Abstract:
The invention provides a liquid pump using an elastomeric piston. The liquid pump has a piston assembly ( 10 ), which comprises: a piston rod ( 11 ); a piston head ( 12 ); a piston engaging portion ( 15 ) which is between the piston rod and the piston head in a necking shape; and a piston ( 13 ) made of an elastomeric material and sleeved around the piston engaging portion between the piston rod and piston head. The piston rod and piston head are formed separately and then assembled together. The piston engaging portion ( 15 ) is composed of a piston-engaging-portion formation part of at least one of the piston rod and the piston head. The invention also provides a method of manufacturing the piston assembly ( 10 ). The invention further provides a piston assembly ( 10 ) for a liquid pump. The piston assembly has a piston ( 13 ) which is provided with an annular notch ( 133 ) formed on the periphery of an upper end of the piston ( 13 ) where an annular inner flange ( 131 ) is formed. The piston assembly and its manufacture method are helpful to improving product precision and quality and facilitate automated assembly, and are specially used for the liquid pumps using an elastomeric piston.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to a liquid pump, and particularly to a liquid pump which incorporates an elastomeric piston. 
       BACKGROUND OF THE INVENTION 
       [0002]    A liquid pump is generally mounted at an opening of a container filled with an oil-like liquid, and used for dispensing the liquid out of the container by its user&#39;s manual press operation. Liquid pumps of this type commonly incorporate a piston made of an elastomeric material (such as Nitrile Butadiene Rubber (NBR)) but not of a plastic material, because the elastomeric materials are less susceptible to the oil-like liquids than the plastic materials which may generate deformation under the action of the oil-like liquids. 
         [0003]    At present, there are various liquid pumps of the above type that incorporate an elastomeric piston and are specially used for dispensing oil-like liquid products. However, conventional liquid pumps of this type mostly have problems such as difficulty to manufacture and assembly and difficulty to improve the product precision. 
         [0004]    A liquid pump of the above type is disclosed in JP2004-131163. As shown in  FIGS. 1   a - 1   e,  the liquid pump comprises mainly a press head  120 , a cylinder  130 , an integrated threaded sleeve-cylinder cap  140 , a spring  150 , a lower non-return ball valve  160  and a piston assembly  110 . The liquid pump is mounted at an opening of a container of liquid product, which opening is commonly an open top end of the container, and is used for dispensing the liquid product out of the container. The piston assembly  110  is moved up and down in the cylinder  130  in a well-known manner under the drive of the press head  120 , thereby a desired pumping effect takes place. The piston assembly  110  includes a piston rod  111  and a piston head  112  which are integrally formed to be a one-piece longitudinally extending member, and a piston  113  which is fitted over a piston engaging portion  115  between the piston rod  111  and piston head  112 . The piston engaging portion  115  is made to be a cylindrical necking portion between the piston rod  111  and piston head  112 , and the cylindrical outer profile thereof mates with the cylindrical inner profile of the piston  113  which is fitted over the piston engaging portion. In the center of the integrated piston rod  111 -piston head  112  there is formed a fluid passage  114 , whose lower end is closed at the piston head  112  and whose upper end is open at the top end of the piston rod  111 . At the piston engaging portion  115  there is formed a transverse passage  116 , which extends radially through the piston engaging portion  115  substantially perpendicular to the fluid passage  114  and communicates with the fluid passage  114 . The passage  116  can be regarded as an extension of the fluid passage  114 , and the liquid in the liquid reservoir of the cylinder  130  can be pumped out of the container through the passages  114  and  116 . The piston rod  111 , at its joint with the piston engaging portion  115 , has a conical step surface  1111  tapering towards the piston engaging portion  115 , while the piston head  112 , at its joint with the piston engaging portion  115 , has a steep step surface  1121  which is substantially perpendicular to the longitudinal direction. The piston  113  is substantially in the form of a cylindrical sleeve, and is formed with an annular inner flange  1131  on its inside wall. The annular inner flange  1131  defines a round central hole, which is for the passage of the piston engaging portion  115  of the integrated piston rod  111  and piston head  112  and substantially surrounds the piston engaging portion  115 . Opposite ends of the piston  113  are flared so as to form a lower sealing lip  1132  and an upper sealing lip  1133 . 
         [0005]    In the static assembled status of the liquid pump, the piston assembly  110  is slidably contained within the cylinder  130 , with the annular inner flange  1131  of the piston  113  that is fitted over the piston engaging portion  115  closing off the transverse passage  116 , and both the lower sealing lip  1132  and the upper sealing lip  1133  of the piston  113  abutting against the inside wall of cylinder  130 . In this status, the fluid communication between the fluid passage  114  and the liquid reservoir of the cylinder  130  through the transverse passage  116  is blocked up by the annular inner flange  1131  of the piston  113 , and therefore, the liquid in the liquid reservoir of the cylinder  130  cannot be dispensed out through the fluid passage  114 . 
