Patent Publication Number: US-11383775-B2

Title: Track assembly for power machine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a divisional of U.S. application Ser. No. 15/747,577, filed on Jan. 25, 2018, which published as US 2018-0215425 A1 on Aug. 2, 2018, which is scheduled to issue as a U.S. Pat. No. 10,793,210 on Oct. 6, 2020, which is a Section 371 National Stage Application of International Application No. PCT/US2016/049676, internationally filed on Aug. 31, 2016 and published as WO 2017/040643 A1 on Mar. 9, 2017, in English; which claims priority to U.S. Provisional Patent Application No. 62/212,290, filed on Aug. 31, 2015, the contents of which are hereby incorporated by reference in their entireties. 
    
    
     FIELD 
     Disclosed embodiments relate to power machines and more particularly to power machines that have endless tracks for tractive elements. 
     BACKGROUND 
     Some power machines, such as mini-loaders or other loaders have tracks mounted on either side of a frame. Endless tracks are powered about track frames on which the tracks are carried. One or more idlers are typically employed to maintain a desirable tension on the endless tracks. In addition, a plurality of rollers or bogie wheels are typically attached to the track frames and engage the endless tracks under the track frames to distribute the weight of the machine over that portion of the track that is in engagement with the ground. 
     The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. 
     SUMMARY 
     Disclosed embodiments include track assemblies for attachment to a power machine. Each track assembly includes a track frame, at least one idler, a sprocket, and a plurality of rollers over which an endless track is carried. The plurality of rollers is spaced along the bottom of the track frame to optimize the power machine&#39;s ride. 
     In one illustrative embodiment, a power machine is disclosed. The power machine has frame, an engine supported by the frame, a transmission powered by the engine, and a track assembly coupled to the frame and to an output of the transmission. The track assembly includes a track frame with a primary portion and a secondary portion that is moveable with respect the first portion under the influence of a tensioning member. A track is carried on the track frame and is moveable about the frame to move the power machine over a support surface. A first idler pivotally mounted to the primary portion of the frame and positioned against track. A second idler is pivotally mounted to the secondary portion of the frame and positioned against the track. Each of the first and second idlers is positioned to engage the track to provide tension to the track. A plurality of rollers is pivotally mounted on a bottom side of the track frame. Each of the rollers is positioned to engage the track. Movement of the secondary portion of track frame adjusts the tension applied against the track. When the secondary portion of the track frame is fully extended, a perimeter of the second idler intersects a cylinder defined by a perimeter of one of the plurality of the rollers. 
     In another illustrative embodiment a track assembly for a power machine, is disclosed. The track assembly includes a track frame having a main portion and a secondary portion that is moveable with respect to a first portion. A track is carried on the track frame and capable of being driven around the frame. A first idler is rotatably mounted to the main portion of the track frame and positioned to engage the track and rotate as the track is driven about the first idler. A second idler rotatably mounted to the secondary portion track portion and positioned to engage the track and rotate as the track is driven about the second idler. A plurality of rollers is rotatably mounted to the main portion of the track frame. A first of the plurality of the rollers is positioned close enough to the second idler so that the second idler extends into a cylinder defined by a perimeter of the first roller. 
     In yet another embodiment, a power machine is disclosed. The power machine has a frame, a lift arm coupled to the frame, an engine, and a drive system powered by the engine and coupled to a track assembly for driving the power machine across a support surface. The track assembly includes a track frame, and a track carried by the track frame and driven under power about the track frame. The track has a plurality of reinforcement members extending transversely to a direction of travel of the power machine. The reinforcement members are evenly spaced from each other so that the track has an alternating pattern of reinforced sections separated by unreinforced sections. First, second, third and fourth rollers are rotatably mounted to the track frame and positioned to engage the track along a bottom side of the track frame. When the second roller is positioned above a first reinforced section of the track, the third roller is positioned above a second reinforced section of the track. 
     This Summary and the Abstract are provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating portions of a power machine upon which embodiments of the present disclosure may be advantageously practiced. 
         FIGS. 2-3  are perspective illustrations of a power machine upon which the embodiments of the present disclosure can be advantageously practiced. 
         FIG. 4  is a side elevation view of a track frame attachable to a power machine and about which an endless track can be carried, according to one illustrative embodiment. 
