EP0684108A1

EP0684108A1 - Fastener driving apparatus with improved pneumatic operation

Info

Publication number: EP0684108A1
Application number: EP95107279A
Authority: EP
Inventors: Umberto Monacelli
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-05-20
Filing date: 1995-05-13
Publication date: 1995-11-29
Also published as: CA2149698A1

Abstract

A pneumatic powered fastener driving apparatus is disclosed as comprising a housing (11), and a cylinder (22) disposed within the housing (11) to divide the housing (11) into first and second compressed air chambers (19,21). A piston (23) is disposed within the cylinder (22) to divide the cylinder (22) into first and second portions and movable therein between a first rest position and a second position remote therefrom. A fastener driving element (28) connected to the piston (23). A valve assembly (25) is movable between a closed position for blocking communication between the first compressed air chamber (19) and the first cylinder portion and an open position supplying compressed air from the first compressed air chamber (19) to the first cylinder portion. A passage (33) is formed between the periphery of the piston (23) and the inner wall surface (29) of the cylinder (22) for providing when the piston (23) is disposed at the second remote position the only effective communication for the flow of compressed air from the second compressed air chamber (21) to the first cylinder portion.

Description

Field of the Invention:

This invention relates to a fastener driving apparatus, and more particularly to an improved pneumatic operation of such an apparatus.

Background of the Invention:

Pneumatic apparatus or tools for driving fasteners such as nails, staples, brads and the like are commonly used in the commercial work place. All of these tools have standard components comprising a source of compressed air, a cylinder in which a piston and driver assembly is driven reciprocally through a drive stroke from a rest position and a return stroke back to its rest position, a valve assembly to provide pressurized air to the piston and a fastener carrier to position the fastener underneath the driver prior to initiating its drive stroke. The completion of the drive stroke and the return stroke constitutes the cycle of the tool.
Most tools are operated by positioning the tool in contact with the workpiece and manually pulling a trigger which in turn operates the valve assembly that provides compressed air to the top surface of the piston. When the tool is used as a stationary device, the trigger is replaced by a remote actuator.
As the piston approaches the end of its drive stroke, a return air chamber is pressurized to provide air for the return stroke of the piston and driver assembly. After the trigger is released, the valve assembly closes blocking the flow of air into the top of the cylinder and in turn opens an exhaust port to release the air in the upper portion of the cylinder above the piston to the ambient atmosphere. The stored air within the return chamber acts upon the under surface of the piston to return it in its return stroke to its rest position at the uppermost portion of the cylinder.
To provide enough power to drive the fastener, the air must enter the cylinder above the piston quickly. To accomplish this, the valve assembly normally effects two functions. A main valve is located directly above the top of the cylinder and is shifted from a closed position pneumatically by a trigger valve. By utilizing air pressure, the main valve can be held closed tightly and then opened with a snap action when air pressure on a surface of the main valve is reduced.
As the air pressure is applied to the top surface of the cylinder above the piston, the piston moves downward rapidly developing power to drive the fastener into a workpiece. Near the end of the drive stroke, the underside of the piston strikes an elastic element, which is used to absorb any excess energy not utilized in driving the fastener. To prepare for the next fastener to be driven, the piston and driver assembly must be returned in its return stroke to its rest position.
Piston return has been accomplished in several ways. U.S. Patent No. 4,252,261 describes a piston constructed with two external seals operating against a cylinder having two diameters. The piston also has a space between the two external seals and the space is subjected to constant pressure. Since the upper portion of the cylinder is larger in diameter than its lower portion, the piston is always subjected to an upward force whereby the piston returns as soon as the air pressure on the top surface of the piston is eliminated.
U.S. Patent Nos. 4,784,308 and 5,020,712 disclose the use of secondary or additional valves that must be shifted to provide pressurized air to the under surface of the piston. Another fastener driving tool, which is most commonly used, has a chamber that can be pressurized during the drive stroke.
In order to charge the return chamber, a first set of one or more upper holes is placed through the cylinder wall, which allows air to flow into the chamber after the piston passes thereby in its drive stroke. A second set of holes is located in the cylinder wall below that position, which the piston may not pass. The second set of holes provides communication between the return chamber and the under surface of the piston. Since the upper surface area of the piston, which is subjected to pressure, is larger than that of the exposed under surface, due to the piston resting on the shock absorbing elastic element, the piston remains down until the air on its upper surface is reduced when the main valve is shifted to its exhaust position.
The fastener driving tool described in U.S. Patent No. 4,747,338 includes an external O-ring disposed over the first set of upper holes to act as a check valve, whereas the tool described in U.S. Patent No. 5,181,450 has no external seal over the holes.
The force needed to drive the piston and driver through its return stroke to its uppermost, rest position is much less than that to drive the fastener in its drive stroke into the workpiece. Accordingly it is not necessary to charge the return chamber to the same pressure as that applied to the top surface of the piston during the drive stroke of the tool cycle. Since the return chamber is in communication with pressurized air from a power source only during the time that the piston is below the first set of upper holes in the cylinder, the pressure within the return chamber reduces gradually as the piston returns to its uppermost position.
After the piston reaches its upper rest position, any remaining pressure in the return chamber must be exhausted around the driver and out the bottom of the tool housing. This is necessary to prevent pressure from being applied to the under surface of the piston when the next cycle is started; otherwise, the back pressure would reduce the driving power of the tool. Determining the size and location of the upper holes in the cylinder is difficult to obtain correct tool function when the pressure of the compressed air source is adjusted for various type of fasteners.
Several fastener driving tools have been devised, but all have certain drawbacks. If the holes are located just above the piston after it has completed its drive stroke, the piston and driver assembly will cycle so fast that there is not sufficient time to charge the return chamber to a sufficient pressure. Enlarging these holes or moving them higher allows more air to enter into the return chamber. However after the main valve is shifted to exhaust, some of the air in the return chamber escapes back out of these holes before the piston raises enough to block such escape. This results in the pressure dropping within the return chamber below that which is needed to drive the piston and driver assembly fully through its return stroke to its upper rest position.
To eliminate the problem of air escaping from the return chamber, an external seal to block the first set of upper holes is used as described in U.S. Patent No. 4,747,338. The use of this external seal restricts the air entering the return chamber since the pressure of the air trying to pass through the first set of upper holes must be high enough to create a force on the external seal causing it to expand and unblock the holes. For example, the source of compressed air may be pressurized as high as 45 to 50 psi to expand the seal and open the upper holes, regardless of the size of the fastener or the pressure required to move the driver and its fastener through a drive stroke.
In those applications requiring small fasteners, a tool can be connected to a relatively low air pressure source and yet have sufficient power to drive the fastener correctly. The restriction of the external seal on the other hand may require higher pressure to provide sufficient air pressure within the return chamber to return the driver to its upper rest position.
The need for more air pressure to return the piston than that needed to drive the fastener not only adds unnecessary stress and wear on the tool components but also increases operating cost of the tool to produce the additional air pressure.

