Single failure proof crane

A synergistic safety system is incorporated in a crane driven by a high-speed motor which requires both an energy-absorbing torque-limiting device in the speed reduction unit and a drum emergency holding device. The energy-absorbing torque-limiting device transmits the static and dynamic torque required to rotate or hold the drum against the maximum carried load, but will slip at a pre-determined setting to absorb high-speed rotational energy of the drive train and/or torque of the drive motor in the event a two-blocking, load hang-up, overload, or engagement of the drum emergency holding device occurs. The drum emergency holding device is set automatically when the energy-absorbing torque-limiting device is actuated and/or any drive train component fails, which are detected by one or more sensing sub-systems.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention pertains to hoisting cranes, and more particularly, to 
safety features of hoisting cranes to prevent dropping or damaging the 
load because of a failure within the hoist system, and to protect the 
entire system from the forces resulting from load hang-up, or 
two-blocking. 
2. Description of the Prior Art 
Many cranes, such as nuclear fuel-handling cranes, require extreme 
failure-proofing safety measures because the potential consequences of 
dropping a load, due to failure of one of the components of the crane, may 
be disastrous. Two occurrences which can lead to failures, result when: 
(1) the traveling block of the crane reeving system accidentally engages 
the stationary or head block of the reeving system (known as 
two-blocking), or (2) the load or traveling block catches or hangs-up on 
some structural obstruction as the load is being hoisted (known as load 
hang-up). When either of these situations occurs, the crane and the 
reeving components become in effect a rigid system. The kinetic energy of 
the high-speed rotating components and the energy input of the drive motor 
must then be dissipated by elastic and/or inelastic deformation of the 
weakest member of the system--frequently leading to its failure. Often the 
cables are the weakest component of the crane and many times they 
fail--allowing the load to drop uncontrolled. Even if failure does not 
occur, the stresses to which the hoist and other crane components have 
been subjected cannot be determined and thus the remaining factor of 
safety of the crane is suspect. Additionally, the load itself must be 
protected from being torn apart during a load hang-up. The forces required 
to damage the load will vary with its strength, but are generally less 
than those which can be exerted by the crane's machinery. 
In the past, various redundant switches have been placed in the vicinity of 
the crane reeving system so that as the traveling block approaches the 
stationary block the switches will de-energize the motor to bring the 
crane to rest. However, in some cases these switches fail due to improper 
installation, maintenance or wear. Secondly, these switches do not provide 
a safeguard against load hang-up. A second technique has been to provide 
substantial crushable or sacrificial structure between the traveling block 
and the stationary block. The purpose of this structure is to absorb the 
kinetic energy of the crane components so that it will be dissipated prior 
to the formation of a rigid system. Whether used with or without the 
switches, this technique is extremely costly and still does not prevent 
over-stressing and failure in the event of a load hangup or overload, 
during which the motor breakdown torque may be applied. 
Clutches have been utilized in construction-crane drive trains to protect 
against overloads, but have not been utilized in overhead cranes because 
the clutch, or energy-absorbing torque-limiting device, would make the 
crane susceptible to loss of control of the load in the event of a 
mechanical failure of the clutch--an unacceptable risk for cranes used in 
critical service. 
Drum emergency holding devices have been used to hold cable drums in a 
stationary position, but have not been used to suddenly stop hoists' drive 
systems, since the impact loading of the sudden stop on the drive train 
cannot be accurately evaluated. Furthermore, the large amount of 
rotational kinetic energy of the high-speed hoisting equipment would 
probably be absorbed in inelastic deformation of the drive train which 
could lead to failure or subsequent maloperation of the equipment. 
Duplicate drive trains have been relied upon in the past to protect 
critical loads in the event of a single mechanical failure of a drive 
train component. However, merely duplicating the drive train components 
penalizes the hoist design in terms of the cost and weight of the extra 
equipment. Furthermore, the substantial increase in weight of the 
additional equipment that must be supported by the girders contributes to 
even more cost. Also, duplicate drive trains are still subject to 
common-mode failures and thus they do not provide diverse redundancy. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a safety device for uncoupling 
the prime mover and the high-speed, high-kinetic energy components of a 
drive train from the load following accidental stopping of the load by an 
external force, and to protect the load following a discontinuity in the 
safety device or drive system. 
