An automatic transmission case is a very complex casting that includes a sophisticated valve body with many channels and passages. These channels and passages must be precisely connected to perform the necessary control functions; however, it is not always possible to cast all of the interconnections in place. Often, it is necessary to drill through the outer shell of the casting—and one or more internal channel walls—this process is called “cross-drilling”. Doing so leaves a hole in the outer shell, and it must be sealed to prevent fluid loss.
Challenge Faced by the Manufacturer: Prior Sealing Method no Longer Works in New Application
The manufacturer referenced in this case is a long-time Promess customer that used an Electro-Mechanical Assembly Press (EMAP) to press a steel ball bearing and seal the cross-drilled hole. This sealing method worked in previous applications; however, on this particular thin-walled transmission case, it led to cracking of the housing, unacceptable leakage rates and warranty issues.
The EMAP is an electric servo press instrumented to monitor and precisely control peak force and final position. In this case, however, the user programmed the ball pressing application to stop at a specific distance, and only distance was controlled. Neither the dimensions of the ball nor the diameter and surface finish of the hole were held to close enough tolerances. As a result, the same level of force could leave the ball in a fairly broad range of positions within the hole which was causing leakage and cracking the thin-walled case.
Faced with unacceptable scrap, yield issues and warranty problems, the manufacturer decided to replace the steel ball with a Betaplug® expansion plug produced by The Lee Company. This pre-assembled, two-piece tapered expansion plug has an inner pin and an outer plug body with lands and grooves that bite into the housing during installation.
The Betaplug product is designed to be installed in a matching tapered bore that creates a perfect fit. It reduces unnecessary expansion and has a predictable boss stress ideal for brittle materials or thin wall conditions. The installation tool is designed to install the inner pin below flush while staking over the back edge of the plug body.
Promess and The Lee Company Assess the Situation
Use of the Betaplug expansion plug eliminated the cracked housing and production yield issues; however, the manufacturer carried over an improper installation specification that created new manufacturing issues: an unacceptable scrap rate, yield issues and damages in the fixturing. The manufacturer contacted Promess, supplier of the EMAP, and The Lee Company, supplier of the new plug solution. Both companies were asked to examine the complete installation and assembly process and suggest a solution to the high scrap rate.
The Lee Company engineers determined that the Betaplug products were being over-pressed; this produced excessive radial force when the pin moved to expand the plug body and extruded the plug in the installation bore. The manufacturer was reluctant to change the distance-based programming, because the plugs installed successfully were not failing in the field. They were not happy with the scrap rate, but they were willing to accept it.
A proper installation for a Betaplug product should be terminated when the staking is complete, regardless of where the unit is located within the bore. The manufacturer’s engineers wanted to install the plug at a fixed point within the bore regardless of the optimum staking location—that was where the steel ball plug had performed best. However, in a tight bore, it generated the excessive installation force that extruded the Betaplug expansion plug.
Promess engineers recommended that the manufacturer change the programming to measure more than a simple force level or distance. The engineers highlighted the benefit of combining EMAP instrumentation with the sophisticated data processing capabilities of the Promess Motion Controller: the result is the ability to measure and control absolute force and distance and more complex relationships such as the rate-of-change between those measurements.
During installation, the Betaplug product initially moves as a unit until the lands on the outer plug body begin to dig on the bore. When adequate resistance is achieved, the plug body stops moving; but the inner pin continues to move and generates the expansion force that creates leak-tight seals and ensures retention. When the pin is 0.5 to 0.8 mm below flush, the installation tool stakes over the top edge of the plug body.
This transition produces a readily detectable inflection point in the rate-of-change relationship between press force and distance: after it is detected, it is a simple matter to stop the press when the pin is appropriately inserted into the plug body. The result is a properly staked installation that avoids harmful pin over-insertion. As an added benefit, the programming can also detect parts that are upside down, sideways, or missing a pin/component.
Promess engineers and their Lee counterparts performed extensive laboratory testing to validate the new programming. This was done prior to installing the upgraded application in the manufacturer’s plant where further trials were performed. The new rate-of-change based application was put into production after all of the required tests were successfully completed.
A Plug and Program Change Solve the Problem
The new EMAP Program and the corrected installation procedure solved the installation yield issues and reduced the scrap rate. It was in fact determined that many of the scrapped parts—previously discarded because the Betaplug products were not inserted to the originally specified distance— were perfectly acceptable and would not have failed in the field.
There is an advantage to using a programmable device like the EMAP in place of a traditional hydraulic or pneumatic press. The EMAP can install multiple types of sealing solutions, and a simple plug and program change corrected significant scrapping and yield issues. Since the change to the plug and program, over 35 million plugs have been installed and in use in the field without any warranty returns for leakage.
Betaplug® is a trademark of The Lee Company.