Low impact spot welding cylinder using single or double piston

A weld cylinder having a single piston arrangement is provided. The cylinder has a movable retract piston assembly with the piston arranged within the retract piston assembly. The piston supports a rod that is movable between home, intermediate, work, and advanced work positions. The rod moves rapidly from the home position to the intermediate position however, the rod moves more slowly from the intermediate position to the work position to reduce the impact force. A cushion chamber slowly exhausts through a pre-orifice and a hole in the cushion valve while an isolator is in an open position. Once the cushion valve opens in response to a retract-forward pressure on the cushion valve, the cushion chamber exhausts rapidly so that weld force increases rapidly to minimize increases in cycle time. Advance of a retract piston assembly is also cushioned. A weld-forward port is no longer provided on a cylinder barrel.

BACKGROUND OF THE INVENTION

The present invention relates to a weld gun used in spot welding, and more particularly, the invention relates to either a single or double piston welding cylinder for the weld gun generating a low impact force when the weld gun engages the workpiece to perform a spot weld.

A typical weld gun used in spot welding includes opposing arms each having an electrode that applies current to a workpiece to generate a weld nugget, for example, between two sheets of metal. The electrodes include malleable welding caps typically manufactured from a copper alloy. Repeated impact force between the welding caps and the workpiece plastically deforms the welding caps thereby increasing the wear of the welding caps and reducing the service life, which increases operating cost.

Impact force between the welding caps and workpiece also generates workpiece distortion, which can have detrimental effects on the welding process and quality of the weld.

Several prior art weld cylinders have endeavored to reduce the impact force between the welding caps and workpiece to extend the life of welding caps and reduce workpiece distortion. One approach in the prior art has been to slow the advance of the electrodes toward one another by slowing the movement of the pneumatic cylinder. However, while this approach reduces the impact force it also increases the cycle time for a spot weld, which is undesirable. Another drawback is that typically the greater the retract stroke length, the narrower the low impact region becomes relative to the stroke. This is problematic in that the internal components of the weld cylinder must be customized depending upon the application. A more versatile weld cylinder design would provide common components for a wider variety of applications.

Another approach in the prior art is to utilize external devices or peripheral components such as valves, regulators, restrictors, and/or electrical switches to control the pressure, timing, and rate that the air is supplied to the pneumatic cylinder thereby controlling the impact force. However, the addition of these externals devices may be difficult to integrate with existing weld guns and is also costly since additional components must be added to the welding system. It is desirable to use the current industry pneumatic actuating systems so that the inventive weld cylinder may be used with current systems. For example, in four weld port systems, the ports are typically pressurized in pairs to achieve the three different weld cylinder stroke positions.

One of the ports on prior art systems is typically provided on a barrel that extends between the ends of the cylinder. The barrel port is a costly feature in that it requires a two-piece tubular wall with a separator arranged between the walls. The multiple components of the barrel must incorporate several seals. As an alternative to using the separator, the port may be welded to a singular tubular wall, which requires subsequent machining of the inner surface of the barrel. Further, the welding process is difficult to control.

Therefore, what is needed is a pneumatic cylinder that reduces impact force between the welding caps and the workpiece but that does not require additional, costly design features or external devices and increases in cycle time.

SUMMARY OF THE INVENTION

The present provides a weld cylinder having a single or dual piston arrangement. The cylinder has a movable retract piston assembly with the piston arranged within the retract piston assembly. The piston supports a rod that is movable between home, intermediate, work, and fully advanced work positions. The rod moves rapidly from the home position to the intermediate position. However, the rod moves more slowly from the intermediate position to the work position to reduce the impact force. A cushion valve supported by the retract piston assembly and a cushion trip valve supported by a front block cooperate to increase the rate at which the rod moves from the work position to the advanced position so that weld force increases rapidly to minimize increases in cycle time.

A weld-return port is provided on the front block so that weld-ports need not be provided on the barrel. A cushion chamber is pressurized using weld-return air. Notches are provided in the retract piston assembly to fluidly connect the retract-forward air to the cushion valve, which is carried by a rearward flange in the retract piston assembly.

The cushion valve includes a pre-orifice to permit some cushion air to exhaust from the cushion chamber prior to reaching the intermediate position, which enables the rod to advance more smoothly and quickly. An isolator valve opens when it engages the trip valve during the advance of the retract piston assembly. The trip valve is also opened at this time. When both the cushion valve and cushion trip valve move from closed positions to open positions, the rate of fluid flow through the retract piston assembly increases, and so too does the rate at which the piston moves within the retract piston assembly.

