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Fasttrack
07-22-2008, 10:47 AM
Hey guys, I need some opinions.

Right now, I'm working on a transmission for a go-kart. This is entirely a "pet project". Its just for fun and I don't like spending money on things that are purely for fun. Luckily, I've got ALOT of junk lying around ;)

So, I've cobled up a simple four speed manual transmission but I don't have any 5/8" ball bearings handy. I need 5/8" because one of the shafts is an odd-ball splined shaft and its too hard to turn the bearing surfaces smaller and too complicated to reproduce at home. BUT ... I do have bronze bushings with a 5/8 ID. My question is regarding the maximum speed for a bronze bushing.

I'm figuring the shaft isn't likely to ever spin faster than 2000 rpm, and 1500 is much more likely. So, with 2000 rpm that gives me a velocity of 328 fpm. I've got an input of about 10 ft-lbs in torque and I'm figuring maximum load on the bearings would be 100 lbs. This is way larger than it will be practically speaking. Its more like 50 lbs, but in the spirit of "over engineering", I'll call it 100 lbs. Each bearing is .625" long, so that gives me a pressure of 255 psi.

Now, I don't know anything about these bearings. They are not sintered and they have a pretty dark cast to them, but that really doesn't help any :) According to McMaster-Carr, I think 660 bearing bronze would be ok for this application. It's max speed is listed as much higher than my 330 but the combined load and speed is a bit on the high side.

Anyway, I guess I'd like to hear your opinions. How likely is it that a craftsman ride-on lawnmower with a 6 speed transmission from the '80's would use 660 for the bronze bush bearings, and how well do you think said bearings will work in a transmission? My transmission will be spinning many times faster than the lawnmower transmission ever would have, and if they used a low speed bearing I'm in for trouble I think.

I'd really like to have ball bearings in there just for reduced friction since I don't have much HP, but oh well. Next time I make a big purchase at Enco, I'll pick up a bunch of chineese ball bearings for these types of projects.

pcarpenter
07-22-2008, 11:12 AM
Is adding a ball bearing outboard an option? Maybe this is a dumb question since I am struggling to figure out just where the bearing goes.

I think the issue is not one of speed in your case. From what I read, you are in the ballpark there. The issue is of load and lubrication. A plain (also plane-- for Evan) bearing is really a hydrodynamic bearing....a bearing that does its job just by floating the mating shaft on an oil film. These films squish out under load. The solutions are either lots of area reducing the force/unit area that would squish the oil out (think big lathe headstock bearings), or pressure lubrication (think automobile engine).

Paul

kf2qd
07-22-2008, 11:39 AM
An old rule of thumb for bushings was 50lbs of 500 RPM. If either of those number went up the other had to go down. Probably should check a farm supply store or auto parts store for an inexpensive ($5) ball bearing for that application. From my experience (personally and with 3 kids) that go kart will take a beating and be needing enough repairs without having to constantly replace a bushing.

Evan
07-22-2008, 11:57 AM
I have been using 660 bronze bearings I made almost exactly the same size on the countershaft of my 3 hp shaper drive. They are turning about 1000 rpm for hours on end and have been running now for at least 100 hours no problem. As long as they have oil they will hold up to much higher rpms than that.

Surprising to many people is that a lot of electric motors use steel on steel. The bearing is hardened and the shaft is not. They last for 1000s of hours.

Fasttrack
07-22-2008, 12:44 PM
Interesting... I didn't realize that about electric motors, either.

Paul - This is a "custom" built tranny. I salvaged some pieces from the six speed but I'm making a new case and a different shift mechanism to handle shifting at high rpm (thats what destroyed the old one) so I could redesign my box to use some ball bearings.

