Abstract:
A vibration damper for screw-driven mechanical systems includes a nut body and one or more attached damping elements. The nut body includes internal threads for cooperating with a lead screw of a screw-driven mechanical system. The damping element may be a viscoelastic material or other material. The damping element may be arranged on an outer surface of the nut body, and may either be exposed or protected by another element. A tuning mass may be applied to the vibration damper by attachment to the damping element. In some embodiments, the damping element is arranged on a cantilever element of the nut body. In other embodiments, the damping element is segmented and is distributed such that air passages are formed between the damping element segments.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to the field of screw-driven machinery, more particularly to traveling nuts in such screw-driven machinery.  
         [0003]     2. Description of Related Art  
         [0004]     Precision lead screw and nut assemblies are used in positioning and locating applications, such as milling machines, automated metrology systems, factory automation systems, and wafer manufacturing equipment. Lead screw assemblies can also be used purely for the conversion of power or motion, such as jack lifts and elevator doors. Translating screws and nuts make very cost-effective solutions to motion applications. The disadvantage of lead screw assemblies is that they can develop unwanted noise and vibrations. Lead screw assemblies may experience longitudinal, torsional, and transverse oscillations during operation. There are many possible causes for these oscillations. Misalignments in the assembly and friction between the mating surfaces of the screw and nut threads typically may cause unwanted vibrations.  
         [0005]     An attempt to reduce vibration by means of improving alignment is described in U.S. Pat. No. 6,099,166 to Erikson et al. (“Erikson”). In Erikson, a radial-stabilizing bushing that maintains contact with the outer diameter of the screw is incorporated in the nut. The bushing reduces radial fluctuations between the nut and screw when in motion, thereby reducing vibration. One of the drawbacks to this approach is the outer diameter of the screw must be precise. In addition, the bushing is subjected to wear.  
         [0006]     Another method of reducing vibration is by preloading the nut threads against the screw threads in U.S. Pat. No. 6,535,305 to Chang et al. (“Chang”). Chang describes how a nut can be configured to reduce vibration. In Chang, nut halves are forced against screw threads by means of springs. A pre-load in the springs keep the internal thread of the nut in close contact with the external thread of the screw. However, the spring pre-load introduces a drag torque, and the assembly is still subjected to resonant frequencies.  
         [0007]     There have also been attempts to reduce or eliminate vibrations by damping the screw directly. One such attempt is described in U.S. Pat. No. 5,379,660 to Ishikawa and U.S. Pat. No. 4,671,127 to Yamaguchi et al., in which a vibration damping mechanism is loosely fitted on a free end of the screw. The damping mechanism suppresses vibration by means of impact damping (or single particle impact damping). Tests have shown that mounting an impact damping mechanism at the end of the screw can address vibration issues, but there are limitations to that concept. Moreover, the overall length of the screw must be increased when an approach such as these is taken, and special machining and special assembly procedures are required.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     One object of the invention is to provide a simpler and more compact means for damping vibrations within a screw-driven mechanism. Another object of the invention is to provide a means for damping vibrations within a screw-driven mechanism that is effective over a wide range of frequencies and multiple modes of vibration. Still another object is to provide a vibration damper for the above-described purpose that is easy to manufacture and assemble in a system. Yet another object of the invention is to provide a vibration damper for the above-described purpose having no wearing components.  
         [0009]     The present invention relates to a nut for use in screw-driven machinery, and damping in conjunction therewith. The nut travels along a lead screw in a device, and includes a damping element thereon. Depending on the specific embodiment, the damping element may be passive, and further, may be a damping material, such as a viscoelastic material.  
         [0010]     Depending on the embodiment, the damping element is placed in one or more predetermined locations on the nut. In a first embodiment, the damping element is a damping material that surrounds the outer circumference of the nut. In another embodiment the damping element is arranged on one or more ends of the nut.  
         [0011]     In a variation of certain embodiments, a bearing layer, tuning mass or constraining layer is arranged on top of the damping material to distribute forces received by the nut to the damping element.  
         [0012]     As a further variation, the damping element is arranged in separate segments on the damping nut, between the damping nut and the bearing layer. This may be implemented to achieve increased air circulation around the nut and/or the damping element. This also can vary, in conjunction with the specific type of damping element or damping material used, the magnitude of resistance of the damping element to movement.  
