【正文】 998 hydraulic, pneumatic and seals in the import about 200 million . dollars, Hydraulic which about billion dollars, aerodynamic nearly 030 million . dollars, sealed about 030 million . dollars. pared with a slight decline in 1997. By sums, currently imported products on the domestic market share of about 30%. 1998 pieces of the domestic market demand for hydraulic total of about six million, the total sales of nearly 40 billion。 aerodynamic pieces of the total demand of about 5 million, with sales more than 700 million yuan。 Seals total demand of about billion. total sales of about billion. the development trend of the future affect the development of the main factors (1) product development ability, and the level of technological development and speed can not pletely meet the advanced mainframe products, major equipment and technology imported equipment and maintenance support。 (2) the number of enterprises manufacturing technology, the level of equipment and management standards are paratively backward, coupled with a strong sense of quality, resulting in low levels of product performance, quality, Reliability poor services in a timely manner, lack of user satisfaction and trust of the brandname products。(3) industry specialization of production low, scattered strength, low repeat serious, between regions and enterprises of convergence products, blindly pete with each other, driving down prices, the decline of enterprise returns, lack of funds, liquidity difficulties, product development and technological transformation is inadequate and seriously restricted the industry to improve the overall level of petition and the increase of strength。 (4) The internationalization of the domestic market and the increasing degree of foreign panies have entered the Chinese market and participate in petition with the domestic private and cooperative enterprises, individuals, foreignfunded enterprises, such as the rise of stateowned enterprises due to the growing impact. the development trend As the socialist market economy continues to deepen, hydraulic, pneumatic and sealing products in the market supply and demand and there is a greater change, long ago to a shortage of the seller39。s market has basically bee a structural surplus of the characteristics of the buyer39。s market place . Overall capacity, is already in oversupply situation, in particular the general low level of hydraulic, pneumatic and seals, the general oversupply。 and the host of urgent hightech highparameter, high valueadded highend products, and they do not satisfy the market needs, only dependent on imports. China joins the WTO, its impact may be even greater. Therefore, the 15 during the growth of the output value of industry, must not depend on volume growth and the industry should address the structural problems of their own, increase the intensity of the adjustment of the industrial structure and product mix, is, we should rely on the improvement of quality, and promote technical upgrading of products to meet market demand and stimulating, seek greater development.2 The application of hydraulic power sliding stage Power sliding Taiwan introducedThis paper deals with the investigation for slide unit39。s impact and motion stabil172。ity in modular machine tool fay means of the method of power bond graph and state space analysis. The dynamic mathematical model of selfadjusting back pressure speed control system used to drive slide unit is established. Main reasons and affect172。ing factors for slide unit impact and motion unstability are analysed through pu172。ter digital simulation, It is concluded from those that, if the structural dimensions of hydraulic cylinder and back pressure valve are designed rationally, the slide unit39。s dynamics will markedly be improved.NOMENCLATURESf flow sourceSei sliding friction force in slide unitR equivalent viscous friction coefficient in slide unitIi mass of slide unit and cylinderh mass of SABP valve spoolCi,C2 hydraulic capacitances of rod chamber and nonrod chamber in cylinder respectivelyC3 spring pliance of SABP valveR]rR2 hydraulic resistances of damping holesR9 hydraulic resistance of orifice of SABP valveSe2 presetting force of spring in SABP valveI4J5 equivalent liquid inertia in pipe linesC^Cg equivalent hydraulic capacitances in pipe lines equivalent hydraulic resistances in pipe linesVj V^ oilcontaining volumes in nonrod chamber and rod chamber respectivelyP,r:, Pi oil pressures in nonrod chamber and rod chamber respectivelyF load acted on slide unitV slide unit velocity* Department of Mechanical Engineering, Dalian of Technology, Dalian. China. IntroductionDuring operation of modular machine tool, the changes of slid unit39。s speed and load acted on it in both magnitude and direction will affect working to a different extent Particularly the impact caused by sudden vanishing of load and the motion unstability due to periodical change of load in operation will affect the surface quality of the workpiece machined, and the tool would be broken off under serious conditions, By using the method of power bond graph and state space analysis, the dynamic mathematical model of the system used to drive slide unit is established, that is called as selfadjusting back pressure speed control system and abbreviated to SABP system. In order to improve slide unit39。s dynamics, it is necessary to find out the main reasons and affecting factors, that must be based on puter digital simulation and study on the results. Dynamic Mathematica ModeldThe schematic diagram of SABP system is shown in , the system is used to perform the cycle of feeding, stopping and returning. Four way control valve works in the right position during slide unit39。s feeding. The supply pressure of the pump is approximately constant under the action of pressure relief valve, the oil through the control valve and pressure pen