Patent Publication Number: US-6213823-B1

Title: Trim-tilt device for marine propulsion unit

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a trim-tilt device for a marine propulsion unit such as an outboard motor or inboard/outboard motor. 
     2. Description of the Related Art 
     Conventionally, the trim-tilt device for a marine propulsion unit has a cylinder device interposed between a hull and the propulsion unit which is tiltably supported by the hull. By controlling supply and discharge of hydraulic fluid from a hydraulic fluid supply/discharge device to a cylinder device or vice versa, the cylinder device is expanded and contracted to thereby trim and tilt the marine propulsion unit. 
     A prior art trim-tilt device for a marine propulsion unit is described in Japanese Patent Application No. 11-112856. This application provides a cylinder device for a trim-tilt device for a marine propulsion unit comprising a housing connected to one of a hull and a marine propulsion unit and which forms a large-diameter trim chamber. A cylinder is telescopically inserted into the trim chamber of the housing which forms a small-diameter tilt chamber. A large-diameter trim piston is fixed to an end portion of the cylinder within the trim chamber of the housing which partitions the trim chamber into a first trim chamber of a cylinder accommodation side and a second trim chamber of an anti cylinder accommodation side. A piston rod is connected to the other of the hull and the marine propulsion unit and is telescopically inserted into the tilt chamber of the cylinder. A small-diameter tilt piston is fixed to an end of the piston rod within the tilt chamber of the cylinder which partitions the tilt chamber into a first tilt chamber of a piston rod accommodation side and a second tilt chamber of an anti piston rod accommodation side. 
     In the above-mentioned trim-tilt device for the marine propulsion unit disclosed in Japanese Patent Application No. 11-112856, during normal forward sailing, the load of an outboard motor and an external force of a forward driving force act in the direction which contracts the cylinder device. Thus, when the cylinder device expands (up) such that a discharging hydraulic pressure of the pump acts on the second trim chamber and the second tilt chamber (a lower chamber), the pressure-receiving area of a lower chamber side of the trim piston is larger than the pressure-receiving area of a lower chamber side of the tilt piston, so that a tilt-up operation is carried out after a trim-up operation. When the cylinder device is contracted (down) such that the discharging pressure hydraulic pressure of the pump acts on the first trim chamber and the second tilt chamber (an upper chamber), the pressure-receiving area of an upper chamber side of the tilt piston is larger than the pressure-receiving area of an upper chamber side of the trim piston, which is not intercepted by the cylinder, so that the trim-down operation is carried out after a tilt-down operation. 
     However, the cylinder&#39;s expansion (up) involves a disadvantage, such as when a tilt-up operation occurs before a trim-up operation, when an external force, such as a backward driving force during backward sailing or an uplifting force of a wave to raise a marine propulsion unit, acts in the direction for expanding the cylinder device. In the situation that the external force Fa acts, for expanding the cylinder device, the pump is driven and a switching valve is opened. In the situation that the discharging pressure Pa of the pump acts to the second trim chamber (the lower chamber), when the force of Fa is greater than that of Pa, namely, Fa/S&lt;Pa (S: a pressure receiving area of a tilt piston), the piston rod of the tilt piston, to which Fa acts, moves ahead of the trim piston. At that time, a hydraulic pressure flow passage from the first tilt chamber to the pump absorption side is unlocked, since the switching valve is opened. 
     If the external force, as described above, that acts in the direction for expanding the cylinder device, such as a backward driving force during backward sailing or an uplifting force of a wave to raise a marine propulsion unit, is greater, a pressure in the pipe passage from the first tilt chamber to the pump absorption side becomes higher, the pump rotates at this pressure in the pipe passage. As the result of that, the discharge/absorption amount of the pump increases more than usual, so that an up-speed of trim/tilt becomes suddenly high. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to ensure that a tilt-up operation is carried out after a trim-up operation, even when an external force acts in the direction that expands a cylinder device to a marine propulsion unit backward sailing or the like, in a trim-tilt device for a marine propulsion unit. 
     According to the present invention, there is disclosed a trim-tilt device for a marine propulsion unit, wherein a cylinder device is mounted between a hull and the marine propulsion unit freely tiltably supported by the hull. A hydraulic fluid is supplied from a hydraulic fluid supply/discharge device into the cylinder device and is discharged from the cylinder device into the hydraulic fluid supply/discharge device to thereby expand and contract the cylinder device and thereby trim and tilt the marine propulsion unit. 
