US3648863A

US3648863A - Dipper pitch control for shovels

Info

Publication number: US3648863A
Application number: US5807A
Authority: US
Inventors: George B Baron; George J Thompson
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-01-26
Filing date: 1970-01-26
Publication date: 1972-03-14
Anticipated expiration: 1989-03-14

Abstract

A pitch control mechanism for power shovels having a body with a stiffleg pivotally connected at its bottom to the body and a dipper handle pivotally connected to the top of the stiffleg. The dipper is pivotally connected to the dipper handle and has limited arcuate movement relative to the handle. A hoist lifts the dipper to its upward limit, if unrestrained, and raises the dipper and dipper handle. A pantograph linkage with a control to lock the linkage, controllably move it, or release it for free movement, extends from the bottom of the stiffleg to its connection with the dipper handle, and from that connection to the dipper. When the linkage is locked, the dipper angle relative to the ground remains fixed during digging and hoisting movement. When the linkage is positively moved, the dipper angle relative to the dipper handle is changed as desired. When the linkage is released, the dipper moves to its upper angle limit under pull of the hoist and the dipper operates as a fixed angle dipper, the linkage moving automatically as required by stiffleg and dipper handle movement.

Description

United States Patent Baron et al.

Mar. 14, 1972 [54] DIPPER PITCH CONTROL FOR SHOVELS [72] Inventors: George B. Baron, 4358 Powell Road; George J. Thompson, 1000 Marseilles Drive, both of Marion, Ohio 43302 [22] Filed: Jan. 26, 1970 [21] Appl. No.: 5,807

Primary Examiner-Gerald M. Forlenza Assistant Examiner-Jerold M. Forsberg Attorney-Mason, Fenwick & Lawrence [57] ABSTRACT A pitch control mechanism for power shovels having a body with a stiffleg pivotally connected at its bottom to the body and a dipper handle pivotally connected to the top of the stiffleg. The dipper is pivotally connected to the dipper handle and has limited arcuate movement relative to the handle. A hoist lifts the dipper to its upward limit, if unrestrained, and raises the dipper and dipper handle. A pantograph linkage with a control to lock the linkage, controllably move it, or release it for free movement, extends from the bottom of the stiffleg to its connection with the dipper handle, and from that connection to the dipper. When the linkage is locked, the dipper angle relative to the ground remains fixed during digging and hoisting movement. When the linkage is positively moved, the dipper angle relative to the dipper handle is changed as desired. When the linkage is released, the dipper moves to its upper angle limit under pull of the hoist and the dipper operates as a fixed angle dipper, the linkage moving automatically as required by stifileg and dipper handle movement.

6 Claims, 9 Drawing Figures PATENTEDMAR 14 I972 SHEET 1 [IF 5 PATENTEHMAR 14 I972 v 3.648 863 sum 5 0r 5 ATTORNEYS DIPPER PITCH CONTROL FOR SIIOVELS This invention relates to revolvable shovels having improved bucket pitch control mechanisms, of the general type shown copending application Ser. No. 709,304, entitled Power Shovel, George B. Baron, inventor, filed Feb. 29, 1968 now U.S. Pat. No. 3,501,034, granted March 17, 1970. This application discloses the pitch control mechanism of the previous application, a modification of that mechanism, and an adaptation of that mechanism to a fixed boom shovel.

BACKGROUND OF THE INVENTION In the above identified patent application, a shovel was disclosed which had its dipper so mounted that it was capable of a long, horizontal stroke so as to be effective in digging in low banks, where the material to be excavated may be only a foot or two in depth. This condition is often found in strip mining, where thin seams of coal are encountered, and also in quarrying, or other open-pit loading operations. The bucket, or dipper, is pivotally connected to a dipper handle, which is pivoted at its top to the upper end of a stiffleg that is pivoted at its bottom to the machine body. Movement of the dipper handle relative to the stiffleg was accomplished by means of a hoist frame and hoist link, connected to the pivotal connection of the dipper handle and stiffleg and to the bucket. This motion is controlled by a hoist line. Movement of the stifileg about its bottom pivot is controlled by a crowd mechanism. By proper operation of the hoist line and crowd mechanism, an exceptionally long digging stroke can be accomplished. The digging angle of the bucket could be maintained throughout the stroke by special pitch control linkage.