Summary of the Invention

The present invention has taken into account these and other disadvantages and therefore it is a primary object of the present invention to provide an improved pneumatic fastener driving apparatus with an improved operation.
Another object is to provide an improved pneumatic fastener driving apparatus having an improved piston and cylinder capable of returning the piston after the driving stroke at reduced air pressure levels.
A further object of this invention is to provide an improved pneumatic fastener driving tool capable of operating efficiently with air compressed at lower pressure levels than contemplated by similar tools of the prior art.
A still further object of this invention is to provide an improved pneumatic fastener tool in which manufacturing steps and costs are reduced.
In accordance with these and other objects of this invention, there is proved a pneumatic powered fastener driving apparatus comprising a housing, and a cylinder disposed within the housing to divide the housing into first and second compressed air chambers. A piston is disposed within the cylinder to divide the cylinder into first and second portions and movable therein between a first rest position and a second position remote therefrom. A fastener driving element is connected to the piston. A valve assembly is movable between a closed position for blocking communication between the first compressed air chamber and the first cylinder portion and an open position for supplying compressed air from the first compressed air chamber to the first cylinder portion. A passage is formed between the periphery of the piston and the inner wall surface of the cylinder for providing when the piston is disposed at the second remote position the only effective communication for the flow of compressed air from the second compressed air chamber to the first cylinder portion.
In a further aspect of this invention, the cylinder comprises an inner surface and the piston comprises an outer peripheral surface. The inner cylinder surface is spaced from the outer peripheral surface of the piston by a distance selected to trap a cylindrically shaped film of air within the passage to retard the flow of compressed air therethrough when the piston is being driven in the drive and return strokes.