     The cylinder device comprises: 
     a housing connected to one of the hull and marine propulsion unit to form a large-diameter trim chamber; 
     a cylinder telescopically inserted into the trim chamber and forming a small-diameter tilt chamber; 
     a large-diameter trim piston fixed to an end portion of the cylinder within the trim chamber of the housing and serving to partition the trim chamber into a first trim chamber of a cylinder accommodation side and a second trim chamber of an anti cylinder accommodation side; 
     a piston rod being connected to the other of the hull and the marine propulsion unit that is telescopically inserted into the tilt chamber of the cylinder; and 
     a small-diameter tilt piston being fixed to an end portion of the piston rod within the tilt chamber of the cylinder and serving to partition the tilt chamber into a first tilt chamber of a piston rod accommodation side and a second tilt chamber of an anti piston rod accommodation side, 
     wherein a check valve for allowing the hydraulic fluid to flow from a pump to the first trim chamber and the first tilt chamber, and a relief valve, which is opened if the hydraulic pressure of the first trim chamber and the first tilt chamber becomes higher than a fixed value, are connect in parallel to a hydraulic fluid low passage connecting the first trim chamber and the first tilt chamber to the pump. When an external force acts in the direction for expanding the cylinder device to the marine propulsion unit, the cylinder device carries out an expansion operation so that the cylinder device can start the movement of the tilt piston after the movement of the trim piston is finished. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully understood from the detailed description given below and from the accompanying drawings which should not be taken to be a limitation on the invention, but are for explanation and for understanding only. 
     The drawings 
     FIG. 1 is a view illustrating a marine propulsion unit; 
     FIG. 2 is a view illustrating a trim-tilt device; 
     FIG. 3 is a side view of FIG. 2; 
     FIG. 4 is a plan view of FIG. 2; 
     FIG. 5 is a view illustrating a state where a hydraulic fluid supply/discharge device is assembled into a housing of a cylinder device; 
     FIG. 6 is a view illustrating a hydraulic circuit of a trim-tile device; 
     FIG. 7 is a cross sectional view illustrating a relief valve with a check valve; and 
     FIG. 8 is a cross sectional view illustrating a relief valve. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, a marine propulsion unit  10  in the form of an outboard motor, or an inboard/outboard motor, has a damp bracket  12  fixed to a stern board  11 A of a boat hull  11 . To the clamp bracket  12  a swivel bracket  14  is pivoted through a tilt shaft  13  and is tiltable about the substantially horizontal shaft. To the swivel bracket  14  a propulsion unit  15  is pivoted through a steerage-changing shaft that is substantially vertically disposed, and not illustrated, in such a way that the propulsion unit  15 , is rockable about the steerage-changing shaft. An engine unit  16  is loaded at the top of the propulsion unit  15  and a propeller  17  is fitted to a lower part of the propulsion unit  15 . 
     In the marine propulsion unit  10 , the propulsion unit  15  is tiltably supported by the damp bracket  12  fixed to the hull  11  through the tilt shaft  13  and swivel bracket  14 . A cylinder device  21  of a trim-tilt device  20  is interposed between the clamp bracket  12  and the swivel bracket  14 . The cylinder device  21  is expanded and contracted by supply or discharge of hydraulic fluid between a hydraulic fluid supply/discharge device  22  and the cylinder device  21 . The propulsion unit  15  is thereby made tiltable in a trim or tilt region of FIG.  1 . It is to be noted that the marine propulsion unit  10  may take an optimum sailing posture with respect to the water surface by retaining the propulsion unit  15  in a state of a relatively gentle slope within the trim region. 
     (Cylinder Device  21 ) 
     As illustrated in FIGS. 1 and 2, the cylinder device  21  of the trim-tilt device  20  has a housing  31  that is used by being connected to the clamp bracket  12 , the housing  31  having a large-diameter trim chamber  32  formed therein. It is to be noted that the housing  31  is casted-molded using, for example, an aluminum alloy, and is equipped with a mounting-pin insertion hold  33  for mounting the housing onto the damp bracket  12 . 
     Also, the cylinder device  21  has a cylinder  41  which when the trim-up/down operation in the trim region is performed becomes telescopically inserted into a trim chamber  32  from a cylinder guide  34  provided in an open end of the housing  31 , the cylinder  41  having a small-diameter tilt chamber  42  formed therein. The cylinder guide  34  is screwed to an opening end of the housing  31  and is provided with a seal member  35  such as an O-ring or the like which closely contacts with the trim chamber  32  and a seal member  36  such as an O-ring or the like which slides on an outer surface of the cylinder  41 . 
     Also, the cylinder device  21  has a large-diameter trim piston  51  screwed and fixed to an end portion of the cylinder  41  that is situated in the trim chamber  32  of the housing  31 . The trim piston  51  is equipped with a seal member  52  such as an O-ring which slides on an inner surface of the trim chamber  32  and partitions the trim chamber  32  into a first trim chamber  32 A on a cylinder  41  accommodation side and a second trim chamber  32 B on an anti cylinder  41  accommodation side. 
     Also, the cylinder device  21  has a piston rod  61  that is provided by being connected to the swivel bracket  14 . The piston rod  61  is inserted into the tilt chamber  42  for a rod guide portion  43  that is provided in an open end of the cylinder  4 l as to be expanded and contracted when the tilt-up/down operation in the tilt region is performed. The rod guide portion  43  is equipped with a seal member  44  such as an O-ring which is in sliding contact with an outer surface of the piston rod  61 . The piston rod  61  is equipped with a mounting-pin insertion hole  62 A for mounting the cylinder device  21  onto the swivel bracket  14  to an mounting joint  62 . 
