11-10-2015 15:29

asid61

Re: pic: Inverted CIM 2-Speed Gearbox

Looks good. You mounting of the idlers is interesting.
That current draw seems a little low for 17.5fps. What CoF are you using to calculate that?
What is your weight at?

11-10-2015 16:31

KohKohPuffs

Re: pic: Inverted CIM 2-Speed Gearbox

CoF is 1.3
Would it be higher if I'm using the rubber treads that go on the performance wheels?
And the weight is under the specs: 7.87lbs.

11-10-2015 18:46

Chris is me

Re: pic: Inverted CIM 2-Speed Gearbox

I'd lower your low gear quite a bit. For 2 CIM gearboxes, you wanna pull around 40 amps each pushing in low gear, so you don't trip the 40A breakers very easily. Between voltage drop and the breaker safety margin, you can afford to draw a bit more theoretical current, but no real need to push it. You're not driving in low gear to go fast. Around 6 FPS (81% speed loss constant) is roughly where you want to be, not 10.

11-10-2015 23:09

KrazyCarl92

Re: pic: Inverted CIM 2-Speed Gearbox

This is dependent on your design objectives. 2 speed shifting gearboxes are desirable because they allow for design criteria which are often conflicting with a single speed gearbox. There are kind of 2 "schools of thought" as far as the design criteria each gear is intended for.

School A:
-Low gear is intended for pushing matches, and in some cases fine movement (e.g. bridge balancing in 2012). You want this gear to be traction limited such that the wheel slip condition (which is when torque the output from the gearbox overcomes the static friction of the wheels on the floor) occurs around or below 40 A per motor. With 6 CIMs, you might need to begin to consider the Main Breaker 120 A limit, but be sure to check out the spec sheet and understand how long your breaker will last at max current for the drive train.
-High gear is what you use at all times when you're doing anything other than pushing an opponent or doing fine motion which is less effective at a higher speed. The aim should be to minimize the travel time for a given sprint distance that aligns with your style of game play. It's nice if this is traction limited, but don't lose too much sleep over the actual current draw numbers at your traction limit since you should never hit those with good, practiced driving.

School B:
-Low gear is intended for completing game objectives with a short sprint distance, and again some fine movement tasks. The gearing should be chosen to optimize that sprint distance. You want this gear to be traction limited such that the wheel slip condition (which is when torque the output from the gearbox overcomes the static friction of the wheels on the floor) occurs at some value which will allow you to be in a pushing match for some amount of time. You will also use this gear when in pushing matches.
-High gear is is intended for completing game objectives with a farther sprint distance. The aim should again be to minimize the travel time for a given sprint distance that aligns with your style of game play. Similarly to School A, it's nice if this is traction limited, but don't lose too much sleep over the actual current draw numbers at your traction limit since you should never hit those with good, practiced driving.

The merit to School A is that it's often easier on the drivers to have a simple dichotomy of which gear to use when. When you're mindset is "Pushing match = low gear, Every other situation = high gear", it's hard to go wrong. It is also good peace of mind to never worry about losing power during a pushing match, especially in years like 2014 (well...almost never ). Chris is spot on with his 6 ft/s suggestion if your design objectives align with School A. Depending on your efficiency and your CoF, 5-7 ft/s is a general range that gets you down to 40 A per motor at your traction limit for a full weight robot with battery and bumpers.

If my memory serves me correctly, one example of a School B design is the Killer Bees' robot in 2013. Their robot was a floor pick up machine and would often times pick up discs from the floor during teleop if the opportunity presented itself. However, there weren't always discs on the floor. In that case they would drive to the opposite end of the field to get discs from the feeder station. Playing the floor pickup role was a short sprint distance objective, and playing the feeder station role was a longer distance objective. They couldn't necessarily know going into each match what role they would play, and sometimes it would change throughout the match, so having a separate drive train gear ratio for each style of play was an elegant solution. It seemed to work well, they were World Finalists after all.

In general, drive train gearing is a trade-off between how long it takes you to go from point A to point B and how much current you're pushing through your breakers. When going above about 15 ft/s, you start to reduce your pushing force and initial acceleration in high gear at the expense of additional top speed (assuming a full weight robot plus battery and bumpers).

12-10-2015 00:10

Knufire

Re: pic: Inverted CIM 2-Speed Gearbox

I thought 33 2013's robot used shifitng for improved acceleration with their autoshift code.

12-10-2015 09:33

JesseK

Re: pic: Inverted CIM 2-Speed Gearbox

I'm not crazy about the tiny idler gear - seems like it'd wear very quickly and as a result become a detriment to efficiency as the season went on. Is that the largest gear you can get for that initial stage?