It is another object of this invention to provide means for dissipating 
kinetic energy following the accidental stoppage of a hoisted load and 
bring the load automatically to rest rather than to depend on the 
elasticity of the hoisting ropes and structures. 
It is another object of this invention to provide a hoist with diverse- and 
dual-load paths for safely supporting critical loads following any 
credible single failure of the drive train. 
It is also an object of this invention to provide a hoist which is 
protected from excessive stresses and the resultant uncertainty in the 
condition of the hoist following an attempt to lift a load in excess of 
the crane's rated capacity or other overloading, including a load hang-up 
or two-blocking. 
Basically, these objects are obtained by providing in the high-speed end of 
the crane drive train a torque-limiting device which will transmit the 
required running- and static-torques in both directions, but will limit 
the amount of torque which can be imposed on the system by the drive motor 
and will dissipate the kinetic energy of the high-speed end of the drive 
train when the drive train becomes overloaded. Detection of an over-speed 
running condition of the drum or prime mover, or a discontinuity in the 
drive train engages the drum emergency holding device to assure that a 
failure of the torque-limiting device itself can be detected and the drum 
stopped safely. This torque-limiting, failure detection, and drum holding 
combination assures protection against most any condition of single 
failure within the hoisting system. Even more complete failure protection 
can be achieved by combining this invention with the drive-train-failure 
detection system disclosed in commonly assigned co-pending application 
Ser. No. 883,539. 
As compared with sacrificial structure for absorbing kinetic energy on 
impact and redundant gear trains as are commonly used under present 
practice, the system of this invention provides substantial cost savings 
and increased reliability, since only one drive train is required for a 
dual-load path that does not depend upon duplicate equipment for both 
paths. 
The location of the energy-absorbing torque-limiting device in the 
high-speed end of the drive train is critical. It has been discovered that 
typically more than 95% of the kinetic energy is contained in the prime 
mover and the equipment that is rotating at the same speed as the prime 
mover, i.e., the conventional mechanical and electrical brakes, and the 
input shaft to speed reduction unit. Preferably, the location for the 
torque-limiter is between the first input shaft and the second stage idler 
shaft or more specifically between the first large (bull) gear and the 
second stage idler shaft. This bull gear has over 2/3 of the kinetic 
energy that is not associated with the motor and other high-speed 
components. A substantial part of the remaining energy is in the hoist 
cable drum, so little additional energy is isolated if the decoupling by 
the torque-limiter is accomplished in the slower speed shafts of the speed 
reduction unit. More importantly, since the torque transmitted by the 
slower speed shafts is significantly greater, the reliability of a 
torque-limiting device is decreased and may compromise the speed reduction 
unit's load carrying capacity. Uncoupling at the input to the speed 
reduction unit would undesirably leave as much as 20% of the total energy 
to be absorbed by the cables and blocks. 
An additional object of the invention is to provide an adjustable 
energy-absorbing torque-limiting device to prevent damage to loads of 
varying strengths and weights during a load hang-up. Basically this object 
is achieved by adjusting externally the torque limit at which the device 
begins to slip so that hang-up of a light, fragile load will cause the 
device to slip at a torque considerably less than normally would be 
required to make a rated capacity lift. One of the most important benefits 
achieved with the adjustable energy-absorbing torque-limiting device and 
the associated failure detection components of this invention is the 
ability to provide, along with redundant reeving, a diverse and redundant 
crane/load failure protection system at a considerable savings in cost. In 
fact it had been speculated prior to this invention that such a 
diverse-failure-proof system was virtually impossible--requiring either 
costly administrative operating procedures to avoid a crane failure that 
could lead to damage of critical loads, or a relaxation in safety 
standards.

