Bearings vs. Bushings in Gearboxes + All-Belt Gearbox?

[Colin Small]

So I've run into an issue messing around designing gearboxes. I've been trying to make an all-belt gearbox, as pulleys from SDP-SI are way smaller than gears from places like vex, and using pulleys allows you to not be restricted to having the gears tangent to each other. Since that wasn't enough of a weird idea, I thought that it'd be cool to make it two speed via a dog shifter, meaning that the pulleys would have to be machined out to fit the dog.

However, the pulleys are already the thickness of any vex hex bearing. This means that there's no room to machine out the pulley. As far as I understand, this could be solved if I first pressed a hex-broached piece of bushing material into the pulley first and then machined it down.

So my questions are:

1. Will an all-belt gearbox even work, or will I end up slipping/breaking/overstretching belts?

2. Do bushings have low enough friction so that both speed pulleys can spin on the drive shaft no matter the pulley that the dog is engaged with (the drive shaft would be idle when the dog is engaged with neither).

I hope that makes sense. I can upload pictures of the CAD and answer questions if they need clarification.

[Joey Milia]

1 yes. however you should be carful about how you select tooth sizing. 3mm pitch will be better for the higher speed early stages but 5mm pitch might be needed for the later ones. I'm also skeptical of the benefits over gears. I like belts for the first stage with placing motors, but gears have a far higher power transmission capability for a given size.

2. Bushings could work given the proper conditions, after all vex uses bushings in their ball shifting gearbox. However they could be problematic, the shaft would have to be steel and machined well or ground. Aluminum is a horrible bearing material and a rough surface would increase wear. They also would probably have more slop than bearings and that would increase gear wear. I don't like and wouldn't recommend using bushings over bearings in such applications. I would look into smaller bearings or just not have the whole bearing in the gear.

If the pulley your using is just the thickness of a vex hex bearings you're probably using belts that are to narrow.

I also don't understand why you need hex bearings or a hex broached bushing. Wouldn't having the bushing hex broached defeat the point of it spinning?

[Colin Small]

Originally Posted by Joey Milia

would look into smaller bearings or just not have the whole bearing in the gear.

If the pulley your using is just the thickness of a vex hex bearings you're probably using belts that are to narrow.

I also don't understand why you need hex bearings or a hex broached bushing. Wouldn't having the bushing hex broached defeat the point of it spinning?

Thanks for the input!

I'll play around with sizes of pulleys and find something that works. It looks like I have room to run wider belts anyways.

And I might just be misunderstanding bushings, but the idea is that the pulley would rotate around the bushing rather than having the bushing rotate around the shaft

[Joey Milia]

Originally Posted by Colin Small

And I might just be misunderstanding bushings, but the idea is that the pulley would rotate around the bushing rather than having the bushing rotate around the shaft

Ah ok. You're understanding them correctly but I don't think that would be best, assuming you're using aluminum or plastic pulleys. I've already mentioned aluminum is a horrible bearing material and it would be much better to have the shaft steel than the pulley. Having the bushing rotate on the shaft would also mean lower surface speeds, less wear and less friction and be stiffer.

[Colin Small]

Originally Posted by Joey Milia

Ah ok. You're understanding them correctly but I don't think that would be best, assuming you're using aluminum or plastic pulleys. I've already mentioned aluminum is a horrible bearing material and it would be much better to have the shaft steel than the pulley. Having the bushing rotate on the shaft would also mean lower surface speeds, less wear and less friction and be stiffer.

That makes sense. I'll play around with the width of the pulleys/belts and find something that works.

Just curious, what width belt would you recommend? Right now I'm running 6mm wide 3mm GT2 throughout the build, although I might make the latter stages 5mm as you said earlier.

http://imgur.com/8gzwYMM

There's plenty of space right now, so I don't see any reason not to run wider pulleys.

Thanks for the help!

Edit: Do you think it's possible to get away with running 3mm GT3 belts? 5mm increases the size to a point that I'm not very happy with.

[Joey Milia]

Is this a drive gearbox? How many cims do you have on it?

I have no guarantee that 6mm wide GT2 will handle the power of even 1 cim. 9mm wide GT2 and 6mm wide GT3 will work fine for a cim on the first stage. I haven't tried using 3mm pitch on anything other than the first stage and I would worry about torque capacity. I recommend looking at gates belt power tables and compare the values with what you know works. You can't take their values at absolute face value because we don't know what a good correction factor is for FRC use, extremely harsh conditions but also extremely short acceptable lifetime.

[Colin Small]

Originally Posted by Joey Milia

Is this a drive gearbox? How many cims do you have on it?

I have no guarantee that 6mm wide GT2 will handle the power of even 1 cim. 9mm wide GT2 and 6mm wide GT3 will work fine for a cim on the first stage. I haven't tried using 3mm pitch on anything other than the first stage and I would worry about torque capacity. I recommend looking at gates belt power tables and compare the values with what you know works. You can't take their values at absolute face value because we don't know what a good correction factor is for FRC use, extremely harsh conditions but also extremely short acceptable lifetime.

It's a three CIM, two speed drive gearbox.

Anywhere I said GT2, I meant GT3 (and in the case of the design, now 9mm wide GT3). I'm just starting out with belts and didn't realize that they were the replaceable with each other. I'll take a look at the tables.

The other option I see is to double up the 6mm wide GT3 belts and pulleys, since I'm not sure that the 9mm pulleys have enough space for the bearing and dog teeth.