This is our 5th year competeing in FIRST and every year we’ve used the bearings from the kit or pillow blocks for our drive train and all of our other moving parts, but this year our sponsor is getting gung-ho about these oil-impregnated brass bushings. I can’t find any information about the advantages/disadvantages for them and he’s not very forthcoming with information about it. I was wondering if any teams have tried them and if so how they feel about them.
Vince T 418
Here’s a link to the McMaster-Carr website which has a link down the page with decent infomation information about bushing ratings:
And…I cranked some numbers, a 5/8" diameter bushing that’s a half inch long can handle a max RPM of about 800, and the Pmax I calculted on a 130 lbs robot, divided by 4 wheels with two bearings supporting it is about 5… for a PV equal to about 4000, and the bearing has a PV rating of around 50,000. So… technically, these simple bearings could be used in lieu of ball bearings in certain applications.
However, I think that there’s more to the story than that. There are many applications where you’re going to need a RPM that’s higher than 800, especially in gear boxes. In those applications, many ball bearings have RPM ratings much higher, and would be more applicable.
And… I’m late to class for answering this post. Hope it helped.
Matt
We use bushings for most of our low speed mechanisms. One of our sponsors is a hardware store and have quite a supply. If you do use them in low speed mechanisms, you still need to oil them initially. The oil impregnation migrates to the surface under pressure and heat, slow motion may not provide enough heat.
I recommend ball bearings for high speed and your drive. Friction is your enemy here. Reducing friction with bearings will reduce the load to your motors and make your batteries last longer in a match.
In 2002, we used a combination of pillow block bearing supports for wheels and bushings for the gearboxes. The pillow blocks were nice because they self-centered and were easily attachable, but the bushings were easily 1/100th of the weight.
In 2003 we only used bronze bushings on the entire robot. It really kept our weight down in the drive train, and it’s surprisingly low-friction. I think the key to using bushings is having an arbor press, precision hole creation capability (mill precision, not drill press and centerpunch precision), and a reamer to open the bushings after they’ve been pressed. It seems like a lot of work, but the payoff is size, weight, and cost savings that are very appreciable.
If you don’t have precision capabilities, bushings are definitely out. Any misalignment between two pressed bushings supporting one shaft (we’re talking .005"/ft here) can cause some serious friction problems.
Ball bearings for me.
theyr’e not so greacy and sound Realy cool
but If they get dirt in them:( things can get nasty
There is alot of good advice here.
We have considered using more bushings as Gui does. The weight and cost savings would be worth the effort, if rotating friction can be kept to a minimum.
Along with Gui’s advice on making the hole placements accurrate, the surface condition of the shafts are very important when using bushings. The rougher the shafts are, the more friction they will generate while turning in the bushing. There are different ratings of “surface texture”. From what I have heard, a roughness average of 0.1-0.08 micrometers is good for a bushing. This sort of surface texture can be achieved by polishing the shaft on a lathe until imprefections (grooves, etc.) have vanished.
Needless to say, it takes a while to get this sort of finish. But, once this finish is achieved, then friction is low and the need for a ball bearing is diminished.
We have not done this on our robot or on gearboxes, ‘cause we don’t spend the time to get the shafts’ surfaces to this degree of polish… but it is tempting to try.
Andy B.
mabie if we were to put the shaft on a lathe and “POWER POLISH” it for a while ?
I think ball bearings are a better choice if you can fit them in. In the end they are lower friction and lower maintenance, even for low speed devices.
For any kind of precision operation where we are trying to control one of our motors to hold a position or to drive the robot to the same place every time, the low friction of the ball bearing will be an advantage.
Sliding friction is the enemy.
Ken
That’s why taking the time (well, the money really) to get double shielded bearings (that seal the insides from the environment) can really make the difference between things going well in the long run and not.
And from what I noticed… it’s not really “dirt” that’s the problem when things go bad, it’s metals shavings from teams working on their robot and not being careful and clean.
Just a thought.
Matt
Thanks everybody for your help, our robot will be grateful.
