Abstract:
A suspension strut having a piston tube containing a floating piston, slidable within a cylinder tube. A collar is fixed to the inner surface of the cylinder tube and contains a tapered wear ring. As the strut expands by extending the piston tube relative to the cylinder tube, the tapered wear band slides over valve orifices in the outer surface of the piston tube, gradually cutting off fluid flow from the annular space between the two tubes into an end cavity of the strut, thus increasing damping and reducing the likelihood of metal to metal contact at full strut extension. This reduces shock loads on the strut for smoother vehicle operation and increased component life.

Description:
PRIOR APPLICATION 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/224,510, filed Aug. 14, 2000 entitled “SUSPENSION STRUT”. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a hydraulic suspension strut for use in heavy vehicles. 
     BACKGROUND OF THE INVENTION 
     Suspension struts are used in many types of vehicles to absorb and dampen transient forces that a vehicle is subjected to as it travels over terrain. In a typical suspension strut, there are a plurality of cavities that contain a viscous fluid such as hydraulic fluid (oil). As the strut is compressed, fluid is allowed to flow between the cavities through orifices of varying sizes. The viscous fluid flowing through the orifices provides damping within the strut. The amount of damping within the strut can be adapted to the types of transient loads expected by increasing or decreasing the diameters of the orifices through which the fluid flows and by changing the viscosity of the fluid itself. 
     It is known in the art that as a suspension strut reaches its full extension, harmful metal to metal contact may occur between the piston and piston guide in addition to large spike loads being transferred to the vehicle at the strut attachment points. This situation results in unnecessary component wear which will decrease the operating life of the strut. To avoid this situation, it is known in the art to increase the damping within the strut to slow the rate of expansion as the strut nears full extension. For this purpose, mechanical rebound stops are sometimes provided which are attached to the outside of the piston rod and contractible within the rod guide. 
     Generally, mechanical rebound stops consist of a resilient bumper or a piston/chamber arrangement. As the bumper compresses, it absorbs energy that would otherwise be dissipated by the contact of metal components within the strut thus avoiding unnecessary wear. The bumpers are inexpensive but have limited energy storage/dissipation ability. The piston/chamber arrangement relies on expensive machining and surface treatment processes to achieve higher levels of energy dissipation, and is not cost-effective. 
     BRIEF SUMMARY OF THE INVENTION 
     This invention in a preferred embodiment provides a hydraulic strut which can be made in a relatively inexpensive manner and has a sliding seal that can increase the damping rate within the strut as the strut approaches full extension. 
     In one aspect the invention provides a suspension strut comprising: 
     (a) an outer cylinder having an inner surface; 
     (b) an inner cylinder defining a first space therein, said inner cylinder being slidable in said outer cylinder and defining an annular second space between said inner cylinder and said outer cylinder; 
     (c) said inner cylinder including a cap assembly defining a third space between said cap assembly and said outer cylinder; 
     (d) at least one valve on said inner cylinder to permit fluid flow between said annular second space and said third space; and 
     (e) a shutoff attached to the inner surface of said outer cylinder and cooperating with said valve to effect a gradual shutoff of fluid flow from said annular second space through said valve to said third space as said inner and outer cylinders are extended with respect to each other. 
     Further aspects and advantages of the invention will appear from the following disclosure, taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of a suspension strut according to the invention, in full compression; 
     FIG. 2 shows a partial cross sectional view of the FIG. 1 suspension strut, illustrating a piston tube cap and plug; 
     FIG. 3 is a cross sectional view showing detail at one end of the cylinder tube; 
     FIG. 4 is a cross sectional view of a floating piston of the FIG. 1 strut; 
     FIG. 5 is a partial cross sectional view of the FIG. 1 suspension strut showing a fluid path as the strut extends; 
     FIG. 6 is a view similar to FIG. 5 but showing a closed fluid flow path; 
     FIG. 7 shows detail of a second embodiment of the suspension strut of the invention with grooved channels in the piston tube; 
     FIG. 8 is a plan view of a portion of the piston tube of FIG. 1 showing round orifices in the piston tube wall; and 
     FIG. 9 is a cross sectional view showing another embodiment of the suspension strut of the invention, using round orifices in the piston tube with a straight wear band. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is first made to FIG. 1, which shows a suspension strut  20  according to the invention. Suspension strut  20  includes an outer cylinder assembly, shown generally at  22 , having a cylinder tube  24  and an annular collar  26  welded to the inner surface of cylinder  24 . Strut  20  also includes a piston tube assembly  28  having a piston tube  30  which is axially received in cylinder tube  24  and can slide axially therein, with a clearance between the piston and cylinder tubes  24 ,  30 . The clearance between these tubes is divided by the collar  26  into a first annular cavity  32  to the left (as drawn) of collar  26 , and a second annular cavity  34  to the right (as drawn) of collar  26 . 
