Sack Attack: 'Lifter'

[murdomeek]

remember 1103's cleansweep bot?
catapult dumper with a winch that winches it back down
if you can get the drive fast enough to avaoid being blocked (his weakness) it may be extremely effective this year

[aslw26]

I was thinking of something that might be a bit to crazy... Using 2 393's to lift 10 sacks (about 5lbs?) which is like nearly 2kg.... But is it possible? I not really experienced and this is my 1st year in VEX...

[Cojust]

Going by gateway, where NZ bots with 4 269's on the arm(elastically balanced) could lift about 5-7 objects which, assuming 4 balls 3 barrels, weighs about 4(0.24) + 3(0.53) = 2.55 pounds. The sacks are about 0.5 pounds each, so 5 should be ok with a gateway robot. The torque by 2 393's is slightly lesser as compared to 4 269's.

Edited: whoops.

[dontworryaboutit]

Originally Posted by Cojust

Going by gateway, where NZ bots with 4 269's on the arm(elastically balanced) could lift about 5-7 objects which, assuming 4 balls 3 barrels, weighs about 4(0.24) + 3(0.53) = 7.32 pounds. The sacks are about 0.5 pounds each, so 14 should be ok with a gateway robot. The torque by 2 393's is slightly lesser as compared to 4 269's, so should be fine with lifting about 10 sacks...

That's 2.5 pounds, not 7.32.

You could always look at the numbers:
The stall torque of a 393 is 13.5in-lbs and if you're using 2, your combined stall torque is 27in-lbs.
If you're geared 7:1 for torque, now you have 189in-lbs.
If your arm is 18 inches long, that's 10.5lbs at the end of the arm.
Your elastics would probably only support the weight of the arm and sack holder itself, so it wouldn't help with lifting those 10 sacks.
The 10.5lbs maximum force at the end of the arm (when the arm is parallel to the ground) means lifting 5lbs of sacks should not be a problem at all.

[Rick TYler]

Originally Posted by dontworryaboutit

That's 2.5 pounds, not 7.32.

You could always look at the numbers:
The stall torque of a 393 is 13.5in-lbs and if you're using 2, your combined stall torque is 27in-lbs.
If you're geared 7:1 for torque, now you have 189in-lbs.
If your arm is 18 inches long, that's 10.5lbs at the end of the arm.
Your elastics would probably only support the weight of the arm and sack holder itself, so it wouldn't help with lifting those 10 sacks.
The 10.5lbs maximum force at the end of the arm (when the arm is parallel to the ground) means lifting 5lbs of sacks should not be a problem at all.

That's maximum torque at stall, so you probably don't want to use that figure for a working number. I usually use 50-60% of maximum values as a usable rule of thumb. As Dontworraboutit writes, the math should work for five pounds, but I wouldn't count on much more than that.

[dontworryaboutit]

Originally Posted by Rick TYler

That's maximum torque at stall, so you probably don't want to use that figure for a working number. I usually use 50-60% of maximum values as a usable rule of thumb. As Dontworraboutit writes, the math should work for five pounds, but I wouldn't count on much more than that.

Yup that's why the 10.5lbs will work for 5lbs (I guess I didn't make that clear, thanks for pointing it out).

The motors are also a lot happier if they run at higher speeds, so gearing 9:1 would have the motors run at 70% of top speed ish, a nice place to be.

[IGC_Alinator]

What do you mean when you say the motors are happier when they run at higher speeds?

[JVN]

Originally Posted by IGC_Alinator

What do you mean when you say the motors are happier when they run at higher speeds?

They use less power since they are doing work at a slower rate (i.e. lifting the 50 lb load at a slower speed).

This means, they are drawing less current.

In open-air motors (not really applicable here) some motors have internal cooling fans which help cool the motor while it is running, and work best when the motor is spinning at high speed (like in a power drill).

-John

[dontworryaboutit]

Originally Posted by JVN

This means, they are drawing less current.

-John

And drawing less current means they don't heat up as quickly (or as much), so you're prolonging the life of the motor as well as increasing the maximum current draw, should you need it. You get a larger max current draw because the PTCs "trip" at a lower and lower current the more times they are "tripped."