The preferred embodiment of the invention is illustrated in FIGS. 1-6 and 
best describes the principles of the interaction between the 
energy-absorbing torque-limiting device 97 and a drum emergency holding 
device 100. The unique feature of this invention provides drum stoppage in 
a short distance brought about by the combination of the energy-absorbing 
torque-limiting device in the drive train to isolate the majority of the 
kinetic energy of the drive train and a drum emergency holding device 
acting directly on the drum. The drum emergency holding device can be used 
only because the energy-absorbing torque-limiting device protects the 
drive train from excessive impact loading when the drum emergency holding 
device engages. The energy-absorbing torque-limiting device in the drive 
train can be used only because the drum emergency holding device protects 
the load from a mechanical failure or abnormal slippage of the 
torque-limiting device. Thus the two components are uniquely interrelated 
such that neither can be provided without the other and each enhances the 
function of the other. This provides the unique synergistic result of 
having a much safer, less expensive hoisting system which avoids the 
problems of two-blocking, load hang-up, overload, and load damage. 
In this preferred form of the invention the drum emergency holding device 
100 is a positive engaging pin or pawl 119 which is normally held in a 
retracted position, to the left in FIG. 2, by an energized solenoid 127 
and is brought into engagement with the hoist drum flange 34 by a spring 
121. The flange is provided with notches 113 which will engage the pin and 
pull against a piston rod 116 of a cylinder 114. The cylinder brings the 
load to rest quickly after some malfunction or hazard condition has 
occurred. It would be hazardous because of the kinetic energy of the 
system to impose such a stopping force on the drum in the absence of the 
energy-absorbing torque-limiting device 97. 
As a typical example, assume that a hoist with a 4 part line two-blocks 
while raising a 50,000 lb. load at 7.75 feet per minute. The kinetic 
energy of the moving load is about 13.0 ft-lb. This small amount of 
kinetic energy can be safely absorbed by the traveling and stationary 
blocks as they contact. The kinetic energy of the drive components between 
the load and the energy-absorbing torque-limiting device is only about 42 
ft-lb compared to almost 3000 ft-lb. in the rest of the drive train. The 
line can be safely stressed to 0.4 of its breaking strength 
(0.4.times.79,600 pounds in this example). Thus the difference between the 
elongation of the cable resulting from the static load and the elongation 
corresponding to 0.4 of its breaking strength is available to absorb the 
kinetic energy of the rotating machinery (in this example only about 300 
ft-lb of the kinetic energy can be safely absorbed in the event a 
two-blocking occurs while a rated capacity lift is being made). 
In the preferred form the solenoid 117 is energized to hold the pin to the 
left so that in the event of an electrical malfunction, which de-energizes 
the solenoid, the pin is automatically drawn into the drum. The load at 
which the cylinder 114 resists movement of the drum is easily controlled 
by a relief valve 125 which couples one side of the piston 115 of the 
piston rod to an accumulator 126. A return spring 117 will restore the 
piston rod to its retracted position after the pin has been reset by the 
solenoid. 
The hoist drum 111 is provided with the flange 34 and with a drum gear 133. 
Cable 112 is wrapped on the drum in a conventional manner. The drum gear 
is rotated by the drum pinion 122 through gear box 18 which includes the 
energy-absorbing torque-limiting device 97. The speed reduction unit 
within the gear case is driven by a high-speed input shaft driven through 
a coupling 120 by the motor 119 with its brakes 128. 
Recognition of a hazardous condition, de-energizing the solenoid 127, and 
thus setting the stop pin 119 can be accomplished by any number of 
techniques already discussed. These would include detecting an 
out-of-synch movement between the input and output halves of the 
energy-absorbing torque-limiting device, out-of-synch movement between the 
drum and the motor, some change in commanded-speed or -direction relative 
to actual-speed or -direction, or a simple overspeed detection device. 
For example, assume that the motor in the crane described above starts 
lowering the load at an excessive speed. If the drum over-speed switch 
engages pin 119 when the drum rotational velocity corresponds to a load 
speed of 10 feet per minute the kinetic energy of the load will be about 
21.6 ft-lb. and the kinetic energy of the drive train between the load and 
the energy-absorbing torque-limiting device will be about 71 ft-lb. The 
positive drum locking device must be able to safely absorb the sum of 
these two kinetic energies, i.e., 93 ft-lb, to bring the load to rest. 