         [0013]     In a further embodiment, a cantilever member is arranged on the nut body to flex in response to applied forces. Such cantilever member may protrude longitudinally from the nut body or radially from the nut body, depending on the embodiment. Similarly to the above, a damping element may additionally be incorporated with the nut having a cantilever member.  
         [0014]     The damping element is, in a preferred embodiment, a viscoelastic material (VEM).  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1   a  is a cross-sectional side view of a first embodiment of the present invention;  
         [0016]      FIG. 1   b  is an isometric view of the first embodiment of the present invention;  
         [0017]      FIG. 2  is a cross-sectional side view of a second embodiment of the present invention;  
         [0018]      FIG. 3   a  is a cross-sectional side view of a third embodiment of the present invention;  
         [0019]      FIG. 3   b  is an isometric view of the third embodiment of the present invention;  
         [0020]      FIG. 4  is a cross-sectional side view of a fourth embodiment of the present invention;  
         [0021]      FIG. 5  is an isometric view of the fourth embodiment of the present invention;  
         [0022]      FIG. 6  is a cross-sectional end view of a fifth embodiment of the present invention;  
         [0023]      FIG. 7  is an isometric, partially cut-away view of a sixth embodiment of the present invention;  
         [0024]      FIG. 8  is a cross-sectional side view of a seventh embodiment of the present invention;  
         [0025]      FIG. 9  is a cross-sectional side view of a eighth embodiment of the present invention; and  
         [0026]      FIG. 10  is a cross-sectional side view of a ninth embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0027]      FIGS. 1   a  and  1   b  illustrate a first embodiment the vibration-damping nut, which comprises a nut body  110  with internal threads  120  engaged with the threads of a lead screw  130 . A damping element, which may be a passive-damping element  140 , is in contact with the nut body  110  at its outer surface  150 . In a preferred embodiment, the damping element is a viscoelastic material (VEM) that transfers vibration-energy into thermal heat. In such embodiment, strain energy is transferred to the VEM through the surface  150  of the nut body  110 . An adhesive can be used to attach the damping element  140  to the nut body surface  150 , and a recess may alternatively or additionally be formed in the nut body  110  to hold the damping element.  
         [0028]     In this and in other embodiments, the damping element is linked to a surface of the nut, which linking can be accomplished in a variety of manners, including friction fitting, by an adhesive, or other means. Contact between the damping element and the nut allows the transfer of vibration-energy to the damping element.  
         [0029]     A preferred passive damping element is manufactured from a viscoelastic material (VEM). Viscoelastic materials damp vibrations by converting vibration-energy into thermal heat. Referring to  FIGS. 1   a  and  1   b , a VEM can be placed directly on the body of the nut with or without an adhesive or mechanical fastener. In use, vibration-energy that is transferred to the VEM will dissipate in the form of heat, thereby damping the vibration and eliminating noise that may occur in a machine, such as those having a rotary lead screw device. The vibration energy and the consequent strain experienced can be in any dimension. The strain may be realized in torsional, axial, or radial strain, or in a combination thereof.  
         [0030]      FIGS. 1 through 10  illustrate various possible embodiments of a nut accommodating a damping element, which may be a VEM.  
         [0031]     It is to be noted that in this and in all embodiments, the term “nut” refers both to what is conventionally thought of in the field of mechanical systems, and also to any component that travels along a lead screw in a mechanical system. Typically these components are roughly cylindrical, and since they ride on threads of a screw, the term nut is generally used. It is to be understood, however, that the invention is not limited only to use with a “nut.” 
         [0032]      FIG. 2  illustrates a second embodiment of a nut having a ring-shaped damping element  270  positioned such that it is in contact with the nut  260  at a radial surface  280 , which is radial relative to a central axis of the nut body. This axis, of course, is essentially collinear with a central axis of the lead screw  130 . In this specific embodiment, the radial surface  280  is at the end of the nut body  260 , but need not be, but depends on the specific embodiment.  
         [0033]      FIGS. 3   a  and  3   b  illustrate a third embodiment of the present invention. In this, as well as in other selected embodiments, a mass  311  is in contact with a damping element  310 . In this embodiment, the mass  311  is substantially tubular with a substantially circular cross section, though other cross-sections are very much possible in this and in other embodiments.  
