US3773296A

US3773296A - Pneumatic balancing hoist

Info

Publication number: US3773296A
Application number: US00187870A
Authority: US
Inventors: L Mckendrick
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-10-08
Filing date: 1971-10-08
Publication date: 1973-11-20
Anticipated expiration: 1990-11-20

Abstract

A pneumatic hoist for raising, lowering or balancing a variable weight load in a suspended position by regulating air pressure in a power chamber. A hand-held vent valve is employed to select the pressure necessary to balance the weight of each load as it is being raised.

Description

United States Patent McKendrick Nov. 20, 1973 [5 PNEUMATIC BALANCING HOIST 2,919,109 12/1959 Minty 254 150 FH 3,384,350 5/1968 Powell 254/168 X [76] 3,428,298 2/1959 Powell 254/168 Surfwood Dr., M1lford, M1ch.

[22] Filed: Oct. 8, 1971 Primary ExaminerRichard E. Aegerter [21] Appl' Assistant ExaminerMerle F. Maffei Related U.S. Data Attorney-Robert C. Hauke et a].

[63] Continuation of Ser. No. 640, Jan. 5, 1970,

abandoned.

52 us. Cl. 254 189, 137/116.5, 254/150 FH, 1 1 ABSTRACT 254/168 [51] Int. Cl B6641 3/08 A pneumatic hoist for raising, lowering or balancing a [58] Field of Search 254/168, 186, 150 FH; variable weight load in a suspended position by regu- 137/ 1 16.5 lating air pressure in a power chamber. A hand-held vent valve is employed to select the pressure necessary [56] References Cited to balance the weight of each load as it is being raised.

UNITED STATES PATENTS 2,500,879 3/1950 Smallpiece 254/150 PH 5 Claims, 3 Drawing Figures PATENTEUNHV 20 ms SHEET 1 [1; 2

FIG-2 FIG-I INVENTOR LORNE J No KENDRICK PAIENTEI] NOV 20 ms II m l INVENTOR LORNE J. Mc KENDRICK 72 M yww/b/a PNEUMATIC BALANCING *IIOIST This is a continuation of application Ser. No. 640 filed Jan. 5, 1970 now abandoned.

BACKGROUND OF THE INVENTION 1. Field Of The Invention This invention relates to pneumatically-operated hoists, and more particularly to an air balanced hoist having a hand control for regulating the air pressure employed to raise each load and to balance it in a suspended position.

2. Description Of The Prior Art Hoists employing a pneumatic balancer are useful in industrial processes in which the load is raised to a suspended position, and then manually positioned in a fixture or the like by the operator. The balancer has a piston connected to the load. The piston is in a power chamber in which air pressure is regulated to move the piston and thereby either raise or lower the load.

Certain commercially available air-balanced hoists employ a pilot-operated valve for regulating the power chamber pressure. The general principles of such regu- 'lators are well known to those skilled in the art. Examples have been disclosed in U.S. Pat. Nos. 2,384,774 issued to C. D. P. Smallpeice, 3,384,350 issued to E. R. Powell, and others. Typically, a portion of the air at line pressure is passed through a pilot regulator which reduces the line pressure to a maximum pilot pressure that depends upon the maximum weight that is to be hoisted.

A valve which is responsive to pilot pressure changes is disposed at the inlet of the power chamber to control its pressure and thereby control the motion of the piston in the following manner. When the load is connected by a hoisting line to the piston the operator raises pilot pressure. The pilot-operated valve responds by raising power chamber pressure until the piston begins to move. Since pilot pressure is chosen to slightly overbalance the load, the load is hoisted as the piston moves.

Upon being raised to a suspended position, the load can be manually positioned by the operator because the load weight is balanced by air pressure. He can easily stop its upward motion since only a slight effort is sufficient to compensate for the slight overbalance of the power chamber pressure. To manually lower the load, he need only apply a slightly greater downward effort. The resulting piston motion tends to compress the power chamber air which causes the pilot-operated valve to open a relief connection between the power chamber and the atmosphere.

A problem with some air-balanced hoists of the prior art is that the pilot regulator must be readjusted to accommodate changes in the load weight. This disadvantage is due to the overhead location ofthe pilot regulator which requires the operator to interrupt his hoisting activity to make the necessary load adjustment.