         [0006]    As the press head  120  is pressed downwards against the force of the spring  150  and the piston assembly  110  is driven to move down therewith relative to the cylinder  130 , both the sealing lips  1132  and  1133  of the piston  113  slide downwards while abutting against the sidewall of the cylinder  130 . Owing to a combined action of the downward driving force exerted on the annular inner flange  1131  by the piston rod  111  and the piston engaging portion  115  and the upward friction force exerted on the sealing lips  1132  and  1133  of the piston  113  by the inside wall of the cylinder  130 , as well as owing to a deformation space provided for the piston  113  by the conical step surface  1111  of the piston rod  111 , the piston  113  is forced to deform toward the side of the conical step surface  1111 , i.e., toward the side of the piston rod  111 , as shown in  FIG. 1   b . The deformation causes the annular inner flange  1131  of the piston  113  to offset from the piston engaging portion  115  so that the transverse passage  116 , which is originally closed by the piston  113 , is opened, thereby allowing the liquid in the liquid reservoir of the cylinder  130  to be pumped out through the transverse passage  116  and the fluid passage  114  under the effect of the downward pumping of the piston assembly  110 . And then, as the press head  120  is released from the downward pressing force, the piston assembly  110  moves upwards relative to the cylinder  130  under the action of the spring  150 . At this time, because the steep step surface  1121  of the piston head  112  does not provide a space for the piston  113  to deform towards the piston head  112 , the piston  113  cannot deform considerably to allow the annular inner flange  1131  of the piston  113  to offset from the piston engaging portion  115  to release the sealing of the transverse passage  116 . To sum up, the piston assembly  110  is configured as follows: when the piston assembly  110  is maintained stationary with respect to the cylinder  130 , the piston  113  remains in its engaging position with the piston engaging portion  115 , thereby to close the transverse passage  116  and prevent the fluid communication of the liquid reservoir of the cylinder  130  with the outside of the container through the fluid passage  114 ; as the piston assembly  110  is driven to move downwards relative to the cylinder  130 , the piston  113  deforms and offsets from its engaging position to open the transverse passage  116  in the piston engaging portion  115 , allowing the liquid reservoir of the cylinder  130  to communicate with the outside of the container; as the piston assembly  110  is moved upwards relative to the cylinder  130 , the piston  113  still remains in its engaging position so as to close the transverse passage  116  and to prevent the fluid communication of the liquid reservoir of the cylinder  130  with the outside of the container through the fluid passage  114 . Thus, in the piston assembly  110 , the piston  113  fitted on the piston engaging portion  115  plays the role of an upper non-return valve, which cooperates with the lower non-return ball valve  160  to allow for the liquid loading and discharging of the liquid reservoir of the cylinder  130 . 
         [0007]    Also, JP52-112107 discloses a liquid pump of the above type. As shown in  FIGS. 2   a - 2   e,  the pump is substantially the same in basic configuration as that in JP2004-131163 discussed above, with the difference only lying in the structure of the piston  213 . Accordingly, the members in  FIGS. 2   a - 2   e  are given the reference numerals similar to those in  FIGS. 1   a - 1   e  just with the initial number “1” replaced with “2”, therefore, the same numerals, except for the initial number, denote the like members. In particular, the pump of JP52-112107 mainly comprises a press head  220 , a cylinder  230 , an integrated threaded sleeve-cylinder cap  240 , a spring  250 , a lower non-return ball valve  160  and a piston assembly  210 . The piston assembly  210  includes an integrally formed and longitudinally extending one-piece piston rod  211 /piston head  212 , and a piston  213  which is fitted over the piston engaging portion  215  between the piston rod  211  and piston head  212 . The piston  213  has a different configuration from that of the piston  113  in JP2004-131163, and is generally composed of an upper portion and a lower portion, the upper portion  2135  substantially presenting a cylindrical shape, and the lower portion  2136  substantially presenting a shape of frustum. The upper portion  2135 , at its top end, is formed with an annular inner flange  2137  which defines a round central hole. The hole is for the passage of the piston engaging portion  215  between the piston rod  211  and the piston head  212 , and surrounds the piston engaging portion  215 . The lower portion  2136 , as it extends downwards, gradually flares and gradually becomes thinner so as to form a sealing element of the piston  213  in contact with the inside wall of the cylinder  230 . The thickness of the upper portion  2135  is significantly greater than that of the lower portion  2136 . 
         [0008]    The pump disclosed in JP52-112107 is operated in a similar manner to that in JP2004-131163. The piston assembly  210  is configured as follows: when the piston assembly  210  is stationary with respect to the cylinder  230 , the piston  213  remains in its engaging position with the piston engaging portion  215 , thereby to close the transverse passage  216  and prevent the fluid communication of the liquid reservoir of the cylinder  230  with the outside of the container through the fluid passage  214 ; as the piston assembly  210  is moved downwards with respect to the cylinder  230 , the piston  213  deforms and offsets from its engaging position to open the transverse passage  216  in the piston engaging portion  215 , allowing the liquid reservoir of the cylinder  230  to communicate with the outside of the container through the fluid passage  214 ; as the piston assembly  210  is moved upwards with respect to the cylinder  230 , the piston  213  still remains in its engaging position so as to close the transverse passage  216  and to prevent the fluid communication of the liquid reservoir of the cylinder  230  with the outside of the container through the fluid passage  214 . In particular, as the press head  220  is pressed downwards so as to drive the piston assembly  210  to move downwards relative to the cylinder  230  against the force of the spring  250 , the piston  213  is made slide downwards with the end edge of its lower portion  2136  abutting against the sidewall of the cylinder  230 . Owing to a combined action of the downward driving force exerted on the annular inner flange  2137  by the piston rod  211  and the piston engaging portion  215  and the upward friction force exerted on the end edge of the lower piston  2136  by the inside wall of the cylinder  230 , as well as owing to a deformation space provided for the piston  213  by the conical step surface  2111  of the piston rod  211 , the piston  213  is forced to deform toward the side of the conical step surface  2111 , i.e., toward the side of the piston rod  211 , as shown in  FIG. 2   b . The deformation causes the annular inner flange  2131  of the piston  213  to offset from the piston engaging portion  215  so that the transverse passage  216 , which is originally closed by the piston  213 , is opened, thereby allowing the liquid in the liquid reservoir of the cylinder  230  to be pumped out through the transverse passage  216  and the fluid passage  214  under the action of the downward pumping of the piston assembly  210 . And then, as the press head  220  is released from the downward pressing force, the piston assembly  210  moves upwards relative to the cylinder  230  under the action of the spring  250 . At this time, because the steep step surface  2121  of the piston head  212  does not provide a space for the piston  213  to deform towards the piston head  212 , the piston  213  cannot deform considerably to allow the annular inner flange  2131  of the piston  213  to offset from the piston engaging portion  215  to release the sealing of the transverse passage  216 . 