Thus if a 75,000 lb retarding force is applied by cylinder 114, pin 119 
will displace about 0.015 inches in bringing the load to rest. The 
retarding force applied can of course vary depending upon the stroke or 
movement desired of the piston rod 116. 
It is a unique feature of this invention that an energy-absorbing and 
-decoupling clutch is provided in the high-speed end of the drum's drive 
train and is uniquely combined with the over-speed detector and drum 
emergency holding device to provide a synergistic total brake and 
shock-absorbing hoisting system not heretofore known in the art. The 
energy-absorption component of this invention is best shown in FIG. 4 
which illustrates a fragmentary section through the speed or gear 
reduction unit 18. The first reduction gears 18a are also the high-speed 
end of the drive train and as described earlier contain most of the 
kinetic energy of the drive train but must transmit a relatively small 
percentage of the torque carried by the low-speed shafts in the drive 
train. Thus by placing the energy-absorbing torque-limiting device in the 
first speed reduction of the drive train it is possible to transmit the 
required torque while still having overload protection by slippage to 
dissipate the high-speed kinetic energy of the drive train upstream of the 
first speed reduction. For this purpose, the first speed reduction 18a is 
provided with a pinion 70 keyed to the motor shaft 20 which meshes with a 
bull gear 71. The bull gear is rotably mounted by bearings 72 on a second 
reduction idler shaft 74. The bull gear is provided with a cylindrical 
flange 75 having friction plates 76 splined thereon in a conventional 
manner. Keyed to the shaft 74 is a stationary clutch hub 77 having a 
plurality of metal separator plates 78 splined to the hub 77 in a 
conventional manner. The separator plates are positioned between alternate 
friction plates 76. A pressure plate 80 is secured to the flange 75 by a 
plurality of circumferentially spaced bolts 82. The bolts are separated 
from the pressure plate by belleville springs 84 which provide a pre-set 
spring pressure which clamps the pressure plate 80 toward the gear 71 thus 
providing a pre-set friction force to transfer the torque between the bull 
gear 71 and the second reduction idler shaft 74. In practice this setting 
would be determined by trial and error since hoisting requirements may 
vary greatly for different hoisting applications. At this location for one 
example given with a 740:1 total gear reduction ratio it is expected that 
the friction discs must be able to transfer approximately 5,000 
inch-pounds of torque for a 25 ton hoisting load powered by a 15 
horsepower motor. 
In operation, should a load which is being hoisted hang-up on a structure 
surrounding the hoist or should the traveling block engage the fixed 
block, and thus in either case provide an overload, the motor 119 will 
normally be de-energized either by conventional overload detecting 
switches, or excessive deviation between the drum and the motor encoder 
outputs as described in said co-pending patent application, or by 
redundant electrical safety switches in the case of the traveling block 
approaching the stationary block. The positive engaging pin will be 
brought into engagement by de-energizing the solenoid 127, so that the 
load will be held if the energy-absorbing torque-limiting device continues 
to slip after the kinetic energy has been dissipated. The overload before 
the motor is completely at rest will be absorbed by the friction plates 
and separator plates slipping relative to one another generating heat. The 
heat will normally be of only a short time duration and can be easily 
carried away by the oil circulating within the speed reduction unit 18 
since the entire energy absorption mechanism will be immersed in the gear 
lubricating oil. 
As shown in FIG. 6 another level of safety is achieved by providing 
external adjustment of the energy-absorbing torque-limiting device. In 
this embodiment the speed reduction unit 18 is provided with a 
first-reduction bull gear 90 keyed to an axially shiftable support shaft 
91. Shifting of the shaft is provided by an air cylinder and piston 93 
which is operated against a compression spring 94 by operator controlled 
air from 0-100 psi, for example. The bull or input gear is releasably 
coupled to a clutch hub 96 by a conventional clutch pack 97 of the type 
which transmits torque between the gear 90 and clutch hub 96 by 
compressing the clutch discs and spacers 98 within the clutch pack. The 
clutch hub is integral with an output pinion to the second reduction. In 
operation the operator determines the torque at which a critical load will 
fail. If it is greater than the torque setting that is required to provide 
the necessary safety factor for the crane components, the torque is set 
the same as for preventing a failure of the crane system. If the torque to 
protect the load is less, then the lower setting is applied by admitting 
air pressure against the piston 93 to offset part of the force of spring 
94 and thus cause the clutch to slip at a lower torque. 
