AU2002256127B2 - Compression spring rod - Google Patents

Description

COMPRESSION SPRING ROD

BACKGROUND OF THE INVENTION

The present invention relates to compression spring rods, and more particularly, to a

spring and rod assembly that exerts an expansional force which increases at a linear rate.

The invention relates to a mechanism for biasing hoods, tops, doors, hinged covers, and

other elements from a closed to an open position. The invention involves the use of springs in

conjunction with a rod member to exert the driving force on the elements to be displaced. The

following patents are incorporated herein by reference as background information with regard

to spring mechanisms: United States Patent No. 6,199,843 to DeGrace; United States Patent No.

5,810,339 to Kuspert, et al.; and United States Patent No. 4,962,916 to Palinkas.

Compression spring rods are used in various applications, for example, to assist in lifting,

opening, and damping. Typical applications include lifting a lid hinged to a stationary base.

Other applications include lifting and/or balancing elements for the trunk or hatchback of an

automobile. Still another application includes a damping spring for closing a door hinged to a

stationary frame. Most applications involve the use of a pneumatic or gas spring to assist the

opening motion. Many of these types of compression spring assemblies contain either gas or

hydraulic fluid to control forces and piston speeds. Consequently, because these products

contain a gas and/or fluid, they are subject to premature failure, due to the leakage of the gas or

fluid over time. The leakage results in a loss of control forces and a subsequent loss of spring

life.

SUMMARY OF THE INVENTION

The present invention provides an improved compression spring rod which overcomes the above referred-to difficulties and others with regard to such rods heretofore available. More

particularly in this respect, a compression spring rod in accordance with the invention is

particularly adapted for lifting or pivoting one component relative to another component at a

controlled rate. In accordance with one aspect, the invention provides a lift mechanism for

hinged covers and the like that operates automatically upon release of the cover, or a lift

mechanism for a loaded platform wherein the platform is elevated, progressively, as the load

thereon is reduced. Advantageously, the compression spring assembly applies a constant and

controlled force to open the cover or lift the platform. The mechanism is able to support

significant loads while maintaining strength over a greater number of operating cycles than

existing pneumatic or gas spring designs. Further, the invention provides a purely mechanical

compression rod assembly that can yield controllable forces over a long period of use and control

the spring forces during both extension and compression.

A compression spring rod according to the invention is comprised of multiple

compression springs. The compression spring rod assembly includes a rod which is adapted to

extend and retract relative to a housing. In one application, for example, the compression springs

will build potential force as the springs are compressed, and release that force once the springs

are allowed to expand. This extension of the springs imparts a force to the parts connected to the

ends of the rod and housing and, advantageously, multiple end configurations can be used to

adapt the spring rod to a variety of mounting applications. The compression springs of a spring

rod according to the invention are interrelated to produce a linear load versus deflection curve.

The encased springs minimize load losses over time, and the mechanism does not contain any

fluid or gases within the lift body. This advantageously eliminates the inevitable problem of leakage and subsequent loss of utility.

It is accordingly an outstanding object of the present invention to provide an improved

compression spring rod for exerting an operating force on a displaceable member at a controlled

rate.

Another object of the invention is the provision of a compression spring rod that supplies

a consistent force over an extended period of time and maintains strength over a greater number

of cycles compared to compression spring rods heretofore available.

Yet another object of the invention is the provision of a compression spring rod having

at least two compression springs interrelated to produce a linear load versus deflection curve.

Yet another object of the invention is the provision of a compression spring rod having

at least two compression springs interrelated to minimize load losses over time.

A further object of the invention is the provision of a mechanical compression spring rod

assembly that provides an operating force that increases at a linear rate.

