EP0573144A1

EP0573144A1 - Compressor unloader

Info

Publication number: EP0573144A1
Application number: EP93303142A
Authority: EP
Inventors: Mark W. Wood; Michael J. Kosmyna
Original assignee: DeVilbiss Air Power Co
Current assignee: DeVilbiss Air Power Co
Priority date: 1992-06-01
Filing date: 1993-04-22
Publication date: 1993-12-08
Also published as: AU3837493A; CA2095772A1; TW223140B

Abstract

An unloader (11) to facilitate starting and idling an air compressor motor (12). In one embodiment, the compressor (10) has a valve (29), such as an inlet or suction valve, having one or more reeds (28) which are biased to a normally closed position. An end of a wire or spring or a similar mechanism (37) is mounted to hold the valve open to unload the compressor when the motor is stopped or operated below a predetermined minimum speed. The compressor motor (12) also drives a fan (35) which cools the compressor. When the motor (12) reaches the predetermined minimum speed, cooling air flow from the fan moves a vane (42) which is connected to move the wire, spring or other mechanism (37) clear of the valve (29), allowing the valve to close to load the compressor (10). In a modified embodiment of a starting unloader, an opening (110) is provided to vent the high pressure output from the compressor (90) when the motor (91) is stopped or idling. When the compressor motor reaches a predetermined speed, a flow of cooling air from a fan (93) driven by the motor moves a vane (111) to close the vent passage (110).

Description

Technical Field

The invention relates to air compressors and more particularly to an unloader for an air compressor which is responsive to a compressor motor reaching a predetermined minimum speed before leading the compressor.

Background Art

Air compressors are commonly used, for example, for operating pneumatic tools, for inflating tires, etc. A common type of air compressor has a motor connected to reciprocate a piston in a cylinder. The piston reciprocates alternately between a suction stroke and a compression stroke. One or more intake valves direct a flow of ambient air into the cylinder during the suction stroke and one or more outlet valves direct pressurized air from the cylinder to an air tank or through a hose to a tool, for example, during the compression stroke. In some compressors, the electric motor is strained when the motor is started while the compressor is subject to a load. For example, where the compressor is connected to an air tank, a switch turns the motor off when the tank pressure reaches a preset maximum pressure and turns the motor on when the tank pressure drops to a preset minimum pressure. The motor will be subjected to a high starting torque if it is started against the load of the preset minimum tank pressure on the compressor. Starting problems are especially of concern under low voltage conditions, as might occur, for example, at a construction site where long extension cords are often used to connect the compressor motor to the power source. For compressors powered by an internal combustion engine, engine starting is more difficult while the compressor is subject to a load. In addition, with internal combustion engines, it is desirable to let the engine and compressor run continuously during normal usage. During continuous operation, when air is not being used and the tank pressure reaches maximum, it is desirable to idle the engine and reduce the compressor torque to save fuel and reduce wear on the engine and compressor.
In order to reduce the starting and idling load demands on a compressor motor, valves are often used to unload the compressor motor when the motor is stopped. Various designs are known for unloader valves. One type of unloader valve operates m response to the motor speed through a mechanism activated by centrifugal force. Electric timers also have been used to unload a compressor for a predetermined time while the motor comes up to speed. In still another design, the motor output is coupled through a magnet to a cam. The cam opens a valve to unload the compressor when the motor is stopped. When the motor speed is sufficiently high, the magnet rotates the cam to close an unloader valve.

Disclosure Of Invention

According to the invention, a compressor unloader is designed to respond to the flow of cooling air from a fan driven by the compressor motor. In one embodiment of the invention, the suction or inlet valve is held open during both the intake and the compression strokes of the piston to unload the compressor while the motor is stopped or operating at a low speed during startup and idle. When the motor speed is sufficiently high, the flow of cooling air from the fan moves a vane and a connected mechanism which allows the suction valve to close. Once the suction valve can close during the compression stroke, the compressor will have a normal compressed air output.
In a second embodiment of the invention, a vent passage is connected either to the compression chamber or to the outlet side of a valve head on the compressor. When the compressor motor is stopped or idling, the passage is open to vent high pressure. When the motor reaches a predetermined speed, cooling air from a fan driven by the motor moves a vane to close the vent passage. When the vent passage is closed, the compressor and connected drive motor become fully loaded.
It is an object of the invention to provide an improved low cost unloader to facilitate starting and idling for an air compressor motor.
Other objects and advantages of the invention will be apparent from the following detailed description and the accompanying drawings.

