Patent Application
20090095574
Traction sheaves are used in a variety of load-lifting applications such as elevated platforms for building maintenance. Such a platform can be used in conjunction with a motorized hoisting device, whereby the hoisting device is attached to the platform of an elevated platform or basket that may then be raised or lowered using the hoisting device. The hoisting device and associated rigging typically comprises a traction sheave, whereby the load of a continuous cable or rope passes through the sheave.
The traction sheave is generally designed to operate with significant tension in the cable or rope encircling the sheave. One characteristic of traction sheaves and in particular the single sheave “V” groove type is that the cable or rope's points of contact with the sheave may have different radii at different positions around the sheave. As a result, the velocity of the wire rope may vary according to the instantaneous radius at which each point is in contact with the sheave. Consequently, the wire rope will tend to be either tensioned or bunched as adjacent portions of the wire rope are at lesser or greater radii than the average radius around the traction sheave.
At the points where the traction sheave contacts the cable or rope, friction creates driving tension in the cable or rope. Generally the wire rope will tend to bunch in a known region of the sheave. When sufficient wire rope has bunched and laterally lifted off the tension sheave, the friction between the sheave and wire rope in that region decreases to the point where the tensioned cable or rope may slip or develop slack. The cable or rope may then tighten under the applied load and suddenly resume full contact and friction with the sheave. This event may produce a loud and objectionable popping impulse noise generated when the cable or rope snaps back against the traction sheave. Generally, the tendency to generate the popping noise increases with a poorly maintained dry, non-lubricated cable or rope as the incidence of the bunching may increase.
Several methods of restraining wire rope movement relative to the traction sheave on traction hoists are known. For example, U.S. Pat. No. 4,681,301 discloses a series of rollers restraining the wire rope to the proximity of the sheave groove. U.S. Pat. No. 5,082,248 discloses a segment of rollers restraining the wire rope to the proximity of the sheave groove. U.S. Pat. No. 4,706,940 discloses a pair of rollers restraining the wire rope to the proximity of the sheave groove. A disadvantage of the above disclosed methods is the generally increased cost and complexity in implementing the disclosed devices, and the potential for an increased incidence of rope jams.
Other methods of restraining wire rope displacement relative to the traction sheave have been developed such as that disclosed in U.S. Pat. No. 4,193,311. However, the method disclosed performs a function different to the situation described above. What is needed is a restraint assembly that can reduce or eliminate the lateral rope or cable movement that can result in the objectionable popping noise, and can further be easily removed and replaced without excessive disassembly of the hoisting device and traction sheave and the removal of the lines, ropes or cables.
In various embodiments, a restraint device is disclosed that limits the lateral movement of the cable or rope relative to the traction sheave, thus reducing or eliminating the popping noise associated with the cable or rope snapping back against the traction sheave. The disclosed restraint device further provides the features of low cost, low wear, and simple removal and replacement.
In one embodiment, the restraint device comprises a molded component which is fitted to encompass the region of the traction sheave where the separation of the cable or rope from the traction sheave is most likely to occur. The restraint device may be mounted so that it is removable and replaceable with minimal dismantling of the hoist and rigging associated with the sheave. In a further embodiment, the restraint device may be symmetrical and reversible or rotatable such that one restraint device may be used for two or more life cycles before being replaced. Abrasion and wear of the restraint device is minimized by allowing a small space between the cable or rope and the restraint device, thereby minimizing contact with the restraint device without increasing the incidence of wire rope jams.
Because the restraint device has no moving parts, contamination from the wire rope does not adversely affect the device's function in eliminating popping as compared to a roller type system in which contamination from corrosives or particulates may more easily accumulate. Where contaminants are present in and around the traction sheave, a further feature of the disclosed restraint device is that abrasion with the cable or rope may provide a self cleaning function.
In another embodiment, indication of the need for replacement is provided by including a second color material at the depth where wear reaches an allowable limit, thereby providing a visual signal to initiate replacement as can be observed during service and maintenance. Alternatively, by placing a hole transversely in the molding at the position of maximum wear, when the wear reaches the hole, the hole will break through and the wear limit will be signaled as a break in the wire rope track.
The foregoing summary, as well as the following detailed description of preferred embodiments, is better understood when read in conjunction with the appended drawings. For the purposes of illustration, there is shown in the drawings exemplary embodiments; however, the present disclosure is not limited to the specific methods and instrumentalities disclosed. In the drawings:
It is to be understood that the embodiments disclosed herein are not limited in application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The disclosure is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.
