US 3628115 A
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Description (OCR text may contain errors)
United States Patent  inventor Dana L. Pruitt Martinsville, Ind.
] Appl. No. 829,503
 Filed June 2, 1969  Patented Dec. 14, 1971.
 Assignee Uni-Light Hoist and Reel, lnc.
 VARIABLE-SPEED l-IOIST CONTROL 3 Claims, 6 Drawing Figs.
 US. Cl 318/345, 254/171, 318/349, 318/378, 318/507, 318/631 [51 Int. Cl. 1102p 7/28  Field of Search 254/168,
 References Cited UNITED STATES PATENTS 2,981,880 4/1961 Momberg et a1 318/345 X 3,184,671 5/1965 Riggs 318/345 x 3.34s,1 13 10/1967 Vichr 318/345 2,500,326 3/1950 Shaff 254/168 2,695,086 11/1954 Parker 254/187 3,292,069 12/1966 Evans, .lr. 318/345 3,461,371 8/1969 Klayman et a1. 318/345 Primary ExaminerT. E. L ynch Assistant Examiner-H. Huberfeld AtrorneyTrask, Jenkins & Hanley ABSTRACT: A hoist of light weight and high capacity with infinitely variable speed and holding characteristics. A sprocket engaged with a roller chain is driven through a multiple-stage spur gear reduction train by a high-speed universal reversible electric motor energized through a modified silicon control rectifier. The reduction gears are of high-tensile alloy steel, mounted on roller hearings to give an efficient drive, and the gear reduction is of such high ratio as to make the gear train irreversible under the rated load of the hoist, so that the SCR control of the high-speed motor gives positive variable-speed lifting, lowering, and holding control of the rated load. The motor control circuit includes a leakage resistor connected across the conducting electrodes of the SCR to inhibit any reversal of the motor.
Patented Dec. 14, 1971 3,628,115
3 Sheets-Sheet 1 NVENTOR DANA L. PRUITT BY i ZJ-M, :M
ATTORNEYS Patented Dec. 14, 1971 I 3,628,115
5 Sheets-Sheet 2 4: l6 I: ill! DANA '3. L J I FT K BY C M 7 1 ATTORNEYS 1 VARIABLE-SPEED I-IOIST CONTROL This invention relates to an electrical hoist which provides infinitely available speed control of lifting and lowering the load and positive holding of the load in suspended position. The invention provides a light weight hoist of high capacity, for example, a hoist weighing less than pounds with a load capacity of 500 pounds and a variable lift speed up to 16 feet per minute. Such a hoist, providing infinitely variable and accurate control of the speed and positioning of the load, meets an existing need in industry and can replace at less cost and greater convenience a variety of hydraulic and air-operated lift and positioning controls now in use.
In accordance with the invention, a lift rotor, preferably a sprocket, mounted in a hoist yoke body is positively engaged with a lift element, preferably a roller chain, for exerting a predetermined maximum lift equivalent to the rated hoist load. The rotor is driven through a multiple-stage spur-gear reduction train from the shaft of a reversible high-speed universal electric motor of fractional horsepower and small size. The reduction gears are mounted on antifriction bearings in closely stacked relation on a single main axis and a single countershaft axis, to provide a compact efficient drive from the motor to the lift rotor. The gear reduction ratio is sufficiently large to make the gear train positively irreversible under full static torque exerted on the lift rotor by the rated hoist load. The electric motor has regenerative braking characteristics to aid the gear'train in braking a moving load. The motor is energized through and controlled by a silicon controlled rectifier (SCR) circuit which is provided with a reversing switch. Desirably the SCR circuit is modified to provide a high-resistance leakage path across the speed control sufficient to maintain a magnetic field in the motor but insufficient to drive it. Such modification improves the smoothness and variability of the speed control by reducing the stepping action which otherwise tends to occur at low speeds under the control of the silicon controlled rectifier, especially in loadlowering operation where the load tends to cause the motor to overrun.