More and more I find myself learning from the masters in School B for certain games. 2011/12 would more have a School A thought - the field was congested or blocked and objectives needed slow/meticulous movements. 2013/14 required much less precision in control (so long as the intake made up for margin of error) so School B would have dominated.

As a result
- School A needs the best high-traction treads available to maximise performance in the situations the design is made for.
- School B needs tread that WILL slip under enough torque so as to not trip the breakers. Colsons fit that spec nicely, imo.

12-10-2015 18:02

asid61

Re: pic: Inverted CIM 2-Speed Gearbox

I've found colsons tend to be rather grippy if you're aiming for slippage under high traction. Something like sky wheels or the white KOP wheels would work better IMO.

12-10-2015 19:23

KohKohPuffs

Re: pic: Inverted CIM 2-Speed Gearbox

So the main purpose of the idler gear is to get the gear space in between the CIMs because the way the gearbox is designed. If we do end up doing idler gears, then we could just replace them with new ones if they ever wear down. On the other hand, the big gear is a 72T gear, so I might be able to replace it with an 84T and remove the idlers. However, I remember on another gearbox I was working with another person, we had trouble with the gear sizes with an 84T.

I plan on making another gearbox of a similar design, and perhaps take a look at Andymark dog gears, which come in smaller diameters, so hopefully that will eliminate the need for dog gears.

I was thinking this gearbox to be one of those that goes decently fast on high gear, and slow for pushing matches on low (I guess this would be School A?). With that being said, I was considering the nitrile rubber treads that go on Andymark performance wheels.

Hopefully this is the right way to go if I'm going for a maneuverable and defensive bot?

12-10-2015 20:56

MichaelBick

Re: pic: Inverted CIM 2-Speed Gearbox

Switching to belts might be a good solution here. Plus, you'd get a nice efficiency boost.

12-10-2015 21:02

Ari423

Re: pic: Inverted CIM 2-Speed Gearbox

You know which wheels aren't too grippy and work really well for slippage? Lunacy wheels

12-10-2015 21:54

GeeTwo

Re: pic: Inverted CIM 2-Speed Gearbox

If it weren't for the emoji, I'd have to report you for trolling. My right eye is twitching, even though I never heard of FRC until April or May of 2011.

12-10-2015 22:56

KrazyCarl92

Re: pic: Inverted CIM 2-Speed Gearbox

You may be right. You likely had more interaction with them that year than I did. The information from my post was what I remember from a conversation I had with one of their members at Championship when I asked how they arrived at their drive train gear ratios (they seemed odd to me, and I wanted to learn more).

I would not be surprised at all to hear that they incorporated autoshifting in 2013, given that they developed a 4-speed autoshifting drive train and code as far back as 2004. However, the logic for gear ratio selection for autoshifting would still probably be consistent with the thought of having good sprint distance for both floor pick up and cycling. Maybe someone from Killer Bees could fill us in with more details?

Interesting...I would have classified 2014 into School A given the prevalence of defense and the fact that just about every team would be playing D at one point or another. That is to say, sustaining a pushing match for 15+ seconds would be more valuable than shaving a few tenths of a second off of a 10 foot sprint. But like we were saying before, depends on which criteria you're aiming to meet.

12-10-2015 23:06

EricH

Re: pic: Inverted CIM 2-Speed Gearbox

Actually, I'm not entirely sure what the CoF of a Lunacy wheel is on carpet. I'm pretty sure it's more than on that pesky Glassliner FRP (AKA regolith), though.


For those that don't get the reference... 2009 (Lunacy) was played on a field of Glassliner FRP with about a foot of carpet on all sides next to the rail. If you want to know what that looks/feels like, there's probably something similar in your nearest school/park restroom (as an anti-graffiti/easy-clean sort of measure). All robots were required to use certain wheels for their floor-contacting propulsion--the CoF between said wheels and the floor was something just under 1 as I recall, while your typical nitrile wheels are 1.something-or-other. Unlimited quantity... but that was the ONLY type allowed! Low-traction game, low-speed, low-friction...And then there were the trailers, but I'll end there.

12-10-2015 23:46

asid61

Re: pic: Inverted CIM 2-Speed Gearbox

I was far under 1 iirc. The regular tread wheels get around 1.

13-10-2015 00:01

Chris is me

Re: pic: Inverted CIM 2-Speed Gearbox

Just under 1 is the CoF of the 2008 Kit wheels, if memory serves, which were a bit slicker than other years. The CoF of lunacy wheels on FRP was reportedly 0.06, though it was probably 2 or 3 times that on a worn field.

