US20090060643A1 - Wedge mechanism - Google Patents

Abstract

A mechanical wedge mechanism comprising base member, output member, and movable wedge member in which frictional connections between mutually movable mechanical members are replaced with shear deformations in elastomeric shims connecting respective surfaces of the members, thus effectively reducing frictional losses in the mechanism.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application is a continuation of U.S. patent application Ser. No. 10/113,524 filed Apr. 1, 2002.
FIELD OF THE INVENTION
• The present invention relates to force and motion-transformation mechanisms.
BACKGROUND OF THE INVENTION
• Wedge mechanisms are widely used in mechanical devices. The most important applications of the wedge mechanisms are as force amplifiers. Thus, the wedge mechanisms and their analogs are universally used in clamping mechanisms wherein relatively small forces applied manually or by means of relatively small and low power motors/actuators can be transformed into much larger clamping forces. The basic conventional wedge mechanism (the Prior Art) inFIG. 1 comprisesbase member1,movable wedge member2, andoutput member3. These members have sliding frictional contacts along flat or curvedconformal surfaces4 betweenmembers1 and2 and along flat or curvedconformal surfaces5 betweenmembers2 and3. Usually the respective contact surfaces ofmembers1 and2 and ofmembers2 and3 are separated by a thinner or thicker layer of a lubricating material (e.g., oil).Output member3 may apply the output force and/or motion to workorgan6, or may have itself the role of the work organ. If the former is true, there is contact surface7 betweenoutput member3 andwork organ6. The motion ofoutput member3 orwork organ6 is constrained/guided byguideways8 of various embodiments. Application of input force F_ito wedgemember2 initiates movement of this wedge member along thecontact surfaces4 and5 after the static friction force in thefrictional contacts4 and5 are overcome. If there is no friction incontacts4 and5 (friction coefficient f=0), application of input force F_iresults in development of output force F_oacting onoutput member3,
• 
  F_o=F_i/tan α,  (1)
• and also of reaction force N normal to contactsurfaces4 and acting onbase member1,
• 
  N=F_i/tan α.  (2)
• Thus, for α<45°, F_o>F_i, the output force is greater than the input force. For small angles α, the effect is increasing so that F_o>>F_i. The displacement Δ_iofwedge member2 is causing displacement Δ_oofoutput member3 guided byguideways8. If the vertical displacement ofmember3 is allowed as shown inFIG. 1, then
• 
  Δ_o=Δ_itan α.  (3)
• For α<45°, Δ_o<Δ_i, and for small α, Δ_o<<Δ_i; F_iΔ_i=F_oΔ_ofor f=0.
• When the friction coefficient f>0, the equation (1) is changing and becomes
• $\begin{array}{cc}{F}_o=\frac{F_i}{\tan \left(\alpha +\rho \right)},& \left(4\right)\end{array}$
• where ρ=tan⁻¹f is the friction angle. Equation (3) is not influenced by presence of friction, but if displacement Δ_iof movingwedge member2 is very small and angle α is small (such combination is typical for clamping devices), the very small displacement Δ_ois not physically occurring and Δ_ois accommodated by elastic deformations in the mechanism.
• Usually, for lubricated steel contact surfaces f=0.1-0.2, or ρ=5.7-11.3°. As a result, for small wedge angles α the ideal large magnitude of the mechanical advantage per (1) does not materialize, and actual mechanical advantage F_o/F_ifor a given f deteriorates to a larger and larger degree the more the wedge angle α is reduced. For α=10° the mechanism with f=0 would deliver the output force F_o=F_i/tan 10°=5.7 F_i. However, for ρ=7° (f=0.12), from (4) F_o=F_i/tan 17°=3.3 F_i, 40% less than the ideal mechanical advantage 5.7. For α=5°, the ideal mechanism described by (1) would deliver the output force F_o=F_i/tan 5°=11.4 F_i, more than ten times force amplification. However, for ρ=7°, f=0.12, from (4) F_o=F_i/tan 12°=4.7 F_i, 60% less than the ideal mechanical advantage. Even worse deterioration from the ideal efficiency/mechanical advantage would develop for more realistic larger values of f. As a result, wedge angles smaller than α<˜5° are seldom used in practical designs and relatively high driving (input) forces should be used, thus increasing size and weight of the mechanisms, requiring two-stage mechanisms, etc. The noted above lack of mobility in the mechanism at small displacements due to static friction forces, leads to a need to increase stiffness of the mechanism and thus further increase its size, weight, and cost of the devices employing wedge mechanisms.
