Patent Publication Number: US-2009232613-A1

Title: Machine Tool Spindle Balancer

Description:
TECHNICAL FIELD 
     The present invention relates to a machine tool spindle balancer for pulling and biasing the spindle unit upward by a gas spring to reduce the load of the lift means for moving the spindle unit vertically. 
     BACKGROUND TECHNOLOGY 
     Conventionally, a machine tool such as a machining center is provided with a spindle unit having a spindle on which a tool is mounted and an electric motor for rotating the spindle. The spindle unit is supported and moved vertically by a lift mechanism. Such a lift mechanism generally comprises a vertical ball screw shaft, a ball screw nut screwed on the ball screw shaft and coupled to the spindle unit, and an electric motor. The electric motor rotates the ball screw shaft to move the ball screw nut and thus the spindle unit vertically. 
     In the above machine tool, the lift mechanism has to support and move a heavy spindle unit vertically. Therefore, problems occur such as that the accuracy of the vertical position of the spindle unit may deteriorate. It is difficult to move the spindle unit vertically at much higher speeds, and the lift mechanism is increased in size, leading to higher production cost. Hence, various spindle balancers designed to reduce the load of the lift mechanism of the machine tool are in practical use (for example, see Patent Documents 1 to 4). 
     The spindle balancer of Patent Document 1 comprises a hydraulic cylinder provided above the spindle unit, a direction switching valve connected to two oil chambers of the hydraulic cylinder that are separated by a piston, an oil pressure supply mechanism supplying an oil pressure to the hydraulic cylinder via the direction switching valve, and a control unit controlling the direction switching valve. The hydraulic cylinder has a piston rod extending downward and connected to the spindle unit. When the spindle unit is moved vertically by the lift mechanism, in which the piston rod of the hydraulic cylinder is extended/withdrawn in sync with the vertical movement of the spindle unit and an upward force nearly equivalent to the weight of the spindle unit is generated by the hydraulic cylinder. 
     The spindle balancer of Patent Document 2 comprises a tension spring, a chain, and a sprocket. The tension spring is provided vertically in a column. The chain is passed around the sprocket in a U-letter shape above the spindle unit. The bottom end of the tension spring is coupled to the bed. The chain is coupled to the top end of the tension spring at one end and to the spindle unit at the other. The tension spring pulls and biases the spindle unit upward via the chain and sprocket. This spindle balancer requires a damping apparatus for suppressing the vibration of the tension spring. 
     On the other hand, the applicant of this application has proposed and realized a spindle balancer having a gas spring as in Patent Documents 3 and 4, in which the gas spring has a cylinder body filled with a compressed gas and a rod inserted in the cylinder body to reciprocate. The gas spring is provided downward between the spindle unit and the underlying bed. The cylinder body is fitted in the spindle unit. The compressed gas biases the rod downward in relation to the cylinder body. The bottom end of the rod is received by the bed. The gas spring pushes and biases the spindle unit upward. 
     Patent Document 1: Japanese Laid-Open Patent Application No. H06-297217; 
     Patent Document 2: Japanese Laid-Open Patent Application No. 2003-191145; 
     Patent Document 3: Japanese Laid-Open Patent Application No. 2002-96229; and 
     Patent Document 4: Japanese Laid-Open Patent Application No. 2002-254265. 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     In the spindle balancer of Patent Document 1, it is difficult for the control unit to control the direction switching valve so that the hydraulic cylinder generates an upward force equivalent to the weight of the spindle unit while the lift mechanism moves the spindle unit vertically. Therefore, it is difficult to move the spindle unit vertically at a high speed because of the restriction by the hydraulic cylinder. Provision of the direction switching valve, hydraulic supply mechanism, and control unit in addition to the hydraulic cylinder leads to a complex structure and higher production cost. 
     In the spindle balancer of Patent Document 2, the spindle unit is pulled and biased upward by the tension spring. The tension spring elastically deforms in association with the vertical movement of the spindle unit. It is difficult to increase the vertical stroke of the spindle unit while maintaining the initial function of the tension spring. Furthermore, the tension spring has to have an initial tension. The ends where stress concentration occurs may more easily be deteriorated or damaged, whereby the spring may fail to pull and bias the spindle unit upward in a stable manner. In addition, a damping apparatus for the tension spring is required, which is disadvantageous for simplifying the structure and reducing the production cost. 
     In the spindle balancers of Patent Documents 3 and 4, the gas spring is used to bias the spindle unit upward, overcoming the problems associated with Patent Documents 1 and 2. However, the gas spring pushes and biases the spindle unit from below and is interposed between the spindle unit and the underlying bed. The gas spring may be an obstacle to the machining operation. Furthermore, the cylinder body has to be mounted on the spindle unit, which makes it difficult to apply to existing machine tools. 
