Positive engagement safety mechanism and lift belt construction for long stroke, well pumping unit

A self-energizing, positive engagement safety mechanism for long stroke, well pumping units, whether powered mechanically or hydraulically, and which employ a lift belt and counterweight. Upon failure of the sucker rod, polish rod or lift belt, a latching mechanism is actuated to engage a rack on both sides of the counterweight thereby to arrest and lock the counterweight against free fall. Actuation of the latching means is controlled by sensing belt tension below the counterweight thereby to enhance the sensitivity and reliability of the safety mechanism.

BACKGROUND OF THE INVENTION 
This invention relates generally to well pumping units and more 
particularly to an improved safety mechanism for shutting down operation 
of the pumping unit in the event of failure of one or more components on 
the lift side of the pumping unit, when under load. Such failures, 
although rare, have disastrous consequences both for personnel in the area 
and the equipment being used. 
The present invention has utility with a wide variety of well pumping 
units. One such well pumping unit includes a tower or mast mounted on a 
base platform, a source of power and a winding drum on the base platform, 
a lift belt made of conveyor belting from the winding drum up the tower 
and over a crown spool mounted atop the tower and then extended downwardly 
and connected to the polish rod of a well pump, and a reversing mechanism 
associated with the power means reciprocate the belt and thus the polish 
rod, and thereby to operate the pump. A counterweight or weight box is 
interposed in that portion of the drive belt between the spool and the 
winding drum so that power requirements of the pumping unit are kept to a 
minimum. An idler spool is provided at the base of the tower and that 
portion of the lift belt between the counterweight and the winding drum is 
trained beneath the idler spool so as to restrict movement of the 
counterweight to a generally vertical direction within the tower during 
operation of the pump. 
The need for a safety mechanism is particularly acute during a lifting 
stroke of the pumping unit. The polish rod load may well be in the area 
of, for example, 30,000 pounds and the counterweight will be weighted only 
somewhat less than the polish rod load. If that portion of the lift belt 
between the spool on the top platform and the polish rod should fail or if 
one or more of the polish rod, rod string and sucker rod components of the 
well pump should fail, the counterweight will fall to the base platform 
and the lift belt will unravel from the spool; the possible, disastrous 
consequences are self-evident. Accordingly, this invention provides a 
mechanism for immediately arresting and locking the counterweight in place 
in the event of a failure as just described. 
A brief description of the development of well pumping units is in order. 
In the early life of a well, reservoir pressure alone may be sufficient to 
raise the oil to the surface, providing local regulatory authorities 
permit such a procedure. However, such pressure is eventually exhausted 
and the oil must be pumped to the surface to be recovered. The most common 
variety of pump employed for this purpose is a walking beam pump having a 
nominal stroke distance of from about seven to twelve feet. A walking beam 
pump is suitable for shallow to medium depth wells, but such a pump 
becomes inefficient as stroke frequency increases. Specifically, rod 
stretch, dynamics and pump volumetric efficiency combine to decrease 
efficiency as stroke frequency increases. 
Thus, long stroke well pumping units, particularly useful in deep wells, 
have been developed, some of which have stroke lengths of thirty-two feet 
or more. One example of such a prior art long stroke pumping unit is the 
"Oilwell" Model 3534 Long Stroke Pumping Unit, manufactured by Oilwell, a 
division of United States Steel. The unit includes a central tower having 
multiple braces to stabilize the structure, a complex multi-strand cable 
crown block assembly suspending the rod string and a variable capacity 
counterweight and, of course, a prime mover. Several safety systems are 
provided, including an automatic air brake system controlled by an 
overspeed governor flyweight and actuated when the counterweight exceeds a 
predetermined, acceptable downward speed. Other safety features include 
interlocked controls and automatic breaking in the event of an air loss or 
power failure. Both the pumping unit and the safety features provided are 
complicated and quite expensive. 
