Material choices improve valve assembly

• increased integrity

• simplified assembly

The transmission check valve function is critical to the proper operation of tractors and other off-road vehicles. The valve allows pressurization of the transmission to between 1 and 2 psi, enabling evacuation of oil from specific transmission chambers for shifting. Valve component integrity is extremely important since the components are inaccessible to service in the end application. Component failure can inhibit transmission shifting which can result in transmission damage and significant warranty issues.

Previously, the individual parts of the six-component assembly were manufactured by multiple suppliers in batch lots, shipped to and assembled by the Tier One customer. Potential quality issues were a concern so the customer looked for a better solution. Bringing to bear their fluid and pneumatic sealing experience with related applications, Minnesota Rubber and QMR Plastics provided an improved design and simplified assembly solution. For the transmission’s operating environment, PPS (Polyphenylene Sulfide) was determined as the most suitable plastic material for the valve body. An NBR (nitrile) ground rubber check ball, an NBR o-ring, a coil spring and a PTFE stamped washer are also included in the assembly. The ball and spring are assembled into the body and the retaining spring is welded in place. An outer o-ring and PTFE washer are then assembled onto the valve body. A final functional test is performed on each completed check valve assembly prior to shipment.

Multiple challenges were present for this assembly, the most important of which were long term strength requirements and the ability to seal correctly in a harsh operating environment. Component materials had to withstand transmission oil exposure under alternating pressures for thousands of cycles. By choosing the correct materials, required temperature and fluid and wear resistance along with close tolerance sealing features were achieved. By unifying rubber and plastic molding and subsequent assembly, critical checks and tests are performed at the molding press where any non-compliance issue is immediately identified and corrected, thereby ensuring quality at the customer’s point of assembly.