Abstract:
An agricultural implement includes a frame, a grow medium manipulation surface configured to manipulate and move a growing medium and at least one elastomeric torsion element coupled to the surface. The growing medium manipulation surface moves between a first growing medium engagement position in which the surface manipulates and moves the growing medium and a second growing medium non-engagement position. The elastomeric torsion element resiliently biases the surface towards the first growing medium engagement position. In one exemplary embodiment, the growing medium engagement surface comprises the surface of a furrow opening device on an agricultural implement. In one exemplary embodiment, the implement additionally includes an adjustment mechanism coupled to the elastomeric torsion element. The adjustment mechanism moves between a first adjustment position in which the torsion element biases the surface towards the first engagement position with a first force and a second adjustment position in which the element biases the surface towards the first engagement position with a second force. In another exemplary embodiment, the adjustment mechanism also enables the elastomeric torsion element to apply a reverse force biasing the surface away from the first growing medium engagement position.

Description:
FIELD OF THE INVENTION 
     The present invention relates to agricultural implements used to engage and manipulate a growing medium to enable seed, fertilizer, insecticide or herbicide to be deposited into the growing medium. In particular, the present invention relates to a mechanism which forces a growing medium manipulation surface, such as a surface of a furrow opening device, towards and into the growing medium or soil. 
     BACKGROUND OF THE INVENTION 
     Many different types of agricultural implements are known for engaging and manipulating a growing medium or soil to enable seed, fertilizer, insecticide or herbicide to be deposited onto or into the soil or to prepare the soil beforehand. Examples of such implements include planters, drills, disks, plows, cultivators and the like. Each such implement includes surfaces which engage and manipulate the soil. For example, planters typically include a pair of spaced disks which are forced into the soil and which sever and separate the soil to create a furrow into which seed is deposited. In such devices, large metal springs are usually employed to apply a downward force to the furrow opening disks or to whatever soil manipulation surface or edge is used. The spring is usually held in place by a bracket which provides discrete spring positions intended for the application of down pressure. 
     In the case of planters, the amount of downward force applied to the furrow opening disk is critical in that it establishes the depth of the furrow and the depth at which seed is planted. Accurate seed depth placement plays a critical role in crop yields. Unfortunately, adjusting the amount of downward force applied by the springs to the furrow opening disks in conventional planters is tedious and time consuming. In many situations, the metal springs and the brackets retaining such springs rust and become jammed with debris, making such adjustment physically challenging. Adjustment of the springs to alternatively create an upward force, such as when planting in lighter soils, generally requires that the entire spring assembly be reassembled. Moreover, such planters provide only a few discrete positions and only a few discrete force levels. Moreover, such planter spring assemblies include multiple parts which increase the complexity, manufacturing cost and assembly time of the planter. 
     Thus, there is a continuing need for a force applying system for planters and other soil engaging agricultural implements: (1) which is easy to adjust, (2) which provides an infinite range of force adjustment between both upper and lower down pressure or force boundaries, (3) which is adjustable between upward force and downward force states seamlessly, without disassembly and (4) which is simple and has relatively few parts, reducing manufacturing and assembly time and cost. 
     SUMMARY OF THE INVENTION 
     The present invention provides an agricultural implement which includes a frame, a furrow opening device having at least one surface and at least elastomeric torsion element coupled to the at least one surface. The surface moves between a first cutting position in which the surface is configured to cut into the soil to create a furrow in a second non-cutting position. The at least one torsion element resiliently biases the surface towards the first cutting position. 
     The present also provides an agricultural implement which includes a frame, a growing medium manipulation surface configured to manipulate and move a growing medium and at least one elastomeric torsion element coupled to the surface. The surface moves between a first growing medium engagement position in which the surface manipulates and moves the growing medium and a second growing medium non-engagement position. The at least one torsion element resiliently biases the surface towards the first growing medium engagement position. 