         FIG. 5  is a bottom perspective view of the track frame of  FIG. 4 . 
         FIG. 6  is a diagram illustrating the relative placement of idlers and rollers on the track frame of  FIG. 4  relative to the center of gravity of a power machine to which it is attachable. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. 
     Disclosed embodiments provide track assemblies and power machines with track assemblies with track frames about which an endless track is carried and powered. A plurality of rollers are pivotally attached to a bottom of each track frame. The rollers are positioned to apply and distribute pressure on an endless track that is carried by the track frame. Before discussing specific embodiments of the present disclosure, however, a brief overview of power machines on which the embodiments can be advantageously employed are discussed. 
       FIG. 1  is a schematic diagram of portions a power machine  100  upon which the embodiments disclosed below can be advantageously employed. Power machine  100  is representative of a number of different types of power machines including, but not necessarily limited to, mini loaders, where an operator controls the loader from a position that is behind or substantially behind the loader. In some of these types of loaders, an operator can walk behind the loader; in others, the operator can ride on the loader. Power machine  100  includes, in an exemplary embodiment, left and right joystick controllers or joystick input devices  102 L and  102 R located on a control panel  105 , a power source  104 , a control system  106  including left drive pump  115  and right drive pump  120 , a left drive motor  125  and a right drive motor  130 , and left side tractive elements  108 L and right side tractive elements  108 R. 
     In some embodiments, power source  104  is an internal combustion engine, though other power sources can also be used, such as those using electricity or other sources of energy. Control system  106  is operably coupled to the power source  104 , receives power from the power source  104 , and converts the received power to signals that operate functional components of the power machine. In some embodiments, the control system  106  includes hydraulic components such as one or more hydraulic pumps (e.g., left and right drive pumps  115  and  120 ) that are configured to provide pressurized hydraulic fluid to various motors, actuators, and valve components that are illustratively employed to control the flow of hydraulic fluid to some or all of the motors and actuators used to control functional components of the power machine  100 . Other types of control systems are contemplated. For example, the control system  106  can include electric generators or the like to generate electrical control signals to power electric actuators. For the sake of simplicity, the motors and actuators disclosed herein are referred to as hydraulic or electrohydraulic motors and actuators, but other types of motors and actuators can be employed in some embodiments. 
     Left and right joystick operator inputs  102 L and  102 R located on a control panel  105  are operatively coupled to control connections  110  to provide inputs or input signals, indicative of the actuation of the inputs by an operator, to the control system  106  to control left and right drive pumps  115  and  120 . Control connections can include electrical, mechanical or other connections, a controller, or other devices. Left and right drive pumps  115  and  120  are illustratively variable displacement pumps that are in hydraulic communication with left drive motor  125  and right drive motor  130 , respectively. Control connections  110  illustratively provide output signals to control the left and right drive pumps  115  and  120  to provide hydraulic fluid to the left and right drive motors  125  and  130  to cause the left and right drive motors to rotate in a clockwise or counterclockwise direction at a rate determined by the drive control operator inputs  102 L and  102 R. Left drive motor  125  is coupled to tractive elements  108 L and right drive motor  130  is coupled to tractive elements  108 R. Tractive elements  108 L and  108 R can be track assemblies with one or more such track assemblies on each side of the machine. The left and right drive pumps  115  and  120  can be energized to cause the tractive elements  108 L and  108 R to act against each other by operating at different speeds and/or directions to accomplish steering by skidding. 
       FIGS. 2-3  illustrates a loader  200 , which is one particular example of a power machine of the type illustrated in  FIG. 1  where the embodiments discussed below can be advantageously employed. Loader  200  is a tracked loader and more particularly, a mini-loader. A mini-loader for the purposes of this discussion is a small loader without an operator cab that can be operated from an operator station at the back of the loader. Some mini-loaders have a platform on which an operator can ride on. Other mini-loaders can be operated by an operator who walks behind the loader. Still other mini-loaders have a platform that is moveable or removable to allow an operator to alternatively ride on the platform or walk behind the loader. 
