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issue: January 2004 APPLIANCE Magazine

Production - Water Heaters
An Assembly Challenge

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When A. O. Smith Water Products Company redesigned its line of gas-fired residential water heaters to meet new safety standards, it utilized the services of a fastener solutions supplier to assist with assembling an access door to the appliance's combustion chamber.

Pictured is the combustion chamber ring of A. O. Smith's C3 Technology water heater.

A. O. Smith Water Products Company's (Ashland City, TN, U.S.) C3 Technology™ gas water heater has been engineered to meet the new standard and testing protocols (ANSI Z21.10.1) established by the U.S. American National Standards Institute (ANSI), which took effect in July 2003. The standard requires that all new gas-fired, atmospherically-vented residential water heaters incorporate new design features to make them resistant to igniting flammable vapors outside of the water heater, according to the Gas Appliance Manufacturers Association.

The C3 flammable vapor-resistant water heater features a sealed combustion chamber with an air inlet, where air flows through a perforated-steel corderite flame arrestor; a removable and reportedly easy-to-clean air intake screen at the bottom of the unit for filtering out airborne lint, dust, and oil, while providing air for the combustion chamber; and a thermal cutoff device to shut off gas flow during a flammable vapor ignition or excessive internal temperatures.

From the outset, a daunting assembly challenge for the new water heaters was how to attach and secure an access door to the combustion chamber. Several different methods initially were attempted, but each posed particular problems.

According to Austin Taylor, project engineer for A. O. Smith, the initial plan was to utilize weld nuts on the water heater's round chamber ring. A mating screw would then be inserted through the access door. This proved unfeasible, however, because thick insulation on the back of the door made it difficult for installers to locate the nut with the screw to complete the job.

A switch was then made to weld studs, but Mr. Taylor says that too was unsuccessful. "We found the studs could not be held consistently perpendicular to the chamber ring, which impeded subsequent mating of the stud, door, and nut," Mr. Taylor recalls.

The challenges in finding usable hardware were ultimately resolved with PEM® Type HFH self-clinching flush-head studs from PennEngineering® Fastening Technologies (Danboro, PA, U.S.). Installed permanently into the inside of the chamber's metal ring, these high-strength steel fasteners provided the consistent and required perpendicularity. Further, the fastener installation reportedly leaves the outside surface (and appearance) of the chamber ring undisturbed and without protrusions.

According to Ed Yeh, an engineering manager at Penn, the fastening capability of the PEM fastener relies on its unique "clinch geometry." He says the fastener, in this case a stud, is inserted into a pre-punched hole in the work piece until lugs in a star-shaped pattern at the base of the fastener head is in contact with the metal of the work piece. During installation, the lugs force the metal of the work piece inward and into a preformed groove in the fastener. "What that does is cold flow all of the metal around the hole into the complicated geometry of the fastener," says Mr. Yeh. "It can't rotate anymore, so it has torque-out performance, and it can't be pushed out because the metal has buried itself in the groove. There is a good clinch, and performance is very strong. It can replace welding in many applications."

Attention next turned to the process of fastener installation. Expected production rates suggested the need for automatic equipment. It was clear that a PEMSERTER Series 2000 press from Penn could easily handle the task, but engineers realized that the press's usual top-feed method of pressing studs from above would be impossible within the confines of the chamber's 14-in diam metal ring.

According to Mr. Yeh, Penn engineers could see that the fasteners would have to be installed from inside the cylinder, which would require a bottom-feed installation system. While bottom-feed systems exist in presses to install self-clinching nuts, such systems for studs posed a new set of challenges to overcome for safe and productive installations.

By modifying the Series 2000 press' injector tooling package to use as the press' anvil instead of a punch, Penn engineers were able to develop safe and effective bottom-feed stud tooling for the application. According to Mr. Yeh, the anvil functions as an upside-down injector. The injector holds the stud by the head as the press operator places the combustion chamber in position over the stud, compressing the injector and pushing the threaded section and clinch feature of the stud into the combustion chamber. The operator then activates a footswitch to actuate the press and install the stud. "That's why this process is backwards," says Mr. Yeh. "Normally, the ram contacts the head and pushes it in. But here, we're actually pushing the material down onto the fastener." The tooling and press work together to assist the operator in aligning the stud and chamber so that each stud can be installed with one actuation of the press's footswitch.

"The PEMSERTER press with the customized tooling is the only press system we found capable of handling this type of fastener installation - inside-out with a ring," says Mr. Taylor of A. O. Smith.

Should access to the combustion chamber be required for service, Mr. Taylor notes that the permanently installed PEM self-clinching studs "preserve the fasteners with the unit." Two PEM studs (thread sizes #10-32) are located in each ring and mated with a nut for secure attachment of the access door.

The Series 2000 presses handling the installations are fully automated systems incorporating a state-of-the-art programmable logic controller (PLC) and touch screen controls to minimize operator training and simplify use. A self-diagnostic system reportedly allows an operator to work with increased speed, accuracy, and confidence. Enhanced programming ensures streamlined operation, according to Penn. The press system also features a 24-in (61-cm) throat depth and can perform up to 1,500 actual installations per hr while delivering a ram force of 400 to 16,000 lb (2.7 to 71.2 kN).

Operator safety is handled by a standard redundant safety system. The Lightstream™ optical sensor component of the safety system allows the operator to set a safety "set point" by the distance from the ram's home position to the point of fastener insertion with a precision of 0.004 in, according to Mr. Yeh. If the ram contacts any object before reaching the designated point, the stud or chassis would be touched with only a few pounds of spring pressure before dual safety valves close, and the ram automatically returns to the home position. The press is also set with an overshoot parameter. If the ram travels too long without encountering the work piece, the press will not go into force. The operator can continue with minimal delay.

With a safe, efficient, and effective assembly system and reliable fasteners in place, full production of the new C3 Technology gas water heaters is under way at A. O. Smith manufacturing facilities in Tennessee, U.S., South Carolina, U.S., and Mexico.


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