US3748370A

US3748370A - Spacer-damper for conductors of overhead electric lines

Info

Publication number: US3748370A
Application number: US00273512A
Authority: US
Inventors: P Dalia
Original assignee: INNOCENTE RIGANTI DI INNOCENTE
Current assignee: INNOCENTE RIGANTI DI INNOCENTE; OFFICINE MECCANICHE INNOCENTE RIGANTI DI INNOCENTE E FR RIGANTI IT
Priority date: 1971-09-07
Filing date: 1972-07-20
Publication date: 1973-07-24
Anticipated expiration: 1990-07-24
Also published as: FR2152533B1; ATA717572A; DE2234721A1; ES404705A1; BE786073A; AT333364B; BR7205831D0; AU473092B2; AR200243A1; ZA724911B; AU4557272A; FR2152533A1; GB1396750A; NL7208464A

Abstract

A spacer for conductors of overhead electric lines has a rigid frame on which two or more arms are mounted equally spaced from each other. Corresponding to each arm, there is provided an elongated polygonal bar mounted on the frame. Each arm has a ring member having an inner surface arranged so as to substantially surround the bar - the inner surface having a polygonal crosssection with each corner of the inner surface being arranged opposite a different side of the bar. Rubber members are placed between the sides of the bar and the inner surface so as to resiliently support the arm on the bar and while the rubber members deform permit movement of the arm while being urged to a predetermined position by the restoring forces of the deformed rubber members.

Description

United States Patent 1 Dalia [111 3,748,370 1 July 24, 1973 i 1 SPACER-DAMPER FOR CONDUCTORS 0F OVERHEAD ELECTRIC LINES [75] Inventor: Piero Dalia, Varese, Italy I [22] Filed: July 20, 1972 [21] Appl. No.: 273,512

[30] Foreign Application Priority Data Sept. 7, 1971 Italy 28291 A/7l [52] U.S. Cl 174/42, 174/146, 287/85 R 51y Int; Cl. .r. .1102; 7 14, HOZg 7/12 [58] 'Field of Search..... 174/42, 146;

[56] References Cited UNITED STATES PATENTS 3,475,544 10/1969 Reed 174/42 3,567,841 3/1971 Kobayshi l74/42 Bradshaw r r 174/42 'luttle l74/42 Primary Examiner-Laramie E. Askin Attorney-Michael S. Striker [57] ABSTRACT A spacer for conductors of overhead electric lines has a rigid frame on which two or more arms are mounted equally spaced from each other. Corresponding to each arm, there is provided an elongated polygonal bar mounted on the frame. Each arm has a ring member having an inner surface arranged so as to substantially surround the bar the inner surface having a polygonal cross-section with each comer of the inner surface being arranged opposite a different side of the bar. Rubber members are placed between the sides of the bar and the inner surface so as to resiliently support the arm on the bar and while the rubber members deform permit movement of the arm while being urged to a '20 Claims, 5 Drawing Figures PATENTEDJUL24I975 SHEET 1 [IF 3 u n n m 1*... u I

SPACER-DAMPER FOR CONDUCTORS OF OVERHEAD ELECTRIC LINES BACKGROUND OF THE INVENTION The present invention relates to spacers, and particularly to a spacer for use with bundled conductors of overhead electric lines, and more specifically to a device of the type which simultaneously serves as a spacer of at least two conductors as well as a damper of the mechanical vibrations which may arise in the conductors.

Single conductors of bundled conductors, also called subconductors hereinafter, of overhead electric lines must be maintained at a required mutual distance from each other. This mutual distance must generally be so maintained all along the electric line, i.e. all along any span between adjacent towers. However, because of atomospheric conditions it is often difficult to maintain the required mutual distance between adjacent conductors when these extend long distances between adjacent towers. Phenomena, in principle similar to those arising in lines equipped with one single conductor for each circuit, may occur in bundled conductors, which phenomena often act to prevent the maintenance of the required mutual distances. The first phenomenon produces relatively small vibrations of the subconductors, generally caused by light winds, these vibrations having high frequencies and small amplitudes. Another pheriomenon results in the substantial vibration of the entire conductor bundle or of a single conductor. This vibration is characterized in violent and very wide or great amplitude oscillations and is often caused by strong winds striking the conductors, particularly when the same are covered by ice or snow. As a result of the high amplitude oscillations or vibrations, the bundle of conductors may be displaced such large distances at' any particular points'as to cause contact between conductors of different circuits and, consequently, result in short-circuit discharges.