     Cylinder tube  24  is connected at its right-hand end (as drawn), by thread  40 , to a cylinder head  42 . The other end of cylinder tube  24  is welded to an end ring  44 . 
     Cylinder head  42  includes an outer end  46  and an inner end  48 . Located at the inner end of cylinder head  42  is a bowl-shaped cavity  50 , positioned centrally within the cylinder head  42 . Located near the outer end  46  of cylinder head  42  is a pivot bearing  52  which typically receives an axle from a road arm assembly (not shown). Bearing  52  is slidably received in cylinder head  42  and is held in place with locking rings  54  as is commonly known in the art. A bored hole  56  which extends to the exterior of cylinder head  42  from bearing  52  allows grease to be pumped into the bearing for lubrication purposes as required. 
     Also located within cylinder head  42  is a vent plug  58  which is used primarily during the manufacturing phase, to bleed air from the interior of the suspension strut. The vent plug  58  communicates with the interior of suspension strut  20  via cavity  50 . 
     As mentioned, one end of cylinder tube  24  is welded to end ring  44 . End ring  44  has an axially inner end  64  and an axially outer end  66 . The axially inner end  64  of the end ring includes a recess  68  in its outer surface, which recess has a depth equal to the thickness of the wall of cylinder tube  24 . The cylinder tube  24  is slid over the recess  50  and is then welded in position on the end ring  44 . When the two components are welded together, the outer radial surface of the axially outer end  66  of end ring  44 , and the outer radial surface of cylinder tube  24 , are axially aligned and together form a smooth, continuous surface. 
     The cylinder tube  24  is covered over part of its length by a cylindrical protection tube  70 . The protection tube  70  is mounted to the outer radial edge of a piston tube head  74  assembly, which is in turn connected to the piston tube  30  by thread  76 . The protection tube  70  acts as a shield to reduce the likelihood of debris entering the interior of the suspension strut  20 . 
     The axially outer end  66  of end ring  44  includes (see FIG. 3) an annular channel  78  on its outer radial surface which receives an annular scraper  80 . The scraper  80  prevents debris from entering the interior of the suspension strut  20  between the protection tube  70  and end ring  44 . 
     The inner radial surface of end ring  44  includes annular channels  82 ,  84 ,  86  and  88 . Channel  82  contains an excluder seal  89  which prevents contaminants from being dragged into contact with the working fluid. Channels  84 ,  86  contain O-ring seals  90 ,  92  which bias step seals  94 ,  96  radially inwardly as shown in FIG.  1 . These seals prevent high pressure fluid located in annular cavity  32  from escaping from the suspension strut by travelling between the inner radial surface of end ring  44  and the outer radial surface of piston tube  30 . 
     Located within channel  88  is a guide ring  98  which can be made of Teflon™ or nylon or any other suitable material. Guide ring  98  helps to provide a smooth axial movement of piston tube  30  within cylinder tube  24 . 
     Annular collar  26  (see also FIG. 5) includes an annular channel  104  in its inner radial surface. Channel  104 , which is open at one end, receives a floating wear band  106  which can typically be made from any suitable material, e.g. suitable grades of steel, aluminum, brass or plastic. Wear band  106  has a tapered end  108  and a thick end  110 . At the tapered end  108 , the radially inner surface of wear band  106  tapers radially outwardly, as shown. 
