Pneumatic telescopic hoist having three or more steps of extension

Claim 1

A compressible fluid-operated multi-stage hoist for operating up to a given load limit comprising: first, second, third and fourth vertically oriented cylindrical members of decreasing cross-sectional area telescopically arranged within each other, said first cylinder member being closed at the lower end thereof, said second cylinder member having a piston member secured thereto in sliding and sealing engagement with said first cylinder member and extending across the interior of said second cylinder member, and having a downwardly facing surface exposed to fluid pressure therebelow, said piston member being closed to passage therethrough from the interior of said first cylinder member to the interior of said second cylinder member during at least the major portion of extension and retraction of said third cylinder member, said third cylinder member having a piston member secured thereto in sliding and sealing engagment with said second cylinder member and extending across the interior of said third cylinder member, and having a downwardly facing surface exposed to fluid pressure therebelow, said piston member being closed to passage therethrough of fluid from the interior of said second cylinder member to the interior of said third cylinder member during at least the major portion of extension and retraction of said foUrth cylinder member, said fourth cylinder member having a piston member secured thereto in sliding and sealing engagement with said third cylinder member and extending across the interior of said fourth cylinder member and having a downward facing surface exposed to fluid pressure therebelow, means forming a first conduit from the exterior of said first cylinder member into the lower end thereof, means forming a seal between the upper end of said third cylinder member and the upper end of said second cylinder member, means forming a second conduit from the exterior of said second cylinder member into the upper end thereof, said second conduit extending down between said second and third cylinder members and extending through and terminating below said piston member of said third cylinder member, means forming a third conduit from the exterior of said fourth cylinder member into the upper end thereof, said third conduit extending through and terminating below said piston member of said fourth cylinder member, a source of compressible fluid at a pressure substantially greater than that required for the extended hoist to support said given load limit, control means for supplying compressible fluid from said source to said first, second and third conduits to cause sequential extension of said second, third and fourth cylinder members and for maintaining the interior of said first cylinder member at a pressure greater than the interior of said second cylinder member during extension of said third cylinder member, and for maintaining the interior of said second cylinder member at a pressure greater than the interior of said third cylinder member after extension of said third cylinder member and during extension of said fourth cylinder member, vent means for venting compressible fluid and pressure from said first, second and third conduits to cause sequential retraction of said fourth, third and second cylinder members and for maintaining the pressure in said second cylinder member higher than the pressure in said third cylinder member during retraction of said fourth cylinder member, and for maintaining the pressure in said first cylinder member higher than the pressure in said second cylinder member during retraction of said third cylinder member.