     The present invention also provides an agricultural implement which includes a frame, a row unit support, a furrow opening device coupled to the support, a furrow closing device coupled to the support and a force applying system coupled between the frame and the support to force the support and the furrow opening device into the soil. The force applying system includes a four bar linkage having a first end pivotably coupled to the frame and a second end pivotably coupled to the support and further having a first and second parallel upper links and first and second parallel lower links. The force applying system further includes a first torsion arm pivotably coupled to the lower links, a bracket pivotably coupled between the upper links, a second torsion arm, a screw rotatably coupled to the bracket and movably supporting the second torsion arm, a tube fixedly coupled to one of the first and second torsion arms, a shaft coupled to the other of the first and second torsion arms and extending through the tube and an elastomeric torsion element disposed in the tube between the tube and the shaft. The elastomeric torsion element is resiliently deformed such that the element applies a torque to the first and second arms in attempting to resiliently return to its original shape. As a result, the four bar linkage applies a force to the support. 
     The present invention also provides an agricultural implement including a frame, a row unit support, a furrow opening device coupled to the support, a furrow closing device coupled to the support, and a force applying system coupled between the frame and the support to force the support and the furrow opening device into soil. The force applying system includes a four bar linkage having a first end pivotably coupled to the frame and a second end pivotably coupled to the support. The four bar linkage includes first and second parallel upper links and first and second parallel lower links. This system further includes a first torsion arm coupled to the frame, a second torsion arm pivotably coupled to the first torsion arm, a tube fixedly coupled to the second torsion arm, a shaft fixedly coupled to at least one of the first and second parallel upper links or the first and second parallel lower links and an elastomeric torsion element. The shaft extends through the tube. The elastomeric torsion element is disposed in the tube between the tube and the shaft. The elastomeric torsion element is resiliently deformed such that the element applies a torque to at least one of the first and second upper links and the first and second lower links in attempting to resiliently return to its original shape. As a result, the four bar linkage applies a force to the support. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an agricultural implement including a force applying system of the present invention. 
     FIG. 2 is a fragmentary sectional view of the implement of FIG. 1 taken along lines  2 — 2 . 
     FIG. 3 is a fragmentary sectional view of the implement of FIG. 2 taken along lines  3 — 13 . 
     FIG. 4 is a fragmentary sectional view of the implement of FIG. 2 taken along lines  3 — 3  and depicting an external upward force applied to the implement. 
     FIG. 5 is a fragmentary sectional view of the implement of FIG. 2 taken along lines  3 — 3  and depicting adjustment of the force applying system such that the system applies a downward force. 
     FIG. 6 is a fragmentary sectional view of the implement of FIG. 3 taken along lines  6 — 6 . 
     FIG. 7 is a fragmentary sectional view of the implement of FIG. 6 taken along lines  7 — 7 . 
     FIG. 8 is a fragmentary sectional view of the implement of FIG. 6 taken along lines  7 — 7  depicting force applying system in the compressed state shown in FIG.  5 . 
     FIG. 9A is a schematic illustration depicting the force applying system in an uncompressed state in solid lines and depicting the force applying system in a compressed state in broken lines to illustrate the change in the force applying system when adjusted from the state shown in FIG. 7 to the state shown in FIG.  8 . 
     FIG. 9B is a schematic illustration depicting the force applying system in a compressed force applying state shown in solid lines and in an uncompressed state shown in broken lines. 
     FIG. 10 is a side elevational view of the implement of FIG. 1 illustrating the force applying system applying an upward force. 
     FIG. 11 is a fragmentary sectional view of the implement of FIG. 2 taken along lines  3 — 3  with the force applying system in a state shown in FIG.  10 . 
