Gravity trolley conveyor with speed control

A gravity conveyor has a work supporting trolley provided with multiple wheels of different diameters fixed to a shaft that operates a retarder for applying a retarding torque to the wheels proportional to their angular velocity. The conveyor track consists of multiple, laterally spaced, resiliently flexible rails on which selective wheels of the trolley are adapted to roll. The axial spacing of the tracks and wheels simplifies the location of the two centers of gravity of the loaded and unloaded trolley.

This invention relates to conveyors and, more particularly, to conveyors 
that utilize work supporting trolleys that are supported by wheels that 
run on inclined trolley tracks and are powered by gravity. 
In the conveyor field where workpieces are conveyed by trolleys running on 
downwardly inclined tracks, such arrangements are known as pallet 
conveyors. In conveyors of this type the speed of the trolley increases 
until its terminal velocity is obtained or until the trolley is stopped by 
some mechanical means, such as a barrier, or by abutting against the last 
trolley in a queue of trolleys at rest. This results in an impact that can 
be destructive to the trolley, the workpiece, or both. 
Various means have been heretofore proposed to control the velocity of such 
trolleys. One of the most commonly used devices is a retarding brake 
torsionally attached to a trolley wheel. In one form of retarding brake 
the rotating member of the retarder is confined in a cavity that contains 
a viscous fluid. The retarding brake can be so designed that the 
resistance caused by the viscous fluid between the stationary wall of the 
cavity and the adjacent surface of the rotating member results in a 
desired low velocity. The use of a retarder of this type is satisfactory 
only in those arrangements where the wheels of all of the trolleys are 
uniformly loaded at all times. However, in most conveyor arrangements of 
this type the trolley wheels are not uniformly loaded at all times. At one 
time the trolley is empty and the load on the trolley wheels is the weight 
of the trolley itself. At other times the trolley carries a workpiece and 
the load on the trolley is therefore the combined weight of the trolley 
and the weight of the workpiece. In order to achieve satisfactory low 
velocity of the trolley under both conditions of loading it is necessary 
to employ a mechanism capable of applying two different retarding forces 
to the trolley. 
In Brems U.S. Pat. No. 4,316,535 dated Feb. 23, 1982, the desired low 
velocity of the trolley under both conditions of loading is obtained by 
employing a trolley wheel having two different diameter portions on the 
same axis connected to a single retarder, the guiding trolley track being 
designed such that when the trolley is loaded the smaller diameter wheel 
engages the track and, when the trolley is empty, the track is engaged by 
the larger diameter wheel. This arrangement has proven to be very 
successful in many applications. 
One of the problems that arises in connection with such speed control 
trolleys is the desirability or the necessity of maintaining the work 
supporting pallet of the trolley substantially level at all times, whether 
loaded or unloaded. The problem of maintaining the pallet level and 
laterally stable can be solved to some extent by utilizing a track in the 
form of a rectangular tube. However, a rectangular tube track presents 
other problems which makes its use impractical. In the first place, 
rectangular tube tracks cannot be readily formed into the desired curved 
and spiral configurations required for the path of travel of most overhead 
gravity conveyors. Thus, the cost of a rectangular tubular conveyor track 
of this configuration would be prohibitive. In addition, where a trolley 
designed for use with a rectangular tube track is provided with 
longitudinally spaced sets of axially opposed wheels, substantially 
difficulty is encountered in attempting to cause the trolley to negotiate 
turns in the track which curve at a relatively short radius. 
A more practical and economical conveyor track for suspended trolleys 
consists of upright rails in the form of thin strips of steel having 
spring characteristics which can be hand formed into smooth curves and 
spirals. Trolley wheels rolling on the upper edges of such rails provide 
an ideal conveyor arrangement. However, the use of a trolley track in the 
form of a thin upright rail with retarders having dual diameter wheels 
presents another problem. 
