A load sensor interconnects the tractor carriage and upper hoist assembly to sense forces created by an operator pulling on the chain, which are used to control an electric motor on the tractor carriage driving a pinion gear engaged with a gear rack on the overhead rail to positively drive the carriage, trolley and upper hoist assembly along the rail.
A stationary dual hoist system is also described in which two hoist assemblies are interconnected by a chain and sprockets to provide synchronised operation.
This application claims the benefit of provisional patent application, Ser. No. 60/663,305, filed on March 18, 2005.
Background
Balancing hoists have long been known in which a drum has a length of cable wound and unwound thereon as the drum is rotated in either direction to position a load held by the cable. This arrangement has utilised pneumatically operated hoists which use regulated air pressure acting on a piston to cause cable wind up or pay out by rotation of the drum. (See US patent, number 3,428,298 for a detailed description of this type of hoist).
Fig 1 – A pictorial view of a hoist and supporting modified overhead system according to the present invention.
The load can be raised or lowered by the operator by exerting a low-level force on the suspended load which increases or decreases the air pressure acting on the piston slightly. Pressure change is made up by a regulator to lower or raise the load accordingly.
The limited stroke of the piston limits the cable travel that can be obtained, and thus electrical motor driven balancer hoists have been developed, as described in US patents numbered 3,921,959 and 4,807,767.
The servo motor typically drives a planetary reduction gear, the output of which drives the cable wind up drum.
Since the cable is elastically stretchable to a significant degree, it has considerable stored energy when heavily loaded.
If the cable breaks, a hazard can be created by whipping of the cable caused by release of the stored energy when the cable breaks or when there is some other failure. Emergency brakes have been employed to prevent rapid unwinding of the cable in this situation.
The mass of the planetary gearing also increases the momentum of the movable components when winding or unwinding is underway. The control of the servo motor is made more complicated by the cable stretch and the momentum of the rotating components, creating complex dynamics, particularly at the high speeds which the electric servo motor drive systems operate.
Fig 2 – An enlarged pictorial view of a hoist upper assembly and trolley tractor drive components included in the hoist system shown in Fig 1 and a portion of an associated overhead rail.
The cable must always be maintained in tension during raising and lowering operation of the hoist in order to avoid loose turns in the cable windings on the drum leading to tangling, which interfers with later unwinding. Sensors and complex software are needed to ensure this does not occur.
Thus, the use of a chain in balancing hoists would be preferable to eliminate difficulties in winding of a cable and the hazards associated with cable stretching.
In some electric motor driven balancer hoists, load sensors sense a change in the load on the cable or chain to cause the electric motor to drive a drum to raise or lower the cable or chain to balance a load in “float” mode.
The weight of an operator’s hand can upset the “float” balance, since the load sensor will react to removal of the operator’s hand from the handle.
Alternatively, manipulation of a handle or grip connected to the cable causes the motor to selectively drive the motor so as to raise or lower the load at a rate proportional to an up or down force applied by the operator to the grip.
Automatic controls can also execute raising or lowering motions to programmed stops as when repetitive motion cycles occur.
Fig 3 – A further enlarged pictorial view of certain components of the upper hoist assembly shown in Fig 2.
Such self balancing hoists have been mounted on trolleys traversed along an overhead aluminium rail track system. In order to assist movement of the trolleys, pulling on the cable by the operator in a given direction is sensed by a power cable angle sensor and powered driving of the trolley in that direction is created in response to sensing such cable pull.
Trolleys have in the past been driven by friction wheels engaging a smooth surface on the aluminum rail. However, friction wheel slippage can sometimes occur especially under heavy loads. Such slippage upsets the accurate functioning of the control system and a commanded movement of the trolley may not occur if slippage is encountered.
Summary
The present invention comprises improvement to a hoist which utilises a chain to support the load, the chain positively driven by an electric servo motor through a low mass self-locking worm gear drive which holds the supported load whenever the motor is de-energised. The chain is not wound up onto a drum but driven linearly by a positive rotary drive hub, the chain optionally able to be routed into a collection receptacle. The use of a hoist chain eliminates the stored energy problem of cable hoists, as a chain does not stretch appreciably compared to a cable, and the low mass of a worm gear drive minimizes the momentum of the rotated components to provide high performance of the balancer function.
