Abstract:
Push-pull controls utilizing linear and rotational inputs to create linear output are disclosed. One control includes an elongate push rod and a roller pin. The rod has an outer surface, a generally circular cross-section perimeter, a center axis, a proximal end with a user input, and a distal end configured to directly or indirectly interact with an apparatus to be controlled. The rod is rotatable and slidable along the rod axis. The roller pin has a center axis and an outer surface that contacts the push rod outer surface. The roller pin center axis is angularly offset from and non-intersecting with the push rod center axis. The amount of angular offset is greater than zero degrees.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     None. 
     BACKGROUND 
     The invention relates generally to the field of manually-operated push-pull controls. More specifically, the invention relates to the field of push-pull controls that operate with both linear and rotational inputs. Push-pull controls are used in various applications, such as throttle controls and controls for fuel mixtures. 
     One common prior art push-pull control  100 , partially shown in  FIG. 1 , operates solely using linear input. In the prior art push-pull control  100 , a panel nut  101  is fastened to a stationary element (e.g., an instrument panel, a housing, et cetera). A push rod  102  extends through the panel nut  101  and is movable relative to the panel nut  101 , and a user input knob  103  is coupled to a proximal end  102   a  of the push rod  102  (e.g., by nut  104 ). A distal end (not shown) of the push rod  102  may be coupled to the apparatus being controlled, either directly or (more commonly) through a cable or other force-transferring device. If a cable is used, swiveling apparatus may couple the cable to the push rod  102 , such that the cable is not crimped by rotation of the push rod  102 . 
     To allow the push rod  102  to temporarily remain in a desired location relative to the panel nut  101 , packing  105  (e.g., leather washers) surrounds the push rod  102 , and a friction nut  106  is used to selectively compress the packing  105 . Threading on the friction nut  106  is generally received by threading (not shown) in the panel nut  101 . The geometry of the friction nut  106  and panel  101  includes internal cones so that the packing  105  is compressed radially inward to increase the friction on the push rod  102 . Loosening the friction nut  106  relative to the panel nut  101  allows the packing  105  to relax. When the packing  105  is compressed, friction is formed between the packing  105  and the push rod  102 ; this friction may allow the push rod  102  to temporarily remain in a desired location relative to the panel nut  101 . It should be appreciated that the amount of friction may be modified by adjusting how much the packing  105  is compressed. Even with maximum compression, however, it is generally possible to overcome this friction by pushing or pulling the push rod  102  (when gripping the user input knob  103 ). Rotating the push rod  102  (e.g., using the user input knob  103 ), on the other hand, generally has no effect, and various structure may optionally be used to restrict the push rod  102  from rotating relative to the panel nut  101 .  FIG. 1  shows the friction nut  106  entirely released from the panel nut  101  for illustration, but in use the friction nut  106  would generally be at least minimally coupled to the panel nut  101 . 
     Another prior art push-pull control  200 , referred to herein as a Standard Vernier Control, is shown in  FIG. 2 . A threaded tube  201  is threadably coupled to a panel nut  202 , and a nut  203 , and the elements  201 ,  202 ,  203  are fixed to a stationary element (e.g., an instrument panel, a housing, et cetera) by sandwiching the stationary element between the panel nut  202  and the nut  203 . As shown, a lock washer  204  may also be included. The threaded tube  201  is generally hollow, and a helical surface  205  extends along the inside of the threaded tube  201 . In the Standard Vernier Control  200 , the helical surface  205  is formed by a spring  206 . 
     A push rod  208  extends through the panel nut  202  and inside the threaded tube  201 , and a user input knob  209  is coupled to a proximal end  208   a  of the push rod  208  (e.g., by a pair of nuts  210 ). A cable  212  is shown coupled to a distal end  208   b  of the push rod  208  by a pair of bearings  213  surrounding an end  212   a  of the cable  212  (or “cable terminal”  212   a ). 
