Many, many years ago - maybe decades ago - I purchased a set of
"muffler pipe" style jack stands with the specific intent of testing
them to failure. At the university I had access to a 120,000 Lb
Tinius Olsen universal test machine (http://www.tiniusolsen.com/) to
run the test. Think of this machine as a BIG bearing press with
instrumentation. The jack stands were the type made of muffler tubing
- splayed legs held with a small strap of steel to keep them from
spreading. An inner pipe has a cradle on top and usually 3 height
adjustments holes through which a pin is placed to make the
adjustment. Something like these - only these are better
http://www.danoland.com/nsxgarage/tools/jackstand.jpg. The ones I
tested had the base strap installed about 1" above the floor and the
3 legs weren't beveled to sit flat on the floor.
Here's what I found.
1) The jacks were able to hold their rated capacity with no problem.
2) They exceeded their rated capacity - I can't remember by how much.
3) When they began to fail, the failure was not catastrophic. There
was plenty of warning that the load was too great. The jacks slowly
buckled when they did fail. The spotwelds held. The adjustment (which
was in double shear) did not fail. The holes in the tubing did begin
to elongate - another warning of excessive load.
4) The jacks rested on 6 points on the legs since the legs weren't
beveled to sit flat on the floor. Those points buckled early in the
failure - a sign of excessive load.
Overall there was plenty of warning that failure was about to happen.
SAFE BUT . . .
1) The load on the jack was applied slowly - no sudden impact which
could have changed how they failed.
2) I only tested 2 jacks out of the thousands made and the
considerable number of variations on how they were built.
3) The Tinius Olsen machine held the jacks squarely - no uneven floor
to introduce odd ball loading.
Conclusion.
These were about the cheapest jacks made in the day yet they did the job.
Just now I had a hard time finding a picture of the jacks - the
danoland.com site above. All the jack stands that came up had better
design features than did the muffler tubing jack stands. This doesn't
mean the materials or workmanship is OK - just that the basic designs
today look better to me.
Finally, don't ever use a concrete block as a jack stand. Concrete is
very weak in tension and will fail suddenly under a relatively small
load. When loaded on the face of the block the concrete is in tension.
Tim Collins
http://www.flickr.com/photos/12702006@N07/
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