Another example is prior U.S. Pat. No. 3,248,958 issued to Emil A. Bender, 
which discloses and claims a wire line deep well pumping apparatus and a 
safety brake system which includes a somewhat complex system for jamming a 
cam against the wire lines in sheaves mounted atop a tower in the event of 
rod string failure, thus preventing the counterweight from falling. 
Another prior U.S. Pat. No. 3,483,828 issued to Emil A. Bender, also 
discloses a deep well pumping unit, and describes generally a braking 
system which is actuated in the event of failure. A more recent example in 
a long stroke pumping unit is yet another invention of Emil A. Bender 
which is the subject of a co-pending application Ser. No. 393,102, filed 
June 28, 1982, and licensed to the Assignee of the present invention, 
Baker Pro-Lift, Inc. The fail safe mechanism described in that application 
is arranged in a lift belt system and includes a wedge and brake shoe 
combination mounted on the top of the tower which is actuated to grasp the 
lift belt in the event of fracture of the belt, polish rod, rod string or 
sucker rod on the polish rod side of the crown spool. 
However, the prior art does not disclose a safety mechanism for well 
pumping units of the type described herein which provides sensitivity and 
consistent reliability over the wide range of load fluctuations that may 
be experienced within even a single cycle of the pumping unit. The load on 
the polish rod side of a lift belt system varies in a representative 
installation from a maximum of approximately 29,700 pounds, when the 
polish rod is at its lowermost position and beginning an upstroke and thus 
experiencing the full fluid and dynamic load, to a minimum of 18,100 
pounds, when the polish rod is at its uppermost position and beginning to 
fall back down through the fluid. The counterweight or weight box in this 
example is loaded with 16,300 pounds or approximately 90% of the minimum 
load on the polish rod side, to reduce power requirements of the system to 
a minimum and yet provide sufficient weight differential to allow the 
polish rod and rod string to fall gently through fluid to its lowermost 
position again. The prior art safety mechanisms all sense off of the 
polish rod side of the system and therefore must withstand the maximum 
load to minimum load fluctuations which requires a heavy duty safety 
mechanism and an attendant sacrifice of sensitivity. Attempts to increase 
sensitivity in such prior art examples results in frequent malfunction 
wherein the brake or safety mechanism prematurely locks up and disrupts 
the operation of the pumping unit although no failure or fracture in the 
lift belt system has occurred. 
Reference to other arts where a similar problem is experienced leads 
inevitably to the elevator art and in particular to prior emergency brake 
systems developed therein to avert human tragedy in the event of cable 
separation. It was recognized in the very early stages of this art that a 
rack and pawl combination was the most dependable safety latch and the 
risk to human life demanded a heavy duty arrangement. Representative 
examples of such prior art safety devices include U.S. Pat. No. 931,211, 
issued to E. E. Moulton, wherein a cross bar at the top of an elevator 
cage is secured to the cable and counter-biased by a coiled spring against 
cable tension. Tension in the cable overcomes the counter-bias and the 
cross bar engages and pivots a pawl arranged at either side of the 
elevator cage out of engagement with corresponding racks mounted in 
alignment therewith in the elevator shaft. The pawls are spring-biased 
toward a position in engagement with the rack, however, so that upon cable 
failure and the attendant release of tension therein, the counter-bias of 
the coiled spring forces the cross bar out of engagement with the pawls 
and allows the pawls to spring into engagement with the corresponding 
racks to arrest free fall of the elevator cage. A variation of this 
concept may be seen in U.S. Pat. No. 1,482,331, issued to J. Vanslett, 
wherein the pawls are spring-biased out of engagement with the 
corresponding racks, and the counter-bias force (provided in this case by 
a pair of opposing leaf springs) drives a wedge arranged between the pawls 
to engage the pawls with the corresponding racks in the event of cable 
failure. In another such example, disclosed in U.S. Pat. No. 1,302,059, 
issued to J. A. Linn, the safety devise is mounted beneath the elevator 
cage and connected to the cable above the cage through an inverted 
U-shaped yoke arranged to pull a pair of pawls in scissors fashion out of 
engagement with corresponding rack when the cable is in tension, and 
counter-bias springs force the pawls again in scissors fashion into 
engagement with the racks upon cable failure. 
