Material Notes: Working with SHAFTS and BALL BEARINGS
A properly made ball bearing based mechanical system is a pleasure to behold, and offers the following advantages over the alternatives:
In the kinetic arts field, ball bearings might be a luxury or a necessity, depending mostly upon the expected lifespan, loading and speeds of the rotating joint in question. Other factors may also come into play, such as the ability of ball bearings to reduce power consumption and thus reduce motor and battery size, and their ability to improve smoothness of operation.
- movement with almost no effort
- little sensitivity to loading
- the ability to run fast for long periods
- quiet operation
The disadvantages of incorporating ball bearings are mainly increased cost, increased joint size, and increased difficulty of design, tooling, and fabrication.
My own perspective is as follows: since I invest substantial effort on projects and want them to operate reliably for many years, ball bearings are are a necessity in rotating joints. The exception is very lightly loaded joints - in which case quality plastic sleeve bearings are used.
The subject of ball bearings is so mature and advanced that the literature, intended for serious mechanical engineers, can be intimidating. But most of this complexity is irrelevant to the kinetic artist. Here, I'll cover the basics of getting projects moving with shafts and ball bearings.
(above) Assembled and disassembled bearing systems. Lathe-machined aluminum cylinders accept pairs of ball bearings. The bearing seats are made about 1 mil over-diameter so the ball bearings easily slide in. As a final step, Loc-tite retaining compound (a type of adhesive) will lock the ball bearings in. These will form part of the Spiroglyph machine (below left, positions indicated by red squares)
The Spiroglyph machine (left) is a good illustration of the power of ball bearing based design. The machine's 4 rotating axes are each supported by pairs of ball bearings. And 4 additional pairs exist: 1 pair in each of the 3 motors, and 1 pair in a concentric transmission. That's 16 ball bearings total.
The machine can run fast and smoothly. The relatively low power motors work almost entirely against air resistance, since mechanical friction is nearly zero.
The ball bearing joints have very little sideways or axial play (aka "slop"), so the machine won't rattle and vibrate when running fast.
WHAT KIND OF BALL BEARINGS ?
There is a bewildering array of ball bearings out there, so it can be confusing to find the "right" type. But really, you just specify a list of parameters, and there will only be one or a few types that fit the bill. For example, on the McMaster website specify: for 3/8" shaft, 7/8" OD, stainless steel, double shielded... only one type comes up, the 6138K24.
SUPPLIERS. I usually buy from McMaster-Carr. Reasons: you know exactly what you're getting, excellent website, fast delivery, good selection, and the ability to get more of the same type many years in the future. For me, these advantages outweigh their disadvantage: higher prices. Don't be tempted by Amazon's ultra low prices - you'll likely get cheap Chinese crap, perhaps counterfeits, that will cause headaches down the road.
PROTECTION. A ball bearing is a precision machine which must be kept clean and oiled; dirt in the little track where the balls roll will most definitely ruin its smooth operation. That's why they usually have covers over the balls. These covers are called "shields" when made of thin metal, and "seals" when made of rubber. Double sealed should be the default choice (double meaning both sides have seals). These are immune to dust, dirt, and splashing liquids and keep the initial lubricant locked inside. Shields, being made of non-contacting metal, can withstand higher temperatures and offer less friction, but can allow some very fine material and liquid to enter. "Open" bearings (the ones where you can see the balls) should only be used in controlled environments, like a box filled with clean oil.
STAINLESS STEEL? Nobody wants a ball bearing to rust - that could kill it. So it seems to make sense to make ball bearings out of non-rusting stainless steel. These do exist, but they are a distinct minority. Why aren't they ALL made of stainless? Because stainless is substantially weaker than the best non-stainless types of steel. So in protected oily environments like inside an engine, non-stainless types are preferred. For kinetic art, or indeed any application where the ball bearing is exposed to humid air, stainless types should be used. They do cost more and there are fewer choices, but they will greatly extend the lifespan of a project. If non stainless types must be used, try to keep a film of oil on them.
WHAT'S ABEC ? ABEC ratings specify precision levels of various dimensions in a ball bearing, for example, how perfect the "sphericity" of the balls, or how close the bore diameter is to the nominal value. The higher the ABEC number, the better. As a general rule of thumb, any ABEC rating means a good ball bearing. ABEC-1 types, although the lowest rating, feel very smooth. In extremely demanding applications like machine tool spindles, ABEC-7 bearings may be used. High ABEC values make bearings cost much more, so they seldom make sense for kinetic art; the exception is "precision miniature ball bearings", which are usually ABEC-5 and not super expensive.
