Electric hoist

An electric hoist includes an electric motor, a driving shaft connected to a rotor shaft of said electric motor, a reduction gear train for transmitting rotation of the driving shaft to a load sheave, and a mechanical brake assembly in the reduction gear train. According to the invention between the driving shaft and the rotor shaft of the motor is arranged an overload safety device comprising a support member fixed to the rotor shaft, a retainer member fitted onto the driving shaft, and a disc arranged between the support and retainer members through friction plates and having a one-way clutch. The one friction plate between the retainer plate and the disc has larger friction transmission force than that of the other friction plate between the support member and the disc. With this arrangement, the electric hoist according to the invention is able to automatically prevent lifting, lowering and dragging a load under overload condition, and enables the once raised load to be securely lowered irrespective of its light or heavy weight.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an electric hoist having overload safety means 
for preventing lifting, lowering and dragging under overload condition and 
capable of securely lowering a load irrespective of its light or heavy 
weight. 
2. Description of the Prior Art 
With electric hoists such as electric chain blocks whose load sheave is 
driven through reduction gears by electric motors, it has been proposed to 
provide on a driven shaft of a reduction gear mechanism a brake assembly 
consisting of a brake support member, a brake retainer member, a brake 
ratchet wheel and friction plates between the members and further provide 
an overload safety device including frictional connections interposed 
between the brake retainer member and a driven gear fitted thereon, in 
order to obstruct raising of a load by slippage in the overload safety 
device when the load sheave is subjected to an overload. 
With such an electric hoist, however, the friction transmission force of 
the overload safety device often becomes less than that of the brake 
assembly owing to its overtightening, so that once raised a load cannot be 
lowered due to slippage in the overload safety device. 
In order to solve this problem, it has been proposed to interpose a one-way 
clutch between the driven gear and the brake retainer member to render 
inoperative the overload safety device when lowering the load, as 
disclosed for example in Japanese Patent Application Publication No. 
42,937/78. In this case, however, as the overload cannot be prevented in 
lowering the load, it is indispensably required to provide a limit switch 
for detecting a lowermost position of the load in view of the safety in 
operation which would possibly make the hoist complicated. 
SUMMARY OF THE INVENTION 
It is therefore a primary object of the invention to provide an improved 
electric hoist which solves these problems of the prior art. 
It is a general object of the invention to provide an electric hoist whose 
rotor shaft and a driving shaft are connected by a friction type overload 
safety device including a one-way clutch and interposing friction plates 
having different friction transmission capacities, thereby automatically 
preventing the lifting, lowering and dragging of a load under overload 
condition and enabling the once raised load to be securely lowered 
irrespective of its light and heavy weight. 
In order to achieve the above objects, the electric hoist including an 
electric motor, a driving shaft connected to a rotor shaft of said 
electric motor, a reduction gear train for transmitting rotation of said 
driving shaft to a load sheave, and a mechanical brake assembly in said 
reduction gear train according to the invention comprises an overload 
safety device arranged between said driving shaft and said rotor shaft of 
said motor, said overload safety device comprising a support member fixed 
to said rotor shaft, a retainer member fitted onto said driving shaft, and 
a disc having a one-way clutch and arranged between said support and 
retainer members through friction plates, said one friction plate between 
said retainer plate and said disc having larger friction transmission 
force than that of the other friction plate between said support member 
and said disc. 
In one preferred embodiment of the invention, the one friction plate 
between the retainer plate and the disc has a larger diameter than that of 
the other friction plate between the support member and the disc, or the 
one friction plate between the retainer plate and the disc is made of a 
material having a larger coefficient of friction than that of a material 
of the other friction plate between the support member and the disc. 
In order that the invention may be more clearly understood, preferred 
embodiments will be described, by way of example, with reference to the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1 illustrating one embodiment of the invention, an 
electric hoist comprises an electric motor 1 including a rotor 12 fixed to 
a rotor shaft 11, a magnetic pole member 13 and an attracted member 15 
made of a magnetic material with a disc 16 provided on its outer periphery 
with a friction plate 17. The magnetic pole member 13 and the attracted 
member 15 are axially slidably fitted on the rotor shaft 11 with spline 
connections or the like. Between the magnetic pole member 13 and the 
attracted member 15 is interposed a spring 18 normally resiliently urging 
the attracted member 15 to the right as viewed in FIG. 1 to force the 
friction plate 17 against a front frame 19. The magnetic pole member 13 is 
made of a non-magnetic material and is provided on its one surface in 
opposition to the attracted member 15 with a number of attractor elements 
14 made of a magnetic material radially embedded in the magnetic pole 
member 13. A reference numeral 20 denotes a stator of the motor. 