Yet a further object of the invention is to provide a mechanical compression spring

assembly that can accommodate, selectively, multiple end configurations, thus adapting the

assembly for mounting in a wide variety of use applications.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and advantages will in part be obvious and in part

pointed out in the following description taken together with the accompanying drawings in

which:

FIGURE 1 is a side elevation view, partially in section, of a compression spring rod

according to the invention in the extended position; FIGURE 2 is a longitudinal cross section view of the compression spring rod in the

compressed position;

FIGURE 3 is a cross sectional view taken along line 3-3 of FIGURE 2;

FIGURE 4 is an exploded perspective view of the component parts of the compression

spring rod shown in FIGURES 1-3;

FIGURE 5 is a side elevation view of the compression springs of the compression spring

rod;

FIGURE 6 is a graph illustrating the relationship between spring force and compression

of the compression spring rod assembly;

FIGURE 7 is a perspective view of a box with a lid pivotable about a horizontal axis and

compression spring rod elements shown in FIGURES 1-3 between the box and lid;

FIGURE 8 is a side elevation view, in section, of a spring rod in accordance with a

second embodiment of the invention;

FIGURE 9 is an exploded perspective view of the component parts of the compression

spring rod shown in FIGURE 8;

FIGURE 10 is a perspective view illustrating a use of the compression spring rod of

FIGURES 8 and 9;

FIGURE 11 is a side elevation view, in section, of a spring rod in accordance with

another embodiment of the invention;

FIGURE 12 is an exploded perspective view of the component parts of the compression

spring rod shown in FIGURE 11 ; and,

FIGURE 13 is an illustration of an application of the compression spring rod of FIGURES 11 and 12.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Referring now in greater detail to the drawings, wherein the showings are for the purpose

of illustrating preferred embodiments of the invention only, and not for the purpose of limiting

the invention, a compression spring rod 10, in accordance with the invention, as shown in

FIGURES 1-6, has an axis 11 and includes a rod member 22 which is axially extendable and

retractable relative to a one-piece tubular housing 24. Rod 22 has an outer end 22a and an inner

end 22b connected to a guide rod 26 as set forth more fully hereinafter. Guide rod 26 extends

axially inwardly of inner end 22b of rod 22 and is surrounded by a first compression spring 28

which is supported by the exterior surface 46 of guide rod 26 against buckling. First

compression spring 28 is surrounded by a second compression spring 30 which is supported

against buckling by the interior surface 48 of housing 24. Alignment of compression springs 28

and 30 relative to one another and axis 11 is maintained by the exterior surface 46 of guide rod

26 in conjunction with the interior surface 48 of housing 24. Housing 24 has a mounting end 23

and an outer or opposite end 25, and compression springs 28 and 30 are axially captured between

a tail bushing 38 at mounting end 23 and a guide member 34 mounted between guide rod 26 and

the inner end 22b of rod 22 as set forth hereinafter. Tail bushing 38 is supported in housing 24

by bending the endmost portion of the housing radially inwardly to define a retaining flange 39.

Compression spring rod 10 involves the use of a one-piece housing 24 which facilitates

smooth movement of lift rod 22 and compression springs 28 and 30 during operation of the

spring rod. As shown in the exploded view of FIGURE 4, tail bushing 38 includes a neck

portion 42 having a diameter sized to be received in the interior of compression spring 28. Tail bushing 38 also has a threaded stud 44 distal to the neck portion 42 which is received in a

threaded recess 17 in a mounting element 18. Guide rod 26 includes a threaded stud 52 at the

outer end thereof which passes through an opening 58 in guide member 34 and into a threaded

bore 54 provided therefor in rod 22. Lift rod 22 passes through an opening 72 through a rod

bushing 32 at outer end 25 of housing 24, and has a threaded stud 70 on outer end 22a thereof

which is received in a threaded recess 27 provided therefor in a mounting element 20. Mounting

elements 18 and 20 have openings 19 and 21 therethrough, respectively, for receiving a variety

of different mounting components common in the industry including, for example, pins, bolts,

swivels, and the like. Advantageously, the threaded studs 44 and 70 at opposite ends of the

spring rod assembly provide for accommodating the use of different mounting elements than

those shown so as to modify the assembly for use in a variety of structural environments.

