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Unread 03-09-2012, 10:29 AM
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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.
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Unread 03-09-2012, 10:47 AM
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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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Unread 03-09-2012, 11:08 AM
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Re: Bridge lowering force calculation

Quote:
Originally Posted by Tom Line View Post
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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Unread 03-09-2012, 11:28 AM
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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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Unread 03-09-2012, 05:30 PM
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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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Unread 03-09-2012, 09:22 PM
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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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Unread 03-09-2012, 09:27 PM
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Re: Bridge lowering force calculation

Figured I'd copy this over, just for reference:
Quote:
Originally Posted by slijin View Post
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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Unread 03-12-2012, 11:52 AM
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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.
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Unread 03-12-2012, 01:51 PM
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Re: Bridge lowering force calculation

Quote:
Originally Posted by Chris Fultz View Post
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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Unread 03-19-2012, 11:01 AM
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Re: Bridge lowering force calculation

Quote:
Originally Posted by Adam Freeman View Post
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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Unread 03-19-2012, 04:29 PM
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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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Unread 03-19-2012, 04:44 PM
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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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