     FIG. 12 is a fragmentary sectional view of an alternative embodiment of the implement of FIG. 2 taken along lines  3 — 3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a perspective view of an agricultural implement  10 , preferably a planter. Implement  10  generally includes tool bar or frame  12 , row unit support  14 , furrow opening disks  16 , depth gauge wheel  18 , furrow closing disks  20 , compaction wheel  22 , and down pressure system  24 . Frame  12  is conventionally known and is configured to support a plurality of such row units along its length. Frame  12  is part of a much larger structure which is movably supported above the ground or soil  26  and is configured for being pulled by a tractor or other work vehicle. As will be appreciated, the exact configuration of frame  12  will vary depending upon the particular agricultural implement. 
     Row unit support  14 , furrow opening disks  16 , depth gauge wheel  18 , furrow closing disk  20  and compression wheel  22  are conventionally known components. Support  14  comprises a subframe assembly coupled to down pressure system  24  and configured to carry each of furrow opening disks  16 , depth gauge wheel  18 , furrow closing disks  20  and furrow compression wheel  22 . As shown by FIG. 1, support  14  includes an adjustment knob  28  connected to a conventionally known adjustment mechanism (not shown), whereupon rotation of knob  28 , the height of support  14  relative to wheel  18 , which rides upon the top of soil  26 , is varied to adjust the depth at which disks  16 , disks  20  and wheel  22  project into the soil. 
     Disks  16  consist of a pair of rotatable disks which are rotatably supported by support  14 . Disks  16  include circumferential edges or surfaces  30  which are angled with respect to one another so as to sever and separate soil  26  to create a furrow into which seed is deposited. Although not shown in FIG. 1, implement  10  additionally includes a seed delivery system which delivers seed from a storage tank through a placement tube and into the furrow created by disks  16 . Depending upon the particular application, implement  10  may alternatively include delivery systems for delivering other types of liquid or particulate material including herbicide, insecticide or herbicide. 
     Furrow closing disks  20  are rotated supported by support  14  behind disks  16  and behind the delivery tube of the material delivery system (not shown). Furrow closing disks  20  manipulate and move the soil over the deposited seed to close the furrow. Compression wheel  22  is supported by frame  12  behind furrow closing disks  20  and further compresses the soil over the deposited seed. 
     Down pressure system  24  is coupled between frame  12  and support  14 . Down pressure system  24  applies a downward pressure or force to support  14  to force surfaces  30  of disks  16  against and into soil  26 . To do so, system  24  includes an elastomeric torsion element  34  which is twisted or deformed such that element  34  applies a torque to support  14  in attempting to resiliently return to its original shape. This torque constitutes a downward force or pressure which forces surfaces  30  downward into soil  26 . 
     Down pressure system  24  additionally includes an adjustment mechanism  36  which adjusts the degree at which element  34  is twisted or compressed and also the direction of the twist or compression applied to element  34 . By adjusting the degree of twist applied to element  34 , mechanism  36  enables the user to vary the amount of downward force applied to support  14  and to surfaces  30  of disks  16 . By adjusting the direction of twist applied to element  34 , mechanism  36  enables the user to modify element  34  such that element  34  applies an upward force to support  14  and to surfaces  30  of disks  16 . Such an upward force may be necessary when planting in or working with lighter soils. 
     FIGS. 2 and 3 illustrate down pressure system  24  in greater detail. As shown by FIGS. 2 and 3, system  24  generally includes four bar linkage  40 , lower extensions  42 , lower torsion arms  44 , tube  46 , upper pivot bracket  48 , lead screw  50 , upper torsion arms  52 , shaft  54  and elastomeric torsion element  34 . Four bar linkage  40  extends between frame  12  and support  14  and includes a first end  58  pivotably coupled to frame  12  and a second end  60  pivotably coupled to support  14 . Four bar linkage  40  includes parallel lower links  62 ,  64  and parallel upper links  66 ,  68 . Each link is relatively rigid and has opposite ends pivotably coupled to frame  12  and support  14 . 
     Lower extensions  42  extend from lower links  62  and  64  and are generally stationary relative to links  62  and  64 . Extensions  42  pivotably support lower torsion arms  44  for pivotable movement about axis  72 . 
