Servo Press Installs Plugs in Transmission Cases

Dec 22, 2023

"Cross-drilling" transmission castings leaves a hole in the outer shell, which must be sealed to prevent fluid loss. Photo courtesy Promess Inc.

This force-distance curve shows the process of inserting an expansion plug into a transmission housing. At point 1, force starts to increase where the cup moves through the bore. At point 2, the cup stops, and the pin starts to move into the cup. Point 3 shows the inflection point when the system starts to stake the part. Point 4, the final inflection point, is when staking has been finished and the rate of change has completed. Source: Promess Inc.

The image on the left shows a good installation at 5 kilonewtons. The image on the right shows a severely over-pressed installation at 11.2 kilonewtons. Photo courtesy Promess Inc.

An automatic transmission case is a complex casting that includes a sophisticated valve body with many channels and passages. These channels and passages must be precisely connected to ensure smooth, accurate shifting. However, it is not always possible to cast all the interconnections in place. Often, manufacturers must drill through the outer shell of the casting—and one or more internal channel walls—in a process called "cross-drilling." Doing so leaves a hole in the outer shell, which must be sealed to prevent fluid loss.

For one leading transmission manufacturer, the process of sealing such holes was simple. The company had long been using an Electro-Mechanical Assembly Press (EMAP) from Promess Inc. to insert a steel ball into the hole and thereby seal the casting.

Recently, however, the company encountered problems when trying to press the balls into a particularly 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 pressing operation to stop at a specific distance. 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.

To solve the problem, the manufacturer replaced the steel ball with a Betaplug expansion plug from The Lee Co. This preassembled, 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 plug is designed to be installed in a matching tapered bore that creates a perfect fit. It reduces unnecessary expansion and produces predictable boss stress that is 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 Betaplug 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 damage to the fixturing. The manufacturer contacted Promess and The Lee Co. for help. Both companies were asked to examine the complete installation and assembly process and suggest a solution to the high scrap rate.

Lee Co. engineers determined that the plugs 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.

Proper installation for a Betaplug should be terminated when 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 excessive installation force that extruded the plug.

Promess engineers recommended that the manufacturer change the programming to measure more than a simple force 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 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 a leak-tight seal and ensures retention. When the pin is 0.5 to 0.8 millimeter 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 over-insertion of the pin. As an added benefit, the programming can also detect parts that are upside down, sideways, or missing a pin or other part.

Promess and Lee engineers 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 tests were successfully completed.

The new EMAP program and the corrected installation procedure solved the yield issues and reduced the scrap rate. In fact, it was determined that many of the scrapped parts—previously discarded because the Betaplugs were not inserted to the originally specified distance—were perfectly acceptable and would not have failed in the field.

Since the change to the plug and program, more than 35 million plugs have been installed and are in use in the field without any warranty returns for leakage.

For more information on servo presses, call Promess at 810-229-9334 or visit www.promessinc.com.

For more information on fluid control products, call the Lee Co. at 860-399-6281 or visit www.theleeco.com.