     Also, the cylinder device  21  has a small-diameter tilt piston  71  that is fixed to an end portion of the piston rod  61  situated within the tilt chamber  42  of the cylinder  41  by a nut  71 B through a washer  71 A. The tilt piston  71  is equipped with an seal member  72  such as an O-ring which is in sliding contact with the inner surface of the cylinder  41 , and partitions the tilt chamber  42  into a first tilt chamber  42 A on a piston-rod  61  accommodation side and a second tilt chamber  42 B on an anti piston-rod  61  accommodation side. 
     The tilt piston  71  has an expansion side buffer valve  73  and a check valve  74 . The expansion side buffer valve  73  is opened by a set pressure for the purpose of guarding a hydraulic circuit when an impact is received in the expansion direction at cylinder device  21 , as for example, when an obstacle such as a floating log or the like collides with the propulsion unit  15 , and transfers the hydraulic fluid of the first tilt chamber  42 A to a side of a free piston  81  as later described situated within the second tilt chamber  42 B, thereby enabling the expansion of the piston rod  61 . At this time, the free piston  81  remains at its own position and only the tilt piston  71  alone works. The check valve  74  is opened when after the valve described opening of the expansion side buffer valve  73  the tilt piston  71  of the piston rod  61  tends to return by the weight of the propulsion unit  15  to the original position to thereby return the hydraulic fluid between the tilt piston  71  and the free piston  81  to the first tilt chamber  42 A. 
     Also, the cylinder device  21  has the free piston  81  which is usually set to the position of its contact with the tilt piston  71  within the second tilt-chambers  42 B and  42 C of the cylinder  41 . The free piston  81  is equipped with a seal member  83  such as an O-ring which contacts the inner periphery of the cylinder  41 . 
     Additionally, the free piston  81  has a reset one-way valve  82  (shown in FIG. 6, not shown in FIG.  2 ). When the marine propulsion unit  10  in a forward motion collides with an obstacle, when the brake is applied to the marine propulsion unit  10  in a forward motion, or when the external force in the expansion direction of the cylinder device  21  is applied for some reason upon start-up of the operation, the reset one-way valve  82  is opened so that it is reset to the former position, in which the tilt piston  71  and the free piston  81  contact each other. Further, when a buffer valve at the expansion side  73  is opened to extend the piston rod  61 , a hydraulic fluid which is transferred from the first tilt chamber  42 A to the second tilt chamber  42 B is inserted between the tilt piston  71  and the free piston  81 . Here, the free piston  81  is at the lowest position and the free piston  81  moves forward while compressing the piston rod  61  by the pump action, the reset one-way valve  82  is opened so that it is reset to the former position, in which the tilt piston  71  and the free piston  81  contact each other. 
     In the cylinder device  21 , the cylinder  41  may be formed of iron material by forging, and on outer pipe  41 B and the above-mentioned rod guide portion  43  may be integrally formed by forging, so that the number of assembling steps is reduced and a high strength is achieved. An inner pipe  41 A is sandwiched between a recessed portion provided at an inner end face of the rod guide portion  43  and a recessed portion provided at an inner end face of the above-mentioned trim piston  51  screwed to the outer pipe  41 B, so that the cylinder  41  is structured as a tilt cylinder assembly. As a result of this, the cylinder  41  has a doubled-pipe structure comprising the inner pipe  41 A and the outer pipe  41 B, where a gap between the inner pipe  41 A and the outer pipe  41 B, is used as a communication passage  46  for communicating the first trim chamber  32 A and the first tilt chamber  42 A with each other. The first trim chamber  32 A is connected directly to a first flow passage  91  formed in the housing  31  while on the other hand the first tilt chamber  42 A is connected to the first flow passage  91  through a passage  91 A formed in the inner pipe  41 A of the cylinder  41 , a passage  91 B formed in the outer pipe  41 B of the cylinder  41 , a passage  91 C formed in the trim piston  51 , and the first trim chamber  32 A. As a result of this, the first trim chamber  32 A and the first tilt chamber  42 A are connected, (a) through the first flow passage  91 , with the supply side of the hydraulic fluid supply/discharge device  22  during the contraction stroke of each of the trim and tilt operations and, (b) through the first flow passage  91 , with the discharge side of the hydraulic fluid supply/discharge device  22  during the expansion stroke of each thereof. 
     In the cylinder device  21 , the trim piston  51  has a through-hole like communication passage  53  for connecting together second trim chamber  32 B and the second tilt chamber  42 C. The second trim chamber  32 B is connected directly to a second flow passage  92  formed in the housing  31  and the second tilt chamber  42 C is connected to the second flow passage  92  through the free piston  81  and the communication passage  53  of the trim piston  51  and the second trim chamber  32 B. As a result of this the second trim chamber  32 B and the second tilt chamber  42 C are communicated (a) for the second flow passage  92 , with the supply side of the hydraulic supply/discharge device  22  during the expansion stroke of each of the trim and tilt operations and (b) through he second flow passage  92  with the discharge side of the hydraulic fluid supply/discharge device  22  during the contraction stroke of each thereof. 
     (Hydraulic Fluid Supply/Discharge Device  22 ) 
     The hydraulic fluid supply/discharge device  22  comprises a reversible motor  23 , a reversible gear pump  24 , a tank  25 , and a switching-valve equipped flow passage  26 , by which the hydraulic fluid can be supplied and discharged, through the first flow passage  91  and the second flow passage  92 , between the hydraulic fluid supply/discharge device  22  and the first trim chamber  32 A, second trim chamber  32 B, first tilt chamber  42 A and a second tilt chamber  42 C of cylinder device  21 . 
