Pushing down the bridge

It works for us, but please note we have a…unique new bridge actuator that allows us to lower the bridge differently than others.

Most likely it will still work.

We weighed our bridge down more than was necessary and used the FP motor and gearbox – not only would it tip the bridge, the bridge barely slowed it down.

Mind you, we haven’t yet used it on an actual competition bridge (and that, as usual, makes me nervous), but it seems to have well more than enough torque to handle the job. (Of course, the length of your moment arm matters, too!)

Might I suggest that you do the math?

And save the world?

The funny thing is we wheelied onto the bridge…

Here’s my math,

We’re planning on using the KOP, Andy Mark PG71 Gearmotor with almost 3: 1 sprocket ratio ( 22 tooth on PG71 attached via chain to 60 tooth gear). 60 tooth has arm mechanism. According to specs PG-71 stall torque is nearly 16.6 ft-lbs (Andy-Mark website) . Someone calculated that 20 foot -lbs was required to tip the bridge (CD website) based on the 2 batteries and distance demonstrated in one of the FIRST kick-off videos.

So, at 60% stall torque (10 ft-lbs) * (60/22) * favorite efficiency ratio (0.8) = 21.7 ft-lbs > 20 ft-lbs should work.

Distance from center of shaft for bridge arm is 6 inches (1/2 foot) on the 60 tooth sprocket , so theorectically placing > 40 ft-lbs on bridge at 60% full torque.

We didn’t build a bridge (small team) but the bridge arm easily picks up the front end of our robot. Hope this helps.

BTW, PG71(am-0914) is powered by RS775 motor and has internal 71:1 gear set. KOP rules allow 2 units for 2012. Andy Mark also sells a 10 mm key hub that couples directly to the 10 mm output shaft (am-0985). You will need to also get a 4 mm key for the hub (am-1249).

So, for our assembly the Bill of Materials , from memory is the following:

  1. PG71, 2) 10mm key hub, 3) 4 mm key, 4) 2 - 375 key hubs (am-0134), 5) 1 sprocket, (22 tooth - am0118), 6) 1 sprocket, (60 tooth - am0057). Need also 7) 3/8 shaft for the 2 - 375 key hubs (recommend 1 foot to start)- Andy Mark does not sell - McMaster-Carr and 8) 3/32 key stock for your 375 key hubs (am-1059, 2 pcs)

I have a second PG71 Gear motor ready to add if necessary (contingency plan).

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.

No, no. And tip the bridge!

We were fortunate enough to travel to another teams location to practice on a competition replica bridge before the kettering event. Our original bridge arm was not strong enough. We eventually lowered the bridge by combining an andy mark PG67 gear and motor combo with a andymark tough box gearbox. the actual bridges are much harder to lower than the plywood version.

AM PG-71 with 12 tooth #35 sprocket on the output, connecting to a 60 tooth sprocket bolted to a ~26" long arm (pivot to tip, wedge shaped). Works well.

In San Diego, we were originally having issues with our ramp lowerer – which was van door motor direct drive. After adding a combination of weight, angle, and robot velocity during the match we were able to force the ramp down. When hitting the ramp, the appendage would get pushed up by the ramp typically.

If I were to rebuild it, I would probably go with the FP+gearbox or window motor to take advantage of the (locking) worm gear in combination with the angle on the appendage to slide the ramp down as the robot moves forward into it.

We are using an extendable arm that locks into a angular horn on our frame when extended. Essentially we are using the weight of our robot to push down the ramp.

How did you do this? Did you just bore a CIM pinion out with a 10mm drill bit and press it onto the shaft?

I saw that and thought it was the greatest thing ever haha. You guys always have a beastly drive every year.

I came up with pneumatic based bridge lower where piston will deploy the straight bars which will slide over the bridge to push it down with robot itself’s torque.

We have no motor, no pneumatics, no arm. We just drive up under the bridge and lower the other side. Then our partners can climp up, or we can back out and when the bridge on our side comes down, we drive up. K.I.S.S. We got 110 bridge points in week one. Funny thing is we had joked about doing it this way before we bagged, and then realized it was the best way for us when we got to the competition.

Are people running their motors at full power when they attempt to lower the bridge, and is any motor doing it ungeared?

From watching several competitions now I have to say that those of you trying to use the drivers depth perception and an arm to lower the bridge have 2 problems. 1. getting the bot at just the right distance from the opposite side of the bridge to allow your arm to lower the bridge is difficult at best.
2. developing enough torque with a motor and coordinating its lowering with your forward motion is a trick in itself.
Solution. just lower a wedge shaped arm at the front of your bot using an over center link(motor or pneumatic) and just drive up the bridge to lower it. see our bot here.


Van door motor straight out of box worked great for us.

In a previous thread from 3601. An adapter was made by the NerdWorks to use a PG71 as an input motor into a toughbox nano. Pictures are attached. Combination worked so well lowering the bridge, they were picked by 3322 at Kettering to be part of the #3 alliance

Team 2468 used a PG71 motor with Tetrix 120 tooth and 40 tooth gears for a 3 to 1 reduction. We have an arm that has a 4" Vex wheel mounted on the end of it. From the edge of the wheel to the frame is 14". The arm is made 3/4" aluminum angle x 1/8" thick. It proved strong enough to push the bridge down or pick it up.


See picture below.

You can see the tipper in action at the 16:07 mark of the following video on Ustream: http://www.ustream.tv/recorded/20856729