Speed limited fan

The invention provides a method of driving a plurality of plastic fan blades (14) integrally molded with a band (16) about and engaging a metal driving hub (12) at an interface by rotating the fan blades (14) in unison with the hub (12) up to a predetermined rotational speed at which the band (16) flows or expands radially to create a space between the band (16) and the hub (12) whereby the fan blades (14) are allowed to rotate at a slower rotational speed than the hub (12) to prevent the fan blades (14) from bursting in response to centrifugal force. The drive connection may optionally include band projections (24) and/or male projections (26) which shear before or as the separation occurs. A film may be disposed at the interface to prevent the plastic of the fan blades (14) and band (16) from bonding to the metal hub (12).

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to controlling the rotational speed of an axial fan
 with particular application to a cooling fan of the type used in
 automotive vehicles to move air through a heat exchanger.
 2. Description of the Prior Art
 A ring fan is typical of cooling fans used in automotive environments. A
 ring fan includes radially extending blades which are connected at the
 distal ends by a ring. The ring provides added stability and rigidity to
 the blades and improves noise performance of the fan. The fan noise is a
 serious problem in automobiles, especially light trucks and sport utility
 vehicles.
 Despite the advantages of noise reduction and cooling performance of ring
 fans, they have not enjoyed widespread use because the burst speed of a
 ring fan is generally lower than the burst speed of conventional axil
 fans, the burst speed being that speed at which the fan will break apart,
 i.e., burst, in response to centrifugal forces generated by the rotation.
 Naturally, automobile manufacturers use fans with very high burst speeds.
 Because the ring adds mass to the outside diameter of the fan, which
 increases centrifugal forces proportionally to the radius squared, engine
 driven ring fans have not met the burst speed requirements of automotive
 manufacturers.
 SUMMARY OF THE INVENTION AND ADVANTAGES
 The invention provides a unique method of driving a plurality of fan blades
 disposed about and engaging a driving hub at an interface by rotating the
 fan blades in unison with the hub up to a predetermined rotational speed
 and rotating the fan blades at a slower rotational speed than the hub in
 response to the hub rotating at a higher rotational speed than the
 predetermined rotational speed to prevent the fan blades from bursting in
 response to centrifugal force.
 The invention may be implemented in a fan assembly comprising a hub adapted
 for driving connection to a rotary drive and a plurality of fan blades
 extending radially from the hub. The fan blades consist of a first
 composition and the hub consists of a second composition different from
 the first composition. The assembly is characterized by a driving
 connection between the fan blades and the hub for rotating the blades in
 unison with the hub up to a predetermined rotational speed and for
 allowing rotation of the hub independently of and at a higher speed than
 the fan blades in response to the hub rotating at a higher rotational
 speed than the predetermined rotational speed to prevent the fan blades
 from bursting in response to centrifugal force.
 Accordingly, the subject invention may be utilized in a ring fan to prevent
 the fan from ever reaching burst speed whereby ring fans, with all of
 their advantages, may be used more extensively without fear of breaking
 apart.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 Referring to the Figures, wherein like numerals indicate like or
 corresponding parts throughout the several views, a fan assembly is
 generally shown at 10. The fan assembly 10 includes a hub 12 adapted by
 holes 13 for driving connection to a rotary drive (not shown), such as the
 engine of an automobile.
 A plurality of fan blades 14 extend radially from the hub 12 and include an
 integral band 16 integrally interconnecting the fan blades 14 and
 extending about the hub 12. The band 16 includes flanges 18 extending
 radially to sandwich the hub 12 therebetween. In other words, the band 16
 is U-shaped as viewed in cross section in FIG. 3 wrap around the periphery
 of the dish shaped hub 12. The fan blades 14 extend to distal ends and
 include a ring 20 extending about and interconnecting the distal ends of
 the fan blades 14.
 The fan blades 14, which include the band 16 and the ring 20, consist of a
 first composition, preferably an organic polymeric material, i.e.,
 plastic, whereas the hub 12 consists of a second composition different
 from the first composition, e.g., a metal or metal alloy.
