US 3784165 A
A hoist of light weight e.g., 10 lbs., and high capacity, e.g., 500 lbs., 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 bearings to give an efficient drive, and the gear reduction is of such high ratio as to give the gear train a high degree of irreversibility. One-way drag braking opposes load-lowering movement of the high-speed and of the gear train, and interacts with the irreversibility to positively hold suspended loads and require the motor to drive the load down as well as up. This combines with the SCR control of the high-speed motor to give positive variable-speed lifting, lowering, and holding control of the rated load.
Claims available in
Description (OCR text may contain errors)
llnited States Patent [1 1 Pruitt VARIABLE SPEED HOIST Dana L. Pruitt, 209 N. Park, Martinsville, Ind.
 Filed: Nov. 13, 1970  Appl. No.: 89,170
Related U.S. Application Data  Continuation-impart of Ser. No. 829,503, June 2,
 U.S. C1. 254/168, 192/12 B  Int. Cl B66d 1/00  Field of Search 254/168, 171, 186; 318/345; 192/12 B  References Cited UNITED STATES PATENTS 2,500,326 3/1950 Shaff 254/168 9 2,991,976 7/1961 Carroll 254/171 X 3,362,685 1/1968 Noye et a1. i 254/168 2,274,175 2/1942 Whitcomb 254/168 X 3,292,069 12/1966 Evans 318/345 3,461,371 8/1969 Klayman et a1. 318/345 X 3,452,964 7/1969 Bibeault 254/168 2,053,055 9/1936 Wadd 254/168 3,523,593 8/1970 Karasick... 192/12 B X 2,629,469 2/1953 Dayton .1 192/12 B Jan. 8, 1974 3,475,991 11/1969 Pilcher 192/12 B Primary ExaminerEvon C. Blunk Assistant Examiner-H. S. Lane Attorney-Trask, Jenkins & Hanley [5 7] ABSTRACT A hoist of light weight e.g., 10 lbs., and high capacity, e.g., 500 lbs., 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 bearings to give an efficient drive, and the gear reduction is of such high ratio as to give the gear train a high degree of irreversibility. One-way drag braking opposes load-lowering movement of the high-speed and of the gear train, and interacts with the irreversibility to positively hold suspended loads and require the motor to drive the load down as well as up. This combines with the SCR control of the high-speed motor to give positive variablespeed lifting, lowering, and holding control of the rated load.
4 Claims, 8 Drawing Figures PATENTEU JAN 819M arm 10f INVENTOR DANA L. PRUITT J ATTOR NEYS SHEU 2 0f INVENTOR DANA L. PRUITT BY A a? 9' M ATTORNEYS PATENTED W PATENTED 8W4 3,784,165
WU 30F &
INVENTOR DANA L. PRUITT ATTORNEYS PATENTED 3.784, 1 65 SEE W 5 INVENTOR DANA L. PRUITT ATTORNEYS 1 VARIABLE SPEED HOIST cation Ser. No. 829,503, filed June 2, 1969.
BACKGROUND OF THE INVENTION This invention relates to an electrical hoist which provides infinitely variable 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 anti-friction 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 highly irreversible under load, and one-way drag braking is preferably applied at the high-speed end of the drive train to oppose load lowering movement; which ensures positive holding of a suspended load and causes the motor to exert positive drive both in lifting and lowering operations. The electric motor has regenerative braking characteristics to aid in braking a moving load. The motor is preferably energized through and controlled by a silicon controlled rectifier (SCR) circuit which is provided with a reversing switch. The required positive drive in both directions improves the SCR control action. 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 load-lowering operation where the load tends to cause the motor to over-run.
The gear assembly of the high-ratio gear reduction train desirably comprises a series of two-gear clusters arranged in inter-meshing stacked relation on a mainshaft axis and a single counter-shaft axis, and held axially between the hoist body and the end wall of a gear case, which provides an especially compact and effective assembly without high inherent thrust loads. The gear case is removable to expose the undisturbed gear assembly, whichfacilitates 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 provide high irreversibility in the gear train 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 one-inch radius sprocket to give a shaft torque of 500 inch-pounds, and have found that a 520:1 over-all gear ratio provides the desired high irreversibility. Changing gears to reduce the ratio to 260 provides a hoist having a rated hoist load capacity of 250 pounds, and provides high irreversibility under that load.