As a result of the above phenomena, it is generally required to equip lines having bundled conductors with suitable means to space the subconductors from one another and it is also desirable that these devices damp the vibrations to. some extent. I

While the above phenomena cause lateral displacement of the parallel conductors, they also cause movements of the conductors in the longitudinal direction of the conductors. The longitudinal movements almost invariably result in stresses at the points where the spacers are fixed to the subconductors these stresses being created by forces of different intensity, direction and frequency and these movements may cause, mostly by fatigue, the breaking of the spacers and/or of the conductor strandsand so involve serious danger to persons and to the line itself.

Consequently, among the most required properties of such spacers is that they allow suitable relative mobility of their attachment points to the subconductors. It is necessary that such mobility can be realized in any direction and to reasonable or suitable amplitudes.

The known spacers do not provide the above mentioned relative mobility and so, very often, cause the breaking of the spacers themselves and/or the breaking of the subconductor strands. Different known spacers have been proposed. Some of these known spacers are in form of an elastic body with rigid clamps. Other known spacers have rigid bodies with elastic or jointed clamps. However, almost all of them, to a greater or lesser degree, are defective inthe above desired characteristic and therefore are not entirely suitable forthe intended purpose. Furthermore, a number of known spacers serve as spacers only and do not simultaneously damp the vibrations. Damping apparatus are also known but these have the disadvantage of requiring previous knowledge of the approximate magnitude'or estimate of the vibration conditions of the conductors this knowledge often not being available and difficult to obtain. The efficiency of these known dampers is a maximum when all of the expected conditions actually prevail, while their efficiency decreases rapidly when the real conditions depart from the expected ones. This may frequently happen as a result of the manner in which the dampers are installed, i.e. whether they are properly positioned along the spans according to previously calculated or empirical data. So, according to probability, one or more dampers installed on each span may fail to achieve their purpose.

SUMMARY'OF THE INVENTION Accordingly, it is an object of the present invent-ion to provide a spacer for conductors of overhead electric lines which is not possessed of the disadvantages of such spacers known in the prior art.

It is another object of the present invention to provide a spacer as described above which is simple in construction and economical to manufacture.

It is still another object of the present invention to provide a spacer of the type under discussion which, in addition to operating as a spacer, operates as a vibration damper. i

It is a further object of the present invention to provide a spacer which can maintain the distances between adjacent conductors of bundled conductors of overhead lines while minimizing the stresses imposed on the spacer as well as on the conductors.

It is still a further object of the present invention to provide a spacer which is relatively free from risks of destruction and which simultaneously minimizes the probability of damage to the conductors which it spaces.

It is yet a further object of the present invention to provide a spacer which accomplishes the above objects while minimizing the criticality of the position of the spacer along the conductors.

According to the present invention, a spacer for bundled conductors of overhead electric lines comprises a rigid support member having first bearing means. At least two spaced arm means are provided, each arm means being oriented in a predetermined position relative to said support member and arranged to hold 'at least one conductor and having second bearing means spaced from said first bearing means. Resilient means are positioned between said first and second bearing means for supporting said arm means on said support member, and arranged to deform in response to relative movements between said arm means and said sup port member from said predetermined positionnAccordingly, said resilient means exerts forces on said bearing means in response to deformation to restore said support member and said arm means to their predetermined relative position.

According to a presently preferred embodiment, said rigid support member comprises a frame, and for each arm a bar mounted on the frame. Said bar has an axis and a plurality of sides parallel to said axis. Said arm means comprises a rigid member having an inner surface arranged substantially coaxially around said bar. Said sides of said bar and said inner surface are spaced from each other and respectively form said first and second bearing means. According to a still further feature of the present invention, said inner surface has a cross-section of a polygon having a number of sides equal to the number of sides of said bar, each two sides of the polygon forming a vertex which is positioned opposite a respective side of the bar. Each vertex is rounded and substantially has a cross-section of a parabola, each parabolic section and each respective pposite side of said bar together defining a seat, and each resilient means being positioned in respective seats in a precompressed state so that said resilient means are free to roll in said seat when said arm means turns relative to said bar about the axis of the latter.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmented front elevational view, partly in section, of a conductor carrying arm forming part of the spacer in accordance with the present invention;