The gear assembly of the high-ratio gear reduction train desirably comprises a series of two-gear clusters arranged in intermeshing stacked relation on a main-shaft axis, and a single countershaft axis, held axially between the hoist body and the end wall ofa gear case, which provides an especially compact and effective assembly without high thrust loads. The gear case is removable to expose the undisturbed gear assembly, which facilitates assembly and allows ready replacement of gears to change the gear ratio and thereby modify the hoist to provide, for example, for higher lift speeds at a lighter rated load capacity.
The reduction gear ratio which is sufficient to make the gear train irreversible under the rated hoist load will vary with the design of the gearing. With the multiple-stage spur-gear arrangement I prefer to use, the gear ratio may be of the same order of magnitude as the torque load on the sprocket shaft. Thus, for a 500 pound rated hoist load, I have used a 1-inch radius sprocket to give a shaft torque of 500 inch-pounds, and have found that a 520:1 overall gear ratio provides the desired irreversibility. Changing gears to reduce the ratio to 260 provides a hoist having a rated hoist load capacity of 250 pounds, and being irreversible under that load.
With such ratios, a high-speed motor gives adequate lift speeds and good control. By high-speed motor, I mean a series connected motor having a free running speed of the order of at least 10,000 r.p.m. by which I mean a speed of 5,000 or more. For higher rated loads, I may use higher gear rates and higher speed motors, up to 25,000 or 50,000 r.p.m. or more.
The accompanying drawings illustrate the invention, and show an embodiment of the best mode contemplated by the inventor for carrying out his invention. In such drawings:
FIG. 1 is a substantially full-scale side elevation of an electric hoist embodying the invention, with parts of the hoist body and gear case broken away to show the gear train;
FIG. 2 is an end elevation of the hoist of FIG. 1 showing an end view ofthe gear case;
FIG. 3 is a section on the line 3-3 of FIG. 1;
FIG. 4 is a section on the line 4-4 of FIG. 1;
FIG. 5 is an electrical diagram showing the motor control circuit; and
FIG. 6 is a view similar to FIG. 1 showing a modification providing a different gear ratio.
The hoist shown in FIGS. 1-4 comprises a cast yoke body I0 formed with a central slot 12 for the reception of a lift sprocket 14 carried by a main shaft 16 mounted in suitable bearings in the walls of the yoke body 10. The left side of the yoke body 10 is formed with a mounting face for a motor 18, and the right face of the yoke body 10 is formed with a mounting face to receive a gear case 20. The top of the yoke body 10 forms an eye 11 which, as shown in FIG. 2, is desirably offset from the centerline of the yoke body to lie in alignment with the load supporting stretch of the chain 22 received on the lift sprocket 14.
The lower end of the lift chain 22 carries a lift hook 21 connected to it by a swivel clamp 23.
As shown in FIG. 3, the bottom of the slot 12 is defined by an arcuate wall 24 which lies close to the periphery of the sprocket l4 and serves to ensure that the chain will not become disengaged from the teeth of such sprocket 14. The gear case 20 is held in fixed and sealed relation against the side face ofthe yoke body 10 by cap screws 15 as shown in FIG. 2.
As shown in FIG. 1, the main shaft 16 is mounted in needle bearings 26 in the walls of the yoke body 10, and carries a wide-face gear 28 within the gear case 20, through which it is driven by a pinion 30 on a countershaft 32. The countershaft is mounted by a needle bearing 34 in the sidewall of the yoke body 10, and has an extension to the right which is mounted in the end wall 36 of the gear case 20 by a ball bearing 38.
The main shaft 26 has a central axial opening through which a drive shaft 40 extends. Such shaft 40 is carried by ball bearings 42 and 44 in the main shaft 26, and its right hand is supported in a ball bearing 46 in the end wall 36 of the gear housing. The leftor motor-end of the drive shaft 40 is coupled to the motor shaft 48 by an axially releasable coupling, here shown as a lap joint between the motor shaft 48 and the drive shaft 40 within the inner race of the ball bearing 42. The right end of the drive shaft 40 is machined to form a pinion gear 50 which is engaged with a spur gear 52 of a two-gear cluster mounted by needle bearings 53 on the countershaft 32, and such pinion 50 and gear 52 form the first lst) reduction stage of the gear train. The gear 52 is in a cluster with a pinion 54 which meshes with a gear 56 of a Cluster carried by needle bearings 57 on the drive shaft 40, and such pinion 54 and gear 56 form the second (2nd) reduction stage of the gear train.