Kfq2d - :D Yep... these are really just for me and my friends to play around with, but I have the occassional niece and nephew who will drive them too. They will probably be much easier on things than I am ;)

<edit> - It sounds like I'll just need to make sure they get plenty of oil and I'll give it a shot with these bearings and see what happens. They had been run in grease and I considered adding a grease zerk. I've been worried the past couple of days on how I'm going to get enough oil to the bushings and still prevent oil from going everywhere. I'm going to need a seal on the output shaft, I think.

lazlo
07-22-2008, 02:14 PM
Surprising to many people is that a lot of electric motors use steel on steel. The bearing is hardened and the shaft is not. They last for 1000s of hours.

Which electric motors use steel on steel? Every machinery motor I've been into (up to 5 HP) has had radial bearings.


Now, I don't know anything about these bearings. They are not sintered and they have a pretty dark cast to them, but that really doesn't help any According to McMaster-Carr, I think 660 bearing bronze would be ok for this application. It's max speed is listed as much higher than my 330 but the combined load and speed is a bit on the high side.

You probably want to calculate the PV rating for the bearing -- it's the product of speed times pressure:

So 328 sfpm x 255 psi is a PV rating of 83,640 -- very high!

The Max PV constant is listed for all the bearing materials in McMaster, MSC et al... 660 Bronze has a Max PV rating of 75,000, which would imply that you'd need to move up to something like Aluminum Bronze. But you've also overestimated several of your parameters, so you might consider derating the pressure or the RPM to put yourself back into bearing bronze.

Fasttrack
07-22-2008, 02:50 PM
Yep, I got 83,000 but the PV rating isn't the maximum load times the maximum speed, its the maximum speed AND load. For my application, the heaviest loads are when it is spinning very slowly. My figure of 100 lbs might be close to the truth if all of the torque generated by the engine was soaked up in the horizontal force pushing the gears apart in the transmission. This would mean that the shaft would be essentially stopped.

Basically, I'm trying to say that if I were to come up with a function that related force to rpm, the maximum on the curve would be under 75,000. For instance, that 2000 rpm corresponds to about 6000 on the engine and lawnmower engines, in my expierence, don't make much torque at 6000 rpm ;) :D

So I checked to make sure that my maximum speed and my maximum load were within the specs for the 660. I think 660 would be ok, now the real question is: is it 660? :p

lazlo
07-22-2008, 03:01 PM
Yep, I got 83,000 but the PV rating isn't the maximum load times the maximum speed, its the maximum speed AND load.

The textbook definition of PV is the product of pressure and velocity. So in your case, the Max PV is as I calculated above: 83,640, which exceeds the SAE 660 specs

There is also a separate maximum pressure and maximum velocity for each bearing material. The maximum pressure rating for 660 is 4,000 psi, and the maximum velocity rating for 660 is 750 sfpm.

Edit: I just re-read your middle paragraph Fastrack, and I think I understand what you're trying to say: that the max pressure is not at the max rpm. Good point :)

Fasttrack
07-22-2008, 03:13 PM
edit:
oop! LOL - I just read your edit!

Yep, the numbers that I gave were the maximum speed and the maximum pressure. They don't both occur at the same time. The PV rating assumes that those numbers and loads occur at the same time.

lazlo
07-22-2008, 03:55 PM
Sure, as long as you don't exceed the max velocity (sfpm) or the max pressure, you're good to go.

Take some pictures! ;)

A.K. Boomer
07-22-2008, 04:20 PM
Sure, as long as you don't exceed the max velocity (sfpm) or the max pressure, you're good to go.

Take some pictures! ;)


Sometimes max pressure is the only thing one needs to consider --- I recall a Kawasaki 750 turbo (factory unit) plain bearing - 200,000 RPM's:eek:

Reason being is their totally floating the shaft on oil - no ifs ands or buts, so plain bearing is the description but its actually a very sloppy but precision fit and they just stoke it with high pressure lube from a major center channel, it floats like an air bearing.

Evan
07-22-2008, 04:51 PM
Which electric motors use steel on steel? Every machinery motor I've been into (up to 5 HP) has had radial bearings.