Pitch controls for dippers of conventional shovels have been known, and their use has been to increase the horizontal digging stroke. These, however, had two problems, the transmission of power out to the handle just behind the dipper, and the requirement that the operator devote much of his time and attention to precise pitch control.

Another area in which difficulty has been encountered has been with the use of fixed boomshovels, such as the conventional knee-action machine of the type shown in U.S. Pat. No. 2,139,254 to K. R. Bixby, issued Dec. 6, 1938, where it is desirable to uncover two seams of coal in the same pit, one being separated from the other by from 20 to 40 feet of overburden. The knee-action machine can have a fixed pitch dipper with the pitch great enough to make a long cleanup, but then the machine is clumsy at short radius because the teeth are raking rather than digging. However, with the dipper fixed at a normal conventional angle, the machine is handicapped because its dipper pitches up above horizontal too soon when it digs horizontally above grade. Therefore, it cannot clean the overburden from the upper seam except at a very short digging radius. To recover the coal efficiently, the machine must uncover it at an operating radius at least as great as the cleanup radius at grade level.

It has been the experience of shovel designers and operators that pitch control systems applied to fixed boom shovels has been impractical because the machine which digs horizontally under, or beyond the point of the boom is generally clumsy and inefficient. Also, the angle between the dipper handle and the hoist lines varies considerably, making it difficult to arrive at a geometry which will neither cause the dipper to roll over forward nor develop unmanageable loads in the linkage.

SUMMARY OF THE INVENTION The general object of the present invention is to provide a revolvable power shovel with improved bucket pitch control mechanism which will permit long horizontal digging movements.

A more specific object is to provide a shovel of this kind having pitch control mechanism which incorporates a pantograph system to maintain bucket angle during digging operations.

Another object is the provision of a shovel having pitch control mechanism which may be locked to hold the bucket against movement, or operated to provide constant pitch variation.

A further object is to provide such a mechanism which can be applied to fixed boom shovels to permit eflicient operation of the shovels at grade level and levels above grade.

Yet another object is to provide pitch control mechanism for the bucket of a fixed boom shovel which will permit the shovel to be operated at grade level as a fixed pitch bucket and function as a variable pitch bucket at levels above grade.

Other objects of the invention will appear from the following description of one practical embodiment thereof when taken in conjunction with the drawings which accompany, and form part of, this specification.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation of a power shovel having dipper pitch control means embodying the principle of the present invention, the dipper being shown in full lines in fully retracted position ready to start a digging stroke, and in dotted lines in several of its positions during digging, hoisting and dumping;

FIG. 2 is a top plan view of the machine shown in FIG. 1, but with the bucket in extended position;

FIG. 3 is a diagrammatic view showing portions of the pitch control system and its control cylinders, and the fluid system which operates the cylinders;

FIG. 4 is a greatly enlarged view of the structure in the region of the shovel head shaft, and is taken substantially on the line 4-4 of FIG. 2;

FIG. 5 is a horizontal section through the structure shown in FIG. 4, and is taken on line 5--5 of FIG. 4;

FIG. 6 is a partial side elevation of a shovel with a slightly modified pitch control mechanism;

FIG. 7 is a fragmentary horizontal section taken on the line 77 of FIG. 6;

FIG. 8 is a section taken on the line 88 of FIG. 6; and

FIG. 9 is a somewhat diagrammatic side elevation of a fixed boom shovel of the knee-action type with the improved dipper pitch control installed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings in detail, FIGS. 1 through 5 are duplicates of drawings which appear in the above identified copending application. Only so much of the shovel structure as is necessary to an understanding of the pitch control mechanism and its operation will be described in detail. The previous application can be referred to for details of the shovel.

