Let's Make Robots!

What can I get away with in bearings?

I'm in the process of building up a drive system from scratch around a couple of high-torque NEMA 17 steppers.  The idea was to use a small V-Belt drive to provide a boost in torque so I could use larger wheels and get up off the floor a bit.  I have a 4:1 reduction going with an intermediate pully in addion to the drive and wheel pulleys.  I'm machining all the parts myself and have the pulleys made.  I wanted the ends of the wheel and intermeadiate pulley to hang free.  Looking at bearings it's become clear that even cheap ones if doubled up on the fixed end of each shaft are going to add up to a lot of cost.

Can I get away with simple *single* radial ball bearings on each shaft in a lightly loaded drive system like this?  The weight would not be more than a few Lbs.  It's not going to be giving my kid piggy back rides or anything.  I've selected LiPo batteries to get lots of juice for the steppers at a low weight.  I anticipate the total weght would not exceed 6 - 8Lbs in the end.  I know that I can keep the loads on the pulleys light by not pulling the belt super tight.  The steppers would stall long before the belt slipped.  I just don't want to make a bearing eating machine in addition to a normal money eating robot.

Keeping it to a single bearing bored hole also keeps the complexity of the bracket to hold the whole thing down.

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Oh sorry, I'm in the U.S.

Thanks for the comments.  I ended up making the bracket to accomodate double ball bearings.  The belt tension is more pronounced than I really anticipated to get any power transmission to the wheel and applying that much side load to a single bearing seemed to be asking for failure.  Instead of machining it all into the bracket I made the bearings fit into cups made on the lathe that bolted onto the bracket.  This made it possible to slide the wheel out to tension that belt as well. I found vxb.com for the bearings and got them at less than $2 each vs $9 at the local hardware store.  So it was $14 for bearings in both units vs nearly $80.

Here's the basic bracket:

And here's the finished drive system:

 

Higher quality than most bots around here have.

Without seeing a picture, it's a bit difficult to visualise. It would also be useful to know what country your in.

With this weight, it should be OK to just use a single bearing - depending on what country your in, you should only be looking at a few pounds or dollars, and if your machining out everything yourself, then it will be a tiny fraction of the cost.

It would defnately be better to use two bearings, and better still to use angular contract bearings but then again, at this scale, and the fact that this bot isn't exactly going to be running constantly, you may find you'll get no additional benefit from using bearings as you would carving up a bit of bearing bronze if you've got some in your scrap tub.

If your worry about cost is the use of "proper"  angular contact bearings, you can get very similar results from just using a couple of cheap ball races, costing pennies, butted up next to each other. I can attest to the quality of the bearings that come out of roller-blade wheels, which you can buy second hand from eBay - they're don't sterling work acting as ACB's on the cross feed of my main lathe - but it depends on shaft sizes.