     The wear band  106  is held within the channel  104  by a retaining ring  112  at the open end of channel  104 . The retaining ring  112  rests in a second annular channel  114  which is deeper than channel  104 . At the other end of collar  26 , end wall  116  of the annular collar holds the wear band  106  in channel  104 . 
     Annular collar  26  is dimensioned so that its inner diameter is slightly larger than the outer diameter of piston tube  30 , providing a clearance between annular collar  26  and piston tube  30 . However, the clearance is largely sealed by the wear band  106 , which nevertheless allows a small amount of fluid flow between annular cavities  32 ,  34 , so that in use, there will be fluid (e.g. hydraulic fluid) in cavity  32  to improve heat dissipation in the strut. 
     Piston tube  30  at its right hand end (as drawn) is fixedly mounted by thread  120  to a piston tube cap assembly indicated at  122  (see also FIGS. 2,  5 ,  6 ). Piston tube  30  also contains a conventional floating piston  124  which defines with cap assembly  122  a cavity  126  within the piston tube for high pressure fluid. 
     The piston tube head assembly  74  (at the left side of piston tube  30  as drawn) includes an outer end  130  and an inner end  132 . Located centrally at the inner end is a bowl shaped cavity  134 . Cavity  134  contains pressurized gas, typically nitrogen, which acts as a gas spring. When the strut is at rest, the pressures at cavities  195 ,  126  and  214  will be very close to being equal. 
     When the compressive force on the suspension strut is reduced, the pressure in cavity  195  will decrease, followed by that in cavity  126 . Once the “stiction” at piston  124  is overcome, piston  124  will move to the right (as drawn), forcing fluid from cavity  126  to cavity  195 . The gas in cavity  214  will continue to expand until its pressure is equal to the force imposed on the strut divided by the area (inner diameter) of the piston tube. 
     At the outer end  130  of piston tube head assembly  74  is a pivot bearing  136  which typically receives a support shaft from a vehicle (not shown). Pivot bearing  136  is slidably received in piston tube head assembly  74  and is held in place with locking rings  138  as is known in the art. A bored hole  140  which extends to the exterior of piston tube head assembly  74  from pivot bearing  136  allows grease to be pumped into the bearing for lubrication, as required. 
     Also located within piston tube head assembly  74  is a filler valve  142  which is used to pump or bleed gas from the cavity  134  as required. Filler valve  142  communicates between the exterior of piston tube head assembly  74  and bowl-shaped cavity  134  via bored hole  144 . With the strut set at mid-stroke, the spring force in the strut is a function of the gas charge pressure. The spring rate, or rate of change of spring force as the strut is extended or collapsed, is a function of the size of cavity  214  with the strut at mid-stroke. Damping forces are produced principally by the pressure differentials across the piston head acting on the annular and bore areas, as is well known. 
     The floating piston  124  (see FIG. 4) includes annular channels  150 ,  152 ,  154  and  156  on its outer radial surface. Channels  150 ,  156  contain guide rings  158 ,  160  to ensure smooth axial motion of the outer radial surface of floating piston  124  on the inner radial surface of piston tube  30 . Channel  152  contains an O-ring seal  160  outwardly biasing a step seal  162 , while channel  154  contains an O-ring seal  164  which outwardly biases a bean seal  166 . These seals prevent fluid from moving between cavities  126 ,  134  while at the same time allowing the floating piston  124  to travel freely within the piston tube  30  in response to displacement of the suspension strut. 
     Piston tube cap assembly  122  (at the right hand side of the piston tube  30  as drawn) includes (see also FIGS. 2,  5 ,  6 ) a cylindrical cap  170  which axially receives the outer radial surface of piston tube  30  on its inner annular surface and is secured thereto by thread  172 . Cap assembly  170  also includes a cylindrical plug  174  which is received in the bore of piston tube  30  and is held in position by plug flange  176  which is trapped between cap  170  and the end of piston tube  30 . 
     Plug  174  includes a first set of three axially aligned poppet valves  180  radially spaced 180° from each other. It will be appreciated that the number of poppet valves can be increased or decreased as desired. The axially outer ends of the poppet valves  180  communicate with a cavity  182  which is defined by axial spacing between plug  174  and cap  170 . The inner ends of poppet valves  180  communicate with radially bored holes  184  through the wall of piston tube  30 . 