The shovel body 1 is supported for rotation upon a base 2, mounted on crawlers 3. A bucket, or dipper, 4 is pivotally connected at 5 to a handle 6, in turn pivotally connected to a stiffleg 7 by a head shaft 8. The stiffleg is pivotally connected to the machine body by foot pins 9. A hoist frame 10 is also connected to the head shaft 8, and a hoist link 11 ties the hoist frame to dipper 4 by

pivots

12 and 13. The hoist frame, hoist link, handle and dipper move as a unit about the head shaft, and there is no relative movement among these parts except when the dipper moves about its pivotal connection 5 with the handle in changing pitch. A hoist line 14 controls movement of the hoist frame about the head shaft, and a crowd mechanism 15 controls movement of the hoist frame and stiffleg about its foot pins.

As mentioned above, dipper 4 is pivotally attached to the end of handle 6 and to the hoist link 11. This enables the dipper to change its angular relationship to the handle and to the bank into which it is digging. There are times when it is desirable to hold the angular relationship of the bucket with respect to the earth, so as to bring about a long, horizontal pass of the dipper. The dipper angular relationship with the earth is either maintained, or changed, under the control of a pantograph linkage 16. The linkage is duplicated on the opposite sides of the dipper and the front end mechanism, so only one linkage will be described.

The linkage 16 consists of a control lever 17, which is in the form of a bent lever pivotally connected at one end to a pivot pin 18 axially aligned with the foot pins 9. The opposite end of the lever is pivotally connected to a pitch link 19 which has its opposite end pivotally connected to one end of an equalizing lever 20 pivotally mounted midway between its ends on the head shaft 8. Actually, link 19 is pivotally connected at 21 to a chain link 22 which, in turn, is connected to the free end of a lever 17 by a pivot 23. Link 19 has its opposite end pivotally connected, as at 24, to a chain link 25 having its opposite end pivotally connected to equalizing lever 20 at 26. Control lever 17 carries a stop 27 for abutment by chain link 22, and the equalizing lever 20 carries a stop 28 for abutment by chain link 25. This arrangement is such that one chain link or the other is in contact with its abutment, and when the linkage moves, one chain link will contact its stop as the other chain link moves from its stop. The opposite end of the lever 20 is connected to a second pitch link 29, which has its opposite end coupled to the dipper 4. Thus, a parallelogram is formed including head shaft 8, pivot between the handle and dipper, pivot 30 between the dipper and pitch link 29, and pivot 31 between pitch link 29 and the equalizing lever 20. The equalizing lever 20, pitch link 19 and control lever 17, and their mountings, form two parallelograms, one when chain link 22 is in contact with stop 27, and the other when chain link 25 is in contact with stop 28. In the first instance, the parallelogram includes pivot 26, head shaft 8, pivot 18 and pivot 21. When the chain link 25 is in contact with stop 28, the parallelogram includes pivot 24, head shaft 9, pivot 18, and pivot 23. Thus, if the control lever 17 is held immovable, the attitude of the bucket relative to the earth will remain unchanged for all movement of the stiffleg and handle. In order to hold the lever 17 against movement, a pitch cylinder 32 is mounted upon a support 33 at the front of the machine body 1, and has its piston rod connected to the control lever 17 intermediate the ends of the control lever.

The movements of the dipper are controlled within certain fixed limits. The dipper carries an abutment 34 for contact with a limit stop 35 which projects from the stiffleg 7 near its bottom, to establish a limit of movement of the dipper in a rearward direction for starting a digging path. The dipper is so weighted with respect to the position of its pivot 5 with the handle that it will assume an upwardly tilted position if not otherwise controlled. The limit of upward pitch is determined by a pitch stop 36, mounted on the handle 6, which cooperates with a pad 37 fixed to the bucket ears 38.

