RennStand - The New Jack Stand for Porsche Enthusiasts
01-05-2017, 01:31 PM
#46
Rennlist Member
Makes sense how you configured them to anticipate jackpoint placement!
As for the pins, increasing the diameter is certainly the most straightforward means. Testing is excellent. Remember, there is a difference between stress and force. Force is mass X acceleration, i.e., what the pin is subjected to when the jack is withdrawn and the weight of the car descends upon the stands. That varies not only with the weight of the car, but also how quickly the car is lowered. Once the car is resting on the stand, the pins are no longer subject to force, and modulus of elasticity becomes more relevant than shear strength.
As for the pins, increasing the diameter is certainly the most straightforward means. Testing is excellent. Remember, there is a difference between stress and force. Force is mass X acceleration, i.e., what the pin is subjected to when the jack is withdrawn and the weight of the car descends upon the stands. That varies not only with the weight of the car, but also how quickly the car is lowered. Once the car is resting on the stand, the pins are no longer subject to force, and modulus of elasticity becomes more relevant than shear strength.
01-05-2017, 03:14 PM
#47
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Makes sense how you configured them to anticipate jackpoint placement!
As for the pins, increasing the diameter is certainly the most straightforward means. Testing is excellent. Remember, there is a difference between stress and force. Force is mass X acceleration, i.e., what the pin is subjected to when the jack is withdrawn and the weight of the car descends upon the stands. That varies not only with the weight of the car, but also how quickly the car is lowered. Once the car is resting on the stand, the pins are no longer subject to force, and modulus of elasticity becomes more relevant than shear strength.
As for the pins, increasing the diameter is certainly the most straightforward means. Testing is excellent. Remember, there is a difference between stress and force. Force is mass X acceleration, i.e., what the pin is subjected to when the jack is withdrawn and the weight of the car descends upon the stands. That varies not only with the weight of the car, but also how quickly the car is lowered. Once the car is resting on the stand, the pins are no longer subject to force, and modulus of elasticity becomes more relevant than shear strength.
Thanks again!
01-05-2017, 07:05 PM
#48
Makes sense how you configured them to anticipate jackpoint placement!
That varies not only with the weight of the car, but also how quickly the car is lowered. Once the car is resting on the stand, the pins are no longer subject to force, and modulus of elasticity becomes more relevant than shear strength.
That varies not only with the weight of the car, but also how quickly the car is lowered. Once the car is resting on the stand, the pins are no longer subject to force, and modulus of elasticity becomes more relevant than shear strength.
If I may be so bold as to offer my services for mechanical testing when you're done. Being basing in Canada the exchange rate is favorable to you guys. Whilst my lab deals with materials characterization up to 15KN the University here has machines that break reinforced concrete, i'm sure i could get my hands on something to flatten your stands
Zymetrix.com
01-05-2017, 09:56 PM
#49
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One point to clarify, gravity is always acting on the car, therefore there is always a force acting on the pins. You're right that acceleration and mass are acting together, but neither go away.
If I may be so bold as to offer my services for mechanical testing when you're done. Being basing in Canada the exchange rate is favorable to you guys. Whilst my lab deals with materials characterization up to 15KN the University here has machines that break reinforced concrete, i'm sure i could get my hands on something to flatten your stands
Zymetrix.com
If I may be so bold as to offer my services for mechanical testing when you're done. Being basing in Canada the exchange rate is favorable to you guys. Whilst my lab deals with materials characterization up to 15KN the University here has machines that break reinforced concrete, i'm sure i could get my hands on something to flatten your stands
Zymetrix.com
This has turned into a very interesting thread. I need to get my engineer son involved except, damn it, he is a nuclear engineer 
01-06-2017, 12:48 AM
#52
Drifting
Regarding the pins, I would suggest two things (you can pay me a flat fee for the design consult or a per-unit royalty ):
1. Pins need to be attached to the center support and the bases with tethers. Loose pins will roll away at best, or more likely, get lost. Lost pins get replaced by whatever is handy--nails, screwdriver shafts, brittle Chinese bolts, etc.