But while we’re on the subject of friction, we ran some numbers on our previous year’s robot, and we’ve found that our wheels are running about 70% of the free speed RPM that they should. We used the 90 degree box that came in the kit, and I was wondering if this was just our robot’s problem or if it was a global problem. Thanks.
Vince T 418
Part of it may be that the free speed of the motors is not an exact science. The motors will have some variation around the mean. You may have just had low end (but perhaps still within spec) motors.
As to your drive train, 90 degree drives are notoriously inefficient compared to parallel axis drives. This may have been part of it.
Also, your gearboxes require some torque just to drive, even if unloaded. This static torque means that the motors will not reach your no-load spec because they have to overcome this torque even when the robot has reached its top speed.
Joe J.
P.S. on the issue of bushings Vs. bearings, I go for bearing for high speed applications (include wheel axles in this catagory) unless I have a good reason not to (one year, my gearbox was just a whole lot smaller if I just had one shaft with one of its sides a bushing – so I used a sintered bushing). The choice between bearings vs bushings need not be a religious debate, I balance the pros and cons and make the best choice with the data I have at the time.
The other place where a bearing might be preferred over a bushing would be a high radial load situation, such as the main drive wheel support as you mentioned.
Hi all
I would strongly suggest using ball rearings.
The way I look at this subject is this;
We all have to start with the same batteries and motors, so we start with the same power into the system. Assuming that we can choose the optimum gear ratio, the difference in output comes down to how much frictional loss you have. I will give myself every advantage I can get.
Matt B
A possibly slightly relevent anecdote, and then my recommendation…
Last week for one of my classes (a fluids dynamics class) we did a pinewood derby truck race at the end of this semester. I am sure lots of you did these in boy scouts. Anyway, it turns out friction between the axle and wheel is a major thing that slows down the pinewood derby car. The default setup is basically a bushing. Since we were required to use the provided wheels and the provided axle (a nail), but there were no rules against wheel modification or using other parts, I decided to install ball bearings 
After about 6 hours in the machine shop I modded the wheels so that I could install these tiny 1/8" ball bearings. Even though the axle (nail) was smaller than the bearing inner diameter (such that the axle could wobble a bit inside the bearing), the truck was flying down the track. We ended up cruising into 2nd place of about 50 teams… With a very non-aerodynamic truck 
So, the moral of the story… If you can use bearings, use them… They have less friction than bushings. On the other hand, if some friction is acceptable (e.g. you are not looking for near 100% efficiency), then bushings might be OK. There are also types of bearings that are smaller than ball bearings… Roller bearings. I have no experience with them, but something to look into if you’re interested.
One more thing… Also consider how much force the shaft will have on the mounting surface. If there will be little stress, a bushing will have little friction. However, if you are supporting the connection between the wheel and robot, for example, there will be tons of friction (F = mu * N). Even though you can get the friction really really low with a good machine finish, it will still be proportional to the load. A ball bearing, on the other hand, is not significantly affected by the load as long as you conform to the loads that it is rated to handle.
We used bushings whereever we could last year, all throughout the trans and drivetrain as well as the wheels and most any other mechanism that moves. Ball bearings are great if you expect to put the machines into extended service, but for the amount of time that we use these machines, the added weight of the bearings doesn’t seem warrented.
Andy is correct in stating you need a good surface finish on the shafts, but you can easily achieve this on a lathe with 400 or 600 grit paper or a centerless o.d. grinder.
We didn’t use your average bronze bushing in most areas either. We used Ampco Bronze with is harder and has better lubricity than the regular bushings. You don’t need real tight tolerance on the shafts either. In fact the looser you can run them, the less friction the system will have. Also use either White Lithium or Graphite for lubrication.
This year it is our mission to eliminate all ball bearing from the machine and use Ampco Bronze instead. I am however, adding Roller Thrust Bearings (which are very lite!) to each side of the hub on the wheels. This should reduce any friction generated by the scrubbing which occurs due to the 6 wheel drive style chasis. 