     Track loader  200  is one particular example of the power machine  100  illustrated broadly in  FIG. 1  and discussed above. To that end, features of loader  200  described below include reference numbers that are generally similar to those used in  FIG. 1 . For example, loader  200  is described below as having a frame  210 , just as power machine  100  has a frame  110 . Track loader  200  is described herein to provide a reference for understanding one environment on which the embodiments described below related to operator controls may be practiced. The loader  200  should not be considered limiting especially as to features that loader  200  may have described herein that are not essential to the disclosed embodiments. Such features may or may not be included in power machines other than loader  200  upon which the embodiments disclosed below may be advantageously practiced. Unless specifically noted otherwise, embodiments disclosed below can be practiced on a variety of power machines, with the loader  200  being only one of those power machines. For example, some or all of the concepts discussed below can be practiced on many other types of work vehicles such as various other loaders, excavators, trenchers, and dozers, to name but a few examples. 
     As mentioned above, loader  200  includes frame  210 . Frame  210  supports a power system  220 , the power system being capable of generating or otherwise providing power for operating various functions on the power machine. Frame  210  also supports a work element in the form of a lift arm structure  230  that is selectively powered by the power system  220  in response to signals from an operator control system  260  and is capable of performing various work tasks. As loader  200  is a work vehicle, frame  210  also supports a traction system  240 , which is also selectively powered by power system  220  in response to signals from operator control system  260 . The traction system  240  is capable of propelling the power machine over a support surface. The lift arm structure  230  in turn supports an implement carrier  272 , which is capable of receiving and securing various implements to the loader  200  for performing various work tasks. The loader  200  can be operated from an operator station  250  from which an operator can manipulate various control devices to cause the power machine to perform various functions, discussed in more detail below. Frame  210  also supports a work element in the form of a lift arm structure  230  that is powered by the power system  220  and is capable of performing various work tasks. 
     Various power machines that are capable of including and/or interacting with the embodiments discussed below can have various different frame components that support various work elements. The elements of frame  210  discussed herein are provided for illustrative purposes and should not be considered to be the only type of frame that a power machine on which the embodiments can be practiced can employ. Frame  210  of loader  200  includes an undercarriage or lower portion  211  of the frame and a mainframe or upper portion  212  of the frame that is supported by the undercarriage. The mainframe  212  of loader  200  is attached to the undercarriage  211  such as with fasteners or by welding the undercarriage to the mainframe. Mainframe  212  includes a pair of upright portions  214  located on either side and toward the rear of the mainframe that support a lift arm structure  230  and to which the lift arm structure  230  is pivotally attached. The lift arm structure  230  is illustratively pinned to each of the upright portions  214 . The combination of mounting features on the upright portions  214  and the lift arm structure  230  and mounting hardware (including pins used to pin the lift arm structure to the mainframe  212 ) are collectively referred to as joints  216  (one is located on each of the upright portions  214 ) for the purposes of this discussion. Joints  216  are aligned along an axis  218  so that the lift arm structure is capable of pivoting, as discussed below, with respect to the frame  210  about axis  218 . Other power machines may not include upright portions on either side of the frame, or may not have a lift arm structure that is mountable to upright portions on either side and toward the rear of the frame. For example, some power machines may have a single arm, mounted to a single side of the power machine or to a front or rear end of the power machine. Other machines can have a plurality of work elements, including a plurality of lift arms, each of which is mounted to the machine in its own configuration. Frame  210  also supports a pair of tractive elements  242  on either side of the loader  200 , which on loader  200  are track assemblies. 
     The lift arm structure  230  shown in  FIGS. 2-3  is one example of many different types of lift arm structures that can be attached to a power machine such as loader  200  or other power machines on which embodiments of the present discussion can be practiced. The lift arm structure  230  has a pair of lift arms  232  that are disposed on opposing sides of the frame  210 . A first end  232 A of each of the lift arms  232  is pivotally coupled to the power machine at joints  216  and a second end  232 B of each of the lift arms is positioned forward of the frame  210  when in a lowered position as shown in  FIG. 2 . The lift arm structure  230  is moveable (i.e. the lift arm structure can be raised and lowered) under control of the loader  200  with respect to the frame  210 . That movement (i.e. the raising and lowering of the lift arm structure  230 ) is described by a radial travel path, shown generally by arrow  233 . For the purposes of this discussion, the travel path  233  of the lift arm structure  230  is defined by the path of movement of the second end  232 B of the lift arm structure. 