A limited solution to this problem was disclosed in the aforementioned Powell patent. A hand-operated control on the lower end of a flexible conduit has a pair of openings for venting the pilot air, each opening being related to a different load weight. The control provides a means for remotely changing the pilot pressure by bleeding off a predetermined portion of its pressure depending upon which of the vent openings is connected to the atmopshere. The problem with this venting arrangement is that the operator is still limited to a certain number of loads unless he interrupts the continuity of his hoisting operation. For loads of other weights, he must readjust either the pilot regulator or the hand control while the hoist is not being used.

SUMMARY OF THE INVENTION The broad purpose of the present invention is to provide an air-balanced hoist having a hand operated control for selecting the power chamber pressure necessary to balance each load as it is being hoisted. The operator can control the balancer to accommodate either large or small variations in load weight while retaining a sensitivity over the motion of the load that allows a load of several hundred pounds to be easily positioned.

The preferred embodiment employs a control needle valve for venting pilot air. The control valve is so mounted on a handle that the operator can precisely adjust pilot pressure with the thumb of one hand as he uses his other hand to position the load. Only one control member need be manipulated to accommodate either a large change in the load weight as when different work pieces are being hoisted, or to accommodate small variances in load weight which occur in certain types of industrial castings. The operator is not limited to only certain pre-adjusted loads.

Still further objects and advantages of the present invention will become readily apparent to those skilled in the art to which the invention pertains upon reference to the following detailed description.

DESCRIPTION OF THE DRAWINGS The description refers to the accompanying drawings in which like reference characters refer to like parts throughout the several views, and in which:

FIG. 1 is a perspective view of an air-balanced hoist illustrating the preferred embodiment of the invention;

FIG. 2 is an enlarged view of the hand-operated control valve, parts being shown in section for purposes of clarity; and

FIG. 3 is a schematic diagram of the preferred hoist and balancer.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, a preferred air-balanced hoist 10, is illustrated in FIG. 1 for hoisting a load 12. The hoist 10 includes an elongated housing 14 having a pair of

ends

16 and 18 connected together by tie rods 20.

An eyebolt 22 is connected to the housing 14 for attaching it to an overhead rail or trolley (not shown). A piston 24 is slidably mounted in the housing 14 to form a power chamber 26 adjacent end 16. A second piston 28 is also slidably mounted in the housing 14, and connected by a pair of rods 30 (only one shown) to the piston 24 such that both pistons move as a unit.

Referring to FIG. 3, a pair of sheaves 32 are mounted in a fixed position adjacent the midsection of housing 14, and a second pair of sheaves 34 are carried by piston 28. Thus, as piston 24 is moved away from end 16, piston 28 and sheaves 34 are moved away from sheaves 32. Similarly sheaves 34 are moved toward sheaves 32 as the pistons 24 and 28 are moved toward end 16.

A hoist line 36 passes over the two sets of

sheaves

32 and 34. One end of line 36 is connected to the housing 14, and its other end is connected to a weight 38. As the two sets of

sheaves

32 and 34 are moved toward one another, the hoist line 36 is extended down from the housing 14 by the weight 38 and as the two sets of sheaves are moved away from one another the hoist line 36 is retracted up into the housing. The arrangement is such that the line is extended or retracted a distance equal to the piston stroke multiplied by the number of sheaves employed. A fixture 40 is fixed to a load carrying member 41 which in turn is connected to a hook 42 carried by weight 38 (FIG. 1'). The load 12 is supported by the member 41 so as to-be easily detached from the hoist. 7

As can best be seen in FIG. 3, a source of air under pressure 44 is connected by a line 46 to the power chamber 26. A valve 48 is disposed between the source 44 and the hoist, to open and close communication between the source and the power chamber 26. A normally closed two way valve 50 is disposed in line 46at the inlet of power chamber 26. A spring 52 biases valve 50 toward its closed position, and air pressure sensed through a line 54 connected to the outlet side of valve 48 biases the valve 50, toward its open position. The arrangement is such that if valve 48 is closed, or if there is a loss of pressure from source 44, because of some other occurrence, the spring 52 closes valve 50. However, valve 50 is opened in response to the pressure of air pressure from the source 44 in line 54. The valve 50 therefore functions as a fail-safe device by automatically closing in response to a pressure loss from the source to trap the air in power chamber 26. The trapped air prevents the load 12 from being suddenly dropped from a hoisted position.

v i A pilot operated valve is disposed be tween valve 48 and valve 50. Such pilot-operated valves are commercially available as a means for maintaining a certain relationship between a controlling pilot pressure and the pressure of a controlled device such as power chamber 26. In the present invention, valve 56 regulates air pressure in power chamber 26 in accordance with variations in the pilot pressure sensed in pilot line 58. If the pilot pressure is increased, the power chamber pressure is raised, and if pilot pressure is reduced, valve 56 operates to reduce the power chamber pressure.

