Braking

#1

Hi, so if you’ve read any of my previous threads, my team is having a bit of a hard time with weight. We would like to add in a mechanical break rather than stalling the motor (on our elevator), but don’t have the weight allowance to add in a whole pneumatic system. Is there some kind of simple-ish electronic brake we could use, ie an electronic disk brake? please post product page / pictures if you have any ideas

#2

If you gear your elevator correctly, you should have no problems relying on stalling the motors for short periods of time. I wouldn’t see any need for you stall your motors for extended periods of time great enough to cause any negative effects.

Can you describe the power transmission system for your elevator (motors, gearing, etc)? I’d be more than happy to help get your system so that you can safely run at stall, which would save both weight and possibly time.

#3

How much air do you need? You don’t have to put the compressor on the robot. A few plastic tanks doesn’t add much weight.

On our lifter, the brake setting on the Talon is enough to keep the totes wherever the lifter stops.

#4

Your best bet is probably to use a servo to push a stationary “tooth” into a gear or sprocket or something. Just make sure it’s got a linkage so the servo doesn’t take excessive load and you should be ok.

Or just gear lower and use motor braking.

#5

1 thin round disc (Frisbee, aluminum, etc), mounted to an appropriate shaft (we use a VersaHub), 1 VEX pancake cylinder, 2 brake pads, two pieces of 1"x1" angle aluminum (~4" length each) properly spaced/mounted and two mount bolts are all it takes for a brake. This is less than half a pound if pneumatics are already used.

The cylinder uses very little air on a single unbrake/brake, but keep in mind that it happens a lot.

#6

Without knowing more specifics about your set up it is hard to give appropriate suggestions. If there are gears in your lift then a servo that engages a parking pawl could be a viable option. One problem with a parking pawl is that it can become torque locked and require a lot of force to unlock it. This can be overcome by powering the motor to drive the lift up slightly while trying to unlock.

Depending on how many positions you wish to be able to hold in driving a “pin” through two elements with a solenoid or a power door lock motor is another option. Again you have the possibility of it being hard to remove but the same solution of driving the lift up slightly while pulling the pin should unlock it.

#7

A ratchet and pawl could be used in this manner with the added advantage of being designed to engage even if the ratchet teeth don’t line up with the pawl when the lock is activated.

As a note, With this system, it might be necessary to drive the lift up a bit to get the rachet to disengage. It is advisable that the program automatically does this to avoid damaging the servo.

#8

Another simple option may be some sort of band brake. You could make drum from a COTS wheel and wrap tread around it connecting one end to a pneumatic cylinder so that when the pneumatic is actuated the tread tightens around the drum. While it may not be strong enough to fully lock up the system it should add enough friction that the motors will be able to handle the load.

#9

If you don’t want to add pneumatics, you could still do a bicycle or disc or drum brake and actuate it electrically. Mount a nut and length of threaded rod (or bolt) in the space where you would have put the cylinder and couple the threaded rod to a motor or gearmotor (e.g. PG27) shaft. I’d advise using limit switches so the motor stops once the brakes are engaged or disengaged.

Another solution is to have an “always on” brake. This is something that provides enough drag to hold your heaviest load without keeping the motor running, but that your motors can still lift through. Many teams will use this sort of solution in the form of a worm gearbox.

An appropriately selected counterweight or counter-spring can reduce the amount of both lift and braking that you will need.

#10

Are your speed controllers already in brake mode? How much passive assistance can you give to your elevator before it starts moving up on its own when it isn’t loaded? This is one instance where friction/drag can be your friend. You might find that an active brake is not necessary.

#11

Currently we have a mini cim through a versa planetary 5:1 driving a 3" winch. We also bought a 3:1 stage and will add it (for 15:1) if we aren’t getting enough torque.

#12

Is there a way to add friction on the end of the shaft not being driven? Note, we are using hex

#13

According to JVN’s calculator Paper: JVN's Mechanical Design Calculator the stall torque on that set will only be ~22lbs. With the 15 to 1 you’ll be good for ~100lbs. Remember to account for the weight of the mechanism and the need for some extra power to overcome the static friction and the sliding or rolling friction in the lift.

#14

Absolutely. Adding friction is about the easiest thing in the world; reducing it is the real trick.

Here’s one way: Put a wheel on the hex shaft, using a standard AM hex hub and wheel (e.g. this wheel and this 1/2" hub or this 3/8" hub. Then, find some way to put drag on the wheel. You could press a scrap of carpet up to it, drive a screw into it, or clamp a cable tie or fence hinge around it.

#15

Another way to add friction is in the lifting assembly. See if you can find a way to make it jam a bit at the top. You could use a wedged shape piece of material at just the right spot, or use friction materials like rubber. The trick is to make the friction stronger than gravity but not stronger than your motor’s ability to yank it back out.