• Since conventional (prior art) wedge mechanisms benefit from low friction and higher stiffness, usually their structural parts, such asmembers1,2,3 inFIG. 1 are made from steel subjected to heat treatment for increasing hardness, the contact surfaces have to be made with high geometric accuracy and high surface finish. The contact surfaces have to be well lubricated and well protected since any scratches would result in increased friction and reduced efficiency. Since the sliding friction coefficients between conforming surfaces depend on vibratory environment, presence of vibrations can change the effective friction coefficients and the mechanical advantage of the mechanism. Consequently, the rated values of the mechanical advantage (clamping force) may change significantly depending on the vibratory environment, thus reducing consistency and reliability of these important mechanisms.
• The friction coefficient in the contact areas can be reduced and its consistency can be enhanced by using rolling bodies (balls, rollers, etc.) between the contact surfaces of the constituting mechanical members. However, such designs require even better materials and heat treatment, higher accuracies, and are more bulky and more expensive.
SUMMARY OF THE INVENTION
• The present invention addresses the inadequacies of the prior art by providing a wedge mechanism having mechanical advantage close to the same for an ideal wedge mechanism without friction.
• The present invention further improves on the prior art by providing a wedge mechanism which has high mechanical advantage while not requiring lubrication.
• The present invention further improves the prior art by providing a wedge mechanism which is constructed as a solid-state mechanical device insensitive to external shocks, vibrations, and requires only a minimal maintenance.
• The present invention improves and simplifies the devices employing wedge mechanisms by making the wedge mechanism largely insensitive to contamination by environmental contaminants such as water and other fluids, dirt, abrasive particles, etc.
• The present invention further improves the devices employing wedge mechanisms by eliminating the need for making contact surfaces in wedge mechanism with high hardness and high geometrical accuracy of their contact surfaces, and by allowing use of light materials for structural parts of lie wedge mechanisms.
BRIEF DESCRIPTION OF THE DRAWINGS
• The present invention can be best understood with reference to the following detailed description and drawings, in which:
• FIG. 1 is a sketch of a basic conventional (prior art) force and motion transforming wedge mechanism.
• FIG. 2 is a sketch of a basic force and motion transforming wedge mechanism according to the present invention wherein the contact surfaces between the constitutive members are flat, and both contacts are realized through elastomeric layers (shims).
• FIG. 3 illustrates the deformation pattern of a rubber cylinder in axial compression.
• FIG. 4 illustrates the deformation pattern in compression of the rubber cylinder ofFIG. 3 divided in the middle.
• FIG. 5 is a cross section of another embodiment of the present invention wherein the contact surfaces between the constitutive members are curvilinear and both contacts are realized through elastomeric layers.
• FIG. 6 shows partial cross section of the6-6 view inFIG. 5.
• FIG. 7 illustrates yet another embodiment of the wedge mechanism according to the present invention wherein the contact surfaces between the constitutive mechanical members are flat but only one contact is realized through elastomeric shim.
• FIG. 8 illustrates yet another embodiment of the wedge mechanism according to the present invention wherein one contact area between the constitutive mechanical members is shaped as a helical thread.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• Referring toFIG. 2, the shown wedge mechanism comprises the same basic mechanical members as the prior art wedge mechanism depicted inFIG. 1, namelybase member1,movable wedge member2, andoutput member3 which can be interacting via surface contact7 withwork organ6 and whose motion can be constrained byguideways8. The wedge mechanism inFIG. 2 differs from the prior art wedge mechanism inFIG. 1 by designs of a firstsurface contact area10 betweenbase member1 and thewedge member2, which is movable along an axis in the direction of an applied force F_ito vary the separation between thesurface contact area10 and a secondsurface contact area11 disposed betweenmovable wedge member2 andoutput member3. Thewedge member2 has a third contact surface15 opposed to and conforming with thefirst contact surface10, and a fourth contact surface16 opposed to and conforming with thesecond contact surface11. The motion of theoutput member3 along the axis in the direction of the applied input force F_iproduces a motion of the wedge member along the axis of the output force F_o, which is substantially perpendicular to the direction of motion of the wedge member. Instead of lubricant filling the surface contact areas in the design inFIG. 1, the conforming surfaces of the above respective mechanical members are separated by thin uniform thickness shims (layers)12 and13 made of an elastomeric (rubber-like) material.