The dipper pitch is controlled by an operator by means of a fluid system, such as that shown in FIG. 3. This includes lines 39 from the tops of pitch cylinders 32 to a fluid tank 40. Check valves 41 in the lines 39, adjacent the tank 40, to prevent flow from the tank to the lines. A control valve 42 bridges the lines 39, and is operated by movement of a pitch pedal 43. Normally, the valve occupies an open position, so that fluid can flow from the cylinders to the tank. When the operator depresses the pitch pedal, the valve closes, holding the fluid within the cylinders and locking the control levers 17 against movement. A relief valve 44 is interposed between line 39 and the tank to prevent excessive are in pressure. All oil flow from the tank to the cylinders is through a strainer 45 in the tank, and line 46, which line includes a check valve 47 to prevent back flow from the line 39 into the tank. A small amount of air pressure in the tank, assisted by the weight of the rams and other parts, returns them to the extended position when the dipper is returned to the starting position, and keeps slack out of the pitch linkage. Pitch pedal 43 has an adjustable stop 48 so that the stroke of the valve can be limited to prevent it from reaching a fully open position when the pedal is released, thus creating a restriction which prevents the dipper pitching up too fast and slamming against the stops 36 on the end of the handle.

In operating the device, the dipper will be brought to starting position near the minimum rise, with the dipper teeth on grade level and the pitch up against stop 36 on handle 6. This is the position shown in full lines on FIG. 1. To begin the normal horizontal digging cycle, the operator will start to take up the hoist and pay out crowd. At this time, the dipper pitch control is released, and the dipper is free to move about its pivotal connection with the handle. As the dipper moves forward away from stop 35, it begins to assume a horizontal position. When the dipper front is nearly horizontal, the operator depresses the pitch pedal 43, locking the fluid in the cylinders 32 and holding the dipper against change in angular movement relative to grade level. As the operator continues to pick up hoist and pay out crowd, the bucket advances in a horizontal direction at a given depth in the material, until the operator decides that the dipper is full. The extent of the horizontal forward movement can be seen in dotted line positions in FIG. I. When the dipper is full, the operator releases the pitch and reverses the crowd simultaneously, but continues to hoist. The dipper then pries itself out of the material and rises almost vertically, due to the fact that the operator is taking up both hoist and crowd so that the stiffleg will be rising while the dipper handle lifts. As soon as the bucket is free of the material, it will swing about its pivot on the handle until the pad 37 on the dipper strikes the pitch stop 36 on the handle. The bucket remains with the pitch up against this stop for the rest of the cycle. The operator continues the lifting of the bucket and swings the machine until the dumping position is reached, whereupon the dipper door 39 will be opened to drop the load. After dumping, the operator moves the bucket back to its starting position.

The above mentioned prior application can be referred to for the details of the shovel operation as a whole. It is to be understood that the cycle is to be carried in as smooth a manner as possible and the pitch control will be released gradually so that the heel of the dipper does not slam down against the grade level, nor does the dipper hit the pitch stops with any appreciable impact.

It is to be noted that due to the use of a pantographic arrangement in the dipper pitch control mechanism, the dipper will maintain its angle with respect to grade level when the control mechanism is locked. This will hold the dipper at grade level during any portion required of the digging stroke of the machine. Of course, at any time, the control can be manually operated to change the dipper attitude as required.

Referring now to that form of the invention shown in FIGS. 6, 7 and 8, the shovel is of the same type as shown in FIGS. 1 to 5, and the machine parts have been given the same reference characters. Only the pitch control mechanism has been modified somewhat and needs detailed description.

The pantograph linkage 50 is composed of two substantially parallelogram configurations as before. In this case, also, the pantograph linkage is duplicated on both sides of the stiffleg and dipper handle, and only that on one side of the mechanism need be described.