     The lift arms  232  are each coupled to a cross member  236  that provides increased structural stability to the lift arm structure  230 . A pair of actuators  238 , which on loader  200  are hydraulic cylinders configured to selectively receive pressurized fluid from power system  220 , are pivotally coupled to both the frame  210  and the lift arms  234  at pivotable joints  238 A and  238 B, respectively, on either side of the loader  200 . The actuators  238  are sometimes referred to individually and collectively as lift cylinders. Actuation (i.e., extension and retraction) of the actuators  238  cause the lift arm structure  230  to pivot about joints  216  and thereby be raised and lowered along a fixed path illustrated by arrow  233 . The lift arm structure  230  shown in  FIGS. 2-3  is representative of one type of lift arm structure that may be coupled to the power machine  200 . Other lift arm structures, with different geometries, components, and arrangements can be pivotally coupled to the loader  200  or other power machines upon which the embodiments discussed herein can be practiced without departing from the scope of the present discussion. For example, other machines can have lift arm structures with lift arms that each has two portions (as opposed to the single piece lift arms  232 ) that are pivotally coupled to each other along with a control arm to create a four-bar linkage and a substantially vertical travel path or at least more vertical than the radial path of lift arm structure  230 . Other lift arm structures can have an extendable or telescoping lift arm. Still other lift arm structures can have several (i.e. more than two) portions segments or portions. Some lift arms, most notably lift arms on excavators but also possible on loaders, may have portions that are controllable to pivot with respect to another segment instead of moving in concert (i.e. along a pre-determined path) as is the case in the lift arm structure  230  shown in  FIGS. 2-3 . Some power machines have lift arm structures with a single lift arm, such as is known in excavators or even some loaders and other power machines. Other power machines can have a plurality of lift arm structures, each being independent of the other(s). 
     An exemplary implement interface  270  is provided at a second end  234 B of the arm  234 . The implement interface  270  includes an implement carrier  272  that is capable of accepting and securing a variety of different implements to the lift arm  230 . Such implements have a machine interface that is configured to be engaged with the implement carrier  272 . The implement carrier  272  is pivotally mounted to the second end  232 B of each of the arms  232 . An implement carrier actuator  237  is operably coupled the lift arm structure  230  and the implement carrier  272  and are operable to rotate the implement carrier with respect to the lift arm structure. Other examples of power machines can have a plurality of implement carrier actuators. Still other examples of power machines of the type that can advantageously employ the disclosed embodiments discussed herein may not have an implement carrier that is capable of accepting and securing a variety of different attachments, but instead may allow only for implements to be directly attached to its lift arm structure. 
     The implement interface  270  also includes an implement power source  235  available for connection to an implement on the lift arm structure  230 . The implement power source  235  includes pressurized hydraulic fluid ports to which an implement can be coupled. The pressurized hydraulic fluid port selectively provides pressurized hydraulic fluid for powering one or more functions or actuators on an implement. The implement power source can, but need not, include an electrical power source for powering electrical actuators and/or an electronic controller on an implement. The electrical power source can also include electrical conduits that are in communication with a data bus on the loader  200  to allow communication between a controller on an implement and electronic devices on the loader  200 . It should be noted that the specific implement power source on loader  200  does not include an electrical power source. 
     The lower frame  211  supports and has attached to it a pair of tractive elements, identified in  FIGS. 2-3  as left track assembly  242 A and right track assembly  242 B (collectively tractive elements  242 ). Each of the tractive elements  242  has a track frame  243  that is coupled to the frame  210 . The track frame  243  supports and is surrounded by an endless track  244 , which rotates under power to propel the loader  200  over a support surface. Various elements are coupled to or otherwise supported by the track frame  243  for engaging and supporting the endless track  244  and cause it to rotate about the track frame. For example, a sprocket  246  is supported by the track frame  243  and engages the endless track  244  to cause the endless track to rotate about the track frame. An idler  245  is held against the track  244  by a tensioner (not shown) to maintain proper tension on the track. The track frame  243  also supports a plurality of rollers  248 , which engage the track and, through the track, the support surface to support and distribute the weight of the loader  200 . 
     An operator station  250  is positioned toward the rear of the frame  210 . A platform  252  is provided for the operator to stand. While standing on the platform  252 , and operator has access to a plurality of operator control inputs  262  that, when manipulated by the operator, can provide control signals to control work functions of the power machine  200 , including, for example, the traction system  240  and the lift arm  230 . 
