Patent Publication Number: US-2017356529-A1

Title: Variable force tensioning assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims benefit of U.S. Provisional Application Ser. No. 62/349,267 filed Jun. 13, 2016, the disclosure of which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The invention relates to a variable force tensioning assembly for use in conjunction with an endless, flexible, power transmission member. More particularly, the invention relates to a variable force tensioning assembly that maintains a constant tension force on an endless, flexible, power transmission member such as a timing belt or a timing chain, encircling a drive sprocket and at least one driven sprocket used for an internal combustion engine of a motor vehicle. 
     A tensioner for an endless loop, such as a timing belt or a timing chain, is used to control the endless loop as the endless loop travels around a plurality of sprockets or gears. The slack of the endless loop varies as the temperature in the engine increases and as the endless loop wears. When wear occurs, the endless loop elongates and the slack in the endless loop increases. The increase in slack may cause noise, slippage, or, in the case when the endless loop is a chain, tooth jumping between the chain and sprocket teeth. If the increase of the slack in the endless loop is not taken up, by a tensioner for example, in an engine that uses the endless loop to drive a camshaft, the engine may be damaged because the camshaft timing will be misaligned by several degrees due to the slippage or tooth jumping. 
     In the case when a hydraulic tensioner is used, the performance of the hydraulic tensioner is based on two primary functions of a check valve. First, oil must flow through a check valve and into a high pressure chamber of the tensioner as the piston extends to take up slack in the endless loop. If the flow restriction of the check valve is too great, the piston will not have enough oil volume to support its extended length. Secondly, as the endless loop begins to push the piston back into the tensioner, the oil wants to flow back out of the check valve. At this point, the check valve ball must move back to seal off the oil passage. If the response time is slow, it takes longer to build up the necessary pressure to support the piston and control of the tension in the endless loop becomes an issue. 
     SUMMARY 
     A variable force tensioning assembly creates and maintains a tension force on an endless loop used to transmit power. The variable force tensioning assembly includes a body having a cylinder. An outer piston is disposed substantially within the cylinder and slidable therewithin. The outer piston includes an outer piston tube and a chamber divider extending through the outer piston tube. The outer piston and the body define a low rate portion. An inner piston is disposed at least partially within the outer piston tube and is slidable therewithin. The inner piston defines a high rate portion and acts with the outer piston against the body to create a variable force to push against the endless loop to maintain a constant tension in the endless loop. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Advantages of the invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a simplified schematic of a variable force tensioning assembly according to one embodiment of the invention for an endless loop used as a power transmission member for an internal combustion engine; 
         FIG. 2  is a perspective view, partially cutaway, of one embodiment of the invention; 
         FIG. 3  is a cross-sectional side view of the embodiment shown in  FIG. 2 ; 
         FIG. 4  is a cross-sectional side view of an alternative embodiment of the invention; 
         FIG. 5  is a partial cross-sectional side view of the embodiment in  FIG. 4  with a ratchet clip allowing a lower force to move the piston assembly out and away from the body; 
         FIG. 6  is a cross-sectional partial side view of the embodiment of  FIG. 4  with a ratchet clip seated, preventing an inward stroke of the piston without receiving a greater force on the piston; and 
         FIG. 7  is an exploded perspective view of the alternative embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The term “loop,” “belt” or “chain,” as used interchangeably herein, is any power transmission member forming an endless loop and constructed of flexible material or of articulated rigid lengths to permit the member to conform to a radius of curvature of a pulley or sprocket drive face and intended, in use, to be driven in an endless path; and, by contact with the pulley or sprocket drive face, to transmit power to or extract power from the pulley or sprocket. The term “pulley” or “sprocket” as used interchangeably herein, is a device rotatable about an axis and having a drive face radially spaced from the axis of rotation for intended power transferring engagement with a belt or chain to drive the belt or chain on an endless path or to extract power from the belt or chain to drive an output load device. The term “guide roll” as used herein is a device rotatable about an axis and having a belt or chain-contacting face radially spaced from the axis of rotation for intended engagement with the belt or chain to aid in directing the belt or chain along an intended path of travel. A guide roll, as distinguished from a pulley or sprocket, is not intended to provide driving power to, or extract power from, a belt or chain. The term “tensioning arm” as used herein is a member other than a pulley or sprocket engagable with the belt or chain, and which is adjustable or relatively movable with respect to the belt or chain in a direction which causes an increase or decrease in tensile stress in the belt or chain or a take-up of any undesirable belt or chain slack to maintain a desirable drive traction between the belt or chain and the pulley or sprocket drive face. A tensioning arm, as distinguished from a guide roll, has a non-rotatable face portion for contacting the belt or chain, whereby the belt or chain slides over the face portion of the tensioning arm. The term “hydraulic tensioner” as used herein applies to a force for actuating the tensioning arrangement and is derived from or transmitted via the exertion of force on a fluid. 
