Tolerances are Stacking Up – Part 1


(Here is the 1st scene from my next story, “Tolerances are Stacking Up.” Join me for this journey into statistical assembly tolerance analysis. Your comments and suggestions are welcome.)

“Nervous?” Blake whispered in Jordan’s ear.

“Are you kidding?” Jordan replied to his mentor on his way to the front of the conference room. He thought, “Now’s my chance to impress the customer in front of my manager and co-engineers. I worked hard on this material. The design is perfect, and my presentation is the best.”

Jordan worked his way through the Preliminary Design Review (PDR) charts like a well-oiled machine, explaining the motor mechanical design to his satisfaction. He addressed every customer requirement and clearly showed how each criteria would be met with his design. When no one responded to his invitation for questions, he closed his mouth, nodded lightly, put away his laser pointer and stepped away from the podium.

“I’m not convinced.” Mr. Bradley, leaned forward in his leather, high-back chair towards the head of the table, lips pressed together, eyes burrowing into Jordan as if they were playing poker. “In fact, I’m worried. What kind of tolerance analysis are you doing? How do you know it’s going to operate without failure for the required number of years or cycles?”

Jordan stopped, turned a little red, and stared at the customer. It got hot in the room suddenly as he looked from person to person sitting around the walnut conference table, hoping for some help; a look, an impression.

“Well sir…” Jordan said, searching his mind and the faces of co-workers for a satisfactory answer. “On the engineering drawings I plan to impose tolerances tight enough so that the overall assembly will fit perfectly with the correct pre-load on the motor bearings.”

Margaret, Jordan’s manager squirmed in her chair, raised an eyebrow, and looked quickly around the room at her team.

“We’ve done this before,” Jordan continued. “It shouldn’t be a problem.”

Mr. Bradley looked at Margaret, then back at Jordan. “As you well know, we plan to build a a million of this car model. If our customers like it, maybe even two or more million. Every window motor must have the correct bearing pre-load. If the overall gap is too large or small, we might ship a bad motor. There is no time on our assembly line for re-work. That’s why we spend huge amounts of money monitoring variations and controlling them. Like I said, there isn’t time to fix an out of tolerance motor, and even if we could fix it, we would have to change our process to…”

“We’ll take another look at our tolerance scheme.” Margaret interrupted.

The team from ElectroTorque including Jordan, Margaret, and Blake, Jordan’s mentor, travelled all the way from Los Angeles to Detroit for a two-day Preliminary Design Review to finalize the design solution and get approval from the auto maker to move forward with detailed design and the purchase of long-lead parts and tooling.

One year earlier, ElectroTorque fought hard for this contract competing with other reputable motor companies. It was a must-win for the company. ElectroTorque designed electric motors. In this case, each car would use four identical motors to raise and lower the driver and passenger windows. The customer was concerned about the internal motor tolerances having had bad experiences with motors overheating, wearing out, and having to replace too many on similar models.

Each motor included a rotary shaft or rotor, mounted in a generally cylindrical housing by two ball bearings, one at each end of the shaft. The housing had a mechanical cap at each end supporting the bearings. Bearing preload was accomplished by the constant-pressure method, or the inclusion of a wave spring washer between one of the shaft end caps and the outer raceway of the adjacent bearing. If the preload was too large, bearing stress and gouging may cause failure; if the preload was too small, bearing and shaft vibration may also result in failure.

“Automobile ‘recalls’ are not an option,” the customer declared while tapping four fingers on his copy of the contract. He looked around the room for a moment as if to say, don’t talk, I’m thinking. After a moment he said, “I think we may have to post pone development for a couple months until you have a better story.”

Margaret jumped causing her chair back to oscillate. “Delays will cost us both money,” Margaret caught her balance and leaned forward gripping the thick, well rounded edge of the table.

Jordan wiped the back of his hand across his forehead, then reached in his pocket for a handkerchief or napkin he knew wasn’t there and thought, “This isn’t good. This can’t be happening. They wouldn’t pull the contract from us would they? My career at this company would be over.”

“I’d rather delay production than have to tell my superiors,” Mr. Bradley paused and with elevated tone said, “I don’t want to explain to the shareholders why motors are failing prematurely.”

MotorAndWasher

(To be continued)

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2 Responses to Tolerances are Stacking Up – Part 1

  1. Pingback: Tolerances are Stacking Up – Part 1 | Margaret Holden Hardman

  2. Anonymous says:

    Dang, just as it was getting good!!!

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