Yetter Manufacturing, Colchester, Ill., U.S., needed a solution to a prematurely wearing knife fabrication an agriculture product designed for high-speed, high-volume fertilizer applications.
“When we had the fabrication part, it worked better than anything we had like it,” said Lonnie Lucas, engineer for Yetter. “But the knives were one of the first parts to wear on the assembly.”
The company also was dissatisfied with the knife’s fabrication time and dimensional instability. Several components welded to the knife also wore prematurely and required increased inventory for replacement. When worn, the complete knife assembly required significant cost and downtime to replace.
“The part requires so many small pieces, and each had a tolerance,” Lucas said. “The knife is made to run at an angle, and that tolerance is sensitive.”
Over the years, Yetter had sourced many cast components from Lethbridge Iron Works, Lethbridge, Alberta, Canada, so it knew converting to a casting could help reduce the number of parts on the assembly, decrease cost and improve dimensional stability.
But the agriculture equipment manufacturer wasn’t sure which iron alloy would provide the necessary hardness in the application.
The fertilizer knife is attached to the Coulter disc to keep the blade free of residue and protect the fertilizer delivery tubes from wear. It creates a 4-in. trench in the soil for fertilizer or ammonia placement. In operation, the knife shatters the trench cavity’s hard side walls to help close it quickly so the liquid nitrogen delivered to the soil via tubes is not exposed to the air to become gas. The knife’s application requires high wear resistance due to the severe conditions of the application.
To achieve optimal performance out of the cast conversion, Yetter worked with Lethbridge to produce prototypes in three materials: 27% high chrome iron, austempered ductile iron (ADI) and carbidic austempered ductile iron (CADI).
CADI is a relatively new family of ductile iron that few North American metalcasting facilities pour. It contains carbides in its microstructure to increase toughness and wear resistance after austempering. Abrasion resistance and hardness increase as carbidic content increases. In field trials, the CADI knife prototype exhibited increased impact resistance compared to the 27% high-chrome iron prototype and superior abrasion and wear resistance over ADI grade 5. Yetter and Lethbridge selected CADI as the production material.
Yetter wanted the new knife design to offer better protection for the assembly’s anhydrous ammonia delivery tubes and beavertail components, which were prone to wear on the original fabrication. Consolidating various parts into a single casting also would eliminate the need for additional part numbers. Other non-critical components could be offered as optional add-ons.
Lethbridge produced the final design in green sand with two cores and supplied it to Yetter as-cast, without any machining required. Shell coring was chosen for the mounting ears and holes, which kept the ears parallel to each other and provided dimensional stability for final assembly. A second core was used to produce the cavity for the fertilizer delivery tube and incorporate a mounting hole for the beavertail, which had been a separate part welded onto the knife. A square carriage bolt hole for the tube assembly was incorporated into the mold to eliminate machining requirements.
At Lethbridge’s suggestion, the knife features a carbide insert on the leading edge for superior wear over an as-cast edge. Lethbridge grinds the parting lines robotically with a diamond-coated wheel for superior dimensional control.
With the final cast design, Yetter achieved a 60% cost savings and reduced its part numbers to two for right and left hand versions of the one-piece casting. One casting may be used for several end-products, such as a single tube, double tube or vapor tube. The cast knife provides greater protection of the delivery tubes from the soil, leading to significantly longer wear life. Yetter’s order accuracy also increased due to the decrease in part numbers and ability of the same knife to be used with various tube kits.
“We needed to clean the part up [by casting it],” Lucas said. “We cleaned it up in every aspect, including inventory. We were able to cut down on the number of parts, which helps maintain our tolerance.”