Bridge material question

[koo_04]

I was not sure where to post this exactly, so it can moved by the moderators later.

I would like to know a good place to buy the HDPE. I look places and most are 300+ bucks. I was hoping maybe there is a cheaper place?

[Peter Matteson]

I was not sure where to post this exactly, so it can moved by the moderators later.

I would like to know a good place to buy the HDPE. I look places and most are 300+ bucks. I was hoping maybe there is a cheaper place?

FYI
The bridge surface is lexan not HDPE. The key is HDPE.

[dez250]

Also if you don't know Lexan is a brand name for the material from SABIC. The generic name is polycarbonate which you can get fairly inexpensively for thin sheets.

[Gray Adams]

We bought a sheet from our local plastic supplier for $96.

[Tom I]

Another option for teams with a tighter budget, Lexan (or other brand of polycarb) will mimic the behavior of HDPE pretty closely. The question becomes do you need it to be EXACT? or is CLOSE ENOUGH good enough for what you're doing. I'd say for driver testing, you won't need anything more accurate than Lexan.

[Phyrxes]

FYI
The bridge surface is lexan not HDPE. The key is HDPE.

The offical FIRST field drawings say its HDPE. But in the video tour of the bridge they call it polycarbonate.

For what its worth a 48 inch by 48 inch sheet of 1/16th hdpe is about $20 from McMaster + shipping.

[RufflesRidge]

The offical FIRST field drawings say its HDPE. But in the video tour of the bridge they call it polycarbonate.

For what its worth a 48 inch by 48 inch sheet of 1/16th hdpe is about $20 from McMaster + shipping.

The bridge deck is HDPE, that's the colored part. The bridge surface is a sheet of polycarbonate velcroed on top of the deck.

IMO, the important part to recreate is the surface, which is polycarb. This is what will affect driving characteristics the most.

[Phyrxes]

You are right, I got the bridge deck and surface mixed up.

[IceStorm]

The offical FIRST field drawings say its HDPE. But in the video tour of the bridge they call it polycarbonate.

For what its worth a 48 inch by 48 inch sheet of 1/16th hdpe is about $20 from McMaster + shipping.

Would you be able to provide the part number at mcmaster? I haven't went looking yet but know that there catalog is quite vast in what they carry.

[Gdeaver]

Watch out for shipping on a full 4 x 8" sheet of polycarb.

[davidrk13]

so im trying to build the seesaw (bridge) and i cant seem to find any plans/blueprints anywhere . can anyone help me out?

[Phyrxes]

Would you be able to provide the part number at mcmaster? I haven't went looking yet but know that there catalog is quite vast in what they carry.

8619K427 is the part number for the 48 inch by 48 inch by 1/16 of an inch thick smooth sheet in white.

http://www.usfirst.org/sites/default/files/uploadedFiles/Robotics_Programs/FRC/Game_and_Season__Info/2012_Assets/2012TeamDrawings.pdf is the "low cost" (Read: lots of plywood and 2x4's field elements).

[Flimsor]

so im trying to build the seesaw (bridge) and i cant seem to find any plans/blueprints anywhere . can anyone help me out?

It's all here:

http://www.usfirst.org/roboticsprograms/frc/2012-Official-FIRST-Field-Drawings

[Alan Anderson]

so im trying to build the seesaw (bridge) and i cant seem to find any plans/blueprints anywhere . can anyone help me out?

The Competition Manual page on the FIRST web site points to the Field Drawings (http://www.usfirst.org/roboticsprograms/frc/2012-Official-FIRST-Field-Drawings).

[PhantomPhyxer]

According to FTC Robotics, 2011 -2012 Edition either US Plastics or Mc Master Carr are good sources for this Plastic. We have used TAP Plastic. We have not purchased the plastic for the Bridge yet. We are still building Game pieces. We hope to finish tomorrow. FIRST Team #2643, Mentor

[Bill_B]

I would also try screen-door and awning installers to find polycarb sheets. they use them instead of glass for their doors.

[davidrk13]

thank you

[davidrk13]

hey so i pretty much finished making the bridge and have some concerns.

