US3167131A - Controllable pitch propeller having low pitch stop - Google Patents

Description

Jan. 26, 1965 w. B. VOISARD 3,167,131

CONTROLLABLE PITCH PROPELLER HAVING LOW PITCH STOP Filed Sept. 5, 1963 2 Sheets-Sheet l FIG-l H 20 21 12 l H12, 43 23 l t as A i 2 g fi I I!!! W1! I? 34 1 as '7 26 (6 I7 2924 R: k F 40 L so FIG-2 50 7a sz 1N VEN TOR WALTER B. VOISARD ATTORNEYS Jan. 26, 1965 w. B. VOISARD 3,167,131

CONTROLLABLE PITCH PROPELLER HAVING LOW PITCH STOP Filed Sept. 3, 1963 2 Sheets-Sheet 2 FIG-3 30 zz INVENTOR.

WALTER B. VOISARID WM mfrz ATTORNEYS United States Patent s 167 131 CUNTRQLLABLE PI'iTQii rnorstrsn HAvrNo Low rrrcn sror Walter Bryan Voisard, Dayton, @hio, assignor to lihe This invention relates to controllable pitch propellers for aircraft.

The invention has particular relation to a propeller control system of the type wherein a hydraulic motor rcsponsive to engine speed is arranged to urge the propeller blades in the direction to reduce their pitch against the biasing force of a control spring and counterweight urg ing the blades towards a maximum pitch position, which maybe full feathered position or whatever other maximum pitch position is desired.

In a propeller control system of this type, whenever the hydraulic pressure fails or is otherwise insufficient to counter-balance the force of the control spring and counterweight, the blades will automatically move to their maximum pitch position unless some provision is made to the contrary. This is a desirable result for a feathering propeller in the event of engine failure in flight, since it will automatically feather the propeller upon such failure of the hydraulic control pressure. it is less desirable, however, if the same action occurs when the engine is shut off with the plane on the ground, since this means that the next time the engine is subsequently started, it must be done against the load created by having the propeller blades in their maximum pitch position in which they offer maximum resistance to turning over the engine.

It is a primary object of the present invention to provide a novel, simple and effective stop mechanism for incorporation with a propeller control system of the type outlined above which is so constructed and arranged that it will offer no interference to operation of the control system when the plane is in flight, but when the engine is shut off While the plane is on the ground, it will hold the blades in a desired low pitch position facilitating restarting of the engine.

It is particularly an object of the invention to provide a stop mechanism as outlined above which functions in response to the pressure conditions within the main hydraulic control cylinder for the propeller independently of the actual engine speed so long as the engine is operating.

A more specific object of the invention is to provide a stop mechanism as outlined above wherein a latching member is mounted in the rotary hub structure for movement from and to a latching position wherein it will engage a fixed stop to block movement of the piston in the direction to increase blade pitch beyond a predetermined low pitch, and wherein movement of this latching lTlH1- her to its latching position is possible only when the engine is shut off.

It is also an object of the invention to provide a stop mechanism as outlined above which is of such structure and operating characteristics that under all normal operating conditions of the engine, it is out of latching position and offers no resistance to feathering of the propeller.

Additional objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.

In the drawings FIG. 1 is a partially diagrammatic view generally in axial section through an airplane hub structure constructed in accordance with the invention and showing the parts in normal operating condition;

FIG. 2 is an enlarged fragmentary view in section on the line 2-2 of FIG. 1; i

PEG. 3 is a view showing the piston assembly and associated parts in the positions they occupy when the engine -is stopped, the view being taken as indicated by the line 3 -3 of FIG. 4;

FIG. 4 is a fragmentary section on the line 44 of FIG. 3;

PEG. 5 is an end View of the supplemental piston looking in the opposite direction from FIG. 4; and

FIG. 6 is a fragmentary view similar to FIG. 3 but showing the parts in the relative positions which they occupy when the propeller is in minimum pitch position.

Referring to the drawings, which illustrate a preferred I embodiment of the inventiomFlG. 1 shows at 10 the hub of an airplane propeller which is secured at 11 to the forward end of the engine shaft 12. The hub 10 carries the mountings 13 for the roots of the propeller blades indicated diagrammatically at 15, and the mountings 13 are rotatable on the hub lb to change the pitch of the blades. PEG. 1 also illustrates diagrammatically at 16 a counterweight such as should be attached as at 17 to each blade mounting in such manner as to overcome the centrifugal twisting movement of the blade.