The cushion valve is a differential pressure valve that is exposed to the pressure within the cushion chamber and retract-forward air pressure. As the pressure in the cushion chamber falls as the chamber is exhausted and the retract-forward air pressure rises, the cushion valve will open. The opened cushion valve enables the cushion chamber to be exhausted even more rapidly so that the weld force can build quickly.

A second cushion chamber is provided between the retract piston assembly and the front block to smooth engagement between the retract piston assembly and the front block. A sealed chamber is created between the rearward flange and the front block as the retract piston assembly approaches the front block. A passage fluidly connects the second cushion chamber and the retract-return port, which is vented to atmosphere at this cylinder position.

Accordingly, the present invention provides a pneumatic cylinder that reduces initial impact force between the welding caps and the workpiece, but that does not require additional, costly external devices and increases in cycle time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An inventive weld gun10including an inventive pneumatic cylinder18is shown inFIG. 1. The pneumatic cylinder18is a four port arrangement ports (P1-P4) that can be easily integrated into existing weld guns utilizing four ports. That is, the plumbing for prior art weld guns having four ports can be used with the inventive pneumatic cylinder18without modification to the welding system. The ports P1-P4are fluidly connected to a compressed air source22. The ports P1-P4are connected to the compressed air source22through valves that control the timing of the air signals provided to the pneumatic cylinder18by selectively opening and closing the valves.

The weld gun10includes opposing arms12that each include an electrode14. The electrodes14include welding caps16that are typically manufactured from a malleable copper alloy. The welding caps16engage a workpiece (not shown) to apply a welding current to the workpiece to generate a weld nugget, as is well known in the art. The arms12are typically supported by the weld gun10and interconnected to one another at various pivot points. A rod20of the pneumatic cylinder18is typically connected to one of the arms12to actuate the electrodes14and welding cap16between electrode positions corresponding to home (fully returned), intermediate, and work (caps16and workpiece engaged) positions. Current is applied to the electrodes14when in the work position using a current source24that is electrically connected to the electrodes14.

Port P1provides weld-forward air when pressurized. Port P2provides weld-return air when pressurized. Port P3provides retract-forward air when pressurized, and port P4provides retract-return air when pressurized. Typically, the ports are pressurized in pairs to achieve a desired position with the other, non-pressurized ports vented to atmosphere. In the example shown, the weld-return and retract-return ports, P2and P4, are pressurized to obtain the home position. The weld-return and retract-forward ports, P2and P3, are pressurized to obtain the intermediate position. The weld-forward and retract-forward ports, P1and P3, are pressurized to obtain the work position. The fully advanced work position is the travel limit of the work position. The rod20may be actuated between the above three positions or directly from the home position to the work position by actuating the desired valves. It should be understood that the work position is not necessarily a discrete position, but rather, may be any distance along which the rod20moves between the intermediate position and the fully advanced work position. The various positions and the condition at the ports is expressed in the following table:

The pneumatic cylinder18includes a cylinder body26that has a barrel34and an end cap32arranged at one end. A front block28is arranged at the other end of the barrel34. The fasteners36secure the components of the cylinder body26together so the pneumatic cylinder18can be pressurized at ports P1-P4without leakage from the cylinder body26. There is no port (typically the weld-return port) provided on the inventive barrel34, as in some of the prior art and Applicant's other weld cylinders. Instead, the weld-return port is provided on the front block28. It should be understood that the ports may be arranged on the cylinder other than described and shown. For example, the ports may be repositioned to provide desired packaging of the weld cylinder.

The multiple component cylinder body26and its configuration described above are exemplary of a four/port pneumatic cylinder having a single piston arrangement. However, it should be understood that other configurations may be used and still fall within the scope of the present invention. For example, a dual cylinder incorporating the present invention is shown inFIG. 9.

In the example embodiment of the inventive pneumatic cylinder18, three cylinder positions are generated.FIGS. 2,3A and3B depict a home position of the pneumatic cylinder18.FIGS. 4 and 5show an intermediate position of the pneumatic cylinder18, andFIGS. 6 and 7depict a work position of the pneumatic cylinder18. The work or weld position may lie anywhere between the intermediate position and the fully advanced position depending on the mechanical system and the workpiece. The position variation is used in the process to ensure the weld force is maintained as the electrodes deflect under mechanical load, the electrode wear, and as the workpiece becomes distorted during the welding process. In the fully advanced position, a weld flange64is in close proximity to or engages a rearward flange44.