     Referring to  FIG. 1 , a variable force tensioning assembly is generally indicated at  10 . The variable force tensioning assembly  10  is schematically illustrated as a hydraulic tensioner used in conjunction with an endless loop, flexible, power transmission member  12  (“endless loop  12 ”) for an internal combustion engine of a motor vehicle (neither the internal combustion engine nor the motor vehicle are shown). The endless loop  12  is shown as a chain having links  13  that encircles a drive sprocket  14  driven by a driveshaft, such as a crankshaft of the internal combustion engine, and at least one driven sprocket  16  supported from a driven shaft, such as a camshaft of the internal combustion engine. A guide roll can also be provided, if desired. The endless loop  12  passes over the drive sprocket  14  and driven sprocket  16  when driven in rotation as shown by arrow  18 . At least one tensioning arm  20  is positioned with a face assembly including a shoe for sliding engagement with the endless loop  12 . The tensioning arm  20  may pivot about pivot  22  in response to a force exerted by the variable force tensioning assembly  10 . By applying a force in a clockwise direction when viewing  FIG. 1 , the tensioning arm  20  pivots about the pivot  22  and applies tension to the endless loop  12  to remove excess slack. It should be recognized that the variable force tensioning assembly  10  disclosed below can be used in other alternative configurations of tensioning arms without departing from the spirit or scope of the invention, and that the illustrated configuration is by way of example only, and is not to be considered a limitation of the invention. 
     Referring to  FIGS. 1 through 3 , the variable force tensioning assembly  10  includes a body  24 . The body  24  includes at least one aperture  26  that allows a fastener (not shown) to secure the body  24  to a portion of the motor vehicle such that the variable force tensioning assembly  10  will be disposed adjacent the tensioning arm  20  and, as such, operatively engageable with the endless loop  12 . The body  24  defines a cylinder  28 , which defines a cylinder inner diameter  30  (shown graphically in  FIG. 1 ). The cylinder  28  is open at a tension arm facing surface  32  of the body  24  and is closed at a non-facing surface  34 . Because it is closed, the cylinder  28  defines an abutment surface  36  that extends perpendicular to the cylinder  28  along the non-facing surface  34 . In the embodiment shown, the abutment surface  36  includes a spring relief  38 , which receives a spring (discussed subsequently) therein. 
     The variable force tensioning assembly  10  also includes an outer piston, generally shown at  40 . The outer piston  40  is disposed substantially within the cylinder  28  of the body  24  and is slidable therewithin. The outer piston  40  includes an outer piston tube  42  and a chamber divider  44 . The chamber divider  44  extends through the outer piston tube  42 . In the embodiment shown, the chamber divider  44  extends perpendicularly to the outer piston tube  42  and parallel to the abutment surface  36  of the cylinder  28 . In the embodiment shown in the Figures, the outer piston tube  42  has a circular cross section. It should be appreciated by those skilled in the art that the outer piston tube  42  may have a cross sectional shape other than a circle without deviating from the concepts of the invention. The chamber divider  44  includes chamber passage  46 . In the examples shown in  FIGS. 1 through 3  the chamber passage  46  is a fluid passage. The space between the outer piston tube  42 , the chamber divider  44  and the abutment surface  36  of the cylinder  28  defines a chamber described as a low rate portion  48 . The low rate portion  48  receives fluid therein and houses a low rate spring  50 . The low rate spring  50  takes up any elongation in the endless loop  12  by asserting a force against the tensioning arm  20 , which is translated into reduced slack in the endless loop. A check valve  52  is disposed between the chamber divider  44  and the low rate spring  50 . The check valve  52  selectively opens and closes the chamber passage  46  based on the pressures of the fluids within the low rate portion  48  and a high rate portion  54 , discussed subsequently. The check valve  52  also releases air from the low rate portion  48 , if necessary. 
