PDA

View Full Version : Request 4 Help: Bearing design (bike)



Tony
12-10-2007, 12:51 PM
Greets all.
Went from lurker, to poster, and back again. I keep up to snuff with
you guys with my afterdinner coffee and a ciggie.. tonite I have a
question and was hoping to pick a few brains.

I used to be better at smoking and typing at the same time...

Need help designing a bearing (arrangement) for a therapeutic exercise
bike. This is somewhat of a charity project -- the clinic is paying for
materials -- great bunch of people ... but not a great budget.

This is essentially an gym-style exercise bike with DC motor assistance..
the motor warms them up (pedalling).. then cuts out.. and in some cases
will pose resistance.

Someone else gets to work out the controller... but I've got to build them
a bike on the cheap.

Anyway, I've got most of it worked out (and built) and now need to
source the bearings for the pedal hub.

Problem is, the pedal hub needs a huge hole in the middle -- 4 inches --
in order to mount various orthopedic accessories, etc. That means I need
a bearing with an ID of at least 5". And its got to be slim -- say, 0.75"
thick at the most. (OD isn't a problem, within reason)

This size bearing costs about $200 here. And I need two. so unless I
can find a really cheap source for these suckers ($80 for the pair), I'm
going to have to design around them.

This hub will see almost no load (maybe 15lbs max -- sadly, these people
aren't in great shape).. and speeds are around the 30-60 rpm range,
probably slower in most cases.

So what are my options?
Think some teflon or bronze bushings might do the trick? Hard(er) starts
shouldn't be a problem since it'll be the motor (200W, 60V) doing the work..
but I'm not sure how to be reduce the 'free-spinning' resistance.

Turn some soft bearing seats and grease-in a handfull of ball bearings?
(homebrew 5" bearing).

Oilite?

Any tips would be great. Credit will be given where credit is due, or course.
Its a neat project that will eventually be upgraded with an electrical
(medical) stimulator -- which basically makes people (paraplegics) peddle
via electrical shocks to the muscles on a doctor-prescribed program.
I believe its called "FES".

These bikes exist, but they cost something like $20k.

Long-winded as always,
-Tony

SGW
12-10-2007, 12:57 PM
Home-brew ball bearings is an interesting idea. You're not talking about high speed here. 100 rpm is probably optimistic. Because of the o.d., the load will be spread out over a large area. I suspect if you got a "decent" surface finish on the grooves for the balls, it would work, even if you made the races out of some free-machining steel.

Ian B
12-10-2007, 01:06 PM
Tony,

Could you machine up a a steel ring with a 5" bore, maybe 6" OD with a square groove on the outside face. Mount this in (at least) 3 much smaller ball races, each held by a short spindle or bolt. The ball races would run in the groove and locate the ring. Small ball races are cheap on Ebay, and this would effectively give you a large diameter ball race.

Something like these:

http://cgi.ebay.co.uk/MINIATURE-BALL-RACES-BALL-BEARINGS-10-PCS_W0QQitemZ280182297686QQihZ018QQcategoryZ98641Q QssPageNameZWDVWQQrdZ1QQcmdZViewItem

or a bit bigger should do.

Ian

Your Old Dog
12-10-2007, 01:12 PM
I have a rotator motor for my ham antennas that is about 8-9 inches in diameter and made of about 5/16 thick aluminum. They simply cut a grove into both sides, slop some grease in the groove and lay 96 ball bearings about 1/4" in diameter in the grease and put the unit haves together. Maybe you could experiment in that direction. Very simple, cheap and effective. This setup holds antennas in the wind weighing up to 75lbs and 20 feet long if memory serves me correctly.

Here's two pics of the halves that fit together.

http://www.wallawalla.edu/frohro/Rotor/bolts.jpg

http://www.wallawalla.edu/frohro/Rotor/Inside.png

kendall
12-10-2007, 05:09 PM
lazy susan bearings.

http://www.klockit.com/products/dept-93__sku-GGGDD.html

Great for LOTS of projects. Not designed for speed, but used them in displays which turned at 200rpm and weighed in at 130lbs.