         [0009]    In the above two pump configurations of the prior art, the piston rod, the piston head and the reduced piston engaging portion between the two formers are formed to be a one-piece member, with a transverse passage in the piston engaging portion, therefore, it is necessary to use a composite mould including half moulds and a core to manufacture such a one-piece piston rod/piston engaging portion/piston head member. Such a manufacture method, using a complicated composite mould and thus involving multiple procedures such as withdrawing cores and splitting moulds, not only increases the equipment costs but also limits improvement of the productivity. Moreover, it is well known that the manufacture using half moulds has shortcomings such as low productivity and poor precision, wherein there exist inevitable problems of mould misalignment and flashing production due to the parting surface, which adversely affects the dimension precision of products, and finally, may lead to leakage of a finished assembly. 
         [0010]    Another problem with the pump configurations of the prior art lies in that: in the one-piece piston rod/piston engaging portion/piston head, the outer diameter of the piston engaging portion for engaging with the piston is smaller than those of both the piston rod and the piston head, and the former is located between the two latter, thus, during engaging the piston onto the piston engaging portion, it is necessary to forcibly broaden the central hole of the annular inner flange of the piston to a size enough for the piston rod to pass so as to fit the piston onto the piston engaging portion. Such a fitting way is time- and labor-consuming, and is not suitable for an automatic assembly line. Moreover, in the course that the piston rod or piston head with a big diameter is inserted through the small central hole of the annular inner flange of the piston, the inner circumference of the hole is prone to be damaged, thereby adversely affecting the operation life of the piston assembly and thus of the liquid pump. 
         [0011]    In addition, in regard to the piston configuration disclosed in JP52-112107, when the press head is pressed and moved downwards, under the effect of the upward friction force of the inside wall of the cylinder and the liquid pressure in the cylinder on the lower portion  2136  of the piston  213 , the piston  213  should per se rapidly deform upwards so as to open the transverse passage  216  which is closed by the annular inner flange  2137 . However, the upper portion  2135  of the piston  213  is rather thick and thus is not easy to deform, which may result in that the transverse passage  216  cannot be opened or can be opened but not in time, thereby causing the pump to be unable to operate normally. 
       SUMMARY OF THE INVENTION 
       [0012]    In view of the shortcomings of the liquid pumps with an elastomeric piston of the prior art, the main object of this invention is to provide a liquid pump using an elastomeric piston that has improved product precision and quality and facilitates automatic assembly. 
         [0013]    Another object of the invention is to provide a manufacture method of a piston assembly for liquid pumps that facilitates automatic production and is advantageous to ensure product precision and quality. 
         [0014]    Still another object of the invention is to provide a piston assembly for liquid pumps that has improved operation reliability. 
         [0015]    In order to attain the above object, in accordance with one aspect of the present invention there is provided a liquid pump using an elastomeric piston, for mounting on a liquid container to pump liquid in said container out of said container, said liquid pump comprising a piston assembly which is disposed in a cylinder slidably in a longitudinal direction of the pump, said cylinder defining a liquid reservoir for accommodating liquid therein. Said piston assembly comprises: a piston rod, formed with a fluid passage in a center of the piston rod in the longitudinal direction; a piston head, connected to a lower end of said piston rod; a piston engaging portion in the form of a neck between said piston rod and said piston head, said piston head being connected with said piston rod via said piston engaging portion, said piston engaging portion being formed with a transverse passage that diametrically extending through said piston engaging portion and is in fluid communication with said fluid passage so as to provide a passage for the liquid accommodated in said liquid reservoir to be pumped out of said container; and a piston made of an elastomeric material, fitted over said piston engaging portion between said piston rod and said piston head for releasably closing said transverse passage. Said piston assembly is configured such that: when said piston assembly is stationary with respect to said cylinder, said piston remains in its engaging position with said piston engaging portion so as to close said transverse passage and block the fluid communication of said liquid reservoir with the outside of said container through said fluid passage; as said piston assembly is moved downwards relative to said cylinder, said piston deforms to offset from said engaging position so as to open said transverse passage in said piston engaging portion and allow said liquid reservoir to be communicate with the outside of said container through said fluid passage; as said piston assembly is moved upwards relative to said cylinder, said piston remains in said engaging position so as to close said transverse passage and block the fluid communication of said liquid reservoir with the outside of said container through said fluid passage. The liquid pump of the present invention is featured in that said piston rod and said piston head are formed separately and then assembled together, said piston engaging portion being composed of a piston-engaging-portion formation part of at least one of said piston rod and said piston head. 
         [0016]    By the way, it is noted that the words “upper” and “lower” and the like used herein are taken with respect to the orientation in which a liquid pump is normally used (i.e., the liquid pump is oriented such that the longitudinal direction of the liquid pump coincides with the vertical direction). 
         [0017]    With the liquid pump using an elastomeric piston of the present invention, as the piston rod and the piston head are separately provided and then assembled together, the problems of complicated manufacture process and low productivity that are associated with the production of the integrated piston rod-piston engaging portion-piston head needing the complicated composite mould can be avoided. Moreover, it is unnecessary for the invention to use half moulds, so the invention avoids the problems of low productivity and poor precision which are usually associated with the employment of half moulds. 