The electric hoist further comprises a driving shaft 2 having at its one 
end a pinion 21 and at the other end an overload safety device 3. The 
overload safety device 3 comprises a connecting shaft 31 having one end 
fitted in the rotor shaft 11 through a key, spline grooves or the like and 
the other end connected through a bearing 32 to the driving shaft 2. The 
overload safety device 3 further comprises a friction plate support member 
33 fixed to the rotor shaft 11 by means of a spline connection or the 
like, a friction plate retainer member 34 fitted on the other end of the 
driving shaft 2 through spline grooves or the like, and a disc 35 fitted 
on the connecting shaft 31 between the members 33 and 34. Between the disc 
35 and the connecting shaft 31 there is provided for example a one-way 
clutch 36 as shown in FIG. 2. While the one-way clutch 36 is being 
operated, when the connecting shaft 31 is rotated in a winding-off 
direction or an clockwise direction as viewed in FIG. 2, the one-way 
clutch 36 is actuated to transmit the rotation to the disc 35, but does 
not transmit the rotation of the disc 35 when the connecting shaft 31 is 
rotated in a winding-up direction or a counterclockwise direction as 
viewed in FIG. 2. 
Friction plates 37 and 38 are respectively interposed between the disc 35 
and the members 33 and 34. The friction plate support member 33 is urged 
through the rotor shaft 11 by resilient members 11a such as dish-like 
springs to the left as viewed in FIG. 1 to force the friction plates 37 
and 38 against the disc 35 and the friction plate retainer member 34. The 
friction plate 38 interposed between the member 34 and the disc 35 has an 
effective diameter larger than that of the friction plate 37 interposed 
between the friction plate support member 33 and the disc 35, thereby 
enabling the friction plate 38 to transmit a higher torque than that 
transmitted by the friction plate 37. 
A load sheave 4 having at its one end a shaft 41 is loosely fitted on the 
driving shaft 4 and journaled in bearings in frames 5 and 5a. A driving 
gear 42 is fixedly fitted on the shaft 41 of the load sheave 4. A chain 
guide 43 is arranged in opposition to the load sheave 4. 
A screw type mechanical brake assembly 6 comprises as shown in FIGS. 3 and 
4 a driven shaft 61 arranged in parallel with the driving shaft 2 and 
formed with a screwthreaded portion 62 and a pinion 63, a brake support 
member 64 having a boss 65 fixedly engaged with the threaded portion 62 of 
the driven shaft 61, a brake ratchet wheel 66 loosely or rotatably fitted 
on the boss 65 of the brake support member 64, a driven gear 67 engaged 
with the threaded portion 62 of the driven shaft 61 and adapted to be in 
mesh with the pinion 21 of the driving shaft 2, and friction plates 68 and 
69 respectively interposed between the ratchet wheel 66 and the driven 
gear 67 and brake support member 64. As can be seen from FIG. 4, the 
pinion 63 of the driven shaft 61 is in mesh with the driving gear 42. 
A brake pawl 71 is pivotally mounted on a shaft 70 fixed to the frame 5a as 
shown in FIG. 1 and urged by a spring (not shown) so as to engage the 
ratchet wheel 66. A reference numeral 81 denotes a rear frame. A hook 82 
serves to hang the electric hoist from a ceiling or cross girder. The 
rotor shaft 11 and driving shaft 2 are journaled in bearings in the front 
frame 19, frames 5 and 5a and rear frame 81. The driven shaft 61 is also 
journaled in bearings in the frames 5a and the rear frame 81. 
The operation of the electric hoist constructed as above mentioned 
according to the invention will be explained hereinafter. When the motor 1 
is energized for lifting or winding up a load, the attractor elements 14 
in the magnetic pole member 13 are magnetized to attract the attracted 
member 15 to the magnetic pole member 13 against the force of the spring 
18 so that the disc 16 is moved to the left as viewed in FIG. 1 to release 
the braking action of the friction plate 17 so as to permit the rotor 
shaft 11 to be rotated. The rotation of the rotor shaft 11 is transmitted 
through the support member 33, friction plate 37, disc 35, friction plate 
38 and retainer member 34 to the driving shaft 2. 