     Furthermore, as the spindle unit is increased in size, it becomes heavier. The lift mechanism moves the spindle unit along with the cylinder body. In other words, the gas spring pushes and biases the significantly heavy spindle unit and cylinder body upward. Then, the gas spring has to be increased in size. Even if the gas spring is provided upside down and the cylinder body is attached to the bed, the same problem occurs. 
     The purpose of the present invention is to provide a machine tool spindle balancer that stably pulls and biases the spindle unit upward and thus reliably and effectively reduces the load of the lift means for moving the spindle unit vertically, is easily applicable to existing machine tools, highly durable, downsizable, and has a simple structure advantageous in production cost. 
     Problem Resolution Means 
     The machine tool spindle balancer of the present invention is a machine tool spindle balancer for a machine tool comprising a spindle unit having a spindle for mounting a tool, an electric motor for rotating the spindle and a lift means for moving the spindle unit vertically, characterized in that: the machine tool spindle balancer comprises a gas spring having a cylinder body, a rod reciprocating in relation to the cylinder body over a specific stroke at least nearly equal to a vertical stroke of the spindle unit and having at least one end portion protruding from one end of the cylinder body, and a compressed gas contained in at least a part of the cylinder body and biasing the rod in a retracting direction in which the one end portion of the rod that protrudes from the cylinder body is reduced; the rod is coupled to the spindle unit at one end so that the spindle unit is pulled and biased upward by a biasing force of the gas spring that is applied on the rod; and the biasing force of the gas spring serves to reduce a load of the lift means. 
     Preferably, the present invention may adopt the following various constitutions. The rod has at an end a piston part fitted in the cylinder body in an airtight and slidable manner, a compressed gas operation chamber containing the compressed gas is formed in the cylinder body on a rod side of the piston part, and a compressed gas filled chamber that is independent from the compressed gas operation chamber, is filled with the compressed gas and communicates with the compressed gas operation chamber via a gas communication passage is provided (claim  2 ). 
     The compressed gas filled chamber can be provided separately from the gas spring or integrally with the gas spring. In the latter case, the cylinder body can have an inner barrel in which the piston part is fitted and an outer barrel provided radially outside the inner barrel and the compressed gas filled chamber is formed between the inner and outer barrels (claim  3 ). 
     The rod has one end rod part forming the one end portion and the other end rod part having a larger diameter than the one end rod part and coaxially coupled to an end of the one end rod part to protrude from the other end of the cylinder body, and the compressed gas operation chamber containing the compressed gas is formed nearly in an entire area of the cylinder body; and a coupling part between the one end rod part and the other end rod part is housed in the cylinder body and the coupling part has an annular pressure receiving part receiving the gas pressure of the compressed gas in the compressed gas operation chamber (claim  4 ). 
     The cylinder body of the gas spring is provided vertically above the spindle unit and one end of the rod is coupled to the top end of the spindle unit (claim  5 ). Alternatively, one end of the rod can be coupled to the top end of the spindle unit via a chain and sprockets (claim  6 ). 
     In another possible structure, the other end of the cylinder body may be coupled to the spindle unit so that the spindle unit is pulled and biased upward by the biasing force of the gas spring. 
     ADVANTAGES OF THE INVENTION 
     According to the machine tool spindle balancer of the present invention, the extendable gas spring reliably pulls and biases the spindle unit upward while assuring the vertical movement of the spindle unit of a degree of freedom. An upward force nearly equal to the weight of the spindle unit is produced even when the spindle unit  10  is in vertical motion, effectively supporting the spindle unit. In other words, the size and shape of the gas spring and the pressure of the compressed gas are appropriately determined according to the weight of the spindle unit. Then, the gas spring uses the pressure of the compressed gas to stably pull and bias the spindle unit upward, reliably and effectively reducing the load of the lift mechanism. Consequently, the lift mechanism can be downsized, the accuracy of the vertical positioning of the spindle unit by the lift mechanism  20  or the machining accuracy can reliably be improved, and the spindle unit having a further high vertical moving speed can be realized. 
     Furthermore, the gas spring is not provided between the spindle unit and underlying bed. The gas spring is not an obstacle to the machining operation. It is unnecessary to mount the gas spring on the spindle unit. The spindle unit does not need to have any particular mounting structure for the gas spring. The rod can easily be coupled to the spindle unit. It can easily be applied to an existing machine tool. The gas spring is highly durable unlike a tension spring. If the compressed gas in the cylinder body is reduced, the compressed gas is easily refilled and the gas spring is restored. It is unnecessary to mount the gas spring on the spindle unit. The spindle unit does not need to be increased in size more than necessary. The gas spring biases only the spindle unit. The gas spring can be downsized and simplify the structure, which is advantageous in production cost. 