Trim the square plate as needed for your purpose, or make 'aero-looking' teardrop shaped cranks

Ken.

winchman
12-11-2007, 07:19 AM
Unfortunately, the lazy Susan bearings are for thrust only. They won't tolerate much of a radial load. But, you might get more radial capacity by increasing the preload so the balls will stay in the grooves.

Are you sure the ortho devices have to go through the bearing? Would a three-piece crank with smaller bearings work if you could attach them on either side after the crank arms were in place? Something like this:
http://cgi.ebay.co.uk/OLD-MID-SCHOOL-BMX-CHUNKY-3-PIECE-CRANK-PEDALS_W0QQitemZ200182711329QQihZ010QQcategoryZ561 97QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Roger

wirewrkr
12-11-2007, 10:11 AM
Tony, Here is the name of a couple of cheap bearing sources, I use these guys allada time. Save lotsa bucks and get good service as well. A good combination.
www.vxb.com
www.youngssurplus.com
Good luck to you and your project.
Robert

A.K. Boomer
12-11-2007, 10:30 AM
Thats a tough one, I had the same task at hand with my bike cranks --- except I had to do it with aluminum, its such a vast surface area that you could almost get by with anodizing aluminum and running a roller or needle directly on it, what i did was bore the aluminum oversize and install apropriate feeler guage material inside the OD (after heating) along with locktight bearing mount, its held up good enough to run needle bearings on as my internal piece is steel, I also had to make up a needle cage for the entire loop out of UHMW and its held up very well and is a great material to use if you need to make a cage -- I have a different situation though, its not for the main crank thrust and I only had one to have to do it to -- so mine was not subject to side loads --- If I were you I would put in some time looking for stuff like people are mentioning --- simply making something of that size out of tool steel for ball races would be great except for the quenching process where you will watch all your work warp to hell -- maybe if you know of a heat treat place with controlled ovens but I couldnt tell you if thats a gaurentee against it going haywire,

Here is one thought I might try in this situation --- I bought some mystery plate from my local scrap yard, the stuff turned out to be very tough and hard, but carbide would still cut it,

Iv made many little friction free linkage's for my bike project but they were tool steel and small enough to quench (they did warp but i still got to use most)
if you had a fairly hard plate material like this it would be posible to come up with the appropriate sized ball end mill and go almost half depth of the ball your going to use, using your rotary table on your mill or your cnc (I actually prefer a rotary table for this -- CNC's leave a little dig where the start and finish unless you do some fancy footwork) make up four --- they will handle both direct thrust and side thrust --- there is a standard of the minimum hardness rating at which you can run ball or rollers on --- you can cheat a little and re-write this rule book however - if you have a vast amount of surface area and your ball or roller unit is of continual motion, (stagnant fretting under pressure will decrease the life drastically)

Tony
12-11-2007, 02:43 PM
wow.. now those are some clever solutions. I'm going to whip a few
together in 3D CAD and see which one catches my eye.

talk about thinking out of the box.

wire -- thought about the lazy susans but like kendall says, thrust only.
i was worried preload would have to be so high to make free-spinning a
little tough.

I'm partial to IanB's and YOD's suggestion.

YOD's solution seems like it would make for an easy fix as I'm already
machining the disks and the mount so I could just throw in a 'gutter' --
drop some balls in -- and have a nice angled bearing (thrust and axial).

BUT... this is all aluminum construction. might not take long for steel balls
to chew through the aluminum races. I noticed that McMaster sells
UHMW and Teflon spheres with pretty decent tolerances... how do you
guys think that might hold up?

I'm working with a 100mm diameter (we're metric out here).. and using
10mm balls, I'd have to load about 31 balls into each side to fill the races
(I'd have no ball cages in there). (bag of 100 UHMW balls = less than $10)

Am I setting myself up for a wreck?