     A release shaft  215  extends inside the push rod  208 , and a release button  216  is coupled to a proximal end  215   a  of the release shaft  215 . A spring  217  biases the button  216 , and thus the release shaft  215 , to an extended configuration (as shown). A distal end  215   b  of the release shaft  215  has a wedge-shaped configuration forming a cavity  218 , and a ball  220  is positioned inside the cavity  218 . When the button  216  and the release shaft  215  are at the extended configuration, the wedge-shaped configuration of the release shaft distal end  215   b  forces the ball  220  to interact with the helical surface  205  (formed by the spring  206 ); this interaction prohibits the push rod  208  from being pushed or pulled relative to the threaded tube  201  and the panel nut  202 . The ball  220  may travel along the helical surface  205 , however. As such, the user input knob  209  may be rotated, causing the push rod  208  to move inwardly/outwardly relative to the threaded tube  201  and the panel nut  202 . Depending particularly on the amount of incline in the helical surface  205 , inward/outward movement of the push rod  208  may be finely controlled by rotating the user input knob  209  in this manner. 
     It is not always desirable to rotate the user input knob  209 , however, as (for example) it may be desirable to quickly move the push rod  208  a relatively large distance or to move the push rod  208  a relatively large distance without the effort of continuously rotating the user input knob  209 . To operate the Standard Vernier Control  200  with linear—instead of rotational—input, the button  216  may be pressed to overcome the force of the spring  217 . When the button  216  is pressed, the button  216  and the release shaft  215  are no longer at the extended configuration, and the ball  220  is released and allowed to separate from the helical surface  205 . Without the ball  220  interacting with the helical surface  205 , the push rod  208  may be pushed or pulled relative to the threaded tube  201  and the panel nut  202 . However, rotational input may not be used to move the push rod  208  until the button  216  and the release shaft  215  return to the extended configuration. 
     While the Standard Vernier Control  200  has been generally well-received by the market, there are at least four disadvantages of the Standard Vernier Control  200 . First, the distance that the control  200  can operate using rotational input is limited by the length of the helical surface  205 , which can result in a threaded tube  201  that is unacceptably long for some applications. Second, the button  216  must be pressed to allow the push rod  208  to be moved using linear input. Third, if the button is not completely depressed, the linear motion is “ratchety” such that resistance is detected as the ball contacts each thread of the helical surface. Fourth, the ball  220  can “jam”, causing it to not automatically release when the button  216  is pressed and the release shaft  215  is no longer at the extended configuration. When this happens, there is a delay before linear input may be used to move the push rod  208 . If the Standard Vernier Control  200  is being used to control an aircraft throttle, for example, such a delay could be life threatening or even deadly. 
     SUMMARY 
     The present invention is defined by the claims below. According to one embodiment, a push-pull control utilizing both linear and rotational inputs to create linear output (without requiring a user-operated release for switching from rotational input to linear input) includes an elongate push rod, a base, a roller pin, and a rotational assist cartridge. The elongate push rod has proximal and distal ends, a smooth outer surface, a circular cross-section perimeter, and a center axis. The push rod is rotatable and slidable along the push rod axis, and the push rod distal end is configured to directly or indirectly interact with an apparatus to be controlled. A user input is at the push rod proximal end. The base has an opening through which the push rod passes, and rotational and sliding movement of the push rod along the push rod axis is movement relative to the base. The roller pin has a smooth outer surface and a center axis and is rotatable about the roller pin center axis. The rotational assist cartridge is adjacent the push rod and rotatable relative to the base. The rotational assist cartridge positions the roller pins such that: (a) at least one point of the roller pin outer surface contacts the push rod outer surface; and (b) the roller pin center axis is angularly offset and non-intersecting with the push rod center axis. 