     Lower torsion arms  44  are pivotably pinned to extensions  42  by pins  74  and are fixedly coupled to tube  46  which extends between arms  44 . Arms  44  pivotably support tube  46  relative to shaft  54 . 
     Tube  46  comprises an elongate hollow tube configured to receive shaft  54  and to also receive elastomeric torsion element  34 . 
     Pivot bracket  48  comprises an elongate rigid member extending between upper links  66  and  68  and pivotably coupled to each of upper links  66  and  68 . Bracket  48  is preferably pivotably pinned to upper links  66  and  68  by pins  76 . Pivot bracket  48  carries lead screw  50 . 
     Lead screw  50  comprises an elongate threaded screw having an upper portion  78  rotatably supported by and axially retained relative to pivot bracket  48  and a threaded portion  80  threadably engaging upper torsion arms  52 . In the exemplary embodiment, upper portion  78  of lead screw  50  is axially retained relative to pivot bracket  48  by bushings  81  or other similar retention mechanisms. Rotation of lead screw  50  causes upper torsion arms  52  to axially move along threaded portion  80 . 
     Upper torsion arms  52  extend from lead screw  50  to shaft  54  for supporting shaft  54 . Shaft  54  extends through tube  46  between arms  52 . Shaft  54  is fixedly secured to arms  52  and is preferably configured such that shaft  54  cannot be rotated relative to tube  46  without compression or twisting of elastomeric torsion element  34 . 
     Elastomeric torsion element  34  comprises a resilient elastomeric material, such as rubber, disposed between tube  46  and shaft  54 . Torsion element  34  compresses or twists upon relative rotation of shaft  54  and tube  46  and applies a torque to both shaft  54  and tube  46  in attempting to resiliently return to its original position or shape. As discussed in brief with respect to FIG. 1, this torque is ultimately transmitted to support  14  and surfaces  30  of furrow opening disks  16 . 
     FIGS. 3 and 4 best illustrate the functioning of down pressure system  24  in an intermediate or neutral state. As shown in FIG. 3, when system  24  is in such a neutral state, lead screw  50  is rotated to a point such that pivot bracket  48  is spaced from pivot arms  52  by distance Y. As a result, elastomeric torsion element  34  resiliently biases tube  46  and shaft  54  to the relative position shown in FIG. 3 such that four bar linkage  40  extends outward from frame  12  perpendicular to plane  84 . As a result, links  62 ,  64 ,  66  and  68  of four bar linkage  40  perpendicularly extend from plane  84 . In this neutral state, support  14  and furrow opening disks  16  carried by support  14  are pressed towards soil  26  with a force substantially equal to the weight of implement  10  less those forces distributed to and among the wheels (not shown) supporting implement  10  above soil  26  and those components of implement  10  in engagement with the underlying soil  26 . 
     FIG. 4 illustrates system  24  reacting to upward force from support  14  in the direction indicated by arrow  86 . This upward force causes four bar linkage  40  to pivot in a counterclockwise direction as indicated by arrow  88  in FIG.  4 . As a result, tube  46  and shaft  54  rotate relative to one another to compress elastomeric torsion element  34  as shown in FIG.  4 . Elastomeric torsion element  34  attempts return to its initial shape shown in FIG.  3  and to return four bar linkage  40  to the original configuration shown in FIG.  3 . In doing this, torsion element  34  applies torque to four bar linkage in the clockwise direction indicated by arrows  92 . This torque results in a force applied to support  14  in the generally downward direction indicated by arrow  94 . 