For many applications, such high gear reduction ratios permits a high-speed motor to give adequate lift speeds and load control without additional braking. 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 ratios and higher speed motors, up to 25,000 or 50,000 R.P.M. or more. 1
In a preferred form of hoist embodying the invention, the high speed motor and high gear-reduction drive train is combined with braking means which applies its braking effect at the high-speed end of the drive train and which operates to brake the drive train against load-lowering movement. Such braking means interact with the high gear reduction and its high degree of irreversibility to provide positive holding of suspended loads. Preferably, the braking means may be a one-way drag brake means effective to apply braking effort against load-lowering movement of the drive train and ineffective on load lifting movement. The braking effort applied desirably is, and may be, sufficient to positively stop and suspend a lowering load but insufficient to overcome positive downward drive of the motor, so that actuation of the motor in either direct-ion will produce drive in such direction and the braking action can be automatic and will not require actuating mecha- I nism.
To effect such braking action, I have found that by mounting the output end of the counter shaft in roller bearings and slightly misaligning such shaft relative to the bearing bore in a suitable direction, the bearing will I cause such shaft to be urged endwise on load-lowering movement in a direction to press one or more of the gears into frictional braking relation with each other and/or with the gear case, and thus produce an effective braking actionnln addition or alternatively, such drag braking action may be provided by a one-way drag brake on the input shaft, conveniently between such input shaft and the output shaft.
BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings illustrate the invention, and show embodiments 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 of the 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;
FIG. 6 is a view similar to FIG. 1 showing a modification providing a different gear ratio;
FIG. 7 is a substantially full-scale side elevation, with parts broken away, of a modified embodiment of the invention, including drag braking means; and
FIG. 8 is a section taken on the line 88 of FIG. 7.
DESCRIPTION OF THE PREFERRED EMBODIMENTS The hoist shown in FIGS. 1-4 comprises a cast yoke body 10 formed with a central slot 12 for the reception ofa 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 center line 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 14 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 of the yoke body 10 by cap screws as shown in FIG. 2.
As shown in FIG. 1, the main shaft 16 is mounted in needlebearings 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 counter shaft 32. The countershaft is mounted by a needle bearing 34 in the side wall of the yoke body 10, and has an extension to the right which is mounted in the end wall 36 of the gear case by a ball bearing 38.
The main shaft 16 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 16, and its right end 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 ofa 2-gear cluster mounted by needle bearings 53 on the countershaft 32, and such pinion 50 and gear 52 form the first (1st) 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) reduction 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 16 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 32 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 125,000 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 anti-friction bearings, so that an efficient drive is provided in the direction from the drive shaft 40 through the gear reduction to the main shaft 16.
The multiple-stage gear reduction provides a gear ratio sufficiently large to give the gear train a high degree of irreversibility under torque exterted on the lift sprocket 14 by the load on the lift chain 22. For some applications, such irreversibility is sufficient to provide adequate load holding action for service requirements. I prefer, however, to supplement such holding action by the use of braking means such as shown in FIGS. 7 and 8. The high degree of irreversibility and the effi cient 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 of the stage.
TABLE I Gear Teeth Stage Gear Nos. Face Width Ratio Ist 52-50 0.l56 in. 130214 2nd 56-54 0.156 in. 36:18 3 rd 60-58 0.156 in. 36:18 4th 64-62 0.187 in. 36:18 5th 68-66 0.375 in. 36:18 6th 28-30 0.750 in. 28:8
The gearing in accordance with Table 1 gives an overall reduction ratio of 520:1. With this gear reduction, and a lift sprocket 14 having a pitch radius of one inch, the hoist will normally support a load of 500 pounds on the lift chain 22 without producing reverse drive through the gear train to the drive shaft. For best control of a lowering load in response to adjustment of the motor speed, and to ensure stopping a lowering load if the motor is turned off, 1 preferably also employ braking means as shown in FIGS. 7 and 8. The 500 pound load exerts a torque of 500 inch pounds on the main shaft 16. The drive train provides high frictional resistance to oppose 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 2:1 gear reduction stages is replaced by a 1:1 gear set, this reduces the overall gear ratio from 260:1 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 one-fifth h.p., 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 one-inch pitch radius, 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 of such motors that their speed is loadresponsive.