One obvious distinction in the foregoing and similar examples of prior 
safety mechanisms in the elevator art lies in the fact that the lift cable 
or cables are exclusively tensioned above the elevator cage; thus, each of 
the various safety mechanisms conceived to respond to cable failure must 
of necessity have an operative link to the cable above the cage for 
actuation. Another significant distinction between such examples and the 
lift belt, well pumping units previously described is that the maximum 
load and load fluctuations experienced in normal duty is significantly 
less. Therefore, the responsiveness and sensitivity are not compromised by 
the mass of the system or extreme load differentials and the attendant 
sizing difficulties for actuating mechanisms such as the springs 
described. The foregoing distinctions are best put in perspective by 
considering the complexity and near impossibility of sizing actuating 
mechanisms such as the springs described for heavy duty service wherein 
loads approaching 30,000 pounds and load fluctuation of 12,000 pounds, or 
more, are experienced under normal operating conditions, which actuating 
mechanisms must be sensitive to load failure and yet avoid premature 
actuation due to normal load fluctuations. 
Accordingly, the prior art does not disclose a safety mechanism for well 
pumping units and the like which provides sensitivity and consistent 
reliability over a wide range of severe loads and load fluctuations 
without premature actuation, and which is of uncomplicated structure and 
requires no power means in order to be operated. Additionally, the prior 
art does not disclose the safety mechanism herein disclosed and claimed in 
a lift belt system for transmitting reciprocating motion to the polish rod 
in a long stroke, well pumping unit. 
SUMMARY OF THE INVENTION 
Therefore, it is a principal object of this invention to provide a safety 
mechanism for a well pumping unit or the like which is completely 
mechanical in structure and operation and thus has no power requirements 
and which is operable to immediatley shut down operation of the pumping 
unit in the event of failure of one or more of the components of the 
pumping unit. 
It is another object of the invention to provide a safety mechanism for a 
well pumping unit which includes a base platform, tower and top platform 
upon which a spool is mounted, a lift belt being trained thereover and 
connected at its ends to the polish rod and winding drum of the pump, the 
lift belt being the connective component for imparting reciprocation to 
the polish rod and rod string of the pump, the lift belt being provided 
with a counterweight between the spool and winding drum and the safety 
mechanism being located beneath the counterweight and operable upon 
failure of one or more of the rod string components to arrest and lock the 
counterweight against falling. 
It is yet another object of the invention to provide a safety mechanism in 
a lift belt system for long stroke, well pumping units such as that 
described, wherein the safety mechanism is beneath the counterweight and 
senses off the lift belt below the counterweight and thus is subjected 
only to the load differential between counterweight and the polish rod 
load at any given point in a cycle of the pumping unit. 
It is still another object of the invention to provide a safety mechanism 
in such a long stroke, well pumping unit, which safety mechanism is 
sensitive to load failure and provides positive responsiveness, yet is not 
prematurely actuated by widely disparate load fluctuations experienced 
under normal operating conditions. 
