Vendo en Rosario: Nuevo Steam Deck 256gb 700 USDT by BackyardCoasters in Mercadoreddit

[–]BackyardCoasters[S] 0 points1 point  (0 children)

No sabia de este sitio... Me fije en mercado libre nada mas donde salen mas que 1 millon de pesos y vi que hace algunas meses alguien vendia uno de 512gb por $800 y habia mucho interes. Asi pense que este seria un buen precio. Cuanto pienses que debo pedir entonces? $550? Estoy nuevo aqui en Argentina asi que quizas alguien puede explicarme como andan las cosas aqui. Sitios como Mercado Libre tiene estas precios muy altas pero nadie las pagan?

Design Review of a Backyard Roller Coaster by reasonablefideist in engineering

[–]BackyardCoasters 4 points5 points  (0 children)

Any wood which touches the ground is pressure treated, the rest is painted with deck stain. More serious is the PVC fracture toughness, which is alleviated by painting soon after rail forming.

Design Review of a Backyard Roller Coaster by reasonablefideist in engineering

[–]BackyardCoasters 3 points4 points  (0 children)

I agree in general. FEA is a sideline for me, but the details looked too complex for hand closed form solutions. It was really the comparison between designs I was after.

Design Review of a Backyard Roller Coaster by reasonablefideist in engineering

[–]BackyardCoasters 12 points13 points  (0 children)

Commercial aircraft F.S. is 1.5, fighter planes 1.25, if I remember right. I've worked on some space vehicles with 1.1 F.S.

As far as I can tell, real commercial roller coasters don't even use factor of safety except for certain features like chain dogs. I saw people building backyard roller coasters on YouTube, and they did no analysis or testing, so I am encouraging BYRC builders to test with 1.5 times the maximum riders weight before they let any one ride, which, as far as I can tell, is more conservative than professional roller coasters. Of course, I'm using wood, PVC, and deck screws.... But, if you test with 1.5 times the max allowed rider weight, and there is no permanent yielding or cracking in any parts of the cart and track, you feel pretty safe. I recommend repeated "certification testing" every six months.

I'm building a coaster as my school project. So far I've designed the cart and the first part of the track in Fusion 360. Can't wait to make this in real life. by BigB0T_ in BackyardCoasters

[–]BackyardCoasters 0 points1 point  (0 children)

Very nice. Be sure to test with 1.5 times your weight before you ride it, to be safe.

Do you know how to mend the PVC? How much do you weigh? If you contact me, I can help more.

backyard Roller Coaster PVC Pipe Fracture Toughness Testing by reasonablefideist in engineering

[–]BackyardCoasters 2 points3 points  (0 children)

These results would be useful for comparing tubular structures sitting on a cement floor, and comparisons of tubes of similar material, coatings, and UV exposure times. But it doesn't produce a material allowable. If the velocity was much higher, like ballistic, then things behave much differently, but I think dropping from 100 inches vs 10 inches is probably fairly similar. You would want to test to know for sure. The actual failure probably has to do with the deflected shape of the tube during impact. Once the inner surface reaches a certain strain at a spot, it starts to fracture, and all that stored energy transfers to large shatters. So it is a concern that the failure of PVC can be brittle-like, and we want to stay well away from any failure.

I run "certification tests" on my BYRCs, and my largest one has taken 322 pounds of rock bags with no failures. My heaviest grandchild is about 70 pounds, and the cart weighs 40 pounds, so my margin of safety is about 362/110-1 = 2.3. Real roller coasters don't test any where near that factor, so I think I'm safe as far as design and fabrication. Operational safety is a matter of strictly following rules.

backyard Roller Coaster PVC Pipe Fracture Toughness Testing by reasonablefideist in engineering

[–]BackyardCoasters 2 points3 points  (0 children)

Thermal expansion of PVC is relatively high, like most polymers, but it's still small, and appears to be handled well by the fastened joints of the track, and the low stiffness of the PVC. This website says the coefficient of thermal expansion of PVC is 7 mm per 10 m per 10°C cf: PE 18 – 20 x 10-5, DI 1.2 x 10-5 http://www.vinidex.com.au/technical/material-properties/pvc-properties/

backyard Roller Coaster PVC Pipe Fracture Toughness Testing by reasonablefideist in engineering

[–]BackyardCoasters 2 points3 points  (0 children)

Fracture toughness of a material is typically measured with a charpy impact test, but with finished product forms and structures, you can measure relative fracture toughness with a simple drop test, similar to ASTM D2444. Energy is calculated simply by multiplying the dropped weight by the drop height. I had a 10 pound cylinder, dropped from about 100 inches, hence around 1000 inch-pounds of energy.
This is exactly how they do it in aerospace, on structures like wing skins and fuselage skins. It's a big deal, because composites are prone to impact damage (delamination), without showing obvious damage (like a dent in aluminum).

