What
makes a good skate bearing?
Abec rating? Lubrication? Seals? Cage/retainer? The
answer is all of these. 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.
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.
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, with an Abec5 bearing,
it can as small as 7.995mm. For Abec7 its 7.996mm so not much difference!
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). As for
Abec9, there very few manufacturers in the world capable of making
genuine Abec9 bearings and they are very, very expensive!
Speed
A standard skate bearing iis rated
up to about 24,000 rpm. This is the basic speed rating less 40 percent
as the bearing outer ring is rotating. Ours are rated at 49,000
rpm or 59,000 rpm for the ceramic bearings. However, with 75mm diameter
wheels, you would spin the bearings at less than 7000 rpm at 60
mph (96 kph) so you will never even approach the maximum speed of
a standard bearing.
Torque and drag
Bearing torque relates to the amount
of force required to rotate a bearing from rest and keep it running.
With skate bearings, this turning force is your weight and force
acting on the skate causing it to roll. 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 UNDER
LOAD will not spin as freely because
it is not as round. With oil lubrication, you can spin it for longer
by hand as there very little load but this does not replicate skate
conditions which is why the finger spinning test is misleading.
A bearing with good roundness and surface finish on the balls and
raceways requires less effort to rotate under load.
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.
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, after testing, 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. A lack of effective
lubrication greatly increases wear. The most effective lubrication
for a sealed or shielded bearing is grease. Oil is used in bearings
where rotation is very slow as it does not stay in place for long
at speed. Grease normally creates more drag than oil but only very
slightly more UNDER LOAD
if the grease is low viscosity. The difference is even less once
the bearing has been running for a short time as the grease warms
up and gets softer.Grease can also help to seal out dirt.
Our chrome steel 608 type skate bearings contain
is a high speed, low torque grease manufactured by Klueber who make
lubricants for some of the most demanding applications in industry
and aerospace. 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. The amount
of grease in our skate bearings is carefully chosen to provide excellent
lubrication but minimal drag. We offer tubes of grease for those
who want to clean and re-grease their own bearings.
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.
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, you can perform regular bearing maintenance to remove dirt
from inside the bearing and replace any contaminated grease. REMEMBER
- dirt in the bearing usually means damage to the surface of the
balls and raceways. Cleaning is unlikely to restore the bearing
to its original state but it will prevent further damage (until
more dirt gets into the bearing).
Checking for wear
Its not difficult to to check your skate bearings for wear.
Over a period of usage, wear will cause flaking 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. 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 but always
follow the safety instructions when using any such product. Agitating
or spinning the bearing in the solvent will help speed up the process.
The bearing will spin freely if its clean. 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 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.
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 15 - 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 on to the raceways (these are the grooves
or tracks inside the inner and outer rings that the balls run in).
Once the greasing is done, rotate the bearing several times by hand
to spread the grease around the raceways.
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). |
|
