Developments with a bearing on motion

Mark Fletcher delves into the world of bearings and takes a look at some of the most recent advances and product offerings

Tolerances created during the manufacture of bearings can play a crucial role in their long-term operational life. It is for this reason that research has been undertaken, relating to the machining of bearing components, to try to give them as long a life span as possible. One technology being investigated by SKF is ‘ultra high pressure jet assisted turning’. This process, as its name suggests, uses a small jet of high pressure fluid entrained onto the point of cutting as opposed to flooding the entire work piece with coolant. Numerous advantages stem from this approach – the main one being that tool temperature decreases with the pressure of the cooling jet. With conventional coolant-based turning on stainless steel (SS 2541-03) to 3mm at a feed rate of 0.3mm/rev the temperature of the tool can sit between 600 and 700 degrees C. With ultra high pressure jet assisted turning, at 25Mpa, the temperature falls to around 400 degrees C and at 300Mpa it falls further to just over 300 degrees C. There is also a commensurate improvement in tool life with SKF offering figures of up to 50%. The temperature measurements are taken with a gold/platinum thermocouple 0.5mm below the edge of the clearance face of the tool. With regard to the work piece, surface roughness and chip breaking is improved creating a finish which is visibly different to conventional methods. As well as offering higher cutting speeds the method also uses significantly less coolant, with a reduction of up to 90% claimed by the company. SKF says that even though the trend in manufacturing is towards dry machining, pressure machining is still a welcome new technology because many modern materials can’t be dry machined due to the high temperatures in the cutting zone which significantly reduce tool life and the capability of the process. Solid technology Bearing design has also moved on in leaps and bounds. NSK has continued the development of its Spacea series of bearings with the introduction of two new types for vacuum environments. The bearings, which use innovative solid lubricant technology, can be used at pressures down to 10-10 Pa and in temperatures up to 350 degrees C. Both are available with self lubricating cages with spacer joints expanding their performance envelopes. One incorporates an MoS2 (molybdenum disulphide) cage which, according to the company, extends operating life in demanding vacuum applications by as much as ten times compared to conventional bearings. Applications are seen in robotics, vacuum pumps, sputtering equipment, solar cell manufacture, liquid crystal display and semiconductor manufacturing. The bearings exhibit inner and outer races and balls made from high performance martensitic stainless steel with protective shields in austenitic stainless steel. MoS2 is preferred for the cage not only because of its lubrication performance but also because of its low out gassing – a feature important for vacuum applications. Plain specification As companies expand their offerings the shear range of bearings becomes immense. This can make some ranges difficult to specify so most companies offer a great deal of “hand holding”, helping designers arrive at the optimum selection for their application. With this in mind, INA has recently released a new 70-page design guide which covers “one of the world’s most extensive ranges of plain bearings”. The guide gives the designer a superb insight in to all of the factors that must be considered prior to plain bearing selection. It starts with four pages of conversions factors and symbols and then gives designers an eight page foray into the murky world of bearing life calculations. Designs of bearing arrangements is closely followed by details of clearances and tolerances before leading the reader neatly into the pressing in of bushes. Substantial coverage of the Permaglide material completes the introduction, all of which takes place before the product range is even mentioned. INA then elaborates on its range before highlighting some of the special designs available. This is an eminently useful publication. As the first half of the book is non-product specific it would make a valuable addition to the reference section of any designer’s book case. Linear bearings are also seeing advances in development as they face the same stringent demands as their rotary counterparts. One particular development which caught our attention was the development of a range of linear bearings developed to run on carbon fibre shafts. Carbon fibre seems to be developing into the material of choice for many applications as engineers seek to lose weight without any strength and performance penalties. The bearings in question, the DryLin R range from Igus, can run on almost any shaft material. One of the most recent developments has been the company’s lightweight, hard anodised aluminium shaft range. This has been taken one step further with the development of the carbon fibre shaft range. According to Igus, weight savings of over 90% can be achieved when compared to conventional steel solutions, however the wear and friction characteristics remain the same. The shafts by their very nature are resistant to corrosion and exhibit excellent chemical resistance. The company also tells us that the expected lifetime of the DryLin bearings, in combination with carbon fibre, can be calculated precisely – giving designers confidence when selecting bearing solutions. With a customer list which reads like a Who’s Who of high technology applications, FAG/Barden has developed the X-Life Ultra, which it claims is “a new design philosophy for precision bearings”. Made from Cronidur 30, a patented high-nitrogen steel, the X-Life bearings offer 100 times the corrosion resistance of conventional stainless steels, they also claim a fatigue life 10 times longer than conventional hybrid bearings. Their design marries a number of important factors: specialist bearing design, advanced materials, superior surface engineering technology, extreme tolerances (P4S), advanced lubrication and unique non-contact sealing.And the bearings offer better load carrying characteristics and higher speeds.