Tuesday, August 29, 2006

It never occurred to me…

When we go out of town we turn off the water to the house. We once had a flush valve on a toilet fail and flood the bathroom. Fortunately we were home when it happened. I know people who had the valve on their icemakers fail when they weren’t home and have their homes flooded. So to avoid a potential problem we simply turn the water off.

We also unplug the coffee maker and toaster. A friend of ours had a plugged in coffee maker catch on fire. Fortunately it was on a tile countertop against a tile backsplash. The result was that it tripped the breaker and burned itself out leaving a gooey stinky mess on the countertop. So after hearing that story we have decided that prudence dictates we unplug the small appliances.

Coming back late at night from our vacation this summer we were greeted at the front door with a foul odor. We quickly discovered the source of the odor. We went into the kitchen and on the floor was a puddle of foul brown liquid that has oozed out of the fridge and onto the floor. We were somewhat baffled as to why the fridge had failed. It was only ten years old. New enough that it should not have failed yet.

We quickly got the kids in bed so as to begin dealing with the putrefying mess. We opened up the freezer on top and dumped its contents into the trash. Then we opened up the lower refrigerator compartment to deal with its contents. When we opened the fridge we saw that it had been turned off. We had a good guess as to what had happened.

We finally got ourselves into bed at two in the morning. The next morning my wife asked our three year old why he turned the refrigerator off.

He looked down at the floor, laughed, looked back at my wife and said, “I was going to turn it on when we got home.”

What can you do? Apparently, after seeing us turn off the water and unplug the small appliances, he thought he would help us get ready by turning off the refrigerator.

It never occurred to us to check that refrigerator was left on before leaving town.

When doing reliability reviews of equipment imagination is required to consider how equipment might fail or how users might abuse equipment.

Doing a quick informal failure mode and effects analysis (FMEA) on our refrigerator prior to our trip would have resulted in something that would have looked like this:

Item – refrigerator
Function – keep food cold
Potential failure mode – compressor fails due to age
Effect of failure – food spoils
Mitigation action – none, failure will occur at a random point in time

After our trip and our son’s action our FMEA would look like this:

Item – refrigerator
Function – keep food cold
Potential failure modes1)compressor fails due to age
2) refrigerator deliberately turned off
Effect of failure – food spoils
Mitigation actionfor failure mode 1, none, failure will occur at a random point in time
For failure mode 2, verify that fridge is on and plugged prior to leaving

From prior experience we have decided turn off our water and unplug our small appliances before leaving town. Before our trip it never occurred to us that our fridge might be deliberately turned off.

What failure modes are lurking in your equipment that you have not considered? Use your imagination. You don’t want to say to an injured employee or disgruntled customer is “It never occurred to me that….”

Thursday, August 17, 2006

Omer Blodgett on weldments

At work I have a copy of Omer Blodgett's Design of Weldments. I occasionally refer to it when designing weldments. I have found it quite useful because it stresses developing a mental image of the flow of loads in welded structures. It helps to develop a way of thinking about weldment design beyond just presenting formulas, though it has those too.

Omer Blodgett has a column, "Blodgett's Basics," in Welding Design & Fabrication magazine. I have found it worthwhile reading. His columns can be found here.

Wednesday, August 09, 2006

Robots meant to increase efficiency slow production

New technology improperly implemented can be worse than old technology that is already functional. From the Rockford Register Star:

BELVIDERE — Last winter, contractors installed more than 700 robots in the
DaimlerChrysler assembly plant’s body shop as part of a $419 million investment
to make the plant the most flexible and automated of the company’s 14 North
American factories.

The Swedish-made ro-bots were intended to make the place more efficient. Instead, it turns out they are the main reason the Belvidere operation has fallen behind on production of the Dodge Caliber and Jeep Compass, according to the person in charge of manufacturing for the Chrysler Group.

The fact the plant isn’t meeting production targets came to light July 3 when
DaimlerChrysler officials announced sales of the Caliber dropped 2.6 percent
from May to June. The sales dip was not because the car isn’t popular; it may be
the most sought-after domestic new car in the past three years. It was because
workers at the plant aren’t turning out enough of them.

Monday, August 07, 2006

Overall Equipment Effectiveness (OEE)

Overall Equipment Effectiveness (OEE) is a metric that captures how well a piece of equipment or an entire plant is operating. The formula for OEE is given below:

OEE=equipment availability X performance X quality

By monitoring a single value, OEE, changes in any of the three factors in the formula above can be detected.

For a fuller description and a discussion on how to assign numerical values to the factors equipment availability, performance, and quality go to the links below

Wikipedia

Modern Machine Shop

OEE.com FAQ

OEE.com Glossary

I would start by reading the first two articles and then follow with the two at OEE.com. The articles at OEE provide definitions and assume some basic knowledge of OEE.

Thursday, August 03, 2006

Mistakes I have made


Some time back I designed a machine with a stepped shaft. The shaft was stepped to make assembly easier. In order to mount the shaft two different size bearings were specified. The two bearings were by the same manufacturer and in the same series. Both bearings were shown in the local bearing distributor catalog as being stocked. A schematic of the arrangement is shown above.

The drawings were sent off to the machine shop for manufacture and assembly. It was at that point that trouble arose. The machine shop called the bearing distributor and found that the smaller bearing had a thirty week lead time. I had made the assumption that since the bearings were shown in the catalog they were readily and quickly available.

We recovered by using two of the larger bearings. A sleeve was mad to fit over the stepped shaft so a larger bearing could be used where the shaft necked down. The diameter for the smaller bearing was then bored out to accept a large bearing.

The moral is never assume that you can easily get what is shown in the catalog. Always investigate the lead time for your parts.

Remember: If you don't do anything you never make mistakes.

Wednesday, August 02, 2006

Quote for the day

"...[T]he mathematician lives in an ideal world, and the practical importance of his conclusions must depend on the essential similarity of his world to ours. Now as practical engineers it is our business to assess this similarity and never to mistake the mathematical abstraction for reality.... "

page 3, The Design and Use of Instruments and Accurate Mechanisms, T.N. Whitehead, MacMillan, 1934, Reprinted by Lindsay Publications