Many fractional hp motors use steel bushings. The starter motor I was using on my ebike was steel bushings but they are also used on continuous duty motors. Steel shaft bushings are common as dirt on low end consumer equipment.

lazlo
07-22-2008, 05:31 PM
Many fractional hp motors use steel bushings. The starter motor I was using on my ebike was steel bushings

A starter motor is ultra low duty cycle, so I'm not surprised that it would have steel on steel bushings. But even cheap PC fans and treadmill motors have sleeve bearings.

Evan
07-22-2008, 06:35 PM
Whatever. It's common. There isn't a source of replacement parts I can point to since they aren't intended to be replaced and the manufacturers don't exactly brag about it. Steel on steel is also used in a lot other places such as the counter shafts in dryer pulley assemblies. By using a really hard bearing on a soft shaft they hold up pretty well if they run dry. The golden rule for plain bearings is hard against soft.

lane
07-22-2008, 06:55 PM
Fasttrack use what you got and have fun Now . If it where`s out fix it then. Will probably wear something else out first. Ball bearings would be nice but if you aint got you aint got so build with what you have it will work for a long time

Fasttrack
07-22-2008, 08:22 PM
Thanks Lane! Will do!

I've seen steel on steel in other places too, come to think of it. Most of the bearings on smithy (for the cross feed screw and lead screw etc) are steel on steel.

ulav8r
07-22-2008, 08:49 PM
The bearings in a dryer motor may look like steel, but Emerson was shipping about 30,000 per day that had sintered iron bearings. I've heard that their volume has dropped off quite a bit lately.

Evan
07-22-2008, 10:12 PM
I have seen plenty of sintered iron too. Many of the gears and sprockets including the ones in my bike trans are sintered iron powder metallurgy and a lot of them run straight on the shaft. They machine beautifully, a lot like Durabar cast iron. That isn't what I was talking about though as the steel bearings are hard as a bike sprocket and probably made from similar material. I have also very occasionally seen aluminum bearings. I don't know what alloy but the first time I saw them I was pretty surprised. Aluminum is one of the last metals I would pick for a friction bearing.

lynnl
07-23-2008, 09:39 AM
A year or two ago I dismantled a old, non-working motor of some type, seem to recall it was a hand drill, but I'm not sure. What I do recall, was my surprise at finding aluminum bushings in it.

The bushings were not the cause of failure. They looked to be in original condition.

Gavin
07-23-2008, 06:45 PM
As a reference, the Bombardier Rotax Max engine, widely used in sprint and road race karting world wide uses a plain bushing to support the centrifugal clutch drum on the end of the crankshaft. The engine generates around 30 bhp, the clutch takes up at around 3,500 rpm.

Lubrication of this bushing is (hopefully) done between races with a smear of grease and they seem to survive well in these harsh circumstances and handle the loads well.

lazlo
07-23-2008, 10:01 PM
Sometimes max pressure is the only thing one needs to consider --- I recall a Kawasaki 750 turbo (factory unit) plain bearing - 200,000 RPM's:eek:

Reason being is their totally floating the shaft on oil - no ifs ands or buts, so plain bearing is the description but its actually a very sloppy but precision fit and they just stoke it with high pressure lube from a major center channel, it floats like an air bearing.

Sure, but that's a completely different animal AK -- that's a hydrodynamic bearing: they usually have a series of pie slices with slots in between to hold a film of oil at ultra-high RPM.

lazlo
07-23-2008, 10:02 PM
As a reference, the Bombardier Rotax Max engine, widely used in sprint and road race karting world wide uses a plain bushing to support the centrifugal clutch drum on the end of the crankshaft. The engine generates around 30 bhp, the clutch takes up at around 3,500 rpm.

What material is the bushing?