Fig 4 – An enlarged pictorial view of some of the internal components of the control box and grip.
This avoids the disadvantages of a cable hoist, such as the need for sophisticated control over winding and unwinding of a flexible cable on a drum, the hazards of stored energy in a stretched cable and the other disadvantages described.
Two load sensors are used in the hoist up-down control, held in a control box supported on the lower end of the chain. The number one load cell is connected between separate upper and lower load shafts passing through the control box. The lower load shaft connected to the load hook or eye generates signals corresponding to the weight of the load signals used to drive the load up or down when the operator directly pulls up or presses down on the load attached to the hook or eye.
The number two load sensor is used when the hoist control system is switched to a manual control as by activation of a push button switch on the control box. A handle grip is mounted to be slidable on the lower load shaft and connected via the number two load sensor to the upper load shaft. The number two load sensor creates signals in response to up or down pressure exerted on the control grip by the user causing up or down hoist operation in correspondence to up or down force applied to the grip.
Forces applied to the grip do not affect the number one load sensor since it is connected below the upper connection point of the number two load sensor support, and since the handle is slidable on the lower load shaft so as to prevent any possible effects on the system if the grip is held or released when the hoist controls are set to the balance mode.
To improve performance of the trolley drive system, steel gear rack sections are clamped onto standard overhead rails and engaged with a pinion gear driven by electric motor powered tractor carriage connected to a hoist trolley. This creates a positive drive for powered positioning of the hoist trolley along an overhead rail.
Technical illustrations
Referring to the drawings and particularly Fig 1, a hoist system (10) according to the present invention includes an upper hoist assembly (12) supported on a trolley (14) able to be traversed along an overhead rail (16) by a trolley tractor drive (18) pulling its upper hoist assembly when activated.
A hoist chain (20) is driven up and down by a chain drive arrangement in the upper hoist assembly, described below. The hoist chain (20) is connected to a lifting eye (22) on which the load (24) is hung.
A control grip (28) extends below the control box (26).
Two alternately selected basic control modes may be provided. In the first mode, a “float” mode may be provided in which the weight of the load is held stationary and up or down movement of the load is produced by lifting or downwardly pushing on the load itself to cause up or down driving of the chain to raise or lower the load in response to the forces applied to it.
In the second or manual mode, upward pulling or downward pushing on the grip (28) causes up or down driving of the hoist chain and thus of the load at a rate and in a direction corresponding to the magnitude and direction of the forces exerted on the load or grip.
The signals generated by components in the control box are transmitted to the hoist controls (29), which may be comprised of a suitably programmed industrial controller as is well known in the art, which in turn controls activation of the hoist motor (25).
Figs 2 and 3 show further details of the upper hoist assembly (12).
An electric servo motor (25) is enclosed within housing (23) which drives reversible right angle gearing, comprising a worm gear (30) irreversibly engaged with a worm wheel (32), which is connected to a shaft (34) on which is affixed a chain driving hub (36) of a commercially available type that drives the chain in either direction.
It is noted that other types of electric motors can be used, other than an electric servo motor, such as a VFD motor.
The upper hoist assembly (12) also includes a trolley support piece (40), having linear bearings (42) affixed there to engage with a bearing way (44) of the trolley (14). An upright web (46) supports two pairs of trolley wheels (48).
The trolley wheels roll along rail tracks (50) formed in the conventional overhead rail (16).
The tractor drive carriage (18) is connected to the trolley (14) by links (52). The tractor carriage includes an electric servo motor (19) driving a pinion gear (54) by means of a worm gear (55) and worm wheel (57) engaged with a steel gear rack (56).
The tractor carriage includes a central plate (21) mounting tractor carriage wheels (48A) rolling on rail tracks (50). The gear rack (56) is held against the underside of one of the tracks (50) of rail (16) by clamping plates (58) affixed to the side of the gear rack (56) by bolts (60) threaded into a hole in the gear rack and into retainer blocks (62) in T slots in the side of the rail.
The reaction to driving by the pinion gear (54) tends to force the gear rack (56) more tightly against the underside of one track (50) of the rail (16) to be quite securely held against the same. Conventional existing aluminium rails can be quickly and easily modified in this way.