13-10-2015 08:58

Andy Brockway

Re: pic: Inverted CIM 2-Speed Gearbox

You could eliminate those idlers by making your own gear sets. A quick search in the white papers using my name or "2005 716" will show a similar gearbox that we have been using off and on since 2005. The 12 tooth intermediate low gear has never been a problem for wear. This gearbox can be made on manual machines. One change is the that we use the stock CIM gear instead of the fancy tapered thread version in the plans.

13-10-2015 21:13

asid61

Re: pic: Inverted CIM 2-Speed Gearbox

As a student machinist for my team, I would recommend against making gears if you can avoid it at all. Indexing and cutting 2-3 times per tooth takes a very long time, not to mention all the work of taking a vise off the table for an indexing head or rotary table and centering everything. If you have the resources, go for it, but personally I can think of few situations where you would really want to do that (a 26t or 16t gear being those few situations ). I've designed swerve drives that can be manually machined, but not ones that you would really want to.
Buying spur gear stock or something similar would be good for making many custom gears.

14-10-2015 07:54

Andy Brockway

Re: pic: Inverted CIM 2-Speed Gearbox

I also do not recommend machining your own gears unless you have the time. I have been using Martin spur gears and modifying the hubs as needed. They come in almost every gear size and are available on line and from McMaster.

14-10-2015 13:52

cbale2000

Re: pic: Inverted CIM 2-Speed Gearbox

I dunno, we run about 3.5 fps in games where we're doing pushing and it works pretty well for us. I've yet to find a robot (or two at once) our 2014 machine couldn't push.

14-10-2015 15:47

Darkseer54

Re: pic: Inverted CIM 2-Speed Gearbox

Bet I can name one...

14-10-2015 16:13

Chris is me

Re: pic: Inverted CIM 2-Speed Gearbox

Once your wheels slip under load, gearing any slower doesn't do anything to increase your pushing force, it just decreases your power consumption. You're traction-limited rather than torque-limited.

For a 150 lb robot with roughtop tread, that "magic number" is around 6 feet per second, depending on your efficiency, exact weight, wheel design, etc.

15-10-2015 01:16

KohKohPuffs

Re: pic: Inverted CIM 2-Speed Gearbox

I've been punching some numbers into JVN design calculator, and I managed 16.85fps high, and 6.76fps low.

This will be a new design (but I'm keeping the inverted CIMs), since to do this I had to have the dog gears be the driven gears rather than the driving gears in the second stage (shifting on upper shaft). Also, in order to have the gear space in the gearbox, I'm most likely going to keep the idlers:

1. The idler gears are easily replacable in case they wear out
2. Using my logic, the idler gears are made of steel, so I doubt wearing down is going to be significant

As for a belted gearbox, I think our team wants to move away from belts. It's a bit complicated, but even if it is better than idlers, I'd rather work on the current style I have going.

Thoughts?

15-10-2015 01:43

asid61

Re: pic: Inverted CIM 2-Speed Gearbox

Those speeds are much better, you should be able to withstand a pushing match for a least a little while (I'm getting about 50A current draw with 4.2" wheels, 150lbs, and 1.3 CoF).
Removing the idlers can only do you good, regardless of wearing out. Reduces complexity and weight. If you have to keep them, that makes sense, but I would remove them if at all possible.
What are you guys moving away from belts for? The epitome of flipped-cim gearboxes (192 in 2014) used belts for their first stage and it reportedly went swimmingly. Saves a lot of weight and space in these things IME.

15-10-2015 01:51

KohKohPuffs

Re: pic: Inverted CIM 2-Speed Gearbox

I might be completely wrong about my team and belts, but we've been using belts for a while on our DT, and this offseason we're experimenting with chain DTs.

I mean... this only applies to the chassis, so perhaps I would attempt a belt system for the first stage. However, I'm using a large ratio for that stage, so I'm not even sure if there are the right pulleys for my configuration.

-Is there an online store that specializes in pulleys? It's been a while since I last been there, and my memory is fading away... -_-
-Also, is it possible to make your own pulleys? If so, is there a link to how to do that?

15-10-2015 02:02

asid61

Re: pic: Inverted CIM 2-Speed Gearbox

SDP-SI has them, but seeing that in this case you have a very large reduction you wouldn't be able to pull it off and still maintain adequate belt wrap without idlers.