     According to the machine tool spindle balancer of claim  2 , the rod has at an end a piston part fitted in the cylinder body in an airtight and slidable manner. The rod is reliably guided to reciprocate in the cylinder body. The compressed gas operation chamber containing the compressed gas is formed in the cylinder body on a rod side of the piston part. The compressed gas in the compressed gas operation chamber reliably biases the rod in the retracting direction. The compressed gas filled chamber that is independent from the compressed gas operation chamber, filled with the compressed gas and communicates with the compressed gas operation chamber via the communication passage serves to suppress changes in the pressure of the compressed gas while the compressed gas operation chamber is subject to large volume changes as the rod reciprocates. Hence, the spindle unit is pulled and biased by nearly a constant biasing force. 
     According to the machine tool spindle balancer of claim  3 , the cylinder body has an inner barrel in which the piston part is fitted and an outer barrel provided radially outside the inner barrel. The compressed gas filled chamber is formed between the inner and outer barrels. Therefore, the gas spring has an integrated compact compressed gas filled chamber. Then, the spindle balancer having a compact overall structure is advantageously mounted on an existing machine tool. 
     According to the machine tool spindle balancer of claim  4 , the rod has one end rod part forming one end portion and the other end rod part having a larger diameter than the one end rod part and coaxially coupled to an end of the one end rod part to protrude from the other end of the cylinder body. The rod can reliably be guided to reciprocate in the cinder body. The compressed gas operation chamber containing the compressed gas is formed nearly in the entire area of the cylinder body. The coupling part between the one end rod part and other end rod part is housed in the cylinder body. The coupling part has an annular pressure receiving part receiving the gas pressure of the compressed gas in the compressed gas operation chamber, reliably retaining the rod and which is biased in the retracting direction by the compressed gas. The volume change of the compressed gas operation chamber as a result of the rod reciprocating, namely the change in the gas pressure of the compressed gas, can be suppressed to pull and bias the spindle unit by a nearly constant force. Furthermore, the rod does not slide directly on the inner surface of the cylinder body. The cylinder body does not need to have a precise mirror-finished inner surface, which is advantageous in producing the cylinder body. 
     According to the machine tool spindle balancer of claim  5 , the cylinder body of the gas spring is provided vertically above the spindle unit. One end of the rod is coupled to the top end of the spindle unit. In this simplified structure, the spindle unit is reliably pulled and biased upward. 
     According to the machine tool spindle balancer of claim  6 , one end of the rod is coupled to the top end of the spindle unit via a chain and sprockets. The spindle unit can be pulled and biased upward with an increased degree of freedom of positioning the gas spring. 
    
    
     
       BRIEF EXPLANATION OF THE DRAWINGS 
         FIG. 1  is s side view of a machine tool including the spindle balancer of embodiment 1 of the present invention. 
         FIG. 2  is a vertical cross-sectional view of the gas spring of embodiment 1. 
         FIG. 3  is a vertical cross-sectional view of the gas spring of embodiment 2. 
         FIG. 4  is a vertical cross-sectional view of the gas spring of embodiment 3. 
         FIG. 5  is a vertical cross-sectional view of the gas spring of embodiment 4. 
         FIG. 6  is a side view of a machine tool including the spindle balancer of embodiment 5. 
     
    
    
     EXPLANATION OF REFERENCE NUMERALS 
     
         
         
           
             M machine tool 
               10  spindle unit 
               13  spindle 
               14  tool 
               15  electric motor 
               20  lift mechanism 
               30 ,  30 A,  30 B,  30 C,  30 D machine tool spindle balancer 
               31 ,  31 A,  31 B,  31 C,  31 D gas spring 
               32 ,  32 A,  32 B,  32 C,  32 D cylinder body 
               33 ,  33 B,  33 C,  33 D rod 
               34  compressed gas 
               40  inner barrel 
               40   a  gas communication passage 
               40   b  gas communication port 
               41  outer barrel 
               50 ,  50 B,  50 C compressed gas operation chamber 
               51 A,  51 B compressed gas filled chamber 
               51   a  gas hose 
               55  piston part 
               80 ,  80 C one end rod part 
               81 ,  81 C the other end rod part 
               82 ,  82 C coupling part 
               85 ,  85 C pressure receiving part 
               95  chain 
               96  sprocket 
           
         
       
    
     BEST MODE FOR IMPLEMENTING THE INVENTION 
     The machine tool spindle balancer of the present invention comprises a gas spring having a cylinder body, a rod reciprocating in relation to the cylinder body over a specific stroke at least nearly equal to the vertical stroke of the spindle unit and having one end portion protruding from one end of the cylinder body, and a compressed gas contained in at least a part of the cylinder body and biasing the rod in the retracting direction in which the one end portion of the rod that protrudes from the cylinder body is reduced; the rod is coupled to the spindle unit at one end so that the spindle unit is pulled and biased upward by the biasing force of the gas spring that is applied on the rod. 