Brief Description Of The Drawings

Fig. 1 is a fragmentary cross sectional view showing a portion of an air compressor incorporating an unloader according to one embodiment of the invention;
Fig. 2 is a fragmentary top view showing an unloader according to a modified embodiment of the invention;
Fig. 3 is a fragmentary cross sectional view illustrating details of the unloader of Fig. 2;
Fig. 4 is an enlarged cross sectional view taken along line 4-4 of Fig. 2;
Fig. 5 is a fragmentary top view showing an unloader according to a further modified embodiment of the invention;
Fig. 6 is a fragmentary cross sectional view showing details of the cylinder, the valve plate and the unloader of Fig. 5;
Fig. 7 is an enlarged fragmentary cross sectional view showing the closed compressor inlet valve as taken along line 7-7 of Fig. 5;
Fig. 8 is an enlarged fragmentary view, similar to Fig. 7, but showing the compressor inlet valve held open by the unloader; and
Fig. 9 is a fragmentary cross sectional view, similar to Fig. 1, but showing a compressor incorporating a further modified embodiment of an unloader according to the invention.

Best Mode For Carrying Out The Invention

Referring now to Fig. 1 of the drawings, a fragmentary portion of an air compressor 10 is shown incorporating an unloader 11 according to one embodiment of the invention. The housing and support frame for the compressor 10 have been omitted for better illustrating the invention. The air compressor 10 generally includes an electric motor 12 or an internal combustion engine (not shown) having an output shaft 13. Unless otherwise specified, the term "motor" as used herein shall mean either an electric motor or an internal combustion engine. The output shaft 13 is connected to drive an eccentric plate 14 having an eccentric shaft 15 which is offset from the axis of the output shaft 13. The eccentric shaft 15 is connected through a bearing 16 to a free end 17 of a connecting rod 18. The connecting rod 18 and a piston head 19 connected to an opposite end 20 of the connecting rod 18 form a wobble piston 21. As the motor drives the eccentric shaft 15 about a circle, the piston head 19 reciprocates in a cylinder 22. At the same time, the piston head 19 wobbles or rocks back and forth because it is rigidly connected to the connecting rod end 20. A piston seal 23 establishes a sliding seal between the piston head 19 and the cylinder 22 as the piston reciprocates and rocks.
An end 24 of the cylinder 22 is closed by a valve plate 25. The valve plate 25 has one or more inlet or suction ports 26 and one or more outlet ports 27. A resilient reed 28 covers the inlet port 26 to form a suction valve 29 and a resilient reed 30 covers the outlet port 27 to form an outlet valve 31. When the piston head 19 is moved downwardly in the cylinder 22, ambient air is drawn from an inlet chamber 32 in a head 33, through the inlet port 26, past the suction valve 29, and into the cylinder 22. When the piston head 19 is moved upwardly in the cylinder 22, the suction valve 29 is closed and the air in the cylinder 22 is compressed and forced through the outlet port 27, past the outlet valve 31 and into an outlet chamber 34 in the head 33. As the air flows through the outlet port 27, the valve reed 30 deflects as shown by the dashed lines. When the air flow from the cylinder 22 to the chamber 34 ceases, the resilient valve reed 30 returns to its original position closing the outlet port 27.
In order to prolong the operating life of the compressor 10 and to prevent overheating during operation, a fan 35 is mounted on the motor shaft 13. During operation of the compressor 10, the fan 35 draws cooling air over the motor 12 and blows the cooling air around the exposed exterior portions of the cylinder 22 and the piston 21. A suitable housing (not shown) for the compressor 10 and baffles within the housing (not shown) may be provided to direct the cooling air from the fan 35 to the locations needing cooling.
According to the invention, an improved unloader 11 is provided for holding open the suction valve 29 while the motor 12 is stopped, just starting or idling. A rocker arm 36 is mounted to rock within the inlet chamber 32. A push rod 37 extends from one end 38 of the rocker arm 36 through the inlet port 26 to the valve reed 28. A spring 39 tends to rock the arm 36 until the push rod 37 contacts and holds the valve reeds 28 away from the inlet port 26, as shown. Consequently, the suction valve 29 is held open and air is free to flow through the inlet port 26 during both the intake and the compression strokes of the piston 21. So long as the suction valve 29 is held open, the load and the starting torque requirements for the motor 12 are at a minimum.
