US3672122A

US3672122A - Bag holder and expander unit

Info

Publication number: US3672122A
Application number: US56364A
Authority: US
Inventors: Leonard Berger; William E Fitzpatrick; James H Mcginley; Adolf Pfund; Ira G Schwartzberg
Original assignee: International Playtex Inc
Current assignee: Playtex FP Inc; INTERN PLAYTEX CORP
Priority date: 1970-07-20
Filing date: 1970-07-20
Publication date: 1972-06-27
Anticipated expiration: 1989-06-27

Abstract

A bag expander is provided for mounting an open ended flexible bag positioned thereon on to a hollow, reusable holder which terminates in an annular edge. The expander has a base member and a head member which are adapted to move axially with respect to each other. A plurality of fingers are housed in the base member and biased toward the head member such that the head member provides a camming action on the fingers when axial movement takes place. A bag is placed over the head member and ends of the fingers of the expander, which is positioned on the holder and depressed, thereby camming the fingers outwardly and thus folding the bag outwardly and over the edge of the holder. Excursion limitation means are provided to limit the relative movement between the head member and base member of the expander to prevent the fingers from contacting the bag at the edge of the holder during the bag seating operation to prevent tearing of the bag, especially if the expander is rotated during the seating operation. In a preferred embodiment the excursion limitation means is comprised of a skirt extension of the head member which is of a predetermined length and prevents further movement when the skirt reaches the base member. The skirt is preferably of such a configuration so as to perform the additional function of supporting the fingers on their initial movement. An improved base construction is provided having compartments which seat the fingers for pivotal movement therein, as well as preventing lateral movement of the fingers, which simplifies the fabrication of the expander.

Description

United States Patent Berger et al.

[ 51 June 27,1972

[54] BAG HOLDER AND EXPANDER UNIT [72] inventors: Leonard Berger, Ridgewood; William E. Fitzpatrick, Oakland, both of N..l.; James H. McGlnley, Smyrna, Del.; Adolf Plund, Milford, N..I.; Ira G. Schwartzberg, Dover, Del.

International Playtex Corporation, New York, NY.

July 20, 1970 [73] Assignee:

[22] Filed:

[2i] Appl.No.: 56,364

[52] US. Cl ..53/384, 53/390, 93/36.0l

Primary Examiner-Theron E. Condon Assistant Examiner Neil Abrams Attorney-Stewart J. Fried and Jeffrey A. Schwab ABSTRACT A bag expander is provided for mounting an open ended flexible bag positioned thereon on to a hollow, reusable holder which terminates in an annular edge. The expander has a base member and a head member which are adapted to move axially with respect to each other. A plurality of fingers are housed in the base member and biased toward the head member such that the head member provides a camming action on the fingers when axial movement takes place. A bag is placed over the head member and ends of the fingers of the expander, which is positioned on the holder and depressed, thereby camming the fingers outwardly and thus folding the bag outwardly and over the edge of the holder. Excursion limitation means are provided to limit the relative movement between the head member and base member of the expander to prevent the fingers from contacting the bag at the edge of the holder during the bag seating operation to prevent tearing of the bag, especially if the expander is rotated during the seating operation. in a preferred embodiment the excursion limitation means is comprised of a skirt extension of the head member which is of a predetermined length and prevents further movement when the skirt reaches the base member. The skin is preferably of such a configuration so as to perform the additional function of supporting the fingers on their initial movement. An improved base construction is provided having compartments which seat the fingers for pivotal movement therein, as well as preventing lateral movement of the fingers, which simplifies the fabrication of the expander.

l2 Claims, 10 Drawing Figures BAG HOLDER AND EXPANDER UNIT BACKGROUND OF THE INVENTION This invention relates to a bag holder and expander unit for mounting a disposable bag in a reusable holder.

