Knife shaft assembly for document shredders

A knife shaft assembly (1) for a document shredder formed from a series of cutting disks (3) arranged on a shaft (2) with spacer rings (4) disposed between them. The spacer rings (4) are compressible, and a securing mechanism, such as snap rings (6) received in grooves (5), is provided on the shaft to secure the cutting disks (3) on the shaft (2) under compression. During assembly, the spaces between the cutting disks (3) are adjusted to a predetermined size by compression in the axial direction. This arrangement enables material costs to be kept low while assuring high dimensional accuracy of the knife shaft assembly.

BACKGROUND OF THE INVENTION 
This invention relates to a knife shaft assembly for a document shredder, 
which is formed by a plurality of cutting disks arranged on a shaft with 
spacer rings between them. 
Document shredders have as a rule two knife shaft assemblies whose cutting 
disks intermesh in contact with one another and which are driven to rotate 
in opposite directions. Documents fed in between the cutting disks are 
shredded. If the cutting disks are thin and contact one another on only 
one side where they cut the paper, it is important for trouble free 
operation that the disks be held tightly against one another and that the 
spacing of the cutting disks arranged on the knife shafts and held tightly 
against one another be the same on both knife shaft assemblies. Otherwise 
stresses occur between the two knife shaft assemblies resulting in 
increased friction, lowering the output of the document shredder, and 
making it impossible to assemble an effective cutting mechanism. On the 
other hand, dimensional variations may cause the cutting disks to fail to 
contact one another, so that no cutting occurs. 
Any variations in the spacing between the cutting disks on the knife shafts 
can also lead to problems if one piece strippers are used with fixed 
spacing between the fingers as described, for example, in German Published 
Patent Application No. DE 36 16 554. In such a case, if there are 
dimensional differences between the cutting disks and the stripper, 
harmful friction will result, and in some cases it may even be impossible 
to assemble the cutting mechanism. 
Inaccuracies in the knife shaft assemblies of the kind described can very 
easily result if the cutting disks are stamped and then ground from a 
material of slightly varying thickness. The same applies to the spacer 
rings which are usually made by turning. Over the length of the knife 
shaft assemblies, slight differences in the material thickness can be so 
great that the adverse effect described above can very easily occur. To 
prevent such undesirable inaccuracies it has heretofore been necessary to 
engage in a great deal of trouble and expense in selecting the material 
and in manufacturing the components. 
SUMMARY OF THE INVENTION 
It is the object of this invention to avoid the foregoing disadvantages and 
provide knife shaft assemblies having accurate dimensions at reasonable 
cost. 
These and other objects of the invention are achieved by providing a knife 
shaft assembly for a document shredder comprising a series of cutting 
disks arranged on a shaft with spacer rings disposed between them and 
means disposed on said shaft for securing said cutting disks and spacer 
rings thereon, wherein said spacer rings are formed of a compressible 
material, such that the intervals between the cutting disks can be 
adjusted to a predetermined size by compressing the spacer rings in the 
axial direction of the shaft. 
Inasmuch as the spacer rings of the knife shaft assembly are made 
compressible, and means are disposed on the shaft to secure the cutting 
disks and spacer rings on the shaft such that the spacing between the 
cutting disks can be adjusted to a predetermined desired width by 
compression in the axial direction, it is possible to produce knife shaft 
assemblies having the correct dimensions with great precision. 
It has been found that spacer rings manufactured from a deformable plastic 
can be used. Such spacer rings can be manufactured economically by 
injection molding and are particularly suitable for use in smaller 
document shredders where the knife shaft assemblies are subject to 
relatively low stress during the shredding process. For the knife shaft 
assemblies of larger, high output document shredders, it has proven 
advantageous to make the spacer rings from a high strength material, such 
as even a metal that is deformable. In this way the spacer rings can be 
protected against being deformed by the higher cutting forces encountered 
at the cutting disks. 
It has proven advantageous for the spacer rings to be conically configured 
or beveled on at least one axial end face, and preferably on both axial 
end faces. If the greatest axial width of the spacer rings is adjacent 
their outer circumferential edge, the deformation will occur only in the 
radially outer region of the spacer rings. The material will then be 
displaced radially outwardly when the rings are compressed, rather than 
inwardly against the shaft, thus assuring that the freedom of the spacer 
rings to shift along the surface of the knife shaft will not be impaired. 
Then, when the rings and cutting disks are compressed together lengthwise 
of the shaft, displaced ring material will not be able to cause additional 
friction against the shaft which might require greater force to deform the 
rings. If the circumferential surfaces of the spacer rings are spherically 
rounded, it will be possible to prevent the displaced material from 
increasing the outside diameter of the spacer rings. The material 
displaced by the compression will accumulate instead at the circumference 
of the spacer rings in the portions adjacent the areas of deformation. The 
sphericity is preferably selected such that the bulge produced by 
compression in the sections adjacent the areas of deformation will not 
exceed the maximum diameter of the spacer rings in the area of greatest 
rounding. 