Since the drive train is highly irreversible under the rated torque load on the sprocket 14, it is not necessary for all applications to provide the motor or the drive train with braking means. 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 often be sufficient to slow and stop the load being lowered. In addition, the motor 18 hasregenerative braking characteristics which will oppose overrunning of the motor under a lowering load. If the motor is completely de-energized, the opening of the motor circuits 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, l-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 rely on the irreversible character of the high ratio gear reduction train to provide a basic degree of control of the hoist operation, of movement of a lowering load, and of holding 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 2-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 armature-brush 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 lead of the arma' ture-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 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 81 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 movable contact 89 of a 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 by-pass resistor 98 which has the function of providing a low voltage 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 non-conducting, 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 full-wave AC provided by the by-pass 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 92 3,900 ohms, the resistor of the potentiometer 90 1,000 ohms; the regulating resistor 83 1,000 ohms; and the by-pass resistor 98 5,000 ohms.
In operation, the hoist described provides positive lifting, lowering, and holding of limited loads 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 16 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 high degree of irreversibility of the high-ratio gear reduction train contributes to the control of lowering operations, and acts to effect braking and holding of a load in response to speed regulation by the SCR control. The high gear ratio and high speed motor combine with the SCR control to give 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.
As previously indicated, the hoist shown in FIGS. 1-4 has a lift sprocket with a one-inch pitch radius, uses a one-fifth h.p., 15,000 R.P.M. motor, and has a gear reduction ratio of 520:1 in its drive train, and has a rated lift capacity of 500 pounds. The motor is of small physical size, as is the transmission, so that the entire basic hoist is contained within a small envelope.
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 1:1 ratio in this second stage of the gear train. The gear train is otherwise the same as in FIG. 1. The 121 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.
The modification shown in FIGS. 7 and 8 comprises a yoke body 210 with a central slot 12 for the reception of a lift sprocket 14 carried by a main shaft 216 mounted in needle bearings 226 in the walls of the yoke body. The left side ofthe yoke body carries a motor 18, and the right side is formed to receive a gear case 220. The right face of the yoke body carries a rib 213 on a circle concentric with the axis of the main shaft 216, and the gear case 220 has at its edge a flange 215 which mates with the rib 213 to locate the housing 220 with respect to the axis of the shaft 216.
The gear case 220 is held in fixed and sealed relation against the side of the yoke body 210 by a pair of cap screws 217 which extend through openings 219 in the case and are screwed into threaded holes in the yoke 210. For purposes described below, the holes 219 are elongated in an arcuate direction about the axis of the main shaft 216 to permit the case 220 to be rotated slightly about such axis.
The main shaft 216 carries a wide-face gear 28 through which it is driven by a pinion 30 on the countershaft 32 which is driven by a gear train identical with that shown in FIG. 1 and described above. The left end of the countershaft is mounted by a roller bearing, preferably a needle bearing 34, in the side wall of the yoke body 210, and its right end is mounted in the end wall 236 of the gear case 220 by a ball bearing 238. The innerface of the end wall 236 is a planar face parallel with the side face of the gear 52 mounted on the countershaft 32.
The main shaft 216 has a central axial opening through which a drive shaft 240 extends. Such shaft 240 is carried by ball bearings 242 and 244 mounted in the main shaft 216 and its right end is supported in a ball bearing 246 in the end wall of the housing 220. The
left end of the drive shaft 240 is coupled to the motor shaft 48 by an axially releasible coupling, shown as a lap joint between the two shafts within an encircling collar 250. The right end of the drive shaft 240 is machined to form the pinion gear 50 as in the modification of FIG. 1.