     According to another embodiment, a push-pull control utilizing both linear and rotational inputs to create linear output (without requiring a user-operated release for switching from rotational input to linear input) includes a base, an elongate push rod, a plurality of roller pins separated into first and second groups, and a rotational assist cartridge. The base has an opening through which the push rod passes, and movement of the push rod is movement relative to the base. The elongate push rod has proximal and distal ends, a smooth outer surface, a circular cross-section perimeter, and a center axis. The push rod distal end is configured to directly or indirectly interact with an apparatus to be controlled. A user input is at the push rod proximal end. Each roller pin has a cylindrical central portion, a first spherical end, a second spherical end, a center point, and a center axis; and each roller pin is rotatable about the roller pin center axis. The rotational assist cartridge is adjacent the push rod, and is at least partially housed in the base. The rotational assist cartridge includes a pin cage; first, second, and third pin cups; a resilient member relatively biasing the first and second pin cups toward one another; and a tension nut. The pin cage has a hollow central area and a plurality of pin slots that are through openings extending to the pin cage hollow central area. The pin slots are equi-angularly spaced about a center axis of the pin cage, and each pin slot extends along a respective helical path over a length of the pin cage. One roller pin of the first group and one roller pin of the second group are positioned in each pin slot. The center points for each roller pin in the first group lay on one plane generally perpendicular to the pin cage center axis, and the center points for each roller pin in the second group lay on another plane generally perpendicular to the pin cage center axis. The first pin cup is positioned between the first group of roller pins and the push rod proximal end, and has a tapered end abutting the first spherical ends of the roller pins in the first group. The second pin cup is positioned between the second group of roller pins and the push rod distal end, and has a tapered end abutting the first spherical ends of the roller pins in the second group. The third pin cup is positioned between the first and second groups of roller pins. The third pin cup has one tapered end abutting the second spherical ends of the roller pins in the first group, and has another tapered end abutting the second spherical ends of the roller pins in the second group. The tension nut is threadably received by a threaded portion of the base and is movable to adjust spacing between the first, second, and third pin cups. Decreasing spacing between the first, second, and third pin cups causes an increase in forces between the roller pins and the push rod, and increasing spacing between the first, second, and third pin cups causes a decrease in forces between the roller pins and the push rod. At least two points of each roller pin central portion simultaneously contact the push rod outer surface. Each roller pin center axis is angularly offset from and non-intersecting with the push rod center axis, and the amount of angular offset is generally the same for each roller pin and is greater than zero degrees. The push rod is slidable along the push rod axis using linear input regardless of the forces between the roller pins and the push rod, and the push rod slides along the push rod axis upon receiving rotational input when sufficient forces exist between the roller pins and the push rod. 
     According to still another embodiment, a push-pull control utilizing both linear and rotational inputs to create linear output includes an elongate push rod, at least one roller pin, and a rotational assist cartridge. The elongate push rod has proximal and distal ends, an interaction portion having an unthreaded outer surface, and a center axis. The push rod distal end is configured to directly or indirectly interact with an apparatus to be controlled, and a user input is at the push rod proximal end. Each roller pin has an outer surface and a center axis and is rotatable about the roller pin center axis. The rotational assist cartridge is adjacent the push rod interaction portion, and movement of the push rod is movement relative to the rotational assist cartridge. The rotational assist cartridge positions the at least one roller pin such that: (a) at least two points of each roller pin outer surface simultaneously contact the push rod outer surface; and (b) a central medial plane of each roller pin is angularly offset from a central medial plane through the pushrod that is perpendicular to a theoretical center point of contact between the push rod and each roller pin. The amount of angular offset is generally the same for each roller pin and is greater than zero degrees. Means for adjusting forces between the at least one roller pin and the push rod are also included. The push rod is slidable along the push rod axis using linear input regardless of the forces between the at least one roller pin and the push rod, and the push rod slides along the push rod axis upon receiving rotational input when sufficient forces exist between the at least one roller pin and the push rod. 