The system for controlling the pitch of the blades 15 includes a hydraulic motor comprising a main cylinder 20 which is mounted on the frontend of the hub 10 as by bolts 21 to form a portion of the rotary hub structure. The complementary main piston 22 in cylinder 2%] is secured on the forward end of ahollow piston rod 23, as by a snap ring 24 and cooperating shoulder 25,. and the piston rod 23 includes a tubular extension 26 which projects into the engine shaft 32. The piston rod 23 includes ears 27 which are connected by links 28 with the blade mountings 13 in such manner that when the piston 2.2 is at its limit position of movement outwardly of the cylinder 2%, to the right in FIG. 1, the blades 15 are at their minimum pitch, and when the piston 22 is at its limit position of movement inwardly of the cylinder, the blades are in full feathered position.

The low pitch stop which limits movement of the piston 22 outwardly of the cylinder 2t is formed by an annular disk 3b which is bolted between the cylinder 29 and hub iii. The disk 3d includes a flange 31 projecting radially inwardly beyond the inner wall of the cylinder for engagement by the outer skirt portion of the piston 22, and this flange has notches at opposite locations for receiving the links 28. The high pitch stop which limits movement of the piston 22 inwardly of the cylinder 20 is shown as a block 33 mounted on the interior or forward end Wall of the cylinder 29 as by a screw and nut 34. Stop 33 is preferably proportioned for engagement by the piston 22 when the piston is moved sufficiently to shift the blades llSinto feathered position. v

T he piston 22 is continually biased in the direction to increase the blade pitch by one or more control springs 35 mounted in surrounding relation with the piston rod 7 23 and partially housed in a guide tube 36 located at the rearward end of the hub It) through which the piston rod 23 extends for reciprocation with the piston, and the action of the springs 35 is supplemented by the counterweight to. The force opposing the springs 35 and counterweight l-G to urge the piston 22- in the direction to decrease pitch is provided by hydraulic fluid supplied through the hollow crank shaft 12 and the piston rod 23 to the interior of the cylinder 2d from the usual pump 4:? and reservoir il through the line 42 and transfer bearing 43.

A governor 4d and governor valve 45, which may be of conventional construction, control the pressure supply to the cylinder 20 in accordance with engine speed to change the blade pitch as required to maintain the engine peed constant. Thus so long as the engine speed is below the control speed for which the governor is set, the pres sure fluid will be supplied to cylinder 20 to maintain the piston 22 in the minimum blade pitch position. Whenever the engine speed rises above the control speed, the

governor 44 will shift the valve H to the position relieving the pressure supply to the cylinder 2% and thereby allowing the piston 22 to move inwardly of the cylinder and thus to increase the blade pitch until the resulting increased load has slowed down the engine to the control speed range.

If the engine should fail in flight, or if the hydraulic fluid supply should fail for any other reason such as a broken oil line, the resulting rapid loss of pressure in cylinder 25) will allow the piston 22 to move all the way to its maximum or feather pitch position. Similarly feathering can be initiated by the pilot by shifting the governor valve 45 through the manual control 46 2 7 to its feather position wherein it connects the cylinder 2h directly with the reservoir 41 and thus relieves the entire hydraulic force opposing the control springs 35 and counterweight 16. However, the same result wiil occur when the engine is shut off with the plane on the ground and the hydraulic fluid leaks back to the reservoir. This would mean that the blades would be in maximum pitch position when the engine is to be started again, which would offer the maximum resistance to rotation of the engine shaft, and it is this result which the present invention prevents by the provisions of :a latch mechanism assuring that the blades remain in a relatively low pitch position after the engine is shut off.

The latch mechanism of the invention comprises two latch members 56 which are pivoted within the piston 22 on pins 51 and project axially beyond the skirt portion of the piston for relative latching engagement of hook portions 52 thereof with the low pitch stop flange 31. Movement of the latch members St} between latching id release positions with respect to the flange 31 is effected and controlled by differential pressure means responsive to the changes in pressure within the cylinder Ztl when the engine is operating and 'when it is shut ofl, as now described.

Referring particularly to FIGS. 25, the interior of the piston 22 is constructed to form a supplemental cylinder 55 of annular shape which is connected through an orifice 56 with the interior of the cylinder 2a. A supplemental piston 60 of complementary annular shape is movable within the cylinder 55 through a range limited at the inner end of the cylinder by the common end wall portion of the piston 22, and at the outer end of the cylinder 55 by the ear portions 27 of the piston rod 23. The piston so is biased to its inner position by a plurality of springs of. (four being shown) each of which is mounted on a guide rod 62 secured to the piston 22 and extending axially therefrom into the interior of the hub structure lit), and the inner ends of the rods 62 are interconnected by an annular guide plate 63.