     The variable force tensioning assembly  10  also includes an inner piston  56  disposed substantially within the outer piston tube  42  and slidable therewithin. The inner piston  56 , together with the portion of the outer piston tube  42  between the tension arm facing surface  32  and the chamber divider  44  defines a chamber described as a high rate portion  54 . It is the inner piston  56  that acts, in conjunction with the outer piston  40 , against the body  24  to create a variable force to push against the endless loop  12  vis-a-vis the tensioning arm  20  to maintain a constant tension in the endless loop  12 . The inner piston  56  includes a cap surface  58 , which abuts the tensioning arm  20 , and an inner cylinder  60  having an open end  62  disposed adjacent the chamber divider  44 . In addition to the fluid which is received by the inner piston  56 , a high rate spring  64  is disposed within the inner piston  56  between the cap surface  58  and the chamber divider  44 . The inner piston  56  is slidable with respect to the outer piston  40 . A small air vent  66  allows air to escape the high rate portion  54 . 
     A fluid inlet passage  68  extends between an outer surface  70  of the body  24  and the cylinder  28 . The fluid inlet passage  68  is the passageway through which the fluid, typically a hydraulic oil, is delivered to the cylinder  28  as well as into the low rate portion  48  and the high rate portion  54 . As is shown best in  FIG. 3 , the fluid reaches the high rate portion  54  through a chamber passage  72  cut through the outer piston tube  42  near the chamber divider  44  but within the high rate portion  54 . The chamber passage  72  is selectively closed when the inner piston  56  is forced down into the high rate portion  54 . When this happens, the high rate portion  54  provides the hydraulic fluid to the low rate portion when the engine cannot supply oil to the variable force tensioning assembly  10  due to the fact that the chamber passage  72  has been closed by the inner cylinder  60  of the inner piston  56 . Assisting in the transfer of the hydraulic fluid from the fluid inlet passage to the high rate portion  54  is an outer surface relief  74 . The outer surface relief  74  is a portion of the outer piston tube  42  that has a diameter that is less than the cylinder diameter  30 . This creates a passageway through which the fluid can pass from outside the body  24  into the cylinder  28  and the inner  56  and outer  40  pistons via the chamber passage  72 . 
     The clearance  76  between the open end  62  of the inner cylinder  60  and the chamber divider  44  controls the stroke of the piston as the endless loop  12  provides a force on the variable force tensioning assembly  10  vis-à-vis the tensioning arm  20 . 
     When an input force is received by the endless loop  12  forcing the compression of the variable force tensioning assembly  10 , the inner  56  and outer  40  pistons compress or reduce the volume inside the low rate  48  and high rate  54  portions. The fluid in the low rate chamber  48  flows out due to its decreasing volume. The reservoir provides fluid to the low rate chamber  48  using negative pressure upon its return to a larger volume. Generally, the low rate chamber  48  keeps enough fluid therein to generate a reaction force when necessary. 
     An alternative embodiment to the variable force tensioning assembly  110  is shown in  FIGS. 4  though  7 , wherein like elements similar to those disclosed in the first embodiment in  FIGS. 1 through 3  include reference numerals similar to those shown in the first embodiment with an offset of  100 . The description of these elements in the first embodiment are hereby incorporated into the alternative embodiment. 
