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Design Features Improve Performance

Jiten Shah

Click here to read in the March/April 2018 digital edition.

An OEM needed a higher pressure rating performance of an existing oil gear pump. It tasked casting supplier Keystone Foundry Division of United Brass Works Inc. (Erie, Pennsylvania) to produce a cast version in C83600 red brass to achieve the needed requirements. The result was a better performing pump for a fluid transfer application in the oil and gas industry.

Casting Profile
Cast Component: Oil gear pump.
Process: Horizontally-parted green sand casting.
Material: Red brass C83600.
Weight: 19.15 lbs.
Dimensions: 7.5 x 5.62 x 4.68 in.

As-cast cored holes eliminate additional machining.

  • Proper tooling design, core setting in the mold and assembly allows as-cast cored holes with high precision tolerance and reproducibility.  As-cast holes in copper castings, in particular, reduce the mass and provide better properties around the cored hole as opposed to casting them solid and drilling afterwards.
  • Depending upon the hole diameter, desired tolerances; section thickness and its location, the design engineer will have a choice for such holes either to be as-cast or drilled/machined. 
  • The molding and core making processes and overall foundry process capability will dictate the decision of as-cast vs machined, in addition to the design feature’s size, location, and orientation.

To make casting more durable under fatigue pressure loading as well as for better fluid performance, it is important to have smooth transitions at intersecting junctions and web corners by generous radii and fillets. 

  • The generous fillet and radii also allow smoother entry of the liquid metal into the mold cavity, thereby reducing the turbulence and dross.
  • Most of the structural components under pressure loading induce hoop stresses and bending, and the stresses are highest on the surface, so it is important to have a sound design without sharp corners and edges and a smooth surface in the high stress areas.

Strengthening ribs are integral to casting design. They must be placed, whenever possible, normal to the parting plane of the pattern and core box, with generous draft and smooth transitions at the junctions, eliminating the tooling complexity. 

  • For better soundness of the casting, the ribs must be carefully sized; thinner than the base casting section.
  • Carefully placing the strengthening ribs can enable designers to avoid making the casting thick walled and heavier. The selection of the parting plane will allow the position and location of strengthening ribs. The casting manufacturing process allow design engineers to position ribs as required and with complex blends.