Wheel direct drive

[molly]

Hi, I am new to this. Are there any disadvantages from driving the wheels directly from the motor/planetary gearbox - not through a spocket and chain.

thanks

[DonRotolo]

Just t start, there are always advantages and disadvantages to every approach, in everything.

Anyway, just a few thoughts about direct drive:

1. Less control over the drive ratio, so you might not get the speed or power you want/need.

2. You have to locate the transmission and motor in a place that may not be convenient.

3. You do end up with less weight, which is a good thing, because you don't have sprockets and chain.

Don

[Billfred]

I'll echo Don's comments about ratios; one of the reasons we're using chain in our setup is so that we have a fallback in case we completely misread the way the game would be played. Since we're Week 3 this year, we have a clue as to which way the game will be played. If we need to adjust our drive system, we just need to order some new sprockets, switch them out on Thursday at Chesapeake, and we're golden.

The other advantage to chain is that chain is an easy-to-fix place to have a drivetrain fail. Ideally you would have zero failures, but I'd much rather throw a chain than bend a shaft or otherwise bust a part.

[Kevin Sevcik]

Originally Posted by Billfred

I'll echo Don's comments about ratios; one of the reasons we're using chain in our setup is so that we have a fallback in case we completely misread the way the game would be played. Since we're Week 3 this year, we have a clue as to which way the game will be played. If we need to adjust our drive system, we just need to order some new sprockets, switch them out on Thursday at Chesapeake, and we're golden.

The other advantage to chain is that chain is an easy-to-fix place to have a drivetrain fail. Ideally you would have zero failures, but I'd much rather throw a chain than bend a shaft or otherwise bust a part.

On the other hand, a good direct drive shaft should be less likely to fail than a chain and sprocket setup.

[BillB]

Just one thing. Direct drive is a lot like driving your car around in only first gear. once you hit that point, it just will not go any faster. if you drive hard, it will reduce your motor and battery life as well. It works, but you have to do it right.

[DarkFlame145]

I have to agree with everyone. The only advantage i can think of is it's lighter. But you lose top end speed, and bat. life

[Cory]

Originally Posted by DarkFlame145

I have to agree with everyone. The only advantage i can think of is it's lighter. But you lose top end speed, and bat. life

I'm confused.

I assume you're talking about direct driving a motor which is never a good idea.

Plenty of teams successfully direct drive their wheels, from a transmission.

Our team was very tempted to go for DD this year because of all the chain problems we've been having in years past (they have been randomly falling off, for example: last year we had 8 idlers and 4 chain tensioners and we were still having probs)

Then we saw 2 disadvantages to DD and we decided to stick with chain:

1. Set gear ratio/speed from gearbox (with chain you can change sprockets)

2. If wheel is damaged, the shaft/gearbox will likely be damaged too (with chain, there will be no further damage than wheel)

[DarkFlame145]

Originally Posted by Jaybee1405

Our team was very tempted to go for DD this year because of all the chain problems we've been having in years past (they have been randomly falling off, for example: last year we had 8 idlers and 4 chain tensioners and we were still having probs)

Then we saw 2 disadvantages to DD and we decided to stick with chain:

1. Set gear ratio/speed from gearbox (with chain you can change sprockets)

2. If wheel is damaged, the shaft/gearbox will likely be damaged too (with chain, there will be no further damage than wheel)

I remember you guys having probs last year.

haha yea, if you saw random chains lying in the middle of the playing field then you know who they belonged to.

We figured at least part of the problem was our axles (the 5 inch stainless threaded rod we used was only connected on one end of the frame, so our 6 in. IFI traction wheels were kinda like cantalevers)

[usbcd36]

We used direct drive last year. We had some custom hubs with key slots that fit inside of the IFI traction wheels we were using, which then slid right onto the shafts of the BaneBots gearboxes. On the other side, we put bearing blocks, which BaneBots sells for most of their gearboxes.

We had some major control problems last year, but they weren't related to the direct drive (as we discovered a few months ago). Everything else worked out pretty well, but we did have to take the gearboxes off every time something went wrong with the wheels (loose screws in the hubs, broken spacers, etc…), which meant taking screws out of hard-to-reach places.

We're going back to chain drive this year, since it allows us to manipulate our center of gravity much more easily.

[Al Skierkiewicz]

Just to add a word of caution, direct drive will require some additional bearing support if you plan on adding wheels to the output shafts. Failure to provide adequate support will increase the bearing friction at the output of the transmission which results in high current and early failure of the bearings. Many teams that use chains forget this point as well. Unsupported side loads, transfer the load to the bearings and internal mechanics resulting in significant wear and friction. The friction is in turn reflected as high current in the drive motor which in turn causes excessive heat.

[JesseK]

I was just about the echo that sentiment Al. Usually to do a direct drive from the transmission to the wheel you need good pillow blocks built into the frame or mounted below it. The weight savings of the chain/sprockets becomes negligible at that point.

For 1885's design, I find that using the flat AM aluminum sprockets and #25 chain is a few ounces lighter and less expensive than buying pillow blocks.

[Deep Dark]

Someone may have already posted something similar, but I personally like direct driving at least one wheel per drive side. That way, if the chain does come loose during a match, you aren't limping around on the field driving in circles. In this year's game, if this occurs near the lap readers, you can get docked points for driving in circles, depending on robot position.

My personal favorite design features a 6 wheel drive, with the center wheel directly driven with two sprockets, each going to their own respective outer drive wheels. That way, the whole robot consists of 4 chain loops, and even if all of the chain breaks or falls off the sprockets, you can still drive fine.

And a switching gear transmission helps with varying speeds, since many, like the AndyMark Supershifter, can have alternative gears that can be switched out to achieve different drive ratios. In many cases, this becomes faster than having to change sprockets on all 4 or 6 wheels, since you 'simply' need to remove, disassemble, and reassemble both drive transmissions.

Anyway, that's my $0.02

[Molten]

Direct Drive is awesome. You get great speed (we did at least). You don't have to worry about the chain breaking. And you will weigh less. Regardless, do not use direct drive unless you hate your designers. We did that last year and to put it simply, you have about 1" of clearence if you use a direct drive(assuming you don't offset your wheels). Though it works great, don't give your designers all headaches. I still remember the pain.