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Neenah, WI 54957
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Web site: www.nfco.com
One of the significant developments coming out of the load test program is the development of the Platen Lid. The Platen Lid is an improved manhole cover. The advantages include an increase in ultimate load, an increase in deflection, an increase in energy absorption, an increase in design efficiency, and a decrease in the weight of the casting. Typically weight is a direct indicator of relative cost. The lighter the lid the less expensive it will be.
Lets look into why the R-1740-B platen lid is a stronger casting than the ribbed lid. Through the strength of materials equations, one can determine that the moment of inertia for the ribbed lid is (13.9 inches4) and for the platen lid it is (2.64 inches4). When determining the section modulus, which is accomplished by dividing the distance from the neutral axis to the extreme fiber in tension, one finds that the distance is much greater for the ribbed lid than for the platen lid. This results in a section modulus thats approximately the same for the platen lid (5.31 inches3) as that of the ribbed lid (5.43 inches3). Part of this analysis corresponds with the load test results, i.e. the deflection characteristic of the platen lid is considerably greater because it has a much lower moment of inertia. But, the load capacity of the platen lid is higher because shifting of the neutral axis has a more pronounced impact on its geometric configuration. The combination of these two characteristics provides the platen lid with its most significant advantage, impact resistance. Impact resistance is a function of both deflection and load capacity, and is one of the significant factors in construction casting design.
Actual tests indicate the R-1740-B Platen Lid has a load capacity 30% greater than the standard. The Platen Lid weighs approximately 90% of the standard and has an impact resistance 400% greater than the standard lid. It can be noted that the ribs on the underside of the R-1740-B make it weaker than if they were not there at all.
The Platen Lid is less expensive than the standard lid and is a superior casting design. Many municipalities and governmental bodies have adopted the Platen Lid design and now are specifying it on their plans.