Iron Conversion for Cement Mixer
The McNeilus Co., (a subsidiary of Oshkosh Truck), Dodge Center, Minn., a manufacturer of concrete mixer trucks for the heavy construction industry, converted a portion of its pour chute to a one-piece casting after a proposed redesign showed it would reduce costs and improve the part.
The casting design team, consisting of McNeilus, the metalcasting facility, pattern shop and machine shop, focused on three imperatives: design for performance, design for production and design for cost. Three casting design issues played a major role in meeting these design imperatives:
- select a ductile iron grade that meets the mechanical requirements;
- optimize the design for stress reduction and weight savings;
- develop a casting tool design that produces flaw-free frames at the best cost.
Each component of the McNeilus’ trucks has to be rugged and durable to withstand mixing and pouring stresses as well as road shock and vibration over the life of the truck. The metal pour chute on the rear of the truck carries the concrete mix from the drum down to the concrete forms. The chute is designed to swing across a 160 degree arc to facilitate easy pouring into the forms without repositioning the truck.
An integral part of the pour chute is the pivot plate frame. Originally, the pivot plate frame was a steel assembly consisting of nine fabricated and machined parts that were welded together. The welded assembly performed to mechanical specification, but was less than ideal for cost, assembly, variation in form-and-fit and inventory management.
A near-net shape one-piece casting design was proposed that would cost less, improve form and fit and reduce stresses for longer life and improved durability.
The pivot plate frame supports the pouring chute of a concrete mixer truck, serves as a pivot point and locks in position during the pour. The critical and production requirements for the cast plate frame were:
- meet or exceed the strength and durability of the original weldment;
- choose an alloy grade for sufficient toughness and ductility for fatigue and impact resistance;
- design the casting with the selected alloy grade to keep stresses below the maximum design stress;
- minimize machining steps and costs;
- include the critical dimensional features—machined center hole in the hub, machined mounting holes in the triangular frame and as-cast locking holes on the locking plate;
- achieve general tolerances of 0.03 in. (0.76 mm), as-cast hole tolerances of 0.02 in. (0.51 mm) and machined tolerances of 0.005 in. (0.13 mm);
- achieve a clean and smooth surface finish with no visible surface defects or grinding marks.
The resulting one-piece ductile iron casting weighed 40 lbs. (18.14 kg) and consisted of a central hub, a flat locking plate, a square mounting frame and a triangular support frame (Fig. 2). The envelope dimensions of the pivot frame casting were 19 x 14 x 6 in. (48.3 x 35.6 x 15.2 cm), the center hub had a diameter of 3 in. (7.6 cm) and a height of 3.5 in. (8.9 cm) and the minimum wall thickness was 0.375 in. (0.95 cm).
The center hub of the plate frame secured the pivot shaft on which the plate and chute rotate. The locking plate had seven lock holes for securing the chute at different swing angles, and the two bushings on the top of the triangular frame secured the pouring chute to the pivot frame.
Choosing the Material
The original assembly was welded from steel plates and bar stock, but ductile iron was chosen for the casting because it met the performance requirements while featuring a lower casting production cost compared to a steel alloy. Ductile iron exhibits a linear stress-strain relation, a considerable range of yield strengths and ductility. It was clear that ductile iron was the metal of choice, but the casting design team had to choose among three grades of the alloy A536.
The ASTM A536 ductile iron casting specification is based on demonstrated mechanical properties that depend on the proper cast iron microstructure. High ductility grades have a ferritic microstructure, intermediate grades have a mixed ferritic and pearlite microstructure and high strength grades have a primarily pearlite microstructure.
The performance and production requirements of the pivot frame called for an alloy grade with sufficient toughness and ductility for fatigue and impact resistance. The microstructure of A536 Grade 2 is primarily ferritic, so it has more than enough ductility/elongation and just misses the hardness requirement. However, it doesn’t meet the minimum requirements for ultimate tensile strength and yield strength.
A536 Grade 3 and Grade 4 both feature mixed ferrite/pearlite microstructures and both meet and exceed the requirements for tensile strength, yield strength and hardness. But, Grade 4 falls short on ductility and elongation. Because Grade 3 A536 fulfills all the requirements, including ductility, it was selected as the best choice alloy for this casting.
Optimizing Design with FEA
One of the design advantages of metalcasting is the freedom to optimize cross-sections and shapes beyond the limits of welded plates and bar stock. The design team used this advantage to design the triangular support frame of the pivot plate frame for the pouring chute. The new design needed to meet two objectives:
- keep the tensile stresses in critical sections below the calculated stresses in the original welded design;
- minimize the weight of the overall casting to keep production costs down and save weight on the truck.
Three-dimensional computer-aided design is the key to rapidly optimizing the design for mechanical performance and weight reduction. This also reduces the “first part” time. The design team developed three designs and used finite element analysis (FEA) to evaluate and optimize the stresses in the triangular support frame (where the stresses were highest). The designs included a plate with perimeter ribs, a frame with cross ribs and a plate with ribs and cut outs.
Design A, which used a flat plate in the support section with perimeter ribs for stiffening and stress control, met target stress requirements at the joint where the triangular frame met the straight bars, but the weight of the design was higher than desired. Design B, which used a heavy frame in the support section with cross ribs for stiffening and stress control, markedly reduced the weight, but the stresses were excessive at the joint where the triangular frame met the straight bars.
Design C used a flat plate in the support section with perimeter ribs and two long ribs in the back for stiffening. In addition, three cutouts in the center panel reduced the overall weight without increasing stresses in the critical sections. This design was selected as the final design because it kept the stresses within limits and met the weight target.
Rapid prototyping using a high-speed 3-D printing system was used to check form-fit function on the final design.
Saving in the Final Steps
Green sand molding was chosen as the best-value mold method to cast the pivot frame chute. Metalcasting engineers used orientation and risers to produce the desired directional flow in the mold. After casting, the component was shot-blasted to remove residual sand on the surface, flash lines and a riser stub were ground off, and the casting was painted prior to machining.
Three features on the pivot frame casting required separate machining steps. The inner diameter (2.5 in. [6.35 cm]) of the center bore was rough drilled and finish drilled to specification. The inner diameters (0.89 in. [2.26 cm]) of the two bearing holes were finish drilled to specification.
The new casting, which converted a nine-piece weldment into a one-piece component, produced a 50% cost savings on each part, compared to the original assembly, welding and machining costs. The tool pay-back was four months of production. The one-piece design reduced part inventory, technical data management costs and production to delivery time.
Quality management principles were applied at each stage of the casting process. Emission spectrographic analysis of the furnace and ladle chemistry assured precise control of the alloy composition, dimensional checks were performed on all features, visual examinations tested the surface appearance and soundness checks were performed with sectioning on prototype castings. The converted component featured better form and fit with less dimensional variation, lower stresses and longer life, compared to the weldment. METAL