Valve 56 includes a housing 60 having an inlet 62 for receiving air from source 44, and an outlet 64 for passing air from the inlet to power chamber 26. A valve seat 66 is disposed in the housing for fluidly connecting inlet 62 to outlet 64. A spacer 68 is supported by a pair of diaphragms 70 and 72 in housing 60 to form a movable wall between the outlet 64 and an internal wall 74. The spacer 68 has a valveseat 69 for connecting outlet64 to a relief passage 76 which opens to the atmosphere. A pilot chamber 78 is disposed between the spacer 68 and oneside of wall 74, and a control chamber 80 is disposed. 95 the an s si of Wall A passage 82 connects control chamber 80 to outlet 64 so that outlet pressure is present in the control chamber 80. An opening 84in wall 74 connects the control msaiber Wit 5516i Ehambrlfi, anaaa'a em 86 connects pilot chamber 78 to the atmosphere.

A hollow pressure-sensitive device 88 is disposed in control chamber 80 with its outer surface being exposed to control chamber pressure, and its interior being connected to the pilot line 58 so as to be expanded or collapsed depending upon the differential between pilot pressure and control chamber pressure. A valve 90 is carried by device 88 to close opening 84 as the device 88 is expanded by pilot pressure, and to unblock opening 84 as the device 88 is collapsed by control chamber pressure.

An elongated valve 92 is mounted between the spacer 68 and inlet 62. One end of valve 92 is adapted to open or close communication between outlet 64 and the relief passage 76, and its other end is adapted to open or close valve seat 66 to control communication between inlet 62 and outlet 64.

Pilot-operated valve 56 operates in the following manner. Assuming a given pilot pressure, if the outlet pressure is greater than either pilot pressure or inlet pressure, the control pressure in control chamber will be at outlet pressure such that the device 88is collapsed by the greater control pressure to open valve 90. Since opening 86 is smaller than opening 84, the pressure in pilot chamber 78 will be substantially the same as both control chamber pressure and outlet pressure.

Under this condition, both sides of spacer 68 are biased by substantially the same air pressure, and a spring 94 disposed between wall 74 and the spacer 68 biases the spacer 68 toward valve seat 66 such that the upper end of valve 92 closes communication between outlet 64 and relief passage 76. The lower end of valve 92 ps as m un i betwe inlet 2 and tlay;

With valve seat 66 open, air at the higher inlet pressure a s tlr hputlet 64 toward power chamber 26 As outlet pressure decreases, control chamber pressure also decreases to gradually expand device 88 and move valve to its closed position. As soon as valve 90 is closed, the pressure in pilot chamber 78 rapidly drops as the air exhausts through opening 86. Spacer 68 is then moved toward wall 74 by the outlet pressure and thereby allows the lower end of valve 92 to move to its closed position as the outlet pressure reaches pilot pressure.

If outlet pressure thereafter falls below pilot pressure, the resulting drop in control chamber pressure causes device 88 to expand even moreso and force valve 90 down through opening 84 to open valve 90 and increase the pressure in pilot chamber 78. The increased pressure in pilot chamber 78 then moves spacer 68 downward to open valve seat 66 and the foregoing process repeats itself. I k V V Assuming a condition occurs that increases outlet pressure above pilot pressure when both valve seat 66 and opening 84 are closed, the spacer 68 is biased away from the upper end of valve 92 by the outlet pressure until relief passage 76 is opened. Outlet air then exhausts itself to the atmosphere until outlet pressure is reduced to pilot pressure. As the outlet pressure is reduced, spacer 68 moves toward valve 92 until relief passage 76 is closed. Thus, pilot-operated valve 56 continuously operates to maintain outlet pressure, and

thereby power chamber pressure, in accordance with pilot pressure.