         [0018]    According to a preferred embodiment of the invention, said piston head comprises a base and a guide stub which extending upwards form said base, said piston rod being formed with a guide stub receiving hole at its lower end, said guide stub receiving hole being formed as part of said fluid passage, said guide stub portion being inserted into and fitted to said guide stub receiving hole so as to assemble said piston head onto said piston rod, said guide stub being formed with at least one longitudinal slot along a longitudinal length of said guide stub, said longitudinal slot constituting a longitudinal passage in said guide stub receiving hole, said piston engaging portion being formed with said transverse passage corresponding to said longitudinal slot in number, with said longitudinal slot correspondingly communicating with said transverse passage in said piston engaging portion, thereby providing a passage from said transverse passage to said fluid passage. 
         [0019]    With such a configuration, it is very simple to fix the piston rod and piston head together by virtue of their own structures without needing any separate fixing means, which makes assembly process simpler and easily achieves the automatic production. Furthermore, once the piston rod and piston head are connected together, the necessary fluid passage, transverse passage and longitudinal passage will all be ready without needing any additional processing procedures, thus minimizing the production cost. 
         [0020]    According to another preferred embodiment of the invention, said piston engaging portion is provided by both of said piston rod and said piston head, wherein said piston rod, at its lower end, is formed with a cylindrical reduced diameter tube portion which serves as one of said piston-engaging-portion formation parts, said reduced diameter tube portion defining part of said guide stub receiving hole, and wherein said piston head, at an upper end of its base, is formed with a cylindrical reduced diameter boss which serves as the other of said piston-engaging-portion formation parts, said reduced diameter tube portion and said reduced diameter boss having the same diameter, whereby said reduced diameter tube portion and said reduced diameter boss abut against each other to cooperatively constitute said piston engaging portion when said piston head is assembled to said piston rod. 
         [0021]    According to the invention, said transverse passage of said piston engaging portion can be formed in said reduced diameter boss. Said transverse passage of said piston engaging portion can be formed integrally with said longitudinal slot of said guide stub. Of course, said transverse passage of said piston engaging portion can be formed in said reduced diameter tube portion, alternatively, can be formed in both said reduced diameter tube portion and said reduced diameter boss. 
         [0022]    According to the invention, said piston engaging portion can be provided only by said piston head, wherein said piston head, at an upper end of its base, is formed with a cylindrical reduced diameter boss which serves as said piston-engaging-portion formation part, when said piston head is assembled to said piston rod, said reduced diameter boss abutting against a lower end of said piston rod to cooperatively constitute said piston engaging portion. Of course, said piston engaging portion can be provided only by said piston rod, wherein said piston rod, at its lower end, is formed with a cylindrical reduced diameter tube portion which serves as said piston-engaging-portion formation part, said reduced diameter tube portion defining part of said guide stub receiving hole, when said piston head is assembled to said piston rod, said reduced diameter tube portion abutting against an upper end of said base of said piston head to constitute said piston engaging portion. 
         [0023]    According to the invention, said guide stub can be formed with a recess or protrusion along its periphery, and a mating protrusion or recess is formed along an inner circumferential surface of said guide stub receiving hole, helping to assemble said piston head to said piston rod. 
         [0024]    According to the invention, in the case that the liquid pump comprises an integrated threaded sleeve-cylinder cap and a press head, said cylinder can be connected with said integrated threaded sleeve-cylinder cap on a lower side thereof, said press head being connected to an upper end of said piston rod, said press head having an upper stop position and a lower stop position of its press stroke with respect to said integrated threaded sleeve-cylinder cap, said integrated threaded sleeve-cylinder cap including an inner sleeve and an outer sleeve which extend in the longitudinal direction, said outer sleeve for engaging an open neck of said container so as to mount said liquid pump on said container, said inner sleeve for engaging an upper end of said cylinder so as to connect said cylinder to said integrated threaded sleeve-cylinder cap on the lower side thereof and thus to form a closed cylinder, when said press head is at the upper stop position of said press stroke with respect to said integrated threaded sleeve-cylinder cap, a lower end of said inner sleeve being engaged in said annular notch of said piston. 
         [0025]    According to the invention, in the case that the cylinder is formed in its sidewall with a balancing vent for balancing pressures within and outside the container, when said press head is at the upper stop position of said press stroke with respect to said integrated threaded sleeve-cylinder cap, said balancing vent can be above an abutment point of said sealing lip of said piston with the sidewall of said cylinder, and below the abutment point of said inner sleeve of said integrated threaded sleeve-cylinder cap with said piston. 
         [0026]    According to the invention, said piston rod, at its joint with said piston engaging portion, can have a conical step surface tapering towards said piston engaging portion, and said piston head, at its joint with said piston engaging portion, has a steep step surface which is substantially perpendicular to the longitudinal direction, said conical step surface providing a space for said piston to deform. Said piston engaging portion, at its joint with said conical step surface, can have an increased diameter portion. 
         [0027]    Moreover, in accordance with another aspect of the present invention, there is provided a method of manufacturing said piston assembly of the above liquid pump, said method comprises: separately molding and providing said piston rod, said piston head and said piston; fitting said piston over one of said piston rod and said piston head that is suitable for receiving said piston; assembling said piston rod and said piston head together, with said piston being fitted around said piston engaging portion between said piston rod and said piston head. 
         [0028]    The method of the invention does not need any complicated composite mould and does not involve the step of enlarging the piston&#39;s inner diameter which is necessitated in the traditional process, therefore, the piston assembly can be produced easily and at a lower cost, and with premium quality and higher reliability of the final product. In addition, the method facilitates the automatic production and thus achieves improved productivity. 
         [0029]    In the method of the invention described above, in the case that each of the piston rod and the piston head has the piston-engaging-portion formation part, the piston can be received by the piston-engaging-portion formation part of either of the piston rod and the piston head, that is, both of the piston rod and the piston head are suitable for receiving the piston; in the case that the piston head has a guide stub for inserting into the guide stub receiving hole of the piston rod, it is preferred that the piston be received by the guide stub of the piston head. 