A second end 40 of the rocker arm 36 is connected through a rod 41 to a vane 42. The vane 42 is mounted to pivot about an axis 43. The vane 42 is mounted where it is subjected to air flow from the fan 35. When the motor 12 is started and the fan speed increases to a sufficient level, the air discharged by the fan acts on the vane 42. The air flow causes the vane 42 to pivot about the axis 43 in the direction of the arrow to pull on the rod 41. This in turn rotates the rocker arm 36 to lift the rod 37 away from the valve reed 28, allowing the suction valve 29 to close. Thus, the suction valve 29 is held open by the rod 37 both when the motor 12 is stopped and during startup for the motor 12. When the motor reaches a desired minimum speed, air flow from the fan 35 causes the rod 37 to be drawn away from the valve reed 28 and the suction valve 29 becomes operational to load the compressor motor 12.
Figs. 2-4 are fragmentary views showing a modified embodiment of a compressor unloader 48 according to the invention. A valve plate 49 is mounted to close a cylinder 50 in which a piston 51 reciprocates. The valve plate 49 defines a plurality of inlet or suction ports 52 and a plurality of outlet ports 53. The inlet ports 52 are closed by resilient valve reeds 54 and the outlet ports 53 are closed by resilient valve reeds 55. A resilient flat spring 56 is secured to the valve plate 49. The spring 56 has at least one finger 57 (two shown) which extends through at least one of the inlet ports 52 and contacts the adjacent reed 54. The spring 56 is stronger than the resilient reed 54 and, consequently, the fingers 57 can deflect the adjacent reeds 54 away from the valve plate 49, as shown in Fig. 4. So long as the fingers 57 hold one or more of the inlet valve reeds 54 away from the valve plate 49, the compressor will be unloaded and minimum torque is required to start the compressor motor.
A rigid wire 58 is secured by suitable mounts 59 to rotate about an axis 60. The wire 58 has an S-shaped end 61 which extends between a tab 62 on the spring 56 and the valve plate 49. The wire 58 is generally U-shaped and has a side 63 which is spaced from an edge 64 of the valve plate 49 and is generally parallel to the axis 60. A vane 65 is secured to the wire side 63. If the cylinder 50 is generally vertically oriented, the vane 65 will fall through gravity to the position shown in dashed lines in Fig. 3. A finger 66 extending from the vane 65 will contact the cylinder 50 to limit travel of the vane 65. When the vane is in this position, the end 61 of the wire 58 will be in the position shown in Fig. 4 and the spring fingers 57 will hold at least one valve reed 54 open to unload the compressor. When the compressor motor is started and reaches a predetermined speed, air flow from a fan driven by the motor (such as the motor 12 and fan 35 of Fig. 1) will deflect the vane 65 to the position shown in solid in Fig. 3. This vane deflection rotates the wire 58 about the axis 60 and the wire end 61 lifts the spring tab 62 and fingers 57 to allow the previously open suction valve reeds 54 to close. Thus, the compressor begins to operate and the motor is loaded after the motor is started.
Figs. 5-8 are fragmentary views showing a further modified embodiment of a compressor unloader 70 according to the invention. As with the previous embodiments, a piston 71 is reciprocated in a cylinder 72. The cylinder 72 is closed by a valve plate 73 having a plurality of inlet ports 74 and a plurality of outlet ports 75. Resilient valve reeds 76 normally close the inlet ports 74 and resilient valve reeds 77 normally close the outlet ports 75.
The unloader 70 includes a rigid wire 78 attached to rotate in mounts 79 about an axis 80. The wire 78 has an end 81 which is bent to extend into one of the inlet ports 74. The wire 78 has a second end 82 which is spaced from an edge 83 of the valve plate 73 and extends substantially parallel to the axis 80. A vane 84 is attached to the wire end 82. The vane 84 is located to be moved in response to a predetermined flow of air from a fan driven by the compressor motor (such as the fan 35 driven by the motor 12 in Fig. 1). When the motor is off, gravity moves the vane 84 to the position shown in dashed lines in Fig. 6. While the vane 84 is in this position, the bent wire end 81 pushes an adjacent inlet valve reed 76 open, as shown in Fig. 8, to unload the compressor motor. While any of the inlet valve reeds 76 is held open, air will flow through the inlet port 74 on both suction and compression strokes of the piston 71. Consequently, the motor is subjected to only a minimum torque during startup. When the motor reaches a sufficient speed during startup, the air flow moves the vane 84 to the position shown in solid in Fig. 6 and the wire end 81 is moved away from the adjacent inlet valve reed 76, as shown in Fig. 7. At this speed and at higher motor speeds, the inlet valve reeds 76 are all allowed to close during the compression stroke of the piston 71 and the compressor will operate.