A load sensor (64) and an orthogonally arranged pair of yokes (66, 68) interconnects the upper hoist assembly to the tractor carriage. When an operator pulls on the chain in either direction, the resultant compressive or tensile load exerted on the load sensor is detected, and the tractor carriage is positively driven to null the signal generated by the load sensor to controllably move the upper hoist assembly in either direction at a rate corresponding to the magnitude of the pull detected by the load sensor.
The electric servo motor (19) is activated in a direction and at a rate tending to null the load sensor signals, and thus positively drive the tractor carriage (18) and upper hoist assembly through the worm gear (55) and worm wheel (57) along the rail (16) until the operator determines the desired location has been reached and discontinues pulling on the hoist chain.
Fig 4 shows further details concerning the control box (26) and control grip (28). The hoist chain is connected to an upper portion of a load support including a shaft (70) also connected to the top (27) of the control box.
The shaft is connected to a lower portion of a load support comprising a shaft (72) and lifting eye (22) by an intermediate number one load sensor (74).
The lower shaft (72) is threaded to a lifting eye (22 or hook) on which the load may be hung. Thus, the load sensor (74) generates electrical signals corresponding to the weight of the load. An emergency stop button (82) is also provided to enable complete stoppage of the servo motor (25).
A number two load sensor (84) is also provided which has an upper end connected to the upper shaft (70) via a self aligning connection (86) and has a lower end to the control grip (28) suspended from the shaft (70) via another self aligning connection (88) and bracket (90) attached to the top of the grip (28). The control grip slidably receives the lower shaft (72) which passes freely through an opening in the same as shown.
Programming
Many modes of operation are possible by suitable programming of the hoist controls.
The basic modes of operation includes a “float” mode, in which the weight of the load is just balanced by the hoist drive. That is, lifting or pushing down on the load directly, as is done in final positioning of a load, will cause the chain to be driven up or down by activation of the servo motor so as to allow positioning of the load in that manner. This mode may be set by a programmed event, such as by pushing the lower button (80B) briefly.
A “manual” mode may be selected as by pushing the upper control button (80A). In this mode, the hoist chain will be driven up if the grip is pulled up, and will be driven down if the grip is pushed down, at rates corresponding to the level of force exerted on the grip. The load is held by the irreversible engagement of the worm gear and worm wheel if no force is exerted on the grip.
Upper and lower limits may be optionally preset by suitable programming of the hoist controller.
A lower limit is set by pushing down on the grip (28) until a desired lower limit is reached, and programmed in by holding lower control button 80B until light 86B flashes.
Fig 5 – A rotated pictorial view of the two axis sensor arrangement.
Other control features could be programmed into the controller.
Fig 5 show a two axis chain pull sensor (114) mounted in a housing (23).
A tube (116) is held and restrained at its upper end by a mounting comprising of two adjustable clamp collars (134A, 134B) on either side of a bracket (136). A clearance (C) is set so that the tube (116) is constrained only by load sensor rods described below when the hoist chain (20) is pulled. An anti-rotation screw (138) is threaded into the upper collar (134A) through a slot (140) in the bracket (136).
The tube receives the hoist chain which passes through to the chain drive hub (36) aligned so that the chain does not normally exert any pressure on the tube. When the hoist chain is pulled in the direction of either axis, this causes force to be applied in either direction to a respective load sensor (124A, 124B).
The tube (116) has a pair of spaced plates (118) which 120receive self aligning eye connections (120A, 120B) aligned along each orthogonal axis connecting a respective rod (122A, B) to load sensor (124A, 124B).
A second rod (126A, 126B) is held by a fixed mounting block (132A, 132B) receiving another self aligning pivot connection (128A, 128B).
The signals generated by load sensors (124A, 124B) are sent to the hoist controls (29) activating the respective tractor drives (18A, 130A, 130B) to drive the hoist assembly (12) along rail (16) or rails (16A) to position the hoist assembly at points along either axis.
About the patent
This patent application was filed as number 11/385,011. The US patent, number 7,467,723, was awarded on December 23, 2008.
Disclaimer
As an edited version of the original, this article and accompanying drawings may omit legally or technically important detail. To see the full patent visit www.uspto.gov/patft/index.html.
