C
On 16 Feb 2007 11:29:38 -0800, [email protected] wrote:
>On Feb 16, 12:41 am, [email protected] wrote:
>> On 15 Feb 2007 20:04:07 -0800, [email protected] wrote:
>>
>>
>>
>> >On Feb 15, 6:46 pm, Tim McNamara <[email protected]> wrote:
>>
>> >> Under ideal circumstances, jim's assessment that the retention provided
>> >> by the QR should exceed the ejection force is probably reasonable. But
>> >> non-ideal circumstances are all too readily encountered in real life.
>>
>> >Jim seems to think a retention force three times the ejection force
>> >(IF everything is done perfectly) is obviously adequate.
>>
>> >In other words, he's satisfied with a design factor (or safety factor)
>> >of three.
>>
>> >But in mechanical design, all common parts are always designed with a
>> >safety factor. That is, parts are not designed to be precisely as
>> >strong as necessary. If that were done, then any tiny error in
>> >evaluation of force, material property, manufacturing technique, or
>> >operating condition would cause failure.
>>
>> >For ordinary industrial design using well-known materials, well
>> >understood loads, ordinary controlled conditions of operation, etc.
>> >safety factors of three are quite common.
>>
>> >For situations where adjustments have to be made in the field by
>> >operators using their own feel and judgement, where loads are
>> >difficult to assess, and where failure can result in serious bodily
>> >harm, a safety factor of three is entirely inadequate.
>>
>> >> Since the fork can be readily redesigned to eliminate the ejection
>> >> force, there is no reason not to do it- and is already being done to a
>> >> degree by the fork makers.
>>
>> >Exactly.
>>
>> >- Frank Krygowski
>>
>> Dear Frank,
>>
>> What is an entirely adequate safety factor "for situations where
>> adjustments have to be made in the field by operators using their own
>> feel and judgement, where loads are difficult to assess, and where
>> failure can result in serious bodily harm"?
>>
>> Possibly rock climbing equipment will give some insight?
>
>Try http://www.usrigging.com/SafetySnapHooks.pdf for one example.
>
>And note that with these, there's no fiddling with a screw adjustment
>to achieve proper strength. There's no chance of a partial
>engagement, by design. But there's still a safety factor of ten.
>
>- Frank Krygowski
Dear Frank,
Interesting.
I looked at climbing ropes (the weakest link).
Here's a description of the rope tests:
"The Union Internationale des Associations d'Alpinisme (UIAA)
certifies climbing equipment. The manufacturer pays dearly to have its
products certified but, because of its rigorous testing, UIAA
certification is a guarantee of high quality. One test determines the
number of falls the rope can withstand with a load of 80 kg (55 kg for
half ropes) before breaking. The length of rope used in the test is
approximately 2.8 m. The weight, attached to one end of the rope, is
released from a height of 5 m. This test simulates a severe fall over
twice the distance between the climber and his or her last piece of
protection. The impact force is also very high since, in the
relatively short length of the fall, the rope's elasticity does not
particularly come into play."
"The impact force test verifies whether a rope's elasticity meets
minimum standards. Ropes have a certain measure of elasticity so that
they can absorb and dissipate the force of impact. If the rope is too
elastic, the climber will bounce uncontrollably after a fall, with
disastrous results. Impact force is measured in the first fall in the
test described above. The impact force must be less than 1200 kN (800
kN for a half rope). So, practically speaking, the smaller the impact
force indicated on the manufacturer's label, the more the rope will
absorb energy in a fall and minimize the impact on your belay system."
http://www.lacordee.com/en/conseils/details/achat/?id=18
And here's a page with some actual ropes and ratings:
http://www.metoliusclimbing.com/ropes_dynamic.htm
If you browse around, you'll notice that there's another extremely
interesting test--the number of UIAA sharp edge falls that the rope
can withstand.
The maximum on that page is 1 such fall for any rope. Apparently, if
you fall and let your rope hit a sharp edge on the mountain, all the
other safety factors are not going to be very helpful.
