we are trying to think of a good gear ratio to use on our lift using 2 269 motors anyone have a CAD drawing for this?
2 x 269 is a weak lift if you are doing a two sided lift, whatever gear ratio you go with would have to be high in my opinion …
its a two sided lift
em … well depends on mass of arms, and sacks etc, what lift exactly ?
its just 18" bars that tilt up with a moving bucket
em … i would probs select either a 1:15, or a 1:21 … i am currently running a 1:7 on mine using 393s … and it isn’t great
what would a 1:21 look like?
compound gear ratio of a 1:3 then 1 :7 … i will cad a version for you
1:21 would look like this … i suggest putting the gears between c-channel … and before someone says, use axle bars …
There are some compound gear examples here
https://vexforum.com/showpost.php?p=317337&postcount=1
The examples are using a turntable but can easily be adapted to use a standard gear at the output. My current favorite ratio for a simple 2 motor arm using two 393 motors is 8.33:1. The 269 motor has about half (depending on rpm) the power of a 393 so if you had two 269 motors then 15:1 would be about right. If you had 4 269 motors then you may be OK with the 8.33:1.
The roundup robot my students built used this configuration of gears.
https://vexforum.com/showpost.php?p=236578&postcount=4
The general consensus was that we had out compound gearing “backwards” in that we should not have used the 12 tooth gear on the output of the motor but instead the 36 tooth gear. The 12 tooth would then go where we had used the 36 tooth. This redesign would have reduced torque on the intermediate axle and reduced it tendency to twist.
You mean the shaft-lock bars? It’s been said many times around the forum that it’s better to bolt your arm directly to the large gear so that the axle does not have to bear the rotational load of the entire lift. If you use the shaft-locks, then you will probably end up twisting your axle.
Why did they say that? My team’s robot this year uses a 12-tooth metal gear as the motor-driven gear, which then turns a 60-tooth gear. This is screwed together (as well as square inserts) to a 36-tooth gear, which turns the 60-tooth gear our arm is attached to. It’s the same setup as your 8.33:1, with the intermediate gears screwed together.
Because the motors must turn a shaft, the torque has no choice to be driven by an axle from the motors. Thus, we put the smallest gear, the 12-tooth, in that position. We then did everything we could to avoid axle twisting. We screwed the intermediate gears together so that no torque was on the axle (we actually had a big problem with this - screwing them together was the best thing we’ve done).
With a couple other precautions we took to prevent the axle bending (because of the torque on the arm - not twisting), our arm is now pretty darn solid, capable of lifting 10+ sacks (though we’d prefer not to go that far) without any gears skipping. Our axles haven’t twisted since we screwed the gears together, and now have very little room to bend.
That’s the difference, you screwed the intermediate gears together.
The roundup robot I showed used 11.66:1 gearing, 12 drives 60 then 36 drives 84. I don’t remember which motors were used but lets say they had 10 in-lbs of torque. The intermediate axle sees 50 in-lbs in our setup, if we had reversed the 12 and 36 tooth gears it would only see 16.6 in-lbs and the axle would be less likely to twist. That’s the theory anyway, that intermediate axle was always twisting, it would be nice to have the torsional strength for the axles, something to figure out one day.
I see, the individual axle strain was less.
I was referring to your post with the several gear ratios explained, just to show our setup. We like to have as little torque as possible on the axles, and put it on the actual gears (hence our screwing them together). We should be putting up a reveal here, soon, and so we’ll show it off then.