The supplemental piston 6i) includes a circumferential flange portion 64 which is engaged by the ends of the springs 61 and is provided with holes 65 through which the rods 62 extend. The skirt 64 is relieved at opposite sides to provide clearance for the latch members 50, and the edge 66 of each of these relieved portions is beveled to form a cam arranged to engage a complementary shoulder 67 on the adjacent latch member so to urge the latter radially outwardly into its position of latching engagement with the flange 31. In addition, each latch member 50 is formed with a cam shoulder '79 which is inclined radially outwardly and leads to an axially extended slot 71 in the latch member, and the piston 66? includes a pair of axially extending cam fingers or cars 72 arranged for cooperation with each cam shoulder 7t} to urge the latch members 50 radially inwardly in response to movement of the piston 60 outwardly of the cylinder 55.

Movement of the piston 64 outwardly of the cylinder 55 is eilected by the hydraulic pressure fluid which enters the cylinder 55 through the orifice 56 in opposition to the biasing pressure of the springs 61. In accordance with the invention, these springs 61 are calibrated to have a pressure equivalent which is slightly in excess of the pressure equivalent of the control springs 35, for example 10 psi. Thus for convenience in explaining the operation of the system, the hydraulic pressure equivalent of the s rings will be designated X, and the pressure equivalent of the springs 61 will be considered as (X+10) p.s.1.

In normal operation of an airplane having propellers constructed in accordance with the invention, the pressure tending to move the blades toward high pitch is a combination of the forces of the springs 35 and the counterweights l6, and the opposing hydraulic pressure in the cylinder 24 will vary over a range of approximately (X+85) to (X-l-lZO) p.s.i. FIG. 2 shows the relative positions of the parts of the latch mechanism under such normal conditions. Since the pressure in the cylinder 20 substantially exceeds the pressure equivalent of the springs 61, the supplemental piston 60 will be moved to its maximum outward position in the cylinder 55, and the cars '72 will accordingly hold the latch members St) in their retracted or released positions. It will therefore be p os sible for the piston 22 to operate through its full range of movement, including engagement with the low pitch stop 30, without any engagement of the latch members with their keeper flange 31.

The stop mechanism of the invention operates to hold the propeller at a low pitch position close to minimum pitch when the engine is shut off while the plane is standing on the ground. Under these conditions, the control handle 47 is retained in the position causing the hydraulic fluid to be continuously supplied to the cylinder 24), and the governor 44 will also be in the position supplying maximum pressure to the cylinder 2t) so long as the engine is turning over and driving the pump 40. The piston 22 will accordingly be held in engagement with the low pitch stop 39 as shown in FIG. 6 until the engine is completely dead, and the pressure in the cylinder 26} will remain effective for an appreciable interval until it is finally lost as a result of gradual leakage of the oil back to the reservoir through the transfer bearing 43.

While the pressure is dropping in the cylinder 20 after the engine is shut off, the parts of the latch mechanism will remain in the relative positions shown in FIG. 6 until the pressure in supplemental cylinder drops below (X+10) p.s.i. At this point, the piston will begin to move under the pressure of the springs 61 until it reaches its innermost position shown in FIG. 3, in which it has completed camming of the latch members 50 into their radially outward or latching positions. During this interval, the pressure fluid in cylinder 55 will be forced back through the orifice into the main cylinder 20 under the pressure of springs 61, and since this pressure is higher than that of springs 35, the pressure within cylinder 20 will remain in excess of X p.s.i. until the piston 60 has bottomed in its cylinder 55 to the extent allowed by engagement of the cam surfaces 66 and 67.

The latch members 50 are so proportioned with respect to their associated parts that they have a small clearance with respect to the flange 31 when the piston is in minimum pitch position. The smaller this clearance, the lower the pitch at which the blades are held while the engine is shut off, and satisfactory results have been obtained with this clearance of the order of .050 inch, which will provide for holding the blades at only 1.5 above minimum pitch. After the latch members 50 have been moved into latching position, the pressure in the cylinders 20 continues to drop until it falls below X p.s.i. At this point, the piston 22 begins to move inwardly of the cylinder Ztl under the force of the springs 35, but it is able to move only the small amount provided by the clearance between the hook portions 52 and the flanges 31. In the resulting latched position, as

shown in FIG. 3, the latch members 50 are held against accidental release by the complementary undercutting indicated at 75 on both the hook portions 52 and the cooperating surface of the flange 31.