A fluid line 94 connects the inlet of pilot line 58 to the outlet of the air source 44. A conventional pilot regulator valve 96 in line 94 provides means for the operator to reduce line pressure as received from source 44 to a certain level that depends upon the maximum weight expected to be hoisted. A trim valve 98 is disposed in line 94 at the outlet of pilot valve 96 to control the rate that air can be vented through a flexible control conduit or line 100 from line 94. Pilot valve 96 and pilot-operated valve 56 are preferably mounted on one end of hoist housing 14 which is normally mounted in an overhead position. Control line 100 extends down from the housing 14, and a control valve 102 is disposed at the lower outlet end of the control line.

As can best be seen in FIGS. 1-2, the control valve 102 is mounted on fixture 40 to allow the operator to manually position the load 12 as it is being hoisted or balanced in a suspended position. Control valve 102 includes a body 104 having an orifice 106 for venting pilot air to the atmopshere. A needle valve 108 is mounted on body 104 for regulating air flow through orifice 106.

A knob 110 is carriedon needle valve 108 adjacent a handle 112. The handle provides means for the operator to manually position fixture 40. Knob 110 is so mounted adjacent handle 112 such that the operators thumb can easily rotate needle valve 108 to precisely control the rate of air flow through orifice 106.

It is apparent that by operating control valve 102, the operator can raise and lower pilot pressure. When orifice 106 is closed, pilot pressure in pilot lines 58 is normally the same as the outlet pressure of the pilot valve 96. As the control valve 102 is manipulated to open orifice 106, pilot air is vented to the atmosphere thereby reducing the pressure in pilot line 58 below the outlet pressure of the pilot valve. The reduction in pilot pressure in pilot line 58 is proportional to the opening of orifice 106.

A spring 114 is mounted between needle valve knob 110 and body 102 to provide a slight frictional resistance to the turning effort and thereby provide the operator with a sensitivity in regulating the air flow rate. In addition, trim valve 98 (FIG. 3) provides an additional sensitivity by governing the manner in which pilot pressure is reduced. It provides an inherent time delay in the rate that pilot pressure responds to the opening of control valve 102. Thus although the operator may quickly fully open control valve 102 from a fully closed position, the change in pilot pressure sensed by the pilot-operated valve 56 will be at a lesser rate to prevent the load 12 from being suddenly dropped by an inexperienced operator.

In summary, the operator controls the motion of load 12 by controlling pilot pressure. Because he can precisely vary pilot pressure, he can easily raise, lower, or balance the load 12 in a suspended position. Assuming the load 12 is attached to fixture 40 and is to be raised to a balanced position, the operator closes control valve 102 to increase pilot pressure. Power chamber pressure is accordingly increased to move piston 24 and retract the line 36 into housing 14. As the load 12 is raised, the operator manipulates control valve 102 to lower pilot pressure until piston 26 is balanced between the bias of the power chamber pressure in one direction, and the bias of the load in the opposite direction. When the piston 24 is so balanced, the load 12 is balanced in a motionless suspended position.

The operator can easily compensate the power chamber pressure for either large or small variations in the weight of individual loads. He can perform the necessary adjustment in pilot pressure as each load is being hoisted. He need not interrupt his hoisting activity to adjust the pilot valve for large changes in load weight. He easily controls the pilot pressure with his thumband without special tools.

To lower the load 12 from its suspended position, the operator opens control valve 102 to lower pilot pressure. Since the control valve is open and venting pilot air when the fixture 40 is in its lower position between loads, the pilot valve 96 is preferably adjusted for the maximum weight that is expected to be hoisted during a given hoisting activity in order to conserve compressed air. If the maximum weight that is normally hoisted is changed, the pilot valve 96 is appropriately adjusted.

Having described my invention, I claim:

1. A hoist suitable for hoisting a series of loads of different weights, said hoist comprising:

a pneumatically-operated device having a fluid power chamber;

a piston disposed in the power chamber for movement according to fluid power chamber pressure;

a hoisting member having means at one end for connection to a load, and the other end connected to hoist the load according to piston movement;

a source of air underpressure;

an elongated fluid conduit having one end connected to the source of air pressure;

a pilot operated valve comprising:

means defining an inlet for receiving air from said source of air under pressure;

means defining an outlet communicating through said pilot operated valve with said inlet, said outlet operatively communicating with the fluid power chamber of the pneumatically operated device to provide pressurized air thereto;

means defining a control chamber;

means defining a passage connecting said control chamber to said outlet so that outlet pressure is communicated to said control chamber;

means defining a pilot chamber, said pilot chamber having a passage to the atmosphere;