     According to still yet another embodiment, a push-pull control utilizing both linear and rotational inputs to create linear output (without requiring a user-operated release for switching from rotational input to linear input) includes an elongate push rod a roller pin. The elongate push rod has proximal and distal ends, an outer surface, a generally circular cross-section perimeter, and a center axis. The push rod is rotatable and slidable along the push rod axis, and the push rod distal end is configured to directly or indirectly interact with an apparatus to be controlled. A user input is at the push rod proximal end. The roller pin has a center axis and an outer surface that contacts the push rod outer surface. The roller pin center axis is angularly offset from and non-intersecting with the push rod center axis. The amount of angular offset is greater than zero degrees. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative embodiments of the present invention are described in detail below with reference to the attached drawings. 
         FIG. 1  is a side view of a part of a PRIOR ART push-pull control. 
         FIG. 2  is a side view of another PRIOR ART control, with sections removed for illustration. 
         FIG. 3  is a side view of a push-pull control according to an embodiment, with a section removed for illustration. 
         FIG. 4   a  is a top view of the rotational assist cartridge shown in  FIG. 3 . 
         FIG. 4   b  is an exploded view of the rotational assist cartridge of  FIG. 4   a.    
         FIG. 5  is a partial view of the push-pull control of  FIG. 3 , with sections removed for illustration. 
         FIG. 6  is a section view taken from area A of  FIG. 5 . 
         FIG. 7  is a broken view of the push-pull control of  FIG. 3 , with the panel nut and other elements omitted to illustrate interaction between the rotational assist cartridge and push rod. 
         FIG. 8  is a section view taken from line A-A of  FIG. 7 . 
         FIG. 9  is a section view taken from area B of  FIG. 7 . 
         FIG. 10  is a partial view of a push-pull control according to another embodiment, with a section and elements removed for illustration. 
         FIG. 11  is a section view taken from line A-A of  FIG. 10 . 
         FIG. 12  is a section view of the push-pull control of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention provide push-pull controls that utilize both linear and rotational inputs without a user-operated release for switching from rotational input to linear input. 
       FIGS. 3 through 9  disclose a push-pull control  300 . The control  300  may, for example, be used for an aircraft throttle control, a control for fuel mixtures, or countless other applications where a push-pull control is used. While the disclosure focuses generally on use in the aviation industry, it should be clearly understood that the control  300  is not limited to such applications. 
     As shown in  FIG. 3 , the push-pull control  300  includes a push rod  302 , a base  310 , and a rotational assist cartridge  330 . The push rod  302  has proximal and distal ends  302   a ,  302   b  and is rigid and generally linear. It may be desirable for the push rod  302  to have a generally smooth outer surface  302   c  with a generally circular cross-section perimeter ( FIG. 8 ). The cross-section perimeter (e.g., outer diameter) may be constant over the length of the push rod  302 , or different portions may have different outer diameters. It should be appreciated, however, that the rotational assist cartridge  330  may only operate in conjunction with a portion having a generally constant outer diameter. 
     Numerous materials may be used to construct the push rod  302 , and material selection may be based at least in part on the intended operating environment. For example, metals, woods, plastics, composites, and other materials may be appropriately used in different applications. Depending, for example, on the intended operating environment and material(s) used for construction, the push rod  302  may be have a generally solid interior  303 , as shown in  FIG. 8 , or may have a hollow interior, a honeycomb-like interior, et cetera. 
     A knob  305  ( FIG. 3 ) or other user input (e.g., handle, wheel, et cetera) extends from the push rod  302  generally at the proximal end  302   a . The knob  305  may be fastened to the push rod  302  using adhesive, welding, mechanical fasteners, or any other appropriate fastener; or the knob  305  and the push rod  302  may have a unitary construction. Similar to the push rod  302 , the knob  305  may be constructed of various materials, including metals, woods, plastics, and composites. 
     The distal end  302   b  of the push rod  302  may be coupled to the apparatus being controlled, either directly or (more commonly) through a cable or other force-transferring device. If cable  10  is used, for example, the cable  10  may be coupled to the distal end  302   b  in a way that allows the push rod  302  to rotate relative to the cable  10 , such that the cable  10  is not wound and crimped. For example, as shown in  FIG. 3 , a terminal end  11  of the cable  10  may be housed in a hollow area  304  of the push rod  302  such that the push rod  302  is rotatable relative to the terminal end  11 . Though not shown in  FIG. 3 , bearings or other swiveling apparatus may couple the cable  10  to the push rod  302 , as will be readily understood by those skilled in the art. 