FIG. 2 is a side view, partly in section, of the arm shown in FIG. 1 taken on line XX of FIG. 1 with the arm rotated approximately 45; and

FIGS. 3, 4 and are front elevational views of spacers in accordance with the present invention, for use on twin, three or four conductor lines, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The spacer in accordance with the present invention is provided with a plurality of articulated arms each of which is constructed identically as the others one of these arms is illustrated in FIG. I and identified with reference numeral 1. This arm clamps a conductor, as is to be described. The arm 1 comprises an upper portion, as viewed in FIG. 1, which is in the form of an annular member or ring 2 having an inner surface 2a. The inner surface 2a surrounds a passage which advantageously has a cross-section resembling a regular polygon having a number of internal sides selected in accordance with considerations still to be described. Adjacent ones of the sides of the polygon bounded by the internal surface 2a join to form a corner. In accordance with the presently preferred embodiment, the corners are rounded in the form of parabolic sections 3.

The arm 1 is mounted for movement on a frame 4, the latter carrying all the arms, as can best be seen in FIGS. 3-5. Referring to both FIGS. 1 and 2, it will be noted that the frame 4 is arranged to receive a bar 5 which is rigidly fixed to the frame by means of a bolt 50. The bar 5 is fixedly mounted in the frame 4 against movement relative to the latter. In the presently preferred embodiment, the bar 5 has a square crosssection and therefore has four sides. Each of the sides is longitudinal and substantially parallel to the axis of the bar 5 which is coaxial with the axis of the bolt 5a.

As described above, the inner surface 2a is provided with a number of side surface portions which is selected in a specific manner. This method of selection is to provide the inner surface 2a with as many sides as has the bar 5. However, the bar 5 is so oriented in relation to the inner surface 2a so that the parabolic sections 3 are located oppositely to the sides of the bar 5.

Between each side of the bar 5 and a corresponding parabolic section 3, which together form a space or seat, there is provided a resilient means in the form of a rubber member 6. The rubber members 6 are desirably precompressed to some extent and resiliently support the ring 2 on the bar 5 of the frame 4.

It will be noted, particularly from FIG. 1, that the ring 2, and therefore the arm 1, can rotate angularly about the axis of the bar 5. In the normal or predetermined position of the arm, as shown in FIG. 1, the spaces or seats formed between each bar surface or side and a parabolic section 3 have a maximum size. However, upon angular rotation of the arm 1 about the axis of the bar 5, each seat decreases in cross-sectional area. Such a decrease in the area causes further compression of the rubber members 6 such further compression generating restoring forces which act upon both the bar 5 and on the inner surface 20 in such a manner so as to restore the relative position of the articulated arm 1 to its predetermined position.

In view of the angular movements which the arm 1 may make relative to the bar 5, stop means are provided which comprise a stop 7 mounted on the ring 2 for movement therewith and stops 8 to each side of the stop 7 which are fixedly mounted on the frame 4. In accordance with the presently preferred embodiment, the stop 7 is made of rigid material or hard material and the stops 8 are made of resilient material.

The lower portion of the arm 1 shown in FIG. 1, includes a subconductor clamp 9 which is arranged to clamp and hold at least one conductor in a wellknown manner. Each clamp 9 has an aperture or cylindrical opening 9a whose axis is substantially parallel to the axis of the bar 5. Referring to FIG. 2, the clamp 9 is shown to hold a conductor 10, as during normal use.

The rubber members 6 may have the shapes of cylindrical sections. However, the rubber members may take any other preferred form including spheres, etc. In FIG. 2, a rubber member 6 is shown in section and positioned between a side of the bar 5 and the inner surface 2a. In order to prevent the rubber member 6 from being forced out from the space or seat by relative movements of the bar and the arm, a pair of plates 11 is provided which extend across the clearance between the inner surface 2a and the sides of the bar 5.