Where the dual diameters of the wheels are on the same axis, the center of 
gravity of the empty pallet should be substantially directly below the 
center of the profile of the larger diameter wheel in order to maintain 
the pallet level. Likewise, the workpiece should be located on the pallet 
so that the center of gravity of its mass plus the mass of the trolley 
will be substantially directly below the profile of the smaller wheel. 
When these two conditions are met the empty pallet and the loaded pallet 
will both be suspended in a level condition during the travel down the 
incline of the conveyor track. In practice determining these centers of 
gravity with sufficient accuracy is very difficult. This problem is 
compounded by the fact that, when the trolley is designed, the workpiece 
itself to be conveyed is not available, but only dimensioned drawings 
thereof. 
The primary object of this invention is to provide a conveyor of the above 
described type which utilizes thin flexible rails for the trolley track 
and which is designed so that a determination of the centers of gravity of 
the loaded and unloaded pallets is not critical. 
A more specific object of the invention resides in the provision of a 
trolley having a pair of laterally spaced thin upright rails 
simultaneously engaged by wheels of the same diameter so that the loaded 
or unloaded trolley is supported by two laterally spaced tracks. With such 
an arrangement, as long as the center of gravity falls some where between 
the two laterally spaced tracks, the trolley will hang vertically and the 
pallet will remain level.

A trolley conveyor arrangement illustrated in FIGS. 1 through 3 is 
generally similar to one form shown and described in the aforesaid Brems 
U.S. Pat. No. 4,316,535. The trolley, generally designated 10, is 
supported on a thin rail track 12 by two sets of wheels. Each set of 
wheels comprises a smaller diameter wheel 14 and a large diameter wheel 
16, both fixed on an axle 18 journalled in a retarder housing 20. Track 12 
is inclined downwardly and is suspended from a suitable overhead structure 
by hangers 22. Each trolley also includes a pair of depending arms 24 
fixed at their upper ends to the retarder housings 20 and supporting a 
pallet 26 for a workpiece W at their lower ends. It will be noted that 
pallet 26 extends laterally outwardly beyond wheels 14,16 and that the 
workpiece W in the arrangement shown overhangs each side of the pallet. 
Within each retarder housing 20 there is arranged on the inner end of axle 
18 a disc 28 located within an enlarged circular cavity 30. Cavity 30 is 
closed at one end by a cover plate 32 and is sealed from the tubular 
portion of housing 20 in which axle 18 is journalled by a suitable seal 
34. Cavity 30 is filled with a viscous fluid which exhibits substantially 
Newtonian characteristics, such as silicone oil. It will be understood 
that in such a Newtonian fluid the shear force required to shear the fluid 
is directly proportional to the time rate of shear exerted on the fluid. 
Therefore, the retarding force exerted on axle 18 by disc 28 is directly 
proportional to the angular velocity of axle 18. The factor of 
proportionality is dependent upon the viscosity of the fluid, the diameter 
of disc 28 and the thickness of the fluid space on opposite sides of the 
disc within cavity 30. 
In FIGS. 1 and 3 trolley 10 is shown loaded with a workpiece W and 
suspended from track 12 by the small diameter wheels 14. When trolley 10 
is empty, it will be supported on a similar track 36 by the larger 
diameter wheels 16. Tracks 12 and 36 can be the same track, in which case 
means are provided for shifting the trolley laterally when loaded or 
unloaded. However, tracks 12 and 36 can be laterally offset extensions of 
each other. The use of different diameter wheels enables independent 
control of the speeds of the empty and the loaded pallets. 
The effect of the rolling diameter on the speed of descent along the 
downwardly inclined track may be understood by reference to FIG. 4, a 
schematic velocity and force vector diagram of a wheel supporting a weight 
W.sub.1 having a diameter D.sub.1, and rolling down a track inclined to 
the horizontal at a constant angle .alpha., and moving with an angular 
velocity .omega. and linear velocity V. It can be seen that the force, 
parallel to the track, causing the wheel to move forward is: 
EQU F.sub.1 =W.sub.1 sin .alpha. 