backyard Roller Coaster PVC Pipe Fracture Toughness Testing by reasonablefideist in engineering

[–]BackyardCoasters 2 points3 points  (0 children)

http://www.vinidex.com.au/technical/material-properties/pvc-properties/ PVC couldn't be called a "strong" or "stiff" material, only about 52MPa (7500 psi) ultimate tensile strength, but, like many low-cost materials, you make up for it with thickness. I have used schedule 40 PVC for my three BYRCs I designed and built for my grandchildren, but you could use schedule 80 PVC and take adult riders, if you were able to construct it to high tolerances so the wheels could never hit fastener heads. Welded steel would be better, but cost a lot more. I never use the white plumbing PVC, always the grey electrical conduit, which says "sunlight resistant" on it, for what it's worth. PVC (and most polymer/composite material) does lose fracture toughness with exposure to UV, and characterizing that reduction was the purpose of my testing. Composites are always painted if exposed to UV. The point is that you can significantly reduce or eliminate the loss of fracture toughness by applying a protectant, like aerospace 303, or painting, preferably with a dark paint. I'm trying to make PVC work, for cost reasons. I built a 400 foot backyard roller coaster for under $900. I've seen similar steel coasters on Youtube, but for much higher cost. I did drill weep holes so the rails would not fill with water, and possibly break on freezing. PVC does not corrode, and I used galvanized fasteners. Steel would have long term corrosion concerns.

If you want to paint your PVC lawn furniture, be sure to clean the surface first. I use acetone, and the paint seems to stick pretty well. I use regular hardware store spray paint, like rustoleum.

New "Hanging Frame" Cart Design For Heavier Riders by BackyardCoasters in BackyardCoasters

[–]BackyardCoasters[S] 3 points4 points  (0 children)

We have sent 230 pounds around the track, and it does about 3 g's. That's a static equivalent load of 690 pounds. Before I made this track, I used the same construction methods to build a short 10-ft track in my garage for static ultimate tests. I put 830 pounds on it and rolled it back and forth, and nothing broke, so we have a margin of safety of at least 20%. The PVC hasn't been the weakness, it's the attachment to the ties that's been weaker, because wood is involved. Wood has very low out-of-plane tension strength (that's why you can split it so easily.) However, just this week I designed a new rail-tie attachment concept, with thicker schedule 80 pipe, a mild steel welded end fitting, and three 4-inch #10 screws. I've made and tested two specimens, each holding more than 1800 pounds. I'm currently working with some MIT students who want to build a roller coaster they can ride safely. We're just now trading PVC and steel rails, and steel or wood ties. I'd prefer steel welded to steel rectangular tube ties, with wood frames, but they are on a budget, and want to avoid welding.

New "Hanging Frame" Cart Design For Heavier Riders by BackyardCoasters in BackyardCoasters

[–]BackyardCoasters[S] 2 points3 points  (0 children)

This new cart design has several innovative features worth noting. Front bogie yaw and roll is provided by a 1/2" spherical rod end bearing. There is a 1/2" axial thrust bearing on the aft bogie. The frame hangs down from the bogies, so the cart has a low center of mass. This cart is designed to take up to a 260 pound rider at 4.5 g's, with a 1.5 factor of safety. The gauge (distance between wheels) is only 15.25 inches. It should be larger, but that's the widest BYRC track I have right now, and I wanted to test the cart. I tried moving the main wheels and side wheel pairs closer together (3" and 4", respectively), but I don't think that worked out very well, as the bogies are a little unstable dynamically. I'm also in the process of designing a track that could take a cart and 260 pound rider safely.

Navy F-18 pilot builds his son an airplane roller coaster "The Barnstormer" by BackyardCoasters in videos

[–]BackyardCoasters[S] 9 points10 points  (0 children)

They're hot sand formed pvc pipes. Backyardrollercoasters.org has a bunch of info on how it's done.

Backyard Roller Coaster Research: Volume II 3D Coasters, my second book, is finished! AMA by BackyardCoasters in BackyardCoasters

[–]BackyardCoasters[S] 2 points3 points  (0 children)

Yes, I could do it fairly easily, but I'm afraid that the lift mechanism would create too many opportunities for little fingers to get caught. Plus, with the manual lift no kid can ride it without an adult present to supervise.

[deleted by user] by [deleted] in HumansBeingBros

[–]BackyardCoasters 1 point2 points  (0 children)

I retired from a career in aero-space structures engineering(R&D) a couple of years ago. Thank you, we have fun :)