a wall disposed between said control chamber and said pilot chamber, said wall having an opening which connects said control chamber to said pilot chamber;

a hollow pressure-sensitive device housed in said control chamber, the interior of said hollow pressure-sensitive device being operatively connected to said elongated fluid conduit between its ends so that the interior of said hollow pressure-sensitive device is subjected to said air pressure from said source, and the outer surface of said hollow pressure-sensitive device being exposed to the outlet pressure in said control chamber, so that said pressure-sensitive device expands and contracts in response to a differential pressure between the outlet pressure in said control chamber and the air pressure in the interior of said pressure-sensitive device;

said inlet of said pilot operated valve being operatively connected to said elongated fluid conduit between said end of said elongated fluid conduit connected to the source of air pressure and said operative connection of said hollow pressure-sensitive device to said elongated fluid conduit;

a valve operatively connected to the outer surface of said pressure-sensitive device, said valve being disposed in said opening in said wall between said control chamber and said pilot chamber to control the communication between said control chamber and said pilot chamber to close said opening in said wall when said pressure-sensitive device expands due to a differential higher internal pressure, and to open said opening when said pressure-sensitive device contracts due to a differentially higher pressure on its outer surface;

a spacer forming a movable wall between said outlet and said wall, said pilot chamber being defined by said spacer and said wall, said spacer having a relief passage connecting said outlet to the atmosphere:

a valve disposed in said relief passage to control the opening and closing of said relief passage;

a valve disposed between said inlet and said outlet to close and open the communication therebetween,

so that when the outlet pressure is less than either the pilot pressure or the inlet pressure, the air pressure in said control chamber is equal to the outlet pressure causing said pressure-sensitive device to open said valve disposed in said opening in said wall between said control chamber and said pilot chamher, and as the outlet pressure increases, the pressure in said control chamber increases to a value greater than the air pressure interior to said pressure-sensitive device causing said pressuresensitive device to contract closing said valve in said opening in said wall between said control chamber and said pilot chamber; and control member mounted on the opposite end of the fluid conduit adjacent said load connecting means of said housing member for venting air pressure from said elongated fluid conduit to thereby control the pressure of air admitted to said pressure-sensitive device thereby controlling the pressure of the air admitted to the power chamber from the outlet of the pilot-operated valve, the control member consisting of a single needle valve cooperating with a port so that said control member is infinitely variable between fully opened and fully closed positions of said needle valve, a knob carried by said needle valve and a handle disposed beneath said knob whereby said handle can be manipulated by the thumb of the user to rotate said needle valve to permit selective variation of pilot fluid pressure according to the weight of a load being hoisted bythe hoisting member.

2, A hoist as defined in claim 1, in which the pilotoperated valve is responsive to a differential between power chamber pressure and pilot fluid pressure.

3. A hoist as defined in claim 1, including a relief valve means mounted on said pneumatically-operated device so as to be responsive to power chamber pressure being greater than pilot fluid pressure.

4. A hoist as defined in claim 1, including a handle structure connected to the hoisting member so as to be movable with the hoisting member, and in which the control member is mounted on the handle structure.

,5. In combination, a pneumatically operated hoist having a hoist housing, a piston assembly movable in said housing, a first set of pulleys fixed within said housing, a second set of pulleys within the housing supported by said assembly for movement therewith towards and away from said first set of pulleys, a cable wound around said first and said second set of pulleys having one end anchored in said housing, the other end of said cable extending out from said housing for attachment to and support of a load therein, a fluid pressure chamber defined between the front end of said pisset of pulleys towards or away from said first set of pulleys, said means comprising a a pilot operated valve including:

means defining an inlet connected with a source of fluid pressure;

means defining an outlet communicating through said pilot operated valve with said inlet, said outlet operatively communicating with the fluid pressure chamber of the said hoist to provide pressurized air thereto;

means defining a control chamber;

a wall disposed between said control chamber and and said pilot chamber, said wall having an opening which connects said control chamber to said pilot chamber;

a hollow capsule housed in said control chamber, the interior of said hollow capsule being operatively connected to said elongated fluid conduit between its ends so that the interior of said hollow capsule is subjected to said air pressure from said source and the outer surface of said hollow capsule being exposed to the outlet pressure in said control chamber, so that said hollow capsule expands and contracts in response to a differential pressure between the outlet pressure in said control chamber and the air pressure in the interior of said hollow capsule;