Guide member 34 is slidable in housing 24 and includes a guide ring 35 of suitable

material to facilitate such sliding movement. Rod 22 is slidably supported at end 25 of housing

24 by rod bushing 32 which is secured to the housing by a pair of set screws 40 having inner ends

received in an annular recess 41 in the rod bushing. Rod bushing 32 is further axially retained

in housing 24 by bending the outermost part of end 25 radially inwardly to provide a retaining

flange 33. At full extension, rod 22 is cushioned by rod bushing 32 and an impact absorbing

metal spring ring 36 received in a recess 55 at inner end 22b of rod 22 adjacent the axially outer

face of guide member 34. When rod 22 is fully extended, spring ring 36 engages in a recess 51

in the axially inner end of rod bushing 32. Lubrication can be provided in housing 24 to facilitate

the sliding movement of guide member 34 therein. As will be appreciated from the foregoing

description, guide member 34 and rod bushing 32 support rod 22 for reciprocation in housing 24 such as to maintain minimal breakaway forces for rod 22. Additionally, guide member 34 and

rod bushing 32 keep rod 22 coaxial with axis 11 and decrease the effect of side loading on the

assembly.

Compression spring rod 10, through the multiple spring rate characteristics of

compression springs 28 and 30, serves to provide smooth extension forces to the movement of

lift rod 22 from the retracted to the extended position thereof relative to housing 24. Depending

upon the application, the appropriate load versus deflection can be determined and the

corresponding physical and elastic properties of the combination of compression springs 28 and

30 can then be ascertained. The compression springs 28 and 30 can each be fabricated from

spring material, such as music wire, and, for example, ASTM A228 or 302 stainless steel.

Each compression spring 28 and 30 has a different stress and strain characteristic. If the

spring is considered to be a one-dimensional object, the only stress will be extensional (or

compressional, which will be the negative of extensional) and the units of stress will be force per

unit of extension. Within a range of compression, each spring obeys "Hook's Law", which states

that for forces in a defined range, the stretch of a material is proportional to the applied force:

F = -kΔL

The proportionality constant, k, is known as the spring constant with dimensions of force over

length, and ΔL is the amount of compression. The negative sign indicates that the force is in the

opposite direction of extension: if the spring is extended, the force tries to restore it to its

original length. Likewise, if the spring is compressed (ΔL < 0), the force attempts to expand the

spring, again to its original length. The spring constant depends on both physical and elastic

properties of the material being stretched. Hook's Law is fairly intuitive at a basic level, and can be illustrated by everyday experience in which it is known that a thin wire will stretch more than

a thick wire or rod of the same material when the same stretching force is applied to both. The

formula U= V k(ΔL)², gives the work of extension (U) or alternatively, the amount of potential

energy stored in the spring.

As shown in FIGURES 3 and 5, compression spring 28 has a free length LI which is

greater than the free length L2 of spring 30, and spring 28 has an outer diameter greater than that

of compression spring 30. Also, the wire diameter of spring 28 is less than that of spring 30, and

the spring rate of spring 28 is less than that of spring 30. As an example of one particular

application, the specific physical characteristics of compression spring 28 are: wire diameter

.055", inside diameter .5444", outside diameter .6544", free length 17.2", and a spring rate of .95

lbs./inch; and the physical characteristics of compression spring 30 are: wire diameter .081",

inside diameter .675", outside diameter .837", free length 13.8", and a spring rate of 3.37

lbs./inch. FIGURE 6 displays the load versus deflection curve for compression springs 28 and

30 having the foregoing specifications, and for the combined springs in the assembly shown in

FIGURES 1 and 2. It is to be noted that springs 28 and 30 are oppositely wound and that this

interrelationship together with the dimensional characteristics of the springs produces the

combined linear load versus deflection graph depicted in FIGURE 6. The different free lengths,

as shown in FIGURE 5, of springs 28 and 30 is one component that helps to control the forces

and stabilize the guide member 34 and rod 22 during initial displacement thereof from the

position shown in FIGURE 1 to the position shown in FIGURE 2 and during the termination of

the movement from the position shown in FIGURE 2 to the position shown in FIGURE 1. In

this respect, the longer spring 28 is, in the free state of the spring 30 shown in FIGURE 1, slightly compressed to the length of the latter spring and, therefore, exerts a stabilizing force on

the components which eliminates any free play during initial and terminal displacement thereof

during use.