     Embodiment 1 
     As shown in  FIG. 1 , a machine tool spindle balancer  30  of the present invention (the spindle balancer  30 ) is applied to a machine tool M such as a vertical machining center. In  FIG. 1 , the arrow A points to the front. 
     The machine tool M comprises a column  1 , a spindle unit  10  vertically guided by the column  1 , and a lift mechanism  20  moving the spindle unit  10  vertically. The machine tool M is provided with the machine tool spindle balancer  30  (the spindle balancer  30 ) comprising a gas spring  31  supporting the spindle unit  10  to reduce the load of the lift mechanism  20 . 
     The column  1  has a vertical guide rail  2  at the front. The spindle unit  10  has a frame member  11 , to the rear end of which sliders  12  are attached. The sliders  12  are engaged with the guide rail  2 , whereby the spindle unit  10  is guided vertically. 
     The spindle unit  10  has a frame member  11 , a spindle  13  supported at the front portion of the frame member  11  in a rotatable manner about a vertical axis to detachably mount a tool  14  at the bottom end thereof, and an electric motor  15  for rotating the spindle  13 . The electric motor  15  is provided on the front portion of the frame member  11  at the top and directly connected to the spindle  13 . The electric motor  15  is electrically connected to and controlled by a control unit  16  to rotate the spindle  13  and thus the tool  14 . 
     The lift mechanism  20  has an electric motor  21  provided at the top of the column  1 , a ball screw shaft  22  directly connected to and vertically extending from the electric motor  21 , a ball screw nut  23  screwed on the ball screw shaft  22  and coupled to the spindle unit  10  via a coupling part  24 . The electric motor  21  is electrically connected to and controlled by the control unit  16  to rotate the ball screw shaft  22 , whereby the ball screw nut  23  and thus the spindle unit  10  moves vertically. 
     The spindle balancer  30  will be described in detail hereafter. 
     As shown in  FIGS. 1 and 2 , the spindle balancer  30  pulls and biases the spindle unit  10  upward by the gas spring  31 ; thus, the biasing force of the gas spring  31  serve to reduce the load of the lift mechanism  20 . 
     The gas spring  31  is vertically provided above the spindle unit  10 . The gas spring  31  has a cylinder body  32 , a rod  33 , and the compressed gas  34  (for example compressed nitrogen gas). The cylinder body  32  is secured to the front wall of the column  1  for example by a bracket  35 . The rod  33  extends from the cylinder body  32  downward. 
     As shown in  FIG. 2 , the cylinder body  32  has an inner barrel  40 , an outer barrel  41  provided radially outside the inner barrel  40 , a top end wall  42  and a bottom end wall  43  fixed to the top and bottom ends of the inner and outer barrels  40  and  41 , and a rod guide member  44  fixed to the bottom end wall  43 . Sealing members  45  and  46  seal between the outer barrel  41  and the top and the bottom end walls  42  and  43 . 
     The rod  33  has a piston part  55  which is fitted in the inner barrel  40  in an airtight and slidable manner. To this end, the inner barrel  40  has a mirror finished inner surface. The inner barrel  40  has at the bottom end portion a gas communication passage  40   a  consisting of multiple through-holes. The top end wall  42  has a breathing hole  42   a . The bottom end wall  43  has a rod guide mounting bore  43   a.    
     The rod guide member  44  has a barrel part  44   a  and an abutment part  44   b  in the form of a flange extending from the bottom end of the barrel part  44   a . The barrel part  44   a  is fitted in the rod guide mounting bore  43   a . A sealing member  47  seals between the barrel part  44   a  and the bottom end wall  43 . The abutment part  44   b  abuts against the bottom end wall  43  from below and is fixed there. In this state, the barrel part  44   a  protrudes above the bottom end wall  43 . The top end of the barrel part  44   a  is slightly above the gas communication passage  40   a . When the piston part  55  is locked on the top end of the barrel part  44   a , the piston part  55  cannot move down. Therefore, the gas communication passage  40   a  is never blocked by the piston part  55 . 
     The rod  56 , barrel part  44   a , bottom end wall  43 , and inner barrel  40  form a compressed gas operation chamber  50  containing a compressed gas  34  inside the cylinder body  32  below the piston part  55 . The inner and outer barrels  40  and  41  and top and bottom end walls  42  and  43  form a compressed gas filled chamber  51  filled with the compressed gas  34  between the inner and outer barrels  40  and  41 . In this way, the gas spring  31  has the compressed gas operation chamber  50  and the compressed gas filled chamber  51  that is independent from the compressed gas operation chamber  50  in a communicated manner. The compressed gas filled chamber  51  communicates with the compressed gas operation chamber  50  via the gas communication passage  40   a.    
