ABOUT THE BEARINGS…
After many years of specialising in miniature bearings, thin-section bearings and corrosion resistant bearings NPB® Bearings now offer custom designed skate bearings (part numbers below) for roller skates, skateboards, inline skates and Heelys.
NPB® Bearings have used our extensive experience and knowledge of miniature bearings to produce four types of skate bearing to satisfy the most demanding skate and skateboard specifications. Our two most important design criteria were high performance and value for money. The only difference between our various specialized skate bearings are the material of the bearing rings and the ball material. Read the explanation below the part number table for more information on bearing technology.
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.
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!
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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).