FIGURE 7 illustrates two compression spring rods 10 according to the invention

connected between a box 12 and a lid 14 therefor. While not shown in detail, lid 14 is suitably

mounted on box 12, such as by hinges, to be pivotable about an axis A relative thereto. The

mounting elements 18 and 20 of compression spring rods 10 are suitably secured to box 12 and

lid 14, respectively. A latch 15 is shown on lid 14 for engagement with a keeper 16 on box 12

to releasably hold the lid closed relative to box 12. Latch 15 may be of various types common

in the industry, and the method for releasing latch 15 may be by hand, foot, key, remote, etc.

Subsequent to releasing the latch 15, compression spring rods 10 automatically extend from the

position shown in FIGURE 2 to the position as shown in FIGURE 1, during which the spring

rods 10 expand, releasing the stored compressive force in compression springs 28 and 30 to

displace lid 14 from the closed to the open position thereof.

FIGURES 8 and 9 illustrate another embodiment of a compression spring assembly

according to the invention. In this embodiment, compression spring rod 100 has an axis 101 and

includes a rod 102 which is axially extendable and retractable relative to a one-piece tubular

housing 104. Rod 102 has an outer end 102a and an inner end 102b connected to a guide rod 106

as set forth more fully hereinafter. Guide rod 106 extends axially inwardly of inner end 102b of

rod 102. A first compression spring 108 is supported against buckling by the exterior surface

103 of rod 102. Spring 108 is surrounded by a second compression spring 110 which is

supported against buckling by the interior surface 105 of housing 104. Coaxial alignment of compression springs 108 and 110 relative to one another and axis 101 is maintained by the

exterior surface 103 of rod 102 in conjunction with the interior surface 105 of housing 104.

When assembled, compression springs 108 and 110 are axially captured between a rod bushing

112 at end 114 of housing 104 and a guide member 116 secured to inner end 102b of the rod

between the latter and guide rod 106. Guide rod 106 includes a threaded stud 107 at the outer

end thereof which passes through an opening 115 in guide member 116 and into a threaded bore

137 provided therefor in rod 102. A tail bushing 120 is supported in end 124 of housing 104 by

set screws 122 received in an annular recess 126 in the tail bushing 120. For the purpose set

forth hereinafter, the component parts of spring rod 100 are cushioned during operation of the

compression spring assembly by a cushioning spring 130 which surrounds guide rod 106. Spring

130 is axially captured between the tail bushing 120 at end 124 and the guide member 116. Tail

bushing 120 includes a neck portion 123 having a diameter sized to be received in the interior

of cushioning spring 130. Tail bushing 120 also has a threaded stud 125 distal to neck portion

123 which is received in a threaded recess 127 in a mounting element 132. Guide member 116

is slidable in housing 104 and includes a guide ring 117 of suitable material to facilitate such

sliding movement. Rod 102 is slidably supported at end 114 of housing 104 by rod bushing 112

which is secured to housing 104 by a pair of set screws 122 having inner ends received in an

annular recess 113 in bushing 112. Rod 102 passes through an opening 111 in bushing 112 at

outer end 114 of housing 104, and has a threaded stud 139 on outer end 102a thereof which is

received in a threaded recess 140 provided therefor in a mounting element 134. As will be

appreciated from the foregoing description, guide member 116 and rod bushing 112 support rod

102 for reciprocation in housing 104 such as to maintain πύnimal breakaway forces for rod 102. Mounting elements 132 and 134 have openings 133 and 135 therethrough, respectively, for

receiving a variety of different mounting components common in the industry including, for

example, pins, bolts, swivels, and the like. Mounting element 132 is fixedly attached to tail

bushing 120 for mounting the compression spring assembly to a work supporting surface.

FIGURE 10 illustrates four compression spring rods 100 each connected between a

corresponding fixed support 142 and a platform or work supporting table 140. The spring rods

100, as shown in FIGURE 10, are designed to expand in the direction of arrow z in response to

a load applied to platform 140, thus compressing springs 108 and 110. Compression springs 108

and 110 then expand and retract the spring rods in the direction of arrow y as the load is

progressively removed from platform 140. Compression springs 108 and 110 have the same

physical characteristics as compression springs 28 and 30 described in the first embodiment. The

arrangement of spring rods 100 and platform 140 as shown in FIGURE 10 is suitable, for

example, as a progressive load lifter, such as for metal plates. As plates are progressively stacked

on platform 140, the spring rods 100 extend in the direction of arrow z, whereby the compression

springs 108 and 110 are progressively compressed. As stated, when the springs are compressed