• Since the elastomeric materials have their Poisson's ratios μ very close to 0.5, usually in the range of μ=0.49-0.499, they can be considered as volumetric-incompressible materials. Thus, compression of an elastomeric specimen involves only redistribution of the specimen's volume (e.g., by bulging at the non-loaded surfaces).FIG. 3 shows acylindrical specimen30 comprisingrubber cylinder31 bonded to upper32 and lower33 covers, and subjected to axial compression force P_z; height h−diameter d ratio of this rubber cylinder is h/d=˜1.13, Since the volume does not change, compression deformation is accompanied by bulging of rubber on the free (not loaded by forces) surfaces, thus creatingconvex bulges34. The deformed conditions of the specimens inFIGS. 3 and 4 are shown by broken lines. Effective compression modulus E of the specimen having hardness H30 (soft rubber) is
• 
  E≈3G(l+S²),  (5)
• e.g., seeE.I. Rivin, Stiffness and Damping in Mechanical Design, Marcel Dekker, Inc.,1999. Here G is the shear modulus (not dependent on the specimen geometry), and S is the “shape factor” equal to ratio between the surface area A_lof the loaded surface (A_l=πd²/4 for theFIG. 3 cylindrical specimen) to the surface area of the free-to-bulge area A_f(A_f=πdh for theFIG. 3 cylindrical specimen). Thus, for the specimen inFIG. 3
• 
  S=A_l/A_f=(πd²/4)/(πdh)=d/4h≈0.22,E≈3.15G.  (6)
• If an intermediaterigid plate45 is bonded at the mid-height of the specimen inFIG. 3, as shown inFIG. 4, thus resulting in two identicalshorter cylinders41 bonded to upper42, lower43 and intermediate45 plates, respectively (h′=h/2), the bulging is constrained tosmaller bulges44, thus obviously increasing the compression stiffness. This statement can be quantified by computing the shape factor and the effective compression modulus for the specimen inFIG. 4 as
• 
  S′=A_l/A′_f=(πd²/4)/[πd(h/2)]=d/2h≈0.44,E≈3.6G.  (7)
• Thus, the compression stiffness of the specimen has increased by ˜15% by dividing its height. This process of “division” can be continued thus resulting in a progressive increase of compression stiffness. With eight intermediate plates (resulting in height of each layer h″=h/9 and d/h″=˜10), E=˜22G, or compression stiffness becoming many times greater than the shear stiffness. The shear deformation (and stiffness) of the specimen, related to the shear force P_x, is not associated the volume change and does not change after the specimen is divided. WhileFIGS. 3 and 4 depict a cylindrical specimen, the same effects can be observed in specimens of other shapes, e.g. in a parallelepiped [width w, length l, height t, A_l=wl, A_f=2wt+2lt, and S=wl/(2wt+2lt)]. For w>10t, l>10t, E>22G. If the specimen does not have a rectangular cross section, width w and length l represent dimensions of the smallest rectangle surrounding the actual cross section, thus representing the outline dimensions of the cross section.
• The increasing compression stiffness with reduction of thickness of elastomeric specimens and increase in shape factor S are accompanied with increasing tolerance for the compression forces. It is shown inE.I. Rivin, “Properties and Prospective Applications of Ultra Thin Layered Rubber-Metal Laminates for Limited Travel Bearings,” Tribology International,1983, Vol. 16, No. 1, pp. 17-25, that thin rubber layers (thickness in the order of ˜1 mm) bonded to rigid (e.g., metal) surfaces can endure specific compressive forces up to 250 MPa (˜37,000 psi) while maintaining low shear stiffness. It was recently demonstrated that even higher compression forces can be allowed for properly designed bonded thin elastomeric layers.
