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Tech Info - Bearings

Needle roller and ball bearing typesOver a fair number of DIY vehicle builds I've moved towards using needle roller bearings almost exclusively. I prefer these because of their low cost and compact size relative to the shaft sizes they run on - in particular their small outside diameters are very helpful and the fact that several standard needle roller bearing sizes have outside diameters that match available standard wood and metal dill bit sizes. This matters - they have to be housed in snug fitting housings and these are straightforward to drill if the hole size matches the drill bits you have. Aside from size and cost needle rollers used without the vendors inner races will fit easily on standard sizes of bright steel shafting without the need for press or other "forced" fits. This is suitable for our types of DIY leisure vehicles which will not see years of heavy duty. For industrial use they are intended to be pressed into metal housings which causes the roller PCD to compress slightly and so result in a close fit on the shaft. With wooden housings, as used in many of our vehicles, this isn't possible and the shafts run slightly looser than strictly speaking intended - but still quite happily for the light duties seen by our designs.

There are one or two disadvantages though. The rollers run directly on the shaft and if the shaft material is soft it will needle roller bearingscompress and mark under the contact stresses from the loaded rollers - if possible use hardened shafts. The shaft also has to have a good (smooth) surface finish to make life easy for the tiny needle rollers. These types of bearings are not designed to carry any axial loads, so sideways loads from wheels etc have to be supported by other means, usually lubricated plain washers between the wheels and the bearing housing will do the job. If the sideways forces are large and friction arising from reacting them has to be avoided then other types of bearings need to be considered. A further downside of simple needle roller bearings is the lack of protection for the rollers from ingress of dirt or water. The commonly available low cost versions I've come across do not have seals, however a generous dollop of grease will help keep moisture out - if they are to be subjected to wet conditions it may be better to use a sealed bearing type. You also need to make sure they remain lubricated. Their main advantages are low size and low cost!

The least expensive type of rolling element bearing which also has some axial load capacity and can be sealed (to keep lubricant in and deep groove ball bearings with metal shieldswater and dirt out) is the deep groove ball bearing. These are commonly available and are supplied (as with most other types of rolling element bearing) as a unit with both inner and outer races, balls and internal cage (interestingly the cyclists call them "cartridge" bearings, although most of the engineering world simply call them bearings). They are not as compact as needle rollers but do have good load bearing capacity and do not run directly on the shaft - so shaft hardness and surface finish is less important (at least as far as the bearing is concerned). For the same shaft size their outside diameter is much larger than the matching needle roller bearing's so drilling housings can be more problematical. Deep groove ball bearings can be bought either with integral rubber seals or with metal shields. They are more expensive than needle rollers but will react axial loads.

rubber sealed deep groove ball bearingsFor some low speed non friction critical jobs plain (sliding) bearings might be considered. You can get these in both bronze and plastic materials. Bronze types can be bought impregnated with lubricating oil (oilite bearings), several types of plastic bearings can be used "dry" i.e. without external lubrication and can be used as steering column bearings etc. You might be surprised to find though that some plain bearings actually are more expensive than their equivalent sized needle roller bearings.

There are other types of rolling element bearings available (e.g. tapered, spherical, cylindrical etc), but most cost more than the above types and aren't really warranted for light duty DIY applications where cost is such an important issue. Standard size bearings are now an engineering commodity and there are several bearing manufacturers all with excellent and informative web sites where you can read-up if you want more technical detail. Try SKF Bearings here or Timken Bearings here.


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