(ΔL < 0) the resultant force attempts to expand the spring rod to its original length. Thus, as the

plates are progressively removed from the platform, the compression springs 108 and 110 expand

thereby causing the platform 140 to move in the direction of arrow y. In this manner, the springs

provide controlled forces by which the top plate in the stack on the platform remains at a given

level as the platform moves first in the z direction and then in the y direction. If the entire load

is suddenly removed from the platform, the spring rods retract rapidly and cushioning spring 130

cushions the retracting movement to protect the spring rods against damage. Additionally, it will be appreciated that this embodiment is particularly well suited as a counterbalance system,

conveyor chain tensioner, door lift assist, and dampener.

FIGURES 11 and 12 illustrate another embodiment of a compression spring assembly

according to the invention. In this embodiment, compression spring rod 180 has an axis 181 and

includes two rods 182 and 184 which are alternately axially extendable and retractable together

relative to a one-piece tubular housing 186. Rod 182 has an outer end 182a and an inner end

182b and rod 184 has an outer end 184a and an inner end 184b connected to inner end 182b of

rod 182 together with a guide member 200 as set forth more fully hereinafter. Rod 182 extends

axially inwardly of end 186a of housing 186 and is surrounded by a first compression spring 188

which is supported by the exterior surface 183 of rod 182 against buckling. First compression

spring 188 is surrounded by a second compression spring 190 which is supported against

buckling by the interior surface 187 of housing 186. Rod 184 extends axially inwardly of

opposite end 186b of housing 186 and is surrounded by a third compression spring 192 which

is supported by the exterior surface 185 of rod 184 against buckling. Third compression spring

192 is surrounded by a fourth compression spring 194 which is supported against buckling by

the interior surface 187 of housing 186. Alignment of compression springs 188, 190, 192, and

194 relative to one another and axis 181 is maintained by the exterior surfaces 183 and 185 of

rods 182 and 184, respectively, in conjunction with the interior surface 187 of housing 186.

Compression springs 188 and 190 are axially captured between a rod bushing 196 at end 186a

and the guide member 200, and compression springs 192 and 194 are axially captured between

a rod bushing 202 at end 186b of the housing and the guide member. Rod bushing 196 is

supported in housing 186 by set screws 206 at end 186a thereof which extend into an annular recess 197 in bushing 196. Similarly, rod bushing 202 is supported in housing 186 by set screws

206 at end 186b thereof which extend into an annular recess 203 in the bushing. Axial retention

of bushings 196 and 202 is further enhanced by bending the corresponding end of housing 186

radially inwardly of the bushings.

The compression spring rod 180 involves the use of a one-piece housing 186 which

facilitates smooth movement of rods 182 and 184 and compression springs 188, 190, 192, and

194 during operation of the spring rod. As shown in the exploded view of FIGURE 12, rod 184

includes threads 230 and 231 at opposite ends of the rod. Threads 230 are received in a threaded

recess 232 in a mounting element 220. Threads 231 pass through an opening in spring ring 210,

an opening through guide member 200, and an opening in spring ring 208 and are received in a

threaded recess 235 in rod 182. Rod 182 includes threads 234 distal to recess 235, which are

received in a threaded recess 233 in a mounting element 222. As will be appreciated from the

foregoing description, guide member 200 and rod bushings 196 and 202 support rods 182 and

184, respectively, for reciprocation in housing 186 such as to maintain minimal breakaway forces

for rods 182 and 184 in use of the spring assembly.

Spring rod assembly 180 is adapted to apply an extension force, alternately, in axially

opposite directions at a controllable rate. At full extension from housing 186, rods 182 and 184

are cushioned by rod bushings 196 and 202, respectively. In addition, impact in the direction of

extension is absorbed by metal spring rings 208 and 210 which are received in recesses 212 and

214, respectively, at inner end 182b of rod 182 and inner end 184b of rod 184. Spring rings 208

and 210 are adjacent the axially outer faces of guide member 200 and respectively engage rod

bushings 196 and 202 upon full extension of the rods in the respective direction of extension. Lubrication can be provided in housing 186 to facilitate the sliding movement of guide member

200 therein.