This in turn creates a forward torque T.sub.F on the wheel 
EQU T.sub.F =F.sub.1 (D.sub.1 /2)=(D.sub.1 /2) W.sub.1 sin .alpha. 
As the wheel rolls down the track and substantial equilibrium is reached, 
i.e., the wheel is neither accelerating nor decelerating, the forward 
torque T.sub.F and the retarding torque T.sub.R are equal (and opposite). 
The retarding torque may be expressed as: 
EQU T.sub.R =K.sub.1 .omega. 
##EQU1## 
where V is the equilibrium velocity, 
EQU T.sub.R =2K.sub.1 V/D.sub.1 
EQU Since T.sub.F =T.sub.R 
EQU (D.sub.1 /2)W.sub.1 sin .alpha.=2K.sub.1 V/D.sub.1 
Solving for V 
EQU V=(Sin .alpha./4K.sub.1)W.sub.1 D.sub.1.sup.2 
where W.sub.E and W.sub.F are the empty and full weights respectively and 
D.sub.E and D.sub.F are the rolling diameters for the empty and full 
conditions respectively. This condition can be expressed also as: 
EQU D.sub.E =D.sub.F (W.sub.F /W.sub.E) 
This equation indicates that the equilibrium forward rolling velocity is 
proportional to the weight and the square of the rolling diameter. In the 
specific case where it is desired to have the rolling velocity of an empty 
pallet equal the rolling velocity of a full pallet, it can be seen that: 
EQU W.sub.E D.sub.E.sup.2 =W.sub.F D.sub.F.sup.2 
In FIG. 2 where pallet 26 is shown empty and the trolley is supported on 
the large diameter wheel 16, pallet 26 is suspended from housing 20 so 
that its center of gravity lies in a vertical plane within the profile of 
wheel 16. When the pallet is so located relative to wheel 16, it will 
remain substantially level in a lateral sense as it travels down the 
track. Thus, we can assume that the center of gravity of the empty trolley 
is approximately at the location designated 38 in FIG. 2. 
In FIG. 3 where the pallet 26 carries the additional load of workpiece W 
and is suspended on the track by means of the small diameter wheel 14 an 
additional problem confronts the conveyor designer. In order to maintain 
pallet 26 horizontally level during its travel along the track, the 
workpiece locator 40 on pallet 26 must be precisely positioned so that the 
center of gravity of the total mass of the loaded trolley will be 
substantially directly below the outline of small diameter wheel 14. As 
mentioned previously, the difficulty of this determinatin is compounded 
because at the time of design all of the trolley components and even the 
workpiece are represented only by dimensioned drawings. Thus, the precise 
relative location of wheels 14,16 and pallet 26 is virtually impossible. 
Even a very close approximation is very difficult. 
In FIGS. 5 through 9, which show various embodiments of the present 
invention, a complete trolley is not illustrated since it differs from the 
trolley 10 illustrated in FIGS. 1 through 3 only in the construction and 
arrangement of the track engaging wheels. In FIG. 5 the wheel arrangement 
consists of two large diameter wheels 42 separated by a flange 44 and 
located between two small diameter wheels 46. Although four separate 
wheels may be used, they are shown formed as a single multiple diameter 
wheel fixedly mounted on the axle 18 of the retarder. The larger diameter 
wheels 42 are arranged for rolling engagement with tracks 48 when the 
trolley pallet is empty and the smaller diameter wheels are arranged for 
rolling engagement with the tracks 50 when the trolley pallet is loaded. 
It will be appreciated that, when the pallet is not loaded, the center of 
gravity of empty trolley need not fall between the two tracks 48 and, when 
the pallet is loaded, the center of gravity of the loaded trolley may fall 
anywhere between the two outer tracks. 