The present invention provides improvements over die structure shown in U.S. Pat. No. 2,900,779 granted Aug. 25, 1959 to T. R. Baxter et al. and entitled Tool for Opening and Expanding the Mouth of a Flexible Receptacle." In the aforesaid patent, the open end of a thin walled flexible plastic bag-like container is positioned on an expander device and inserted in the open end of a reusable rigid holder whose opening terminates in an annular edge to form the top edge of the bottle holder. The expander functions when in operative contact with the bag and holder to expand the open end of the bag outwardly and over the edge of the holder, thereby attaching the bag to the holder for subsequent filling and use. When used as a nurser unit, a nipple fits over the bag and top edge of the bottle holder, for example as shown and described in U.S. Pat. No. 3,075,666. The expander was characterized by having a base member housing a plurality of radial pivoting fingers and a head member mounted thereon in a manner to provide relative axial movement between the head and base members. The fingers were biased inward on the head member and axial movement between the head and base member caused the free ends of the fingers to be cammed outward by the head member. Thus, with the bag placed over the head member and the free ends of the radial pivoting fingers, with the head of the expander positioned on the open end of the holder, on depression, the fingers folded the bag outwardly and over the outer edge of the holder and came to rest on the bag bearing against the outer edge of the holder. The propensity of some users at this point is to rotate the expander to make sure the bag is properly seated. Although unnecessary, such rotation by the user caused the outer portions of the fingers to grip and tear the bag along the outer edge of the holder. Such a result is particularly undesirable when the unit is used as a nurser since the formula could leak out.

In the aforesaid expander, the fingers were positioned in the base member on a plurality of upstanding molded pins which complicated the molding of the base element as well as requiring openings in the fingers to accommodate the pins. The above cited expander also offered no support for the intermediate portion of the fingers on their initial movement over the head surface, putting a substantial stress on the fingers. The fingers had to be made of a very strong material (e.g., steel), which in practice was difficult to fabricate.

Accordingly, it is an object of the present invention to provide an improved bag holder and expander unit which overcomes the aforesaid problems.

A further object of this invention is to provide an improved bag holder and expander unit which is easier to fabricate and less likely to inflict damage on the bag during installation.

Another object of this invention, in one illustrative embodiment thereof, is to provide an improved expander construction which supports the initial excursion of the fingers to remove undue stress thereon, and permits their manufacture from a more easily fabricated material.

SUMMARY OF THE INVENTION In carrying out this invention in one illustrative embodiment thereof, an expander device is provided having a base member and a head member which are mounted to move axially with respect to each other. The base member carries a plurality of fingers which are biased toward the head member which provides the camming action on the fingers on relative axial movement between the head and the base members. Excursion limitation means are provided which predeterrninedly limits the relative movement between the head and the base members, thereby preventing the fingers from contacting the upper outer edge of the holder on which the thin walled plmtic bag has been interposed during the bag seating operation. If the expander is then inadvertently rotated in the bag seating operation, the fingers are prevented from contacting a bag along the outer edge of the holder, thus preventing the gripping of the bag by the fingers and the tearing which may result therefrom. In a preferred embodiment, an annular skirt of predetermined length depends from the head member for limiting the movement between the head member and the base member when the skin contacts the base member. The annular skirt preferably also provides the additional function of supporting the fingers on an intermediate area thereof during their initial movement along the head against static friction. The base member is provided with side walls of sufllcient height to form compartments for seating the fingers in the base for pivotal movement therein. The compartments are shaped to conform to the shape of the base of the fingers which are seated therein to prevent lateral movement of the fingers in the compartments.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is an exploded perspective view of the bag holder, bag, and expander device embodied in this invention.

FIG. 2 is a crosssectional view of FIG. I which illustrates the bag holder and bag positioned on the expander during the initial step of inserting the bag on the holder.

FIG. 2a is a partial cross-sectional view of FIG. 2 showing a finger in contact with the annular skirt of the expander during the initial movement in the expander.

FIG. 3 is the cross-sectional view shown in FIG. 2 with the expander fully depresed, showing the bag being fully mounted on the holder.

FIG. 4 is a top view of the base of the expander device.

FIG. 5 is a perspective view of a pivotal finger utilized in the expanders of FIGS. 2 and 3.

FIG. 6 is a cross-sectional view taken along lines 6-6 of FIG. 3.

FIG. 7 is a cross-sectional view of the holder and expander unit similar to FIG. 3 showing another embodiment of an excursion limitation means.

FIG. 8 is a perspective view of a pivotal finger utilized in the embodiment shown in FIG. 7.

FIG. 9 is a cross-sectional view of the bag holder and expander unit showing another embodiment of an excursion limitation means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following descriptions the bag holder and expander unit embodied in this invention may at times be described in connection with its use as part of a nursing unit, but it will be understood that the invention may be utilized in any application requiring a flexible bag to be mounted over the rigid end of a bag holder. In the following description, like elements will be identified by the same reference numerals.