In the embodiments shown in Figs. 2-8, the vanes 65 and 84 move through the action of gravity to open one or more of the inlet valves when the motor is stopped. In tested compressors, gravity has been found sufficient to move the vane and unload the compressor with the cylinders arranged from a vertical orientation to tilted up to about 45°. If the cylinder is tipped too far from the vertical, a spring may be needed to move the vane to assure positive action of the unloader.
Fig. 9 is a fragmentary cross sectional view of a modified embodiment of an air compressor 90 according to the invention. The compressor 90 includes an electric motor 91 having an output shaft 92 which rotates both a cooling air fan 93 and an eccentric plate 94. An end 95 of a wobble piston 96 is connected to an eccentric pin 97 on the eccentric plate 94. As the motor 91 rotates the eccentric plate 94, the piston 96 is caused to reciprocate in a cylinder 98. A valve plate 99 and a head 100 are secured to a top 101 of the cylinder 98. An inlet chamber 102 in the head 100 is connected through a port 103 in the valve plate 99 and a intake check valve 104 to a compression chamber 105. The compression chamber 105 is connected through an outlet port 106 in the valve plate 99 and an outlet check valve 107 to an outlet chamber 108. A tube or hose 109 receives compressed air from the chamber 108.
According to the invention, the compressor 90 is unloaded during starting either by venting the outlet chamber 108 (as shown) or by venting the compression chamber 105 (not shown) through a normally open small vent hole 110. A vane 111 is mounted to pivot on a bracket 112 which is illustrated as being secured to the head 100. The vane 111 is influenced by air flow from the fan 93. Either through gravity or through the action of a spring (not shown), the vane will be in the position illustrated by the solid lines in Fig. 9 when the motor 91 is stopped or operating at a low startup or idle speed. When the motor speed reaches a predetermined level, the flow of cooling air from the fan 93 will be sufficient to pivot the vane 111 to the position 113 shown in dashed lines. A stopper 114 is secured to the vane at a location to plug the vent hole 110 when the vane 111 pivots to the position 113. Consequently, when the compressor motor reaches a predetermined minimum speed, the vent hole 110 will be plugged and the compressor 90 will supply compressed air to the tube 109. At lower motor speeds, the compressed air will be vented to atmosphere through the open hole 110.
It should be noted that the flow of cooling air required to close the vent hole 110 is determined by various factors. If, for example, the vent hole is 0.05 inch (1.27 mm) in diameter and the maximum output pressure from the compressor is 100 psi (0.0703 Kg/mm²), a maximum force of 0.2 pounds (0.00089 Kg) is required to stop the vent hole 110. This force is significantly reduced by the leverage exerted by the vane 111 due to the difference in distances between the pivotal connection of the vane 111 to the bracket 112 and the stopper 114 and between the pivotal connection of the vane 111 to the bracket 112 and an end 115 of the vane 111 on which the cooling air flow acts. The required air flow for stopping the vent hole 110 may be further reduced by increasing the area of the vane 111 impinged by the air flow. Thus, the motor speed at which the vent hole 110 will be closed will be set by adjusting various design factors for the vane 111.
The compressors shown and described above are of a type sometimes called oilless compressors because the wobble piston does not require an oil sump for continuous lubrication. It will be appreciated that the compressor unloader of the invention will be equally applicable to a compressor having a purely reciprocating piston of the type which is hinged to a connecting rod by a wrist pin. The invention is also applicable to other known compressor designs, such as rotating piston compressors. Various modifications and changes may be made to the above described preferred embodiments of the invention without departing from the spirit and the scope of the following claims.