     FIGS. 5-9B illustrate system  24  adjusted to apply a greater downward force to support  14  and to surfaces  30  of furrow opening disk  16  (shown in FIG.  1 ). As shown by FIG. 5, lead screw  50  has been rotated to move upper torsion arm  52  along threaded portion  80  to shorten the distance separating pivot bracket  48  and torsion arm  52  to the distance Y′. As a result, pivot bracket  48  and upper torsion arm  52  pivot in a clockwise direction as seen in FIG. 5 about axis  89  and lower torsion arms  44  pivot in a counterclockwise direction as seen in FIG. 5 about axis  89 . In turn, tube  46  and shaft  54  rotate relative to one another in the directions indicated by arrows  102  and  104 , respectively, from the positions shown in FIGS. 6 and 7 where element  34  is an uncompressed, untwisted state to a compressed and twisted state shown in FIG.  8 . As shown by FIG. 9A, tube  46  and shaft  54  are rotated relative to one another such that angle X is reduced to angle X′, compressing elastomeric torsion element  34 . Because element  34  is elastomeric and resilient, elastomeric torsion element  34  applies a force to both tube  46  and shaft  54  in the directions indicated by arrows  108  and  110 , respectively, as element  34  attempts to resiliently return to the uncompressed state shown in FIGS. 6 and 7. 
     The end result of this compression of element  34  is schematically shown in FIG.  9 B. To return to an uncompressed state, element  34  applies force to tube  46  and shaft  54  to attempt to pivot lower torsion arms  44  and upper torsion arms  52  from angle C to angle D in which element  34  is no longer compressed. In other words, element  34  continues to apply a force to arms  44  and  52  until four bar linkage attains the position shown in broken lines in which four bar linkage  40  obliquely extends from plane  84 . Since surfaces  30  of furrow opening disks  16  are in engagement with soil  26  and prevent four bar linkage from pivoting to the position shown in broken lines, elastomeric torsion element  34  continues to apply a downward force to support  14  and to furrow opening disks  16  in the direction indicated by arrow  116 . 
     FIGS. 10 and 11 illustrate lead screw  50  rotated in an opposite direction as indicated by arrow  95  to move upper torsion arm  52  along threaded portion  80  so as to increase the distance between pivot bracket  48  and arms  52  to the distance Y″. Distance Y″ is greater than distance Y. As a result, elastomeric torsion element  34  is compressed and twisted in an opposite direction such that element  34  applies forces to tube  46  and shaft  54  in generally opposite directions to those shown in FIG.  8 . The force applied by element  34  to support  14  and to disks  16  is in an upward direction as indicated by arrow  120  in FIG.  10 . Although the upward force applied by element  34  to support  14  and the components carried by  14  is insufficient to lift support  14  and to carry components above the ground or soil  26 , this force does reduce the overall force pressing support  14  towards the ground due to the weight of support  14  and the components it carries. 
     FIG. 12 is a side elevational view of down pressure system  124 , an alternative embodiment of down pressure system  24  shown in FIG.  3 . Down pressure system  124  is similar to down pressure system  24  except that down pressure system  124  includes extensions  142 , torsion arms  144 , tube  146 , pivot bracket  148 , lead screw  150 , torsion arm  152  and shaft  154  in lieu of lower extensions  42 , lower torsion arms  44 , tube  46 , upper pivot bracket  48 , lead screw  50 , upper torsion arm  52 , and shaft  54 , respectively. In addition to elastomeric torsion element  34 , those remaining elements of down pressure system  124  which correspond to like elements of down pressure system  24  are numbered similarly. Extensions  142  generally comprise a pair of rigid bars or other structures stationarily affixed to and extending rearwardly from frame  12  on opposite sides of pivot bracket  148 . Extensions  142  pivotally support pivot bracket  148  to permit bracket  148  to pivot about axis  157 . 
     Pivot bracket  148  comprises a rigid bar or other structural member extending between extensions  142  and pivotally coupled to extensions  142 . In the exemplary embodiment, pivot bracket  148  includes a pair of opposing bores which receive a corresponding pair of inwardly extending bosses projecting from extensions  142  to permit bracket  148  to pivot about axis  157 . As will be appreciated, bracket  48  may be pivotally coupled to and between extensions  142  by various other pivotal support arrangements. 