15-10-2015 02:02

Naveed Riaziat

Re: pic: Inverted CIM 2-Speed Gearbox

We are moving away from belt in drivetrains because the tenioning was too much work for the benefit. However, tensioning similar to 192's 2014 gearbox is feasible, and in fact, 649 has cadded a similar idea already. It might be tough to get such a large first reduction however with belt. SDP-SI is basically the go-to for pulleys, and 649 already does make their own pulleys, either through our laser cutter or through our wire-edm sponsor.

26-10-2015 03:11

KohKohPuffs

Re: pic: Inverted CIM 2-Speed Gearbox

I must bring back this discussion to ask some questions regarding this design, mainly because I'm considering making another inverted CIM gearbox with more desirable high/low speeds.

1a. How can I provide enough space in between the CIMs for the gears without using the original idler design, but not go into belts (This is something I'll consider later on) or custom gears?

1b. So what if I decide to stick with my first first stage design? Because I don't think that anything bad will happen if I do it. For one, the gears are made of steel, and even if it does wear out, the design is made so that those gears are easily replaceable.

2. How can I bring down the weight to a minimum? I came across a topic on CD regarding delrin gearbox plates, and I was thinking about polycarbonate plates beforehand, but I'm not sure if either of those are good ideas.

26-10-2015 06:55

cbale2000

Re: pic: Inverted CIM 2-Speed Gearbox

We've used 0.25in polycarbonate for gearboxes before with decent success. So long as you have a solid way to mount the gearboxes and watch out for over-tightening screws you should be fine. You're already using VexPro gears from the looks of it so you should save a decent amount of weight just by doing that instead of using steel.

26-10-2015 07:54

Chris is me

Re: pic: Inverted CIM 2-Speed Gearbox

The biggest thing is that I would use bigger idlers. I don't think it makes any sense that you're using 14 tooth pinions on your motors and 12 tooth gears as your idlers. 14 tooth pinions are a pain to use with a gearbox because you can't install them in advance - they are bigger than the hole size needed to fit around the CIM boss, and it's best to design for a tight slip fit around this boss rather than a bigger clearance hole. 12 tooth pinions are great for this reason. The downside is that the gears are small so the tooth loads are higher. Using them as idlers is just getting all of the drawbacks and none of the benefits.

I would use something like a 20 tooth idler that you can put small (1/4" ID) bearings into, then mount them on a shoulder bolt or something like that. You really do want a ball bearing in your idler or you're just throwing efficiency away, and you ideally want them mounted to a shoulder bolt instead of just a screw or something so you have a simple robust round shaft for them.

I wouldn't do either of those things, especially since you are cantilevering the first stage. You don't want the extra flex that these materials will add to your gearbox. Spend the few ounces of weight where it matters; I guarantee you something else will be on your robot that you would rather cut weight from than your gearbox plate. If you really want to minimize gearbox weight, pocketing the gears is where you could start shaving a few ounces.

Also, don't forget to add some fillets to your lightening pattern - even if you waterjet these plates, fillets avoid the stress risers of sharp corners and also just look better. If you mill these plates, obviously you can't do interior hard corners.

26-10-2015 09:39

JesseK

Re: pic: Inverted CIM 2-Speed Gearbox

(Apologies, just noticed this comment)
It seemed to me that the better defense for 2014 was a 'pillaring' technique. Pillaring is a tank warfare term, where the tank drives back & forth perpendicular to the cannon's aim. It requires planning & setup, but it makes the tank much harder to hit while making it relatively easy for it to maintain sighting on a target. This is prevalent in the Battlefield series of games.

This same concept works for defense on the FRC field. Sprint into position, then pillar back/forth and force the other team to either push you sideways or drive fast enough around you to get to their goal. The likelyhood of them pushing you is high - yet it's time consuming and usually not as effective as one would thing since it still doesn't solve the problem of them getting to their desired spot for an open shot.

Faster low gear speeds on an open field also give more opportunities to clip/turn a corner of a shooting bot - much more effective than raw pushing.

26-10-2015 09:47

Chris is me

Re: pic: Inverted CIM 2-Speed Gearbox

While this is a great defensive technique for many years, 2014 included, it was a better technique for years such as 2013, when one had to navigate the length of the field, around obstacles, and pushing a robot into a specific area was a liability.

In 2014, pushing was much less risky as there were no safe zones. T-bone pinning a robot had a bit more risk than "pillaring" but a lot more benefit - the robot is essentially immobile for the duration of the pin. I don't think "pillaring" was definitively better in 2014 just because of the T-bone pin and the relatively wide space to drive around. It is an important part of a defensive strategy but not the end-all.