The pantograph linkage includes a central equalizing lever 51 mounted on the head shaft 8. In this case, the equalizing lever is a double member, having rearwardly extending, spaced plates 52 and forwardly extending, partial sheaves 53, concentric to the head shaft 8. A pitch link 54 extends between the equalizing lever and the bucket, and has its ends pivoted at 55 to the bucket, and at 56 between the rearward ends of the spaced plates 52 of the equalizing lever. This connection establishes a parallelogram which has its pivots at 5, 8, 56 and 55. Cables forming pitch links 57 are dead ended at 58 at the edge of the partial sheaves 53 of the equalizing lever. The opposite ends of the links extend around sheaves 59 mounted on the foot pins 9, and over the top of a bridge secured to the upper end of the piston rod of a pitch cylinder 61. By using a single cable on each side of the cylinder as the pitch link 57, the ends of the cable can be dead ended to the equalizing lever and an equalized pull can be obtained on both flights of the cable. Vertical movement of the pitch cylinder rod will exert a pull on the pitch link and rock the equalizing lever 51. By using sheaves 59 and partial sheaves 53 on the equalizing lever, an equal power moment is maintained throughout all rocking positions of the equalizer. At the same time, it is possible to mount the pitch cylinder in a vertical position which enables a more rigid mounting and dispenses with the use of flexible hoses to carry the fluid to and from the cylinder. The parallelogram formed between the equalizer and sheave 59 will be from head shaft 8 to a point on a radius of the partial sheave 53 which is at right angles to the flight of the pitch link 57 and at the point of intersection with that flight, the foot pin 9, and the intersection of a similar radius of sheave 59 with the flight of the pitch link. In this form, this parallelogram remains a fixed one.

It will be noted that the rearward ends of the spaced plates 52 of the equalizing lever has an added opening 56' for alternate connection of the pitch 54. While connection of the pitch link to the opening 56' causes a departure from the exact parallelogram described above, the resulting configuration is generally that of a parallelogram. With this modified parallelogram arrangement, the dipper actually pitches down a couple of degrees during its horizontal pass, with the control held fixed, and obtains the practical advantage of a little better bite and compensation for deflections due to increasing loads on the linkage.

Dipper 4 has a pad 62 for engagement with a stop 63 on the dipper handle 6, as in the previous form. The bucket also has a pad 64 for engagement with a stop 65 on the stiffleg 7, as in the previous form.

Although the second disclosed form of the pitch control mechanism has certain structural and operational advantages over the first described form, the operation of the control is in the same manner as that described for the first form. In this form, vertical movement of the pitch cylinder 61, through the pitch links 57, will rotate the equalizing lever and determine the angular position of the dipper. As before, the dipper can be caused to maintain a fixed angle relative to grade level by holding the pitch cylinders against movement.

Turning now to that form of the invention shown in FIG. 9, the pitch control linkage is substantially the same as that in the form just described, but it is illustrated in connection with a fixed boom shovel of the knee-action type.

The shovel has a body 66 rotatably mounted on a base 67, supported on crawlers 68. The body carries a gantry, or A- frame, 69. A boom 70 is attached to the body by foot pins 71, and is normally held at a fixed angle by means of pendants 72 extending from the gantry 69 to a point adjacent the boom tip. The body also pivotally supports a stifi'leg 73 on foot pins 74, and the stifileg is pivotally connected at its top, by means of a head shaft 75, to a dipper handle 76 and a crowd handle 77. The crowd handle has its opposite end conventionally mounted in the top of the gantry, and has a suitable crowd mechanism by which it can be reciprocated to move the stiffleg about its foot pins. The dipper handle carries a dipper 78, and the dipper and dipper handle are movable as a unit about the head shaft 75 by means of a hoist line 79 which passes over suitable sheaves in the tip of the boom. The dipper is pivotally mounted to the handle at 80.

The pitch control mechanism for this form, as in the other forms, is duplicated on opposite sides of the stiffleg and dipper handle. Only one side need be described.

