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—which is a process called “cross-drilling”. Doing so leaves a hole in the outer shell, and it must be sealed to prevent fluid loss.
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 tight 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 cracked housings and yield issues, 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.
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 into 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.
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 registered trademark of The Lee Company.
The Lee Betaplug® is a pre-assembled, one-piece, tapered expansion plug specifically engineered to seal fluid passages in metal castings and plastic housings leak-tight, without the use of threads or sealants. Controlled expansion during installation causes the lands and grooves on the O.D. of the plug body to bite into the wall of the fluid passage, creating a leak-tight seal and ensuring retention. The unique tapered design eliminates the need for tight manufacturing tolerances and allows the designer to minimize the wall thickness required around the plug, even for brittle housing materials.
The Promess Electro-Mechanical Assembly Press (EMAP) is a fully electric, programmable motion controlled ball screw press. The Promess EMAP offers you the most competitive, accurate, and efficient solution to any assembly requirement. The Promess EMAP provides solutions to many applications including: