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 Chief Delphi Bridge lowering force calculation
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#1
03-09-2012, 10:29 AM
 Chris Fultz My Other Car is a 500 HP Turbine FRC #0234 (Cyber Blue) Team Role: Engineer Join Date: Jan 2002 Rookie Year: 1942 Location: Indianapolis, IN Posts: 2,558
Bridge lowering force calculation

For a quick calculation of the force needed to lower the bridge, you can use this equation.

F is the vertical force, in pounds, required.
D is the distance, in inches, that your device contacts the bridge, measured from the end edge.

F = 21 + D * 0.75

So if your contact point is 1" from the end, you would need 21 + 1*.75, or about 21.75 pounds of force.

If your contact point is 12" from the end, you would need 21 + 12*.75, or about 30 pounds of force.

Once you know this number, you can select the right motor and gearing combination to give you that force at the end of your lever arm (remember the motors are mostly rated in oz-in so you have to convert).

If you are using a pneumatic, remember that you need to determine the vertical component of the force, based on the angle it contacts the bridge.
(i.e. 50 pounds at a 45 degree angle is only about 35 pounds vertical).

* As noted below, the "starting point" is around 21 pounds and may be a little less. Bridges vary and the underside construction has an impact.
Use this as a guide and design margin into your system.
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Last edited by Chris Fultz : 03-09-2012 at 11:31 AM.
#2
03-09-2012, 10:47 AM
 Tom Line Raptors can't turn doorknobs. FRC #1718 (The Fighting Pi) Team Role: Mentor Join Date: Jan 2007 Rookie Year: 1999 Location: Armada, Michigan Posts: 2,070
Re: Bridge lowering force calculation

Chris, how accurate is this? I was under the impression that it only took about 16 pounds of force at the end to lower a competition bridge.
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#3
03-09-2012, 11:08 AM
 Cory Registered User AKA: Cory McBride FRC #0254 (The Cheesy Poofs) Team Role: Engineer Join Date: May 2002 Rookie Year: 2001 Location: Redwood City, CA Posts: 6,239
Re: Bridge lowering force calculation

Quote:
 Originally Posted by Tom Line Chris, how accurate is this? I was under the impression that it only took about 16 pounds of force at the end to lower a competition bridge.
Yeah, but why would you design your mechanism to provide the exact amount of force required?

21 lbs is a ~50% margin of safety. Sounds reasonable to me.
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#4
03-09-2012, 11:28 AM
 Chris Fultz My Other Car is a 500 HP Turbine FRC #0234 (Cyber Blue) Team Role: Engineer Join Date: Jan 2002 Rookie Year: 1942 Location: Indianapolis, IN Posts: 2,558
Re: Bridge lowering force calculation

It depends on how you assume the weight of the bridge itself is distributed.

The intent was to give a target for teams who do not have a way to test in advance.
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#5
03-09-2012, 05:30 PM
 PAR_WIG1350 &amp;amp;gt;e, 7h3 5up3rn47ur4l AKA: Alan Wells FRC #1350 (Rambots) Team Role: College Student Join Date: Dec 2009 Rookie Year: 2009 Location: Rhode Island Posts: 1,091
Re: Bridge lowering force calculation

This does seem rather misleading. The post says to design a safety margin into the system, but the formula appears to do this already.
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#6
03-09-2012, 09:22 PM
 Chris Fultz My Other Car is a 500 HP Turbine FRC #0234 (Cyber Blue) Team Role: Engineer Join Date: Jan 2002 Rookie Year: 1942 Location: Indianapolis, IN Posts: 2,558
Re: Bridge lowering force calculation

The forumula doesn't add any margin, but depending on some assumptions it could be a high starting value. One of our early designs was pushing about 10" up the ramp with about 28 pounds vertical, and that was "just" enough.

The calculations say it takes about 28 - 29 at that point.