         [0030]    Furthermore, in accordance with still another aspect of the present invention, there is provided a piston assembly for use in a liquid pump for mounting to a liquid container to pump liquid in said container out of said container, said liquid pump comprising a piston assembly which is disposed in a cylinder slidably in a longitudinal direction of the pump, said cylinder defining a liquid reservoir for accommodating liquid therein. Said piston assembly comprises: a piston rod, formed with a fluid passage in a center of the piston rod; a piston head, connected to a lower end of said piston rod; a piston engaging portion in the form of a neck between said piston rod and said piston head, said piston head being connected with said piston rod via said piston engaging portion, said piston engaging portion being formed with a transverse passage that diametrically extending through said piston engaging portion and is in fluid communication with said fluid passage so as to provide a passage for the liquid accommodated in said liquid reservoir to be pumped out of said container; and a piston made of an elastomeric material, fitted over said piston engaging portion between said piston rod and said piston head for releasably closing said transverse passage. Said piston assembly is configured such that: when said piston assembly is stationary with respect to said cylinder, said piston remains in its engaging position with said piston engaging portion so as to close said transverse passage and block the fluid communication of said liquid reservoir with the outside of said container through said fluid passage; as said piston assembly is moved downwards relative to said cylinder, said piston deforms to offset from said engaging position so as to open said transverse passage in said piston engaging portion and allow said liquid reservoir to be communicate with the outside of said container through said fluid passage; as said piston assembly is moved upwards relative to said cylinder, said piston remains in said engaging position so as to close said transverse passage and block the fluid communication of said liquid reservoir with the outside of said container through said fluid passage. The piston assembly is featured in that said piston comprises a substantially cylindrical body with opposite open ends, said body, at its upper end, being formed with an annular inner flange radially protruding inwardly for fitting around a periphery of said piston engaging portion so as to releasably close said transverse passage, and, at its lower end, being formed with a sealing lip slidably abutting against a sidewall of said cylinder, said sealing lip being an end edge of a lower portion of said body which is gradually flared and thinned as said lower portion extends downwards, said body, at its upper end where said annular inner flange is formed, being formed with an annular notch along a peripheral edge of said body. 
         [0031]    With the above piston assembly of the invention, because the piston made of deformable elastomeric material, at its upper end where the annular inner flange is formed, is circumferentially formed with the annular notch along its peripheral edge, which reduces the rigidity of this portion of the piston, the increment of rigidity and difficulty to deform owing to the transverse inner flange of this portion of the piston is avoided, thereby improving the operation reliability of the piston and thus of the piston assembly. Moreover, the annular notch can be engaged by the inner sleeve of the integrated threaded sleeve-cylinder cap so as to form an annular sealing interface, which, in cooperation with the sealing between the lower sealing lip of the piston and the cylinder sidewall, helps to prevent the liquid in the container from leakage through the balancing vent formed in the cylinder sidewall. 
         [0032]    Additional features and advantages will be apparent as the exemplary embodiments of the invention are described below in reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
         [0033]      FIG. 1   a  is a sectional view of a liquid pump of the prior art; 
           [0034]      FIG. 1   b  is a sectional view of the liquid pump shown in  FIG. 1   a,  illustrating its piston assembly being in the course of moving downwards; 
           [0035]      FIG. 1   c  is a side elevation view of an integrated piston rod-piston engaging portion-piston head used in the liquid pump of  FIG. 1   a;    
           [0036]      FIG. 1   d  is a sectional view taken along the line A-A in  FIG. 1   c;    
           [0037]      FIG. 1   e  is a sectional view of the piston used in the liquid pump of  FIG. 1   a;    
           [0038]      FIG. 2   a  is a sectional view of another liquid pump of the prior art; 
           [0039]      FIG. 2   b  is a sectional view of the liquid pump shown in  FIG. 2   a , illustrating its piston assembly being in the course of moving downwards; 
           [0040]      FIG. 2   c  is a side elevation view of an integrated piston rod-piston engaging portion-piston head used in the liquid pump of  FIG. 2   a;    
           [0041]      FIG. 2   d  is a sectional view of the integrated piston rod-piston engaging portion-piston head taken along the line B-B in  FIG. 2   c;    
           [0042]      FIG. 2   e  is a sectional view of the piston used in the liquid pump shown in  FIG. 2   a;    
           [0043]      FIG. 3   a  is a sectional view of a liquid pump of an embodiment of the invention, with the press head shown in the upper stop position of its press stroke with respect to the integrated threaded sleeve-cylinder cap; 
           [0044]      FIG. 3   b  is a sectional view of the liquid pump shown in  FIG. 3   a,  illustrating its piston assembly being in the course of moving downwards; 
           [0045]      FIG. 4   a  is a side elevation view of a piston rod used in the liquid pump of  FIG. 3   a;    
           [0046]      FIG. 4   b  is a sectional view of the piston rod taken along the line C-C in  FIG. 4   a;    
           [0047]      FIG. 4   c  is a perspective view of the piston rod of  FIG. 4   a;    
           [0048]      FIG. 5   a  is a top view of the piston head used in the liquid pump of  FIG. 3   a;    
           [0049]      FIG. 5   b  is a sectional view of the piston head taken along the line D-D in  FIG. 5   a;    
           [0050]      FIG. 5   c  is a side elevation view of the piston head of  FIG. 5   a;    
           [0051]      FIG. 5   d  is a perspective view of the piston head of  FIG. 5   a;    
           [0052]      FIG. 6   a  is a side elevation view of the piston used in the liquid pump of  FIG. 3   a;    
           [0053]      FIG. 6   b  is a sectional view of the piston taken along the line E-E in  FIG. 6   a;    
           [0054]      FIG. 6   c  is a perspective view of the piston of  FIG. 6   a;    
           [0055]      FIG. 7  is a sectional exploded view of the piston assembly used in the liquid pump of  FIG. 3   a;    
           [0056]      FIG. 8  is a perspective view of the piston assembly of  FIG. 7 , with part of the piston being cut away to clearly show the configuration of the piston engaging portion; 
           [0057]      FIGS. 9-10  are views similar to the views of  FIGS. 3-4 , showing another embodiment of the piston rod and the piston head of the invention; and 
           [0058]      FIGS. 10-11  are views similar to the views of  FIGS. 3-4 , showing still another embodiment of the piston rod and the piston head of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0059]    Referring to  FIGS. 3-6 , a liquid pump of the invention mainly comprises a piston assembly  10 , a press head  20 , a cylinder  30 , an integrated threaded sleeve-cylinder cap  40 , a spring  50  and a lower non-return ball valve  60 . The liquid pump can, through its integrated threaded sleeve-cylinder cap  40 , be mounted at an opening (usually an open neck) of a container (not shown) of a liquid product. Of course, the threaded sleeve and cylinder cap can also be of the separate type. As the invention focuses on the piston assembly of liquid pump, other components of the liquid pump, which may be of the conventional structures, are not described in detail in this application. 
