Abstract:
A fluid power linear drive including a housing having an inner surface defining a piston receiving space, a piston reciprocally movable within the piston receiving space and a cooperative engagement means provided on the inner surface of the housing and an outer surface of the piston to prevent the piston from rotating relative to the housing. The cooperative engagement means may take the form of a projection means and a cooperating longitudinal groove, wherein the inner surface of the piston receiving space has the projection means extending radially inwardly into the piston receiving space and the piston has the groove cooperatively engaging the projection means for preventing the piston from rotating within the piston receiving space.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to pneumatic and hydraulic equipment, and more particularly to a linear drive device having a piston that is prevented from rotating with respect to the device. 
     BACKGROUND OF THE INVENTION 
     Linear drive units or double acting cylinders are known in the art for imparting linear reciprocating motion for driving a power transmitting member or the like. Such devices typically include an elongated fluid power cylinder housing in which a piston is arranged able to be slid by fluid actuation in a longitudinal direction. Usually, the piston is connected with a piston rod extending out of a front end of the cylinder housing, which in turn is coupled to a power transmitting member. 
     In such devices, the piston and piston rod are commonly circular in cross-section and are slidingly seated in a circular bore and/or bushing of the cylinder housing. Due to their circular design, it is possible for these pistons and piston rods to rotate to some extent during operation. However, in certain applications, it is desired or necessary to prevent the piston and/or piston rod from rotating as it linearly traverses. 
     One method by which conventional drive units accomplish this goal is by utilizing non-circular pistons and/or piston rods seated in correspondingly sized bores or barrels, whereby the piston and/or piston rod is prevented from rotating by its non-circular geometry. Typical non-circular geometries include square and elliptical cross-sections. For example, EP 0346716 discloses an actuator unit having a non-circular piston rod prevented from rotating by a bearing component fastened to the outside of the cylinder housing. 
     Another method for preventing rotation of the piston involves the use of one or more guide rods which are connected in parallel with the piston and/or piston rod and slidingly traverse in a separate bore spaced apart from the main piston chamber. The guide rods are generally fixed to the piston by a yoke plate which prevents the piston from rotating. 
     However, such methods are not without their drawbacks. For example, non-circular piston rods have limited torque and are difficult to seal at sharp corners to protect against contamination and other environmental influences. It is also more expensive to manufacture high-precision non-circular pistons and piston rods from hardened stainless steel rod material, as compared to circular pistons and piston rods. It is also often difficult to precisely match non-circular pistons with mating complex geometrical bores or barrels. With respect to the use of guide rods, such external guide rods can easily bind and further require the device to overcome higher frictional forces during operation. Moreover, guide rods mean additional parts and extra space is required on the device to accommodate the guide rods and yoke plate. 
     Accordingly, it would be desirable to maintain a standard circular piston and piston rod within a linear drive yet prevent the circular piston from rotating without the need for guide rods. It would be further desirable to provide a compact linear drive unit that utilizes a minimum number of inexpensive components to prevent the piston from rotating. 
     SUMMARY OF THE INVENTION 
     The present invention is a fluid power linear drive including a cylinder housing having an inner surface defining a piston receiving space, a piston reciprocally movable within the piston receiving space and a cooperative engagement means provided on the inner face of the piston receiving space and an outer surface of the piston for preventing the piston from rotating relative to the housing. The cooperative engagement means may be provided in the form of a projection means and a cooperating groove, wherein the inner surface of the piston receiving space has a projection means extending radially inwardly into the piston receiving space and the piston has a longitudinal groove engaging the projection means for preventing the piston from rotating within the piston receiving space. Alternatively, the projection means may be provided on the piston and the cooperating groove may be formed in the housing. 
     In a preferred embodiment, the longitudinal groove is defined by a bottom wall and two side walls extending from the bottom wall and is formed in the piston between two longitudinally spaced seals provided on the piston. Also, the projection means is preferably a ball press-fit within a hole formed in the inner surface of the piston receiving space, wherein the ball engages the piston groove. Alternatively, the projection means can be a pin fixed within a hole formed in the inner surface of the piston receiving space, or a raised portion integral with the inner surface of the piston receiving space. 