26-10-2015 09:57

JesseK

Re: pic: Inverted CIM 2-Speed Gearbox

When determining where to set low gear when executing defense is a consideration, is an effective T-bone pin mutually exclusive of an effective pillar defense?

Do (e.g.) sailcloth bumpers change this consideration at all?

This plays into the original topic a bit - shaft spacing is usually determined by the gear availability and the desired difference between high gear and low gear (e.g. the dog gear choices). School A wants a larger gearing difference, School B wants a smaller gearing difference.

26-10-2015 11:41

cbale2000

Re: pic: Inverted CIM 2-Speed Gearbox

To clarify my previous comment, you CAN use polycarbonate for gearboxes generally speaking, but you would probably not want to use your current design if you went this route.

A quick redesign using solid (not pocketed) plates and eliminating cantilevered shafts (by simply supporting them on both sides) might still be a worthwhile weight savings.

27-10-2015 06:19

Chris is me

Re: pic: Inverted CIM 2-Speed Gearbox

Not really.

Somewhat, particularly if you're using solid core pool noodles with sailcloth. In that case, your bumper doesn't deform or grip enough to effectively T-bone anyone (or be T-Boned, which is the point), so you can only really play pillar defense.

I don't really get school B. The advantage in short acceleration is usually so minor that it's not important, and I feel like using low gear for more precise movement is using hardware to solve a software / controls problem. While pushing matches should be avoided, low gear to me exists so offensive teams have the option to push through defense if necessary and defensive teams can themselves push all day long against the strongest drivetrains. So it doesn't get used all that much, but it's more of a safety net.

27-10-2015 11:49

MichaelBick

Re: pic: Inverted CIM 2-Speed Gearbox

In our experimentation with a "school A" drivetrain, we've actually found that we like to t-bone and play the "pillar defense" both in high gear. We found that teams can create separation from the t-bone when we are t-boning in low gear. When we are playing "pillar defense", high gear allows us to keep up with opposing robots to effectively block the field. We now teach drivers to only use low gear in head to head pushing matches.

27-10-2015 14:37

JesseK

Re: pic: Inverted CIM 2-Speed Gearbox

Mind giving traction details? I would expect that sufficiently high traction & gearing, in combination, would lead to popped breakers. I have no experience here myself, but I've read plenty of it on CD from 2014.

27-10-2015 15:18

MichaelBick

Re: pic: Inverted CIM 2-Speed Gearbox

The robot was geared for 20FPS @ 100% efficiency, had 4 CIMs and 2 550s, and had 2" wide roughtop traction wheels. The robot never stalled while t-boning, which probably helped in preventing blown breakers.

27-10-2015 23:13

KrazyCarl92

Re: pic: Inverted CIM 2-Speed Gearbox

To give an example where breakers DID pop...

Team 20's 2014 drive train had 3 CIM WCP dual speed shifters, with 4" colson wheels, and was geared for about 5.5 fps and 16 fps free speed (theoretical).

Here is video of our second match of the season: http://www.thebluealliance.com/match/2014nytr_qm13

In our pre-match strategy, we adopted the role of post-auto hounding of any opponents that missed their auto shots while our partners cleared missed auto shots of our own alliance. At the very start of teleop we go to play defense on 116 and set an open field T-bone pin on them which they fail to break free from for 26 seconds. The pin ends because we popped our main breaker.

Post-match after discussion with our drive team and some napkin math in the pits, we decided the following events likely led to the issue:
-The driver switched to low gear after the pin was initially set
-The shifting cylinder did not have sufficient force to shift the dog from high gear to low gear under the traction limit condition, so the dog remained engaged in high gear throughout the pin
-Our driver did not let up full throttle on the pin (we wanted to pin at full throttle without worrying about popping breakers as a design objective)
-We would have been pulling around 400 A or something crazy through the main breaker in this condition, which should only last a max of about 8 seconds according to the breaker spec sheet, so I am surprised we lasted this long before popping the breakers.

To mitigate the issue, we had the drivers always let up at the best opportunity early in the pin so the dog could shift. The very next match we popped the main breaker again, and after replacing it never saw a tripped main breaker the rest of the season (they tend to become easier to trip after tripping the first time).

When hounding teams on defense we could almost always maintain a pin once we set it, regardless of the fact that we were in low gear. The only exception that comes to mind is the Killer Bees being able to slip away well due to their drive train and driver skill.

Side note: I am unsure of whether we had changed this yet or not in the above scenario, but at one point early in the season we switched from 6 CIMs in the drive train to 4 CIMs and 2 MiniCIMs to up the torque in our catapult gearbox due to an increase in the pre-load of the torsion springs.