     At this time, as illustrated in FIG. 5, the hydraulic fluid supply/discharge device  22  is arranged with a mounting base  23 A for mounting a motor  23  installed on a motor installation surface  31 A formed in the housing  31  of the cylinder device  21 . The mounting base  23 A is fixed thereto by bolts  27 , whereby the motor  23  is laterally juxtaposed with the cylinder  41  of the cylinder device  21 . 
     The hydraulic fluid supply/discharge device  22  has a void space portion that forms a side of the trim chamber  32  in the housing  31  of the cylinder device  21  in such a way as to surround the same and uses this void space portion as the tank  25  in which the hydraulic fluid is stored. As opening  25 A is formed in the portion within the tank  25  of the housing  31  which corresponds to a lower portion of the motor  23 , whereby a fitting portion  23 B that connects to the mounting base  23 A of the motor  23  is fluid-tightly fitted into the opening  25 A through a seal member  25 B such as an O-ring. Under the motor  23  within the tank formed in the housing  31 , the pump  24  is fixedly disposed in a state of essentially constant immersion in the fluid, whereby an output shaft  23 C protruding from the fitting portion  23 B of the motor  23  is connected to a driven shaft  24 A of the pump  24 . 
     Additionally, in the embodiment of the present invention, a portion of the cylinder  41  of the cylinder device  21  which projects outward from the cylinder guide  34  of the housing  31  in the trim operation area is covered with a sub-tank housing  28  constituting the hydraulic fluid supply/discharge device  22 . The sub-tank housing  28  is made of, for example, resin, a lower end opening portion of the sub-tank housing  28  is fitted on the cylinder guide  34 , and a lower end flange portion  28 A of the sub-tank housing  28  is fluid-tightly fastened to an opening end face of the housing  31  through an O-ring  29 A by bolts  30 . An upper end opening portion of the sub-tank housing  28  is provided with a seal member  28 B such as an oil seal or the like which allows sliding of the piston rod  61  fluid-tightly. As a result of this, the sub-tank housing  28  is provided in a standing manner along the longitudinal directions of the cylinder  41  and the piston rod  61  with a constant clearance about the cylinder  41  and the piston rod  61 , thereby forming a sub-tank  28 C. The sub-tank  28 C communicates with the above-mentioned tank  25  of the housing  31  through a passage  28 D formed in the cylinder guide  34  and a passage  28 E formed in the housing  31 . In FIGS. 3 and 4, reference numeral  28 F denotes an oil syringe plug. 
     The switching-valve equipped flow passage  26  of the hydraulic fluid supply/discharge device  22  which connects the pump  24  to the first flow passage  91  and second flow passage  92  is built in the housing  31 , the switching valve equipped flow passage  26  being provided with a shuttle type switching valve  101 , check valves  102  and  103 , contraction side relief valve  104 , expansion side relief valve  105 , contraction side buffer valve  106 A and manual switching valve  107 . 
     The shuttle type switching valve  101  has a shuttle piston  111  and a first check valve  112 A and second check valve  112 B that are located on both sides of the shuttle piston  111 , and defines a first shuttle chamber  113 A on the first check valve  112 A side of the shuttle piston  111  and defines a second shuttle chamber  113 B on the second check valve  112 B side of the shuttle piston  111 . The first check valve  112 A is opened by the pressure of the transmission fluid applied to the first shuttle chamber  113 A through a pipe passage  93 A by the pump  24  rotating in the forward direction. The second check valve  112 B can be opened by the pressure of the transmission fluid applied to the second shuttle chamber  113 B through a pipe passage  93 B by the pump  24  rotating in the reverse direction. Also, the shuffle piston  111  opens the second check valve  112 B by the pressure of the transmission fluid resulting from the forward rotation of the pump  24  and can open the first check valve  112 A by the pressure of the transmission fluid resulting from the reverse rotation of the pump  24 . 
     The first check valve  112 A of the shuttle type switching valve  101  is connected to the first flow passage  91  and the second check valve  112 B is connected to the second flow passage  92 . 
     A check-valve  102  is mounted on a connection pipe passage  94 A between the pump  24  and the tank  25 . In the tilt-up stage of the marine propulsion unit  10 , the internal volume of the cylinder  41  is insufficient in volume and the piston rod  61  is retracted, with the result that the circulating amount of the hydraulic fluid becomes deficient by that extent. Therefore, the check valve  102  is opened to thereby supplement from the pump  25  to the pump  24  the portion which corresponds to the deficiency of the circulating amount of fluid. 
     A check valve  103  is mounted on a connection pipe passage  94 B between the pump  24  and the tank  25 . The pump  24  is still in an operative stage at the point in time when, at the trim-down stage of the marine propulsion unit  10 , the trim piston  51  reaches its position of maximum contraction at which the trim-down stage is completed. As a result the return fluid from the second trim chamber  32 B to the pump  24  stops, and the check valve  103  is opened whereby the hydraulic fluid can be supplied from the tank  25  to the pump  24 . 