Claims

An unloader for a motor driven air compressor, said motor also driving a cooling air fan, said compressor having a valve, said unloader comprising means for holding said valve open, and means responsive to a predetermined flow of cooling air from said fan for disabling said holding means to allow said inlet valve to close.
An unloader for a motor driven air compressor, as set forth in claim 1, and wherein said means responsive to a predetermined flow of cooling air includes a vane mounted to move between first and second positions, means urging said vane towards said first position, said vane moving from said first position to said second position in response to a predetermined flow of cooling air from said cooling air fan, and means for disabling said holding means when said vane is in said second position.
An unloader for a motor driven air compressor, as set forth in claim 2, wherein said disabling means is a flat spring mounted to hold said valve open when said motor is below a predetermined minimum operating speed, and wherein said means for disabling said holding means comprises means which lifts said spring clear of said valve when said vane is in said second position.
An unloader for a motor driven air compressor, as set forth in claim 3, wherein said valve is an inlet valve having a valve reed, and wherein said spring has a finger located to deflect said reed to an open position.
An unloader for a motor driven air compressor, as set forth in claim 1, wherein said disabling means comprises a rigid wire mounted to rotate about an axis, said wire having a first end attached to said vane, said vane rotating said wire about said axis when said vane moves between said first and second positions, and wherein said holding means comprises a second end of said wire located to hold said valve open when said vane is in said first position and to not interfere with operation of said valve when said vane is in said second position.
An unloader for a motor driven air compressor, as set forth in claim 5, wherein said valve is an inlet valve having a valve reed, and wherein said second wire end pushes said valve reed open when said vane is in said first position.
An unloader for a motor driven air compressor, said motor also driving a cooling air fan, said compressor having a compressed air outlet, said unloader comprising a normally open passage venting said outlet to atmosphere, and means responsive to a predetermined flow of cooling air from said fan for closing said normally open vent passage.
An unloader for a motor driven air compressor, as set forth in claim 7, and wherein said means responsive to a predetermined flow of cooling air comprises a vane, means mounting said vane to pivot to a first position when the flow of cooling air from said fan is less than said predetermined flow, said predetermined flow of cooling air causing said vane to pivot to a second position, and means for closing said normally open vent passage when said vane is in said second position.
An unloader for a motor driven air compressor, as set forth in claim 8, wherein said means for closing said normally open vent passage is a stopper secured to said vane.
An unloader for a motor driven air compressor, said motor also driving a cooling air fan, said cooling air fan establishing a predetermined air flow when said motor is operating above a predetermined speed, said compressor having a chamber which is pressurized during normal operation of said compressor, said unloader comprising a passage connected to said chamber, said passage venting pressure from said chamber when said passage is open, and means for maintaining said passage open to vent said chamber when the air flow from said fan is less than said predetermined air flow and for closing said passage when the air flow from said fan is equal to or greater than said predetermined air flow.
An unloader for a motor driven air compressor, as set forth in claim 10 and further including a check valve arranged to allow air flow into said chamber while preventing air flow from said chamber, and wherein said means for maintaining said passage open includes means for holding said check valve open when the air flow form said fan is less than said predetermined air flow and for allowing said check valve to close when the air flow is equal to or greater than said predetermined air flow.