         [0034]     The mass  311  acts as a constraining layer to assist in creating strain in the damping element  310 . This is accomplished because the mass  311  helps distribute loads evenly to the damping element  310 , due to its rigidity. The mass  311  also acts as a tuning mass to assist in damping vibration. The magnitude of the mass (weight) of mass  311  is pre-selected for a desired behavior under anticipated loading conditions of the nut  300 . By pre-selecting the mass (weight) of the mass  311 , the natural frequencies (rotational and linear) can be adjusted, as can the natural frequencies of the entire system attached thereto. As such, undesired resonance can be avoided. Energy from the nut body  309  is transferred to the damping element  310  through surface  312  of the nut body  309 , and the energy is dissipated. Mass  311  also assists in protecting the damping element  310  from the environment.  
         [0035]      FIGS. 4 and 5  illustrate nut  400 . A damping element  415  is in contact with surface  413  of a flange  417  on the nut body  401 , and is constrained axially by a tuning mass  416 . Mass  416  also assists in protecting the damping element  415  from the environment. The structure of nut  400  is similar to the structure of nut  300  in  FIG. 3 . However, this embodiment includes the tuning mass  416  at the end of the nut body, rather than around its circumference. As with nut  300 , the tuning mass can be used to accomplish various goals, as set forth above.  
         [0036]      FIG. 6  illustrates a nut  600 , representing a fifth embodiment of the invention. Nut  600  is variant of the nut  300  illustrated in  FIG. 3 . A nut body  618  is mounted onto a screw  621 . Damping element segments  619 , are in contact with the nut body  618  and with a tuning mass  620 . Each damping element segment  619  is separated by a gap  622  to assist heat dissipation. The damping element segments  619  are constrained between the nut body  618  and a rigid mass  620 , which is in the form of a tubular sleeve. The damping element performance is improved in a constrained state, as compared with an unconstrained state, for example, as shown in  FIG. 1 , because the loads can be distributed across essentially all of the viscoelastic material. In addition, segmenting the damping elements  619  improves manufacturability and eases assembly over using a solid damping element.  
         [0037]      FIG. 7  illustrates a nut  700 , representing a sixth embodiment of the present invention. A nut body  724  is mounted on a screw  723 , and has a flange  725  that is in contact with damping element segments  726 . The damping element segments  726  are separated by a gap. A tuning mass  727  constrains the damping elements  276  axially. Unlike the embodiment of nut  400  shown in  FIG. 4 , which uses a solid ring-shaped damping element  415 , this configuration utilizes damping element segments  726 . Such segments  726 , and the resultant gaps therebetween, promote airflow around and cooling of the damping segments  726 , as well as other components.  
         [0038]      FIG. 8  illustrates a nut  800 , representing a seventh embodiment of the present invention. A nut body  829 , mounted to a screw  830 , has a cantilever member  831  extending from one end. The cantilever member  831  is thinner in cross-section than the nut body  829 . When vibrated, the cantilever member  831  flexes, transferring vibrational energy to a damping element  832 , via surface  833 . A tuning mass  834  constrains the damping element  832  such that it experiences shear strains substantially uniformly across its whole surface.  
         [0039]      FIG. 9  illustrates nut  900 , which is an alternate embodiment to nut  800  shown in  FIG. 8 . Nut  900  has a nut body  935  and an arm  936  that extends out of the body of the nut body  935 . The arm functions as a flexing member to transmit vibrational energy to a damping element  937 .  
         [0040]      FIG. 10  illustrates nut  1000 , which is a variant of nut  900  shown in  FIG. 9 . A nut body  1038  has a flexure member  1039  that is in contact with a damping element  1040 , which in-turn, is also in contact with the nut body  1038 . The surface  1041  of the nut body  1038  helps to constrain the damping element  1040 . Vibrational energy is transmitted from the flexure member  1039  through surface  1042  to the damping element  1040 . With the damping element  1040  positioned between surface  1041  and  1042 , the damping element  1040  is also shielded from the environment, and thus also from damage.  
         [0041]     While the embodiments of  FIGS. 8-10  only show one portion of the nut as a cantilever, it is to be understood, that depending on the embodiment, there may be multiple cantilevers. Alternatively, such cantilever may be circumferential and surround all or a segment of its respective nut body.  
         [0042]     As one alternative to the use of viscoelastic material as the damping element, it is possible to utilize impact damping in accordance with the invention to achieve a desired result.  
         [0043]     As another alternative to the use of viscoelastic material as the damping element, it is possible to utilize friction damping in accordance with the invention to achieve a desired result.  
         [0044]     It is to be understood that though not specifically set forth herein, other embodiments are possible while still keeping with the spirit of the invention.