Gear Box Design

I am designing a single speed 775 pro gear box with four motors. I was wondering what the best way to design the casing is. I originally made it with a open design (similar to 2451) but then was wondering whether I should close in (2485) so that the grease stays in the gears are not hit with anything. However this blocks the vents and makes it heavier.

I am not sure whether it should be closed or open.

Also, I noticed that in many analysis belts are better than chains (234) but teams like 254 use chains. Are chains or belts better?

For CIM-style motor gearboxes I prefer closed gearboxes exactly for that reason. For 775pros, you probably want to leave the gearbox open to allow for better airflow. Either way, you can close a two plate style gearbox by wrapping a thin sheet of plastic around the perimeter between the plates and attach it with double-sided tape; it barely adds weight. IMO the two plate style is the easiest way to make a custom gearbox, open or closed.

In the belts vs chains debate, there is no real “better” option: each has its tradeoffs. Belts are lighter, quieter, and don’t stretch over time. Chains are stronger, thinner, and are much harder to skip. Either will work just fine if done properly; it’s a matter of what you have more experience with and which tradeoffs you like better for your design.

The short answer to both is the same: it depends.

As you noted, enclosed gearboxes are less likely to lose oil or take in foreign materials like carpet fibers, loose nuts, fingers, and small game pieces. They are also heavier (materials being otherwise equal), harder to inspect and work on, and may be harder to cool esp if using a low-efficiency (e.g. worm gear) design. You evaluate the benefits and risks and make a call.

Belts are lighter and they are dimensionally more stable than chains over FRC robot lifetimes. For a given load level, chains are narrower but heavier. They also tend to “stretch” as the rollers wear in, so you will want to have a way to tension them unless you use rather short runs, enclose it in a tube where the chain can’t wander, or break in the chain before installation. Chains are also generally better for prototyping because you can add or subtract a few links pretty easily, whereas changing a belt length means buying another belt. While I’ve never tested it and don’t plan to do so, I expect that a finger getting caught between a chain and sprocket would result in greater injury than a belt and pulley. You evaluate the benefits and risks and make a call.

There are plates that you can mount between the 775 and your gearbox plate to allow the motor to vent, while keeping the gearbox plate partially pocketed so nothing can get into the gearbox.

That being said, I personally haven’t use the vent plates at all so I can’t speak to their effectiveness, though I’m sure they are just fine.

Enclosed gearboxes are great because you remove the possibility of something getting in there to mess it up. The downside is that you can’t diagnose an issue you may have with the gearbox since you cannot see inside. I suppose some kind of poly-carbonate housing could be used to get around that, but chances are it would be translucent, not transparent.

I prefer open gearboxes, so that you can visually inspect them easily. Keep in mind that even if you enclose your gearbox, the grease will still fly off of the gears and stick to your enclosure. Putting a case around your gears doesn’t keep them greased, it just makes it hard to see them. Grease is really cheap and it’s easy to put more into your gearbox when needed.

Belts and chains both work in pretty much any application, best to pick the one that is most available (as in, cheaper) for your team. Chain can handle more tension before skipping on a sprocket, so if you are looking at a high load mechanism then chain may be better.

Also, what kind of mechanism are you making that needs 4 775 motors?

Pretty sure this is for a drive gearbox; he used 2451’s from the last couple years as an example.

As far as belt v chain, I agree it’s probably just best to use what your team is most comfortable with. Personally, I have a lot of experience using 25 chain in drives, and the weight savings when switching to belts is negligible or non-existent depending on the need for tensioners, so I just haven’t had the desire to switch to belt.

However, if you have used the kitbot belt system the last couple years, and you’re super comfortable with that, then you should probably roll with that. Both can be done well, so you just need to choose what works best for your team as far as previous knowledge, as well as resources.

Belt versus chain comes down to strength and packaging in my opinion.

5mm Pitch, 9mm HTD Belt: 36.1 LB Max Working Load, .04 LB/Ft
5mm Pitch, 9mm GT2 Belt: 56.6 LB Max Working Load, .04 LB/Ft
5mm Pitch, 15mm HTD Belt: 60.2 LB Max Working Load, .07 LB/Ft
5mm Pitch, 15mm GT2 Belt: 94.4 LB Max Working Load, .07 LB/Ft
ANSI #25 Chain: 140 LB Max Working Load, .087 LB/Ft
ANSI #25H Chain: 190 LB Max Working Load, .108 LB/Ft
ANSI #35 Chain: 480 LB Max Working Load, .21 LB/Ft

Chain usually wins our decision matrix for drivetrain applications as it is a slight increase in weight for a significant increase in strength. The efficiency gains belt provides can be useful for different mechanisms in FRC, but I’ll take robustness over efficiency in a FRC drivetrain.

Also keep in mind that belt pulleys are generally heavier than chain sprockets. So weight ends up being pretty close to a wash with 25 chain. We’ve been using belts in our drive train since 2014 and have had mixed results. We have failed belts 5mm pitch 15mm HTD belts in 2014 (debris in DT - probably would have knocked a chain loose too) and 2017 (PTO hang drastically exceeded working load limit of belt). In 2015 DT loads were a joke. In 2016 we used chain. In 2018 we had no failures. Belts are certainly doable but I recommend doing the math on working load on smaller pulleys. Overall we’re still of the mindset that not having to deal with chain tension is preferable when dealing with a dead axle sheet metal chassis. But I think this depends more on packaging and team preference. If I was running WCD I’d be doing chain in tube.

For non DT applications belts are usually better than chains at very high surface speeds.

I like that you put “stretch” in quotes, because chains don’t stretch. They wear. That wear between the pins and rollers is what creates the change in length. A pre-stretched chain is nothing more than a chain that has been run at it’s specified load point in order to seat the pins and rollers and wear in any significant ridges / high points / inaccuracies.

All grease is an oil mixed with a soap to make it cling to surface and remain in place. For FRC gearboxes I recommend white lithium grease with PTFE; a 6 ounce tube is enough for many years of use.

You should just use a little dap on the face of one mating gear pair, enough to form a very light lubricant layer, but not enough to have a lot of excess. Best to start very light, and add a little extra to the pair if needed. Hand rotate the gearbox to distribute the grease evenly. Wipe up any excess, and run the gearbox slowly to high speed. At that point it shouldn’t have any noticible grease liberating with use.

This video from Vex seems to disagree. This amount of grease recommendation was a surprise to us.

For fully enclosed gearboxes you can and should pack in a lot of grease with no potential issues. However, only the grease on wear surfaces provides any benefit. The gear mating surfaces will squeeze out all but a very small amount of grease.

I was specifically addressing this open gearing topic, where there are separate sealed bearings.

We typically use an open gearbox with a piece of clear polycarb wrapped around it to keep grease in and debris out. I usually bend it to the curvature of the plates (Easy to do with a heat gun) and zip tie it through both ends and around one of the gearbox standoffs. Makes it super easy to remove/reinstall if you need to inspect and keeps things light