FIG. 7 illustrates an alternate embodiment of the invention which utilizes 
the Cable Drum Safety Brake System disclosed in commonly assigned 
co-pending application Ser. No. 883,539. In this alternate embodiment a 
hoist drum 10 is mounted for rotational movement in bearings 12 and 
carries a hoist cable 13. The cable is shown as having dual reeving 13a 
and 13b for redundant safety, if desired. The drum is provided with a drum 
gear 14 that is powered by a pinion 15. The pinion is keyed on an output 
shaft 17. In one typical example, the speed reduction between the pinion 
15 and the gear drum 14 is 6.1:1. A gear reduction unit 18 is provided 
with an additional three stages of speed reduction which include a first 
stage 18a, for example, having a reduction of 6.2:1; a second stage 18b 
having, for example, a reduction of 4.9:1; and a third stage reduction 18c 
having, for example, a reduction of 4.0:1. The exact reductions and the 
number of reductions is, of course, subject to considerable variation 
depending upon the requirements of the particular installation. An input 
shaft 20 is powered by a motor M which includes conventional electric and 
mechanical brakes. 
The hoist system has an additional braking system comprising a disc 34 
attached to the drum 10, a caliper brake 36 opened by pneumatic lines 37 
and 38. The pneumatic lines are coupled to a braking mechanism control 
system 40 which releases the air pressure input to the lines 37 and 38 to 
actuate the caliper brake 36 when the number of pulses from a drive motor 
encoder 41 deviate by a sufficient amount from the number of the pulses 
produced by a cable drum encoder 42 which measures the rotation of the 
drum. A conventional overspeed switch 44 will actuate the braking 
mechanism when the drum is rotating in excess of some predetermined speed. 
The brake 36 is pneumatically powered from a motor 50 which drives a 
conventional compressor 52. The compressor supplies pressurized air to a 
tank 54. A conventional solenoid valve 56 directs the pressurized air to a 
cylinder 62 for releasing the caliper brake 32. The solenoid valve is 
energized by a control circuit which includes the over-speed switch 44, 
normally open relay contacts 63 and a solenoid coil 64. The contacts 63 
are kept open when the rotation of the drum 10 and the motor M are 
in-synch with respect to one another. Thus the solenoid can be 
de-energized by a deviation from the preset compared values or by an 
over-speed condition or by total electrical failure of the system. 
The details of the foregoing hoist system apparatus are more fully 
disclosed in the commonly assigned patent application entitled "Cable Drum 
Safety Brake" Ser. No. 883,539 filed herewith which description is hereby 
incorporated herein by specific reference therto. However, application of 
the "Cable Drum Safety Brake" can be extended by utilizing the synergistic 
relationship established by this invention. Because of adverse 
environmental conditions, such as contaminates, some applications require 
the drum caliper brake to have excess braking capacity over that which 
would be required to stop or hold the load under normal operating 
conditions. In these applications, the energy-absorbing torque-limiting 
device is required to protect the drive train from impact loading 
following engagement of the caliper brakes for the same reasons that use 
of the drum emergency holding device depended upon use of the 
energy-absorbing torque-limiting device. The caliper brakes serve the same 
function as the drum emergency holding device in protecting the load from 
undesired slippage of the energy-absorbing torque-limiting device. 
As is apparent the total combination of energy-absorption, failure 
detection, and drum holding thus provides a relatively inexpensive and 
considerably safer total-braking and energy-absorption system than was 
heretofore possible in the prior art. With the adjustable setting to 
uncouple the drive train, the load also is protected from over-loads 
applied to it. While the preferred embodiments of the invention have been 
illustrated and described it should be understood that variations will be 
apparent to one skilled in the art without departing from the principles 
expressed herein. For example, the adjustability of the decoupling torque 
can be provided mechanically, electrically by other means within the skill 
of the art. Accordingly, the invention is not to be limited to the 
specific embodiments illustrated in the drawing.