1. is it alright to NOT include a pinch guard? whats the worst thatll happen?

2. the real field will be made out of a kind of plastic which has a different friction than plywood. is the frictional difference that great that lets say if the bot goes fine on the plywood bridge. that it wont go as smoothly on the real field or vice versa?

[Retired Starman]

hey so i pretty much finished making the bridge and have some concerns.

1. is it alright to NOT include a pinch guard? whats the worst thatll happen?

2. the real field will be made out of a kind of plastic which has a different friction than plywood. is the frictional difference that great that lets say if the bot goes fine on the plywood bridge. that it wont go as smoothly on the real field or vice versa?

The competition field does not include the pinch guard, but it is good to have it on your practice bridge to keep people from getting their hands under the bridge. The worse that an happen might include having a finger cut off or some hand bones broken. This won't happen on the field since there won't be people getting their hands under the bridge. The most dangerous place is in the hinge assembly. A second danger point is right at the end of the bridge where the bridge meets the floor. If you don't want to build the entire guard, consider putting a guard over both sides of the hinge.

There is a significant difference between the slickness of a plywood bridge and one with polycarbonate on it. If you can't afford polycarbonate, consider something e styrene or acrylic which might bee a little less expensive but would be durable enough for practice.

Dr. Bob

Chairman's Award is not about building the robot. Every team builds a robot.

[davidrk13]

The competition field does not include the pinch guard, but it is good to have it on your practice bridge to keep people from getting their hands under the bridge. The worse that an happen might include having a finger cut off or some hand bones broken. This won't happen on the field since there won't be people getting their hands under the bridge. The most dangerous place is in the hinge assembly. A second danger point is right at the end of the bridge where the bridge meets the floor. If you don't want to build the entire guard, consider putting a guard over both sides of the hinge.

There is a significant difference between the slickness of a plywood bridge and one with polycarbonate on it. If you can't afford polycarbonate, consider something e styrene or acrylic which might bee a little less expensive but would be durable enough for practice.

Dr. Bob

Chairman's Award is not about building the robot. Every team builds a robot.
thank you very much retired starman just one question concerning the bridge. i already finished the plywood version of the bridge and the most i can do is to use a coating to try to mimic the slickness of the actual competition bridge what do u recommend i should get to coat the plywood to accomplish a like slickness to that of the polycarb?

[Tristan Lall]

thank you very much retired starman just one question concerning the bridge. i already finished the plywood version of the bridge and the most i can do is to use a coating to try to mimic the slickness of the actual competition bridge what do u recommend i should get to coat the plywood to accomplish a like slickness to that of the polycarb?
There's no substitute for the real thing. There aren't any practical coatings that will simulate polycarbonate at reasonable cost. Get as thin a sheet of polycarbonate as you can find, and attach it to the surface.

[davidrk13]

so now that the bridge is completely done i realized some weird things, like that when its perfectly level the weight force it takes to push one side down is waaaay less then the other side so i just screwed some blocks of wood to the harder to push down side bottom to balance out the weight and it worked to fix that problem but its now completely not level when there's nothing on it so im scared that it wont be accurate when it comes to testing did anyone have a similar problem? or know if it will effect the testing?

[slijin]

so now that the bridge is completely done i realized some weird things, like that when its perfectly level the weight force it takes to push one side down is waaaay less then the other side so i just screwed some blocks of wood to the harder to push down side bottom to balance out the weight and it worked to fix that problem but its now completely not level when there's nothing on it so im scared that it wont be accurate when it comes to testing did anyone have a similar problem? or know if it will effect the testing?

That's a combination of 1) plans modified on the fly and 2) the construction material used. I know that 148 reported a balancing bias when they constructed their bridge, about a couple lbf's worth. We haven't checked ours for such, but I wouldn't be surprised. Anyone who used the team drawings to construct their bridge will also have a bridge that's easier to tip than the one at competition (about 56 lbft of torque is necessary for the real bridge, as opposed to 30 lbft for the wooden one) because of the differencee in moment of inertia. Most teams have rectified this by adding steel plates or some other heavy, dense material to the underside of their bridges.