A one-way drag brake is mounted between the drive shaft 240 and the main shaft 216. The main shaft 216 is counterbored from the left to receive a roller clutch assembly 252 of known construction, consisting of a plurality of rollers 254 mounted within a cam ring 256. The cam ring is formed with cam ramps against which the rollers are locked upon rotation of the shaft 240 in one direction, namely, the load-lowering direction, and from which the rollers are released upon rotation of the shaft 240 in the opposite or load-lifting direction. The cam ring 256 is rotatably received within the bore of the main shaft 216, and is held between two sets of friction washers at its opposite ends. Each washer set includes an inner hardened steel washer 258 adjacent the cam ring 256, an intermediate bearing washer 260 of oil-impregnated sintered bronze material, and an outer hardened steel washer 262. At the right such outer washer 262 bears against a stop shoulder at the bottom of the counterbore in the main shaft 216. At the left, the outer washer 262 of the washer set is engaged by a compression spring 264 held in place by an annuar plug 266 threaded into the counterbore. The two sets of washers form a friction drag brake between the clutch ring 256 and the main shaft 216 which serves to apply a friction braking effort to the input shaft 240 when that shaft rotates in a direction to be clutched to the cam ring 256. The ball bearing 242 has its outer race seated in a larger counterbore in the main shaft 216, and held in place by a snap ring 243.
Lift operation of this modification of FIGS. 7 and 8 is similar to that of the modification of FIGS. 1-5. The high speed motor 18 drives the input shaft 240 at high speed and this drives the gear train with a gear reduction of the order of 500:1 to drive the main shaft 216 carrying the lift sprocket 14 engaged with the lift chain 22. The high gear-reduction ratio permits the small motor 18 to lift relatively large loads in the same manner as described in connection with the earlier modifcation. The friction brake 252-262 is not operative during load-lifting operation, since the shaft 242 runs free in the roller clutch 252 when rotated in load-lifting direction.
In load lowering and holding operation of the hoist of FIGS. 7 and 8, the high gear reduction ratio of the gear train cooperates with the braking means to stop and hold a full load on the hoist with a high factor of safety. The braking means opposes load-lowering operation. On the one hand, the braking effort is sufficient under reverse drive conditions through the gear train to provide more than adequate braking to positively stop and hold a full load. On the other hand, the braking effort is small enough to be overcome by motor operation in forward drive conditions through the gear train. Accordingly, the combined effect is that on load-lifting, no braking effort is applied, whereas when the hoist is suspending a load, the braking effort is sufficient, in combination with the high gear reduction ratio, to positively stop and hold a full load, but will be overcome by motor operation in forward drive through the gear train to produce a positively driven load-lowering action.
The hoist of FIGS. 7 and 8 contains two separate braking means, either of which may be used separately, or both used together.
One such braking means is the one-way drag brake 252-262 between the input shaft 240 and the main shaft 216. This exerts no braking effort in load lifting operations, for the roller clutch 252 is so oriented that it permits the shaft 240 to rotate freely in its load-lifting direction of rotation. In load-lowering rotation of the shaft 240, however, the rollers 254 clutch the cam ring 256 to the shaft 240 so that such ring rotates with the shaft 240. Such rotation causes relative rotation of the washers of the two sets of washers 258-262 at opposite ends of the cam ring 256, and this applies a frictional drag opposing rotation of the cam ring 256 and of the shaft 240 relative to the main shaft 216. The amount of braking effort thus produced is regulated by the tension in the spring 264 with which the washers are pressed against each other. This braking effort so produced is made of such value that in combination with the high gear reduction ratio of the gear train it will stop lowering movement ofa full load on the hoist chain 22 if positive forward drive of the hoist in a lowering direction is interrupted by shutting off the motor 18. On the other hand, the braking effort is such that motor operation in a lowering direction overcomes the drag of the one-way drag brake and causes positive lowering operation of the gear train and the hoist.