     The base  310  is, in general terms, a stationary element relative to which the push rod  302  travels. Accordingly, the base  310  may have an opening  312  ( FIG. 5 ) through which the rod  302  passes. The base  310  shown in the accompanying drawings further houses a portion of the rotational assist cartridge  330  and includes threading  313  such that a nut  314  and a lock washer  315  may be used to fasten the base  310  to an environmental element (e.g., a dashboard, control panel, et cetera). More specifically, the environmental element may be placed between the base  310  and the nut  314  and washer  315 , and the nut  314  may be tightened relative to the base  310 . However, other methods and apparatus for coupling the base  310  to an environmental element may alternately be used, such as other threading arrangements, adhesive, welding, screws/bolts, rivets, or other mechanical fasteners; or the base  310  and the environmental element may have a unitary construction. As shown in  FIG. 3 , a hollow shaft  317  may be coupled to (e.g., by a jam nut  318  and a lock washer  319 , as shown, or through any other appropriate fastening method), and extend from, the base  310 ; the push rod distal end  302   b  may accordingly travel inside the shaft  317 . 
     Attention is now directed to the rotational assist cartridge  330 , shown in  FIGS. 3 through 9 . The rotational assist cartridge  330  has a hollow pin cage  332 , one or more roller pins  342 , and pin cups  346 . The pin cage  332  includes one or more pin slots  334 , each extending along a respective helical path over a length of the pin cage  332 , and each pin slot  334  (if more than one is included) is equi-angularly spaced about a center axis  332   a  ( FIG. 8 ) of the pin cage  332 . The center axis  332   a  also corresponds to a center axis of the push rod  302 . In the embodiment  300 , three pin slots  334  are included, and each helical path is angularly spaced one hundred and twenty degrees about the axis  332   a  of the pin cage  332 , as shown in  FIG. 8 . Each pin slot  334  is a through opening extending to a hollow central area  333  ( FIG. 8 ) of the pin cage  332 . 
     The roller pins  342  are positioned inside the pin slots  334 , and the embodiment  300  includes two roller pins  342  in each pin slot  334 , such that two groups  343   a ,  343   b  of pins  342  are formed. The center points for each roller pin  342  in the first group  343   a  are at a common distance along the push rod  302 , and the center points for each roller pin  342  in the second group  343   b  are at another common distance along the push rod  302 . In other words, the center points for each roller pin  342  in the first group  343   a  lay on one plane perpendicular to the center axis  332   a  (and the center axis of the push rod  302 ), and the center points for each roller pin  342  in the second group  343   b  lay on another plane perpendicular to the center axis  332   a  (and the center axis of the push rod  302 ). The roller pins  342  have cylindrical or concaved center portions  342   a  and generally rounded ends  342   b.    
     As shown in  FIG. 6 , a center axis  343  of each pin  342  may be, for example, offset seven degrees from a line (e.g., line  332   b ) that is parallel to the axis  332   a . In other words, each pin&#39;s central medial plane may be angularly offset from a respective central medial plane through the pushrod  302  that is perpendicular to the theoretical center point(s) of contact between the push rod  302  and the pins  342 . Though other configurations may also be used, about a seven degree offset is currently preferred. The pin slots  342  allow the pins  342  to rotate freely about their axes  343  without changing the angle of the pressure plane  390  (discussed below) generated onto the push rod  302 . 
     The pin cage  332  and the roller pins  342  may be constructed of various materials, including metals, ceramics, plastics, composites, woods, and other materials, depending for example on the intended operating environment and application. Materials may also be selected to minimize friction and/or wear between the pin cage  332  and the roller pins  342 , and to provide effective interaction between the roller pins  342  and the push rod  302 , and between the roller pins  342  and the pin cups  346 . If the control  300  is intended for use as an aircraft throttle control, the pin cage  332  may be constructed, for example, of 6061-T6 aluminum or 2024-T4 aluminum, and the pins may be constructed, for example, of 440C stainless steel. 