IanB -- the whole crank mechanism will be mounted in a bent sheetmetal
frame.. two mirrored sheets.. with a 100mm hole in the middle.
(1/8" sheet with 1" standoffs keeping them separated) Hub on each side,
and my pulley in the middle.. I'm worried that I won't be able to line
up the two sheetmetal supports well enough to get a steel sleeve to run true.

in YOD's design, I can make the races "float" in the 100mm holes...
preload would squeeze the outside hubs onto the balls, onto the inner
race, and against the sheetmetal.. but again, I'll be making a ball beaing
with aluminum races. :)

Thanks all... I'll keep you posted.
-Tony

Roy Andrews
12-11-2007, 06:00 PM
i have made a couple of bearings with large ids for different reasons all low speed, i made the inner and outer races first then drilled and tapped a hole in the center of th outside race big enough to install the balls thru. i then thred in a plug that sits flush with the inside. i grind it smooth on the outside and mark it for allignment( a slot for a screwdriver works well and facilitates removal). i then ground the grooves in the races(with the plug in). put them together and filled with balls. i made 2 of these with the inside race made with hard plastic for a spin fixture for welding and it worked very well.

Tony
05-01-2008, 05:27 AM
Maybe someone might remember this thread?

I went forward with YOD/IanB's suggestions and turned out a
large diameter inner & outer race from aluminum. Went easy
on the grease (just to hold the balls in place -- 56 in all) and
slipped the other race on. The balls are 10mm Delrin.

When the bearing is horizontal (flat on the table -- right after
assembly) it spins real smooth and quiet. When I turn it upright
and subject it to radial force (via V-Belt) it makes the most
god-aweful noise I've ever heard -- like a coffee mill.

Its still easy to turn (depending on the preload I give the balls
during assembly) -- its just really really noisy.

Not what I expected from Delrin balls in an aluminum race.

I think I might have the contact angles wrong -- basically each
race has a 5mm radius and a 90 degree contact angle on the
ball. When assembled, of course, the ball is seeing 180 degree
contact... inner race covers the 3 o'clock to 6 o'clock area and
the outer race covers the 9 o'clock to 12 o'clock position.

I hesitate to fill it with more grease.. and if I reduce the contact
angles 'on a whim' I may end up scrapping these parts.

SKF specs for their smaller bearings claim 20-30 degree contact.

Any thoughts? esp. why they might make more noise in one
position and not the other?

-Tony

I'll post some pictures soon.

Evan
05-01-2008, 06:17 AM
Several things come to mind.

Fully filled commercial ball bearings are designed for high loads at low rpm. The balls scuff each other so rpms are very limited.

Contact angles are limited to less than 180 because it causes scuffing of the race and ball if the contact angle is too great.

Delrin is less than ideal as a bearing if Delrin to Delrin contact occurs. PTFE is far better, especially if glass filled.

Suggestions:

Buy PTFE balls instead.

Buy fewer PTFE balls and alternate with the Delrin balls.

Best options technically: Reduce race contact only at the equator of the balls. Ball bearings designed for radial loads do not make contact at the equator. Equator in this context means the section of the ball in the radial plane.

Unfill the bearing and use a cage. In place of a cage slightly undersize plain PTFE balls should do.

Most important is to eliminate ball to ball contact by some means. The ideal is a partially filled bearing with a cage.

derekm
05-01-2008, 08:38 AM
Maybe someone might remember this thread?

I went forward with YOD/IanB's suggestions and turned out a
large diameter inner & outer race from aluminum. Went easy
on the grease (just to hold the balls in place -- 56 in all) and
slipped the other race on. The balls are 10mm Delrin.

When the bearing is horizontal (flat on the table -- right after
assembly) it spins real smooth and quiet. When I turn it upright
and subject it to radial force (via V-Belt) it makes the most
god-aweful noise I've ever heard -- like a coffee mill.

Its still easy to turn (depending on the preload I give the balls
during assembly) -- its just really really noisy.

Not what I expected from Delrin balls in an aluminum race.

I think I might have the contact angles wrong -- basically each
race has a 5mm radius and a 90 degree contact angle on the
ball. When assembled, of course, the ball is seeing 180 degree
contact... inner race covers the 3 o'clock to 6 o'clock area and
the outer race covers the 9 o'clock to 12 o'clock position.

I hesitate to fill it with more grease.. and if I reduce the contact
angles 'on a whim' I may end up scrapping these parts.