     The lengths of the rod  33  and cylinder body  32  are determined so that the rod  33  reciprocates in relation to the cylinder body  32  over a specific stroke at least nearly equal to the vertical stroke of the spindle unit  10 . The rod  33  has a piston part  55  at the top end and a rod part  56  extending from the piston part  55  downward. The piston part  55  is fitted in the inner barrel  40  via a sealing member  57  in an airtight and slidable manner. The rod part  56  is inserted in and protrudes from the rod guide member  44  via a sealing member  58  in an airtight and slidable manner. 
     The piston part  55  has an underside forming an annular pressure receiving part  55   a  around the rod part  56 . The pressure receiving part  55   a  receives the gas pressure of the compressed gas  34  in the compressed gas operation chamber  50 , whereby the rod  33  is biased upward. Here, W (the biasing force of the gas spring  31 )≈P (the gas pressure of the compressed gas  34 )×S (the area of the pressure receiving part  55   a ). The gas pressure P of the compressed gas  34  and the area S of the pressure receiving part  55   a  are determined so that W is nearly equal to the weight of the spindle unit  10 . 
     The rod part  56  has a screw shaft portion  56   a  at the bottom. A lock nut  59  is screwed on the screw shaft portion  56   a . On the other hand, the frame member  11  of the spindle unit  10  has a screw hole  11   a  at the top. The screw shaft portion  56   a  is screwed in the screw hole  11   a , whereby the lock nut  59  is pressed against the top surface of the frame member  11  and the bottom end of the rod  33  is coupled to the top end of the spindle unit  10 . The screwing position of the lock nut  59  on the screw shaft portion  56   a  can be adjusted for fine tuning of the coupling position between the rod  33  and spindle unit  10 . 
     The operation and advantages of the above spindle balancer  30  will be described hereafter. 
     The gas spring  31  has the cylinder body  32 , rod  33 , and compressed gas  34 . The gas spring  31  is provided vertically above the spindle unit  10 . The bottom end of the rod  33  is coupled to the top end of the spindle unit  10 . Then, the gas spring  31  pulls and biases the spindle unit  10  upward. 
     When the lift mechanism  20  moves the spindle unit  10  downward while the gas spring  31  pulls and biases the spindle unit  10  upward, the rod  33  is advanced in relation to the cylinder body  32  of the gas spring  31 , whereby the gas spring  31  is extended. When the lift mechanism  20  moves the spindle unit  10  upward, the rod  33  is withdrawn in relation to the cylinder body  32  of the gas spring  31 , whereby the gas spring  31  is contracted. 
     In this way, the extendable gas spring  31  simplifies the structure in which the spindle unit  10  is reliably pulled and biased upward while assuring the vertical movement of the spindle unit  10  of a degree of freedom. An upward force nearly equal to the weight of the spindle unit  10  is produced when the spindle unit  10  is in rest position and even when the spindle unit  10  is in vertical motion. 
     In other words, the size and shape of the gas spring  31  and the pressure of the compressed gas  34  are appropriately determined according to the weight of the spindle unit  10 . Then, the gas spring  31  uses the pressure of the compressed gas  34  to stably pull and bias the spindle unit  10  upward and thus reliably and effectively reduce the load of the lift mechanism  20 . Consequently, the lift mechanism  20  can be downsized, the accuracy of the vertical positioning of the spindle unit  10  by the lift mechanism  20  or the machining accuracy can reliably be improved, and the spindle unit  10  having a further high vertical moving speed can be realized. 
     Furthermore, the gas spring  31  is not provided between the spindle unit  10  and underlying bed. The gas spring  31  is not an obstacle to the machining operation. It is unnecessary to mount the gas spring  31  on the spindle unit  10 . The spindle unit  10  does not need to have any particular mounting structure for the gas spring  31 . The rod  33  can easily be coupled to the spindle unit  10 . It can easily be applied to an existing machine tool M. 
     The gas spring  31  is highly durable unlike a tension spring. If the compressed gas  34  in the cylinder body  32  is reduced, the compressed gas  34  is easily refilled and the gas spring  31  is restored. It is unnecessary to mount the gas spring  31  on the spindle unit  10 . The spindle unit  10  does not need to be increased in size more than necessary. The gas spring  31  biases only the spindle unit  10 . The gas spring  31  can be downsized and simplify the structure, which is advantageous in production cost. 