Turning now to FIG. 1, the bag holder and expander unit are shown in perspective, comprising the three principal components, which are a receptacle or bag holder 10, a flexible bag I6, and an expander device 20. The bag holder 10 is generally cylindrical and hollow, and terminates at its open end in an annular edge 14. An annular flange 12 is provided to accommodate the mounting of a nipple thereon for a nurser bottle of the type shown and described in U.S. Pat. No. 3,075,666. Of course the flexible bag 16, being mounted in the open end of the holder 10, is also positioned over the annular flange 12, but it will be appreciated that, although the invention is described in connection with the use of an annular flange, that it is not necessary, and that the flexible bag would be capable of being mounted over the open end of other types of holders which have an end surface on which the bag may be mounted. Flexible bag 16 is a thin walled disposable bag having an open end 18 formed by the edge I7 which is expanded and mounted on the annular edge 14 of the holder 10.

The expander 20, as shown in FIGS. I and 2, includes a base 22, a head member 40 having a conical nose 45, and radially pivotal fingers 60 which are seated in the base 22 and ride on the head member 40. The aforesaid parts are preferably of molded plastic, and preferably of the thermosetting variety for use in a nurser unit so that it may withstand boiling water temperatures for sterilization. The base 22 of the expander, as shown in FIG. 4, has a central post 24 with an opening 23 therein, and is separated into pie-shaped compartments 25 by upstanding side walls 26. These pie-shaped compartments have sufficient depth and a shape which conforms generally to the shape of the bases of the fingers 60, such that when the fingers 60 are seated therein, minimum lateral movement of the fingers takes place. Each compartment 25 is provided with projections 28 which act as pivot points for the fingers 60. As will be seen in FIG. 2, the opening 23 of the central post 24 has a guide pin retainer means 29 firmly mounted therein. A guide pin 30 is mounted in the guide pin retainer means 29 by threading or another suitable means. The guide pin 30 extends through a channel 42 in the head member 40, and has a pinhead 32 thereon for retaining the head member 40 on the guide pin 30. The guide pin 30 carries a spring 34 which in its extended position biases the head member 40 against the guide pinhead 32. The spring 34 is separated from the base member 22 by a bellville spring 35 which is also mounted on the guide pin 30 below the spring 34. The bellville spring 35 rests on projections 61 of the fingers 60, thereby transmitting the force of spring 34 to bias the fingers 60 inward toward the head member 40. As will best be seen in FIG. 6, the head member is hollow, but is provided with reinforcing quadrature ribs 44.

The head member 40, as seen in FIG. 2, has a conical nose which terminates at its vertex in an opening of a shape to accommodate the pinhead 32 which retains the head member 40 on the guide pin 30. The conical nose 45 terminates at its base end in an annular surface 46. The annular surface 46 has a diameter slightly less than the opening of the bag holder 10. The annular surface 46 forms a shoulder 47 with a first annular ridge 48 near the base of the conical nose 45. The annular shoulder 47 is adapted to rest on the annular edge 14 of the bag holder 10 when the expander 20 is positioned on the holder 10 for performing the bag seating operation. The first annular ridge 48 is separated by neck portion 50 from a second annular ridge 52, and the head member 40 terminates in an annular skirt 54. The annular skirt 54 is of predetermined length with a free end 55 and provides support for the fingers in their initial movement, as will be more fully described hereinafter.

The radially pivotal fingers 60, as best shown in FIG. 5, have a base 62 with a configuration which conforms to the shape of the pie-shaped compartments 25 in which they are seated with a minimum of lateral play to prevent any lateral shifting or lateral movement therein. Prior expander structures, as dis closed in the U.S. Pat. No. 2,900,779 required molded pins which fit through openings in the base for positioning and seating the fingers on the base. The necessity for such pins complicated the molding of the base member. The side walls 26 of the compartments 25 have sufficient height to retain the bases 62 of the fingers in the base member without the need for additional molded parts, which simplifies the fabrication of the base member.

Projections 61 on the ends of the base of the fingers 60 engage the bellville spring 35 which holds the fingers 60 in their compartments 25 by the force transmitted from the spring 34 against the bellville spring 35. This force also biases the fingers about pivot projections 28 inward toward the head member 40. The base members 62 of the fingers 60 meet with an upright body portion 63 to form a pivot surface 65, forming a generally V-shaped configuration which sits on the base pivot projection 28.

