Custom Bearing - Calculate # of Ball Bearings
Custom Bearing - Calculate # of Ball Bearings
(OP)
Greetings:
We are designing an inspection fixture where a round part will sit on a plate and rotate about its center. The size is fairly large, so we are looking at designing custom thrust type bearings. General information: Ball bearing diameter is .250". We are cutting (2) grooves where these balls will ride, one radius is 34.500 and one is 36.500. I know how to determine the circumference and then divide by .250, and this gives me a perfect fit number of ball bearings; however, is there a rule of thumb to reduce that number so that the ball bearings have sufficient space to perform their job?
Thanks in advance for your assistance,
Brent
We are designing an inspection fixture where a round part will sit on a plate and rotate about its center. The size is fairly large, so we are looking at designing custom thrust type bearings. General information: Ball bearing diameter is .250". We are cutting (2) grooves where these balls will ride, one radius is 34.500 and one is 36.500. I know how to determine the circumference and then divide by .250, and this gives me a perfect fit number of ball bearings; however, is there a rule of thumb to reduce that number so that the ball bearings have sufficient space to perform their job?
Thanks in advance for your assistance,
Brent





RE: Custom Bearing - Calculate # of Ball Bearings
Please define exactly why you can't choose one out of a catalog?
Those will have cages if necessary, and are engineered according to years of experience and real-life testing, I doubt you'll improve on that process for a one-off.
http://www.fusionpoint.be
http://be.linkedin.com/in/fusionpoint
RE: Custom Bearing - Calculate # of Ball Bearings
- 19.7 OD w/ a 17.7 ID
- 15.0 OD w/ a 13.0 ID
Can you point me in a direction where simple bearings like this can be found?
Thanks again for your time.
RE: Custom Bearing - Calculate # of Ball Bearings
RE: Custom Bearing - Calculate # of Ball Bearings
RE: Custom Bearing - Calculate # of Ball Bearings
I do not have access to my machine design textbook at the moment.
As I recall, the round bearing ball contacts the curved race surface with a contact area of zero, resulting in infinite stress. There are formulas for this. What happens in reality is that the ball and race flatten a bit, creating a finite contact area, and a stress a bit below infinite. This works fairly well because the balls and races are very accurate, and they are hardened.
I assume you are buying the bearing balls. How accurately can you machine the races?
If this were my problem, I would use as many balls as I could fit. The rule of thumb is that journal bearings wear out, and ball bearings fatigue. More balls, less stress and fatigue. I would want to know how I was going to install them, and retain them into position.
--
JHG
RE: Custom Bearing - Calculate # of Ball Bearings
Can you use crowned profile cam followers or needle bearings arranged around the periphery?
How about curved linear rail?
RE: Custom Bearing - Calculate # of Ball Bearings
These are my thoughts. I appreciate everyone's feedback. I am continuing to research catalog bearings that are large enough for this application diametrically, but are also small cross section.
RE: Custom Bearing - Calculate # of Ball Bearings
RE: Custom Bearing - Calculate # of Ball Bearings
RE: Custom Bearing - Calculate # of Ball Bearings
RE: Custom Bearing - Calculate # of Ball Bearings
RE: Custom Bearing - Calculate # of Ball Bearings
https://www.vxb.com/500mm-Lazy-Susan-Aluminum-550-lbs-Turntable-p/kit11283.htm
https://www.vxb.com/392mm-Lazy-Susan-Aluminum-420-lbs-Turntable-p/kit11281.htm
RE: Custom Bearing - Calculate # of Ball Bearings
Avoiding the friction between two consecutive balls, especially for high rpm's, is the main reason of the separator in normal ball bearings.
The number of balls in your case should leave some room for the lubricated balls to naturally find their place in the tracks, IMHO.
I would recommend you to observe the following:
- Provide your bearing with a flat surface as a solid base, which eliminate any tendency to the twisting of the racks and the need for extra energy to induce rotation. Due to the dimensions, your tracks may not be very rigid by themselves.
- Keep the lubricated tracks and balls protected from contamination with sand, wind driven dirt or water.
- Rather than trying to copy the radius of the balls in the cross sections of your tracks, machine V-shape tracks. This will make both tracks self-centering under weight, eliminating any undesired lateral movement. Bonus: each ball will have two points of contact with each track and small debris will go to the bottom of the track, not interfering with smooth rolling for moderate contamination.