         [0060]    The piston assembly  10  is composed of a piston rod  11 , a piston head  12  and a piston  13  fitted around a piston engaging portion  15  between the piston rod  11  and the piston head  12 . Both the piston rod  11  and the piston head  12  can be made of a relatively rigid plastic material, for example, polypropylene, and the piston  13  is made of a relatively elastic elastomeric material, for example, NBR and thermoplastic polyurethane. The piston rod  11  is, in its center, formed with a fluid passage  14  having plural sections of length of different diameters. The piston rod  11  is, at its upper end, formed with a press head connecting portion  1101 , which is circumferentially formed with annular ribs  1107  on its periphery. The annular ribs are engaged with corresponding annular grooves formed in an inner circumference of a connecting tube  21  of the press head  20 , thereby connecting the piston rod  11  to the press head  20 . The piston rod  11 , at its lower end, is formed with a reduced diameter tube portion  1103 , which serves as a piston-engaging-portion formation part to constitute the piston engaging portion  15  which engages the piston  13 . A conical step surface  1104  is provided immediately adjoining and tapering towards the reduced diameter tube portion  1103 , and is provided by an annular projection formed on the piston rod  11  to provide a space for the piston  13  to deform towards the side of the conical step surface  1104 . The reduced diameter tube portion  1103  which constitutes the piston engaging portion  15  can, at its joint with the conical step surface  1104 , have a length of increased diameter tube portion  1108  with a lightly increased diameter. A shoulder  1102  is formed on the periphery of the piston rod  11  for engaging a corresponding portion provided on the integrated threaded sleeve-cylinder cap  40  so as to assist defining the upper stop position of press stroke of the press head  20  relative to the integrated threaded sleeve-cylinder cap  40 . 
         [0061]    The piston head  12  includes a base  121  and a guide stub  123  extending upwards from the base  121 . The guide stub  123  can be inserted and secured into a stub receiving hole  1105  that is formed at the lower end of the piston rod  11 , which stub receiving hole constitutes part of the fluid passage  14 . An annular groove  126  is circumferentially formed in the periphery of the guide stub  123  to be engaged with a corresponding annular rib  1106  formed in the stub receiving hole  1105  of the piston rod  11  so as to enhance fixture of the guide stub  123  in the stub receiving hole  1105 . A cylindrical reduced diameter boss  122  with its diameter being smaller than that of the base  121  is formed on the upper side of the base  121 , and serves as another piston-engaging-portion formation part  15 . The diameter of the reduced diameter boss  122  is greater than that of the guide stub  123 , and is the same as that of the reduced diameter tube portion  1103 . Therefore, when the piston head  12  is fitted to the piston rod  11 , the reduced diameter tube portion  1103  of the piston rod  11  and the reduced diameter boss  122  of the piston head  12  abut against each other to cooperatively constitute the piston engaging portion  15 . 
         [0062]    Two (or one or any other suitable number) longitudinal slots  124  are symmetrically formed in the outside of the guide stub  123  and constitute longitudinal passages within the stub receiving hole  1105 . Corresponding number of transverse passages  125  are formed at corresponding positions in the reduced diameter boss  122 , and extend inward from the periphery of the reduced diameter boss  122  so as to communicate with the longitudinal slots  124 , whereby the longitudinal slots  124  provide passages from the transverse passages  125  through the longitudinal slots  124  to the fluid passage  14 . For easy molding, the transverse passages  125  are integral with the longitudinal slots  124  without any clear boundary therebetween. The base  121  is provided, at its joint with the reduced diameter boss  122 , with a steep step surface  127  that is perpendicular to the longitudinal direction and is used to limit deformation of the piston  13  towards the side of the steep step surface  127 . 
         [0063]    The piston  13  comprises a substantially cylindrical body with opposite open ends. A radially inwardly extending annular flange  131  is formed at an upper end of the body, and forms an inner circumferential surface for fitting around the outer circumferential surface of the piston engaging portion  15  to releasably close off the transverse passages  125 . The body is formed at its lower end with a sealing lip  132  for slidably abutting against the sidewall of the cylinder  30 . The sealing lip  132  is a lower edge of a lower portion of the body which is gradually flared and thinned as the lower portion extends downwards. An annular notch  133  is circumferentially formed, at an upper end of the body where the annular inner flange  131  is formed, along a peripheral edge of the body. Provision of this annular notch  133  appropriately reduces the rigidity of this portion of the piston  13  and thus appropriately counteracts the increment of rigidity of this portion due to the presence of the annular flange  131 , thereby improving the deformability of the piston  13 . Of course, a conventional piston without this annular notch  133  (such as those discussed in the Background of the Invention) can also be used in the piston assembly of the invention. 