2. Pins need to have a positive latch like a square wire lock pin, Clovis or a spring-loaded ball. With a traditional stand, gravity always holds the pin in place. With your design, jacking and lowering the car causes the pin to be acted on alternately from the top and bottom, increasing the chance that the pins could slip either partially out or completely drop out, especially if the jack is not perfectly flat.
Just thinking like a plaintiff here.
HTH, James.
):1. Pins need to be attached to the center support and the bases with tethers. Loose pins will roll away at best, or more likely, get lost. Lost pins get replaced by whatever is handy--nails, screwdriver shafts, brittle Chinese bolts, etc.
2. Pins need to have a positive latch like a square wire lock pin, Clovis or a spring-loaded ball. With a traditional stand, gravity always holds the pin in place. With your design, jacking and lowering the car causes the pin to be acted on alternately from the top and bottom, increasing the chance that the pins could slip either partially out or completely drop out, especially if the jack is not perfectly flat.
Just thinking like a plaintiff here.
HTH, James.
01-06-2017, 02:30 PM
#53
I like the idea of the shear pins being tethered. Lessons the chance of someone subsituting a pin that does not have the required shear strength. A latch with a positive or semi-positive retaining system is also a good idea. There are some SS ones I use on the standing rigging on my C270 sailboat. I will look up the model and technical info and pass it on. I come from an EE background, we use to have a joke in our design processes. 'You can try and design in factors to cover you for the idiots, but it is much harder to design for the ******* user'
01-06-2017, 03:47 PM
#54
After some thinking, I would not suggest the locking clevis pins I use on my sailboat. While they provide a rock-solid positive lock, they are fussy to use and are expensive. Plus no method to tether them. I only use them twice a year and only while the boat is on the hard. If you do want to have a look, they are called Jurgan Tool Free Clevis Pins. There are made in the US. Just Google Jurgan Clevis Pins and you will find a few sellers, and links to the manufacture.
01-06-2017, 04:37 PM
#55
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Regarding the pins, I would suggest two things (you can pay me a flat fee for the design consult or a per-unit royalty ):
1. Pins need to be attached to the center support and the bases with tethers. Loose pins will roll away at best, or more likely, get lost. Lost pins get replaced by whatever is handy--nails, screwdriver shafts, brittle Chinese bolts, etc.
2. Pins need to have a positive latch like a square wire lock pin, Clovis or a spring-loaded ball. With a traditional stand, gravity always holds the pin in place. With your design, jacking and lowering the car causes the pin to be acted on alternately from the top and bottom, increasing the chance that the pins could slip either partially out or completely drop out, especially if the jack is not perfectly flat.
Just thinking like a plaintiff here.
HTH, James.
):1. Pins need to be attached to the center support and the bases with tethers. Loose pins will roll away at best, or more likely, get lost. Lost pins get replaced by whatever is handy--nails, screwdriver shafts, brittle Chinese bolts, etc.
2. Pins need to have a positive latch like a square wire lock pin, Clovis or a spring-loaded ball. With a traditional stand, gravity always holds the pin in place. With your design, jacking and lowering the car causes the pin to be acted on alternately from the top and bottom, increasing the chance that the pins could slip either partially out or completely drop out, especially if the jack is not perfectly flat.
Just thinking like a plaintiff here.
HTH, James.
Also a good observation on the pins being acted on in alternating directions and how that could pose an issue. We are currently using the detent type with the spring loaded ball at all four attachment points. The two pins in the legs are also assisted by gravity since they angle down in the direction of insertion, but the ones in the center component don't have that benefit. So far the detent pins are working great, but we were a bit concerned that over time the ball could waller out the holes, making the detent pin less effective. After having this concern assessed, it evidently will take thousands of uses before that could become an issue. They've been the most convenient to use given it's a single motion to put them in or out, but even so we're still open to other options before they go into production.
Great that you came at it from that point of view. Safety should be the #1 priority.
Much thanks!
01-06-2017, 04:46 PM
#56
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After some thinking, I would not suggest the locking clevis pins I use on my sailboat. While they provide a rock-solid positive lock, they are fussy to use and are expensive. Plus no method to tether them. I only use them twice a year and only while the boat is on the hard. If you do want to have a look, they are called Jurgan Tool Free Clevis Pins. There are made in the US. Just Google Jurgan Clevis Pins and you will find a few sellers, and links to the manufacture.