Best luck with your bearing!
"Engineering is achieving function while avoiding failure." - Henry Petroski
RE: Custom Bearing - Calculate # of Ball Bearings
RE: Custom Bearing - Calculate # of Ball Bearings
Machining your own races within 0.001" met your goals.
Sounds like a much smaller bearing would suffice.
http://www.l-07d.com/images/plinth1.JPG
http://www.vxb.com/Hardware-Turntables-Lazy-Susans...
fancy -
http://www.kaydonbearings.com/RK_turntable_bearing...
RE: Custom Bearing - Calculate # of Ball Bearings
You might also have more luck if you do your search for "rotary stages" rather than just thrust bearings. Lots of companies make rotary stages.
Also, have you considered using cam followers to support your upper plate? Does it absolutely have to be supported 100% all the way around?
RE: Custom Bearing - Calculate # of Ball Bearings
RE: Custom Bearing - Calculate # of Ball Bearings
Filling a race with balls, no cage, is certainly not necessarily a problem. It's done all the time in bicycle crank and wheel bearings. In bikes, the rule of thumb is to fill the race, then remove one. It's easy to put in too many if you try to maximize the number in there.
Ok, now for the crazy idea...
Ground flat upper and lower plates. Upper plate rotates and is located by a smallish vertical shaft in bushing, ground hole, or similar. Fill the space between the ground plates with a sea of balls. Some sort of superficial fence out near the edge of the plates would keep the balls contained.
Small ball bearings are very inexpensive and quite uniform. The flat ground plates would also be inexpensive and very uniform, especially compared to big precision circular grooves.
I have seen a crude application of this on a drive-on car hoist. In that case, the purpose was to allow easy pivoting of the front wheels. The plates were on a bed of balls, plastic balls, if I remember correctly.
RE: Custom Bearing - Calculate # of Ball Bearings
Based on this requirement, you probably want to use a catalog bearing. Grinding large diameter ball bearing race surfaces having a level of precision acceptable for an inspection fixture is not a simple thing. Here are a couple catalogs for large diameter, thin section ball bearings. It looks like there are standard sizes that should meet your requirements.
http://www.kaydonbearings.com/downloads/catalog390...
http://www.rbcbearings.com/literature/pdfs/RBC-ts....
http://www.jp.nsk.com/app01/en/ctrg/index.cgi?rm=p...
To improve the axial/radial runout accuracy and rigidity of your inspection fixture bearing system I would recommend using some preload. If there is axial space available, you can use a duplex pair of back-to-back angular contact ball bearings (Kaydon Reali-Slim type A) manufactured with a controlled gap between the inner race faces. Clamping the inner races together at assembly will produce the required amount of preload. If axial space is limited in your fixture base, you can also use a single 4-point ball bearing assembled with preload (Kaydon Reali-Slim type X). You can order catalog bearings from most manufacturers with whatever amount of preload you require. Either of the bearing arrangements described will handle combined radial/axial/moment loads.
While these are standard catalog bearings, just remember that they are also fairly expensive. A 16" bore, .50" section, 4-point ball bearing will probably cost well over $1000 new. If you only require a single bearing, you might search e-bay for one that is new-in-box.
RE: Custom Bearing - Calculate # of Ball Bearings
For all the screwin around... A difficult design problem, difficult machining problems, heat treating problems, one of a kind special, very thin, thrust bearings, significant costs, etc.; you have told us very little about what the test piece weights, its size, how it’s tested, how it is fixed to the turn table, etc.; how the turn table is driven, speed/rpm, cycles per day, etc. Would something a little cleaner and simpler work as well as the ball bearing thrust bearing arrangement? A fairly heavy turn table plate, with a centering bearing over a vert. axis on a heavy base plate, for centering the turning table. The underside of the turn table plate has a 34.5" i.d. and 36.5" o.d., flat bottomed, washer shaped, shallow groove machined up into it. This groove takes a Teflon/fiberglass (Fiberglide?) flat thrust washer or a metal backed flat plastic bearing thrust washer (GGBearings.com). The base plate gets a similar shallow groove for a chrome plated steel, flat washer shaped, thrust washer. I don’t know exactly what vertical, thickness and flatness tolerance this system can achieve.