As shown in FIGURE 13, spring rod assembly 180 is capable of self-centering a load

which, as illustrated by way of example only, is in the form of two workpieces 230 and 232

having ends 230a and 232a pivotally attached to a fixed support member 231. Spring rod 180

has the outer ends of rods 182 and 184 thereof respectively pivotally connected to ends 232b and

230a of the workpieces. Spring rod 180 is supported centrally between workpieces 230 and 232

by a bracket 224 rigidly secured to support member 231 by a support arm 225. The springs of

each pair of compression springs 188 and 190 and 192 and 194 have the same physical

characteristics as compression springs 28 and 30 described in the first embodiment. In the

arrangement shown in FIGURE 13, spring rod 180 is a load centering assembly. In this respect,

it will be appreciated that if either workpiece 230 or 232 is displaced in the direction of arrow

c, rod 184 extends relative to housing 186 and the springs 192 and 194 are compressed. The

resultant force of springs 192 and 194 in the direction of arrow e attempts to expand the springs

to their original length. It will be appreciated that the opposite is true when either workpiece is

displaced in the direction of arrow d. In this respect, springs 188 and 190 are compressed and

springs 192 and 194 are totally relaxed. The resultant force in the direction of arrow e attempts

to expand springs 188 and 190 to their original length. During return movement of workpieces

230 and 232 to the central position thereof, the relaxed pair of springs cushion the return

movement. As with the earlier embodiments, the springs 188 and 190 and 192 and 194 provide

controlled forces to self-center workpieces 230 and 232 when either is deflected from the neutral

position. It will be appreciated that this embodiment is particularly well suited as a centering device in a steering mechanism, linkage mechanism, gating mechanism, and dampener.

While considerable emphasis has been placed herein on the structures and configurations

of the preferred embodiments of the invention, it will be appreciated that other embodiments, as

well as modifications of the embodiments disclosed herein, can be made without departing from

the principles of the invention. In this respect, it will be appreciated that the spring rod can be

used in applications other than those disclosed herein. Similarly, multiple combinations of

coaxial and surrounding springs (i.e. three, four, etc.) may be configured to meet the desired load

versus deflection for a particular application. Likewise, it will be appreciated that a spring rod

according to the invention can be secured to relatively displaceable components in any number

of different ways. These and other modifications of the preferred embodiments, as well as other

embodiments of the invention, will be obvious and suggested to those skilled in the art from the

disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter

is to be interpreted merely as illustrative of the present invention and not as a limitation thereof.

Claims (27)

Having thus described the invention, it is claimed:

1. A compression spring rod comprising a housing having an axis and axially

opposite ends, a rod member coaxial with said axis and having an inner end in said housing and

an outer end axially outwardly of one of said opposite ends, means including a guide member

on said inner end of said rod member supporting said rod member for reciprocation axially of

said housing between retracted and extended positions relative thereto, and first and second

compression springs each extending between said guide member and the same one of said

opposite ends of said housing, said first and second springs being coaxial with one another and

with said axis.

2. The compression spring rod of claim 1, wherein the direction of winding of said

first compression spring is opposite to the direction of winding of said second compression

spring.

3. The compression spring rod of claim 1, wherein the free length of said first

compression spring is different from the free length of said second compression spring.

4. The compression spring rod of claim 1 , wherein the outside diameter of said first

compression spring is less than the outside diameter of said second compression spring.

5. The compression spring rod of claim 1, wherein the wire diameter of said first

compression spring is less than the wire diameter of said second compression spring.

6. The compression spring rod of claim 1, wherein the outside diameter and wire

diameter of said first compression spring are respectively less than the outside diameter and wire

diameter of said second compression spring.

7. A compression spring rod comprising a housing having an axis and axially

opposite ends, a rod member coaxial with said axis and having an inner end in said housing and

an outer end axially outwardly of one of said opposite ends, means including a guide member

on said inner end of said rod and a bushing at said one of said opposite ends supporting said rod

for reciprocation axially of said housing between retracted and extended positions relative

thereto, and first and second compression springs each extending between said guide member

and the other of said opposite ends coaxial with said axis for biasing said rod toward the

extended position thereof.