Reinforcing triangular shaped ribs 64 are provided to add rigidity to the fingers 60 The free end portion ofthe fingers 60 have an intermediate surface 67, a groove 66, and an end portion 68 which are shaped generally to conform to the contour of the annular skirt 54, the second annular ridge 52, and the neck portion 50 of the head member 40 respectively.

With this configuration and the expander in its static, or extended position, as shown in FIG. 2, the fingers 60 are positioned with their free ends 68 resting on the neck portion 50 of the head 40. The groove 66 overlies the second annular ridge 52 with the intermediate surface 67 of the finger contacting the skirt 54 below the annular ridge 52. On the initial depression of the expander, as shown in FIG. 2a, support for an appreciable portion of the intermediate surface 67 of the fingers is provided by the annular skirt 54, while the initial static friction is being overcome, to prevent any appreciable stress from being applied to the fingers which ride over the camming surface provided by the annular ridge 52. The free ends 68 of the fingers 60 are also provided with a plurality of ridges 69 which perform a bag-gripping function during the seating operation.

In performing the bag seating and mounting operation, the open end 18 of the flexible bag 16 is positioned with its outer edges 17 extending over the conical nose 45, the first annular ridge 48, and the ridges 69 on the free ends 68 of the fingers 60, as shown in FIG. 2. The annular edge 14 of the bag holder 10 rests on the annular shoulder 47 of the head member 40.

As is illustrated in the drawings, the holder 10, bag 16, and expander 20 are inverted to provide a horizontal surface for the base 22 of the expander 20, but it will be understood that the reverse, or right-side-up configuration can be utilized if it is more convenient for the user. As initial downward pressure is applied to the bag holder, as was pointed out with respect to FIG. 2a, the annular skirt 54 contacts the intermediate portion 67 of the fingers 60 to provide support while the static friction of the expander is being overcome. On full depression, as will be seen in FIG. 3, the head member 40 has slide down the guide pin 30, compressing the spring 34. At the same time radial fingers 60 are pivoted radially outward by the camming action of the second annular ridge surface 52 on the head member 40. The fingers 60 are pivoted about surface 65 on the projections 28 in the b$e 22, with the projections 61 of the fingers bearing on the under side of the bellvile spring 35, slightly forcing the spring 34 upward by still retaining the fingers in their compartments 25. As the fingers are forced outward, the bag edge 17 is expanded and forced outwardly and over the annular edge 14 of the bag holder It). In the prior art expander, complete depression of the expander allowed the free end portions 68 of the fingers 60 to contact the bag 16 along the annular edge 14 of the holder 10. It was found that after a user had reached this point, there was a propensity to rotate the expander to make sure that the bag was firmly seated, although this procedure was in fact unnecessary and undesirable because of the tendency of the fingers to grip and tear the bag along the edge 14 of the holder 10. In accordance with the present invention, this undesirable contacting of the bag along edge 14 is prevented by the annular skirt 54. The length of the annular skirt 54 is of a predetermined amount to closely control the extent of the relative movement between the head member 40 and the base 22. The free end 55 of the skirt 54 coming into contact with the compartment sides 26 of the base 22 governs the extent of movement between the head member 40 and the base 22, and accordingly the relative movement of the fingers 60 with respect to the holder 10 so that they are now allowed to contact the bag 16 on the annular edge 14 of the holder 10. That is, the skirt acts as a camming stroke stop providing selective orientation between the fingers 60 and the head 40. Note on FIG. 3 the distance marked "x between bag 16 overlying the annular edge 14 of the holder [0 and the base of the groove 66 on the fingers. It should be pointed out that the bag edge 17 is expanded on the outward pivotal movement of the fingers as the fingers are moved over the camming surface formed by the second annular ridge 52 of the head member, and then moved inwardly by the bias provided by the spring 34 through the bellville spring 35. This inward excursion releases the expanded bag and folds it over the annular flange I2 of the holder 10. The contraction of the bag holds it firmly lapped over the flange I2 of the holder 10. As has been pointed out previously herein, once the expander has been fully depressed, the tendency on die part of the user to rotate the expander on the holder no longer allows the fingers 60 to grip and tear the bag along the outer edge 14 of the holder 10, as was the case in the prior art where the extent of the excursion of the fingers was not so limited.