Generally speaking, the long stroke, well pumping unit with which the 
invention may be used includes a base platform, a tower or mast on the 
platform, and a rotatable winding drum located on the platform with a 
mechanical or hydraulic drive to impart rotation to the winding drum. A 
flexible lift belt is attached at one end to the winding drum and at its 
other end to the upper end of the polish rod of a well pump. A freely 
rotatable spool is located atop the tower and the lift belt is trained 
over the rotatable spool. A counterweight or weight box is interposed in 
the lift belt between the winding drum and the spool. A reversing 
mechanism is associated with the hydraulic or mechanical drive for the 
winding drum to thereby provide reciprocating movement through the lift 
belt to the polish rod. The self-energizing, positive engagement safety 
mechanism of the invention, which terminates operation of the pumping unit 
in the event of failure by fracture of the lift belt, polish rod, rod 
string or sucker rod, includes a cross bar securely fastened to the upper 
end of the lift belt beneath the counterweight, a guide member dependent 
from the counterweight and arranged to receive the cross bar and permit 
only limited movement thereof longitudinally in the plane of the belt, 
mechanism arranged on the guide member for exerting a predetermined 
counter force on the cross bar in opposition to tension on the lift belt, 
a pair of racks mounted within the tower one on either side and in the 
plane of the belt, a latch mechanism located on the guide member adjacent 
to each of the racks in opposing relationships for movement into and out 
of engagement therewith, mechanism arranged on the guide member and 
responsive to movement of the cross bar longitudinally in the plane of the 
lift belt to move the latch means out of engagement with said racks when 
tension on the lift belt overcomes the predetermined counter force and for 
moving the latch means into engagement with the racks when tension on the 
lift belt is reduced below the level of the predetermined counter force. 
The predetermined counter force is selected so as to permit disengagement 
of the latch mechanism from the racks under belt tension corresponding to 
the normal operating loads of the pumping unit whereupon failure by 
fracture as described reduces tension below the level of the counter force 
on that portion of the lift belt beneath the counterweight thereby to 
allow the cross bar counter force exerting mechanism to displace the cross 
bar longitudinally in the plane of the lift belt and cause the latch 
engagement mechanism to move the latch means into engagement with the 
racks, thereby to arrest and lock the counterweight against free fall. 
Further novel features and other objects of this invention will become 
apparent from the following detailed description, discussion and the 
appended claims taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings by reference character, and in particular to 
FIG. 1 thereof, an improved, long stroke well pumping unit is illustrated. 
A skid mounted base platform 10 supports a tower structure or mast 12 and 
a top platform 14 surmounts the mast 12. The mast 12 is composed of two 
parallel I-beams 16--16, pivotally mounted to the base platform 10 and 
structurally stabilized intermediate their lengths by a series of cross 
members and struts 18--18, the beams 16--16 being further stabilized 
vertically with respect to the base platform 10 by two parallel mast 
supports 20--20. A rotatable winding drum 22 is located on base platform 
10 and is driven from a suitable power source 24 which may be mechanically 
or hydraulically driven and is also located on base platform 10. A 
reversing mechanism (not shown) is also provided in association with the 
power source for periodically reversing rotation of the winding drum in a 
manner described in greater detail hereinbelow. An otherwise conventional 
well pump (not shown) includes a rod string and sucker rod therein, topped 
by a conventional polish rod 26. A flexible lift belt 28 is secured at one 
end to rotatable winding drum 22 and at the other end to a yoke assembly 
30 from which polish rod 26 is centrally suspended. Flexible lift belt 28 
is reaved beneath an idler pulley 32 on base platform 10, then upwardly 
through mast 12 to and over a crown spool 34, freely rotatably mounted 
atop the top platform 14 and then vertically downwardly to yoke assembly 
30. A counterweight or weight box 36 is interposed in lift belt 28 and 
reciprocates generally vertically, with movement of lift belt, between the 
upper and lower ends of the mast 12. During operation of the pumping unit, 
the reversing mechanism (not shown) allows belt 28 to be wound upon and 
unwound from winding drum 22 thus to impart reciprocating movement to 
polish rod 26 and the well pump. 
As mentioned above, commercially available conveyor belting may be employed 
as the material for lift belt 28. One available brand of conveyor that 
might be used is that sold under the trademark "Unilok" as "PolyVinylok" 
conveyor belting. One particular material found to be useful is Unilok's 
PVK-350 material, a belting that is 10/32 inches thick, 15 inches wide and 
has an ultimate tensile strength at rupture of 3500 pounds per inch. 
Similar belting materials sold under the Unilok mark are available up to 
15/32 inches thick and having an ultimate tensile strength at rupture of 
up to 9000 pounds per inch. Belt widths may vary from fifteen inches to 
twenty-four or more inches. The particular belting material chosen will 
depend on the requirements of the particular well pumping unit. 