From some of the posts in other threads, it seemed that some teams were not sure at all what it took to lower the bridge - I was giving some guideance.
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#7
03-09-2012, 09:27 PM
 slijin Pockets AKA: Samuel Lijin FRC #0694 (StuyPulse) Team Role: Mentor Join Date: Jan 2010 Rookie Year: 2010 Location: New York City Posts: 537
Re: Bridge lowering force calculation

Figured I'd copy this over, just for reference:
Quote:
 Originally Posted by slijin The video said 28" to 30", so assuming 29" is the tipping point, 26" is the effective tipping point (CoG of battery isn't at the end). Keeping in mind that the bridge rotates on the edges of the bump and not the exact center, subtract another two more inches to get 24". Two batteries are 27 lb. That gives 27 lbf * 24" * (1'/12") = 54 lbf-ft = 648 lbf-in. However, because of the double hinge, it's not a perfect lever, so aim for a bit more than that. (88"/2) - 2" = 42"; 648 lbf-in / 42 in = 15.4 lbf, so approx. 16 lbf to tip.
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#8
03-12-2012, 11:52 AM
 Kevin Thorp Registered User AKA: Kevin Thorp FRC #3489 (Category 5) Team Role: Mentor Join Date: Dec 2002 Rookie Year: 1998 Location: Charleston, SC Posts: 520
Re: Bridge lowering force calculation

At the Orlando Regional we saw a rookie team with an underpowered tipping arm. It just tapped the top of the Bridge with no effect.
#9
03-12-2012, 01:51 PM
 Adam Freeman Einstein OR BUST! FRC #0067 (The HOT Team) Team Role: Coach Join Date: Jan 2007 Rookie Year: 2005 Location: South Lyon, MI Posts: 437
Re: Bridge lowering force calculation

Quote:
 Originally Posted by Chris Fultz If you are using a pneumatic, remember that you need to determine the vertical component of the force, based on the angle it contacts the bridge. (i.e. 50 pounds at a 45 degree angle is only about 35 pounds vertical).

I think what Chris explained here is one of the biggest reasons why many teams bridge manipulators do not function as well as required.

We saw lots of pneumatic manipulators at Waterford that originally did not take into account the breakdown of horizontal and vertical components of force being applied to the bridge.
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#10
03-19-2012, 11:01 AM
 Peck worse then failing: proving murphy FRC #1619 (Up-A-Creek Robotics) and FTC # 4633) Team Role: College Student Join Date: Mar 2012 Rookie Year: 2007 Location: Colorado Posts: 237
Re: Bridge lowering force calculation

Quote:
 Originally Posted by Adam Freeman I think what Chris explained here is one of the biggest reasons why many teams bridge manipulators do not function as well as required. We saw lots of pneumatic manipulators at Waterford that originally did not take into account the breakdown of horizontal and vertical components of force being applied to the bridge.
Also keep in mind that:
• the torque created shifts as the bridge moves. This formula happens to provide a built in quantity that partly allows for the shift. However, It dose not accommodate a shifting contact point (having the same point of contact for the entirety is hard to design and have strong enough to take the abuse of FRC).
• your motors/pneumatic won't be a constant force for all times used and will become weaker over time. The greater the buffer amount built in, the slower It will deteriorate and the easier It will be to compensate.
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#11
03-19-2012, 04:29 PM
 sandiegodan FTC Mentor, FRC Inspector AKA: Dan Glenn FTC #6565 (Stuffed Dragons) Team Role: Mentor Join Date: Mar 2010 Rookie Year: 2008 Location: San Diego, CA Posts: 40
Re: Bridge lowering force calculation

Make sure that whatever force you are designing to push down on the bridge with can be counteracted by the weight of your bot. I've already seen several designs that can tip their own bot with their bridge manipulator before they move the bridge, especially if they try to tip an occupied bridge and it's heavier than anticipated. Go carefully!

I've seen several teams desperate to add legal ballast to their bots while at the regional. It isn't an easy last minute modification.

Good luck!
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#12
03-19-2012, 04:44 PM
 compwiztobe Always Aspiring AKA: Aren Siekmeier FRC #2175 (The Fighting Calculators) Team Role: Mentor Join Date: Apr 2008 Rookie Year: 2008 Location: Woodbury, MN Posts: 444
Re: Bridge lowering force calculation

There is something to be said for being able to push down with the weight of your robot. Then if something is keeping the bridge from going down, you just climb up on top of it.

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