The pitch control mechanism is a pantograph linkage 81, which includes as its central member an equalizing lever 82 pivotally mounted on the head shaft 75, Pitch link 83 connects the equalizing lever and dipper 78, and has a pivotal connection 84 to a lever end 85 of the equalizing lever, and a pivotal connection 86 to the dipper. A second pitch link 87 is in the form of a cable, hearing at its center on the top of a pitch cylinder 88 fixed to the shovel body 66, and standing vertically with respect to the body. The cable flights extend vertically downward and around sheaves 89 mounted on the foot pins 74 of the stiffleg and are dead ended at 90 on partial sheaves 91 forming part of the equalizing lever 82, as in the previous form. Thus, vertical movement of the pitch cylinder will rotate the equalizing lever and move the bucket. In this form, as in both previous forms, the dipper, dipper arm, equalizing lever,

stiffleg, and the pitch links provide parallelograms to give the pantograph action to maintain the bucket at a fixed angle relative to the grade level when the pitch cylinders are maintained in fixed position. A stop 92 is provided on the dipper and a companion stop 93 is mounted on the dipper handle. When the pitch control mechanism is released, the pull of the hoist line will tilt the dipper about its pivot to the handle and bring the

stops

92 and 93 into engagement. The relative positions of

stops

92 and 93 are such that when the dipper swings about its pivot to bring the stops into engagement, the dipper angle with respect to the dipper handle is approximately that conventionally used on a fixed dipper stripping shovel.

The knee-action shovel of FIG. 9, equipped with the dipper pitch control mechanism, is very effective in digging at two levels. It can dig at grade level in the manner of a conventional stripping shovel, and it can strip overburden from a second coal vein located some distance above grade level. Normally, the pitch angle when stripping overburden from a vein considerably above grade level would be such that the dipper angle will move above horizontal very shortly after starting its movement.

With the pitch control mechanism of the present invention, the angle of the dipper can be set to that of the plane of the second vein level and maintained at that angle relative to the vein through an appreciable forward movement of the dipper before starting the hoisting movement. At the start of the digging operation at the upper level, the stilfleg and dipper handle will be drawn well back, so that the digging stroke can extend over an appreciable distance without moving beyond the end of the fixed boom to an impractical digging position. (See upper level dotted dipper positions in FIG. 9)

This form of the pitch control mechanism can be operated through a fluid control system substantially the same as that shown in FIG. 3 of the drawings, where the pitch cylinders are connected by equalizing lines to a fluid supply tank. A manual valve controls flow of fluid to and from the cylinders and the tank.

In operation of the shovel of FIG. 9, the manual control valve of the hydraulic system will be opened when the shovel is to operate at grade level. Therefore, the pull of the hoist line will cause the dipper to swing about its pivot to bring the

stops

92 and 93 into engagement. The continued movement of the dipper handle under these conditions will cause the handle and dipper to maintain their angular position relative to one another, and the dipper to move and act in the same way as a fixed pitch dipper. During the outward movement of the dipper, the control linkage will be moved reversely, that is by maintaining a fixed condition of the dipper, the pitch link 83 and the equalizing lever 82, the pitch link 87 will be drawn up and the pistons of the pitch cylinders will be moved down. The fluid in the cylinders will flow back to the tank. The length of the stroke of the cylinders will be designed to have the pistons reach full downward position as the dipper reaches its fully extended upward position. The operator can control the pitch of the dipper during the hoist operation if he so desires, and he can pitch the dipper forward when it reaches the dump position to cause a gentle deposit of the spoil. Thus, in the normal use of the shovel at grade level, the machine will function as a fixed pitch dipper shovel.

When it is desired to clear overburden from a vein some distance above the grade level, the angle of the bucket will be adjusted by operation of the control linkage to assume a position parallel to the top of the upper vein. The valve of the control system will then be closed, locking the pistons of the pitch cylinders against movement, and the dipper will then be moved through its digging cycle. During forward movement of the stifileg and dipper handle, the dipper will maintain its prefixed attitude due to the pantograph linkage. When the dipper is full, the operator can move the control valve to adjust the dipper pitch, or to allow the dipper to swing about its pivot until the

stops

92 and 93 are abutting.