The present invention pneumatic cylinder18moves the rod20quickly from the home position (FIG. 2) to the intermediate position (FIG. 4) in which the welding caps16are in close proximity to the workpiece. However, the rod20moves more slowly from the intermediate position (FIG. 4) to the work position (FIG. 6) in which the welding caps16sufficiently forcefully engage the workpiece. The rod20moves forward and additional distance to increase the weld force so that current can be applied to produce a weld nugget. The slower movement from the intermediate position (FIG. 4) to the work position (FIG. 6) ensures that the impact force between the welding caps16and the workpiece is minimized. However, the preceding, more rapid movement of the rod20ensures that cycle time is not unduly compromised. The present invention reduces the impact force of the caps engaging the workpiece while not significantly increasing cycle times. Further, the invention decouples the retract stroke length of a cylinder from low impact performance. That is, the invention can meet desired low impact targets using the same components for cylinders having various retract stroke lengths.

Referring toFIG. 2, ports P1and P3are provided by the end cap32. Ports P2and P4are provided by the front block28, which also slideably supports the rod20with bushing59. A retract piston assembly40is slideably supported by the barrel34. The retract piston assembly40includes a barrel46having forward and rearward flanges42and44secured at opposite ends using retaining ribbons48. The retract piston assembly40separates a cavity50provided by the barrel34into first and second chambers52and54. The retract piston assembly40is shown in a retract-return position inFIG. 2and a retract-forward position inFIGS. 4 and 6.

The retract piston assembly40includes a weld flange64, which is threadingly secured to an end of the rod20opposite an end60of the rod20. The end60is secured to one of the arms12. The weld flange64is arranged within the retract piston assembly40between the forward and rearward flanges42and44in a cavity66that is separated by the weld flange64into third and fourth chambers68and70. The fourth chamber70acts as a cushion chamber to reduce the impact between the caps and the workpiece. The weld flange64is shown in a weld-return position inFIGS. 2 and 4and moving toward a weld-forward position inFIG. 6. The work position depicted inFIG. 6includes a range of positions once the rod20begins to move. The rod20continues to advance a distance as the pressure builds to allow the weld-force pressure to build quickly.

Referring toFIG. 3A, the inventive pneumatic cylinder18includes a cushion or differential pressure valve78supported by the rearward flange44of the retract piston assembly40. A seal82is arranged between the cushion valve78and the return flange44. A spring84is arranged between a plate86, which is secured to the rearward flange44by screws88(only one shown), and the cushion valve78. The spring84biases the cushion valve78to a closed position, which is shown inFIGS. 2,3A and4. The cushion valve78has a pre-orifice73that provides a predetermined, controlled leak of the pressurized cushion chamber70through passage72.

The retract piston assembly40includes notches75. Wear bands67are arranged at the notches75to keep the retract piston assembly40centered within the barrel34. An annular space77is provided between the barrels34and46. The rearward flange44carries a seal76that provides a seal between the barrel34and the rearward flange44. A passage74in the rearward flange44provides a fluid connection between one side of the cushion valve78and the retract-forward port P3.

An isolator80is arranged concentric with and inside of the cushion valve78. The isolator80is sealed against the cushion valve78with seals79. A spring81acts against a retainer that is secured to the isolator80to bias the isolator80to the closed position shown inFIGS. 2 and 3A. The pressure in the cushion chamber70and spring81apply sufficient force to the isolator80to maintain the isolator80in the closed position with the retract-return air applied to the isolator from the second chamber54.

Referring toFIG. 3B, the front block28supports a trip valve98that is biased to a closed position against a plate90by a spring92. The plate90is secured to the front block28by screws (not shown). A seal100is arranged between the rod20and the trip valve98.

With continuing reference toFIGS. 2,3A and3B, which depict the home position, the cushion chamber70has already been pressurized by weld-return air subsequent to welding the workpiece. Specifically, from the work position shown inFIG. 6, the weld-return port P2is actuated (along with the retract-return port P4) to obtain the home position ofFIG. 2. However, in the home position ofFIG. 2, the weld-return port P2is pressurized, but pressurized air is prevented from acting on the retract piston assembly40with the trip valve98in the closed position. The retract-return port P4provides pressurized air to the second chamber54to retain the retract piston assembly40in the retracted position.