     In an alternative embodiment, the fluid powered linear drive includes a cylinder housing defining a piston receiving space and a piston rod receiving space, a piston reciprocally movable within the piston receiving space and a piston rod axially connected to the piston for reciprocal movement therewith. The piston rod receiving space has an inner surface with a protuberance or projection means extending radially into the piston rod receiving space. The piston rod extends through the piston rod receiving space and has a longitudinal groove engaging the projection means for preventing the piston rod from rotating within the piston rod receiving space. In this embodiment, the cylinder housing may also include a piston rod bearing which defines the piston rod receiving space therein and wherein the piston rod extends longitudinally outwardly from the housing. 
     The present invention further involves a method for guiding a piston reciprocally movable within a piston receiving space of a fluid powered linear drive cylinder housing. The method generally includes the steps of providing a radially extending projection means on one of an inner surface of the housing piston receiving space and an outer surface of the piston and cooperatively engaging the projection means with a longitudinal groove formed on the other of the inner surface of the housing piston receiving space and the outer surface of the piston for preventing the piston from rotating within the piston receiving space. 
     The preferred embodiments of the linear drive with a non-rotating piston as well as other objects, features and advantages of this invention, will be apparent from the following detailed description, which is to be read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top perspective view of a linear drive with non-rotating piston, formed in accordance with a preferred embodiment of the present invention, with the housing shown partially cut away. 
         FIG. 2  is a cross-sectional view of the linear drive shown in  FIG. 1 , taken along line  2 — 2 . 
         FIG. 3  is a cross-sectional view of an alternative embodiment of the present invention. 
         FIG. 4  is a cross-sectional view of another alternative embodiment of the present invention. 
         FIG. 5  is a top perspective view of yet another alternative embodiment of the present invention. 
         FIG. 6  is a longitudinal cross-sectional view of still another alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to  FIGS. 1 and 2 , the fluid power linear drive device  10  of the present invention generally includes an elongated cylinder housing  12  and a front and a rear housing cover  14  and  16 . The cylinder housing  12  is preferably a tubular body of any external geometry extruded from a durable metal material. The housing covers  14  and  16  are respectively mounted on the front and rear end faces of the cylinder housing  12  and secured thereto, for example, using bolts or by ties. 
     The cylinder housing  12  defines a piston receiving space  18  extending in the interior of the housing in the longitudinal direction  20 . This piston receiving space  18  has a generally circular cross-sectional configuration and extends between the two end faces of the cylinder housing  12 . The piston receiving space  18  is closed at the ends by the housing covers  14  and  16 . 
     A piston  22 , which is able to be reciprocally slid in the direction of the longitudinal axis  20  of the cylinder housing  12 , is located in the piston receiving space  18 . The piston  22  has a generally circular cross-sectional configuration and divides the piston receiving space  18  into a front working space  24  adjacent to the front housing cover  14 , and a rear working space  26  adjacent to the rear housing cover  16 . The piston  22  is provided with seals  23 , such as O-rings or any other known seal arrangement, which cooperate with the inner surface  25  of the piston receiving space in a sealing, fluid-tight manner. 
     A piston rod  28  is preferably permanently connected with at least one end of the piston  22  and extends coaxially with the piston. The device  10  shown in  FIG. 1  is a double-acting cylinder wherein the piston  22  has one piston rod  28  extending from the front of the piston through the front working space  24  and through the front housing cover  14  and another piston rod  28   b  extending from the rear of the piston through the rear working space  26  and through the rear housing cover  16 . The piston rods  28  and  28   b  are preferably slidingly supported by bearings  29  fixed within the piston receiving space  18  or within respective housing covers  14  and  16 . The ends  30  of the piston rod  28  are disposed outside the cylinder housing  12  and may be provided with attachment means  32 , such as a screw thread or the like, which permits attachment to an object to be moved by the linear drive device. 
     The cylinder housing  12  is further formed with front and rear fluid ducts  34  and  36 , which are in respective fluid communication with the front working space  24  and the rear working space  26  of the piston receiving space  18 . The front and rear fluid ducts  34  and  36  may, for example, be longitudinally formed in the front and rear housing covers  14  and  16 , respectively, as shown in  FIG. 1 , or the ducts may be formed perpendicularly through the wall of the cylinder housing  12 . Of course other arrangements and combinations thereof can be utilized so long as each of the front and rear working spaces  24  and  26  is provided with a fluid duct. 
     By way of the fluid ducts  34  and  36 , connected to fluid lines (not shown), an actuating fluid, such as compressed air, is alternately supplied and exhausted from the working spaces  24  and  26 . Operation utilizing a hydraulic fluid is also contemplated by the present invention. As a result of such fluid action in the working spaces  24  and  26  and, in turn, on the piston  22  dividing the working spaces, there is a linear movement of the piston and the piston rod  28  in one direction or the other along the longitudinal axis  20  indicated by a double arrow  38  shown in  FIG. 1 . 