     The contraction side relief valve  104  is connected to the first shuttle chamber  113 A. The contraction side relief valve  104  is intended to permit the hydraulic circuit pressure to be relieved into the tank  25  under a set pressure in order to return to the tank  25  the amount of fluid corresponding to the volume of the rod, which is to remain at the time of the tilt-down and trim-down operations, and in order to guard the hydraulic circuit while continuing to operate the pump  24  even after the trim-down operation has been completed. 
     The expansion side relief valve  105  is built into the shuttle piston  111 . The valve  105  is intended to permit the hydraulic circuit pressure to be relieved into the tank  25  under a set pressure in order to guard the hydraulic circuit while continuing to operate the pump  24  even after the tilt-up operation time is completed, and the piston rod  61  has reached its position of maximum expansion. 
     The contraction side buffer valve  106 A is intended to relieve the hydraulic circuit pressure under a set pressure into the tank  25  when an impact has been applied to the propulsion unit  15  in the contraction direction of the piston rod  61 , for example, when an obstacle has bumped against the propulsion unit  15  from behind, with the tilt piston  71  and free piston  81  of the cylinder device  21  being located at an intermediate position of the tilt chamber  42 . 
     The manual switching valve  107  is interposed on a connecting passage  95  between the first flow passage  91  and the second flow passage  92 , and by connecting the first flow passage  91  and the second flow passage  92  and the tank  25  with each other. This permits manual expansion and contraction of the cylinder device  21  to thereby make the propulsion unit  15  tiltable in each of the trim and tilt regions. 
     Further, in order to avoid that a tilt-up operation is carried out faster than a trim-up operation when the cylinder device  21  expands (up), even when an external force, such as a backward driving force during backward sailing or an uplifting force of a wave tending to raise the marine propulsion unit  15 , acts in the direction for expanding the cylinder device  21 , the marine propulsion unit  10  has a check valve  121  and a relief valve  122  in the hydraulic fluid supply/discharge device  22 . In the hydraulic fluid supply/discharge device  22  the check valve  121  allows the hydraulic fluid to flow from the pump  24  to the first trim chamber  32 A and the first tilt chamber  42 A and the relief valve  122  which is opened if the hydraulic pressure of the first trim chamber  32 A and the hydraulic pressure of the first tilt chamber  42 A become higher than a fixed value are connect in parallel to the first flow passage  91  connecting the first trim chamber  32 A and the first tilt chamber  42 A to the pump  24 . Therefore, even when an external force, such as a backward driving force during backward sailing or an uplifting force of a wave to raise a marine propulsion unit  15 , acts in the direction for expanding the cylinder device  21 , the cylinder device  21  is provided so as to operate telescopically so that it can start a tilt-up movement of the tilt piston  71  after the cylinder device  21  finishes the trip-up movement of the trim piston  51 . 
     Additionally, FIG. 7 illustrates a relief valve  122  comprising a check valve  121 , a valve body  123 , a valve sheet  124  which is pressed into the valve body  123 , a blind plug  125 , a ball  126  of the check valve  121 , a ball  127  of the relief valve  122 , a spring sheet  128  and a relief spring  129 . 
     The marine propulsion unit  10  has a relief valve  131  in the hydraulic fluid supply/discharge device  22 , to avoid an abnormal application of pressure of the tanks  25  and  28 C caused during forward sailing, where the operation of the pump  24  of the trim-tilt device  20  stops in a trim operable region, the state in which the cylinder device  21  does not finish the trim up and an obstacle in the water such as a floating log etc., collides with the propulsion unit  15 . When the hydraulic fluid supply/discharge flow passages  91  and  92 , from the hydraulic fluid supply/discharge device  22  to the cylinder device  21 , are locked, a piston rod  61  and the cylinder  41  carry out a trim-stroke in a body to the housing  31 , the tank housing  28  and the tanks  25  and  28 C. The relief valve  131  is provided on a bottom in which a pump  24  is fixed on a lower part of a motor  23  of the tanks  25 . The relief valve  131  is connected to a flow passage  95  ( 92 ) communicating with the second trim chamber  32 B. The relief valve  131  is opened due to the boosting of the hydraulic pressure in the tanks  25  and  28 C beyond a fixed value so that it can transfer the hydraulic fluid of the tanks  25  and  28  into the second trim chamber  32 B. Thus, an abnormal application of hydraulic pressure of the tanks  25  and  28 C by transfer of the cylinder  41  (trim-stroke) can be avoided when the obstacle in the water collides with the propulsion unit  15  and the impact is added in the expansion direction of the cylinder  21 . 
     FIG. 8 illustrates the relief valve  131 , a valve body  132 , a valve collar  133  which is pressed into the valve body  132 , a ball  134  and a relief spring  135 . 
     The operation of the trim-tile device  20  will hereafter be explained. 