• These unique characteristics of thin elastomeric layers are utilized in the design shown inFIG. 2 whereinelastomeric shims12 and13 comprising thin elastomeric layers are inserted intocontact area10 betweenbase member1 andmovable wedge2 and intocontact area11 betweenmovable wedge member2 andoutput member3, respectively. These elastomeric shims can be bonded to the appropriate contact surfaces, glued, held by friction or by other known means. Application of input force F_itomovable wedge member2 causes shear deformations in thinelastomeric layers12 and13 and a corresponding displacement Δ_iofmember2. This displacement also results in generation of output force F_oapplied tooutput member3 and reaction force N applied tobase member1. Although these forces can be much larger than F_i, they induce only minimal compression deformations oflayers12 and13 if w, l>˜10t, and geometry of the mechanism does not change noticeably.
• In some cases, the condition w, l>˜10t can be too stringent and lower aspect ratios can be beneficially used.
• Since it is desirable for better functioning of the wedge mechanism inFIG. 2 to have as low shear stiffness as possible, and since the allowable compression loads on thin elastomeric layers (up to and exceeding 250 MPa) are very high,elastomeric layers12 and/or13 inFIG. 2 may be designed with surface areas less than the total surface contact area betweenmembers1 and2,2 and3, respectively. The preferred, but not the only, way to achieve such area reduction by using two or more elastomeric shims satisfying the above stated aspect ratio condition to be inserted into the surface contact areas between the interactingmembers1 and2,2 and3 inFIG. 2. The total surface area of these shim segments may be much less than the total contact surface area between the respective members.
• It is shown in above quoted paper by Rivin that increase of the compression force applied to thin elastomeric layers does not lead to increasing resistance to the shear deformation.
• Since wedge mechanisms like ones shown inFIGS. 1,2, as well as described below in reference toFIGS. 5,7,8 are usually working in the range of very small displacements ofmovable wedge member2 inFIGS. 1,2 or its equivalents inFIGS. 5,7,8, and shear resistance of rubber layers for small deformations is very low, the wedge mechanism inFIG. 2 can be considered as a mechanism with reduced friction and zero static friction. This statement was confirmed by comparative testing of wedge mechanisms ofFIG. 1 andFIG. 2 designs which demonstrated ˜35% increase in mechanical advantage for mechanism perFIG. 2 having same geometry as mechanism inFIG. 1.
• It is apparent that mechanism inFIG. 2 is not sensitive to contamination of the contact surfaces, and its performance is not influenced by external vibrations and shocks.
• The wedge mechanism inFIG. 2 is a basic embodiment per the present invention. The embodiments illustrated below as depicted inFIGS. 5,7,8 illustrate some important design modifications possible within the confines of the present invention.
• FIG. 5 depicts a clamping device for rotating tools (collet chuck) utilizing a modification of wedge mechanism per the present invention. Tool51 (end mill is shown) has to be clamped intoolholder52 while assuring precise concentricity (coaxiality) between the tool and the toolholder. The clamping wedge mechanism comprisesbase member53 which is a segment oftoolholder52,movable wedge54 andoutput member55 contacting work organ (rotating tool)51. Contact surfaces betweenmembers53 and54 are conformingcylindrical surfaces56 and57, respectively, separated byelastomeric shim58. Contact surfaces betweenmembers54 and55 are conformingconical surfaces59,60, respectively, separated by elastomeric shim61. Althoughoutput member55 is physically connected to toolholder/base member52/53 inarea62 in order to insure high concentricity,output member55 can be considered as a free moving component of the wedge mechanism since the performance displacement of the output member in this mechanism is its small radial deformation not noticeably affected byconnection62. The external surface ofoutput member55 in itsarea62 can be made cylindrical in order to provide guidance for and concentricity withmovable wedge member54.
• Whileelastomeric shims58 and61 are shown as integral inFIG. 5 because of the relatively small scale of the drawing, their actual design is shown in the enlarged partial cross section6-6 inFIG. 6. It can be seen inFIG. 6 that eachshim58 and61 are comprised from two thinelastomeric layers58aand58band61aand61b, respectively, bonded to thin intermediate rigid (e.g., metal)layer65,66, respectively, thus increasing shape factors of the shims. Such construction allows enhancing of compression (normal to contactsurfaces56,57 and59,60, respectively) stiffness of therespective shims58 and61, which is important for performance of the clamped tool, while maintaining low shear stiffness, which is important for operation of the clamping wedge mechanism.