     Lead screw  150  comprises an elongate threaded screw having a lower portion  178  coupled to torsion arm  152  and an upper portion  180  rotatably journaled and axially fixed to pivot bracket  148 . In the exemplary embodiment, upper portion  180  is rotatably journaled and axially fixed to pivot bracket  48  by means of a pair of bushings  181  that allow rotation but axially secure lead screw  50  to bracket  148 . As noted above, rotation of lead screw  150  causes torsion arm  152  to axially move along lower portion  178  of lead screw  150 . 
     Torsion arm  152  comprises an elongate U-shaped member extending between torsion arms  44  and coupled to lower portion  178  of lead screw  150  intermediate torsion arms  144 . Torsion arm  148  includes an internally threaded bore  1   59  which threadably engages external threads of lead screw  150  to retain torsion arm  152  in any one of a plurality of positions along the axial length of the externally threaded portion of lead screw  50 . Torsion arm  152  is pivotally coupled to torsion arms  144 . In the exemplary embodiment, torsion member  152  includes a pair of bosses or pins  179 , wherein ends of the pins are rotatably journaled within a corresponding bore in each of torsion arms  144 . The pins  179  may either be non-rotatably affixed to torsion arm  152  or may be rotatably positioned within a bore extending through ends of torsion arm  152 . 
     Torsion arms  144  comprise a pair of spaced arms extending on opposite sides of lead screw  150 . Each arm  144  has a first end  183  pivotally coupled to torsion arm  152  and a second end  185  fixedly coupled to tube  146  such as by welding. As will be appreciated, torsion arms  144  may alternatively comprise various other structures for coupling tube  146  to torsion arm  152 . 
     Tube  146  comprises an elongate hollow tube fixedly secured between torsion arms  144  and configured to receive shaft  154  and to also receive elastomeric torsion element  34 . 
     Shaft  154  comprises an elongate shaft extending through tube  46  between links  66  (shown in FIG.  2 ),  68 , wherein the axial ends of shaft  154  are non-rotatably coupled to links  66 ,  68 . Shaft  154  is preferably configured such that shaft  154  cannot be rotated relative to tube  146  without compression or twisting of elastomeric torsion element  34 . 
     Elastomeric torsion element  34  comprises a resilient elastomeric material, such as rubber, disposed between tube  146  and shaft  154 . Torsion element  34  compresses or twists upon relative rotation of shaft  154  and tube  146  and applies a torque to both shaft  154  and tube  146  in attempting to resiliently return to its original position or shape. This torque is ultimately transmitted to support  14  and surfaces  30  of furrow opening disks  16  (shown in FIG.  1 ). 
     In operation, down pressure system  124  functions similarly to down pressure system  24 . In particular, FIG. 12 illustrates down pressure system  124  in an intermediate or neutral state where lead screw  150  is rotated to a point such that elastomeric torsion element  34  resiliently biases tube  154  and interconnected to links  66 ,  68  to the relative position shown in FIG. 12 such that four bar linkage  40  extends outward from frame  12  perpendicular to plane  84 . In this neutral state, support  14  and furrow opening disk  16  carried by support  14  are pressed towards soil  26  with a force substantially equal to the weight of the implement  10  less those forces distributed to and among the wheels (not shown) supporting implement  10  above soil  26  and those components of implement  10  in engagement with the underlying soil  26 . In response to an upward force from support  14 , shaft  154  rotates relative to tube  146  in a counter-clockwise direction to twist and compress elastomeric torsion element  34 . As a result, elastomeric torsion element  34  attempts to return to its initial shape by applying torque to shaft  154  and four bar linkage  40  in a clockwise direction. As with down pressure system  24 , the down pressure applied by system  124  may be adjusted by rotation of lead screw  150 . In addition, rotation of lead screw  150  may also result in elastomeric torsion element  34  applying an upward force to four bar linkage  40  to reduce the overall force pressing support  14  towards the ground due to the weight of support  14  and the components it carries. 