Brief Description of the Drawings

FIG. 1 is a partial, cross-sectional view of a pneumatic fastener driving tool according to the present invention, illustrating the position of its piston at its rest position prior to start of its tool cycle;
FIG. 2 is a partial, cross-sectional view of the tool of FIG. 1 showing its valve assembly actuated to apply pressure to the upper surface of the piston driving it in its down stroke; and
FIG. 3 is an enlarged scale, cross-sectional view of a lower portion of the tool of FIGS. 1 and 2 showing the piston at rest on a shock absorber in a position wherein a return chamber of the tool is being pressurized.

Detailed Description of Preferred Embodiments:

Referring now to FIG. 1, there is shown a pneumatic tool for driving a supply of fasteners 18 one at a time into a workpiece (not shown). The tool includes a housing 11 for enclosing a piston 23 and a driver 28 which is fixed to the piston 23 and functions together as a unit. The housing 11 comprises a body 12, a handle 13 and a cap 14. The size and shape vary considerably depending on the type off fastener and application, but all have in common an internal space used as a compressed air source or chamber 19. The air chamber 19 is pressurized from an air line source by an inlet connection (not shown) attached to the handle 13.
There are numerous means and configurations of attaching the driver 28 to the piston 23, one of which is shown in FIG. 3. The piston 23 is provided with a slot 43 in which the driver 28 is inserted. A pin 44 is pressed into the piston 23 and through a hole in the driver 28. A pocket 45 formed in top portion of the piston 23 is for the sole purpose of reducing the weight of the piston 23.
In the embodiment shown in FIG. 1, the cap 14 is attached to the body 12 with screws (not shown). The body 12 and cap 14 are joined by a seal 20 to prevent compressed air escaping to the ambient atmosphere. The body 12 contains an internal space, which is divided into two sections, i.e., the first air chamber 19 and a second or return air chamber 21. The pressurized air within the air chamber 19 is used to provide the power to drive the fastener 18 in its drive stroke downwardly as shown in FIG. 2 into the workpiece, and the pressurized air within the return chamber 21 is used in its return stroke to drive the assembly of the piston 23 and its driver 28 from a second or remote position back to their first or rest position as shown in FIG. 1. The sequence of pressurizing the return chamber will be described in detail below.
The lower portion of the housing 11 is provided with a fastener carrying rail 15. The front of the rail 15 is defined by a nose piece 16 and includes a guide cavity 17 shaped to match that of the fastener 18. A pusher assembly (not shown) delivers one of the fasteners 18 at a time into the guide cavity 17 underneath the end of the driver 28.
A cylinder 22 is mounted in the body 12 as a unit with the piston 23 slidably disposed therein for reciprocating movement through its drive stroke and return stroke. To control the movement of the piston 23, a valve assembly is employed comprising a trigger valve 24 positioned near the handle 13 and a main valve 25, which is located above the cylinder 22 as seen in FIG. 1.
The trigger valve 24 is controlled by a manual lever 26. Pulling or closing of the lever 26 causes the trigger valve 24 to exhaust pressurized air in a chamber 37 above the main valve 25 through a series of passageways 27 and 27a whereby the main valve 25 is shifted upwardly as shown in Figure 2. Releasing of the lever 26 pressurizes the chamber 37 and passageways 27 and 27a connected thereto. In FIG. 1, the tool is manually operated; however, if the tool is part of a stationary application, the trigger valve could be a remotely located valve and operated by something other than the lever 26.
Referring now to FIG. 3, the cylinder 22 has an inside surface 29. A cylindrically shaped passage 33 exists between the inside surface 29 and an outer, peripheral surface 30 of the piston 23. The passage 33 is free of any sealing element disposed between the surface 29 and the piston 23. The piston 23 having an outside diameter less than the inside diameter of cylinder 22 and having no seal attached thereto will move through the full drive and return strokes without any frictional contact with the cylinder 22.
At the end of the drive stroke as shown in FIG. 3, the piston 23 is stopped by a shock absorber 31. The shock absorber 31 prevents damage to the tool that may occur should the piston 23 be allowed to strike the housing 11 directly. A hole 32 is provided in the shock absorber 31 to allow the driver 28 to pass therethrough. In addition to preventing damage to the tool, the top surface of the shock absorber 31 and bottom surface of the piston 23 form a temporary seal therebetween when the piston 23 has been driven to its remote position, as shown in FIG. 3, thereby preventing air from escaping through the hole 32 in the shock absorber 31, around the driver 28 and out through the bottom of the body 12.