The bar 5 has an axial length which is greater than that of the ring 2 so that the bar extends axially beyond the ring, as can best be seen in FIG. 2. To avoid contact between the edges of the bar 5 and the inner surface 20 or the plates I 1, grooves 12 are provided opposite each edge of the bar. In this manner, the am 1 can move in the directions generally indicated by the lines XX and YY as well as angularly rotating about the axes of the bar 5. As a result of the length of the bar 5 being greater than the length of the ring 2, a recess or space 13 is provided by spacing the walls 4a from each other a distance which is greater than the length of the ring 2. With this construction, the ring 2 can move axially towards the left or the right, as viewed in FIG. 2, until the plates 11 come into contact with a respective inner wall 4a of the frame. During such axial movement, the rubber member 6 rubs against the faces or sides of the square bar 5.

The operation of the spacer will now be described. It will be understood that when the device is at rest or when the arms are in their predetermined positions, the space available to the rubber members 6 is at a maximum. As soon, however, as the arms 1 start vibrating, as a result of vibrations of the conductors in planes transversely to the center lines of the conductors, the rubber members 6 roll and the spaces or seats between the parabolic sections 3 and the opposite sides of the bar 5 become smaller in response to angular displacement of the arm 1 about the axis of the bar 5. The greater the angular displacement of the arm 1, the smaller the spaces or seats will become with resultant elastic deformation of the members 6. At the beginning of the movement of the arm, the resistance of the rubber members 6 to deformation is relatively low. However, the resistance increases rapidly due to the rapid reduction of the spaces in which the rubber members 6 are confined, as a result of the increased compression of these members.

The construction of the joint is suchflthat the natural elastic hysteresis of the rubber members is increased by the friction occurring during compression while the rubber members roll and are compressed in their seats simultaneously with the rotation of the arms. The friction also causes a lag in the elastic response of the rubber members the more these rubber members are compressed or confined into smaller spaces or seats, the more is the elastic hysteresis and the efficiency of the damping effect.

- Furthermore, the transition from the area in which the rubber members 6 are compressed normally to the area in which the samerubber members are still more compressed causes a flex point in the strength curve of the device. This feature, in particular, prevents the device or spacer from remaining in resonant or oscillating conditions.

Obviously, depending on the mechanical characteristics of the conductor bundle, the spacer-damper is made differently for different applications, i.e., insofar as shape, dimensions and weight are concerned.

Another feature of the present invention favorably effects the uniform performance of the novel spacers and the resistance of its components to fatigue and the durability .of uniform performance. This feature is achieved in that only the rubber members or resilient means-perform the brunt of the work bybeing compressed. and absorb most. of the stresses.

It will become clear that the, articulated joint in accordance with the present invention makes it possible for the arms to oscillate or angularly turn in a plane perpendicular to the bundle center-line as well as to move in other directions including directions outside the plane of FIG. 1. In order to permit the subconductors to move in a longitudinal plane passing through the pivoting axis of the arm 1 and the clamp centerline, or alternatively, in a plane perpendicular to the abovementioned plane, the grooves 12 of FIG. I are provided, so as to afford room for the edges of the bar in those positions in which the edges are closer to the inner surface 2a of the ring 2.

As a precaution against the event that the device may become subjected to violent actions causing the arm 1 to turn by a greater than normal amount, the articulated joint is also provided'with elastic limit stops, consisting of a stop 7 protruding from the ring 2 which is arranged to engage with the elastic stop 8 fixed on the frame in a suitable position, as described above. This limit means avoids the rigid stopping of the arm at both ends of each oscillation.

A further advantage of the present invention is that its articulated joint makes it possible to slide the arm 1, together with the rubber members 6, along the bar 5' in the space 13 shown in FIG. 2, as described above. Such axial movement may take place in any of the two directions parallel to the bundle center lines. Such sliding of the arm 1 may occur after the installation of the spacerdamper on a bundled conductor. This sliding, which would occur automatically in response to the subconductor action on the arm of the spacer-damper, maintains the clamp aperture 9a or the axis thereof parallel with the axis of the bar 5 as well as with the axes of the other apertures mounted on the frame 4. The flexibility thereby provided by permitting the arms 1 to move slightly in one or the other of the axial directions of the conductors, avoids possible couples on the clamps 9 and on the device itself. These couples could arise in practice, for instance, as a result of the defective'installation of the device on the bundled conductor or of subsequentrelative movements of single conductors in the longitudinal direction either as a result of winds or of elongation of one conductor relative to the others. It will thus be appreciated, that the present invention is able to serve both as a damper of small vibrations and as a subconductor spacer at one and the same time. Also, the invented device, as a damper, is intended to overcome other disadvantages of the prior art. Thus, a number of devices may be installed on each span so that, in addition to essential properties, as above specifled, the probability will beincreased that at least'one of the installed devices will be positioned in an antinode, or in the vicinity of an antinode, of the sinusoidal diagram of the conductor vibration to thereby place it in the best of positions suitable todamp vibrations.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of spacers, differing from the types described above.