     Overall, down pressure systems  24  and  124  enable the force at which surfaces  30  of disks  16  are pressed against soil  26  to be quickly and easily adjusted by simple rotation of lead screw  50 . Because systems  24  and  124  utilize an elastomeric torsion element  34  instead of a coil compression spring, systems  24  and  124  are less susceptible to corrosion and rust and are less susceptible to becoming jammed with debris. As a result, systems  24  and  124  are more easily adjusted. Because systems  24  and  124  provide an infinite range of force adjustments between force boundaries, systems  24  and  124  enable more precise control of the amount of down pressure applied to furrow opening disks  16 . Because systems  24  and  124  utilize torsion element  34 , systems  24  and  124  may be adjusted between upward force and downward force states seamlessly, without disassembly. Moreover, because systems  24  and  124  are simple and include relatively few parts, systems  24  and  124  are more easily manufactured with lower costs and a shorter assembly time. 
     FIGS. 1-12 depict but two exemplary contemplated embodiments. Various other embodiments, although not specifically shown, are also contemplated within the present disclosure. For example, in lieu of being employed on a planter, systems  24  and  124  may alternatively be employed on other agricultural implements in which a ground-engaging and cutting tool or surface must be forced downward into the soil to manipulate the soil. In lieu of utilizing a pair of upper links and a pair of lower links, systems  24  and  124  may alternatively utilize a single upper link and a single lower link. In lieu of tube  46 ,  146  and shaft  54 ,  154  being rectangular, tube  46 ,  146  and shaft  54 ,  154  may alternatively have various other noncircular shapes. Moreover, tube  46 ,  146  and shaft  54 ,  154  may have various circular and noncircular shapes so long as both tube  46 ,  146  and shaft  54 ,  154  do not rotate relative to one another without compressing or uncompressing elastomeric torsion element  34 . For example, elastomeric torsion element  34  may alternatively be keyed, bonded, co-molded or otherwise fixedly coupled to a generally round tube  46 ,  146  or a generally round shaft  54 ,  154  wherein the other of tube  46 ,  146  and shaft  54 ,  154  cannot be moved relative to torsion elements  34  without compressing or uncompressing torsion element  34 . In lieu of tube  46  and shaft  54  being supported by arms  44  and arms  52 , respectively, tube  46  and shaft  54  may alternatively be carried by arms  52  and arms  44 , respectively. In lieu of pivot bracket  48 , lead screw  50  and arms  52  being coupled to upper links  66 ,  68  and arms  44  being coupled to lower links  62 ,  64 , pivot bracket  48 , lead screw  50  and arms  52  may alternatively be coupled to lower links  62 ,  64  and arms  44  may alternatively be coupled to upper links  66 ,  68 . In lieu of utilizing a lead screw  50  to serve both functions of removably supporting arms  52  in a linear fashion between lower links  62 ,  64  and upper links  66 ,  68  and selectively retaining arms  52  in any one of the plurality of positions between lower links  62 ,  64  and upper links  66 ,  68  to provide different force levels, system  24  may include separate distinct components for providing the same functions. For example, arms  52  may alternatively be slidably movable along a shaft of a structure relative to pivot bracket  48  to any one of a plurality of preset positions, wherein one of arms  52  and the bracket are provided with a detent such as a depression or hole and the other of arms  52  and the bracket  48  are provided with a detent-engaging member such as a protuberance or pin, enabling arms  52  to be selectively retained in one of the plurality of positions. As evident from the relatively cursory list of alternatives above, the present disclosure contemplates a multitude of different variations. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. Because the technology of the present invention is relatively complex, not all changes in the technology are foreseeable. The present invention described with reference to the preferred embodiments and set forth in the following claims is manifestly intended to be as broad as possible. For example, unless specifically otherwise noted, the claims reciting a single particular element also encompass a plurality of such particular elements.