RPM RATING. You may think a ball bearing's rpm rating is irrelevant to your project because it's going pretty slow, but this rating actually tells you a lot about the precision or "smoothness" of a bearing. The higher the rpm rating, the smoother it will feel. This often corresponds closely with the ABEC rating.
FLANGES AND SNAP RINGS. When it comes to mounting your ball bearing to a plate, especially a thin plate, flanges and snap rings can make your life much easier. These features extend part of the bearing's outer diameter, allowing it to seat squarely and securely. These features are common on smaller types (for shafts 1/4" or less) but less common on large, high quality ones. Bearing suppliers can get you snap ring types in most sizes (photos left). Snap rings are separate pieces that snap into grooves ground into the sides of a bearing.
EXTENDED INNER RINGS. A handy feature found mainly in small types is having the inner part ("inner race") stick out beyond the outer part ("outer race"). This prevents inadvertent rubbing. Otherwise, a shim is used for this purpose.
ANGULAR CONTACT TYPES. Be aware that angular contact ball bearings may look similar to regular "deep groove" ball bearings, but they are actually very expensive, exotic cousins. They are often employed in machine tool spindles where extreme positional precision is required. They probably won't find much employment the kinetic arts.
Sealed ball bearing with the seal pried off
A 1" bore, 2" OD, shielded snap-ring ball bearing seated in a 1/4" thick plate. See also photo below.
A pair of opposed snap ring ball bearings (same as in above photo) supporting a verticle spindle. Part of the top bearing can be seen protruding out of the upper aluminum plate.
RIDING IN BALL BEARINGS: THE SHAFT.
The thing which passes through the bore of one, or more typically a pair, of ball bearings may be generally referred to as a shaft, even though sometimes it doesn't look like one (photo right).
In the simplest case, the shaft is just a length of metal cylinder. What can you use for this job? In principle, anything that can fit in there will work, but in practice you want a strong, accurate fitting thing that doesn't wiggle around - a "precision shaft". McMaster carries lots. These may be stainless steel (preferred), steel, or aluminum, and are ground to very a accurate diameters slightly UNDER the nominal bore size. For example, a "1/2 precision shaft" might be guaranteed to be between .4995" to .4999" diameter - thus nicely slipping into a ball bearing. In addition to an accurate diameter, precision shaft has an unusually accurate cylindrical shape.
It's easy to confuse "precision ground rod" with "precision shaft" because they look exactly the same. The difference is that shaft is always under the nominal size, and rod may be either under or over. If you try to insert a rod which is even .0001" diameter too big (a tenth of a thousandth of an inch or "a tenth" over) it wont go in at all or it may go a little and then get stuck. You will have to sand it (preferrably while it's spinning) a bit to get rid of that extra diameter. It's best to avoid the hassles and just get the precision shaft - and be sure to label the left overs.
Normal rod, which is not ground smooth, is a poor choice for a shaft in its as-received condition since it has poor diameter accuracy and poor cylinder "shape accuracy". But, if you have a lathe, it can be turned down to fit the ball bearing bores well. Pretty much anything can work if you're going to lathe it - aluminum, Delrin, epoxy, etc. This approach allows for more design freedom, as in the photo upper right.
The majority of precision shaft material is hardened. This makes it near impossible to cut with a hack saw, but it's easy to cut with an abrasive wheel. Edges can be then cleaned up with a grinder or sander, and it may also be lathed with carbide tooling.
Precision shafting fits so closely in ball bearings that any nicks (like those caused by set screws) will prevent it from passing easily, or at all - this situation calls for messy remedial filing. So be carefull not to damage that surface. Brass-tipped set screws help since they won't dig in.
McMaster carries a few usefull variations of precision shafting which can help create more simple, streamlined designs and save labor:
- tubular shafts
- shafts with male threaded end or ends (photo right)
- shafts with female threaded end or ends
- shafts with keyways
- shafts with dual diameters
- D profile shafts
A "shaft" that doesn't look like a shaft... The upper axis shaft of Orbit 1 is a complex form machined out of an aluminum rod. Its 2 ball bearings are in the background along with their associated shims and retaining rings. It incorporates slip rings, belt pulley, and rod clamp into a single structure, saving weight and size.