     Display devices  264  are provided in the operator station to give indications of information relatable to the operation of the power machines in a form that can be sensed by an operator, such as, for example audible and/or visual indications. Audible indications can be made in the form of buzzers, bells, and the like or via verbal communication. Visual indications can be made in the form of graphs, lights, icons, gauges, alphanumeric characters, and the like. Displays can be dedicated to provide dedicated indications, such as warning lights or gauges, or dynamic to provide programmable information, including programmable display devices such as monitors of various sizes and capabilities. Display devices can provide diagnostic information, troubleshooting information, instructional information, and various other types of information that assists an operator with operation of the power machine or an implement coupled to the power machine. Other information that may be useful for an operator can also be provided. 
     The description of power machine  100  and loader  200  above is provided for illustrative purposes, to provide illustrative environments on which the embodiments discussed below can be practiced. While the embodiments discussed can be practiced on a power machine such as is generally described by the power machine  100  shown in the block diagram of  FIG. 1  and more particularly on a loader such as loader  200 , unless otherwise noted or recited, the concepts discussed below are not intended to be limited in their application to the environments specifically described above. 
       FIGS. 4-5  illustrate a track assembly  300  of the type that can be operably coupled to a power machine such as power machines  100  and  200  according to one illustrative embodiment. Track assembly  300  includes a track frame  302  and an endless track (not shown in the FIGs. to more clearly illustrate the track frame and attached components) that is carried on the frame. The track frame, in one embodiment, is rigidly mounted to the power machine so that it cannot move relative to the rest of the power machine. In other embodiments, the track frame may be mounted to allow for movement between the track frame and the machine frame. The track frame  302  includes a first, or primary, portion  304  and a second portion  306 , which is moveable with respect the first portion under the influence of a tensioning member (not shown). The tensioning member can be a spring mechanism, a cylinder, a combination thereof, or any structure suitable for moving the second portion  306  of the track frame  302  relative to the first portion  304 . A pair of idlers are mounted, one each, to the first portion  304  (front idler  308 ) and the second portion  306  (rear idler  310 ). The front and rear idlers  308 ,  310  engage the track to apply tension to the track by extending the second portion  306  of the frame relative to the first portion  304 . It should be noted that the track assembly  300  is shown in  FIGS. 4-5  in an extended or tensioned position. In one embodiment, in order to retract the idler and remove the track, the roller  316 - 1  will have to be removed. A drive sprocket  312  is operably coupled to the track frame  302 . The drive sprocket  312  has a plurality of teeth  314  for engaging a track. The drive sprocket  312  is capable of selectively receiving power from a power source on a power machine and converting the received power into a rotational motion so that teeth on the sprocket engage and drive a track around the track frame  302 . 
     A plurality of rollers  316  are rotatably coupled to the first portion  304  of the track frame  302  in fixed positions so that the second portion  306  of the track frame is capable of movement under influence of the tensioning member. The rollers  316  are individually identified in  FIGS. 4-5  as  316 - 1 ,  316 - 2 ,  316 - 3 ,  316 - 4 , and  316 - 5  for the purposes of identifying their relative positions, but can be collectively referred to using the reference  316 . As is best seen in  FIG. 5 , each of the rollers  316 , except for roller  316 - 1 , has three circular elements or ribs, outer ribs  318  and an inner rib  320  of substantially identical diameters and attached to an axle  322 , which in turn is attached to the first portion  304  of track frame  302 . Each of the outer ribs  318 , are capable of engaging the track to apply pressure to the track and distribute the weight of the machine onto the track. In some embodiments, the relationship between the rollers  316  and the track, center ribs  320  are capable of engaging the track, but in others, the center ribs are not capable of engaging the track. Roller  316 - 1 , by contrast, only has outer ribs  318 ′ attached to an axle  322 ′, with no inner rib. This allows the rear idler  310  to be positioned within the diameter of the roller  316 - 1 , advantageously allowing the roller  316 - 1  to be positioned as close as possible to the rear idler  310 . 