     The rod  33  has at the top end portion the piston part  55  fitted in the cylinder body  32  in an airtight and slidable manner. The rod  33  is reliably guided to reciprocate in the cylinder body  32 . The compressed gas operation chamber  50  containing the compressed gas  34  is formed in the cylinder body  32  on the rod side (lower side) of the piston part  55 . The compressed gas  34  in the compressed gas operation chamber  50  reliably biases the rod  33  upward. The compressed gas filled chamber  51  that is independent from the compressed gas operation chamber  50 , filled with the compressed gas  34 , and communicates with the compressed gas operation chamber  50  via the communication passage  40   a  serves to suppress changes in the pressure of the compressed gas  34  while the compressed gas operation chamber  50  is subject to large volume changes as the rod  33  reciprocates. Hence, the spindle unit  10  is pulled and biased by nearly a constant biasing force. 
     The cylinder body  32  has the inner barrel  40  in which the piston part  55  is fitted and outer barrel  41  provided radially outside the inner barrel  40 . The compressed gas filled chamber  51  is formed between the inner and outer barrels  40  and  41 . Therefore, the gas spring  31  has an integrated compact compressed gas filled chamber  51 . Then, the spindle balancer  30  having a compact overall structure is advantageously mounted on an existing machine tool M. 
     Embodiment 2 
     As shown in  FIG. 3 , a spindle balancer  30 A has a gas spring  31 A and a compressed gas filled chamber  51 A provided separately from the gas spring  31 A. The gas spring  31 A is applied to a machine tool M in place of the gas spring  31  of Embodiment 1. Here, the same components as in Embodiment 1 are given the same reference numbers and their explanation is omitted. 
     The gas spring  31 A has a cylinder body  32 A, a rod  33 , and the compressed gas  34 . The cylinder body  32 A is different from the cylinder body  32  of Embodiment 1 in that the outer barrel  41  is eliminated and the inner barrel  40  and top and bottom end walls  42  and  43  are partly modified. The compressed gas operation chamber  50  is formed inside the cylinder body  32 A. 
     The cylinder body  32 A has a barrel  40 A, top and bottom end walls  42 A and  43 A fixed to the top and bottom ends of the barrel  40 A, and a rod guide member  44  fixed to the bottom end wall  43 A. The barrel  40 A corresponds to the inner barrel  40  of Embodiment 1. The barrel  40 A has a communication port  40   b  facing the compressed gas operation chamber  50 . Sealing members  45 A and  46 A seal between the barrel  40 A and the top and bottom end walls  42 A and  43 A. The top end wall  42 A has an air breathing hole  42   a . The bottom end wall  43 A has a rod guide mounting bore  43   a.    
     A compressed gas filled chamber  51 A filled with the compressed gas  34  is formed in a compressed gas filled unit  51 Aa. A gas hose  51 Ab extending from the compressed gas filled unit  51 Aa is connected to the communication port  40   b . The gas hose  51 Ab and communication port  40   b , which correspond to the gas communication passage, establish communication between the compressed gas operation chamber  50  and compressed gas filled chamber  51 A. The gas spring  31 A of the spindle balancer  30 A can further be downsized compared with the gas spring  31  of Embodiment 1. The gas spring  31 A advantageously require a smaller installation space. As for the rest, the same efficacy and effects as of Embodiment 1 can be obtained. 
     Embodiment 3 
     As shown in  FIG. 4 , a spindle balancer  30 B comprises a gas spring  31 B. The gas spring  31 B is applied to a machine tool M in place of the gas spring  31  of Embodiment 1. The gas spring  31 B has a cylinder body  32 B, a rod  33 B, and the compressed gas  34 . The rod  33  extends from the cylinder body  32 B both upward and downward. A compressed gas operation chamber  50 B containing the compressed gas  34  is formed nearly in the entire area of the cylinder body  32   b.    
     The cylinder body  32 B has a barrel  60 , top and bottom end walls  61  and  62  fixed to the top and bottom ends of the barrel  60 , a top rod guide member  63  fixed to the top end wall  61 , and a bottom rod guide member  64  fixed to the bottom end wall  62 . Sealing members  65  and  66  seal between the barrel  60  and the top and bottom end walls  61  and  62 , respectively. The top and bottom end walls  61  and  62  have rod guide mounting bores  61   a  and  62   a , respectively. 
     The top rod guide member  63  has a barrel part  63   a  and an abutment part  63   b  in the form of a flange extending from the top end of the barrel part  63   a . The barrel part  63   a  is fitted in the rod guide mounting bore  61   a . A sealing member  67  seals between the barrel part  63   a  and top end wall  61 . On the inner surface of the barrel part  63   a , a dust seal  70  is attached at the top, a sealing member  71  is attached under the dust seal  70 , and a recess  63   c  is formed under the sealing member  71 . The recess  63   c  is filled with lubricating grease  72 . A highly slidable rod guide  73  is attached under the recess  63   c . Here, the recess  63   c  can be filled with lubricating oil or nothing. 