     The pin cups  346  maintain the pins  342  in the pin slots  334  and are adjustable to increase and decrease an amount of force that the pins  342  place on the push rod  302 . More particularly, the pin cups  346  have tapered ends  347  ( FIG. 6 ) that interact with the respective pins  342 , and specifically with the spherical ends  342   b . By decreasing a distance between adjacent pin cups  346 , the tapered ends  347  force the pins  342  further into the pin slots  334 ; interaction (and forces) between the pins  342  and the push rod  302  is accordingly increased. By increasing the distance between adjacent pin cups  346 , the pins  342  are allowed to retract in the pin slots  334  from the push rod  302 ; interaction (and forces) between the pins  342  and the push rod  302  is accordingly decreased. The pin cups  346  may be constructed of various materials, including for example metals, ceramics, plastics, composites, and woods. If the control  300  is intended for use as an aircraft throttle control, the pin cups  346  may be constructed, for example, of 17-4PH H900 or H950 stainless steel. 
     To adjust positioning of the pin cups  346 , one portion  330   a  of the rotational assist cartridge  330  is free to rotate relative to the base  310  when in use, and another portion  330   b  is adjustable. For example, a flat washer  352  ( FIGS. 4   a  through  5 ) may interact with a ledge  312   a  of the base  310  to fix the portion  330   a  relative to the base  310 , as shown in  FIG. 5 , and various adjustment structure may be used. In the embodiment  300 , a threaded tension nut  354  interacts with threading  312   b  on the base  310  to move the portion  330   b  relative to the base  310 . To aid in regulating forces placed on the pin cups  346 , wave washers  356  (or other resilient members) may interact with the pin cups  346 . While two wave washers  356  (separated by flat washers  357 ) are shown with each of the outer pin cups  346 , it should be clear that more or fewer resilient members may be included. 
     In use, the push-pull control  300  may generally appear as set forth in  FIG. 3 . The amount of compression on the pin cups  346 —and thus the amount of force between the pins  342  and the push rod  302 —may be adjusted using the tension nut  354 . 
     If rotational input is not desired, the tension nut  354  may be drawn away from the flat washer  352  and the ledge  312   a  ( FIG. 5 ), allowing the pin cups  346  to separate from one another. Separation of the pin cups  346  away from one another in turn allows the pins  342  to retract in the pin slots  334  from the push rod  302 , decreasing interaction (and forces) between the pins  342  and the push rod  302 . Interaction between the pins  342 , the pin cups  346 , and the push rod  302  is best shown in  FIGS. 6 and 8 . With little interaction between the pins  342  and the push rod  302 , the push-pull control  300  may be unable to effectively use rotational input (i.e., turning of the knob  305 ) to release (or “push”) or pull the cable  10 . However, linear input (i.e., pushing or pulling of the knob  305 ) may be used to push or pull the cable  10  even with little or no interaction between the pins  342  and the push rod  302 . Even if rotational input is not desired, it may be desirable to maintain enough interaction between the pins  342  and the push rod  302  to temporarily retain the push rod  302  in a desired location relative to the base  310 ; otherwise, the push rod  302  could be free to move relative to the base  310  and constant user interaction with the push rod  302  (i.e., with the knob  305 ) may be required. 