         [0064]    Now the operation of the liquid pump of the invention is described with reference to  FIGS. 3   a  and  3   b . The liquid pump can be assembled by fitting the upper end  33  of the cylinder  30  over the inner sleeve  42  of the integrated threaded sleeve-cylinder cap  40 , which is then mounted on a container of liquid product (not shown). In the case that the liquid pump is in non-operative condition, i.e., its press head  20  is not pressed, as shown in  FIG. 3   a , the press head  20  is at the upper stop position of press stroke relative to the integrated threaded sleeve-cylinder cap  40 , and the piston assembly  10  is in its uppermost position. In this condition, the shoulder  1102  of the piston rod  11  abuts against the corresponding shoulder formed on the integrated threaded sleeve-cylinder cap  40 , and the top surface of the annular notch  133  of the piston  13  is abutted by the lower end of the inner sleeve  42  of the integrated threaded sleeve-cylinder cap  40 . In this position, the balancing vent  32  formed in the sidewall of the cylinder  30  is below the abutment point between the piston  13  and the inner sleeve  42  while above the abutment point between the piston  13  and the sidewall of the cylinder  30 , that is, the balancing vent is closed between the two abutment points. Therefore, the liquid in the container can be prevented from entering into the cylinder  30  through the balancing vent  32  and then leaking out of the container. The piston  13  maintains properly engaging around the outer circumferential surface of the piston engaging portion  15  of the piston assembly  10 , so that the transverse passages  125  in the piston engaging portion  15  is closed, thereby preventing the liquid in the liquid reservoir  31  from flowing out of the container through the transverse passages  125 , the longitudinal slots  124  in the guide stub  123  and finally the fluid passage  14 . 
         [0065]    As the press head  20  is pressed to move downwards against the upward force of the spring  50 , as shown in  FIG. 3   b , the piston assembly  10  is driven by the press head  20  to move downwards. The sealing lip  132  of the piston  13  slides downwards while abutting the sidewall of the cylinder  30 . Owing to the combined effect of the downward driving force exerted on the annular inner flange  131  of the piston  13  by the piston rod  11  and the piston engaging portion  15  and the upward friction force exerted on the sealing lip  132  of the piston  13  by the sidewall of the cylinder  30 , as well as owing to the deformation space provided for the piston  13  by the conical step surface  1104  of the piston rod  11 , the piston  13  is forced to deform towards the side of the conical step surface  1104 , i.e., the side of the piston rod  11 . The deformation causes the annular inner flange  131  of the piston  13  to offset from the piston engaging portion  15 , so that the transverse passages  125 , which are originally closed by the piston  13 , is opened, thereby allowing the liquid in the liquid reservoir  31  to be pumped out of the container through the transverse passages  125 , the longitudinal slots  124  and the fluid passage  14  under the pumping action of the downward pressing motion of the piston assembly  10 . At this time, the lower non-return ball valve  60  maintains closed by the pressure in the cylinder  30  so as to prevent the liquid in the liquid reservoir  31  from reflowing downwards. 
         [0066]    Subsequently, as the press head  20  is released from the downward pressing force, the piston assembly  10  is moved upwards relative to the cylinder  30  by the upward force of the spring  50 . At this time, because the steep step surface  127  of the piston head  12  does not provide any space for the piston  13  to deform towards the piston head  12 , the piston  13  substantially cannot deform to allow the annular inner flange  131  of the piston  13  to offset from the piston engaging portion  15  to open the transverse passages  125 . As such, the communication from the liquid reservoir  31  to the outside of the container is blocked up. As the piston assembly  10  moves upwards, a negative pressure occurs in the liquid reservoir  31  of the cylinder  30  and thus makes the lower non-return ball valve  60  open, whereby a volume of the liquid of is drawn into the liquid reservoir  31  from the container through the valve  60 , getting ready for being pumped out in next cycle. 
         [0067]    Now production of the piston assembly  10  described above with reference to  FIGS. 3-6  is described in detail with reference to  FIGS. 7 and 8 . Firstly, the piston rod  11 , piston head  12  and piston  13  are formed separately by molding. Then, fitting the piston  13  over the guide stub  123  of the piston head  12 , and making the reduced diameter boss  122  wholly or partially enter into the central circular hole of the annular inner flange  131 . Thereafter, inserting and fixing the guide stub  123  of the piston head  12 , which has passed through the central circular hole of the piston  13 , into the stub receiving hole  1105  of the piston rod  11 , whereby the central circular hole of the piston  13  is properly fitted over the outer circumferential surfaces of both the reduced diameter boss  122  of the piston rod  11  and the reduced diameter tube portion  1103  of the piston rod  11 . A piston assembly thus assembled is shown in  FIG. 8 . 
         [0068]    The method is described above to produce the piston assembly of the embodiment shown in  FIGS. 3-6  (in which the piston engaging portion is provided by both the piston rod and the piston head), wherein the piston is placed over the piston head first and then the piston head is assembled to the piston rod. However, it is understood that it is feasible to place the piston over the piston rod first (as the piston rod has at least the piston-engaging-portion formation part suitable for receiving the piston), and then assemble the piston head to the piston rod. Anyway, as long as the piston rod or the piston head has a portion for receiving the piston, no matter the piston engaging portion is provided by both the piston head and the piston head or only on one of the two, it is feasible to place the piston over the piston rod or the piston head first and then assemble the piston head or the piston rod to the piston rod or the piston head. 