     Also located within plug  174  is a second set of three poppet valves  188  which are axially aligned and spaced 120° from each other. Poppet valves  188  are each located 60° apart from poppet valves  180 . The outer ends of poppet valves  188  communicate with cavity  182 , while the inner ends of poppet valves  188  communicate with axially bored holes  190  in plug  174 . The holes  190  in turn communicate with the cavity  126  in piston tube  30 . 
     Plug  174  also includes a bored hole  192  along its central axis. Bored hole  192  allows fluid communication between cavities  126  and  182 . Similarly, cap  170  includes a bored hole  194  along its central axis. Bored hole  194  allows communication between cavity  182  and a cavity  195  in the cylinder tube  24  at the right-hand side of cap  170 . Cavity  195  is adjacent cavity  50 . 
     Cap  170  also includes six compression check valves  196  which are axially aligned and radially spaced  600  from each other. Compression check valves  196  communicate with cavity  195  at their outer ends, and with annular cavity  34  at their inner ends, by channel  198  as is shown in FIG.  2 . When the suspension strut  20  is placed in compression, fluid can flow from cavity  50  to annular cavity  34  via the flow path indicated by arrow  200  in FIG.  2 . However, when the suspension strut is being extended, fluid cannot flow through the compression check valves  196  from annular cavity  34  to cavity  50  since compression check valves  196  allow only one way flow, as is known in the art. 
     Cap  170  also includes (see FIG. 2) two annular channels  204 ,  206  on its outer radial surface. Channel  204  contains an outer T-ring seal  208  and an inner O-ring seal  210  to place outward pressure on the T-ring seal  208 . These seals prevent fluid from moving between cavities  34 ,  195  by travelling between the outer radial surface of cap  170  and the inner radial surface of cylinder tube  24 . 
     Annular channel  206  contains a guide ring  212  which can be made of Teflon™ or nylon. Guide ring  212  ensures smooth axial movement between the inner radial surface of cylinder tube  24 , and cap  170 . 
     The operation of the strut is as follows: 
     In Extension 
     With reference to FIG. 1, which shows the strut  20  in maximum compression, it will be seen that as the compressive force on the strut decreases, gas contained within cavity  134  and in the piston tube at the left of floating piston  124  (indicated as cavity  214 ) can expand (since the gas will now be under higher pressure than the fluid within cavity  126 ). As the gas expands, floating piston  124  will be driven towards cylinder tube head  42 , expelling fluid from piston tube cavity  126  into cavities  182 ,  195  and  50  via two different flow paths. Along one path, the fluid will flow through plug bore  192  into cavity  182 , and through bore  194 , before reaching cavities  195  and  50 . Along the second path, fluid will flow through poppet valves  188 , into cavity  182 , and through bore  194 , before reaching cavities  195  and  50 . 
     As the fluid enters cavities  195  and  50 , it will place an outward force on cylinder tube head  42  which will drive cylinder tube head  42  and piston tube head  74  away from each other. Consequently, collar  26  will move towards piston tube cap  170  which will reduce the volume of annular cavity  34 . This will force fluid to move from annular cavity  34  through bores  184 , through poppet valves  180 , into cavity  182 , through the bored hole  194  and into cavities  195  and  50  as indicated by the arrow  216  in FIGS. 2 and 5. 
     Eventually, strut  20  will be in equilibrium when the force exerted by the gas spring on the fluid in cavity  126  is equivalent to the compressive force being applied to the strut by the vehicle. 
     If the compressive force on the suspension strut  20  is reduced sufficiently, collar  26  will continue to travel towards piston tube cap  170 . Eventually, the tapered end  108  of floating wear band  106  will begin to overlap bored holes  184 . This will restrict fluid flow through bored holes  184  which will effectively slow the movement of hydraulic fluid from annular cavity  34  into cavities  182 ,  195  and  50 . As the flow of hydraulic fluid from cavity  34  into cavities  182 ,  195  and  50  is reduced, the pressure within annular cavity  34  will increase such that the force exerted on the end wall of the collar  26  (including the floating wear band  106 ) will approach the force applied to the cylinder tube head  42  via the gas spring. This will increase the damping within the strut  20  and will slow the expansion of the strut. 
