Saturday, April 01, 2006

Failure Modes

Beautiful weather again today. I did do some work around the house, including raking up the yards, going over the gardens to see what needs to be done, etc. But since there was some rain this morning, I decided to do some things inside. The guest bed has needed some work for years now. It sits on a folding metal frame that has 4 legs with casters. The two casters at one end came equipped with locking brakes. I forgot to unlock them when moving the bed, and ended up shredding the cheap plastic wheels. So I finally decided to replace them. I took a nice walk down to the local hardware store and found casters I thought would work well.

On the way home, I stopped at the custard shop for the flavor of the day, Puree du Smurf. It was great - bright blue color, tasting of - I don't know. Sherbet? But what made it truly brilliant were the pieces of whipped marshmallow hidden inside. Delicious, and hilarious. Also, they had a drawing of smurf in a blender on the front door.

I got home and set to replacing the casters. Here's the new with the old:

The new, the slightly broken, and the totalled casters

The casters are set in a plastic sleeve that goes into the formed sheet metal frame. Mild steel, I think. And the shaft of the new caster fit perfectly. All done! Let's take her for a test toast.

The bed... settled underneath me. Hmmm.

Turns out the old casters had longer shafts for a reason. The upper end of the plastic sleeve was set into a small metal retaining tab that's welded to the frame. I hadn't noticed this. It's important because the center of the wheel, where the load will be centered, is offset from the axis of the shaft. This means that a moment couple is created, trying to bend the shaft. The shaft is plenty strong - no problem. But the metal tab is what normally resists the bending moment. If the shaft in the sleeve doesn't reach the tab, only the crappy plastic sleeve is there to stop the entire caster from rotating right out from under the frame.

Weld fracture and inelastic deformation at pre-stressed zone Posted by Picasa

As you see, the sleeve failed, the welds holding the bottom plate failed, and the pre-stressed areas where the side plate is bent over to become the bottom plate failed. As an engineer, I really should have caught this one ahead of time.

So the guest bed is now a frameless low-rider, just like my bed.

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ElingFam said...

Re your lack of engineering foresight .......... (I verbalize the following sentence in an imperious and indignant tone)
Sir, I served with Don Lathrop, I knew Don Lathrop, Don Lathrop was a friend of mine. Sir, you are no Don Lathrop.

Kelly said...

Just think, you were smart enough to give it a test ride!! Although, when I saw the picture of the casters, that was my first concern. That stinks! I hope your floor didn't get too sratched.

Dan said...

I may not be a Don Lathrop, but I'm at least a Dave Lathrop. Or maybe... which one was the road guy? Not the bridge guy. I hope I'm not the Other Lathrop... Dwayne?

At least the floor didn't get all messed up. There are some old scratches from a previous tenant, so the few I added from the old broken caster are no big deal. I can buff them out later.