REINFORCING RIBS AND BOSSES:

Definition:

·         RIBS:   A reinforcing member of a fabricated or molded part. It is included to support and strengthen the part.

·         BOSS:  Projection on a plastic part. It is included to: 1) add strength, 2) to facilitate alignment during assembly and 3) to provide for fastening.

Theory:

Increasing the wall thickness throughout a part, will increase the stiffness of a part. However this will also increase the cross sectional moment of inertia, hence a rectangular cross section becomes relatively inefficient in bending. To prevent this, Reinforcing Ribs and Bosses are added to provide increased stiffness of a part.

RIBS: 

SIGNIFICANCE:

·         A T-section results when reinforcing gussets are added. This provides stiffness equivalent to a rectangular section, with less material and therefore less weight.

·         Ribs increase stiffness locally, without adding extra material and weight.

·         Ribs around attaching holes and bosses, efficiently transfers loads from the bolt to the primary structure.

·         Ribs at the unsupported flat plates stiffen it to prevent buckling or increase the natural frequency of vibration.

·         Ribs around the openings 1) increase radial stiffness, 2) maintains concentricity and 3) helps filling Bosses.

DESIGN RULES:

·         Properly designed Ribs have minimal possibility of warping and sink marks after cooling or curing.

·         Gradual thickness transition should be provided with generous radii.

·         Thickness of the Rib should be less than that of the wall to which it is attached. If it is necessary to provide Ribs with thickness greater than that of the adjacent wall, it is better to provide an additional Rib or split the Rib and thus maintaining the maximum thickness limitation.

·         When part requires more than one Rib, the spacing between the Ribs should not be less than twice the thickness of the wall to which it is attached.

·         It will be more advantageous (in terms of stiffness) to increase the height of the Rib than to increase its thickness because stiffness is proportional to the cube of height and proportional to thickness (stiffness α height³ and stiffness α thickness). However, the height should not be large enough to make the Rib unstable and buckle under load.

·         The Rib design for thermoset and thermoplastic slightly differ from each other.

BOSSES:

SIGNIFICANCE:

·         Bosses are protruding pads, commonly used as mounting surfaces for the molded part.

·         It is used to distribute bearing loads caused by attachment bolts preloads and to transfer bolt loads to the main surface.

·         They increase the wall section around the slot.

DESIGN RULES:

·         Excessively thick Bosses should be avoided.

·         When attachment loads are very high, Ribs should be used to connect the bosses to the adjacent wall.

·         Diameter of the Boss should be sufficient to carry the required load without failure.

·         Three Bosses permit better alignment than four or more.

·         Boss height should not be more than twice the diameter.