The means for securing the cutting disks and spacer rings of the knife 
shaft assemblies preferably take the form of grooves arranged at both ends 
of the shaft and engaged by snap rings. In this case the grooves provided 
in the shaft establish precisely the length of the knife assembly. When 
the knife shaft assembly is assembled, the procedure is to string the 
cutting disks and spacer rings on the shaft, push a snap ring into the 
groove at one end, and then with the aid of a tool, compress the cutting 
disks and the spacer rings together to the predetermined length, so that 
the snap ring can be pushed into the groove at the opposite end of the 
knife shaft in order to secure the cutting disks and the spacer rings in 
the correct position.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 shows a knife shaft assembly 1 comprising a shaft 2 on which cutting 
disks 3 and spacer rings 4 are arranged in sequence. In the shaft 2, 
grooves 5 are disposed on both sides of the series of cutting disks 3 and 
spacer rings 4, and snap rings 6 are inserted into the grooves 5. 
Intermediate rings 7 are disposed on the shaft 2 in contact with the snap 
rings 6. Through the intermediate rings 7 the snap rings 6 hold the 
cutting disks 3 and spacer rings 4 in their position in which they are 
urged against one another during assembly by means of a tool, not shown in 
the drawing. The provision of the intermediate rings 7 serves in this case 
merely for an optimum transfer of the compression forces to the snap rings 
6. It is quite possible to omit the intermediate rings 7, if desired. 
FIG. 2 shows a cross-sectional view of a spacer ring 4. As can be seen in 
the drawing, both axial end faces 8 of the spacer ring 4 are conically 
configured or beveled. This causes the adjacent spacer rings to have small 
areas of contact at which great pressures per unit area will be produced 
upon compression. This produces the advantage that, in the initial phase 
of the compression, a relatively great deformation can be achieved by 
means of a relatively low force. As deformation increases, the surfaces in 
contact increase in size, so that finally, after reaching the 
predetermined size, a desired stiffening of the knife shaft assembly 1 is 
achieved. This means that the desired deformation of the spacer rings 4 
will be achieved during assembly without great difficulty, but that the 
knife shaft assembly 1 will be sufficiently stable in the fully assembled 
state to meet the conditions in which it will be used. It is possible by 
selecting the degree of bevel on the sides 8 of the spacer rings 4 and by 
selecting the material used to form the spacer rings, to adapt the spacer 
rings 4 to the conditions required in each case. For a small shredder, it 
has been found desirable to make the spacer rings 4 for the knife shaft 
assembly 1 from a suitable plastic. For knife shaft assemblies 1 subject 
to heavy loads, it has been found desirable to manufacture the spacer 
rings 4 from a deformable metal. The selection of a suitable material is 
governed by the amount of stress on the spacer rings 4 and the 
configuration of the axial end faces 8 of the spacer rings 4. It has been 
found especially advantageous for the axial end faces 8 to be inclined at 
an angle of about 15.degree.. 
Advantageously, the spacer rings 4 are rounded spherically on their 
circumferential surfaces 9. As a result, when the spacer rings are 
deformed at the contacting surfaces, the circumferential areas 9 that 
bulge out in the vicinity of the contacting surfaces will not increase the 
outside diameter of the spacer rings 4. The spherical curvature of the 
circumferential surfaces 9 of the spacer rings is preferably selected such 
that the bulging of the circumferential surfaces 9 will not exceed the 
greatest diameter in the center of the circumferential surfaces 9. 
If the spacer rings 4 are made of plastic, it is possible, for example, to 
injection mold the spacer rings 4 from polyethylene. Spacer rings 4 
subject to greater stresses can be made, for example, from brass alloys. 
It is also possible, of course, to use other deformable materials. 
The knife shaft unit 1 of the invention is assembled by sliding the cutting 
disks 3 and the spacer rings 4 onto the shaft 2. After placing a snap ring 
6 in a groove 5 of the shaft 2, the cutting disks 3 and the spacer rings 4 
are compressed together by means of a tool, not shown in the drawing, to 
such an extent that a second snap ring 6 can be inserted into the groove 5 
at the free end of the shaft 2. The spacing between the grooves 5 
determines the correct length of the knife shaft assembly 1. After the 
tool is removed, the snap rings 6 hold the cutting disks 3 and the spacer 
rings 4 compressed together. 
Due to the deformability of the spacer rings 4, not only is the correct 
total length of the knife shaft assembly 1 achieved in the compressed 
state, but also the correct spacing between the individual cutting disks 
3. Thus great accuracy in the dimensions of the knife shaft assembly 1 is 
assured regardless of variations in the material thicknesses of the 
individual cutting disks 3. This means that the cutting disks 3 of the two 
knife shaft assemblies 1, which intermesh with one another when the 
document shredder is in the assembled state, will always be at the correct 
distance from one another. The cutting disks 3 contact one another such 
that a perfect cutting action is assured, but on the other hand the 
friction between the cutting disks 3 is so slight that the performance of 
the document shredder is not impaired. 
If a stripper made in one piece is used, a sufficient clearance between the 
cutting disks 3 and the fingers of the stripper is thus assured, so that 
here, again, no harmful friction can be produced. The knife shaft assembly 
1 of the invention assures that high performance document shredders can be 
manufactured with uniformly high quality at lower cost. 
The foregoing description and examples have been set forth merely to 
illustrate the invention and are not intended to be limiting. Since 
modifications of the described embodiments incorporating the spirit and 
substance of the invention may occur to persons skilled in the art, the 
invention should be construed broadly to include all variations falling 
within the scope of the appended claims and equivalents thereof.