A modified (cut to length) hardened precision shaft with male threaded end, and its associated clamping collars and shims
SEATING BALL BEARINGS
A crucial, and sometimes difficult aspect of ball bearing based systems is "seating" the bearings in the surrounding structure.
Ball bearings must be held with excellent alignment and stability, which implies a very accurate cylindrical hole of equal diameter (plus or minus a few tenths of mils) to the bearings OD, which may be quite large.
If the hole is made very slightly too small, the bearing can be pressed in, an excellent holding method, but requiring extremely good diameter accuracy.
If the hole is made slightly too big, the bearing may be adhesive bonded in, an acceptable and pretty easy approach, or perhaps allowed to be loose if the hole is very close to the correct diameter.
In some cases, a hole with relatively lax tolerance can be made in a clamping structure which "grabs" the bearing as it's tightened.
So how exactly does one make accurate diameter holes in structures to hold ball bearings? These are the basic methods:
REAMING. For holes roughly 1/2" diameter or less, reamers can be used. Reamers are amazing - they are easy to use and can "fine tune" holes to within .0001". You just drill a little under, then use the correct reamer to finish the hole. Say you want a press fit a 1/2" OD ball bearing into a plate... you simply drill a .484" hole on the drill press or mill, then ream to .4995". The ball bearing will then be easy to press in with a small arbor press, and it will stay put. Of course you must buy the .4995 reamer to do it this way ($30-$70). If you want the ball bearing to insert by hand, a .5000 reamer would probably do the trick, and you'd need to buy that reamer also. If you deal with lots of accurate hole diameters, you will build up a collection of expensive reamers.
In principle, you could use the reamer method on large (say 2" diameter) holes. But these big reamers are hundreds of dollars each and difficult to hold, so it's usually not done in the small shop. There are better ways...
BORING. Boring is a method of producing fairly accurate cylindrical holes with a single point cutting tool. It can be done on the mill or the lathe, and multi-inch diameters can be easily made. I find it difficult to do better than 1/2 mill (.0005") diameter accuracy on my low-end machine tools, and it's easy to make a mistake and go oversize, so attempting to achieve a press fit is risky... unlike with computers, you can't "undo" boring. Reliably achieving 1-2 mils oversize, however, is quite easy. And then you can adhesive mount the bearing. I was initially skeptical about this technique, but professionals do it all the time with Loctite retaining compound (the green stuff), and I've had zero issues with it.
CNC MILLING. Even though CNC mills are working X and Y axes independently, they can cut out reasonably good circles. Some machines (and machine setups with boring heads) will be accurate enough to achieve press fits without distorting ball bearings, but on any CNC milling machine can cut a circular pocket with +1 or +2 mil diameter tolerance allowing seating of a snap ring bearing loose or with adhesive. Do some test cuts on scrap first to size up the situation.
CLAMPING. Clamping structures are pretty nice... you turn an allen key and a hole "shrinks" by up to several mils, grabbing a ball bearing (or any other cylinder) firmly. Turn the allen key the other way and release it for position adjustment or disassembly. Clamping structures are not so hard to make, and the required bore tolerance is lax since there is ample adjustability. They grip evenly and strongly.
FREE STYLE. Epoxy putty hardens into a very strong plastic-like material and can be easily shaped by hand to engage ball bearings. If the goofy looks and compromise in strength can be tolerated, otherwise difficult forms can be very rapidly created.
Hand reamers, straight and spiral types
Clamping structures holding ball bearings
Using a boring head on a milling machine to create an accurate hole.
Adhesive mounting of a ball bearing in an oversized seat with Loctite retaining compound
A PRACTICAL SINGLE AXIS BALL BEARING SYSTEM
A practical single axis ball bearing based system... This is the Transmutascope mechanical core, shown in both assembled and "exploded" states. The spindle of the machine is supported by a pair of R8 ball bearings (1/2" bore, 1 1/8" OD) which are Loctite bonded into bored holes in 1/4" thick aluminum "decks". The upper and lower decks are stood-off by quartets of aluminum spacers.
The axial position of the shaft is locked by a pair of clamping collars which are isolated from the bearing outer races by shims. Rubber O-rings shock mount the shaft.
The shaft carries a puck-like clamping structure which holds a large platter. A large special nut locks the platter down.