     If rotational input is desired, the tension nut  354  may be drawn toward the flat washer  352  and the ledge  312   a  ( FIG. 5 ), forcing the pin cups  346  to move toward one another. Reducing the spacing between the pin cups  346  in turn causes the tapered ends  347  of the pin cups  346  to force the pins  342  further into the pin slots  334 , increasing interaction (and forces) between the pins  342  and the push rod  302 . Once a sufficient amount of interaction is present between the pins  342  and the push rod  302 , rotational input (i.e., turning of the knob  305 ) releases or pulls the cable  10 . More particularly, as shown in  FIG. 9 , the positioning of the roller pins  342  in the pin slots  334 , when combined with friction generated between the rotating pins  342 , produces a pressure plane  390  on the push rod  302 . Due to the pressure plane  390 , the push rod  302  is moved inward or outward depending on the direction of rotation. When the knob  305  is not being rotated, the interaction between the pins  342  and the push rod  302  temporarily retains the push rod  302  in a desired location relative to the base  310 . Additionally, even when the rotational assist cartridge  330  is adjusted to use rotational input, the interaction between the roller pins  342  and the push rod  302  may be overcome by pushing or pulling the knob  305 ; and the cable  10  may therefore be released or pulled using linear input, without having a mechanical release to shift between utilizing rotational and linear input. 
     While linear input may be used at all times, without having a mechanical release to shift between utilizing rotational and linear input, it may nevertheless be desirable to move the tension nut  354  away from the flat washer  352  and the ledge  312   a  when linear input is desired. Doing so may reduce the amount of force necessary to move the push rod  302 , as less interaction between the roller pins  342  and the push  302  has to be overcome. 
       FIGS. 10 through 12  show part of an alternate push-pull control  500 . The push-pull control  500  is similar to the push-pull control  300  in many aspects. For uniformity and brevity, corresponding reference numbers may be used to indicate corresponding parts, though with any noted deviations. 
     The primary difference between the push-pull control  500  and the push-pull control  300  is that the rotational assist cartridge  330  is replaced with rotational assist cartridge  530 . The rotational assist cartridge  530  may, for example, be coupled to the base  310  (which may be altered to receive the rotational assist cartridge  530  in the opening  312 , with threads  532  of the rotational assist cartridge  530  interacting with threads  312   b ). Other couplings and configurations may alternately be used, and the principal requirement is that the location of the rotational assist cartridge  530  is fixed such that the push rod  302  is movable relative to the rotational assist cartridge  530 . 
     The rotational assist cartridge  530  has rollers (or “roller pins” or “pins”)  542  that are positioned in the same way relative to the push rod  302  as the roller pins  342  in the rotational assist cartridge  330 . However, instead of a hollow pin cage  332  and pin cups  346  providing the positioning, the rollers  542  are held by arms  533  coupled to a frame  531 . While the location of the rollers  542  may be fixed relative to the rod  302 , it may be more desirable for the rollers  542  to be biased by springs. For example, the arms  533  may be stationary, and springs may be between the rollers  542  and the arms  533  to bias the rollers  542  toward the push rod  302 ; or the rollers  542  may be fixedly (though rotatably) coupled to the arms  533 , and springs may couple the arms  533  to the frame  531  to bias the rollers  542  toward the push rod  302 . Or, the rollers  542  may have rotatable axes movable along predetermined paths, and spring plungers  545  ( FIG. 12 ) in channels  544  ( FIGS. 10 and 12 ) may interact with the rollers  542  to bias the rollers  542  toward the push rod  302 . 
     Numerous methods may be used to adjust the forces provided by the springs. For example, a pin  546  with an angled end  546   a  may extend from each spring plunger  545 , and a collar  548  with angled faces  549  may be rotated to force the pins  546  toward the springs plungers  545  (and increase pressure provided by the springs  545 ) or allow the pins  546  to retract from the springs  545  (and decrease pressure provided by the springs  545 ). Those skilled in the art will be able to implement alternate methods of adjusting the pressure provided by the springs upon receiving a single user input, or by adjusting the pressure of each spring individually. 
     Many different arrangements of the various components depicted, as well as components not shown, are possible without departing from the spirit and scope of the present invention. Embodiments of the present invention have been described with the intent to be illustrative rather than restrictive. Alternative embodiments will become apparent to those skilled in the art that do not depart from its scope. A skilled artisan may develop alternative means of implementing the aforementioned improvements without departing from the scope of the present invention. It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations and are contemplated within the scope of the claims.