• The high clamping force necessary for the required performance of the collet chuck inFIG. 5 is maintained by spring63 (Belleville spring is shown), while release of the chuck is effected by axial displacement ofmovable wedge member54 againstspring63. The force exerted byspring63 ontomovable wedge member54 is amplified by the wedge mechanism (using conical surfaces ofmovable wedge54 andoutput member55 interacting via elastomeric shim61 instead of flat wedge surfaces inFIG. 2) and applies uniformly distributed radial compression force on sleeve-shapedoutput member55 causing its radial shrinkage and clamping action ontool51. Whilesolid sleeves54,55 are shown inFIGS. 5 and 6, axially slotted sleeves (one or both) can be used, as is the case in standard collet chucks.
• FIG. 7 shows another embodiment of the present invention as incorporated into clamping device for a flat object (e.g., saw blade for a hand-held reciprocating saw). InFIG. 7, sawblade83 plays the role of the output member directly, by contacting alongcontact surface86 withmovable wedge member82 which, in its turn, has contact viaelastomeric shim84 withbase member81. The clamping device is assembled insidehousing85. Use of the elastomeric shim only in one surface contact area allows to establish better directional stability forsaw blade83. While using the elastomeric shim only on one contact surface ofmovable wedge member82 increases motion resistance as compared with the mechanism inFIG. 2 due to presence of sliding friction between contact surfaces86, the friction influence is reduced and the mechanical advantage is increased in comparison with conventional clamps in which all contacts in the wedge clamping mechanism are frictional contacts.
• The clamping device is “normally locked” byspring87 and can be manually (e.g., by finger88) released by pushingmovable wedge82 againstspring87.
• FIG. 8 illustrates yet another embodiment of the wedge mechanism per the present invention. InFIG. 8, a device for coaxial connection betweenshaft91 andexternal component92, such as a pulley or a gear, is shown. The device comprises thininternal ring93 with double-tapered outside surface, which initially is snugly but without interference fit onshaft91;external ring94 with double-tapered internal surface, which is snugly but without interference fit into coaxial cavity inexternal component92; two clamping rings95 and96 having oppositely tapered surfaces on the internal and on the external surfaces; actuatingbolts99 connecting clamping rings95 and96 and uniformly distributed around their circumference. External tapered surfaces of clamping rings95 and96 have identical taper angles with internal tapered surfaces ofring94, thus their tapered surfaces conform with each other; internal tapered surfaces of clamping rings95 and96 have identical taper angles with external tapered surfaces ofring93, thus their tapered surfaces conform with each other. Elastomeric shims97a,97bseparate external tapered surfaces ofring93 and internal tapered surfaces of clamping rings95 and96; elastomeric slims98a,98bseparate internal tapered surfaces ofring94 and external tapered surfaces of clamping rings95 and96.
• This device constitutes a balanced (double-acting) modification of the wedge mechanism per the present invention. Clamping rings95 and96 represent movable wedge members; internal93 and external94 rings represent output members in the wedge mechanism;bolts99 serve both as base members (contactingmovable wedge member95 viawasher100 andmovable wedge member96 along the threaded surface) and as actuators.
• Tighteningbolts99 causes displacements (mutual approach and movement along the bolt) of two movable wedge members/clamping rings95 and96, and these displacements initiate wedge actions in surface contacts between tapered surfaces ofrings95,96 and93, and between tapered surfaces ofrings95,96 and94. These wedge actions are causing uniform expansion ofexternal ring94 and uniform contaraction ofinternal ring93, thus commencing interference fits betweenring93 andshaft91 and betweenring94 andpulley92. These interference fits create gripping action with the respective connected components, and torque can be transmitted from91 to92 via these gripping contacts and via circumferential shear deformation of elastomeric slims97 and98.
• It is readily apparent that the components of the wedge mechanism disclosed herein may take a variety of configurations. Thus, the embodiments and exemplifications shown and described herein are meant for illustrative purposes only and are not intended to limit the scope of the present invention, the true scope of which is limited solely by the claims appended thereto.

