Research On High Efficiency, Heavy-Load, Steel Backing Carbon Fabric/Epoxy Composite Screw Pair

Heavy-duty screw-nuts are the key components applied widely in forming equipments with the advantages of simple structure and high reinforcement ratio. In the other hand, their great friction loss and low transmission efficiency severely restrict the further improvement of the forming equipment performances. Ac-servo driving is the new developing trend for modern forming equipments. Heavy-duty ball screws are generally used at present. They have become a bottleneck of the new forming equipment development due to their low bearing capacity and high cost. The new development of forming equipments demands urgently for a new screw-transmission manner with high efficiency and bearing capacity, favorable dynamic performance, simple structure and low cost.Currently, there are two different developing aspects on screw-nut material research. One is developing new types of metal nut materials and screw surface modification, the other is development of polymer solid nuts. This paper presented a new approach for high efficiency and heavy-duty screw transmission, named steel backing carbon fabric/epoxy (CF/EP) composite screw-nut. This new type of screw-nuts is characterized by steel base, lining material on female thread surface layer with polymer self-lubrication composite. The spiral surface layer was shaped by molding instead of traditional machining. The oil groove was also implemented at the same time in molding. This type of screw-nuts has excellent anti-friction and wear-resistant performances as well as high bearing capacity. By means of load distribution analysis and nut contour structure optimization, the loads on each meshing threads would be well-distributed. Thus actual working stresses would be decreased and the screw-nut working performances could be improved further.Through researching on the influences of composite fabrication methods, material selection, cohesive material batch formula on friction and wear properties, a steady low friction coefficient and high wear resistant CF/EP composite batch formula and the fabrication method have been established. It provided a base for implementation of high efficiency, heavy-duty and precision sliding screw-nut. Research on fabrication methods indicated that semi-dry fabrication method tended to achieve steady friction coefficient-time behavior. Graphite and molybdenum disulphide added in the cohesive material enhanced the composite anti-friction property effectively. The lowest steady mean friction coefficient (0.14) appeared when composite was modified by 30 wt.% lubrication component with the weight ratio 1.5:1 of graphite and molybdenum disulphide. Wear loss of this composite was only 25.6% of pure CF/EP composite. Analysis on the worn surfaces of composites and their counterparts with a scanning electron microscopy (SEM) equipped with an energy-dispersive X-ray analyzer (EDS) attachment indicated that adhesion was the principal wear mechanism for the composite without lubrication components. Molybdenum disulphide made the fatigue wear be the main wear mechanism through suppressing the resin transferring to the steel counterpart working surface. Wear debris generated by the fatigue wear distributed evenly on the wear surface, contributing to the anti-friction function.The composite friction and wear property under oil lubrication was investigated. Experiments indicated that the friction coefficient of optimum composite under oil lubrication decreased to 1/3～2/3 of under dry friction. Oil grooves on the composite friction surface could improve the lubrication effect greatly. The friction coefficient of composite with logarithmic spiral oil groove dropped to 0.026, decreased by 30% of the composite without oil grooves. Starting with light-load and high velocity could further decrease the composite friction coefficient under oil lubrication. Micro-analysis on test ring worn surface indicated that, to the composite reinforced by plain carbon fabric, the gap between the carbon fibers and texture provided by the plain fabric caused the oil infiltrate to the friction surface, improving the lubrication effect between the sliding surfaces. To the composite with logarithmic spiral oil grooves, the central area of the cohesive region on the test ring surface disappeared and absolute fluid lubrication features were shown.Split and integral steel base backing CF/EP composite nuts were fabricated with spiral surface fabric laying and duplication molding method. Both self-lubrication material spiral surface with oil grooves were molded integrally. A screw-transmission efficiency test set was developed. The friction property of the copper nut and the composite nut under different conditions were tested and compared. The tests showed that composite nut exhibited excellent friction properties under the heavy-duty condition. Frictional moment of resistance on the composite nut decreased by 53.5% compared with the copper nut. Transmission efficiency increased by 50% compared with steel-copper screw-nut.The spring set model was established to analyze the load distribution on the threads. Several schemes of uniforming load distribution were presented and finite element simulation was carried out. The results showed that the nut with the optimized stepped or combination of tapered and cylinder contours could effectively make the thread loads well-distributed. Compared with the constant cross-section nut, Axial load distribution percentage mean square deviation decreased by 81.5%, maximum axial load on single thread decreased by32.2%, and the maximum thread load was two times of the minimum thread load. Thus, nut bearing capacity and the service life could be improved.This research has opened a new approach for high efficiency, heavy-duty and precision screw transmission technology. The development of new type sliding screw-nut can highly increase equipment efficiency, lower energy consumption, improve working reliability and extend service life. Not only will it promote the development of servo presses, but also possess the revolutionary significance to all the mechanical equipments with screw-transmission.