In accordance with this invention all of the tracks, such as illustrated at 
48,50, are in the form of thin upright rails, preferably made of a high 
carbon strip steel that has been heat treated so as to have the 
characteristics of a spring. This material is known in the steel industry 
as "blue tempered spring steel." It is relatively hard and difficult to 
machine with conventional cutters, but can be readily pierced and sheared 
with common sheet metal tools. Tracks formed on this material cannot only 
be hand formed into smooth curves and spirals, but is also highly 
desirable because of its unique wearing and scuff resistant qualities. 
These rails have the advantage of being shop-prepared with holes and slots 
and then coiled for shipment to the construction site for conveyor 
assembly and erection. The manner in which these rails can be assembled 
for gravity chuting is well known, extensively used, and shown in U.S. 
Pat. No. 2,815,841. 
In the wheel arrangement shown in FIG. 6 a wide large diameter wheel 52 
formed with flanges 54 at each side thereof is located between small 
diameter wheels 56. When the pallet is empty the trolley is supported by 
wheel 52 on the laterally spaced tracks 58 and when the pallet is loaded 
the trolley is supported by wheels 56 on the outer tracks 60. The wheel 
arrangement shown in FIG. 6 is essentially the same as that shown in FIG. 
5 except that the flanges 54 are located laterally outwardly of the two 
tracks 58 whereas the flange 44 in FIG. 5 is located between the two 
tracks 48. The arrangement shown in FIG. 6 is particularly useful where 
the trolleys are suspended in a storage area without rails. The laterally 
spaced large diameter flanges 54 are adapted to bear on flat surfaces with 
stability. 
In the arrangement shown in FIG. 7, when the trolley is loaded it is 
supported by the laterally spaced small diameter wheels 62 on the 
laterally spaced rails 64. The unloaded trolley, where its attitude and 
stability are not critical, is supported by the single large diameter 
wheel 66 on the single track 68. The arrangement shown in FIG. 8 is the 
reverse of that shown in FIG. 7 in that when the pallet is loaded it is 
supported by the small diameter wheel 70 on the single track 72 and when 
the pallet is empty it is supported by the large diameter wheels 74 on the 
laterally spaced tracks 76. The enlarged peripheral flanges 78 at the 
laterally outer sides of wheels 74 allow for stable storage of the 
trolleys as discussed above with respect to flanges 54 in FIG. 6. 
The arrangement shown in FIG. 9 is functionally the same as that shown in 
FIG. 7. However, one small diameter wheel 80 has a grooved periphery while 
the other small diameter wheel 82 has a straight cylindrical outer 
periphery. With this arrangement the spacing of the two outer tracks 84 is 
not critical. 
Although FIGS. 5 through 9 show both tracks simultaneously engaged by the 
two wheel sets, it will be appreciated that the two tracks are not 
co-extensive in length. The track rails shown in broken lines are actually 
located at a different section of the conveyor from the track rails shown 
in solid lines. For example, in the arrangement shown in FIG. 5 rails 48 
will form one section of the conveyor track traversed by the trolley when 
empty and rails 50 will form a different section of the conveyor track 
traversed by the trolley when loaded. 
It will be appreciated that the preferred form of trolley has two wheel 
assemblies of the type shown in FIGS. 5 through 9, located fore and aft of 
the trolley so as to prevent the trolley from swaying in the direction of 
travel. Likewise, when the trolley is traveling along the laterally spaced 
double tracks, it is stable and not subject to lateral sway. This is 
particularly advantageous when going around corners and curves. Thus, the 
trolley-track arrangement disclosed herein overcomes the practical 
problems encountered with respect to prior art arrangements. The necessity 
for determining the exact location of the center of gravity of loaded and 
empty trolleys is not critical, as is the case with the arrangement shown 
in FIGS. 1 and 3. At the same time, the problems involved with the use of 
a rectangular tube track are completely eliminated.