The intermediate position is shown inFIGS. 4 and 5and is achieved by pressurizing the weld-return and retract-forward ports P2and P3. As the first chamber52is pressurized, the second chamber54is exhausted through vented retract-return port P4. Seals102on the rearward flange44engage the front block28and plate90to prevent the second chamber54from fully exhausting through the large passage104. Instead, the remaining pressure in second chamber54is exhausted through cushion passage106thereby smoothing the engagement between retract piston assembly40and front block28.

The isolator80and trip valve98engage one another forcing both the isolator80and trip valve98to the open positions, best seen inFIG. 5. As a result, an opening108is created between the trip valve98and plate90to fluidly connect the weld-return port P2and newly created chamber110through the opening108.

The opened isolator80provides an opening112between the isolator80and cushion valve78that is fluidly connected to an annular passage114. A hole115in the cushion valve78(in addition to the pre-orifice73) fluidly connects the annular passage114to the passage72so that the cushion chamber70is in fluid communication with the weld-return port P2. Pre-orifice73may be situated on rearward flange44instead of on cushion valve78. Furthermore, pre-orifice73and hole115can each be comprised of one or more holes or passages. The pre-orifice73, of a smaller net flow area than hole115, allows only a small amount of cushion fluid to escape to the vented port P2, just enough to smooth the transition at the intermediate position when stroking from the home to work positions directly. The pressure in the cushion chamber70is “topped off” prior to actuating the cylinder to the work position.

Prior embodiments allow cushion air to flow freely, to the point where insufficient pressure remains in the cushion chamber70for achieving low-impact operation, especially for cylinders with longer retract stroke lengths. The isolator80“isolates” the cushion chamber70from hole115so that during the retract stroke, cushion air can escape through the pre-orifice73. This way, the transition at the intermediate position can be smoothed by the pre-orifice's effect of slightly reducing cushion pressure without compromising low-impact operation that would otherwise occur should cushion air be allowed to escape through hole115during the retract stroke.

One side of the cushion valve78is exposed to pressure from the retract-forward port P3through notches75, annular space77and passage74. However, the retract-forward pressure is not sufficient to overcome the weld-return pressure and the spring84to open the cushion valve78.

The cylinder is actuated from the intermediate position (FIGS. 4 and 5) to the work position shown inFIGS. 6 and 7by pressurizing the weld-forward port P1and the retract-forward port P3. The weld-return port P2is vented while the cushion chamber70and weld-return port P2are still fluidly connected to one another.

Referring toFIGS. 6 and 7, the pressurized fluid enters the third chamber68through passage118in tube116that is fluidly connected to the weld-forward port P1. The tube116is threaded into end cap32and sealed relative to forward flange42by seal117. As the pressure rises in the third chamber68, the weld flange64advances, but initially with the cushion valve78and isolator80in the positions shown inFIG. 5.

With reference toFIG. 5, as the weld flange64advances, the air from cushion chamber70is exhausted slowly through the pre-orifice73and hole115out weld-return port P2to cushion the contact between the weld caps and the workpiece. The pressure in cushion chamber70finally drops to a level at which pressure from the retract-forward port P3at passage74overcomes the pressure in cushion chamber70and spring84to open the cushion valve78, as shown inFIG. 7. Pressure in passage74acts on a side of the cushion valve78exposed to a cavity122. The cushion valve78engages the isolator80in the open position. In the open position, the air in the cushion chamber70is permitted to exhaust rapidly through annular opening124out the weld-return port P2so that welding force can rise rapidly. The effects of the controlled exhausting of cushion chamber70of the inventive cylinder are shown inFIG. 8. By comparison, the harsh impact of other cylinders is eliminated. The “switch” inFIG. 8indicates the triggering of the valves to pressurize ports P1and P3.

The cylinder is actuated to either the intermediate or home positions by pressurizing the cushion chamber70with air from the weld-return port P2. The weld flange64is moved to the returned position. Once the cushion chamber70is filled, the pressure in the cushion chamber70will close the cushion valve78.

The above inventive arrangement may also be used for a dual piston arrangement, shown inFIG. 9, which is similar to the single piston cylinder described above. The dual piston cylinder18′ includes a middle separator30dividing barrels34aand34b. The middle separator30acts as the front block described above relative to the single piston cylinder. A first rod portion20ais supported by the middle separator30, and a second rod portion20bis supported by a front block28′. A second weld flange62is secured between the rod portions20aand20b. A second weld-forward port P1is provided on the middle separator30to provide pressurized air to the second weld flange62. A vent is provided in the front block28′ to permit air to be exhausted from forward chamber130. The cylinder may also be used as a five port cylinder by independently controlling the two ports labeled as P1inFIG. 9.