     According to the present invention, the circular piston  22  is prevented from rotating within the piston receiving space  18  by providing a cooperative engagement means  39  on the outer surface of the piston and the inner surface  25  of the piston receiving space  18 . In a preferred embodiment, the cooperative engagement means  39  is in the form of at least one longitudinal channel or groove  40  formed in the outer surface of the piston, which receives a protuberance or projection means  42  provided on the inner surface  25  of the piston receiving space. It is envisioned that the projection means  42  and the groove  40  can take any geometry, so long as they cooperate to prevent rotation of the piston upon longitudinal translation within the cylinder housing  12 . 
     Referring additionally to  FIG. 2 , the longitudinal groove  40  preferably has a depth “d” and a width “w” and is preferably defined by a bottom wall  43  and side walls  44  extending from the bottom wall. The groove  40  is preferably formed by milling to a precise width “w” and extends longitudinally between the seals  23  of the piston  22 . By positioning the groove  40  between the seals  23  of the piston  22 , the groove will not provide a leak path for fluid in the working chambers  24  and  26 . The piston  22  shown in  FIG. 2  is formed with two longitudinal grooves  40  formed in opposite radial surfaces of the piston, however, other configurations are of course possible. 
     The projection means  42  provided on the inner surface  25  of the piston receiving space  18  can take any form so long as it protrudes to some extent inwardly from the inner face into the piston receiving space. In a preferred embodiment, the projection means is a hardened ball bearing  46  press-fit within a hole  48  formed in the cylinder housing  12 , as shown in  FIGS. 1 and 2 . The ball bearing  46  is pressed into the hole  48  to a depth wherein the ball engages the aligned groove  40  formed in the piston. Thus, the outer surface of the ball bearing  46  will contact the side walls  44  of the groove  40  and will restrict all possible rotational motion of the piston  22 , but will permit longitudinal reciprocation. The ball bearing  46  is preferably fixed within the cylinder housing  12  so there will be a sliding, as opposed to a rolling, friction between the ball and the piston groove  40 . 
     As mentioned above, the projection means  42  can take other forms. For example,  FIG. 3  shows the projection means  42  in the form of a pin  50  press-fit within the hole  48  formed in the cylinder housing  12 . The pin  50  is pressed into the hole  48  to a depth wherein the pin sides engage the aligned groove  40  formed in the piston. In  FIG. 4 , the protuberance  42  is a raised portion  52  integral with the inner surface  25  of the cylinder housing  12 . The integral raised portion  52  has a height sufficient to engage the aligned groove  40  formed in the piston. Again, in each embodiment, the projection means  42  will restrict all possible rotational motion of the piston  22  but will permit longitudinal reciprocation. Also, the projection means  42  is preferably fixed within the cylinder housing  12  so there will be a sliding friction with the piston groove  40 . 
     In another alternative embodiment, as shown in  FIG. 5 , the piston rod  28 , as opposed to the piston  22 , can be formed with a groove or channel  54 . In this case, a projection means  42  can be provided on an inner face  56  of a piston rod receiving space  58  defined by the piston rod bearing  29 . In this regard, the front housing cover  14  would be provided with a seal  60  which cooperates with the outer surface of the piston rod  28  in a sealing, fluid-tight manner. As previously described, the projection means  42  may take any form so long as it engages the groove  54  formed in the piston rod  28  to restrict the piston  22  and piston rod from rotating. 
     In still another alternative embodiment, the projection means  42  may be provided on the piston  22  while the longitudinal groove  40  is formed in the inner surface  25  of the piston receiving space, as shown in  FIG. 6 . It is further envisioned that this reverse engagement means arrangement can also be provided on the piston rod  28  and the piston rod receiving space  58 . 
     As a result of the present invention, a simple, low-cost solution is provided for the problem of preventing a piston from rotating. The present invention allows the piston and piston rod to be fabricated with circular cross-sections, which provides strength and sealing benefits, while at the same time requires a minimum of additional components, such as guide rods. 
     Although the preferred embodiments of the present invention have been described with reference to the accompanying drawing, it is to be understood that the invention is not limited to those precise embodiments, and that other changes and modifications may be made by one skilled in the art without departing from the scope or spirit of the invention.