So quick question, we are trying to scale using a ‘tape measure’ idea with a 16:1 planetary gear. We are stuck on trying to figure out how to keep it locked in place when the power shuts off or the game ends. We are afraid of our 120 lb robot would fall after we scaled. Any ideas?
Yes. Configure it more like a train brake than a bicycle brake. That is, it should be in engaged/brake mode by default, and have to be forcibly actuated to release and hold the brake released. Then, if power is cut, the brake automatically engages.
While there may be more resistance to backdrive in the Andymark worm gear offerings than in your standard spur gearboxes, both have bolded notes that, due to the angles of the worm gears, they are not anti-backdrive. I’m not sure if there are any other COTS worm offerings,.
Worm gears are notoriously inefficient. That is why they don’t backdrive actually. They throw away 60-90% of the power that you put into them (said another way only 10-40% of the mechancal power you put into the worm gear shaft comes out the other side as mechanical power on the worm wheel shaft).
The reason you care is that if you are trying to do a fixed energy task (e.g. lift a 140lbs robot 2 feet in the presence of a 32.2fps^2 gravity field) then your power output is going to tell you how long it takes to complete that task. If only 10% of your input power ends up going to that desired task (i.e. lifting your robot) then you need either put in 10X the power to finish the job in the same time or you need to take 10X as long if you put in the same amount of power (or some combination 5X the power lifts in 2X the time, 3X the power lifts in 3.3X the time, …)
Bottom line, if you can manage age, I believe it is preferable to have a lock or a brake or an over center linkage or 10 other ideas your team could come up with that will keep your robot from back driving that are **not **a worm and wormgear pair.
From my understanding worm gears are not all that inefficient if used in certain ways. Judging by what it says on this site: http://www.roymech.co.uk/Useful_Tables/Drive/Worm_Gears.html
you can achieve a decent efficiency of 70% or higher using a bronze wormwheel and 2-lead steel worm if they are turning ok, and should not backdrive either.
That being said, worms and worm gears are expensive. Furthermore the only place I’ve found that has a decent selection is SDP-SI and wmberg. If you go SDP-SI, they only have 24 pitch worms and worm gears in stock, so you would have to have another stage of reduction after the worm gear to reduce load on it, which would slow you down a lot.
A worm gear’s efficiency is inversely proportional to its tendency to backdrive. A more efficient worm gear will more easily backdrive, and vice versa. Worm gears aren’t really a magic solution to this problem, unfortunately.
First 70% is crazy high for worm gears in the real world, especially worm gears that a FIRST team can afford (both in terms of weight and $). It is really hard to get worm drives that are this efficient. You have to manage the coefficient of friction between the worm gear and the worm wheel. You have to manage the lead angle. You have to worry about the thrust loads (and manage them efficiently). You have to get the tooth forms just so… The list goes on and on. Getting over 50% is crazy good and probably not even possible for most FIRST designs because teams are almost always using worm drives to get high ratios in one gear stage which is at odds with high efficiency.
Second, if you do everything right and you get anywhere close to 50% efficient, that gearbox is going to be easily back driveable. Believe me. This is the case. There is another CD thread about pins on window motors. The REASON that Denso puts those pins in their window motor is because they have made their worm drive efficient enough that it will no longer keep the window from dropping on a bumpy road.
I believe their intention is to be able to drive the device in both directions (they want to use the same motor to extend the device before retracting it). That doesn’t completely rule out a ratchet/pawl, but it would mean they would need some sort of actuator to engage their pawl when necessary and disengage when not.
Head out to your garage and look at your bicycle. The principle is identical - your squeezing something to create friction in order to stop it. The key difference between a train brake and a bicycle brake is that a train brake is closed by default, while a bike is open by default - that way the train will automatically brake if it loses power, rather than be unstoppable until it hits something.