Ball bearings and energy saving
Today there is a growing need for products that have less impact on the environment, particularly with regard to energy saving. Laurent Frésard of Jesa looks at what the role ball bearings can play, and finds that, since they are used on such a large scale in so many different applications, the potential global energy savings are considerable.
Rolling bearings transmit a rotary motion between two bodies with little friction. This is thanks to the ball bearing which performs with minimal relative sliding upon surfaces. The friction, heat and wear is limited compared to other rotation guidance solutions, while also ensuring a high positioning accuracy and low noise. Despite this, certain factors generate friction which translates into a dissipation of energy and heat generation adverse to high performance of the final product.
Standard ball bearings generally offer the best possible capacity for a given dimension. They are suitable for a number of applications, but they represent a compromise which is not optimal for precision applications due to friction which impairs the proper functioning of the application.
The main cause of friction in a sealed ball bearing is generally friction within the joints. This type of seal is often unavoidable since exposure to the industrial environment would quickly impair their proper functioning. Joints generally consist of a thin sheet of coated synthetic elastomer type NBR, HNBR or FKM, selected according to the operating temperature. One or two flexible lips are in radial or axial support on the opposite ring to ensure an air tight seal.
Research into friction and pre-stressing optimisation between the seal can highlight the best possible solution to increase the yield. One can reach up to a 50% reduction in friction thanks to the use of seals with an improved design without diminishing the seal. A special surface treatment is necessary to reduce the coefficient of friction of the material and thus reduce the friction.
Another area of optimisation is the roughness of the metallic surface in contact with the bearings. When there is sufficient space, it is sometimes possible to replace seals and joints that are not making proper contact, which allows for a significantly reduced energy loss by ensuring the required tightness.
When it comes to lubricants, the primary role is to prevent metal-on-metal contact of the rings and balls, by separating the surfaces with a thin film. The asperities of the surfaces are thus excluded and do not come into collision, which allows for significant improvement and a longer service life of the product. The most common lubrication is lifetime lubrication with grease which offers ease of implementation. However, lubrication with circulating oil is sometimes necessary.
The choice of the viscosity and the type of base oil is critical to ensure optimal lubricant film thickness. Too high a viscosity will cause unnecessary friction and temperature increase, while a low viscosity will cause friction of mixed types and limited collision of the asperities of the surfaces which results in a severe reduction in energy and life expectancy. There is no universal grease suitable for all applications. A grease measured accordingly, depending on the application, will optimise the lubricant film thickness and thus reduce the friction as well as extend the life expectancy.
A part of the friction is created by mixing lubricant with the ball bearings and cage - it is during this time that grease is most heavily distributed upon the bearings. The geometry of the cage, the ball size, and the amount of balls influence the mix. The type of NLGI class soap thickener used in the grease, as well as the initial volume of lubricant also have an impact on the friction. The application will also be decisive when determining these separate parameters.
Internal geometry and friction
Because the materials used are not rigid and contact pressures are very high, there are elastic deformations of balls and tracks in the areas of contact. This deformation causes two principle types of friction:
a. Hysteresis losses - when a ball rolls on a surface, it forms a bead of material at the front of the point of contact, which should be deformed elastically so rotation can continue. Energy consumption is partially recovered by the material as it returns to its position at the rear point of contact, but not entirely. The energy difference corresponds to a loss through dissipation.
b. Micro-slipping - the area of contact between the balls and ball tracks has an elliptical shape more or less elongated depending on the radii of curvature selected for the ball in comparison to the diameter of the ball tracks. A relative slipping of surfaces is caused by the elasticity of the material, and only two points of the ellipse are rolling without slipping, in the case of a deep groove bearing radially loaded.
Standard balls generally are created to maximise surface areas of contact between balls and rings, to reduce surface pressures and thus increase the carrying capacity of the bearing. Optimised performance is unfortunately impeded by friction that increases with the contact surface. Thanks to advanced calculation tools, manufacturers are able to determine the optimal internal geometry to minimise friction, guaranteeing the lifetime performance.
The use of a cage allows limited friction by keeping the balls from rolling against each other, as is the case in a full ball bearing. The cage itself is in contact with the ball and sometimes also with the inner or outer ring, which causes energy loss as well as slipping by shearing off lubricant. The quality of manufacture of the cage, its geometry, as well as the manner in which it is manufactured, have an influence on the friction generated.
A range of engineering polymers and a low coefficient of friction are combined by Jesa for the development of cages with low friction, which allows for reduced energy loss compared to a conventional steel cage, while providing better resistance to vibration.
By optimising each element of the design with regard to friction, it is possible to reduce bearing friction by an order of two or even three. As one multiplies this saving though the number of bearings per machine, significant energy savings can be achieved on the final product. The impact of the machine on the environment is reduced and lower operating costs are obtained.
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