     The bottom rod guide member  64  has a barrel part  64   a  and an abutment part  64   b  in the form of a flange extending from the bottom end of the barrel part  64   a . The barrel part  64   a  is fitted in the rod guide mounting bore  62   a . A sealing member  68  seals between the barrel part  64   a  and bottom end wall  62 . On the inner surface of the barrel part  64   a , a dust seal  75  is attached at the bottom, a sealing member  76  is attached above the dust seal  75 , and a recess  64   c  is formed above the sealing member  76 . The recess  64   c  is filled with lubricating grease  77 . A highly slidable rod guide  78  is attached above the recess  64   c . Here, the recess  64   c  can be filled with lubricating oil or nothing. 
     The lengths of the rod  33 B and cylinder body  32 B are determined so that the rod  33 B reciprocates in relation to the cylinder body  32 B over a specific stroke at least nearly equal to the vertical stroke of the spindle unit  10 . The rod  33 B has one end rod part  80  forming the bottom end portion protruding downward from the bottom rod guide member  64  of the cylinder body  32 B and the other end rod part  81  having a larger diameter than the one end rod part  80  and coaxially coupled to the top end of the one end rod part  80  so as to protrude upward from the top rod guide member  63  of the cylinder body  32 B. 
     The one end rod part  80  is slidably inserted in the bottom rod guide member  64  via the rod guide  78 . The dust seal  75 , sealing member  76 , and grease  77  seal between the one end rod part  80  and bottom rod guide member  64 . The other end rod part  81  is slidably inserted in the top rod guide member  63  via the rod guide  73 . The dust seal  70 , sealing member  71 , and grease  72  seal between the other end rod part  81  and top rod guide member  63 . 
     A coupling part  82  between the one end rod part  80  and the other end rod part  81  is housed in the cylinder body  32 B. At the coupling part  82 , a screw shaft portion  83  provided at the top of the one end rod part  80  is screwed in the other end rod part  81  via an locking member  84  in the form of a disc having a larger diameter than the other end rod part  81  to couple the one end rod part  80  and other end rod part  81 . Then, the locking member  84  is fastened to these rods  80  and  81 . 
     When the locking member  84  abuts against the barrel part  63   a  of the top rod guide member  63 , the rod  32 B is at the highest position. When the locking member  84  abuts against the barrel part  64   a  of the bottom rod guide member  64 , the rod  32 B is at the lowest position. The rod  32 B reciprocates between the highest and lowest positions but is retained in between with the rods  80  and  81  remaining inserted in the rod guide members  64  and  63 , respectively. 
     The locking member  84  has an underside forming an annular pressure receiving part  85  around the one end rod part  80 . The pressure receiving part  85  receives the gas pressure of the compressed gas  34  in the compressed gas operation chamber  50 B, whereby the rod  33 B is biased upward. Here, W (the biasing force of the gas spring  31 B)≈P (the gas pressure of the compressed gas  34 )×S (the area of the pressure receiving part  85 ). The gas pressure P of the compressed gas  34  and the area S of the pressure receiving part  85  (namely, the diameters of the one end rod part  80  and the other end rod part  81 ) are determined so that W is nearly equal to the weight of the spindle unit  10 . 
     The one end rod part  80  has a screw shaft portion  87  at the bottom. A lock nut  88  is screwed on the screw shaft portion  87 . As in Embodiment 1, the screw shaft portion  87  is screwed in the screw hole  11   a  of the frame member  11  of the spindle unit  10 , whereby the lock nut  88  is pressed against the top surface of the frame member  11  and the bottom end of the rod  33 B is coupled to the top end of the spindle unit  10 . 
     The rod  33 B of the spindle balancer  30 B has the one end rod part  80  forming the bottom end portion protruding downward from the bottom rod guide member  64  of the cylinder body  32 B and the other end rod part  81  having a larger diameter than the one end rod part  80  and coaxially coupled to the base end (top end) of the one end rod part  80  so as to protrude upward from the top rod guide member  63  of the cylinder body  32 B. Therefore, the rod  33 B can reliably be guided to reciprocate in the cylinder body  32 B. 
     The compressed gas operation chamber  50 B containing the compressed gas  34  is formed nearly in the entire area of the cylinder body  32 B. The coupling part  82  between the one end rod part  80  and the other end rod part  82  is housed in the cylinder body  32 B. The coupling part  82  has the annular pressure receiving part  85  receiving the gas pressure of the compressed gas  34  in the compressed gas operation chamber  50 B. Therefore, the rod  33 B can reliably be retained and biased upward by the compressed gas  34 . The volume change of the compressed gas operation chamber  50 B as a result of the rod  33 B reciprocating, namely the change in the gas pressure of the compressed gas  34 , can be suppressed to pull and bias the spindle unit  10  by a nearly constant force. 
