating shaft to 
the screw element. 
The different process functions require that the work material be adapted 
to these functions, e.g. that it be wear-resistant, anticorrosive or a 
combination of both. Conditional upon the respective connecting systems 
for the shaft/hub (feather key, six-spline, multi-spline or involute tooth 
system (e.g. DE-OS 15 02 337)) stress concentrations tend to occur at the 
notches or grooves of those connecting systems. In particular with 
through-hardened elements made of wear-resistant materials (tool steels, 
cold-working steels, rapid-machining steels) these stress concentrations 
can lead to a breaking of the element, which can result in considerable 
damage in a screw-type extruding machine. 
The use of spring-actuated keys as transfer elements for the rotational 
moment between shaft and screw elements to increase the load capacity of 
the shaft is known from DE 42 06 219 A1. 
Apart from the fact that this measure can be used only when using feather 
keys as transfer elements for the rotational moment and a groove as a 
longitudinal recess (and not when using multi-spline, involuted tooth 
system etc.), it is not possible to purposely reduce the stress peaks (hub 
edge stresses) that occur at the screw elements in the above arrangement 
because the deformation of the feather key always occurs along its total 
length. 
In order to reduce the danger of breaking, screw elements used primarily in 
practical operations are made from a composite material comprising a 
ductile core material into which the profile of the composite system is 
inserted, and a jacket material that serves as a wear and/or corrosion 
resistant material. This composite material is typically manufactured with 
methods such as those used for manufacturing melt-metallurgical cast 
materials, soldered composites or hot-isostatically produced PM 
composites. 
The manufacture of these composite materials is very cost-intensive and 
frequently no longer economical for a wear-resistant part. 
SUMMARY OF THE INVENTION 
The object upon which the invention is based involves designing the 
connection between the shaft or shafts of a screw-type extruding machine 
and the insertable screw elements in such a way that the risk of breakage 
during the transfer of the rotational moment, particularly for screw 
elements made of through-hardened material, is reduced considerably. 
The above object is achieved by providing a screw-type extruding machine 
which includes a housing; a drivable shaft disposed inside the housing and 
including a rotational moment transfer element; and at least one screw 
element. The screw element has two ends and defines a longitudinal groove 
therein for receiving the rotational moment transfer element of the shaft, 
the shaft thereby transferring its rotational moment to the screw element. 
The groove is configured such that it defines two expansion regions, each 
expansion region having an axial extent, the groove having side surfaces 
which diverge with respect to a longitudinal axis of the groove along the 
axial extent of each expansion region in a direction toward a 
corresponding one of the two ends of the screw element, a width of the 
groove thereby increasing in a direction toward its two ends along each 
expansion region. Based on the above arrangement, the point-shaped 
stresses on the fronts (hub edge) of the screw elements are avoided during 
the transfer of the rotational moment because the shaft deformation due to 
the rotational moment of the shaft is distributed over a relatively long 
contact surface by the V-shaped expansion of the side surfaces of the 
groove. 
According to one embodiment of the invention, the side surfaces of the 
groove diverge with respect to one another in a direction away from the 
base of the groove. 
In modifications of the invention, an optimizing of the connection is 
provided for each shaft diameter. Based on these conditions, the optimum 
side expansion can be determined for each application. Thus, according to 
one embodiment of the invention, a difference 2x between the width of the 
groove between its expansion regions and the width of the groove at each 
of its two ends is such that x is less than or equal to 0.02 D. Moreover, 
the axial extent of the expansion regions y may be less than or equal to 
L/2 or D, where L is a length of the screw element and D is the diameter 
of the shaft. 
The extruding machine of the present invention may comprise a plurality of 
wear-resistant screw elements adapted to be fitted over the shaft in an 
optional sequence such that they interlock with one another. 
The preferred application according to the invention is for screw elements 
which are made of a through-hardened metallic or non-metallic material and 
thus have a very high breaking sensitivity.

DETAILED DESCRIPTION OF THE INVENTION 
As seen in FIGS. 1, 2 and 5, in housing 2 of a screw-type extruding 
machine, a continuous, spectacle-shaped borehole 21 is arranged for 
receiving the shafts 1, 1' fitted with screw elements 3. The shafts 1, 1' 
rotate in the same direction and are operated via a motor and gears that 
are not shown. 
As seen in FIG. 3, feather keys 4 are shown as rotational movement transfer 
elements for the rotational moment and grooves 5 as longitudinal recesses 
in the screw elements 3. In place of the shaft/hub connection shown here, 
it is of course possible to provide all other known connection elements 
used for screw elements, such as six-key, multi-spline, serrated and/or 
involute tooth systems with the V-shaped expansion according to the 
invention. 
As seen in FIG. 1, in dependence on the material to be processed, screw 
elements 3-3"" (kneading, screw-type, conveying, pressure build-up element 
and others) are fitted onto the shafts 1, 1' in a freely selectable 
sequence. Screw elements 3-3"" are interlocked with one another at 
interlocking regions "I" as shown schematically in FIG. 1. The number 22 
stands for the feed, 24 for the degasing opening and 23 for the discharge 
of the screw-type extruding machine. 
As seen in FIGS. 4 and 6, in order to reduce edge stresses, sides 51 of 
groove 5 are expanded along expansion regions Ex in a V shape toward the 
ends 31 of the screw element. This expanded side surface 51' is shaped as 
a double bent spatial surface, that is, as a surface which is bent in the 
longitudinal direction on both sides of the longitudinal axis, as shown in 
FIG. 4. Surface 51' is further calculated anew for each application as a 
function of the diameter of shafts 1, 1', which diameter is in turn 
dependent on the rotational movement intended for the shaft. 
The axial extent y of side surface 51' in screw element 3 is such that at 
least one of the following applies: 
EQU y.ltoreq.L/2 and y.ltoreq.D 
where L is the length of screw element 3 in the axial direction, and D is 
the diameter of shaft 1. 
Moreover, the difference 2x between the width of groove 5 at ends 31 and 
the width of the groove at a non-expanded region of the groove between the 
two expansion regions is such that: 
EQU x.ltoreq.0.02 D. 
As seen more particularly in FIG. 6, groove 5 has a base 7, side surfaces 
51' diverging with respect to one another in a direction away from the 
base. 
The preferred area of application for the invention is for wear-resistant 
screw elements, which consist only of a through-hardened work material and 
are very sensitive to breaking as a result of stress. The working 
materials can be hard or hardenable metallic work materials or hard, 
nonmetallic work materials, for example ceramics, zirconium oxide, 
aluminum oxide or the like. With the invention, the risk of breakage for 
these sensitive screw elements can be lowered by more than 50% as compared 
to the traditional shaft/hub connections.