One particular embodiment of the well pumping unit under discussion is 
dimensioned to provide a twenty-five foot stroke in polish rod 26. 
Currently, a unit with a twenty-five foot stroke is most economically 
practical because commonly available, off-the-shelf components may be 
interfaced with the unit. Specifically, a standard long stroke pump is 
thirty feet long and has a plunger five feet in length. Standard polish 
rods and standard sucker rods making up the rod string of the pump are 
made in lengths which match the size demands of a twenty-five foot stroke 
pump unit. A comparison of the production figures of a standard walking 
beam unit with the long stroke pumping unit of this invention yields the 
following interesting results. In pumping a well about two mile deep, a 
standard walking beam unit with a ten-foot stroke and operating at eight 
strokes per minute will produce a net lift per minute of forty feet, when 
a rod stretch of five feet on the lift stroke is taken into account. On 
the other hand, use of a pumping unit as above disclosed with a 
twenty-five foot stroke and operating only at four strokes per minute 
yields a net lift per minute of eighty feet, again taking the five feet of 
rod stretch on the lift stroke into account. Thus, the present unit 
produces twice as much effective lift per minute than a standard walking 
beam unit. Equally importantly, the long, half speed stroke reduces the 
number of cycles required per minute, and extends rod life by reducing the 
number of stress cycles and extends tubing life by distributing wear over 
a greater area. 
The safety mechanism of the present invention is located beneath the weight 
box 36 and is generally indicated by reference numeral 38. Referring now 
to FIGS. 3 and 5A and 5B, the components of the safety mechanism 38 
include a cross bar 40, a guide member 42 dependent from weight box 36, 
and a rack and pawl arrangement indicated generally by the numeral 44 at 
either side of the weight box. The cross bar 40 is secured by conventional 
fasteners 46--46 to the upper end of lift belt 28 beneath weight box 36, 
and is received within guide member 42 between parallel plates 48--48 
separated at their upper edges by a spacer plate 50 and provide with stops 
52--52 to permit only limited movement of the cross bar longitudinally in 
the plane of the belt. The cross bar 40 is provided with camming surfaces 
54--54 at either end and is biased by compression springs 56--56 secured 
at their lower ends in receptacles 58--58 in spacer plate 50 and connected 
at their upper ends to the cross bar as by spindles 60--60 passing axially 
back through the springs and secured to the cross bar. Springs 56--56 
exert a predetermined counter force in opposition to tension on lift belt 
28 and are sized so that belt tension under normal load conditions 
compresses springs 56--56 and forces the cross bar 40 against stops 
52--52. A pair of pawls 62--62 are pivotally suspended between plates 
48--48 on either side of cross bar 40 and are provided with lugs 64--64 
arranged adjacent the camming surfaces 54--54 of the cross bar 40. A pair 
of compression springs 66--66 secured in receptacles 68--68 to resist 
compression, are arranged to bias pawls 62--62 through plungers 69--69 
normally into engagement with a pair of racks 70--70 mounted in opposing 
relationship therewith. Each rack 70 is mounted on the interior web of one 
of the I-beams 16--16 in alignment with a corresponding pawl 62 and spans 
the entire length of travel of weight box 36 during a full cycle of the 
pumping unit. 
As may be viewed in FIG. 5A, the safety mechanism 38 is responsive to 
tension on the lower portion of lift belt 38 which under normal operating 
loads overcome the predetermined counter force and compresses the springs 
56--56 thereby forcing cross bar 40 against stops 52--52. Thus camming 
surfaces 54--54 are driven against lugs 64--64 to pivot pawls 62--62 out 
of engagement with the corresponding racks 70--70 and the weight box 36 is 
free to travel in the mast 12. Failure of the system by fracture of the 
lift belt, polish rod, rod string or sucker rod, reduces tension on the 
lift belt below the level of the predetermined counter force and allows 
springs 56--56 to expand, as shown in FIG. 5B, thereby raising cross bar 
40 and causing camming surfaces 54--54 to recede and allow springs 66--66 
to force plungers 69--69 to drive pawls into 62--62 into locking 
engagement with racks 70--70 and thus arrest and latch the weight box 
against free fall. 
