What
makes a good skate bearing?
Abec rating? Lubrication? Seals? Cage/retainer? The
answer is all of these. The problem is that some retailers will
throw a lot of jargon at you to convince you that their bearings
are superior. You have to decide what is relevant but, not being
a bearing expert, how are you supposed to know? As experts, we will
try to explain in plain simple language in the hope that you will
go away with a better understanding of what to look for. For more
details see our technical
pages.
Material:
Rings: SAE52100 chrome steel is the most commonly used steel for
bearings because it is hard wearing. It will corrode so if you skate
in the wet, clean the bearings after use to avoid rust (see the
section on cleaning below). 440 grade stainless steel is slightly
softer but will still last a long time as, even if you are very
heavy, the bearings are designed to take much greater loads than
you. We do offer stainless steel as an option for those who like
the rain and it should not corrode unless subjected to salt water/salt
spray or certain aggressive chemicals. See Technical
Data - Bearing Material).
Balls: Chrome and stainless steel balls will take high loads and
speeds and are absolutely fine for skaters. Silicon nitride (ceramic)
balls are much lighter than steel balls and about twice as hard.
These factors reduce running temperature and minimise wear meaning
longer bearing life. The extra hardness of ceramic balls means they
are less likely to be damaged if debris gets into the bearing. If
used with a small amount of low torque grease, bearing life can
be significantly extended. These hybrid bearings (bearings with
ceramic balls) are capable of higher speeds too but higher speeds
are not necessary for skate use. As they are lighter and generate
less friction, they require fractionally less effort to accelerate
and maintain speed. However, this will not be a noticeable difference
compared with, for example, using a stiff grease in your bearings.
See more details on hybrid bearings at the Cerbec
website.
ISO/Abec rating (tolerances/precision grade)
ISO ratings are expressed in millimetres whereas Abec tolerances
are measured in inches (see Technical
Data - Tolerances). These standards control the dimensions of
the bearing such as internal and external diameters and width.
For example, the inner ring diameter (bore
size) of a 608 bearinng is 8mm. If the bearing is P0 (Abec1), the
inner diameter can be no bigger than 8mm but it can as small as 7.992mm.
The smallest for P6 (Abec3) is 7.993mm, for P5 (Abec5) its 7.995mm
and for P4 (Abec7) its 7.996mm. The ISO/Abec rating also controls
the roundness of the bearing by limiting the amount of variation allowed
on each inner or outer ring. The ISO/Abec rating DOES NOT control
the looseness of the bearing. That is down to "Clearance"
(see below).To be honest, ISO/Abec ratings are way over-hyped in the
skate world. The difference in inner ring tolerance between P0 (Abec1)
and P4 (Abec7) is a matter of 4 microns or 0.004mm and none of the
other components on the skate/skateboard would be made to such a standard.
There are a lot of much higher precison applications in industry where
P5 or P4 bearings would not be considered necessary. As for Abec9,
there very few manufacturers in the world capable of making genuine
Abec9 bearings and they are very, very expensive! An improvement in
smoothness can be achieved by selecting a low noise bearing (see "Noise
Rating" below). However, many skaters seem convinced that
the ISO/Abec rating gives a certain peace of mind so our skate bearings
are genuine P5. There are factories out there who will stamp Abec1
bearings with "Abec5" and others who make bearings with
high tolerances on the inner and outer diameters but won't worry about
smoothness of the raceways (grooves inside the inner and outer rings
that the balls run in) or the roundness of the rings so making the
whole thing a waste of time (and your money). Speed
A standard bearing is rated up to a certain speed (according to
the size). This can be increased by use of a high speed cage, a
high speed lubricant, ceramic balls or, very slightly, by a higher
precision grade. None of this is relevant here as you will never
even approach the maximum speed of a standard bearing!! As an extreme
skater, you may occasionally approach 2500 to 3000 rpm whereas the
bearing will manage well over 30,000 rpm with a standard cage and
over 60,000 rpm with a high speed cage. There's a lot of talk about
fast bearings. They are all fast. Its about how freely they spin
so what is much more relevant is torque or drag (see below).
Torque and drag
Bearing torque relates to the amount of force required to rotate
a bearing from rest (starting torque) and to keep it running (running
torque). With skate bearings, this turning force is your weight
acting on the skate causing it to roll downhill or you pushing off
on your board to accelerate. Starting and running torque figures
for a bearing are influenced by a number of factors such as raceway
roundness and smoothness; seals; lubricant and cage.
Obviously, a bearing with a very stiff grease
and tight seals will be harder to turn due to lubricant drag and seal
drag and will not allow your skates to roll freely. Less obviously,
a cheap bearing will not spin as freely because it is not as round
although you would need a bearing torque tester as the difference
is too slight to feel by hand. A bearing with good roundness and surface
finish on the balls and raceways requires less effort to rotate.