The second braking means in the hoist of FIGS. 7 and 8 is obtained by adjusting the gear train in a manner to produce an axial thrust on the countershaft 32 in a direction such that, on load-lowering movement, it presses the gears into frictional engagement with each other and/or with the end wall 236 of the gear case 220 to produce an effective braking action.
It is found that when the countershaft 32 is mounted in a roller bearing such as the needle bearing 34 at its left end, if the countershaft 32 is slightly misaligned in a suitable direction with respect to the axis of the bearing, the countershaft 32 will tend to screw itself to the right during load-lowering operation, as indicated by the full line arrow 31 in FIG. 7 and hence to be given an end thrust to the right, and conversely, to screw itself to the left on load-lifting rotation as shown by the dotted arrow 33 in FIG. 7, and hence to be given an end thrust to the left. The end thrust in either direction is relatively weak when the gear train is under forward drive and is stronger when the gear train is driven or tends to be driven in a reverse drive direction, that is, when driven by a load acting on the main shaft 216.
End thrust to the left is transmitted directly to the hoist frame by a low-friction thrust washer 232 mounted on the left end of the countershaft 32, and imposes no significant drag on motor-driven lift rotation of the gear train. The slight misalignment which produces such end thrusts may be obtained by slightly rotating the gear case 220 to move the right end of the countershaft 32 with respect to the left end of such countershaft. The gear case 220 is held concentric with the common axis of the main shaft 216 and the input shaft 240 by engagement of the peripheral flange 215 of such gear case with the circular rib 213 on the yoke 210, and the gear case has a position, indicated by the centerline 221 in FIG. 8 in which the countershaft 32 is parallel with the common axis of the shafts 216 and 240. To produce the end thrust effect mentioned above, the gear case 220 is slightly rotated, clockwise as viewed in FIG. 8, from the position of the centerline 221, to a new position indicated by the centerline 223. The amount of such clockwise rotation required is substantially less than 1 and ordinarily less than A corresponding slight misalignment to produce the desired end thrust can also be obtained by forming the bore for the bearing 34 at a slight angle to the desired position of the axis of the countershaft 32.
The end thrust to the right produced by this slight misalignment has the effect that, on load lowering rotation of the gear train, the countershaft 32 is thrust to the right. This closes the clearance between the fixed gear 68 and the gear clusters 6062 and 52-54, and thrusts the side face of the gear 52 against the inner face of the end wall 236 of the gear case 220. This produces a frictional drag which opposes load lowering rotation. By suitable adjustment of clearances, the end thrust can also produce frictional drag between the gear clusters on the countershaft 32 and the gear clus ters on the input shaft 240, to add additional frictional drag in the gear train. Such frictional drag is especially forceful when the gear train is being reversely driven by torque from the load on the hoist chain 22, but appears to be relieved when the motor exerts forward drive in the load-lowering direction. There is thus produced in the gear train a braking drag which is strongest when it is most needed to overcome a reverse drive from a hoist load but which is relieved when the gear train is forwardly driven by the motor. Such braking drag in the gear train is effective to greatly increase the loadholding capability of the hoist, whereas load-lowering motor operation overcomes the drag and provides positive lowering drive.
The presence of the drag braking provided by either or both of the above-described braking means has the effect of providing a positive drive of the motor 18 for load-lowering, and ofimposing a load on such motor 18 when driven in the load-lowering direction. The presence of that load on the motor has the advantageous effect of improving the smoothness with which the motor operates under speed control from the SCR speed control mechanism.
