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Layout Design of Printing Press Bearings

by:JNSN     2022-03-15
Design requirements for adjacent structures: The bearing of the printing press is a very robust mechanical component, and the surrounding structure is required to have high precision, which modern processing equipment can fully achieve. The main requirements for the surrounding structure are high-precision shaft diameter and precision-machined wall plate holes. Typically, journals are ground and wall plate holes are finished and, if necessary, honed, Figure 1. Internal Radial Clearance of Cylindrical Bore Bearings The internal radial clearances of cylindrical bore printing press bearings are specially designed according to specific applications. Oscillating bearings in three- and four-ring bearings are zero clearance or light preload and are set at the factory. No customer adjustment is required. The Schaeffler Group has advanced computational software to aid design. For optimum functionality, all tolerances must be carefully determined. Axial positioning of printing press bearings The positioning of the outer ring of the printing press bearing DML is very simple because it is not subjected to axial forces during operation. The eccentric intermediate and outer rings of three-ring and four-ring bearings are axially positioned by the shoulders and rings of the bearing. For bearing units DML3E, DML3D and DMLD, therefore, no axial positioning by means of the surrounding structure is required. In general, fixing the eccentric ring to the wall panel by means of peripheral parts is sufficient for axial positioning, page 37, Fig. 1. Thrust bearing arrangement If cylindrical roller bearings are used at both ends of the drum, then another rolling bearing is required to bear the axial force. Since the axial forces in printing presses are usually very low, economical standard bearings suffice. Proven types include two zero-clearance angular contact ball bearings, double-direction thrust needle roller bearings or double-direction thrust cylindrical roller bearings, see page 98. Shaft and Housing Bore Tolerances To minimize bearing operating clearance tolerances, cylindrical shaft diameter tolerances should be kept as small as possible. The prerequisite for optimization is that the tolerance class of the shaft should reach IT4. However, in general the most common shaft fit k5 suffices for all requirements. For tapered shafts, the tolerance can be slightly larger since the working clearance of the bearing can be adjusted. In this case, the coordination of the axis is usually js6. Wall Plate Hole Tolerances For printing press bearings, there is usually a transition fit or tight fit between the wall plate hole and the bearing outer ring. When using bearing units DMLE or DML3D, if the outer eccentric needs to move during operation, a certain fit clearance is required. To minimize possible deformation of the assembly, the tolerance zone should be as narrow as possible. This can be achieved by fitting the bearings into the wall plate holes or by high-precision machining methods such as honing. In order to facilitate optional installation, the bearings of the printing press are measured before leaving the factory, and the actual outer diameter size is recorded on the bearing. For the case of mass production, which often occurs in sheet-fed offset presses, it is more economical to design the outer diameter of the bearing in groups. In this way, the diameter tolerance of the wallboard holes can be relatively large, which is very beneficial to mass production. Geometrical tolerances of bearing mating surfaces If the bearing arrangement is to meet the requirements related to running accuracy, stiffness, zero clearance, operating temperature and ensuring excellent print quality, the adjacent structures must meet certain minimum requirements. Geometric deviations of adjacent parts must be kept to a minimum. In order to achieve the desired fit, the mating surfaces of the shaft and housing bore must meet certain tolerances, Figure 2, Figure 3 and page 42, Table Note Bearing Surface Roughness Requirements, page 43. Accuracy of bearing mating surfaces For the tolerances of shaft and housing bore mating surfaces, the allowable geometric deviations are listed here, see table. Accuracy class See ISO basic tolerances in accordance with ISO 286 on page 43. The permissible geometric deviations must be determined based on the relevant shaft and housing diameters. Example: Shaft If the bearing is of tolerance class P5 and the shaft diameter is 90 mm, the tolerance value of the shaft is as follows, see table: ■ Roundness t u003d IT3 1/2 u003d 6 1/2 u003d 3 m. Roughness of the bearing mating surface The roughness of the bearing mating surface should match the tolerance class of the bearing. The average roughness Ra should not be too large to ensure a limited reduction in interference, see table. The shaft needs to be ground and the housing bore is finished. The design and safety guidelines in the product chapter also give hole and shaft tolerances and allowable roughness.
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