a valve operatively connected to the outer surface of said hollow capsule, said valve being disposed in said opening in said wall between said control chamber and said pilot chamber to control the communication between said control chamber and said pilot chamber to close said opening in said wall when said hollow capsule expands due to a differential higher internal pressure, and to open said opening when said hollow capsule contracts due to a differentially higher pressure on its outer surface;

a spacer forming a movable wall between said outlet and said wall, said pilot chamber being defined by said spacer and said wall, said spacer having a relief passage connecting said outlet to the atmosphere;

so that when the outlet pressure is less than either the pilot pressure or the inlet pressure, the air pressure in said control chamber is equal to the outlet pressure causing said hollow capsule to open said valve disposed in said opening in said wall between said control chamber and said pilot chamber and as the outlet pressure increases, the pressure in said control chamber increases to a value greater than the air pressure interior to said pressure-sensitive device causing said hollow capsule to close said valve in said opening in said wall between said control chamber and said pilot chamber;

the improvement comprising an elongated fluid conduit having one end connected to a source of fluid pressure, the interior of said capsule being connected to the elongated fluid conduit between its ends, said inlet of said pilot operated valve being connected to said elongated fluid conduit between said end of said elongated fluid conduit connected to the source of fluid pressure and said connection of said capsule to said elongated fluid conduit, and

a control member mounted on the opposite end of the fluid conduit adjacent th free end of said cable for venting fluid pressure from said capsule to thereby control the pressure of air admitted to the fluid pressure chamber, the control member comprising a needle valve cooperating with a port connected with said fluid conduit so that said control being hoisted by the hoisting member.

Claims

1. A hoist suitable for hoisting a series of loads of different weights, said hoist comprising: a pneumatically-operated device having a fluid power chamber; a piston disposed in the power chamber for movement according to fluid power chamber pressure; a hoisting member having means at one end for connection to a load, and the other end connected to hoist the load according to piston movement; a source of air under pressure; an elongated fluid conduit having one end connected to the source of air pressure; a pilot operated valve comprising: means defining an inlet for receiving air from said source of air under pressure; means defining an outlet communicating through said pilot operated valve with said inlet, said outlet operatively communicating with the fluid power chamber of the pneumatically operated device to provide pressurized air thereto; means defining a control chamber; means defining a passage connecting said control chamber to said outlet so that outlet pressure is communicated to said control chamber; means defining a pilot chamber, said pilot chamber having a passage to the atmosphere; a wall disposed between said control chamber and said pilot chamber, said wall having an opening which connects said control chamber to said pilot chamber; a hollow pressure-sensitive device housed in said control chamber, the interior of said hollow pressure-sensitive device being operatively connected to said elongated fluid conduit between its ends so that the interior of said hollow pressuresensitive device is subjected to said air pressure from said source, and the outer surface of said hollow pressure-sensitive device being exposed to the outlet pressure in said control chamber, so that said pressure-sensitive device expands and contracts in response to a differential pressure between the outlet pressure in said control chamber and the air pressure in the interior of said pressure-sensitive device; said inlet of said pilot operated valve being operatively connected to said elongated fluid conduit between said end of said elongated fluid conduit connected to the source of air pressure and said operative connection of said hollow pressuresensitive device to said elongated fluid conduit; a valve operatively connected to the outer surface of said pressure-sensitive device, said valve being disposed in said opening in said wall between said control chamber and said pilot chamber to control the communication between said control chamber and said pilot chamber to close said opening in said wall when said pressure-sensitive device expands due to a differential higher internal pressure, and to open said opening when said pressure-sensitive device contracts due to a differentially higher pressure on its outer surface; a spacer forming a movable wall between said outlet and said wall, said pilot chamber being defined by said spacer and said wall, said spacer having a relief passage connected said outlet to the atmosphere; a valve disposed in said relief passage to control the opening and closing of said relief passage; a valve disposed between said inlet and said outlet to close and open the communication therebetween, so that when the outlet pressure is less than either the pilot pressure or the inlet pressure, the air pressure in said control chamber is equal to the outlet pressure causing said pressure-sensitive device to open said valve disposed in said opening in said wall between said control chamber and said pilot chamber, and as the outlet pressure increases, the pressure in said control chamber increases to a value greater than the air pressure interior to said pressure-sensitive device causing said pressure-sensitive device to contract closing said valve in said opening in said wall between said control chamber and said pilot chamber; and a control member mounted on the opposite end of the fluid conduit adjacent said load connecting means of said housing member for venting air pressure from said elongated fluid conduit to thereby control the pressure of air admitted to said pressure-sensitive device thereby controlling the pressure of the air admitted to the power chamber from the outlet of the pilot-operated valve, the control member consisting of a single needle valve cooperating with a port so that said control member is infinitely variable between fully opened and fully closed positions of said needle valve, a knob carried by said needle valve and a handle disposed beneath said knob whereby said handle can be manipulated by the thumb of the user to rotate said needle valve to permit selective variation of pilot fluid pressure according to the weight of a load being hoisted by the hoisting member.

