This invention relates to telescoping tubes, and more particularly to longitudinally extensible and retractable telescoping tubes which extend and retract in a vertical direction for inspection testing of large stationary objects.
Various telescoping crane booms have been developed in the past and have been known in the art for some time. An example of an extensible crane boom is shown in U.S. Pat. No. 3,481,490 issued to Eiler which teaches a telescopic crane jib which is formed by welding two folded metal sheets together. The Eiler patent teaches the use of metal of different sheet thicknesses wherein the bottom section is thicker because it is subjected to heavy compressive loads versus the top sheet which is thinner and under tension loads. Eiler further teaches the use of rollers or thrust elements in between each of the telescopic sections. A similar telescoping tubular boom patent is U.S. Pat. No. 4,004,695 issued to Hockensmith et al. which teaches a tubular boom with a thicker, flat bottom plate and a thinner channel having the free edges of its walls welded to the bottom plate at the lateral edges of the bottom plate. Hockensmith also teaches the placement of wear pads for supporting an outer boom section about the periphery of each prior section to provide substantially columnar support for the walls of the outer section while reducing bottom plate bending forces.
As one can see, these telescoping crane booms involve lifting substantial weights or loads which cause very large bending moments on the crane boom. These crane boom bending moments and forces are offset typically by increasing the boom rigidity and strength through increasing the width of each boom section. This increase in width adds excessive weight in an attempt to combat the moment created about its base. Thus, various alternatives, such as thicker flat bottom plates and thinner surrounding channels have been used in attempts to lighten the weight while increasing the strength.
Vertically telescoping devices, which are similar in appearance to the crane booms involve considerably different design considerations. Instead of being concerned about compressive loads on a lower section versus tension loads on a top section as well as the bending moment about its base as in the crane booms, vertically telescoping tubes, operating generally in the same direction as gravity, are more concerned about accurate movement, higher rigidity, higher dampening, low-cost construction, and linear load capability.
The art area of vertically telescoping tubes or extensible/retractable members typically involves inventions more concerned with accuracy, reliability and control, in contrast to crane booms, which are more concerned with the distance a load can be supported and how large of a load can be supported safely by the boom. British Patent No. 970,441 discloses a power manipulator which vertically positions itself by extending a column assembly comprising a number of telescoping sections adapted to rest one within another. This patent further teaches that control of position is important thus the vertical movement should be as smooth as possible. The British patent discloses a column comprising an assembly of three telescopic tubes which rest one within another and are stowed within a carriage. Three pairs of rollers are carried on the inner surface of a cylindrical shroud at the base of the carriage and these three pairs of rollers cooperate with three equally spaced vertical guides on the outermost tube. The middle and inner tube are similarly guided.
It is desirable to improve vertically telescoping tubes such that a higher accuracy, higher rigidity, higher dampening and low-cost construction exist. Such an improvement is needed so that when testing devices are attached to the end of the vertically telescoping tube, extreme accuracy will result when retracting or extending the vertically telescoping tube such that the tube is always properly positioned thereby allowing a proper recording of its position to be made with the corresponding structural integrity measurements. It is important that the vertically telescoping tube be capable of being used in high precision applications where high repeatability, quick settling times, and no loss of motion are of paramount importance.