Lifting problem

Our team is using 4 269 motors on our lift, geared 1:9 for torque. However, we are having issues lifting 5 sacks, and sometimes 3. We even attempted using rubberbands to lift it, but it didn’t help.

However, there are times when it could lift 5 sacks, but after then, it couldn’t even lift a single sack/no sack. How do we solve this?

Edit: It is a 6-bar lift.

There’s a few things you can do.
-You can lighten up your lift and intake system.
-You can raise your gear ratio.
-If your gears are skipping, you can attach lock plates to connect the axles and use spacers to ensure they are stationary.
-You can apply counterweights (i find steel collars linked with rubber bands good).
-Also check your programming, make sure your motor values are fairly high (100+).
-Also just make sure your batteries are fully charged.

If rubber bands didn’t help, you probably weren’t using them in an optimal placement. If placed correctly, rubber bands should make a huge difference.

what type of lift did you say it was?

How long is your arm because if you double the length it has double the force resting on the motors?

In addition to the suggestions above, try reducing friction in the joints of your lift. That makes a big difference.

WE found that 4 269 would only work for 5 sacks if the arm was light, even with rubber bands. We were using a full size 6-bar as our arm. You need to make sure that your arm is as light as possible. My suggestion would be to replace at least two of the 269 with 393’s.

If I were to replace 2 of the motors with 393’s, wouldn’t the difference in torque strain the 269’s?

Because they both run at 100 rpm, they shouldn’t put any extra load on each other, I’ve mixed them myself. But 5 sacks with 4 269’s seems pretty low, I’ve seen 2 393’s lift 11 sacks on a 1:15 6 bar with rubber band assist, maybe your arm is longer than it needs to be (goes higher than 30"), or there is a lot of friction?

How long is the bar?
What is it made of? (aluminum,steel)
Did you super glue it?

I would also like to ask, what are the differences between the two lifts I have posted below.
Lift.jpg

if you could post pictures or even a video of it lifting sacks and not lifting sacks we could help out a lot more

If I lift the bars up to the maximum height (Reaching 33 inches), it measures 20 inches from the motor to the end of the 6-bar.

It is made of aluminium.

No, we did not superglue it.

We are currently changing the lift to using 4 393 motors.

Well 4 296’s geared 1:9 for torque should have 309.6 in-lbs of stall torque (stall torque is the amount of torque the motor can “hold up” not actually move).

Now in a real application this will be lower due to friction, motor efficiency (VEX rates the motor specs at ± 20%), etc.

5 sacks at 30 inches from the motors would require 75 in-lbs of torque to hold up, more to move. This calculation is for when the arm is parallel to the ground, the arm position requiring the largest amount of torque to hold.

Now if we have a picture of your arm we could calculate in the weight of all the metal to the equation.

However, this is all just theory and in real life the system’s efficiency is going to be nowhere near 100%.

Something to keep in mind: In Gateway, the balls and barrels would line up in your robot (I’m assuming an NZ setup) in a way that the average load was somewhere in the middle of the arm, but if you use the same setup in Sack Attack, the average load will be much closer to the intake, requiring more torque to lift. 3-4 sacks in an NZ bot will require more torque than 3 barrels and 3 balls, even though the total object weight is about the same.

depending on how u have built your arm could have been the problem, we built a 6 bar using 4 X 269s and it can lift 5 sacks easily, with the torque, we only use 1:5 and also, where the axles are on the bar we used lots of washers etc … but we used lots of elastic bands and it works well, i agree with previous posts, may need to re-position elastic bands.

My suggestion would be don’t use a 6-bar. I personally think they are overused and overrated. 90% if the robots at worlds were 6-bars. People are creative. they are more than capable of thinking of impressive and original designs.

The main disadvantage of 6-bars this year is their length. The longer your arm, the less weight it can lift. I think vertical lifts, scissor lifts, and shorter arms will be more successful this year.

I don’t see a problem with using a six bar if it’s the simplest and best way to perform their selected task, unless you rank aesthetics over simplicity and function.

Anyhow, if your willing to make it a bit complex, you can try adapting this into a counterweight system. If you want, you can even turn this into a type of braking system which would be helpful when scoring in the high goal.

But is a 6-bar the best option anymore? They were great for Gateway, when game objects were lighter, and the whole game was based around speed, but this year things are much different. I don’t think a 6-bar is the best option.

Thanks everyone for the help. Currently our robot is able to lift 10 sacks with 4 393 motors.