SKF specs for their smaller bearings claim 20-30 degree contact.

Any thoughts? esp. why they might make more noise in one
position and not the other?

-Tony

I'll post some pictures soon.
horizontally all of the race is loaded and is quiet. vertically only part of the race is loaded and is noisy
Easiest thing first - try more preload to get the balls to rotate all the way round the race and thus keep away from each other.

Listen to bicycle wheel bearings - loose they are noisy then as you tighten them up, they quieten.

Curiously the most damaging thing you can do to a ball/roller bearing is not load it sufficently.


Derek
(worked on Pump bearing life programs based on SKF tables over 30 years ago)

kf2qd
05-01-2008, 08:39 AM
Make tehe bearing groove of a slightly larger radius than the balls - you will get point contact, but will get little, if any - sliding contact. You will get some noise - any bearing makes noise - its just less than the background noise. A 5" bearing with that many small balls is not going to be quiet. Mignt not be any noiseier with metal balls, and they a re less likely to deform. YOu may need to run it in at a higher load make it run smoother - most ball bearings are ground. You may also get some work hardening of the races if you run it in before you install it.

A.K. Boomer
05-01-2008, 09:58 AM
Curiously the most damaging thing you can do to a ball/roller bearing is not load it sufficently.


Derek
(worked on Pump bearing life programs based on SKF tables over 30 years ago)



Yes too much free play results in misalignment and "floating"


Also brinnelling is huge --- even if a bearing is properly adjusted and lubed and overated for its application its still no match for fretting under pressure, A stagnant bearing carrying just a fraction of the load that its rated for is still doomed if it never gets spun, vibrations work at the contact surfaces and metal to metal is experienced --- its why brand new cars would travel by train across the country only to show up at the dealership with trashed wheel bearing...

A.K. Boomer
05-01-2008, 10:19 AM
Unfill the bearing and use a cage. In place of a cage slightly undersize plain PTFE balls should do.

Most important is to eliminate ball to ball contact by some means. The ideal is a partially filled bearing with a cage.




Yes --- you have so much surface area there that you need not worry about losing any of it, still if you did the math and are concerned, I would not use another ball (smaller) for a spacer, this causes them to be forced into the race at awkward angles as two spheres pushing against a smaller one will, (its actually a guarantee)

I would grab my design from a Kubota base swivel bearing that i had to repair,
It had steel balls and nylon ball cups, the cups were basically little straight diameter pieces (but smaller than race to accommodate the arc) with about a 1/3 spherical cup recess in each end, actually the Kubota base bearing had the best of both worlds as it really didnt have to give up every other space for a spacer ball, the recesses on the pieces only left a little wall of nylon in-between --- so almost all the steel balls could still be utilized, the cups made the parts self aligning so they didnt drag against the races like a filler ball would, and they also had a little recess in the middle to hold grease --- they could have just drilled a small hole through the center too....

So maybe the ticket would be the PTFE like Evan said but just get some stock and make up some free floating cup cages.

A.K. Boomer
05-01-2008, 11:14 AM
Analogy of a cage, A well designed bearing cage has multiple tasks --- it not only keeps the bearings segregated in a pre-determined distance, it keeps the bearings from scuffing on one another, this is critical for two reasons because with all bearing design ( not plain mind you, im talking ball, needle, roller ) there is going to be a point to point contact and also these points will be traveling at double speed, when a proper cage is introduced it - one; keeps these point to point running contacts away from each other -- two; cuts the speed down in half as they only get to transfer to the cage, I bring this up because if a simple smaller "filler ball" is used it can intermittently help, but its really a piss poor substitute for a proper bearing cage, the filler ball can keep the load balls from scrubbing -- and they can even rotate between the two acting as kind of a mediator -- however depending on the bearing design and what it will be used for they can fall very short of a friction free cage as even though it can keep the two load balls from contacting and it itself is quite happy doing so as it gets used in the direction on both its sides that it needs to go (it simply acts as a roller between the rollers) the results under loads will inevitably force it into the outer race (be it a slightly smaller ball --- needle -- or roller),,, now were back to the double speed factor as its getting pressed against the outer race (this is where a smaller sphere goes when getting pressured by two lager ones in a circular tube)
This is where the needle or roller could differ here as if the filler is small enough it could fit inbetween the two load contacts and actually be forced into the inner race --- but --- inner -- outer- the results are the same , double speed scuffing... (the filler sphere cannot be trapped on the inner race like this as it will go off to the side and then to the outer race)
But Knuckle heads design isnt an inner or an outer and actually two sides, If the application was used like a "lazy susan" With direct uniform load Filler balls would work great as there is really no varying loads on the load balls to get them unequally loaded and create a discrepancy, But now I have to add, if there is no real reason for a deviance to occur and it is a very equal load then the common practice is to just keep all the balls load carrying and give them a little free space, In this particular application they will find there area and not infringe on others.
--- but - by his description there not used this way, Sooo there will be deviance within the design if filler balls are used in this application.

