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2. Documentation

2.3 Rules and requirements

2.3.3 Load cases

The magnitude of the mass acting on the EGS is defined as the weight on rope. This is the total amount of the moving mass influenced by the acceleration acting normal on the guide rails. For an elevator car in operation condition, the estimated load is considered as the sum of bout the car weights and the rated load. In general, this is the equivalent of an elevator car with a fully loaded cabin, where the rated load, Q, should be based on the standardized relationships between available area and number of passengers listed by DNV in tables from EN 81-20 [11]

presented in Appendix B. In stowed conditions, the cabin is assumed to be empty and the estimated load is considered as the weight of the car components only. Even though this results in reduced weight, the stowed load conditions are still assumed to produce the biggest loads considering the vast requirements for roll and pitch.

The weight of the counterweight is usually set in accordance to Function 13. This weight is the same in both operating and stowed conditions and is therefore expected to produce the largest loads on the EGS.

With respect to the guide rail, the determining loads are defined in two directions, normal on the x-x axis and normal on the y-y axis. This is illustrated in Figure 25, where the force acting on the rail is generated by the direction of the moving mass, through the connected guide shoe.

Since the frame of the car and the counterweight is connected to guide shoes at both the top and bottom, the analytical weight used to determine the load is considered as half the total weight on rope.

Figure 24 Illustration; Weight hanging from cable

Figure 25 Force on guide rail

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Depending on the direction of rotation, the positioning of the guide rails in relation to the hull must be considered. The rails provide support on both sides of the mass, which means that a load acting normal on the y-y axis of the rail is distributed by the two, whiles one rail carries the entire load when normal to the x-x axis. For pitching, load is normal on the x-x axis when the guide rails are oriented in the longitudinal direction and normal on the y-y axis when oriented in the transverse direction. For rolling, the opposite applies according to the illustration in Figure 26. Considering the orientation of the guide rails and following the specifications for the roll and pitch requirements, the determining force can be calculated using Function 14 and 15.

: Effective height of mass [m]

: Acceleration normal on EGS [m/s2] For guide rails, the worst load case is considered to occur when the force is acting in the middle between to bracket supports. In this case, the force is evenly distributed between the brackets and the reaction force is divided by two according to the principle illustrated in Figure 27. This results in the largest bending moment, which can be found using Function 16. It should also be noted that the relative length is defined as half the bracket distance, l.

Figure 27 Force on guide rail centered between two bracket supports Figure 26 Positioning of guide rails in ship and acting forces during roll

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Following these simple principles of beam theory, the maximum bending moment can be calculated in accordance with Function 16.

𝑀𝑏 = 𝑙

After establishing the maximum bending moment, the actual guide rail stress is determined by the characteristic cross-sectional area modulus, V, related to the load direction, for the specific ISO-code.

: Cross-section area modulus related to the x-x axis [cm3] : Cross-section area modulus related to the y-y axis [cm3]

The stress found for the specific load case is compared to the yield stress of the material. If the established stress is less than the yield stress, the criteria should be approved by the notifying body. A measure taken in this research, with regards to safety factors, is the implementation of a stress factor of 0.8 for cold drawn guide rails and 0.68 for machined. The allowable stress in this optimization process is therefore set according to Function 18.

𝜎𝑎𝑙𝑙 = 0.8 ∙ 𝜎𝑦𝑖𝑒𝑙𝑑 𝑜𝑟 𝜎𝑎𝑙𝑙 = 0.68 ∙ 𝜎𝑦𝑖𝑒𝑙𝑑 (18) specific stress case and results in a rather large margin for error.

Another criteria to be fulfilled in the validation of the guide rails is the strict requirement related to the deflection. The geometrical and material properties contributes in determining the magnitude of deflection for the direction in question. Allowable deflection is set to a maximum of 3 mm, which is generally considered as a rather strict criteria. Because of this, no additional safety factor is included in Function 19, when validating the requirement for deflection. In this relationship, the bracket distance, l, is stated in the power of three and will severely influence the result. Therefore, this factor is of vital importance in the optimization process.

𝛿 = 𝐹 ∙ 𝑙3

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