Pushing down the bridge
 

It appears that the window motor (with arm attached) that works on our (and many other teams) practice bridge may not have enough torque to push down a competition bridge.

Would anyone with actual experience with a competition bridge comment on what motor and gearing appear to have the power necessary to push the bridge down?

Re: Pushing down the bridge
 

We used a Van Door motor for our bridge tipper. It works well on comeptition bridge. Gear it down a bit, though, just for the extra torque.

Re: Pushing down the bridge
 

FisherPrice through the kiddy car gearbox works.

Re: Pushing down the bridge
 

same

we have more than enough power

Re: Pushing down the bridge
 

We used the van door motor and 4:1 gearing - so we are probably OK?

TIA

Re: Pushing down the bridge
 

use a force plate and see if you can get about 16lbs of force. That seems to be about what it takes

Re: 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.

Re: Pushing down the bridge
 

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?

Re: Pushing down the bridge
 

And save the world?

Re: Pushing down the bridge
 

The funny thing is we wheelied onto the bridge...

Re: Pushing down 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).

Re: Pushing down the bridge
 

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.

Re: Pushing down the bridge
 

No, no. And tip the bridge!

Re: Pushing down 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.

Re: Pushing down the bridge
 

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.

Re: Pushing down the bridge
 

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.

Re: Pushing down the bridge
 

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.

Re: Pushing down the bridge
 

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

Re: Pushing down the bridge
 

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

Re: Pushing down the bridge
 

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.

Re: Pushing down the bridge
 

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.

Re: Pushing down the bridge
 

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

Re: Pushing down the bridge
 

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.

http://i1121.photobucket.com/albums/...2/100_0011.jpg

Re: Pushing down the bridge
 

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

Re: Pushing down the bridge
 

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

Re: Pushing down the bridge
 

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.



Edit:

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

Re: Pushing down the bridge
 

We just push straight down on it with a 1.5" pnuematic. Worked great at the Suffield scrimmage with the real field during week 0 so we assume it will work fine when we compete at our first regional this week.

Re: Pushing down the bridge
 

As for getting the right depth, you are right. That is why we have our turret servo mounter on the tilt assembly. We can look down to lower the bridge or up to camera track

Re: Pushing down the bridge
 

We are using pneumatic pistons.

Re: Pushing down the bridge
 

We have 2 devices which can lower the ramp. One is an aluminum wedge with a linkage that locks, driven by a window motor. Works great, moves the bridge super smoothly. The other is a ball gatherer on the other side, also lowered by a window motor. It's harder to use because you have to be positioned properly for it to work, but it has no trouble when done from the right position.

Unrelated, but I've been trying to do a wheelie onto the bridge, but I can never get both the right height and be in the right position at the same time.

Re: Pushing down the bridge
 

we used a FP/motor and transmission, then geared that down 4 to 1 using a chain drive with #25 chain.

it worked well at the Granite State Regional

Re: Pushing down the bridge
 

Our team is using a wedge that is raised and lowerd by a window motor, when it drives the bridge down the bridge pushes back on it straight back when the bumper is there tto keep it rigid. We don't have to worry about the motor nat being strong enough because it only puts the wedge in place, however it will back drive if we hit the bridge to hard.

Re: Pushing down the bridge
 

Another way to solve this problem is to have two posts that stop the robot from going too far into the bridge, and then just back up a small bit to deploy an arm. Our team has done this, and it should only take a maximum of 30 minutes, and a few feet of angle.
We are also using a 550 banebot with a 251:1 reduction. We have pushed our entire robot onto two wheels with it :D

Re: Pushing down the bridge
 

Be careful, the widow motor in the kit, and probably many others, use plastic worm gears. The danger here is that if you get hit hard enough, or after many hits, the teeth will shear off of the gear. Removing the locking pins helps (and it sounds like you already have), but have a spare motor handy if you are using one with plastic gears.

Re: Pushing down the bridge
 

^This.

We've already sheared one window motor and have a couple on hand ICE.

Re: Pushing down the bridge
 

Pneumatics

Re: Pushing down the bridge
 

I would ask a different question. I wouldn't worry about how much force it takes to push the bridge down, I would look at how much force it takes to lift up your bot, and size to that. You may even want to intentionally limit it so you don't tip yourself over. It's really the weight of the bot that lowers the bridge, not your mechanism.

What I observe in competition is that tipping an empty bridge is only one of several scenarios. Often you need to tip the bridge with a robot already on or you need to squish a ball (or 2) underneath the bridge far enough to let you climb up. If you can't go over the bump because of low clearance you've got the dual problem of needing to lower the bridge nearly to the floor and pushing past any balls that are stuck underneath. In most cases it will take the full weight of your bot and a partner to lower the bridge far enough to get on. The arm should allow the bot to bring it's full weight into play. Big advantage to the wedge designs here as they are mostly based on this idea anyway.

We are also seeing that if the arm is too ridgid and engaged on the bridge, the bridge can flip the bot back over in short order.

Just my observations.