Evan
05-01-2008, 11:28 AM
The PTFE filler ball idea should work in this case because of the extremely low surface friction and zero stick/slip properties of PTFE. It is also more compliant than Delrin so PTFE to Delrin contact will be quieter if they are rattling around on the top side of the race.

A.K. Boomer
05-01-2008, 11:57 AM
Id much rather "cup" them, you dont need to try and do all the math for the perfect size on all --- you just build them to what You think is the needed minimum and then if the last dont fit just right you can leave a little clearance, if its too much make up a one or two masters that take up the slack,

I think your suggestion is an improvement but in some instances unguided fillers can actually be worse as being smaller they can actually increase leverage ratios in what they are being forced upon and act a little like a wheel block.

Fasttrack
05-01-2008, 01:26 PM
Ditto what Evan said - Out of curiousity, what type of grease are you using? I'm always hesitant mixing "ordinary" lithium grease with plastics. I prefer a lighter, slicker lubricant than ordinary #2 or #3 lithium grease. Seems like heavy grease just adds fricition and won't increase the life of the bearing any. If the loads/speeds are small enough to warrant the use of plastic balls or soft races, than a heavy grease certainly isn't need.

I don't know ... just my thoughts, i'd like to hear other's opinions on the matter.

Evan
05-01-2008, 02:01 PM
In this instance the best bet is a very thin application of ordinary wheel bearing grease. Delrin (and PTFE) are completely resistant to petroleum lubricants and perform better when lubricated with same.

davidh
05-01-2008, 02:01 PM
i have made a couple of bearings with large ids for different reasons all low speed, i made the inner and outer races first then drilled and tapped a hole in the center of th outside race big enough to install the balls thru. i then thred in a plug that sits flush with the inside. i grind it smooth on the outside and mark it for allignment( a slot for a screwdriver works well and facilitates removal). i then ground the grooves in the races(with the plug in). put them together and filled with balls. i made 2 of these with the inside race made with hard plastic for a spin fixture for welding and it worked very well.


turntable bearings for large rotating machinery (cranes etc) are mfg'd nearly like that. check rotec . they may give you insight. but it sounds like you have it solved anyway.

Tony
05-01-2008, 03:16 PM
I'm attaching an image of this first mockup.
This is the "exploded" view.
The bearing assembly is unchanged. I added the
v-belt grooves directly to the pedal hubs and
eliminated the large pulley.. otherwise, the "bearing"
is exactly the same.

Its a bit of a trick getting these together, because all
the balls are held in place only once both sides are
assembled, sandwiched, and bolted.

Orthopedic pedals are bolted onto the outside (purple)
plates. When assembled, the overall width of this thing
is about 80mm give or take.

Maybe this might shed some light on whats going on.
I have all the pieces fab'd -- I'll post pictures of the
actual parts as soon as I get the chance.

Got a lot of replies here and will reread carefully.. for now
just wanted to add this image.

-Tony

http://img.photobucket.com/albums/0903/knucklehead/hubassembly2.gif

Tony
05-01-2008, 03:18 PM
ps.. the aluminum disks (purple) are 260mm diameter.
the inner race is a press fit into the sheet metal
(100mm diameter opening in the 'frame') -- blue part.

balls are 10mm delrin -- about 60 go in each side.

-Tony