The present disclosure relates to the production and service of hoisting devices used to elevate platforms or baskets typically associated with large structure service and maintenance, such at buildings, bridges, towers, and the like. In particular, the disclosure relates to a restraint device used in conjunction with a traction sheave that may be used with a hoisting device.
For controlled travel, platforms typically utilize a motorized hoisting device. A hoist is a device used for lifting or lowering a load by means of a drum or lift-wheel around which a rope or chain wraps.
A traction sheave is typically a wheel with a groove between two flanges around its circumference. The groove normally guides a rope, cable or belt. Traction sheave are used to change the direction of an applied force, transmit rotational motion, or realize a mechanical advantage in either a linear or rotational system of motion.
As mentioned above, traction sheaves are used in a variety of load-lifting applications such as elevated platforms for building maintenance. The traction sheave is generally designed to operate with significant tension in the cable or rope encircling the sheave. One characteristic of traction sheaves and in particular the single sheave “V” groove type is that the cable or rope's points of contact with the sheave may have different radii at different positions around the sheave. As a result, the velocity of the wire rope may vary according to the instantaneous radius at which each point is in contact with the sheave. Consequently the wire rope will tend to be either tensioned or bunched as adjacent portions of the wire rope are at lesser or greater radii than the average radius of around the sheave.
At the points where the sheave contacts the wire rope, friction creates driving tension in the cable or rope. Generally the cable or rope will tend to bunch in a known region of the sheave. When a sufficient length of cable or rope has bunched and laterally lifted off the sheave, the friction between the sheave and wire rope in that region decreases to the point where the tensioned wire rope may slip or develop slack. The cable or rope may then tighten under the applied load and suddenly resume full contact and friction with the sheave. This event may produce a loud and objectionable popping impulse noise generated by the cable or rope snapping back against the traction sheave. Generally the tendency to generate the popping noise increases with a poorly maintained dry, non-lubricated cable or rope. Furthermore, the popping noise may occur with sheaves carrying full, half, or quarter turns.
In one embodiment of the present disclosure, a restraint device is disclosed that comprises a molded component fitted to encompass the region of the sheave where separation of the cable or rope from the sheave is most likely to occur. The restraint device may be mounted to the associated hoist or rigging such that it is removable and replaceable with minimal dismantling of the hoist containing the sheave. In a further embodiment, the restraint device may be reversible or rotatable so that one restraint device may by used for at least two cycles before being replaced. Wear of the restraint device may be minimized by allowing a small space between the wire rope and the restraint device, thereby minimizing contact with the restraint device without increasing the incidence of wire rope jams.
The disclosed restraint device may be designed with no moving parts, thus providing advantageous features compared to a restraint system comprising a roller system. For example, a roller system may collect contamination from corrosives, errosives, and other particles from the cable or rope and the associated environment. By eliminating moving parts, the restraint device of the present disclosure may thus avoid such contamination and other effects of foreign particles. Where contaminants are present and have adhered to the device, a further feature of the disclosed restraint device is that continuous contact and/or abrasion with the wire rope may provide a self cleaning function.
In yet another embodiment, indication of the need for replacement may be provided by providing a visual marking at a suitable position on the restraint device where the wear reaches an allowable limit. Such a marking may provide a visual signal to indicate that a replacement is needed in a manner that can be readily observed during normal service of the hoist. For example, a second color material may be used to indicate the wear limit. Alternatively, a hole may be placed transversely in the molding of the restraint at the position of maximum wear. Thus, when the wear reaches the proximity of the edge of the hole, the restraint may break through and the wear limit will be signaled as a break in the wire rope track.
Referring to
Referring back to
In one embodiment, wear block 310 may be contoured to approximate the contour of said cable or wire 410 encircling sheave 320. It should be noted that such contouring is not required and the wear block may provide a flat or other surface. When cable or wire 410 lifts off sheave 320, a force will be applied to cable or wire 410 to restrain its radial movement and thereby limit the snap-back distance of cable or wire 410 to substantially reduce or eliminate the intensity of the popping noise.
The radial position of wear block 310 relative to cable or wire 410 may be chosen to limit the duration of contact with cable or wire 410 except when lifting occurs. Typically, wear block 310 will experience some wear during initial use, and will also experience wear when a larger diameter wire rope (for example, an 8.2 mm rope) replaces a smaller diameter wire rope (for example, an 5/16 inch rope). Additionally, the position of the cable or wire rope in the sheave groove 520 may change during the course of continued use as the tension and condition of the cable or rope varies.