     (1) Trim-Up 
     When the motor  23  and pump  24  are rotated in reverse, the hydraulic fluid discharged from the pump  24  flows from the pipe passage  93 B to the second shuttle chamber  113 B of the shuttle type switching valve  101 , whereby the shuttle piston  111  moves to the right side in FIG. 6 to thereby forcibly open the first check valve  112 A. Also, the hydraulic fluid that has flown into the second shuttle chamber  113 B of the switching valve  101  forcibly opens the second check valve  112 B by its own pressure and is thereby sent to the second trim chamber  32 B through the pipe passage  92  as indicated by a solid-line arrow. The hydraulic fluid that has flowed into the second trim chamber  32 B in this way tends to push up the trim piston  51 . It is to be noted that the hydraulic fluid of the second trim chamber  32 B not only acts on the trim piston  51  but also acts on the tilt piston  71  in close contact with the trim piston  51  through the through-hole connecting passage  53  of the trim piston  51 . Since the pressure receiving area of the connecting passage  53  is set so that the pressure receiving area of the trim piston  51  may be larger than that of the tilt piston  71 , the trim piston  51  pushes up and moves the tilt piston  71 . At this time, the hydraulic fluid of the first trim chamber  32 A flows out into the first flow passage  91  and further returns to the pump  24 , and therefore the trim piston  51  is moved. Simultaneously, the cylinder  41  and piston rod  61  are caused to protrude outwardly from the housing  31 , whereby trim-up occurs. When the trim piston  51  has collided with the stroke end in the trim-up direction within the first trim chamber  32 A, the trim-up is maximized. 
     (2) Tilt-Up 
     After under the above item (1) the trim piston  51  has been moved up to a level corresponding to the maximum trim-up the hydraulic fluid within the second trim chamber  32 B. The hydraulic fluid is further supplied to the second trim chamber  32 B extends from the through-hole like connecting passage  53  formed in the trim piston  51  to an anti-piston rod  61  side of a tilt piston  71  end surface through the free piston  81 . As a result of this, the hydraulic fluid supplied to the second trim chamber  32 B is filled in to the second tilt chamber  42 C formed while being gradually expanded between the trim piston  51  within the cylinder  41  and the free piston  81 , and the tilt piston  71 . The hydraulic fluid within the first tilt chamber  42 A flow out into the first flow passage  91  through the passage  91 A formed in the rod guide portion  43  of the cylinder  41 , connecting passage  46  of the cylinder  41 , passage  91 B formed in the outer cylinder  41 B of the cylinder  41 , passage  91 C formed in the trim piston  51  and first trim chamber  32 A. Therefore, the tilt piston  71  and the free piston  81  are moved together. As a result of this, the piston rod  61  protrudes outwardly from the cylinder  41 , whereby tilt-up occurs. When the tilt piston  71  collides with the stroke end in the tilt-up direction within the first tilt chamber  42 A, the tilt-up reaches it&#39;s maximum. 
     (3) Tilt-Down 
     When the motor  23  and pump  24  are rotated in a forward direction, the hydraulic fluid discharged from the pump  24  flows from the pipe passage  93 A into the first shuttle chamber  113 A of the switching valve  101 , whereby the shuttle piston  111  is moved to the left side in FIG. 6 to thereby forcibly open the second check valve  112 B. The hydraulic fluid that has flown into the first shuttle chamber  113 A of the switching valve  101  forcibly opens the first check valve  112 A by its own pressure and, as indicated by a broken-line arrow, is sent from the first flow passage  91  to the first tilt chamber  42 A through the first trim chamber  32 A, passage  91 C, passage  91 B, communication passage  46  of the cylinder  41  and passage  91 A. When hydraulic fluid flows into the first tilt chamber  42 A in this way, the hydraulic fluid pushes down the tilt piston  71 , and the free piston  81 . At this time, the hydraulic fluid of the first trim chamber  32 A acts on the trim piston  51 . However, the pressure-receiving area of the tilt piston  71  facing the first tilt chamber  42 A is so set as to become larger than that of the trim piston  51  facing the first trim chamber  32 A and therefore only the tilt piston  71  alone is depressed until the tilt piston  71  collides with the trim piston  51 . As a result of this, the piston rod  61  is retracted into the cylinder  41  and is tilted down. At this time, the hydraulic fluid of the second tilt chamber  42 C flows out from the through-hole connecting passage  53  of the trim piston  51  into the second flow passage  92  through the second trim chamber  32 B and further into the pump  24 . When the tilt piston  71  collides with the trim piston  51  that is kept at the stroke end on the trim-up direction of the trim chamber  32 , the tilt-down is completed. 
     (4) Trim-Down 
     When after the tilt-down described above (3) terminates, the hydraulic fluid is supplied to the first trim chamber  32 A and first tilt chamber  42 A, the tilt piston  71  and the free piston  81  is depressed down to the second trim chamber  32 B side integrally with the trim piston  51 . The hydraulic fluid within the second trim chamber  32 B flows out into the second passage  92 , with the result that the cylinder  41  and the piston  61  are retracted further into the housing  31  for trim-down operation. And when the trim piston  51  collides with the stroke end in the trim-down direction within the second trim chamber  32 B, the trim-down is completed. 