The choice of seal is important. Non contact seals
do not drag on the inner ring as contact seals do so do not increase
bearing torque. As for lubrication, the difference in drag between
low torque grease and a light oil is very small. Skate
bearings do not need to be oiled to run freely with you onboard.
You may notice the difference when you spin the bearing with your
finger. This is because lubricant drag with very low load, turning
force and speed is much more noticable. However, when you're actually
skating with the right bearing, there are only a few grammes/cm
difference in frictional
torque. The difference is even less once the bearing has been
running for a short time as the grease warms up and gets softer.
See the section below on lubrication for more on this.
Cage/retainer
A good all purpose steel cage is fine for skate bearings. We use
a high speed nylon cage simply because, although it does have better
sliding characteristics and slightly better low noise performance,
it is more forgiving if the bearing is contaminated with dirt or
being used with inadequate lubrication which may be important if
your bearings are overdue for cleaning and relubrication. The stainless
bearings have a stainless steel cage.
Seals
Bearings can be supplied with metal shields to keep out larger
particles of dirt or rubber seals (sometimes called contact seals)
which will stop dust too.
Metal shields will allow small particles of
dirt into the bearing. Contact seals rub on the inner ring of the
bearing to create an effective seal but greatly increase bearing drag
which is not great for skate use. A better solution is the non contact
seal. This design gives better protection than shields but without
the drag caused by normal seals. Finally, as they are only a push
fit in the outer ring of the bearing, non contact seals can be easily
removed and re-fitted when you clean and relubricate the bearing.
IMPORTANT - the 688 micro skate bearing has one seal. When the bearings
are fitted, the seal should always be on the outside of the wheel.
Noise Rating
Bearing rings and balls are not perfectly round and the balls
and raceways, even after extensive fine grinding and polishing,
are not perfectly smooth. These machining/grinding imperfections
cause extra vibration. Our skate bearings are EMQ grade which means
they had additional noise testing after production and only the
lower noise P5 bearings were selected. These have better roundness
and smoother raceways which means a smoother running and lower torque
bearing.
Clearance
The internal clearance of the bearing relates to the space between
the balls and the raceways. A bearing is designed to take mainly
radial load (vertical load on skate bearings) and some axial (side)
load. The clearance on our skate bearings is slightly larger than
standard and designed to allow the bearings to take a greater side
load when needed.
Lubrication
Correct lubrication is vital to keep the inner surfaces of the bearing
from getting too hot which causes more rapid wear. The most effective
lubrication for a sealed or shielded bearing is grease. Oil is often
used in bearings but normally where rotation is very slow or where
it possible to continuously lubricate the bearing with, say, an
oil jet or spray as oil does not stay in place for long. Grease
is simply an oil mixed with a thickener to keep it in place. It
normally creates more drag than oil but only slightly more if the
grease is low torque (free-running) and the amount used is quite
small. Low torque greases are based on low viscosity oils and often
work well in low temperatures as they do not stiffen up too much
in the way a high viscosity grease would. Grease will also help
to seal out dirt.
The lubricant in our chrome steel 608 type skate
bearings is manufactured by Klueber who make a range of high grade
lubricants for some of the most demanding applications in industry
and aerospace. "LDS18 Special A" is a high speed, low
torque grease. The grease in our stainless steel 608 type and our
688 type mini skate bearings is also a high speed, low torque grease
made by Kyodo Yushi of Japan. They both contain hard wearing lithium
which is more than tough enough for skate conditions. They possess
good water resistance, useful for skating in wet conditions (stainless
steel bearings recommended). A standard amount of grease for a ball
bearing is about 30 percent of the internal space of the bearing
but we have reduced this to 20 percent. This means minimal drag
on the bearing while giving very effective lubrication. We offer
tubes of LDS18 grease for those who want to clean and re-grease
their own bearings.
Light oils are used for bearings in sensitive instruments where
the force acting on the bearing is likely to be very small and sensitivity
to tiny movements is vital. Oil is not normally recommended in skate
bearings. However, if you race and a split second makes a difference
to your performance, a couple of drops of synthetic instrument oil
in a bearing instead of grease will give very little resistance
but we are talking small reductions in rolling resistance over our
low torque grease and the bearings will need replacing more often
than the greased type. If this is your type of skating, we can provide
the bearings with instrument oil on request. We can also supply
synthetic instrument oil in small bottles for relubrication. The
average skater will find that although the wheel will spin for longer
by hand, there will be no noticeable performance difference when
skating.