In a preferred embodiment, the weight box 36 is guided in its travel within 
the mast 12 to facilitate alignment between the pawls 62--62 and 
corresponding racks 70--70. As may be seen in FIGS. 2, 3 and 4, two side 
wheels 72--72 are rotatably mounted one on each side of weight box 36 in 
diagonally offset relationship, as for example, one side wheel 72 at the 
top front edge and the other side wheel 72 at the bottom rear edge on one 
side of the weight box and one side wheel 72 at the top rear edge and the 
other side wheel at the bottom front edge of the other side. The side 
wheels 72--72 engage and are guided by interior surface 74 of web 76 of 
the corresponding I-beam 16. In addition, front rollers 78--78 are 
rotatably mounted at each of the four corners on the front of weight box 
36 to engage and ride on the interior surfaces 80--80 of front flanges 
82--82 of the I-beams 16--16, and rear rollers 84--84 are rotatably 
mounted at each of the four corners at the rear of the weight box and 
arranged to ride on the interior surfaces 86--86 of rear flanges 88--88 of 
the I-beams. As may be seen in FIG. 1, the mast 12 is normally tilted 
forward at an angle of approximately 96 degrees under ordinary operating 
conditions, with the result that front rollers 78--78 carry the weight box 
load and ride front flanges 82--82, while rear rollers 84--84 are thereby 
held out of contact with rear flanges 88--88. However, if the mast 12 is 
tilted back to an angle of less than 90 degrees, to permit workover of the 
well for example, the weight box load is shifted to the rear rollers 
84--84 and which ride rear flanges 88--88. Also in this preferred 
embodiment, a tilt mechanism is provided, indicated generally by the 
numeral 90 in FIG. 1, for adjusting the attitude of the mast 12 as 
aforesaid. Such a mechanism may include a drive motor 92, screw jack 94 
and carriage 96 which cooperate to tilt the mast forward and back through 
mast supports 20--20. This tilt mechanism, though not essential to the 
present invention, is described in detail and claimed in a co-pending 
application Ser. No. 489,821 filed by the present inventors simultaneously 
herewith and assigned Baker Pro-Lift, Co. 
As may now be more fully appreciated, the safety mechanism of the present 
invention senses off the lift belt below the weight box or counterweight, 
and thus sees only the tension resulting from the differential in load 
between the polish rod side of the mast and the weight box. During an 
upstroke of the pumping unit, the load on the polish rod side includes the 
belt, polish rod, rod string, sucker rod, the fluid being lifted and the 
dynamic load of stroke reversal, which may reach a maximum of 29,700 
pounds in the example previously given. This load reduces to 18,300 in the 
downstroke as the sucker rod drops back down through the fluid. The 
enormous stress of the maximum load and the resulting requirement of heavy 
duty components in a safety mechanism, coupled with the wide fluctuations 
in loads between upstroke and downstroke, makes it virtually impossible to 
design a safety device with proper sensitivity which does not lock up 
prematurely due to load fluctuations alone when a condition of failure 
does not in fact exist. By sensing off the belt below the weight box, the 
safety mechanism sees only the differential in load between the polish rod 
side and the weight box, which in our prior example called for a weight 
box loading of 17,000 pounds, yields a maximum load seen by the safety 
mechanism of 12,700 pounds. Thus, a safety mechanism is provided with the 
necessary positive response to arrest and latch the counterweight against 
free fall without sacrificing critical sensitivity. 
The invention may be embodied in other specific forms without departing 
from the spirit and other essential characteristics thereof. The present 
embodiment is therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than the foregoing description and all changes 
which come within the meaning and range of equivalency of the claims are 
therefore intended to be embraced therein.