     As shown in  FIGS. 4-5 , the spacing between the rollers  316  is not uniform. Generally speaking, rollers  316 - 2  and  316 - 3  are closer together than the spacing between any of the other rollers. Rubber tracks of the type that can be advantageously employed with track frames such as track frame  302  have metal reinforcing strips spaced periodically along the width of the track, the width of the track being defined as the direction between first and second sides  324 ,  328  of the track frame. The track can be the same width as the track frame  302 , but in some embodiments the track width can be wider (or narrower) than the track frame. When rollers  316  are on these metal reinforcing strips, they ride slightly higher than when they are between the reinforcing strips. It has been found that arranging specific rollers to be on pitch with each other can influence the ability of the track frames to compensate for vibrations that might otherwise be introduced by the rollers  316  as they move over the track (or more accurately, as the track moves under the rollers). If two rollers  316  are on pitch, the rollers travel over reinforcing strips simultaneously. If two rollers  316  are off pitch, one of the rollers is travelling over a reinforcing strip while the other is not traveling over a reinforcing strip. The slight variation between on-pitch and off-pitch heights of the rollers introduces a vibration of the machine, especially as the rollers are closer to the center of gravity of the power machine that can cause the power machine to rock slightly fore and aft as the machine travels over the ground, especially if a resultant force from the rollers moves fore and aft of the center of gravity. 
       FIG. 6  illustrates the relative position of elements of the track assembly  300  as attached to a power machine. The track assembly is positioned relative to the machine so that the roller  316 - 1  is fully behind the fore/aft center of gravity (identified as CG) of the machine. In addition, roller  316 - 2  at least partially positioned behind the CG of the machine. Rollers  316 - 2  and  316 - 3  are positioned so that they are in pitch with each other. Rollers  316 - 1  and  316 - 4  are positioned so that they are in pitch with each other, but out of pitch with rollers  316 - 2  and  316 - 3 . With this arrangement, as track  330  travels under the rollers  316 , the resultant force  326  remains in a relatively constant position and always either forward or, alternatively, rearward of (in other words, not oscillating between forward and rearward of) the CG. This position relative to the CG is discussed with reference to an unloaded machine. In some instances, a machine with a resultant force forward of the CG when unloaded may be rearward or aft of the CG when in a loaded condition because the CG of a front loaded machine may move forward when accounting for the load. For clarity&#39;s sake, any discussion in this application of a position of the resultant forces relative to the CG should be considered to be relative to the CG of an unloaded machine. The track  330  illustrates reinforcement members in the form of metal strips  332  embedded along the bottom thereof (it should be understood that understood that the members are embedded along the entirety of the track), showing rollers  316 - 1  and  316 - 4  being positioned over the reinforcement members and rollers  316 - 2  and  316 - 3  being positioned over spaces between the reinforcement members, illustrating the pitch relationship of these rollers. This arrangement improves the stability of the machine. Alternatively, when the rollers  316 - 2  and  316 - 3  being positioned over reinforcement members, rollers  316 - 1  and  316 - 4  are positioned over spaces between reinforcement members. Roller  316 - 5  can be in pitch with either of the pairs of  316 - 2  and  316 - 3  or  316 - 1  and  316 - 4 . Because roller  316 - 5  is relatively far away from the CG, its impact on the system&#39;s performance is relatively negligible. 
     The arrangement shown in  FIG. 6  accomplishes two important objectives: first, it allows for as much positioning of rollers behind the CG of the machine as is practicable. It should be noted that having the CG of the machine toward the rear of the machine is desirable because it increases the lift capacity of the machine. Thus, moving the CG forward is not a desirable option as a practical matter. Part of this is accomplished by allowing the rear idler  310  to move within the diameter of the roller  316 - 1 . Second, the rollers are spaced apart in such a manner that rollers  316 - 2  and  316 - 3  are in pitch while rollers  316 - 1  and  316 - 4  are moved away from rollers  316 - 2  and  316 - 3  so that they are out of pitch with  316 - 2  and  316 - 3  but in pitch with each other to maintain a relatively constant position of the resultant force  326 . 
     The embodiments above provide several advantages. In particular, the position of the rollers are such that vibrations caused by driving over various support surfaces are minimized due to the placement of rollers relative to the rear idler and the center of gravity of the power machine. The inventive placement of these rollers result in an improved experience for an operator, allowing the operator to control the power machine more easily and without experiencing as much fatigue as would otherwise be experienced. 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above.