Design of breast roll retraction system : Redesign of keyway solution for positioning of breast roll in tissue machine

Abstract: This Thesis has been carried out as a final part of the Mechanical Engineering programme (180 credits) at Karlstad University during the spring semester 2022 and comprises 22.5 credits. The work is carried out on behalf of Valmet AB in Karlstad.At the beginning of a tissue machine, a mixture of fibres and water is fed into the machine and formed into a sheet using a forming wire and felt. During maintenance work, the wire sometimes needs to be detached from the machine. In order to release the forming wire, the breast roll retraction system needs to be opened up, which is done using an arm with the breast roll attached to it. This arm can rotate upwards, taking the breast roll with it, and when this is opened, the forming wire can be removed. In the hinge that this arm is attached to there is a gap, in addition to the gap the arm can be bent slightly. When the arm is returned to its original position after replacing of the forming wire, this gap and bending creates some problems. The problem is that the arm needs to return to its original position with great precision to avoid unnecessary adjustments. This is currently done using keys with angled walls and to lock the position eye bolts with associated nuts. The manufacturing of this solution has been identified as expensive due to tight tolerances and the angled walls of the keys and keyways. The aim of this project is to make the design cheaper to manufacture through design modification of this positioning and locking mechanism.Through a feasibility study regarding the current design and manufacturing method a deeper understanding of the function and machining was gained. The project has followed a structured product development process, from pre-study through requirements specification, concept generation and evaluation to detailed design. A requirement specification was developed using the results of the feasibility study together with information from Valmet employees. Two concept generation sessions were conducted, the 6-3-5 method and morphological matrix, these were done together with people at Valmet with different experiences. For concept evaluation and selection, Harris profile and relative decision matrix were used, these were conducted after consultation with experienced people at Valmet.The best concept is a design change that eliminates the milled angled walls in the keyway on the arm. These are replaced by a cylindrical pocket and a threaded hole. A corresponding change is made to the bracket against which the arm rests. Instead of the keys used today, two turned parts with a conical contact surface between them are proposed. This solution achieves all the requirements of the requirements specification as well as all the wishes except that it should fit similar fastening elements in other sections of the machine. The explanation for this is that the other fastening elements have slightly different functions which the proposed solution does not take into account. The price difference between the current design and the design proposed by the project has been obtained by a subcontractor providing price proposals for both.