Pushing down the bridge

[WarehouseCrew]

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?

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.

[wireties]

Originally Posted by SuperNerd256

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.

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

TIA

[JohnSchneider]

Originally Posted by wireties

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

TIA

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

[pfreivald]

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?

Originally Posted by pfreivald

Might I suggest that you do the math?

And save the world?

[pfreivald]

Originally Posted by SuperNerd256

And save the world?

No, no. And tip the bridge!

[slijin]

Originally Posted by pfreivald

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?

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.

Originally Posted by wireties

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

TIA

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.

[AlexH]

FisherPrice through the kiddy car gearbox works.

[dellagd]

Originally Posted by AlexH

FisherPrice through the kiddy car gearbox works.

same

we have more than enough power

[AlexH]

Originally Posted by dellagd

same

we have more than enough power

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

[rcmolloy]

Originally Posted by AlexH

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

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

[marccenter]

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).

[Greg Shaw]

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.