8. The compression spring rod of claim 7, wherein the direction of winding of said

first compression spring is opposite to the direction of winding of said at least second

compression spring.

9. The compression spring rod of claim 7, wherein the length of said first

compression spring is unequal to the length of said second compression spring.

10. The compression spring rod of claim 7, wherein the outside diameter of said first

compression spring is less than the outside diameter of said second compression spring.

11. The compression spring rod of claim 7, wherein the wire diameter of said first

compression spring is less than the wire diameter of said second compression spring.

12. The compression spring rod of claim 7, wherein the outside diameter and wire

diameter of said first compression spring is less than the outside diameter and wire diameter of

said second compression spring.

13. The compression spring rod of claim 7, wherein a guide rod extends from said

guide member toward the other of said opposite ends coaxial with said axis and said first

compression spring surrounds said guide rod.

14. A compression spring rod comprising a housing having an axis and axially

opposite ends, a rod member coaxial with said axis and having an inner end in said housing and

an outer end axially outwardly of one of said opposite ends, means including a guide member

on said inner end of said rod and a bushing at said one of said opposite ends supporting said rod

for reciprocation axially of said housing between retracted and extended positions relative

thereto, and first and second compression springs each extending between said guide member

and said rod bushing coaxial with said axis for biasing said rod toward the extended position

thereof.

15. The compression spring rod of claim 14, wherein the direction of winding of said

first compression spring is opposite to the direction of winding of said at least second compression spnng.

16. The compression spring rod of claim 14, wherein the length of said first

compression spring is unequal to the length of said second compression spring.

17. The compression spring rod of claim 14, wherein the outside diameter of said first

compression spring is less than the outside diameter of said second compression spring.

18. The compression spring rod of claim 14, wherein the wire diameter of said first

compression spring is less than the wire diameter of said second compression spring.

19. The compression spring rod of claim 14, wherein the outside diameter and wire

diameter of said first compression spring are respectively less than the outside diameter and wire

diameter of said second compression spring.

20. The compression spring rod of claim 14, wherein a guide rod extends from said

guide member toward the other of said opposite ends coaxial with said axis and a cushioning

spring surrounding said guide rod and extending between said guide member and the other of

said opposite ends of said housing for cushioning retracting movement of said rod member.

21. A compression spring rod comprising a housing having an axis and axially

opposite ends, first and second rods coaxial with said axis and having an inner end in said housing and an outer end axially outwardly of a different one of said opposite ends, means

including a guide member interconnecting said inner ends of said rods and supporting said rods

for reciprocation axially of said housing between retracted and extended positions relative

thereto, first and second compression springs each extending between said guide member and

one of said opposing ends of said housing coaxial with said axis , third and fourth compression

springs each extending between said guide member and the other of said opposite ends of said

housing coaxial with said axis, said compression springs biasing said rods toward a central

position thereof relative to said housing.

22. The compression spring rod of claim 21 , wherein the direction of winding of said

first and third compression springs is opposite to the direction of winding of said second and

fourth compression springs.

23. The compression spring rod of claim 21 , wherein the length of said first and third

compression springs are the same, and the lengths of said second and fourth compression springs

are the same and different than the lengths of said first and third compression springs.

24. The compression spring rod of claim 21, wherein the outside diameters of said

first and third compression springs are less respectively than the outside diameter of said second

and fourth compression springs.

25. The compression spring rod of claim 21 , wherein the wire diameters of said first and third compression springs are the same, and the wire diameters of said second and fourth

compression springs are the same and greater than that of the first and third compression springs.

26. The compression spring rod of claim 21 , wherein the outside diameters and wire

diameters of said first and third compression springs are less respectively than the outside

diameters and wire diameters of said second and fourth compression springs.

27. The compression spring rod of claim 26, wherein said outside diameters of said

first and third compression springs are the same, the outside diameters of said second and fourth

compression springs are the same, the wire diameters of said first and third compression springs

are the same, and the wire diameters of said second and fourth compression springs are the same.