The continuous maintenance of the cable or rope may generally be inadequate to prevent some degree of corrosion, thus leading to abrasive rust being formed on the cable or rope. As a result, depending upon the level of maintenance of the cable or rope, the abrasive potential may vary. Furthermore, as the cable or rope is used over a long period of time, the wear and/or ovality (i.e., the degree of deviation from circularity of the cross section of the cable or rope) may increase and potentially result in contact with the wear block 310. Because of the reasons described above, wear block 310 may be constructed of an abradable material that can abrade in preference to abrading and damaging the cable or rope. Thus, when the wear block 310 rubs against the wire or rope, the wear block 310 will be worn whereas the wire or rope may experience little or no wear.
Referring back to
The circumferential position of wear block 310 may be chosen at a position where maximum lifting typically occurs. As depicted in
In another embodiment, the wear indicator may be provided by molding wear block 310 with different colored materials wherein the wear indicator position may be marked with a visually distinct color or point of color transition. When wear block 310 wears to the position indicated by the alternative coloration, visual inspection may reveal that sufficient wear has occurred indicating the need for replacement. Alternatively, the wear indication may be provided by inserting or attaching different colored material on the wear block 310. In other embodiments, other methods of wear detection may be implemented such as including a thin wire in the molding that carries an electrical current, the wire being coupled to a monitoring device or other suitable means for monitoring wear indication. When sufficient wear has taken place, the wire will break, thus altering or opening the electrical signal to the monitoring device. Alternatively, the wire may not carry an electrical current and may be mechanically coupled to maintain spring tension with a monitoring device. When sufficient wear has taken place, the wire will break and thus mechanically release the wire, thus providing a mechanical indication of wear.
Typically, providing indicator holes 610 will provide a low cost wear indication as compared to the other embodiments described above. It should be noted that local or national rules or regulations may require inspections of the associated rigging or hoisting device at specified intervals. Although the embodiments disclosed herein do not affect the safety of the hoist and is provided to limit the popping noise described above, during such an inspection the indicator hole may provide a convenient visual indication of wear as the cylinder of the hole becomes visible on the wear surface. Alternatively, other visual wear indicators can provide similar visual cues.
Referring to
Wear indicator holes 610 may be positioned relative to the wear surfaces in order to provide its wear indication function regardless of the specific orientation of the wear block. In a further embodiment, wear block 310 may be shaped such that both longitudinal ends are symmetrically shaped, providing the further possibility that an end-for-end rotation may change the position of the leading edge relative to the holder 710 and thus offer two further uses of the wear block before wear block replacement is required. Note that a multiplicity of wear indicator holes may be disposed along any wear surface.
As a further feature of the present disclosure, wear block 310 may be replaced in the wear block holder with a minimum of disassembly of the hoisting device, traction sheave and associated rigging. Replacement or rotation of the wear block 310 does not typically require removal of the traction sheave or de-reeving of the cable or rope from the traction sheave. Furthermore, installation or removal of the wear block does not typically require special tools, thereby minimizing maintenance time and cost, and maximizing availability of the hoisting device.
In another embodiment, the wear block may further be symmetrical about both the shorter and longer axes and mounted in a similarly shaped wear block holder to provide two additional uses of the wear block.
Furthermore, by further flipping wear block 310A 180 degrees about its horizontal or vertical axes, the movement of the cable or rope is reversed relative to the already worn surfaces and an additional four wear cycles may be provided. Since the nature of the wear is predominantly at the entry point at the left edge in
It will be understood that each of the elements described above, or two or more together may also find a useful application in other types of methods differing from the type described above. Although the more detailed examples provided above relate to traction sheaves in hoisting devices associated with elevated platforms for building maintenance, it should be apparent to one of ordinary skill in the art that the apparatus and methods described herein will find application to other systems that utilize traction sheaves. Additionally, the foregoing description has set forth various embodiments of the apparatus and methods via the use of diagrams and examples. While the present disclosure has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiment for performing the same function of the present disclosure without deviating there from. Furthermore, it should be emphasized that a variety of applications, including marine and transportation systems, are herein contemplated. Therefore, the present disclosure should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the appended claims. Additional features of this disclosure are set forth in the following claims.