     Here, in the trim-tilt device  22 , during a transition process from the trim-up to the tilt-up operation under the above items (1) and (2) and during a transition process from the tilt-down to the trim-down operation under the above items (3) and (4), the effective area of each of the pistons  51  and  71  varies between the large-diameter trim piston  51  and the small-diameter tilt piston  71 . For this reason, the transfer speed of the piston rod  61  is such that transition speed in the trim region is less than that in the tilt region while, on the other hand the force that acts on the piston rod  61  is such that this force in the trim region is greater than in the tilt region. In the above-described embodiment, it is possible, (a) in the trim region, to finely adjust the trim angle while resisting the trust force of the propeller and also to sail in a shallow water area, and (b) in the tilt region, it is possible to quickly perform tilt-up/down operations with a relatively small magnitude of force that is necessary for supporting the weight of the propulsion unit itself. 
     Therefore, the present embodiment has the following effects. 
     (A) Securing a trim-up operation when an external force for expanding the cylinder device  21  acts during backward sailing or the like 
     (1) The check valve  121  and the relief valve  122  are connect in parallel to the hydraulic fluid flow passage  91  in which the first trim chamber  32 A and the first tilt chamber  42 A are connected to the pump  24 . The check valve  121  introduces the discharging fluid of the pump  24  into the first trim chamber  32 A and the first tilt chamber  42 A (an upper chamber) upon a down operation of the cylinder device  21 . The relief valve  122  can carry out an up operation only when the hydraulic pressure of the first trim chamber  32 A and the first tilt chamber  42 A increases upon an up operation to push open the relief valve  122 . Accordingly, if the discharging hydraulic fluid of the pump  24  is supplied to the second trim chamber  32 B (a lower chamber) in order to carry out an up operation of the cylinder device  21  when an external force, such as a backward driving force during backward sailing or an uplifting force of a wave to raise a marine propulsion unit  15 , acting in the direction for expanding the cylinder device  21 , the hydraulic pressure of the second trim chamber  32 B increases the hydraulic pressure of the first trim chamber  32 A and the first tilt chamber  42 A (an upper chamber) through the trim piston  51  and the tilt piston  71 . At this time, the relief valve  122  is dosed until the increased hydraulic pressure is increased to reach the relief pressure so that the hydraulic pressure flow passage from the first tilt chamber  42 A to the pump  24  absorption side is kept unlocked. When the hydraulic pressure of the first trim chamber  32 A and the first tilt chamber  42 A increases to reach the relief pressure, the relief valve  122  is opened and the trim piston  51 , which has a larger pressure-receiving area at the lower chamber side than that of the tilt piston  71 , carries out an up operation ahead of the tilt piston  71 . 
     (2) The set pressure P of the relief valve  122  describe above in ( 1 ) is desirably larger that the external force Fa such as a backward driving force acting on the pressure receiving area S of the tilt piston  71 . The set pressure P satisfies P&lt;Fa/S. For example, it is assumed that P is 30 kgf/cm 2 . In this case, when the relief pressure P is excessively high, the efficiency of the pump  24  is decreased and it suffers from a heavy load. Therefore, for example, it is determined that P=30±10 kgf/cm 2 . However, because of the influence by the lost pressure in a pipe passage or the friction or the like, the relief pressure P may be smaller that the valve in P&gt;Fa/S. 
     (B) Avoidance of an abnormal application of pressure of the tank  25  by transfer of the cylinder  41  (a trim-stroke) according to the collision with an obstacle in the water, 
     (1) The relief valve  131 , that is opened due to the increase of the inner pressure of the tanks  25  and  28 C greater than a fixed value and transfers the hydraulic fluid of the tanks  25  and  28 C to the second trim chamber  32 B, is provided. Accordingly, when the marine propulsion unit  15  collides with an obstacle in the water, the piston rod  61  and the cylinder  41  carry out in a body a trim-stoke in the tank housing  28 . Therefore, even when the internal pressure in the tanks  25  and  28 C suddenly increases, it is possible to let out the inner pressure immediately to the second trim chamber  32 B side by the above-mentioned relief valve  131 . Thus, it becomes possible that an abnormal application of pressure of the tanks  25  and  28 C is avoided, the housing  28  made of resin is prevented from being destroyed, or an end plate which is made of resin of the pump motor  23 , covering the pump chamber which is provided in communicating with this tank  28 C is prevented from being destroyed. 
     (2) In order to absorb an abnormal application of pressure of the tanks  25  and  28 C, it is not necessary to increase the air capacity of the tanks  25  and  28 C, so that the tanks  25  and  28 C are prevented from increasing in size. 
     (3) The value of the set pressure of the relief valve  131  described above may be small if the relief valve  131  is not opened by a small negative pressure of the tanks  25  and  28 C, namely, it may be small enough to get the stability of an opening valve operation of the relief valve  131 . For example, the value of the set pressure of the relief valve  131  may be between 1-3 kgf/cm 2 . 
     The present embodiment has the following effect. 
     (1) Since the cylinder  41  if the cylinder device  21  is covered with the tank  28 C, it is prevented from contacting with outer water and it is easily and reliably made rustproof by hydraulic fluid in the tank housing  28 . As a result of this, even when the cylinder  41  serving as a member for transmitting a propulsion force (axial compressing force) between the hull  11  and the marine propulsion unit  15  is made of metal material such as iron, such that a predetermined strength for accommodating forces can be secured at a small area, it is unnecessary to structure the cylinder  41  with a high grade rustproof material or to perform a rustproof treatment such as coating, so that the number of machining steps for rustproof treatment can be reduced and cost can be decreased. 
