| Pneumatic system has some advantages such as clean conditions,easy maintenance,easy central gas supply and far-distance transportation.So it is widely applied in food and medicine processing,semiconductor and microelectronic device manufacturing,and medical auxiliary equipments.The sealing elements in the piston-type pneumatic cylinders have the function of preventing leakage from chambers and outside but simultaneously introduce the inevitable friction,which plays one of the most important roles to affect the positioning accuracy and stability.A friction reduction phenomenon by high-frequency vibrations between friction couples was found and attracted a broad concern in related field.Therefore,a friction reduction method to improve the friction characteristics of the pneumatic cylinders is proposed in this study.The friction models for general friction couples are analyzed firstly and followed by a comparison between pneumatic cylinder friction models.A mechanism study is carried out for the friction reduction in different vibration directions,which provides theoretical and experimental basis on the friction reduction study by vibrations.The friction force is reduced by the decreased average force while the vibration alters the transient friction direction as the vibration direction is in the plane of friction couples.The influence of vibration/sliding velocity ratio on the friction ratio is studied based on the Coulomb friction model and Dahl friction model,and a preferable model is confirmed to be more suitable to describe the friction discipline with vibrations.In addition,the friction force is reduced by the decreasing real contact area,when the vibration is perpendicular to the friction couple surface.The vibration amplitude is verified to be an important parameters for the friction reduction by theoretical analysis: the bigger the amplitude is,the smaller the friction force is.An optimized design for the excited position is to be crucial for increasing the average vibration amplitude,from the theoretical study of the influence of excited region on the friction reduction effect.The conclusions above will provide theoretical basis for the development of the friction reduction cylinders.The feasibility of friction reduction by vibrations is verified on the friction couple of rubber and aluminum by plane contact.The friction reduction effect between equivalent sealing ring and cylinder barrel couple with vibrations indicates that the contact form between pneumatic elements is confirmed to be valid in vibration assisted friction reduction.The friction characteristics is investigated on a pneumatic cylinder which integrates two piezoelectric stacks,which has a more closer accordance to the real pneumatic cylinder structure.A series of parameters are studied related to vibrations,such as the normal force,vibration amplitude,vibration-sliding velocity ratio and excited region.The Coulumb friction model is verified to be suitable to describe the friction couples of pneumatic cylinder with or without a vibration.The static friction force is smaller as the vibration amplitude is bigger and the friction force can be reduced to 22.6% of its original value at dry friction condition,while the force can be decreased by a further 50% with lubricants.The Dahl friction model is found to be more suitable to describe the real contact friction with vibrations in the study of the influence of vibration/sliding velocity ratio on the friction reduction effect,because this model takes the shear stiffness of friction couples and the microscale pre-sliding displacement into consideration.The best friction reduction effect is found when the friction couple locates at the region which has the largest average vibration amplitude in the study of the friction between rubber/metal plate,which provide a rule in the structure optimization of the future pneumatic cylinders.A bending/longitudinal vibration mode friction reduction pneumatic cylinder is proposed based on the inverse piezoelectric effect of piezo-stacks refering to the analysis results of the factors which affect the friction reduction.A friction force measurement system is established to carry out the tests of the static/dynamic friction force of the pneumatic cylinder working on either vibration mode.The static friction force can be reduced to 33% and 52.6% of its original friction force at bending and longitudinal vibration modes.The dynamic friction at the bending mode has a smaller value due to its vibration amplitude magnifier structure,and the friction reduction ratio reaches around 50%.It is found that the friction reduction effect is weaker as the air pressure is higher,so the pneumatic cylinder is best to work at a lower pressure for a better friction performance.The pneumatic cylinder has a friction reduction effect only when the vibration/sliding velocity ration is above 1 at the longitudinal vibration mode,which means the sliding velocity is to be below a certain value as the vibration amplitude and frequency is constant.A radial vibration mode friction reduction pneumatic cylinder at the piston rod sealing is proposed for a smaller size,simpler and more compact structure.The static/dynamic friction force measurements are carried out by a self-developed tribometer driven by linear motor.The static friction of the pneumatic cylinder is reduced to 53.3% of its original value at a resonant frequency of 144 k Hz,and the friction reduction is weaker at a higher working air pressure.The displacement and dynamic friction curves with high-frequency vibrations show a more smooth trend while the pneumatic cylinder is working at low sliding velocity,and in this way the stick-slip phenomenon disappears with a more stable motion.It reveals that the vibration introduced has a good effect on improving the stability of the motion' s velocity and force,and it will provide some valuable basis on the study of improving the positioning accuracy of the pneumatic clinders. |
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