It was mentioned above that it has been difficult to arrive at a geometry for pitch controls on fixed boom shovels which will neither cause the dipper to roll over forward nor develop unmanageable linkage loads. This has been solved by the present invention by positioning stops 92 and 93 so that the pull of the hoist cable swings the bucket up until the stops engage, in which position the dipper is at the conventional angle relative to the handle of a fixed angle dipper, and operates as such during digging at grade, to eliminate strains on the linkage during that part of the cycle when strains would otherwise be impractically great on pitch control linkage. With the present mechanism, these strains do not exist because the dipper stop is against the handle, even out to the maximum cleanup radius. Even though the machine must dig horizontally beyond the boom point, this occurs during a relatively small percentage of the digging cycles. Even if the machine is a bit slow when it does dig in this manner, the overall efficiency is still good.

It will be observed that in all forms of the invention which have been disclosed, there is a stiffleg having its bottom end pivotally attached to the body of the shovel and its upper end pivotally attached to the upper end of a dipper handle. The dipper is pivotally connected to the dipper handle, and there are stops carried by the dipper and handle to limit the rotative movement of the bucket in an upward direction. In each embodiment, the bucket has a hoist with means connected to the bucket so that the hoisting pull will draw the bucket up against the stops if the bucket is free to move. There is also a pantograph linkage connected to the bucket and having its center movable member attached to the head shaft connecting the stiffleg and dipper handle. The second portion of the pantograph linkage has means for moving the linkage about its several pivots, locking the linkage against movement, and releasing the linkage to permit the dipper to maintain a fixed relation with the dipper handle. This permits operation of the shovel with the dipper serving as a fixed pitch dipper, as an automatically controlled dipper to maintain a fixed attitude relative to grade level, and as a variable pitch dipper manually controlled by the operator.

While several embodiments of the invention has been disclosed, it will be apparent that structure as set out in the preceding paragraph, and operating in the manner stated in that paragraph, can take other forms than those which have been disclosed.

What is claimed is:

1. Pitch control mechanism for dippers of power shovels having a body, a stiffleg pivoted to the body by means of a foot pin, a dipper handle pivoted to the top of the stiffleg by means of a head shaft, and a dipper pivotally mounted on the dipper handle to allow the dipper to swing upwardly of its own weight against a limit stop on the dipper handle, comprising, an equalizer pivotally mounted on the head shaft, a pitch link connected to the equalizer at one side of the head shaft center and to the dipper, a sheave pivotally mounted on the foot pin, a cable connected to the equalizer on the opposite side of the head shaft center from the connection of the pitch link and having its opposite end passing around the sheave, a fluid cylinder connected at one end to the power shovel and having its other end connected to said opposite end of the cable, and means to operate the fluid cylinder, lock it against movement and release it for free movement.

2. Pitch control mechanism for dippers of power shovels as claimed in claim 1 wherein, the equalizer has an arm extending to one side of the head shaft center to which the pitch link is connected, and the equalizer on the opposite side of the head shaft center is a partial sheave about which the cable is wrapped and to which the cable is connected.

3. Pitch control mechanism for dippers of power shovels as claimed in claim 2 wherein, the sheave and the partial sheave are of equal radius.

4. Pitch control mechanism for dippers of power shovels as claimed in claim 2 wherein, the fluid cylinder has a cylinder end fixed to the power shovel body and a rod end extending in a direction generally tangential to the sheave.

5. Pitch control mechanism for dippers for power shovels as claimed in claim 4 wherein, the cable is doubled and has its bight seated upon the fluid cylinder rod end, a second sheave is upon the foot pin about which one flight of the cable passes, a second partial sheave is on the equalizer concentric to the first, and the ends of the doubled cable are anchored to the respective partial sheaves at the partial sheave ends remote from the fluid cylinders.

6. Pitch control mechanism for dippers of power shovels as claimed in claim 5 wherein, the sheaves and the partial sheaves are of equal radius.