The gear 56 is in a cluster with a pinion 58 which meshes with a gear 60 carried by needle bearings 61 on the countershaft 32, and such pinion 58 and gear 60 form the third (3rd) reduction stage of the gear train. The gear 60 is in a cluster with a pinion 62 which meshes with a gear 64 of a cluster mounted by needle bearings 65 on the drive shaft 40, and such pinion 62 and gear 64 form the fourth (4th) reduction stage of the gear train. The gear cluster of the gear 64 includes a pinion 66 which meshes with a gear 68 carried by the countershaft 32, and such pinion and gear 68 form the fifth (5th) reduction stage of the gear train. The sixth (6th) reduc tion stage of the gear train is formed by the pinion 30 on the countershaft 32 and the main drive gear 28 of the main shaft 26.
The face widths of the pinions and the gears in the successive reduction stages desirably increase progressively with the gear reduction and as tooth loads progressively increase. Thus. the gears 52 and 56 in the first two stages of reduction are relatively narrower than the gears 60 and 64, the gears 66 and 68 are still wider, and the pinion 30 and main gear 28 are still wider yet.
The main shaft 26 is wholly supported by the yoke body 10 and provides major bearing support for the drive shaft 40. The countershaft 32 receives major bearing support from the needle bearing 34 in the yoke body 10, and the arrangement is such that the three shafts will all be supported for assembly purposes independently of the bearings in the end wall 36 of the gear case 20. In assembling the gear train, therefore, it is convenient first to mount the shafts, then to mount the several gear clusters on the countershaft 30 and drive shaft 40, and then to install the gear case 20 over them. Correspondingly, the overall gear reduction can be readily modified by removing the gear case 20 to expose the several gears in supported relation from the yoke body 10, lifting off two or more of the gear clusters and replacing them with gear clusters providing different ratios in one or more of the reduction stages.
The gears are desirably made of high tensile, high-alloy steel material. In the embodiment shown, the gears in the first four reduction stages are made of a high-alloy steel having a tensile strength of l25,000 p.s.i., and the gears in the final reduction stages, that is, the gears 68, 30 and 28 are made of high-alloy steel having a tensile strength of 250,000 p.s.i. The several gears are desirably straight spur gears so that no substantial thrust forces are generated and no special thrust bearings are needed beyond the end-face engagements between the adjacent gear clusters, and between them and the end faces of the gear housing. The gears are all mounted on antifriction bearings, so that an efficient drive is provided in the direction from the drive shaft 40 through the gear reduction to the main shaft 26.
The multiple-stage gear reduction provides a gear ratio sufficiently large to make the gear train positively irreversible under full static torque exerted on the lift sprocket 14 by the rated load on the lift chain 22. Such irreversibility and the efficient through drive appears to be enhanced by the use of low single-stage reduction ratios in the intermediate and high stages and a relatively high ratio in the first stage. The following table shows for each reduction stage of the embodiment shown, the gear face width, the number of gear teeth used and, by their relationship, the reduction ratio ofthe stage.
The gearing in accordance with table I gives an overall reduction ratio of 520:1. With this gear reduction, and a lift sprocket 14 having a pitch radius of 1 inch, the hoist will support a load of 500 pounds on the lift chain 22 without producing reverse drive through the gear train to the drive shaft, will provide good control of a lowering load in response to adjustment of the motor speed, and will stop a lowering load if the motor is turned off. The 500 pound load exerts a torque of 500 inch-pounds on the main shaft 26. The drive train provides sufficient frictional resistance to prevent reverse drive under that rated load. A similar irreversible drive train may be provided for different loads by different overall gear ratios. For example, if one of the 2.l gear reduction stages is replaced by a H gear set, this reduces the overall gear ratio from 260:] to provide a hoist which will operate in accordance with the invention with a load rating of 250 pounds.