FIGS. 7 and 8 show another embodiment of the excursion limitation means. In this embodiment the fingers 60 are provided with protrusions 72 on an intermediate portion 67 of the body 63 of the fingers 60. The protrusions 72 are spaced a predetermined distance below the bottom ridge S2 of the head member to limit the relative movement between the head member 40 and the base 22 by the head member contacting the protrusions 72. That is, the second annular ridge 52 of the head member 40 on its downward excursion contacts the protrusions 72 on the fingers 60 to define the extent of movement between the head member 40 and the base 22. As in the case of the embodiment of HO. 3, it will be noted that the fingers do not contact the annular edge 14 along the area previously noted with an ,r" with the bag mounted thereon to thus prevent tearing of the bag, which may result from such contact and rotation of the expander. Although the embodiment of FIG. 7 is illustrated without an annular skirt as shown in FIG. 3, it should be appreciated that a shorter annular skirt could be provided as part of the head 40 to provide the support function for the fingers in their initial movement along the head 40.

FIG. 9 shows another embodiment in which the stopping action is provided by a preselected spring 70 which in total compression has a predetermined length which serves to limit the extent of the movement of the head member 40 toward the base so as to prevent the end portions 68 of the fingers 60 from contacting a bag which is mounted over the upper edge 14 of the holder 10. Hence, the predetermined compression length of spring 70 serves as the excursion limiting means to present bag tearing, as in the case of the previous two embodiments.

The bag holder and mounting unit as described above avoids the problem of damage to the bag during the seating operation, especially if the expander is inadvertently rotated during that procedure. This reliability is particularly important to the type of non-skilled user who is encountered when the bag holder and expander unit are utilized as a nurser unit. Further, the all-plastic construction of the fingers in addition to the base, and movable member, as well as the simplified molding of the base member and improved seating of the fingers in the base member, has reduced the expense of the expander device while increasing its reliability and improving its performance.

Since other modifications, varied to fit particular operating requirements and other environments will be apparent to those skilled in the art, this invention is not considered limited to the examples chosen for purposes of disclosure, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention.

We claim:

I. In combination, a supporting holder and expander unit for mounting the flexible open end of a container over the rigid open end of the supporting holder, comprising:

a hollow supporting holder terminating in an open end having a rigid annular edge;

a container formed of a material having a flexible open end, said container adapted to be mounted in said holder with its open end adapted to fit outwardly and over said annular edge;

an expander for mounting said container open end on said annular edge, said expander including;

a base member, a head member, and a plurality of radially movable fingers; said head member including a portion adapted to fit on said annular edge,

guide means between said head member and said base member slideably joining said head member and said base member for biased relative axial movement of said members towards and away from each other;

said plurality of radially movable fingers positioned between said base member and the outer periphery of said head member,

said fingers including free ends adapted to accept the flexible open end of said container;

means for biuing said fingers inwardly towards the periphery of said head member whereby said head member acts as a camming surface for said fingers during said relative axial movement between said head member and said base member; and

excursion limitation means for limiting the extent of relative axial movement between said base member and said head member towards each other, said excursion limitation means operatively limiting said extent of movement such that the free end portions of said fingers are spaced away from the annular edge of said holder and avoid contact with the underlying flexible material of the container open end at the point of greatest inward axial movement.

2. The combination set forth in claim 1 wherein said excur sion limitation means comprises an annular skirt of predetermined length on said head member, the free end of said skirt normally spaced apart from said base member, said free end of the skirt contacting said base member at the extreme perm issible excursion of said head member towards said base member to limit the extent of axial movement therebetween.

3. The combination as set forth in claim I, wherein said head member includes an annular skirt for supporting an appreciable intermediate portion of said fingers during initial movement of the fingers from a static condition.

4. The combination as set forth in claim 2, wherein said annular skirt includes side walls for supporting an appreciable intermediate portion of said fingers during initial movement of the fingers from a static condition.

5. The combination as set forth in claim 1 wherein said excursion limitation means comprises a plurality of protrusions spaced equidistant a predetermined height above the base and along the inner surface of a plurality of said movable fingers for contacting said head member and limiting the relative axial movement between said base and said head member.