While the invention has been illustrated and described asembodied in a spacer for conductors of overhead electric lines which simultaneously acts as a vibration damper, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art fairly constitute essential characteristics of the generic or specific aspects of the invention, and therefore, such adaptationsshould and are intended to be comprehended within the meaning means including second bearing means, and each of said second bearing means being located adjacent one of the respective first bearing means but spaced therefrom, each of said arm means being adapted to hold at least one conductor; and elastomeric resilient means interposed between the respective first and second bearing means for supporting said arm means on said support member, said resilient means being precompressed and arranged to deform in response to relative movements of said arm means and said support member, whereby said resilient means exerts a biasing action in a sense urging said arm means and said support member to said predetermined position.

2. A spacer as defined in claim 1, wherein said first and second bearing means comprise a plurality of spaced first and second elongated surfaces respectively.

3. A spacer is defined in claim 2, wherein said resilient means is substantially configurated in the form of cylinders having axes substantially parallel to said surfaces.

4. A spacer as defined in claim 1, wherein said resilient means is substantially configurated in the form of spheres.

5. A spacer as defined in claim 1, wherein said resilient means is composed of rubber.

6. A spacer as defined in claim 1, wherein said support member has an axis and said arm means can move angularly about said axis relative to said support member, and further comprising stop means for limiting the degree of relative angular movement between said support member and said arm means.

7. A spacer as defined in claim ll, wherein said support member and said arm means are composed at least in part of metallic material.

8. A spacer as defined in claim 1, wherein said support member and said arm means are made from a metal alloy.

9. A spacer as defined in claim 1, wherein a predetermined number of conductors are to be spaced from each other, and wherein a number of arm means equal to said predetermined number are provided, said arm means being substantially equi-angularly spaced about said support member.

10. A spacer for conductors of overhead electric lines, comprising a rigid support member including a frame and a bar mounted on said frame, said bar having an axis and a plurality of sides which extend parallel to said axis and contribute first bearing means; at least two spaced arm means oriented in a predetermined position relative to said support member, each of said arm means having second bearing means spaced from said first bearing means, said arm means including a ring member having an inner surface arranged substantially coaxially about said bars, said inner surface being spaced from said sides and constituting said second bearing means, each of said arm means being arranged to hold at least one conductor; and resilient means positioned between said first and second bearing means for supporting said arm means on said support member, said resilient means being arranged to deform in response to relative movements between said arm means and said support member from said predetermined position, whereby said resilient means exerts forces on said bearing means in response to deformation to restore said support member and said arm means to their predetermined position.

11. A spacer as defined in claim 10, wherein said inner surface defines a passage which has a cross section which resembles a polygon having a number of sides equal to the number of sides of said bar, each two adjacent sides of the inner surface forming a corner whi is positioned opposite a respective side of said bar.

12. A spacer as defined in claim 11, wherein each corner is rounded and substantially has a cross section of a parabola.

113. A spacer as defined in claim 12, wherein each side of said bar is in a position of a chord of a respective oppositely positioned parabolic section.

14. A spacer as defined in claim 12, wherein each parabolic section and each respective opposite side of said bar together define a seat, and wherein said resilient means are positioned in said seats in a precompressed state, so that said resilient means are arranged to move a limited degree in said seats when said arm means turns relative to said bar about the axis of the latter.

15. A spacer as defined in claim 10, wherein said ring member has two open ends, and further comprising holding means provided at each of said ends for maintaining said resilient means between said sides of said bar and said inner surface of said ring.