     In the alternative embodiment, the variable force tensioning assembly  110  is not a hydraulic assembly. In the place of hydraulics is a ratchet and retainer. Because this embodiment does not incorporate the use of hydraulics, elements such as the check valve  52 , small air vent  66 , fluid inlet passage  68 , and chamber passage  72  are not required. In place of the hydraulics is a ratchet clip  78  and a plurality of ribs  80  that encircle the outer surface relief  174  of the outer piston tube  142 . Each of the plurality of ribs  80  includes a shallow side  82  and a steep side  84 . That is, the ratio of displacement of the ratchet clip  78  as is moves along the shape of the shallow side  82  is further in the x-direction relative to displacement in the y-direction than occurs when the clip  78  moves along the steep side  84 . The ratchet clip  78  is slidably securable to the outer piston  140  and slidable along the plurality of ribs  80  rendering forces required to move the inner piston  156  into the cylinder  128  greater than the forces required to move the inner piston  156  out of the cylinder  128 . The variable force tensioning assembly  110  also includes a retaining ring  86 . 
     In this embodiment, the inner piston  156  includes a solid inner cylinder  160  or inner stem  160 . The inner cylinder  160  extends through the high rate spring  164  and is retained in place by the retainer ring  86  which is disposed on an inner side  88  of the chamber divider  144 . The high rate spring  164  may be implemented using a plurality of disk-shaped springs that collectively act against the inner piston  156 . The disk-shaped springs, sometimes referred to as Belleville washers or spring washers, can be oriented in series by stacking the springs on top of each other in an axially-aligned orientation. Or the disk-shaped springs can be arranged in parallel such that the center line of one disk-shaped spring is offset from another disk-shaped spring. And rather than a single module of disk-shaped springs arranged in parallel or series, multiple modules each comprising a plurality of disk-shaped springs may be used. It should be understood that the number of disk-shaped springs used to implement the high rate spring  164  can be selected based on a desired spring rate for the high rate spring  164 . The quantity, orientation, and/or the size of the disk-shaped springs used to implement the high rate spring  164  can be selected to make the overall stiffness of the spring  164  progressive or regressive. The cap surface  158  is fabricated of a solid material and does not include a small air vent similar to the small air vent  66  found in the first embodiment. Because this embodiment does not include hydraulic fluid, there is no reason to vent air out from the system. The retainer ring  86  is press fit over the inner cylinder  160  to secure the inner piston  156  to the outer piston  140 . 
     The cylinder  128  includes a relief  90  that extends along a portion of the cylinder  128 . The relief  90  provides first  92  and second  94  stops preventing the ratchet clip  78  from moving therepast. The first stop  92  prevents the outer piston  140  and the inner piston  156  from leaving the body  124  of the variable force tensioning assembly  110 . 
     Referring to  FIG. 5 , the ratchet clip  78  is not seated against either of the first  92  or second  94  stops. As the inner piston  156  moves out and away from the cylinder  128 , a lower force is required and low stiffness providing minimal damping occurs. This is to move the inner piston  156  out allowing the tensioning arm  20  to engage the endless loop  12  quickly. In  FIG. 6 , the ratchet clip  78  is seated against the second stop  94 . When this occurs, a higher force is required to move the inner piston  156  back into the cylinder  128  and body  124 . Because the ratchet clip  78  abuts the second stop  94  and is seated, a higher force is required to overcome the steep sides  82  of the plurality of ribs  80 . This provides a high stiffness and the maximum amount of damping so that tension arm  20  does not readily retract with respect to the endless loop  12  such that a force is maintained on the endless loop  12  in a manner to provide smooth transitions between periods of slack and periods of tautness. 
     The invention has been described in an illustrative manner. It is to be understood that the terminology, which has been used, is intended to be in the nature of words of description rather than of limitation. 
     Many modifications and variations of the invention are possible in light of the above teachings. Therefore, within the scope of the appended claims, the invention may be practiced other than as specifically described.