         [0069]    One embodiment of the invention is described above with reference to  FIGS. 3-8 , in which the piston engaging portion  15  is provided by both of the piston rod  11  and the piston head  12 . However, according to the invention, a whole piston engaging portion can be provided by only one of the piston rod and the piston head. 
         [0070]    Another embodiment of the invention is shown in  FIGS. 9 and 10 , in which the piston engaging portion is provided by only the piston rod. In this embodiment, the configuration of both the piston rod and the piston head is substantially the same as that of the embodiment of  FIGS. 4 and 5 , therefore, the like portions are denoted by the like reference numbers but with an additional inverted comma. In comparison to the embodiment shown in  FIGS. 4 and 5 , the difference only lies in that: in the embodiment of  FIGS. 9 and 10 , on the base  121 ′ of the piston head  12 ′, there is not provided a reduced diameter boss  122 , and a full-size reduced diameter tube portion  1103 ′ is provided at the lower end of the piston rod  11 ′. As such, the piston engaging portion  15  is made up of the reduced diameter tube portion  1103 ′ of the piston rod  11 ′ singly, unlike the case of the embodiment of  FIGS. 4-5  where the piston engaging portion is made up of a combination of the reduced diameter boss  122  on the piston head  12  and the reduced diameter tube portion  1103  on the piston rod  11 . At the lower end of the reduced diameter tube portion  1103 ′, there are formed two transverse passages  1110  which are diametrically opposite and extending from the outer circumferential surface to the inner circumferential surface of the reduced diameter tube portion  1103 ′. The transverse passages  1110  can be made with a width substantially the same as the width of the longitudinal slots  124 ′ in the piston head  12 ′. In the condition that the piston head  12 ′ is assembled together with the piston rod  11 ′, the lower end of the reduced diameter tube portion  1103 ′ of the piston rod  11 ′ abuts against the upper side of the base portion  121 ′ of the piston head  12 ′, with the two transverse passages  1110  substantially aligned with the two longitudinal slots  124 ′ so as to make them communicate with each other. Thus, a piston assembly thus assembled presents substantially the same structure as that shown in  FIG. 8 . 
         [0071]    Still another embodiment of the invention is shown in  FIGS. 11 and 12 , in which the piston engaging portion is provided only by the piston head. In this embodiment, the configuration of both the piston rod and the piston head is largely the same as that of the embodiment of  FIGS. 4 and 5 , therefore, the like portions are denoted by the like reference numbers, but with additional double quotation marks. In comparison to the embodiment shown in  FIGS. 4 and 5 , the difference only lies in that: in the embodiment of  FIGS. 11 and 12 , at the lower end of the piston rod  11 ″ there is not provided a reduced diameter tube portion  1103 , and a full-size reduced diameter boss  122 ″ is provided on the base  121 ″ of the piston head  12 ″. As such, the piston engaging portion  15  is made up of the reduced diameter boss  122 ″ provided on the piston head  12 ″ singly, unlike the case of the embodiment of  FIGS. 4-5  where the piston engaging portion is made up of a combination of the reduced diameter boss  122  on the piston head  12  and the reduced diameter tube portion  1103  on the piston rod  11 . At the lower portion of the reduced diameter boss  122 ″ and in the positions substantially aligned with the longitudinal slots  124 ″, there are formed transverse passages  125 ″ extending from the outer circumferential surface of the reduced diameter boss  122 ″ inwardly so as to communicate with the longitudinal slots  124 ″. The transverse passages  125 ″ can be made with a width substantially the same as the width of the longitudinal slots  124 ″ on the piston head  12 ″. In the condition that the piston head  12 ″ is assembled together with the piston rod  11 ″, the upper side of the reduced diameter boss  122 ″ of the piston head  12 ″ abuts the lower end of the piston rod  11 ″, therefore, the reduced diameter boss  122 ″ is immediately adjacent to the conical step surface  1104 ″ on the piston rod  11 ″. An assembled piston assembly thus assembled presents substantially the same structure as that shown in  FIG. 8 . 
         [0072]    In the exemplary embodiments of the invention described above, for           the sake of simplifying description, many aspects of the invention are implemented in combination in one and the same embodiment. However, those skilled in the art can understand that these aspects of the invention can be separately implemented in different embodiments so as to attain their respective objects. For example, in each of the embodiments described herein, the piston assembly, which is manufactured by separately providing the piston rod and the piston head and then assembling them together, in accordance with one aspect of the invention is used with the piston with the annular notch in accordance with another one aspect of the invention, but it is possible to implement the two in separate embodiments so as to realize their respective functions. Of course, those skilled in the art can also understand that the combined use of the technical solutions of these two aspects of the invention may result in even better technical superiority. 
         [0073]    In addition, various changes and modifications can be figured out by those skilled in the art without departing from the spirit of the invention and in the scope thereof For example, in the embodiments described herein, the piston rod and the piston head are configured so that a portion of the piston head is inserted and fitted into a portion of the piston rod, but it is also feasible to configure the piston rod and the piston head so that a portion of the piston rod is inserted and fitted into a portion of the piston head. For another example, in the embodiments described herein, the number of each of the longitudinal passages and the transverse passages in the piston head is two, but the number can be one or more than two. For still another example, in the embodiments described herein, the transverse passages in the piston engaging portion are provided by the reduced diameter boss of the piston head, but the transverse passages can also be provided by both the reduced diameter boss and the reduced diameter tube portion, or provided only by the reduced diameter tube portion of the piston rod. Clearly, these changes will not influence the implementation of the invention and the achievement of the invention object. Therefore, the scope of the invention is not limited to the specific embodiments described herein, but is to be rationally defined by the claims.