16. A spacer as defined in claim 15, wherein said holding means comprise plates extending between said sides of said bar and at least said inner surface of said ring member.

17. A spacer as defined in claim 10, wherein said inner surface is provided with a plurality of longitudinal slots each opposite a respective edge formed by two sides of said support member when the latter and said ring members are in said predetermined relative position.

18. A spacer as defined in claim 10, wherein saidring member has an axial length which is smaller than the axial length of said bar and said ring member is arranged to move in axial direction on said bar, and wherein each arm means is provided with a clamping member, each clamping member having a substantially cylindrical aperture with an axis substantially parallel to said axis of said bar, said axes of said apertures remaining substantially parallel to said axis of said bar in all axial positions of said ring member on said bar, whereby stresses are prevented in said frame as a result of axial movements of one or more ring members.

19. A spacer for conductors of overhead electric lines, comprising a rigid support member having first bearing means and an axis; at least two spaced arm means oriented in a predetermined position relative to said support member, said arm-means being arranged for angular movement about said axis relative to said support member and each having second bearing means spaced from said first bearing means, each of said arm means being arranged to hold at least one conductor; resilient means positioned between said first and second bearing means for supporting said arm means on said support member, said resilient means being arranged to deform in response to relative movements of said arm means and said support member from said predetermined position, whereby said resilient means exerts forces on said bearing means in resaid support member, said stop means including a rigid lected degree. portion extending from said arm means and a resilient 20. A spacer as defined in claim 19, wherein said reportion connected to said support member, said resilsilient portion is composed at least in part of elastoient portion being arranged to engage said rigid portion meric material. when said relative angular movement exceeds a prese-

Claims

1. A spacer for conductors of overhead electric lines and the like, comprising a rigid support member having at least two discrete first bearing means; at least two spaced arm means oriented in a predetermined position relative to said support member, each of said arm means including second bearing means, and each of said second bearing means being located adjacent one of the respective first bearing means but spaced therefrom, each of said arm means being adapted to hold at least one conductor; and elastomeric resilient means interposed between the respective first and second bearing means for supporting said arm means on said support member, said resilient means being pre-compressed and arranged to deform in response to relative movements of said arm means and said support member, whereby said resilient means exerts a biasing action in a sense urging said arm means and said support member to said predetermined position.

7. A spacer as defined in claim 1, wherein said support member and said arm means are composed at least in part of metallic material.

13. A spacer as defined in claim 12, wherein each side of said bar is in a position of a chord of a respective oppositely positioned parabolic section.

14. A spacer as defined in claim 12, wherein each parabolic section and each respective opposite side of said bar together define a seat, and wherein said resilient means are positioned in said seats in a pre-compressed state, so that said resilient means are arranged to move a limited degree in said seats when said arm means turns relative to said bar about the axis of the latter.

18. A spacer as defined in claim 10, wherein said ring member has an axial length which is smaller than the axial length of said bar and said ring member is arranged to move in axial direction on said bar, and wherein each arm means is provided with a clamping member, each clamping member having a substantially cylindrical aperture with an axis substantially parallel to said axis of said bar, said axes of said apertures remaining substantially parallel to said axis of said bar in all axial positions of said ring member on said bar, whereby stresses are prevented in said frame as a result of axial movements of one or more ring members.

19. A spacer for conductors of overhead electric lines, comprising a rigid support member having first bearing means and an axis; at least two spaced arm means oriented in a predetermined position relative tO said support member, said arm means being arranged for angular movement about said axis relative to said support member and each having second bearing means spaced from said first bearing means, each of said arm means being arranged to hold at least one conductor; resilient means positioned between said first and second bearing means for supporting said arm means on said support member, said resilient means being arranged to deform in response to relative movements of said arm means and said support member from said predetermined position, whereby said resilient means exerts forces on said bearing means in response to deformation to restore said arm means and said support member to their predetermined relative position; and stop means for limiting the degree of relative angular movement between said arm means and said support member, said stop means including a rigid portion extending from said arm means and a resilient portion connected to said support member, said resilient portion being arranged to engage said rigid portion when said relative angular movement exceeds a preselected degree.

20. A spacer as defined in claim 19, wherein said resilient portion is composed at least in part of elastomeric material.