     The rod  33 B does not slide directly on the inner surface of the cylinder body  33 B. Therefore, the cylinder body  32 B (barrel  60 ) does not need to have a precise mirror-finished inner surface. This is advantageous in producing the cylinder body  32 B. The cylinder body  32 B of the gas spring  31 B has a relatively large diameter. The pressure receiving part  85  can have a relatively large area to increase the basing force without excessively reducing the diameter of the one end rod part  80 . Then, a heavy spindle unit  10  can reliably be biased upward. As for the rest, the same operation and advantages as of Embodiment 1 can be obtained. 
     Embodiment 4 
     As shown in  FIG. 5 , a spindle balancer  30 C has a gas spring  31 C. The gas spring  31 C is applied to a machine tool M in place of the gas spring  31  of Embodiment 1. The gas spring  31 C has a cylinder body  32 C, a rod  33 C, and the compressed gas  34 . The cylinder body  32 C has a compressed gas operation chamber  50 C, a barrel  60 C, a top end wall  61 C, a bottom end wall  62 C, a top rod guide member  63 C, and a bottom rod guide member  64 C. The rod  33 C has one end rod part  80 C, the other end rod part  81 C, a locking part  84 C, and a pressure receiving part  85 C. 
     The gas spring  31 C is different from the gas spring  31 B of Embodiment 3 in size and shape; however, they basically have the same structure and the detailed explanation is omitted. In the gas spring  31 C, the compressed gas operation chamber  50 C is filled with the compressed gas  34 . As a means for adjusting the gas pressure of the compressed gas  34 , the one end rod part  80 C and top rod guide member  63 C have gas passages  90  and  91  connecting the compressed gas operation chamber  50 C to the outside, respectively. The gas passage  90  has a check valve  92 . 
     A specific gas supply unit (not shown) is connected to the gas passage  90  to supply the compressed gas  34  and control the gas pressure thereof. The gas spring  31 C of the spindle balancer  30 C is downsized and therefore suitable for biasing a light spindle unit  10  upward. As for the rest, the same operation and advantages as of Embodiment 3 can be obtained. The means for refilling the compressed gas operation chamber with the compressed gas and controlling the gas pressure thereof can also be provided to the gas springs  30 ,  31 A, and  31 B of Embodiments 1 to 3. 
     Embodiment 5 
     As shown in  FIG. 6 , a spindle balancer  30 D comprises a gas spring  31 D. The gas spring  31 D has a rod  33 D, the one end of which is coupled to the top end of the spindle unit  10  via a chain  95  and sprockets  96  and  97 . This spindle balancer  30 D is applied to the same machine tool M as in Embodiment 1. The spindle balancer  30 D has a cylinder body  31 D fastened for example to the rear wall of the column  1  via a bracket  35 D and a rod  33 D extending from the cylinder body  30 D upward. 
     The sprocket  96  is provided above the gas spring  31 D and rotatably supported by a bracket  96   a  secured to the column  1 . The sprocket  97  is provided above the spindle unit  10  and rotatably supported by a bracket  97   a  secured to the column  1 . The chain  95  is passed around the sprockets  96  and  97  with one end coupled to the top end of the rod  33 D and the other end coupled to the top end of the spindle unit  10 . The gas spring  31 D can be any of the gas springs  31 ,  31 A,  31 B, and  31 C of Embodiments 1 to 4. 
     In the spindle balancer  30 D, one end of the rod  33 D is coupled to the top end of the spindle unit  10  via the chain  95  and sprockets  96  and  97 . The gas spring  31 D has an increased degree of freedom in positing to reliably pull and bias the spindle unit  10  upward. Therefore, for example, the gas spring  31 D can be provided horizontally on the top wall of the column  1  or in various other orientations at various other positions and coupled to the spindle unit  10  via the chain and sprockets to pull and bias the spindle unit  10  upward. 
     Embodiment 6 
     In another not-shown structure, the other end of the cylinder body can be coupled to the spindle unit so that the biasing force of the gas spring that is applied on the cylinder body serves to pull and bias the spindle unit upward, whereby the biasing force of the gas spring serves to reduce the load of the lift mechanism. In such a case, the gas spring can be any of the gas springs  31 ,  31 A,  31 B,  31 C, and  31 D of Embodiments 1 to 5. 
     The present invention can be realized with various modifications other than those disclosed above without departing from the scope of the present invention. Furthermore, the present invention is applicable to various machine tools having a spindle unit and a lift mechanism for vertically moving the spindle unit.