Metal working – Means to assemble or disassemble – Puller or pusher means – contained force multiplying operator
Reexamination Certificate
2000-09-18
2002-10-15
Watson, Robert C. (Department: 3723)
Metal working
Means to assemble or disassemble
Puller or pusher means, contained force multiplying operator
C029S263000
Reexamination Certificate
active
06463643
ABSTRACT:
CROSS-REFERENCES TO RELATED APPLICATIONS
This application claims the priority of German Patent Application, Serial No. 199 47 439.7, filed Sep. 24, 1999, the subject matter of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for withdrawing a screw from the barrel of a screw extruder.
Typically, a screw extruder has a barrel which defines a bore for receiving a screw. During operation, the screw rotates in the barrel bore to convey a medium, and has a head portion, called screw cap, which is formed with an outwardly pointing undercut. In general, the screw of a single screw extruder or the screws of a multi-screw extruder have a geometric shape that is best suited to the particular properties of a material or material group that is processed in the extruder. As a consequence, each time when the extruder is intended for use with a different material of markedly different properties, the screw must be replaced for a different type to ensure optimum operating conditions. Normally, the screw exchange is carried out in such a manner that the screw is extracted in conveying direction, i.e. the screw is withdrawn at the head portion of the extruder from the barrel bore. Oftentimes, the screw is composed of a plurality of components which are arranged axially behind one another and formed with a longitudinal bore for passage of a shaft. Threadably mounted onto the shaft is the screw cap which forms a closing element in conveying direction and braces the individual components of the screw together. The screw cap thus forms the head portion of the screw. As substantial forces must be applied, especially when extruders of larger size are involved, in order to extract the screw from the barrel bore, particular pull mechanisms, such as chain tackles or hydraulic cylinders, have frequently been used for attachment to the head portion. Normally, the head portion is provided with a circumferential groove in the form of an outwardly facing undercut for positive engagement of gripper claws of an extractor device, and a post-processing device, such as a screen pack changer, a die plate of a granulating machine, slot die or other nozzle for producing extrudate, is operatively connected to the head portion of the extruder. As a consequence of the comparably robust configuration of the gripper claws, conventional extraction apparatus require a dismantling of the post-processing device before a screw replacement can take place so as to create the needed space for attachment of the extraction apparatus. Dismantling and alterations are fairly cumbersome and increase the manufacturing costs, in particular when smaller batches of different material are processed as a screw replacement is needed within shorter intervals compared to larger batches that run at respectively longer operating periods, without requiring a screw replacement.
SUMMARY OF THE INVENTION
It is thus an object of the present invention to provide an improved screw extraction apparatus, obviating the afore-stated drawbacks.
In particular, it is an object of the present invention to provide an improved screw extraction apparatus which is simple in structure and yet is capable to significantly reduce the complexity of replacing a screw of a screw extruder.
These objects, and others which will become apparent hereinafter, are attained in accordance with the present invention by providing at least one gripper having a free end for attachment of a pull mechanism and a grabbing end for interaction with an undercut of the screw cap, with the at least one gripper being formed at grabbing end with a plurality of radially resilient clamping segments which can be elastically bent inwards and/or outwards, and which form parts of a hollow cylinder in longitudinal direction thereof, whereby the hollow cylinder has an inner diameter, which substantially corresponds to an outer diameter of the screw cap, and an outer diameter which is smaller than a diameter of a bore of the barrel, wherein each of the clamping segments terminates in a beaded end which extends radially into the hollow cylinder for formation of a gripping claw, with the beaded end having a wall thickness which is smaller than a difference between a radius of the barrel bore and an outer radius of the screw cap, and, in longitudinal direction of the gripper, a length which is smaller than a width of the undercut of the screw cap in longitudinal direction of the screw.
The novel and inventive approach of the present invention is based on the recognition that the complexity of a screw replacement can be substantially reduced when a dismantling of post-processing devices, positioned downstream of the extruder head, can be eliminated. To date, conventional extraction apparatuses did not allow this approach because, without dismantling of the post-processing device, there was simply not enough space available to attach the extraction apparatus. This problem has now been overcome through the provision of an extraction apparatus according to the present invention, which exhibits a very slim and extremely space-saving configuration of its gripper tools.
Through the provision of a gripper with radially resilient clamping segments in accordance with the present invention, the gripper end can be inserted into the barrel bore and pushed over the screw cap, whereas the beaded ends of the clamping segments are so configured in size and shape as to form gripping claws which are prevented from wedging in the ring gap between the screw cap and the barrel bore. When the gripper end is pushed sufficiently far over the screw cap, the beaded ends can positively engage or snap in the undercut. Depending on the magnitude of the pulling forces applied in axial direction by means of a pull mechanism, such as a hydraulic cylinder, attached to the extraction apparatus, and the magnitude of the spring force exerted by the clamping segments, the positive engagement of the beaded ends in the undercut may be enough by itself in some cases to allow a withdrawal of the screw from the barrel bore. However, normally, it may be more advantageous to provide the gripper with a securing sleeve by which an inadvertent outward deflection of the clamping segments and thus a release of the positive engagement are prevented when axial forces are applied to extract the screw. The securing sleeve is suitably shiftable over the gripper up to the area of the clamping segments to thereby prevent the outward deflection of the clamping segments. Suitably, the securing sleeve has an inner diameter, which corresponds to the outer diameter of the hollow cylinder so as to be easily slideable over the hollow cylinder, and an outer diameter, which is smaller than the inner diameter of the barrel bore so that the securing sleeve can easily be pushed into the barrel with accommodated screw.
Suitably, the gripper has, as an essential component, a tubular portion, whereby the clamping segments are formed by a plurality of slits extending in axial direction from the grabbing end inwardly of the tubular portion. The beaded ends may be formed by selecting for the grabbing end a tubular portion with excess wall thickness, and subsequently turning the wall thickness from the inside to size, except in the area that should form the beaded ends. Of course, there is no need to reduce the wall thickness across the entire length of the tubular portion. It is sufficient to turn the wall thickness of the tubular portion to size along a section that is long enough to allow the inner surface of the tubular portion to contact the screw cap. Suitably, the turned section extends slightly beyond the length of the clamping segments so that the clamping segments have a smaller wall thickness than the rest of the tubular portion.
To facilitate a detachment of the screw extraction apparatus, after withdrawal of the screw and release of the clamping segments through retraction of the securing sleeve, i.e. to allow a fairly easy outward deflection of the clamping segments when applying axial pulling forc
Berstorff GmbH
Feiereisen Henry M.
Watson Robert C.
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