     If the compressive force on the strut  20  is further reduced, the thick end  110  of the floating wear band  106  will begin to seal bored holes  184 , further increasing the damping effect. Eventually the bored holes  184  will be entirely sealed by the thick portion of the floating wear band  106  (see FIG.  6 ), causing cylinder tube head  42  to stop moving with respect to the collar  26 . Extension of the strut will cease since the force on collar  26  exerted by the fluid in annular cavity  34  will be equivalent to the force exerted by the gas spring on cylinder tube head  42 . 
     It will be appreciated that the gradual increase in damping that occurs as the suspension strut approaches full extension has several advantages. Firstly, metal to metal contact between the piston tube cap  170  and the collar  26  is avoided, which increases the. durability of the strut and makes the vehicle operation smoother and quieter. Secondly, reduced loads are transferred to the bearings, which will prolong their useful life. 
     Of course, other techniques can be used to achieve increased damping as the suspension strut approaches full extension, and which are within the spirit of the invention. For instance, the increased damping can be achieved (see FIG. 7) by having a wear band  106 ′ with uniform thickness slide over a tapered channel  220  in the piston tube  30 . The channel  220  can be deepest towards its inner axial end and shallowest towards its outer axial end such that as the wear band  106 ′ slides over the channel  220 , damping will gradually increase. Increased damping occurs because the area through which the fluid travels from annular cavity  34  through bores  182 ′ will be decreased, which will increase the pressure within annular cavity  34 . 
     As another alternative, and as shown in FIGS. 8 and 9, a straight, untapered wear band  106 ″ can be used (as also shown in FIG.  7 ), in cooperation with a round drilled hole  184 ″ in the piston tube wall. This combination will also produce a graduated shutoff of fluid flow from annular cavity  34  through the poppet valves  180 . In fact, it is found that because of restrictions to fluid flow in the poppet valves which are in series with flow through holes  184 ″, little or no additional damping effect occurs from the wear band  106 ″ covering the holes  184 ″ until (for example, with 6 mm diameter holes  184 ″) the holes  184 ″ are approximately half-covered by the wear band  106 ″. It should also be realized that even when the holes  184 ″ are fully covered by the untapered wear band  106 ″ and in fact until the front edge of the wear band is several millimetres or more beyond the front edge of the holes  184 ″, some fluid will flow through the clearance (typically 0.01 to 0.025 mm) between the wear band  106 ″ and the outer surface of piston tube  30 . Of course fluid friction losses increase as the wear band covers the holes  184 ″, until eventually fluid flow is cut off. 
     It will be realized that the damping characteristics of the strut as the wear band covers the holes in the piston tube wall may be tailored as desired, by selecting appropriately the shape of the wear band (straight, or with a desired degree of taper), and by selecting the shape of the holes  184  (which can be round, or axially or even circumferentially elongated, and of a desired depth profile in axial and radial cross section). 
     In Compression 
     Assuming that the compression strut  20  is in a fully extended state as shown in FIG. 3, and a compression force is applied to the strut, fluid will flow from cavities  50  and  195  through bored hole  194  and into cavity  182 . From cavity  182 , the fluid can flow either through plug bored hole  192  into piston tube cavity  126 , or through poppet valves  188  into bored holes  190  and then into cavity  126 . Simultaneously, fluid will also travel from cavities  50  and  195 , through compression check valves  196 , and into annular cavity  34  via channels  198 . As the gas spring cavity reduces in size, eventually the force exerted by the gas spring on. floating piston  124  will be equivalent to the compressive pressure on the strut. These equal and opposite forces will stop further compression of the strut. Assuming that the compressive force on the strut is sufficient to fully compress the strut, metal to metal contact can occur depending on the pressure in the gas spring. 
     It will be understood that preferred embodiments of the invention have been described, and that changes and alternative embodiments can be made within the spirit of the invention as described above.