 The assembly 10 is characterized by a driving connection between the fan
 blades 14 and the hub 12 for rotating the blades 14 in unison with the hub
 12 up to a predetermined rotational speed and for allowing rotation of the
 hub 12 independently of and at a higher speed than the fan blades 14 in
 response to the hub 12 rotating at a higher rotational speed than the
 predetermined rotational speed to prevent the fan blades 14 from bursting
 in response to centrifugal force. The fan blades 14, through the band 16,
 define an interface with the hub 12 and the drive connection is disposed
 at the interface. The drive connection includes hub projections 22
 extending from the hub 12 and band projections 24 extending from the band
 16 and interleaved or meshing with the hub projections 22. Both sets of
 projections 22 and 24 extend radially and are interleaved at the
 interface. The hub projections 22 have greater shear strength than the
 band projections 24 whereby the band projections 24 shear in response to a
 predetermined torque transmitted thereto by the hub projections 22.
 In operation, the first or plastic composition of the fan blades 14 flows
 radially in response to centrifugal forces generated at the predetermined
 rotational speed to separate from the hub 12 at the interface. In order to
 facilitate this separation, it may be useful to include a film disposed
 between the hub 12 and the band 16 at the interface to prevent the band 16
 from bonding to the hub 12. In other words, the integral plastic fan blade
 unit is molded about the hub 12 and to prevent bonding, the hub may be
 coated with a film as by spraying a polymer compound on the hub. The hub
 12 may also include a plurality of female receivers or holes 26 and the
 band 16 includes a plurality of complementary male projections 28
 extending axially from at least one of the flanges 18 and into the female
 receivers 26. The male projections 28 are integral with the band 16 and
 are molded therewith to extend from the legs of the U-shape into the
 female receivers or holes 26 in the opposite sides of the hub 12.
 As the hub 12 rotates toward the burst speed, the male projections 28 shear
 and the centrifugal force moves the band 16 radially outward to create a
 space at the interface with the hub 16. The freedom of the fan blades 14
 and the integral band 16 to rotate slower or not rotate at all, depends
 upon the configuration of the drive connection at the interface between
 the hub 12 and the band 16. If the drive connection relies only upon the
 friction between the band 16 and the hub 12, then the separation at the
 interface will be relatively immediate, whereas if the projections 22 and
 24 are included, the depth of the projections must clear one another.
 Alternatively and preferably, the band projections 24, which are integral
 plastic and weaker than the metal of the hub 12, shear after a
 predetermined degree of separation between the band projections 24 and the
 hub projections 22, as illustrated in FIG. 2.
 An alternative drive connection is shown in FIG. 4 wherein the hub 12 has a
 plurality of concave recesses 30 into which are molded a plurality of
 convex tabs 32 whereby the convex tabs 32 withdraw radially from the
 concave recesses 30 in response to the rotational speed of the hub 12
 reaching a predetermined speed which is lower than the burst speed of the
 fan blade assembly. As will be appreciated, various configurations and
 combinations of drive connections may be employed between the hub 12 and
 the band 16, it only being essential that the band 16 move radially
 relative to the hub 12 to effect a separation at the interface.
 Accordingly, the invention provides a method of driving a plurality of fan
 blades 14 disposed about and engaging a driving hub 12 at an interface
 comprising the steps of rotating the fan blades 14 in unison with the hub
 12 up to a predetermined rotational speed and rotating the fan blades 14
 at a slower rotational speed than the hub 12 in response to the hub 12
 rotating at a higher rotational speed than the predetermined rotational
 speed to prevent the fan blades 14 from bursting in response to
 centrifugal force. The method is further defined as disconnecting the fan
 blades 14 from driving connection to the hub 12 above the predetermined
 rotational speed of the hub 12. This is accomplished by the flow of the
 plastic material of the band 16 and fan blades 14. The additional and
 optional shearing step may be included, i.e., shearing the fan blades 14
 from the hub 12 at the predetermined rotational speed by shearing the band
 projections 24. This occurs after the fan blades 14, i.e., the band 16,
 are separated radially at the interface from the hub 12 at the
 predetermined rotational speed.
 The invention has been described in an illustrative manner, and it is to be
 understood that the terminology which has been used is intended to be in
 the nature of words of description rather than of limitation.
 Obviously, many modifications and variations of the present invention are
 possible in light of the above teachings. It is, therefore, to be
 understood that within the scope of the appended claims, wherein that
 which is prior art is antecedent to the characterized novelty and
 reference numerals arc merely for convenience and are not to be in any way
 limiting, the invention may be practiced otherwise than as specifically
 described.