Some skaters use bearings "dry" (no
lubrication). This will (only marginally) give the lowest resistance
but there is a greatly increased risk of corrosion with a chrome
steel bearing as it will rust. The exterior surfaces are protected
by oil when packed at the factory. The grease or oil lubrication
takes care of the interior surfaces. A dry chrome steel bearing
can rust from a small amount of sweat on your fingers after handling
the bearing so if you insist on running them dry, you might consider
wiping some protective oil on the exterior surfaces but take care
to use it sparingly so as not to get any inside the bearing.
Learn more about Klueber at www.klueber.com
or Kyodo Yushi at www.kyodoyushi.co.jp/eng/
Packaging
Bearings are precision engineered components and secure packaging
is important to keep them from being damaged in transit and prevent
contamination. It should also be cost effective. You should pay
for the bearings, not the packaging so we do not supply our bearings
in fancy tins or supply free stickers.
Cleaning and Regreasing
In order to make bearings last longer, they
should be kept clean. In skate conditions the bearings are often
subjected to dirt and dust so, to save yourself money, perform regular
bearing maintenance to remove dirt from inside the bearing and replace
any contaminated grease.
Checking for wear:
Its not difficult to to check your skate bearings for wear. Over
a period of usage, wear will cause flaking or spalling of the raceways
and fine particles of metal will break away. This causes increased
roughness in the bearing. At this point the wear will accelerate
as the metal debris scores and dents the balls and raceways. This
process is greatly speeded up by any dirt that has worked its way
into the bearing. If you hold the bearing's inner ring between a
thumb and finger and then turn the outer ring with your other hand,
you will be able to feel a roughness or grittiness if the bearing
is worn. OK, this may be due to dirt in the bearing so if the roughness
is still there after cleaning, the bearing is due for replacement.
Worn bearings often feel rougher after cleaning because grease has
a dampening effect.
Seal removal:
As mentioned in the section on seals, the non contact seal can be
easily removed with a thin blade such as a Stanley knife, a trimming
knife, a small screwdriver etc. CAREFULLY slide the tip of the blade
between the seal and the inner ring taking care not to damage the
seal. Push the tip just under the seal (but not too far as you don't
want to catch the bearing cage!!) then gently prise the seal out.
The outer lip of the seal sits in a groove in the outer ring of
the bearing.
Lubricant removal:
It is important to clean the old grease (or oil) out as this may
contain particles of dirt or grit which have an abrasive effect
on the ball and raceways. The grease can be removed by soaking in
a solvent such as white spirit. Agitating or spinning the bearing
in the solvent will help speed up the process. You may need to dip
the bearings and re-spin a few times to remove all the old grease.
The bearing will spin freely if its clean. It will save you money
if you filter the solvent through a lint free cloth to remove larger
dirt particles and grease solids during the cleaning process rather
than using fresh solvent as soon as it gets dirty. You can spin
the bearing to remove most of the solvent and place on a clean surface
to dry. If using a flammable cleaner, make sure you keep well away
from any flames or other possible sources of ignition and UNDER
NO CIRCUMSTANCES should you ever use petrol/benzine/gas (depending
on the country you live in) due to the risk of fire or explosion.
You must always allow the solvent to fully dry off before re-lubricating
or the solvent will break down the lubricant.
You can use WD40 to clean the bearing but you should clean the bearing
in solvent before regreasing as the remaining film of WD40 will
stop the grease from properly adhering to the inner surfaces of
the bearing. Bearings can also be cleaned with a water-based degreaser
but they will need a final rinse in water to remove the degreaser.
Care must be taken to remove any remaining water quickly from chrome
steel bearings or they will rust. This can be done by immersing
in oil or spraying thoroughly with WD40. The problem here is that
you should then remove the oil or WD40 before greasing so adding
another process to the cleaning procedure.
Lubricant application:
Without the right equipment, it is difficult to regrease with exactly
the right amount so guesswork is involved here. You want to fill
roughly 20 percent of the internal space of the bearing with grease.
It is better to evenly distribute the grease so work round the bearing
squeezing a small amount between the balls. If you use too much
grease, don't worry. It will make the bearing slightly stiffer and
you can always carefully remove some of the excess. Once the greasing
is done, rotate the bearing several times by hand to spread the
grease around the raceways (these are the grooves or tracks inside
the inner and outer rings that the balls run in). If you choose
to use oil, a few drops will suffice but you should also spin the
bearing after oiling to distribute the oil evenly.
Seal replacement:
The rubber seals can be refitted by pressing the outer lip of the
seal into the narrow groove in the outer ring of the bearing. This
is a snap fit and is done by hand. When you have replaced the seals,
check that the bearing rotates freely enough. If it feels much stiffer,
the seals may not be correctly fitted.
Protecting your bearings:
To protect against corrosion, apply a light oil coating to outer
surfaces of chrome steel bearings (e.g. 3 in 1 or WD40). |
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