From: paul womack




>>george@g... wrote:
>>
>> "Modern" ones have a built-in safety mechanism: The main lever simply bends
when you've overdone it.
>On a loosely related theme, on a well designed vise, the length
>and strength of the handle is designed to NOT allow a normal
>human to destroy the vise.
> 
>Of course, all this careful design work
>can be circumvented by a fool with a cheater bar.

If only the world was made for us abnormal humans. 
As a much younger man I ended up rebuilding and riding Harley Davidson
motorcycles. The 1940 thru 1956 models that came into my possession were made
with castings brazed to tubing. Because I was spending a great deal of time and
MONEY at Balance Harley Davidson in Battle Creek, MI, I got to know the owner.
He told me that the reason the frames were so heavy was that Harley Davidson
followed a rule. If it breaks double it.
As a 5’ tall 150 lb kid I knew that with a 9” long 9/16” box end wrench I could
pull as hard as I wanted and not break a 3/8 bolt.
At 6’1” and just shy of 300 lbs I have to pay attention to over tightening. 
As an engineer I found that it was necessary to equip the assembly line with
torque limiting drivers that stopped driving and free wheeled when it reached
max torque. The operators had a bad habit of giving the bolt just a bit more
after it seated and the old clicker wrench clicked. A 1/4-20 grade 8 bolt snaps
at approx. 16 foot lbs of torque. (I personally broke 100 of them to establish
the base line.) Prior to installing the new wrenches we had a crew of two
trained people using a tap burner to remove broken bolts.
People will be people. I managed to twist the drive shaft on a bush hog in two.
1. I was using a 5’ diameter bush hog to cut 6’ tall grass and wild rose bushes.
2. I had the bush hog connected to an 8N tractor that had been upgraded to a 6
cylinder engine. The resulting horsepower went from 30 to 95.
3. Some time in the distant past a previous owner operator had grown tired of
replacing the shear pin and replaced it with a grade 8 bolt.
Fortunately the driveshaft was light enough that it failed before the PTO shaft
did.