Concrete mixing drum manufacturing method

#The building industry makes widespread use of concrete mixing trucks for transportation of ready mixed concrete to sites for concrete pours. These trucks typically comprise a large mixing assembly including a mixer drum mounted to the vehicle and which is connected to a mixer drive for mixing concrete contents during transportation and for discharge of the contents on site. The drive system comprises a gear box which takes power from the vehicle motor and which applies a mixing torque to the drum imparting axial rotation to the drum with the torque being adjustable depending upon the operating requirements. The above general arrangement is described in U.S. Pat. No. 4,585,356 which discloses a concrete mixer truck having a mixer drum adapted to be rotated by the traction motor of the vehicle through an auxiliary transmission of the traction motor transmission.

#According to the known vehicle mounted mixing assemblies, the mixing drum is typically of heavy duty steel construction and is disposed at approximately 10 to 15 degrees from horizontal. The drum is fitted with internal vanes or mixing blades defining an archimedian spiral so that as the drum rotates in a first direction the concrete held therein is mixed and as the drum is rotated in the opposite direction, the concrete is discharged from the drum via an elevated discharge orifice under the reverse action of the internal spiral vanes. The drum is disposed such that the drive end is lowest and the discharge end is highest relative to a generally horizontal plane of the vehicle.

#While steel drums have been in use for many years, they suffer from a number of attendant disadvantages, relating to their cost of manufacture and replacement, working life, wear characteristics, weight and volume.

#The present invention will now be described according to a preferred but non limiting embodiment and with reference to the accompanying illustrations wherein:

#FIG. 1 shows a side elevation of a prior art mixing drum;

#FIG. 2 shows a side elevation of a cement mixing drum according to one embodiment of the invention;

#FIG. 1 shows an elevation view of a known steel mixing drum 1 which is typically constructed from separate prefabricated sections 2, 3 and 4 which are welded together at seams 5, 6 and 7. At seams 5, 6 and 7 the welded joints which are subject to concentrated wear due to the change in surface direction at the joint. The concentrated wear points in the prior art steel drums reduces the working life of the drums necessitating costly repair or replacement. Steel drums are fabricated from rolled flat sheets which form cones and a cylinder which are then joined together by welding. Archimedian spirals are then welded to the inner surface of the drum resulting in a high specific gravity vessel whose self weight reduces the amount of concrete which can be carried by the vehicle to which it is attached. As previously indicated, the steel drums suffer from a number of disadvantages including susceptibility to abrasion at the junctions of the cylindrical and conical sections and the tendency for unwanted concrete build up at the sharp corners and crevices formed by the mixing blades. In addition, the smooth internal surface of the steel drum promotes sliding abrasion and inhibits mixing at the boundary layer due to the low coefficient of friction at the concrete/metal interface.

#FIG. 2 shows an external profile of a fibre reinforced composite concrete mixing drum 8 according to one embodiment of the invention. The drum includes an internal archimedian spiral formed by helical blades or vanes which mix concrete during rotation of the drum in one direction and discharge concrete when the drum is rotated in an opposite direction. The drum is generally pear shaped and includes an opening 9 at one end for entry and discharge of concrete. The arrangement of FIG. 2 is arrived at by application of the method aspect of the invention which will be described in detail below. Drum 8 is constructed from a fibre reinforced plastic structural shell with an elastomeric interior having a surface property which imparts abrasion resistance to the concrete but increases the mixing at the boundary layer of the concrete and drum wall by forced rotation of aggregate. A preferred method of construction of the drum will now be described in detail. Whilst the embodiment described employs three mold parts it will be appreciated that the drum may be constructed from a lesser or greater number of parts. It has been found however, that a three part construction is preferable in view of the shape of the finished drum. According to one embodiment, drum 8 is constructed from three mold sections 13, 14 and 15 (shown in FIG. 3) which form portions 12, 11 and 10, respectively, of drum 8. FIG. 3 shows typical profiles of mold sections 13, 14 and 15. Portion 12 of drum 8 is constructed to engage a drive system mounted on a vehicle for rotating the drum. Portion 11 extends between portions 12 and 10. Portion 10 includes a discharge opening through which mixed concrete is discharged. The first step in the construction of the mixing drum is the preparation of the mold from which the drum will be produced. The mold is preferably constructed in three parts as this enables ease of extraction form mold formers and also allows for the formation of mixing drums of different sizes according to requirements. For instance the length of the drums can be increased by changing the size of intermediate section 14. Mold sections 13, 14 and 15 are each formed in separate molds from expanded polystyrene beads. Steam heat is applied through slots in each mold thereby fusing the beads against the mold surface. The polystyrene surface finish of the mold may be improved by the application of a fast drying liquid. The external profile of the mold parts when joined provide the mold for the inner surface of the drum. The mold profile includes helical grooves which are the inverse of helical mixing blades which extend from an inner surface of the finished drum. The mold and partially finished drum are shown at 16.

#FIG. 4 shows an assembled mold mounted on a cantilevered mandrel 17. The mold sections include mating dowels which align corresponding parts of the mold profiles and are clamped together with an adhesive. FIGS. 5a-d show the first stages of preparation of the drum. At the commencement of this stage the mold is mounted on mandrel 17 which is capable of axial rotation. The operation of the mold is computer controlled and employs a robot which rotates the mandrel. The mold surface is prepared with a release agent which will enable release of the mold from the drum on completion of the method steps. A spray head 18 delivers a polyurethane elastomer to the surface of the mold which forms an inner layer of the drum.