Weatherproofing

Spider After 3 Years

In 2001, the spider got rained on, showing that you can only get by for so long. Even if weather were not much of a factor, it's better to be safe than sorry. We really should have built her to be much more water-resistant. That means:

• Use outdoor plywood for the base, not just scraps of whatever happened to be around.
• Wet cloth is heavy. Consider some other form of covering.
• Use better glue to attach the rug scraps that cushion the roof.

Better Glue

Spider After 3 Years

Revised Base

Spider After 3 Years

The spider mechanism and base were built and tested on level ground. We always assumed that if she could rear up on the ground, and she could be mounted on the roof, she could rear up on the roof. This was true, but we forgot that, once mounted on the roof, the resting state would be pointing downhill. This places stress on the two upright "legs". They don't appear ready to break, but they make us nervous. The base should have taken the roof pitch into account. This would reduce stress on the uprights. And perhaps we need more uprights, or to strengthen the existing ones with a metal pipe inside.

For 2002, we ended up making some simple changes to the existing base, in order to strengthen the uprights.

In year three, we also noted that the spider is not as energetic and responsive as she once was. It took more air pressure to activate her than in the past. After examining the structure, we determined that it's not an issue of dirt or wear in the bored-out PVC joints. Perhaps she would benefit in having the upright "legs" spaced a bit further apart. And having the pneumatic cylinder offset from the center line might be torquing things. We originally offset the air cylinder because the center position is occupied by a vertical structural member. Perhaps we should have worked a little harder to fit the pneumatic cylinder in the center.

For 2002, we also replaced the modified screen door closer with a commercial pneumatic cylinder.

Structural Strength

Spider After 6 Years

A piece of PVC pipe has snapped entirely. Even more alarming, this particular component is used to support one of the front legs, which reach out over the porch.

This is a close-up view of the failure.

The PVC pipe is a short piece between two closely-positioned fittings.

The pipe snapped at the point where it exits from the coupler on the right.

This view from farther away shows the position of the failed piece with a red arrow.

The front legs are supported on only one end, and reach out over the porch. This diagonal bar takes most of the punishment as the motion of the spider makes the front legs sway in and out.

Analysis:

• Notice that this piece of PVC pipe is yellowish and discolored. We believe that this was an old piece of scrap PVC that was used. David recalls that when cutting some pieces from the old PVC that it was hard, and when he tried to cut it, it sometimes shattered.
• We believe that the failure is due to the use of old, brittle PVC. Most of the spider is built from new PVC that should not have this problem.

Suggestions:

• Use all new and fresh PVC pipe and fittings.
• Do not let PVC pipe or fittings sit around in the sun. It can make it brittle.
• If we were building the spider again from scratch, we might be inclined to use 3/4" PVC for the entire body, instead of using 3/4" for the bottom ring and 1/2" for the top.
• Inspect your spider regularly so that structural failures can be detected early.

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