1. An electric hoist of small physical size and light weight in relation to its load capacity, such that a hoist having a load capacity in the range of 250 pounds to 500 pounds weighs not more than about l0 pounds, comprising a hoist body, a lift rotor mounted in said body, a chain or like lift element in positive lift engagement with said rotor on a pitch radius,
a multiple-stage spur gear reduction train connected to said lift rotor for driving the same in lift and lowering directions,
a reversible electric motor having a motor shaft con nected to said gear reduction train to drive the same in lift and lowering directions,
said motor being a high-speed series motor having a free-running speed of the order of l0,000 R.P.M. or more,
said gear reduction train having a gear ratio of at least 250:] per inch of pitch radius of said lift rotor so that a load of up to 250 pounds on said lift element corresponds to a torque load on said motor shaft, neglecting friction, of not more than about one inch pound,
and said gear train comprising a countershaft and a rotor mounted thereon adjacent a stationary wall,
and means to cause said countershaft to exert an end-wise thrust on rotation in a load-lowering direction and thereby to thrust said rotor toward said stationary wall to apply frictional braking thereto to oppose rotation of the gear train in a load lowering direction,
said gear reduction train being provided with sufficient frictional resistance to rotation in a loadlowering direction to make the same substantially irreversible under the full static torque load on said lift rotor from the stated load of 250 pounds on the lift element.
2. An electric hoist as in claim 36 wherein said countershaft is mounted at one end in a roller bearing having elongated rollers therein, and means is provided to cause said rollers to be slightly misaligned relative to said countershaft and the rollers thereby tend to screw the shaft end-wise on rotation thereof, said end thrust being produced by the screwing action of the rollers.
3. An electric hoist as in claim 2 in which said relative misalignment between the rollers and countershaft is produced by a slight misalignment of the shaft relative to the axis of the roller bearing.
4. An electric hoist of small physical size and light weight in relation to its load capacity, such that a hoist having a load capacity in the range of 250 pounds to 500 pounds weighs not more than about pounds, comprising a hoist body,
a lift rotor mounted in said body,
a chain or like lift element in positive lift engagement with said rotor on a pitch radius,
a multiple-stage spur gear reduction train connected to said lift rotor for driving the same in lift and lowering directions,
a reversible electric motor having a motor shaft connected to said gear reduction train to drive the same in lift and loweringdirections,
said motor being a high-speed series motor having a free-running speed of the order of 10,000 R.P.M.
said gear reduction train having a gear ratio of at least 250:1 per inch of pitch radius of said lift rotor so that a load of up to 250 pounds on said lift element corresponds to a torque load on said motor shaft, neglecting friction, of not more than about 1 inch pound,
whereby said motor and gear train combine to permit each to be of small physical size and light weight and thereby provide a hoist of small physical size and light weight as set forth,
the driving connection between the motor and the lift rotor comprising an input shaft mounted on a first axis, a hollow main shaft rotatably mounted on said first axis in said hoist body and carrying said lift rotor, a counter shaft mounted on a second axis in spaced parallel relation with said first axis, a pinion on said input shaft and a driven gear on said second axis and meshing with said pinion, providing a first reduction stage of said gear reduction train, a main drive gear on said main shaft and a pinion on said counter shaft axis and meshing with said main drive gear, providing a final reduction stage of said gear reduction train, a plurality of gear sets forming a continuous gear train from said first stage to said final stage and forming intermediate gear reduction stages, each gear set comprising a driven gear on one of said axes meshing with and driven by a driving gear on the other of said axes, the driving gear of each gear set being connected for rotation with the driven gear of the preceding gear set, wherein said driven gear meshing with the pinion on the input shaft lies adjacent a stationary wall, and means to cause said countershaft to exert an endwise thrust on rotation in load-lowering direction and thereby thrust said driven gear toward said stationary wall to cause frictional braking thereof to oppose rotation of the gear train in a load-lowering direction.
CERTIFICATE OF CORRECT-ION Patent No. 3, 784,165 Dated January 8, 1974 Inventor(s) Dana" L. Pruitt I It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
Col. 11, line 13, claim 2, change "claim 36" to --claim l-- SLQI'IGG and sealed this 23rd. day of April 1914; r v
LEU IAED M.FLQSTCIJH, JR.
/1 U At'tsstiug Gfficer IIAHQ'IALL DAMN Commissioner of Patents FORM RO -105D (10-69) UNITED STATESPZATENTOFFICE'Z 1Y