As shown in FIGS 1 and 2, a flexible retaining ring 42 is inserted near the top of the cylinder 22 to hold the piston 23 in its first or rest position prior to the start of the tool cycle. Since the piston 23 has no contact with the cylinder 22 due to passage 33, other than occasional touching during the drive and return stroke, the piston 23 would fall downward when pressure on both sides of the piston 23 was fully exhausted so that the driver 28 rests on top of one of the fasteners 18. This action would reduce the driving stroke and in turn reduce the power of the tool.
To assure that the piston 23 is in a correct position at the start of the tool cycle, the retaining ring 42 has an inside diameter slightly less than that of the outside diameter of the piston 23. The outer peripheral surface 30 of the piston 23 engages and expands the ring 42, whereby the piston 23 slides inside the ring 42. The elastic resistance of the retaining ring 42 causes enough friction between the ring 42 and the piston 23 to retain the piston 23 in its rest position as shown in FIG. 1.
The sequential operation of the fastener driving tool according to the present invention will be described as follows. When an air supply is connected to the tool, the air chamber 19, the passages 27 and 27a, and the chamber 37 are pressurized. The piston return chamber 21, a passage 38 through the main valve 25 and the volume located below the piston 23 in the cylinder 22 remain unpressurized. A fastener 18 is positioned in the guide cavity 17 underneath the driver 28 from a previous tool cycle.
The tool is positioned on the workpiece and the trigger lever 26 is pulled upwardly to initiate the drive stroke. At the start of the tool cycle, the trigger valve 25 is actuated to thereby exhaust air passages 27 and 27a and the chamber 37. The main valve 25, which was previously disposed in its closed position wherein the valve 25 abuts and seals with a top surface of the cylinder 22 as shown in FIG. 1, now shifts upwardly to an open position, as shown in FIG. 2, due to the pressurized air from the air chamber 19 acting upon the bottom surface of the main valve 25.
When the main valve 25 is disposed to its open position, air enters into a first or top portion of the cylinder 22 above the piston 23, while at the same time engaging a seal 41 and blocking the communication to the ambient atmosphere through an exhaust port 40. As the bottom surface of the main valve 25 moves, the air tight seal with the cylinder 22 is broken and air from the reservoir 19 starts to enter the upper portion of the cylinder 22 above the piston 18. The retainer ring 42 is made of an elastic material, thus the piston 23 and ring 42 block any air from flowing past the peripheral surface of the piston 23. The main valve 29 continues to move upwardly until it is in the open position, as shown in FIG. 2. The air pressure on top of piston 23 creates a force larger than the interference engagement between piston 23 and the retainer ring 42, whereby the piston 23 becomes disengaged from the ring 42. The released piston 23 along with the driver 28 is forced down rapidly as shown in FIG. 2 until the lower surface of the piston 23 engages and is stopped by the shock absorber 31, as shown in FIG. 3. The driver 28 then pushes the fastener 18 into the workpiece (not shown).
During the time the piston 23 remains in its second or remote position on top of the shock absorber 31, pressurized air from the first or upper portion of the cylinder 22 above the piston 23 passes through the passage 33 into a cavity 34 formed between a second or lower portion of the cylinder 22, the shock absorber 31 and the lower surface of the piston 23. The lower portion 35 of the cylinder 22 is provided with a passage 36, which allows communication between the cavity 34 and the return air chamber 21. In the preferred embodiment as shown in FIGS. 1, 2 and 3, the passage 36 is formed by setting the length of the lower portion 35 of the cylinder 22 to be short enough and the diameter of the lower portion 35 is made larger than that of the upper portion of the cylinder 22 to prevent a seal with the body 12 or the shock absorber 31. Illustratively, the lower portion 35 is spaced at least 3 to 4 mm from the body 12 or the shock absorber 31 to allow for the free flow of air. The air is thus allowed to pass freely from the cavity 34 underneath the lower cylinder portion 35 and into the return chamber 21, when the piston 23 is disposed in its remote position against the shock absorber 31. It is contemplated that a series of struts (not shown) could be disposed for support purposes about the periphery of the cylinder 22 for interconnecting the cylinder 22 and the inner surface of the housing 11. Alternatively, a series of holes in the lower portion 35 of the cylinder 22 could be utilized to permit communication between the cavity 34 and the return chamber 21, if the mounting alignment of the cylinder 22 requires the lower cylinder portion 35 to be seated in the housing 11.