The motor 18 is a universal-type series motor operable on either AC or DC current. In the embodiment shown, the motor is a 1/5 hp. motor of this type having a free running speed of the order of 15,000 r.p.m. With the gear ratio of approximately 500:1, this gives an output speed of approximately 30 r.p.m. for the lift sprocket 14. Since the sprocket has a 1- inch pitch diameter, this corresponds to approximately 16 feet per minute of lift movement of the chain 22. This lift speed will of course be reduced under load since it is characteristic ofsuch motors that their speed is load-responsive.
Since the drive train is positively irreversible under the rated torque load on the sprocket 14, it is not necessary to provide the motor or the drive train with any mechanical or electromagnetic brake. When the hoist is lifting, the load will tend to brake the hoist to a stop or lower speed if the motor is deenergized or its speed reduced. in lowering operation, gear train drag of the irreversible gearing will ordinarily be sufficient to slow and stop a load being lowered. In addition, the motor 18 has regenerative braking characteristics which will oppose any tendency of the motor to overrun under a lowering load. As the energiiation of the motor is controlled to reduce the motor speed, the regenerative braking effect will follow the motor speed reduction and correspondingly slow the lowering operation. If the motor is completely deenergized, the opening of the motor circuits would prevent further regenerative braking, but I have found that the motor still produces some braking, perhaps by hysteresis effects from residual magnetism in the motor armature and field cores. As mentioned below, I preferably maintain at least some leakage current to the motor at all control speeds, and while this is in part for other purposes, it tends to maintain a magnetic field and a regenerative braking effect in the motor even when the speed control is set at zero speed. However, I mainly rely on the irreversible character of the high ratio gear reduction train to provide positive control of the hoist operation, to positively stop a lowering load, and to hold a suspended load.
The motor 18 is desirably energized through a manual speed-control box 70 connected to the motor by a cable 72 and connected to a suitable source of current by a grounding two-wire cable 74. The control box contains an on-off switch 76, a forward-reverse switch 78, and a speed control dial 80. The speed control dial may actuate any suitable speed-control circuit, but I preferably use the silicon controlled rectifier (SCR) circuit shown in FIG. 5. The motor 18 has a field winding 17 and an armaturebrush assembly 19, each having leads separately connected to the control box 70. The leads from the field winding 17 are connected to the blades 77 of the reversing switch 78. One leadof the armature-brush assembly 19 is connected to a wire 73 which in turn is connected through the on-off switch 76 to one wire of the AC power supply circuit 74. The other lead of the armature-brush assembly 19 is connected to one of two forward contacts 79 of the reversing switch 78. The other forward contact 79 is connected through a speed control circuit to a wire 75 which in turn is connected through a fuse 82 to the opposite wire of the AC supply circuit 74. The switch also has reverse contacts 8! cross connected to the forward contacts 79.
The speed control circuit comprises a silicon controlled rectifier 84 having an anode 85 and a cathode 86 forming its principal conducting electrodes. The rectifier also has a gate or control electrode 87 which is connected through a diode 88 to the moveable contact 89 ofa potentiometer 90 which, with a resistance 92, forms a voltage divider connected through a diode 94 between the wires 73 and 75. The gate electrode 87 may also be connected through a regulating resistor 83 to the electrode 86. A full-on switch 96 is connected across the principal conducting electrodes of the silicon rectifier 84 and is arranged to be closed by the speed control dial 86 when the potentiometer 90 is adjusted to the maximum speed position.