The rotation of the driving shaft 2 is transmitted through its pinion 21 to 
the driven gear 67 (FIG. 4). As the driven gear 67 rotates, it moves to 
the right as viewed in FIGS. 1 and 4 on the threaded portion 62 of the 
driven shaft 61 owing to the threaded engagement therebetween to join the 
driven gear 67, ratchet wheel 66 and brake support member 64 together with 
the friction plates 68 and 69 interposed therebetween into a frictionally 
combined unit. Accordingly, the driven shaft 61 is rotated to cause the 
load sheave 4 to be rotated through the driving gear 42 engaged with the 
pinion 63, so that the load is raised or lifted by a chain (not shown) 
wound about the load sheave 4. It is of course understood in this case 
that the rotating speed of the driving shaft 2 is considerably reduced 
through the reduction gear train including the pinion 21, driven gear 67, 
pinion 63 and driving gear 42 to be transmitted to the load sheave 4. 
If a weight of the load is more than a rated value, the friction plate 37 
of the overload safety device 3 is slipped relative to the support member 
33 or disc 35 and the one-way clutch 36 is brought into an inoperative 
condition, so that the rotor shaft 11 is idly rotated without transmitting 
the torque of the rotor shaft to the driving shaft 2. As the load cannot 
be raised or lifted under such a condition of the hoist, the hoisting or 
dragging of the overload is automatically prevented. 
When the motor 1 is deenergized, the attracted body 15 is returned to its 
original position by means of the spring 18 to urge the friction plate 17 
against the front frame 19 so as to cause the braking action which holds 
the hoisted load in its position. 
When it is required to lower the load, the motor 1 is energized to rotate 
the rotor shaft 11 in a reverse direction. The rotation of the rotor shaft 
11 is transmitted through the overload safety device 3 to the driving 
shaft 2. The rotation of the driving shaft 2 is then transmitted to the 
pinion 21 and driven gear 67 so that the load is lowered with the aid of 
the mutual action of the weight of the load and the mechanical brake 
assembly 6. 
If an overload occurs during lowering the load, the friction plate 37 of 
the overload safety device 3 is subjected to a force slipping it relative 
to the support member 33 or the disc 35. However, the relative rotation 
between the rotor shaft 11 and respectively the support member 33 and disc 
35 is prevented by the one-way clutch 36 in the disc 35, so that the 
friction plate 38 slips relatively to the disc 35 or retainer member 34 
without transmitting the torque on the rotor shaft 11 to the driving shaft 
2. Accordingly, even if a stopper (not shown) provided on one end of the 
chain wound about the load sheave 4 remote from the other end suspending 
the load strikes the chain guide 43 at the lowermost position of the load, 
the lowering of the load is stopped without any damage of the chain guide 
43, thereby automatically preventing the lowering of the overload. The 
electric hoist according to the invention can therefore dispense with a 
limit switch for detecting the lowermost position for winding-off a load. 
Moreover, even if the mechanical brake assembly 6 is clamped to an excess 
extent when a lifting load is stopped, the load can be securely lowered 
without stoppage of the load during lowering because the winding-off 
torque of the overload safety device 3 has been set to be larger than the 
friction transmission force of the mechanical brake assembly 6. 
In the above explanation, one example of the one-way clutch for use in the 
overload safety device has been shown in FIG. 2. However, any other 
one-way clutches may be used for this purpose. Moreover, instead of the 
screw type mechanical brake assembly, a cam type mechanical brake assembly 
may be used. In the above embodiment, moreover, the effective diameter of 
the friction plate 38 interposed between the retainer member 34 and disc 
35 is made larger than that of the friction plate 37 interposed between 
the support member 33 and disc 35 of the overload safety device in order 
to that the winding-off or lowering torque is made larger than the 
winding-up or lifting torque of the overload safety device. However, the 
effective diameters of the friction plates 37 and 38 may be made 
substantially equal to each other for the purpose so long as the friction 
plate 38 is made of a material having a coefficient of friction larger 
than that of a material of the friction plate 37. 
As can be seen from the above description, the electric hoist according to 
the invention comprises a one-way clutch between a driving shaft and a 
rotor shaft of a motor and an overload safety device constructed as its 
winding-off torque larger than its winding-up torque, thereby 
automatically preventing the lifting, lowering or dragging of a load under 
overload condition, and enabling the once raised or lifted load to be 
securely lowered regardless of its light or heavy weight. Therefore, the 
electric hoist according to the invention brings about significant effects 
in actual use. 
It is further understood by those skilled in the art that the foregoing 
description is that of preferred embodiments of the disclosed hoists and 
that various changes and modifications may be made in the invention 
without departing from the spirit and scope thereof.