The return chamber 21 becomes pressurized only by the air that passes through passage 33. Although some air might pass through the passage 33 during the downward movement of the piston 23, most of the air enters the chamber 21 during the time the piston 23 is disposed in its remote position on the shock absorber 31. The differences between the diameters of the piston 23 and cylinder 22 is set less than 0.2 mm. The clearance or spacing between the inner surface 29 and the outer peripheral surface 30 of the piston 23 is thus set less than 0.1 mm. The selective setting of the dimensions of the passage 33 creates a thin air film within the passage 33 between the peripheral surface 30 of the piston 23 and the inside wall surface 29. That air film of cylindrical configuration retards the flow of air as the piston 23 moves rapidly down in its drive stroke or up in its return stroke. Both of the interior surface 29 and the outer, peripheral surface 30 of the piston 23 present a friction to the air flow tending to trap the cylindrical air film therebetween when the piston 23 moves. Further. The height of the piston 23 may be set relatively high with respect to that used in the piston configurations found in other tools. For example, the piston height may be set greater than its height as shown in FIGS. 1, 2 and 3. Such dimensions, particularly of the piston height, create a relatively high cylindrical air film within the passage 33, which further retards the flow of air therethrough when the piston 23 is moving. Generally, as the height of the cylindrical air film is made greater, the retardation imposed by the film is increased. Similarly, the smaller that the clearance between the wall surface 29 and the peripheral surface 30 of the cylinder 23 is made, the more that the cylindrical air film will retard the flow of the pressurized air through the passage 33. The minimum dimension of the clearance is limited by the tolerances need to assure that the piston 23 will move freely within the cylinder 22, as well as the consideration that more air pressure will be needed to force the air through a narrower passage 33 when the cylinder 23 is brought to rest at its remote position.
Although that portion of the cylinder 22 which is disposed above the piston 23 is subjected to an air pressure higher than that portion disposed below the piston 23 as seen in FIG. 2, there is very little air that flows through the passage 33 during the downward or upward movement of the piston 23. The flows of pressurized air from the air chamber 19 and the return chamber 21 attempts to pass through the passage 33 as the piston 23 moves downwardly in its drive stroke and upwardly in its return stroke, respectively. In either case, the flow of the pressurized air and the movement of the piston 23 (and the trapped cylindrical air film carried thereby) are in the same direction, whereby the cylindrical air film significantly restricts the flow of the pressurized air through the passage 33. On the other hand, when the piston 23 is brought to its remote position against the shock absorber 31, pressurized air does flow relatively freely permitting the flow of pressurized air from the chamber 19, through the upper portion of the cylinder 22, the passage 33, the cavity 34 and the passage 36, and into the return chamber 21, whereby that chamber 21 is pressurized to effect the return stroke of the piston 23.
The lever 26 is then released and the trigger valve 24 repressurizes the passageways 27 and 27a, and the chamber 37. Whenever the upper and lower surfaces of the valve 25 are subjected to equal pressure, the main valve 25 is forced toward the cylinder 22 by a spring 39, which is disposed between the cap 14 and the main valve 25. Upon release of the trigger valve 24, the air within the cavity 37 is pressurized to force the main valve 25 against the top of the cylinder 22, i.e., the valve 25 is disposed in its closed position, and the communication between chamber 19 and top portion of cylinder 22 is blocked as shown in FIG. 1.
The shifting of the main valve 25 to its closed position allows the space above the piston 18 to again communicate with the atmosphere and the air within the cylinder 22 above the piston 18 exhausts through passage 38 and the exhaust port 40. When the air pressure in the first or upper portion of the cylinder 22 above the piston 23 drops below that under the piston 23, due to the air exhausting, the air in the return chamber 21 enters the lower portion of the cylinder 22 under the piston 23 through the passage 36 and forces the piston 18 and driver 21 to move upwardly in its return stroke. The return chamber 21 has a fixed volume which is determined as a fraction of the volume of the lower portion of the cylinder 22 below the piston 23 when the piston 23 is at the uppermost position as shown in FIG. 1. Thus as the piston 18 moves upwardly, the pressure in the return chamber 21 is reduced. The return chamber 21 is sized to provide enough air to fully return the piston 23 at the lowest operating pressure, i.e., a pressure nearly reduced to that of the ambient atmosphere prior, to its rest position. Any remaining pressure must be further reduced by the air escaping past the clearance between the driver 28 and the body 12 prior to the next cycle. As the end of the driver 28 raises above the fastener rail 15, the next fastener 18 is positioned into the nose piece cavity 17 ready to be driven during the next tool cycle.
It must be understood that the terms upper, lower, above, downward and the like are used in reference to the illustrations shown in FIGS. 1, 2 and 3 solely for the purpose of clarity.
While preferred embodiment of the present invention have been illustrated and described, it is anticipated those skilled in the art may make numerous changes and modifications without departing from the spirit of this invention, which is intended to be limited only by the scope of the following appended claims.

