(Letter to the editor, Model Engineer ME 4249)
I have been a bearing trader, handling OEM, Import & Export sales, since 1986. About four years ago, we also started selling machine tools to model engineers, as a result of a decline in bearing sales to our OEM customers who were shifting their bases to the Far East, or simply closing due to competition. It was therefore interesting to read Mr. Johnson’s and Mr.Read’s comments in ME4237 on the use of needle roller bearings. On the issue of using drawn cup needle roller bearings, I would agree with many of Mr. Read’s comments, if only to add the following: It is true that drawn cup needle roller bearings may not be suitable for all applications such as Mr. Read’s experience in industry. However, they have been successfully used in model locomotive applications as demonstrated in Neville Evans articles. We sell many for precisely this purpose. One of the key issues with drawn cup needle roller bearings is that they gain their support from their housing. So, you do need a press fit – no Loctite type products. If for example, you were to put a correct size diameter shaft through the bore of the bearing before you fit the bearing into the housing, it will probably be a very loose fit. However, you have to make the housing about 0.0005in” smaller for cast iron/steel and up to 0.001in” smaller for softer materials such as brass, than the bearing manufacturers actual outside diameter specification of the bearing, and then press fit the bearing in the housing. These are not to be used as exact measures, but more as a general observation and guidance. Once the bearing is in the housing, you will find that the shaft does fit properly, as the needle rollers get compressed. This procedure is especially important with drawn cup needle roller clutch bearings used in lubricator drives, to avoid slippage over a period of time. For locomotive axel use, Mr. Read’s comments for rubber sealed drawn cup needle roller bearings to retain grease is true, and these bearings are available with seals, in many sizes, as shown in our catalogue and on our website (which will be going live shortly). If there are any particular sizes you have in mind and which are not covered in our catalogue or website, do ask us or your local bearing dealer. The seals also keep a lot of the dirt out. Although the grease lubrication in sealed units is quite adequate for most of its life, those wishing oil lubrication can follow the example shown in Peter King’s letter in ME4241.
We stock Torrington (now part of Timken) drawn cup needle roller bearings. They are made mainly in USA, France, and Germany. It is true that needle roller bearings are also made in China. However, most of their production for drawn cup needle roller bearings by reputable well know brand manufacturers is consumed within China and Asia, and at present, very little filters through to the West. We still export Torrington needle roller bearings as well as Super Precision Angular Contact ball bearings in large volumes to Dubai, India and Singapore – key export markets for bearings.
I now refer to Mr.Barrett’s comments in ME4238 and Mr.McNee’s comments in ME4241 on the subject of chattering in Chinese mini-lathes. As you may be aware, we sell the C3 mini-lathe, which is of the same family. The issue is raised for chattering due to deep-groove ball bearings being used in these lathes. The solutions are clear as suggested in both these letters. However, this is not prevalent in all lathes of this family. Deep groove ball bearings size 6206 are used by manufacturers of these lathes as a result of lower cost. These are cheaper to use then 30206 – Taper Roller Bearings which were originally used in this family of lathes when they first came out. Till today, the casting of the headstock of these mini-lathes are designed to accept the wider taper roller bearings. You may just have to add a spacer or skim a spacer on the shaft before fitting the taper roller bearings, depending on which factory the mini-lathe came out of. At last count, there are about five factories making this lathe, which I know of in China. It does not mean that the Chinese 6206 bearings are inferior. We have been buying Chinese bearings for nearly 18 years and exporting spherical roller bearings directly from China to Russian made Steel Rolling Mills in East India, through to supplying deep groove ball bearings for OEM applications in the U.K.
The issue of chattering with the deep groove ball bearing 6206 results from where the bearings were purchased from by the lathe manufacturer. One of the best bearing factories in China is based in an interior region, near the Vietnamese border. They are furnished with Russian and Japanese production machinery, as they also produce for a Japanese bearing manufacturer. They have an inspection procedure during which the bearings are checked for clearance, and separated accordingly for specific applications. “Standard” clearance of a deep groove ball bearing raceway is measured as C0. A “looser fit” is C3, and even looser is C4. Exception is C2 which is a tighter then normal C0 fit. Greater the clearance = looser the fit of ball race on the raceway = higher the play in the bearing. However, cost of bearings from this factory are about 30% to 50% higher then rest of China. Other factories have an inspection policy in varying degrees. As a result, for cost reasons, depending on where the lathe manufacturer purchased the bearings from (usually they have no clear idea of where the bearings come from as they buy through dealers due to limited quantity), there is no clear certainty of what will be the clearance for the bearings they fit in the lathe. So you could easily have C0 or C3 clearance bearings or combinations in the lathe. Little bit like Russian Roulette!. This is also clearly not 100% apparent when you first use the lathe. It could become clearer in the fullness of time. This also applies to our C3 mini-lathe, and ALL the same family lathes around the world regardless of which factory has made them. All these lathes are built to a price!. I am aware of many customers who have deep groove 6206 ball-raced mini-lathes and have no problem with chattering. At the same time, there is an equal number who buy 30206’s in pairs from me to cure chattering, and/or before putting a bigger chuck on the lathe, and/or to get a higher precision result. All I would say is see how you get on with your mini-lathe with the existing deep groove ball bearings 6206 fitted, as they are designed for “general” hobby use. If you want to have higher precision and/or if you wish to take on cleaner shaping work using a radius turning attachment like the ones we sell, to make cleaner concave/convex shapes (example for making spinner nuts for model aircraft i.c.engines), then consider changing the ball raced bearings to taper roller bearings.
Whilst writing, I would suggest that C3 & C4 clearance ball raced bearings are ideal for the “Big end” of four-stroke model aircraft engines, as they run better at higher temperatures.
In the coming months, on our website, we may show with pictures and comments, how taper roller bearings can be fitted to mini-lathes, and also show how needle roller thrust bearings can be used to remove backlash on cross travel of lathes. Our website is www.arceurotrade.co.uk
There is only one request I would make of people writing up their projects in various articles. Where ever they refer to bearing sizes used for their project, please do write the dimensions in the format: inside diameter x outside diameter x width and state if the bearing is open, sealed (rubber seals) or shielded (metal shields) on one side or both sides, and state the internationally recognized bearing number. If you don’t know, then ask us or a bearing dealer. Most bearings are classified in an internationally recognized form. This information helps others who intend to tackle the project, when they go to their bearing dealers or if they contact us.
Ketan Swali – Arc Euro Trade Ltd. 29-6-2005