Cheers,
Carl Fogel
>On Feb 16, 12:41 am, [email protected] wrote:
>> On 15 Feb 2007 20:04:07 -0800, [email protected] wrote:
>>
>>
>>
>> >On Feb 15, 6:46 pm, Tim McNamara <[email protected]> wrote:
>>
>> >> Under ideal circumstances, jim's assessment that the retention provided
>> >> by the QR should exceed the ejection force is probably reasonable. But
>> >> non-ideal circumstances are all too readily encountered in real life.
>>
>> >Jim seems to think a retention force three times the ejection force
>> >(IF everything is done perfectly) is obviously adequate.
>>
>> >In other words, he's satisfied with a design factor (or safety factor)
>> >of three.
>>
>> >But in mechanical design, all common parts are always designed with a
>> >safety factor. That is, parts are not designed to be precisely as
>> >strong as necessary. If that were done, then any tiny error in
>> >evaluation of force, material property, manufacturing technique, or
>> >operating condition would cause failure.
>>
>> >For ordinary industrial design using well-known materials, well
>> >understood loads, ordinary controlled conditions of operation, etc.
>> >safety factors of three are quite common.
>>
>> >For situations where adjustments have to be made in the field by
>> >operators using their own feel and judgement, where loads are
>> >difficult to assess, and where failure can result in serious bodily
>> >harm, a safety factor of three is entirely inadequate.
>>
>> >> Since the fork can be readily redesigned to eliminate the ejection
>> >> force, there is no reason not to do it- and is already being done to a
>> >> degree by the fork makers.
>>
>> >Exactly.
>>
>> >- Frank Krygowski
>>
>> Dear Frank,
>>
>> What is an entirely adequate safety factor "for situations where
>> adjustments have to be made in the field by operators using their own
>> feel and judgement, where loads are difficult to assess, and where
>> failure can result in serious bodily harm"?
>>
>> Possibly rock climbing equipment will give some insight?
>
>Try http://www.usrigging.com/SafetySnapHooks.pdf for one example.
>
>And note that with these, there's no fiddling with a screw adjustment
>to achieve proper strength. There's no chance of a partial
>engagement, by design. But there's still a safety factor of ten.
>
>- Frank Krygowski
Dear Frank,
Interesting.
I looked at climbing ropes (the weakest link).
Here's a description of the rope tests:
"The Union Internationale des Associations d'Alpinisme (UIAA)
certifies climbing equipment. The manufacturer pays dearly to have its
products certified but, because of its rigorous testing, UIAA
certification is a guarantee of high quality. One test determines the
number of falls the rope can withstand with a load of 80 kg (55 kg for
half ropes) before breaking. The length of rope used in the test is
approximately 2.8 m. The weight, attached to one end of the rope, is
released from a height of 5 m. This test simulates a severe fall over
twice the distance between the climber and his or her last piece of
protection. The impact force is also very high since, in the
relatively short length of the fall, the rope's elasticity does not
particularly come into play."
"The impact force test verifies whether a rope's elasticity meets
minimum standards. Ropes have a certain measure of elasticity so that
they can absorb and dissipate the force of impact. If the rope is too
elastic, the climber will bounce uncontrollably after a fall, with
disastrous results. Impact force is measured in the first fall in the
test described above. The impact force must be less than 1200 kN (800
kN for a half rope). So, practically speaking, the smaller the impact
force indicated on the manufacturer's label, the more the rope will
absorb energy in a fall and minimize the impact on your belay system."
http://www.lacordee.com/en/conseils/details/achat/?id=18
And here's a page with some actual ropes and ratings:
http://www.metoliusclimbing.com/ropes_dynamic.htm
If you browse around, you'll notice that there's another extremely
interesting test--the number of UIAA sharp edge falls that the rope
can withstand.
The maximum on that page is 1 such fall for any rope. Apparently, if
you fall and let your rope hit a sharp edge on the mountain, all the
other safety factors are not going to be very helpful.
Cheers,
Carl Fogel