Claims

A pneumatic powered fastener driving tool operable in a tool cycle including a drive stroke and a return stroke, said tool comprising:
a) a housing;

b) a cylinder disposed within said housing to divide said housing into first and second compressed air chambers, said first chamber for storing a source of compressed air, said cylinder having an inner surface of a given first diameter;

c) a piston disposed within said cylinder to divide said cylinder into first and second portions and slidably disposed within said cylinder for reciprocating movement between a first rest position and a second remote position, said piston having an outer peripheral surface of a given second diameter; and

d) a main valve operable between a closed position for blocking communication between said first compressed air chamber and said first cylinder portion to release compressed air from said first cylinder portion to the ambient atmosphere to thereby drive said piston in said return stroke from said second remote position to said first rest position, and an open position for supplying compressed air from said first compressed air chamber to said first cylinder portion to drive said piston in said drive stroke from said first rest position to said second remote position;

e) said inner surface of said cylinder and said outer peripheral surface of said piston defining a passage therebetween free of any sealing element therein, said first and second diameters being selected to trap a cylindrically shaped film of air within said passage to retard the flow of compressed air therethrough when said piston is being driven in said drive and return strokes and to permit the relatively free flow of compressed air from said first cylinder portion through said passage and into said second compressed air chamber when said piston is disposed at rest at said second remote position.
The fastener driving tool as claimed in claim 1, wherein said passage providing when said piston is disposed at said second remote position the only effective communication for the flow of compressed air from said second compressed air chamber to said first cylinder portion.
The fastener driving tool as claimed in claim 1, wherein said inner peripheral surface of said cylinder is spaced from said outer peripheral surface of said piston a given distance which is set less than 0.1 mm.
The fastener driving tool as claimed in claim 1, wherein said cylinder has an axis, said piston having diameter and a dimension parallel to said axis set greater than said diameter.
The fastener driving tool as claimed in claim 1, wherein there is further included a shock absorber disposed to abut said piston whereby said piston is disposed at its second remote position, said cylinder having first and second ends, said second cylinder end being supported within said housing so that said second cylinder end is spaced a distance from said housing and said shock absorber sufficient to permit the free flow of air between said first cylinder portion and said second compressed air chamber.
The fastener driving tool as claimed in claim 5, wherein said distance is at least 3 mm.
A pneumatic powered fastener driving apparatus, comprising:
a) a housing;

b) a cylinder disposed within said housing to divide said housing into first and second compressed air chambers;

c) a piston disposed within said cylinder to divide said cylinder into first and second portions and movable therein between a first rest position and a second position remote therefrom;

d) a fastener driving element connected to said piston;

e) valve means movable between a closed position for blocking communication between said first compressed air chamber and said first cylinder portion and an open position supplying compressed air from said first compressed air chamber to said first cylinder portion; and

f) a passage formed between the periphery of said piston and the inner wall surface of said cylinder for providing when said piston is disposed at said second remote position the only effective communication for the flow of compressed air from said second compressed air chamber to said first cylinder portion.
An apparatus according to claim 7, wherein said piston is constructed so as to have substantially no frictional engagement with said cylinder as said piston moves between said first rest position and said second remote position.
An apparatus according to claim 7, wherein said cylinder comprises an inner surface and said piston comprises an outer peripheral surface, said inner cylinder surface being spaced from said outer peripheral surface of said piston by a distance selected to trap a cylindrically shaped film of air within said passage to retard the flow of compressed air therethrough when said piston is being driven in said drive and return strokes.