Bearings, cost, and skimping
photomankc
Posts: 943
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.
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.
Comments
I'm no expert on bearings. I was surprised to see you think they cost a lot, but I have only used bearings up to 5mm. I see as soon as the bearings get larger, the price goes up a lot.
What is the diameter of your axle? I think if you could use an axle the same size skateboards use, the price of the bearings would be a lot less.
I hope you post pictures of your robot.
The OD on those is big and would tend to expand the size of an already large bracket. The 8mm bore is also a bit larger than the stock I was planning to use but I could look at how difficult it would be to alter the desing to use them as that beats the $9 per bearing cost of the local ones I sampled.
Right now there is only a 3D model growing up around the drive system. It uses 1" --> [2":1"] --> 2" v-belt pulleys and 2L v-belts. Wheels are 90mm Pololu wheels that were on sale over Thanksgiving. I'll post up a render of the model later. I was just hoping that to reduce the complexity I could live on 1 bearing per shaft with each only seeing a few lbs of moment loads vs fully supporting at two points.
The steppers will already have bearings in them (at least they should), so it comes down to the pillow block or whatever you're machining for the wheel axles. You can get bearings surplus for just a couple dollars each. You need to go online and not buy from local shops like Graingers (or whatever). Rollerblade/rollerskate bearings are okay, and yes they're cheap, but they tend to sized in metric, so keep that in mind when matching to other components. One of my usual haunts for these is American Science & Surplus. See if they have anything you can use: http://www.sciplus.com/category.cfm/subsection/19. You might also try H&R and Surplus Center. You can find these with a Web search
I don't think the 90mm Pololu wheels are designed for a 5-6 pound robot. I've got them on a 2 pound robot, and that's about as much stress as I'd want to put on them. They're just plastic with five fairly fine spokes from the hub. I don't think they'll hold up long with that weight.
-- Gordon
V-Belts were mainly to test it all out, they are simple to make on the lathe. I can later go to a cogged belt if it's worth the time to CNC out some timing pulleys. I think I may just have to do as you say and design to what I can find that's inexpensive. 8 bearings at $10 is too much but I think it would be best to have two for each of the two shafts. I knew the motor had it's own bearing and they are usually fairly decent so it has it's own support.
Actually, I was wondering if maybe an old school Bronze bushing would work for you?
What RPM's are you expecting?
I only ask because cutting cost and skimping is a specialty of mine...:)
-Tommy
The babbit bearing is..... interesting.
The Bronze bushing could be cut into two or more bushings, for even more thriftiness,
and just regular washers for thrust bearings, or use a couple of layers of plastic packaging, for the ultimate in cheap.
At those RPM's you could go really cheap and try copper tubing and a well fit bolt,
I could suggest Nylon stand-offs too, but the weight could be to much if the Nylon was not thick enough.
They do make aluminum stand-offs, but whats the fun in buying those? when you could make your own...:thumb:
-Tommy
I also found the BaneBot wheels which are inexpensive and in the right size ranges. I could add a little more reduction to make up for slightly larger wheel.
There's the state of the design as of now.
Simply, if your ball bearings require a bigger axel, it will provide an opportunity to design an axel that will carry substantially more payload. In general, electric motors are intended to provide torque and not carry the weight of a payload.
Alright, I spent the day yesterday going back through and adjusting the design to accomodate double bearings on the wheel shaft and the intermediate pulley shaft as well. Still have a few modifications to make since that pushed the wheel outward a little but I wanted to keep any overhang from the bearings to the wheel pulley as short as possible so I placed the wheel bearings out front. Thanks for the suggestions. The bearing blocks are the transparent parts in the image. I'll order my bearings next week and get started on machining the rest of the parts for version 1. I just realized I had Gordon who wrote the book answering in my thread! I love this place.