The speed control circuit described above is a conventional SCR circuit. 1 preferably modify such circuit by the addition of a bypass resistor 98 which has the function of providing a lowvoltage supply of full-wave AC current to the motor 18 even when the speed control circuit is turned to zero speed and the rectifier 84 becomes nonconducting. Such leakage supply of AC current maintains sufficient magnetic field in the motor 18 to permit regenerative braking in the event the hoist tends to overrun and drive the motor 18. The leakage supply of fullwave AC provided by the bypass resistor 98 is of primary value, however, to improve the operation of the SCR at low speeds. Without it, low-speed operation, especially in a lowering direction, tends to be jerky and uneven. The bypass resistor smooths out such unevenness and gives a more accurate and smooth control.
In a specific embodiment of the circuit shown in FIG. 5, the silicon rectifier was a GE. CB 22, 7.4 Amp SCR and the resistor values were as follows: The resistor 923,900 ohms, the resistor of the potentiometer 90 l ,000 ohms; the regulating resistor 83-l,000 ohms; and the bypass resistor 98-5,000 ohms.
In operation, the hoist described provides positive lifting, lowering, and holding of a load suspended on the hook 21 of the lift chain 22, with infinitely variable speed control from zero speed or holding up to a maximum of about l6 feet per minute with a light load. The high speed of the motor 18 in relation to the much lower lift speed of the sprocket 14 permits the speed control circuit to provide smooth operation over the entire speed range, and slow speeds for positioning a load at a desired elevation are particularly smooth to give precise control. The irreversibility of the high-ratio gear reduction train contributes to the control of lowering operations, and acts to brake and hold a load accurately in response to speed regulation by the SCR control, without need for a brake to stop rotation of the motor shaft 48 and drive shaft 40. Brake mechanism would introduce unevenness of control which the present invention avoids. The high-gear ratio and high-speed motor combine with the SCR control to give positively and smoothly controlled hoist operation over a full speed range. The particular and compact structural arrangement permits the hoist to be of small physical size and light weight in relation to its rated load capacity.
The modified hoist structure shown in FIG. 6 is identical with that of FIG. 1 except for the gear ratio. A different gear ratio is obtained by replacing the two outermost gear clusters on the shafts 40 and 32. The drive shaft 40 carries the same pinion gear 50 as in FIG. 1. The gear 152 which that pinion engages is the same as in FIG. 1. The pinion gear 154 formed in the gear cluster with the gear 152 is of larger diameter and the gear 156 which it engages is of smaller diameter than in FIG. 1. Such gears 154 and 156 are of the same diameter and produce a M ratio in this second stage of the gear train. The gear train is otherwise the same as in FIG. 1. The 1:] second stage reduces the overall gear reduction ratio by half, to a value of 260:1. A hoist having this gear ratio is readily produced from the same major parts as the hoist of FIG. 1, and provides higher operating speeds for lower rated loads.
1. A variable speed control means for an electric motor, comprising a silicon controlled rectifier having principal conducting electrodes and a gate electrode. circuit means for connecting the principal conducting electrodes to control the current supplied to the electric motor, means to vary the voltage supply into the gate electrode to thereby control the current supplied through the principal electrodes to the motor, and a leakage resistor connected across the principal conducting electrodes of the silicon controlled rectifier to maintain a lowvoltage supply of full wave AC to the motor for regeneratively braking the motor.
2. A variable speed control means according to claim 1 wherein said leakage resistor is adapted to pass sufficient current to maintain a magnetic field in the motor but insufficient current to drive the motor.
3. A variable speed control means for an AC powered series electric motor, comprising a silicon controlled rectifier having principal conducting electrodes and a gate electrode, a pair of input conductors for connection to an AC supply circuit, a pair of output conductors for connection to the electric motor, means connecting the input connectors to the output connectors and including said conducting electrodes being one input conductor and an output conductor, variable voltage dividing means connected across the input conductors for deriving a control voltage in each voltage cycle of the AC supply circuit and means to apply such control voltage to said gate electrode to cause the conducting electrodes to conduct during a variable portion of said cycle and thereby to supply power from the input conductors through the principal electrodes to the output conductors for operating the motor, and a leakage resistor connected across said principal conducting electrodes to maintain a low-voltage supply of full-wave AC to the motor for regeneratively braking the motor.
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