Claims (6)

1. Means to realize a greater mechanical advantage and reduced sensitivity to vibratory environment in a mechanical force and motion transforming wedge mechanism, said mechanism comprising:

a base member having at least one contact surface;

a movable wedge member having first and second non-parallel contact surfaces, and first contact surface conforming with an being in surface contact with one of said contact surfaces of said base member;

an output member having a contact surface conforming with and in surface contact with said second contact surface of said movable wedge member;

said movable wedge member being capable of being driven simultaneously in relation to both said base member and said output member while maintaining said surface contacts with said base member and said output member thus realizing mechanical force and motion transformations;

at least one of said surface contacts between the wedge member and the base member or the output member being maintained through a thin constant thickness shim separating said contact surfaces and comprising at least one thin layer of elastomeric material so that the relative motion between the contact surfaces is accommodated by internal shear in said elastomeric layers;

said thin elastomeric layers having their length l, width w, and thickness t, such that the ratio of l/t is greater than approximately 10 and the ratio of w/t is greater than approximately 10.

2. A mechanical wedge mechanism ofclaim 1 comprising a mechanical spring whose force is applied to said movable wedge member thus generating continuous specified force onto said output member: said continuous specified force being relieved as needed by application of an external force to said movable wedge member in the direction opposite to said spring force.

3. A mechanical wedge mechanism ofclaim 1 wherein said contact surface between said base member and said movable wedge member is shaped as a threaded connection.

4. A mechanical wedge mechanism ofclaim 1 embodied as a clamping device for cylindrical components, wherein said base member has an internal cylindrical contact surface connected to external coaxial cylindrical contact surface of said movable wedge member via elastomeric shim comprising at least two thin elastomeric layers with an intermediate rigid layer, said movable wedge member having an internal conical contact surface coaxial with said cylindrical surfaces and connected to external coaxial conical contact surface having the same conical angle of said output member via elastomeric shim comprising at least two thin elastomeric layers with an intermediate ridge layer, said output member being deformable in the radial direction under the force generated by a spring acting axially on said movable wedge member and transformed into radial distributed output force by the wedge action, thus realizing the clamping of cylindrical components.

5. A mechanical wedge mechanism ofclaim 1 embodied as a clamping device for flat components, wherein said base member has a planar contact surface inclined to the plane of the component to be clamped and connected to a planar contact surface of said movable wedge member via elastomeric shim, said movable wedge member having another nonparallel planar contact surface parallel to the component to be clamped and frictionally connected to planar contact surface of said output member, said output member performing clamping of said component by the force generated by a spring acting on said movable wedge member in the direction parallel to said component and transformed into lateral distributed output force by the wedge action, thus realizing the clamping of the flat component.

6. A mechanical wedge mechanism ofclaim 1 embodied as a clamping device for interconnecting first and second coaxial mechanical components, comprising:

outer output member whose external cylindrical surface is frictionally connected to concave cylindrical surface of the first mechanical component and whose internal surface has first and second conical rings formed on it with apexes of said conical surfaces directed towards each other;

inner output member whose internal cylindrical surface is frictionally connected to convex cylindrical surface of the second mechanical component and whose external surface has first and second conical rings formed on it with apexes of said conical surfaces directed in the opposite directions;

first and second movable wedge members shaped as rings having conical external and internal surfaces, with apexes of external and internal conical surfaces directed in opposite directions;

with external and internal conical surfaces of said first ring conforming with respective said first conical rings of outer and inner output members, and external and internal conical surfaces of said second ring conforming with respective said second conical rings of outer and inner output members;

said second movable wedge member having a multitude of uniformly distributed around the circumference smooth holes of slightly larger diameters than said threaded holes, whose axes are parallel to the axes of the interconnected mechanical components and coinciding with axes of said threaded holes in said first movable wedge member;

threaded bolts passing through said smooth holes and engaged with said threaded holes, wherein uniform tightening of the bolts induces axial displacements and approaching of said movable wedge members thus causing expansion of the outer output member, shrinkage of the inner output member, and frictional interconnection of said first and second coaxial mechanical components.

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