Fasttrack
07-23-2008, 10:30 PM
Well just to be safe (oh ok just for fun then :) ) I decided to make an oil pump. I settled on a simple plunger type pump driven by a cam on the output shaft. This means it will only provide positive lubrication when in gear, but there was no room anywhere else in the transmission for it. Its better than nothing, I suppose. At least they won't run dry for lack of maintainence (i.e. manually oiling or greasing them)

Its a simple design made from a spring out of an old 700R4 valvebody, some scrap metal and some short pieces of scrap copper tubing plus a few ball bearings out of the bike I pulled apart for sprockets. Maybe it will work, maybe it won't but at least its been fun and good soldering practice! I sure wish I could get it to run all the time, though.

Gavin
07-24-2008, 01:31 AM
lazlo,

It's a very thin bronze shell within the gear bore. The gear used for sprint karting is only 10 tooth hence the need for a bushing as there is no room for a "proper" bearing. Larger sprockets for the engine (12T-16T) use a caged needle bearing but the 10T plain bush sprocket lasts forever and needs (and usually gets) very little lubrication.

Rgds - Gavin

Professor
07-24-2008, 01:47 PM
As Lane suggests, "use what you got and have fun Now." You'll probably wear something else out first. Attached is a pic of a go-cart we built YEARS ago (in the mid 70's). I and my friends used it, our kids used it, the neighbor's kids have used it -- I can't imagine how many hours of use (abuse) it has endured but the plane bearing jackshaft hasn't been one of the things that had to be replaced.

It has a 5hp motor on it with about a 3:1 reduction so the shaft is spinning at approximately 1000 rpm at full speed (somewhere around 30 mph). We just squirt some oil into the cups each time we use it and once an hour or so as it runs.

Larry

http://www.gamesandexercises.com/photogallery/Go-cart.JPG

franco
07-24-2008, 08:38 PM
Evan,

You said: "I have also very occasionally seen aluminum bearings. I don't know what alloy but the first time I saw them I was pretty surprised. Aluminum is one of the last metals I would pick for a friction bearing."

As a matter of interest W. O. Bentley experimented with aluminium big end bearings in the 3 litre Bentley engine about 1923. The experimental engine was run on a test bench non-stop for four days, then taken for a long, fast and hard drive. One of the people involved with the engine road testing said:

"Unfortunately the aluminium big ends were not a success; they developed some suspicious cracks and showed every sign of early fracture, so had to be abandoned. It was a pity, for their conductivity was superior to white metal, while the reduction in reciprocating weight was an enormous advantage."

franco

lazlo
07-24-2008, 08:56 PM
Wouldn't an aluminum bushing seize because of the radical difference in thermal expansion?

Aluminum has one the highest coefficients of thermal expansion of any of the common materials: 13.7 microinches per F. Steel is around 8 microinches per F, and cast iron is 6 microinches per F (did the Bentley's have steel or cast iron hubs?)

Evan
07-24-2008, 11:05 PM
A ring doesn't expand inward.

S_J_H
07-25-2008, 07:18 AM
FYI, Aluminum based bearings are very common in auto engines now. I first got a look at some when I rebuilt a Chevy ls1 years ago.
And that is a high perf engine.
Of course these are full pressure lubricated. But the fact is certain types of aluminum make excellent bearings.
Read more-
http://www.ebearing.com/news2004/040701.htm

Steve

lazlo
07-25-2008, 09:13 AM
A ring doesn't expand inward.

Right, but aren't the Bentley bushings sleeved around a shaft, and then pressed into a mounting hole? Seems like it would expand from the heat and seize into the mounting hole in the hub.

Evan
07-25-2008, 12:19 PM
Got me, I'm not a Bentley fan. Plane bearings in general though are the all time champions on the low velocity end of the PV curve. They are used in the really heavy load bearing applications such as container cranes and other heavy lift machines.

BTW, if anyone doubts my use of the term "plane" bearings just Google it and you will find that is how the manufacturers all refer to the type.

macona
07-25-2008, 03:25 PM
Look at a small gas engine like a briggs. Steel on aluminum bearings thoughout, Connecting rod, camshaft, main crank. No other bearing than the material the crankcase is cast out of. And no pressure lube here either.