Calculation Model And Methods Of Load Distribution Over Threads In The Planetary Roller Screw Mechanism

Planetary roller screw mechanism(PRSM)is a device to transmit rotary motion into linear motion,or vice versa.As PRSM has numerous contact points and has no returning device,it is of great advantages of large applied load,high velocity,long service life,etc.Along with the development of the automatization and intelligence of manufacturing industry,PRSM attracts much attention in engineering,as a device of Electro-Mechanical Actuator(EMA),and has be widely used in aviation,aerospace,navigation and oil production.The load bearing characteristic is one of the most important performance of PRSM,and it plays an important role on the performance of EMA,and load distribution over threads of PRSM is an expression of load bearing characteristic.However,the applied load is always non-uniformly distributed among threads of PRSM,which will limit the promotion of PRSM capacity in definitized nominal diameter of screw,and it will cause transmission noise,excessive wear of threads,decline of service life,and restricts the applications of PRSM.Research on load distribution over threads of PRSM in this paper will provide the technical foundation to parameter design and further improvement of load bearing capacity of PRSM.The phenomenon of load non-uniform distributed over threads of PRSM is systematically investigated in the paper,then the effects of installations,load conditions,material parameters,structural parameters and manufacturing errors on load distribution over threads are revealed.On the basis on effects of pitch deviation on load distribution,the load balance design method of PRSM is proposed.Then the load distribution over threads of dual-nut pre-load PRSM is investigated.Lastly,experiments are carried out to verify the effects of pitch deviation on load distribution over threads of PRSM.1.Based on the structure and load bearing principle,PRSM is dispersed into shaft sections,threads and contact points of threads,with the installations and load conditions considered,model of load distribution over threads of PRSM is developed.The matrix equation to solve load distribution of PRSM is obtained from the deformation compatibility relationship and force equilibrium relationship.As the manufacturing errors are always exist,the developed model is further improved into a model consider manufacturing errors to investigate the effects of manufacturing errors on load distribution over threads.By comparing the results with the finite element model and the existing research,the load distribution model of PRSM is validated.2.The effects of installations,load conditions,material parameters,structural parameters and manufacturing errors on load distribution over threads of PRSM are investigated.The results indicate that load distributions over threads are different under different installations,while they will be the same with different load conditions under a certain installation.Among the structural parameters,external diameter of nut,parameter of thread form(except for pitch)modulus of elasticity,have little effects on load distribution overs threads,while the nominal diameter ratio between screw and roller,number of rollers,number of threads of roller and pitch have great influence on load distribution.Pitch deviation can make load distribution over threads evener or more uneven,and the effects on screw side and nut side are opposite,because the contact of threads are on opposite thread flank on screw and nut side.The non-uniform load distribution over threads of PRSM is caused by the accumulation of deformation of shaft sections,and the shaft section deformation of screw and nut play the leading role.3.The load balance design method for threads of PRSM is proposed on the basis of effects of pitch deviation on load distribution to make the applied load uniformly distributed over the threads of roller.Firstly,two criteria are presented to guide the parameters design of thread form,which include intensity criterion and yield criterion of contact,and based on which,the rated load and limit load of thread and the criteria of parameter design are developed.Secondly,the load balance design method is proposed,through the optimum design of pitch and thread thickness of roller and nut,the contact conditions among threads of roller with screw and nut are improved,and then the load distribution over threads of PRSM are improved.4.Based on the load distribution model of single-nut PRSM,with the structure and load bearing principle of double nut pre-load PRSM,the load distribution model of double nut pre-load PRSM is developed.The load distribution over threads is investigated,results shows that the load distribution is determined by the installations,load conditions,material parameters,structural parameters,stiffness of elastic element and the stiffness of connect bolt set.Under the applied load,preload on one of the nut will lose,and the value of applied load depends on the stiffness of shaft section of nut,elastic element and connect bolt set.After the vanish of preload on one nut,the applied load will be bore simultaneously by the two nuts.The load share between the two nuts depend on stiffness of elastic element or connect bolt set,when PRSM bear compression or tension,respectively.5.The effects of pitch deviation on load distribution over threads of PRSM are experimentally studied,with two PRSM samples from different producers.Firstly,the pitch deviations of thread parts of the two PRSM samples are measured.Then the results are substituted into the load distribution model to evaluate the load distribution over threads with pitch deviations.After the measurement of pitch deviations,the two PRSM samples are assembled correctly for the load bearing test.Afterwards,the wear depths of contact traces on the threads are measured to verify the effects of pitch deviations on load distribution.The pitch deviation,load distribution over threads and wear depths of threads show obvious accordance,which indirectly demonstrate the effect of pitch deviation on load distribution over threads of PRSM.Finally,some conclusions and observation are given and the developmental prospect of the PRSM are discussed and outlined.