     (2) Since the outer surface of the cylinder  41  is made rustproof according to the above (1), rust does not occur thereon. Therefore, even when the cylinder  41  repeatedly slides on the seal member  35  of the cylinder guide  34  provided on the housing  31  in the trim operation area, the outer surface of the cylinder  41  does not scratch the seal member  35 . 
     (3) The tank housing  28  of the hydraulic fluid supply/discharge device  22  covers the entire of the cylinder  41  projecting outward from the housing  31  along the longitudinal direction of the cylinder  41 . As a result of this, the tank housing  28  extends along the longitudinal direction of the cylinder  41  and it does not budge laterally about a proximal portion of the cylinder  41  so that the trim-tilt device  20  can be made compact. 
     Here, the housing  31  of the cylinder device  21  is formed of, for example, aluminum alloy by forging integrally with the tank  25  of the hydraulic fluid supply/discharge device  22 , and it is not prevented from rusting. Also, the sub-tank housing  28  is made of, for example, resin to be rustproof. 
     Incidentally, such a treatment as plating can be performed on the outer peripheral surface of the cylinder  41 , thereby improving sliding performance of the housing  31  with respect to the cylinder guide  34 . 
     (4) Since the first trim chamber  32 A and the first tilt chamber  42 A have been interconnected with each other by the passage  46  provided in the wall of the cylinder  41 , the exposure to the outside of the pipes of hydraulic fluid supplied and discharged from the supply/discharge device  22  to cylinder device  21  can be suppressed. As a result of this, the outer appearance of the cylinder device  21  is compact and there is no likelihood that exposed piping will be damaged and that hydraulic fluid will leak from the connection. At this time, since the connecting passage  46  between the first trim chamber  32 A and the first tilt chamber  42 A is provided within the wall of the cylinder  41 , the cylinder device  21  is simple in construction. 
     (5) The portions where the cylinder device  21  should be fluid-tightly sealed are only four in number and those include a portion (seal member  35 ) where the cylinder  41  slides on the cylinder guide  34  provided in the housing  31 , a portion (seal member  52 ) where the trim piston  51  slides on the inner surface of the trim chamber  32  of the housing  31 , a portion (seal member  44 ) where the piston rod  61  slides with respect to the rod guide portion  43  provided in the cylinder  41  and a piston seal member  72  where the tilt piston  71  slide on the inner surface of the tilt chamber  42  of the cylinder  41 . Therefore, the sealability of the cylinder device  21  is greatly improved. 
     (6) The tilt piston  71  is moved up and down merely by sliding contact of its outer peripheral portion with the inner surface of the tilt chamber  42  of the cylinder  41  which is made during tilt operation. Therefore, the assembling efficiency and slideability thereof are high and so the tilt-operating efficiency is improved. 
     (7) By making the cylinder  41  of the cylinder device  21  into a double-cylinder structure, the double cylinder structure is made up through the connection of the inner pipe  41 A and the outer pipe  41 B and the gap between both pipes  41 A and  41 B can be used as the connecting passage  46  between the first trim chamber  32 A and the first tilt chamber  42 A. As a result of this, construction of the cylinder device  21  is greatly simplified. 
     (8) By building the pump  24 , the tank  25  and the switching valve equipped flow passage  26  of the hydraulic fluid supply/discharge device  22  into the housing  31  of the cylinder device  21 , the connecting flow passages between the hydraulic fluid supply/discharge device  22  and the cylinder device  21  are not outwardly exposed, and it is possible to eliminate all use of exposed piping over the entire trim-tilt device  20  with (4). 
     (9) The hydraulic fluid supply/discharge device  22  is integrally assembled to the cylinder device  21 . By respectively connecting the housing  31  and the piston rod  61  of the cylinder device  21  to the hull  11  and marine propulsion unit  10 , the mounting of the hydraulic fluid supply/discharge device  22  also is simultaneously completed 
     However, in the present invention, the connecting passage that is built into the wall of the cylinder and that connects the first trim chamber and the first tilt chamber with each other may be constructed of a hole-like passage formed in the wall of the cylinder. At this time the cylinder may be formed by using a casting and the hole-like passage may be formed by casting. Or, the cylinder may be formed using a pipe and the hole-like passage may be formed in the wall of the pipe. 
     As heretofore explained, embodiments of the present invention have been described in detail with reference to the drawings. However, the specific configurations of the present invention are not limited to the embodiments but those having a modification of the design within the range of the present invention are also included in the present invention. 
     As mentioned above, according to the present invention, it is ensured that a tilt-up operation can be carried out after a trim-up operation, even when an external force acts in the direction that expands a cylinder device to a marine propulsion unit during backward sailing or the like, in a trim-tilt device for a marine propulsion unit. 
     Although the invention has been illustrated and described with respect to several exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made to the present invention without departing from the spirit and scope thereof. Therefore, the present invention should not be understood as limited to the specific embodiment set out above, but should be understood to include all possible embodiments which can be embodied within a scope encompassed and equivalents thereof with respect to the features set out in the appended claims.