6. The combination set forth in claim 1 wherein said biasing means between said base member and said head member includes a compression spring and wherein said excursion limitation means comprises said spring having a predetermined length under full compression for limiting the extent of relative movement between said head and base members.

7. The combination set forth in claim I wherein said fingers are made of a thennosetting molded plastic material and have annular ridges on the outer surfaces of said free end portions.

8. The combination set forth in claim 1 wherein said base member has a plurality of upstanding side walls forming compartments which seat said fingers therein, and wherein each of said fingers include a base portion, said compartments having a shape which generally conforms to the shape of the base portions of said fingers and said side walls having sufficient height to minimize lateral movement of said fingers in said base.

9. The combination set forth in claim 6 wherein projections are provided in each of said compartments in said base member which act as pivot points for said fingers.

10. An expander unit for receiving the flexible open end of a container and mounting same outwardly and over the rigid annular edge of a supporting holder comprising:

a base member, a head member and a plurality of radially movable fingers, guide means for operatively connecting said base member to said head member in axially spaced relationship,

biasing means for normally maintaining the axial spacing between said base and head members, with said members movable towards each other against the force of said biasing means said radially movable fingers having a seated end mounted in said base member, and a free end biased against an outward surface of said head member,

11. An expander unit as set forth in claim 10 wherein said annular skirt includes side walls for supporting an appreciable intermediate portion of said fingers during initial movement of the fingers from a static condition.

12. An expander unit as set forth in claim 10, wherein said base member has a plurality of side walls forming compartments which seat said fingers therein, said compartments having a shape which generally conforms to the shape of the seated end of said fingers and said side walls having sufficient height to minimize lateral movement of said fingers in said base.

i i i

Claims

1. In combination, a supporting holder and expander unit for mounting the flexible open end of a container over the rigid open end of the supporting holder, comprising: a hollow supporting holder terminating in an open end having a rigid annular edge; a container formed of a material having a flexible open end, said container adapted to be mounted in said holder with its open end adapted to fit outwardly and over said annular edge; an expander for mounting said container open end on said annular edge, said expander including; a base member, a head member, and a plurality of radially movable fingers; said head member including a portion adapted to fit on said annular edge, guide means between said head member and said base member slideably joining said head member and said base member for biased relative axial movement of said members towards and away from each other; said plurality of radially movable fingers positioned between said base member and the outer periphery of said head member; said fingers including free ends adapted to accept the flexible open end of said container; means for biasing said fingers inwardly towards the periphery of said head member whereby said head member acts as a camming surface for said fingers during said relative axial movement between said head member and said base member; and excursion limitation means for limiting the extent of relative axial movement between said base member and said head member towards each other, said excursion limitation means operatively limiting said extent of movement such that the free end portions of said fingers are spaced away from the annular edge of said holder and avoid contact with the underlying flexible material of the container open end at the poiNt of greatest inward axial movement.

2. The combination set forth in claim 1 wherein said excursion limitation means comprises an annular skirt of predetermined length on said head member, the free end of said skirt normally spaced apart from said base member, said free end of the skirt contacting said base member at the extreme permissible excursion of said head member towards said base member to limit the extent of axial movement therebetween.

3. The combination as set forth in claim 1, wherein said head member includes an annular skirt for supporting an appreciable intermediate portion of said fingers during initial movement of the fingers from a static condition.

7. The combination set forth in claim 1 wherein said fingers are made of a thermosetting molded plastic material and have annular ridges on the outer surfaces of said free end portions.

10. An expander unit for receiving the flexible open end of a container and mounting same outwardly and over the rigid annular edge of a supporting holder comprising: a base member, a head member and a plurality of radially movable fingers, guide means for operatively connecting said base member to said head member in axially spaced relationship, biasing means for normally maintaining the axial spacing between said base and head members, with said members movable towards each other against the force of said biasing means said radially movable fingers having a seated end mounted in said base member, and a free end biased against an outward surface of said head member, said head member outward surface configurated to cam said fingers radially outward as said head member is moved towards said base member, said head further including an excursion limiting skirt of a predetermined length extending towards said base member, said skirt including a free end which is normally spaced apart from said base member by a predetermined amount, said free end of the skirt contacting said base member at the extreme permissible excursion of said head member towards said base member to limit the extent of axial movement therebetween to said predetermined amount.