2. A hoist as defined in claim 1, in which the pilot-operated valve is responsive to a differential between power chamber pressure and pilot fluid pressure.

5. In combination, a pneumatically operated hoist having a hoist housing, a piston assembly movable in said housing, a first set of pulleys fixed within said housing, a second set of pulleys within the housing supported by said assembly for movement therewith towards and away from said first set of pulleys, a cable wound around said first and said second set of pulleys having one end anchored in said housing, the other end of said cable extending out from said housing for attachment to and support of a load therein, a fluid pressure chamber defiNed between the front end of said piston assembly and one end of said housing, means supplying said fluid pressure chamber with fluid under pressure to move said piston assembly and said second set of pulleys towards or away from said first set of pulleys, said means comprising a a pilot operated valve including: means defining an inlet connected with a source of fluid pressure; means defining an outlet communicating through said pilot operated valve with said inlet, said outlet operatively communicating with the fluid pressure chamber of the said hoist to provide pressurized air thereto; means defining a control chamber; means defining a passage connecting said control chamber to said outlet so that outlet pressure is communicated to said control chamber; means defining a pilot chamber, said pilot chamber having a passage to the atmosphere; a wall disposed between said control chamber and and said pilot chamber, said wall having an opening which connects said control chamber to said pilot chamber; a hollow capsule housed in said control chamber, the interior of said hollow capsule being operatively connected to said elongated fluid conduit between its ends so that the interior of said hollow capsule is subjected to said air pressure from said source and the outer surface of said hollow capsule being exposed to the outlet pressure in said control chamber, so that said hollow capsule expands and contracts in response to a differential pressure between the outlet pressure in said control chamber and the air pressure in the interior of said hollow capsule; a valve operatively connected to the outer surface of said hollow capsule, said valve being disposed in said opening in said wall between said control chamber and said pilot chamber to control the communication between said control chamber and said pilot chamber to close said opening in said wall when said hollow capsule expands due to a differential higher internal pressure, and to open said opening when said hollow capsule contracts due to a differentially higher pressure on its outer surface; a spacer forming a movable wall between said outlet and said wall, said pilot chamber being defined by said spacer and said wall, said spacer having a relief passage connecting said outlet to the atmosphere; a valve disposed in said relief passage to control the opening and closing of said relief passage; a valve disposed between said inlet and said outlet to close and open the communication therebetween, so that when the outlet pressure is less than either the pilot pressure or the inlet pressure, the air pressure in said control chamber is equal to the outlet pressure causing said hollow capsule to open said valve disposed in said opening in said wall between said control chamber and said pilot chamber and as the outlet pressure increases, the pressure in said control chamber increases to a value greater than the air pressure interior to said pressure-sensitive device causing said hollow capsule to close said valve in said opening in said wall between said control chamber and said pilot chamber; the improvement comprising an elongated fluid conduit having one end connected to a source of fluid pressure, the interior of said capsule being connected to the elongated fluid conduit between its ends, said inlet of said pilot operated valve being connected to said elongated fluid conduit between said end of said elongated fluid conduit connected to the source of fluid pressure and said connection of said capsule to said elongated fluid conduit, and a control member mounted on the opposite end of the fluid conduit adjacent th free end of said cable for venting fluid pressure from said capsule to thereby control the pressure of air admitted to the fluid pressure chamber, the control member comprising a needle valve cooperating with a port connected with said fluid conduit so that said control member is infinitely variable between fully opened and fully closed positions, sAid needle valve including a knob and said control member including a handle portion disposed below said knob whereby said knob can be rotated by the thumb of user while its handle portion is grasped to rotate said needle valve to permit selective variation of pilot fluid pressure according to the weight of a load being hoisted by the hoisting member.