It will be apparent that the parts can be proportioned in accordance with the above principles to provide different operating conditions as desired from the standpoint of the pressures, times and limit positions of the blades. As one operating example, satisfactory results have been obtained in a propeller system wherein the X value for the main control springs was approximately psi' and the pressure equivalent of the plurality of springs 61 was 55 psi. At idling speed of approximately 600 rpm, the piston 22 remained on the low pitch stop with the pressure in the cylinder 2% being approximately 260 psi. When the engine was turned off, the propeller slowed to a dead stop in approximately 6 seconds, and the pressure in the cylinder 25 dropped over the same interval to approximately 75 p.s.i.

The piston 22 remained on the low pitch stop 3%) while the pressure gradually decreased over an interval of the order of 12 seconds until it was below the value of 55 psi. of the springs 61. At this point, the supplemental piston tit began to move in the direction to shift the latch members St into their outer or latching positions, an operation which consumed approximately 6 seconds. The pressure in the cylinder Zil, however, was still more than p.s.i. and did not decrease to 45 psi. for several additional seconds, thus assuring that the latch members 50 were in position to latch with the fiange 31 when the pressure had finally dropped sufficient-ly to allow movement of the piston 22 to begin.

When the engine is re-started, the blades remain latched at the intermediate but relatively low pitch position established by the latch members 5% and flange 3i until the governor pump it has developed sufficient pressure to raise the pressure in the cylinder 2% to X p.s.i., and the control 47 is retained in its minimum pitch position. The piston 22 will then move against the springs 35 until it engages the stop 3%, thereby freeing the latches 50 from the flange 31. As the pressure continues to increase until it is in excess of (X-l-lO) p.s.i., the supplemental piston 60 will similarly move against the spring 61 until it has completed the retracting movement of the latch members 50. Thereafter the latchmembers remain released throughout normal operation of the aircraft in the moved positions of the piston 22..

If a condition should develop during flight which requires feathering of the propeller, the feathering operation is initiated by moving control 47 to its feathering pisition in which it shifts the valve 45 to the position connecting the interior of the cylinder 2t directly with the reservoir 41 in order to effect complete relief of the pressure supply in the cylinder. The springs 35' will therefore be free to move the piston 22 inwardly of cylinder 20, and even if the piston should be bottomed on the low pitch stop when feathering is initiated, the piston will have moved sufficiently for the latches 5h to be beyond the stop flange 31 before the pressure in the supplemental cylinder has dropped sufiiciently for appreciable movement of the piston 6h to take place. The piston 22 will therefore be free to move to its full feathered position established by the high pitch stop 33, and although the latch members 50 will ultimately move outwardly to their latching positions, this movement will be of no eifect.

Unfeathering of the propeller in flight may be accomplished by moving the control handle 47 out of its feathering position and then turning the engine over with the usual starter. As soon as sufiicient pressure is built up in this way in the cylinder 29 to move the piston 22 away from stop 33, the propeller will begin to windmill, and the resulting rapid pressure build-up will move the piston 22 back toward low pitch position.

The positions of the latch members 50 during this-op"- eration are immaterial, but in general they will have been moved to their release positions before the piston 22 can reach the low pitch stop 30. If this were not the case, however, they would be cammed into release position by engagement of the beveled shoulders 77 thereon with the inner edge of the flange31.

The invention accordingly provides a stop mechanism oitering a variety of important practical advantages. It is simple to" construct and install, it requires a minimum of maintenance, and all of the working parts are enclosed in a space facilitating proper lubrication. In particular, its operation is independent of actual engine speed, and the invention could be employed in an air craft having a hydraulic system which includes a pump driven independently of the engine. advantage of the invention that the working parts are readily proportioned to establishthe desired latched position of the propeller at any desired pitch, and speciiically to establish such position at substantially minimum pitch. All of these advantages are provided. without affecting the normal operation of the propeller control system with which it is used.

While the form of apparatus herein. described constitutes a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise form of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in. the appended claims.

What is claimed is:

1. In a control system for the blades of an adjustable pitch propeller having a drive engine and a rotary hub structure including adjustable upporting means for said blades, a hydraulic motor within said hub structure including a cylinder and piston, means connecting said piston with said supporting means to increase and decrease the pitch of said blades in response to movement of said piston inwardly and outwardly of said cylinder respectively, means defining the minimum pitch limit position for said outward movement of said piston, control spring means arranged to bias said piston inwardly of said cylinder, means for supplying hydraulic pressure fluid to said cylinder to move said piston against said control spring means, and selectively operable valve means for relieving said pressure fluid supply, the combination of a stop mechanism for selectively blocking increase in the pitch of said blades beyond a predetermined low pitch, comprising latching means supported on said hub structure for movement relative thereto between a release position offering no resistance to the movement of said piston and a latching position blocking said piston from movement inwardly of said cylinder beyond an intermediate position corresponding to said predetermined low blade pitch, and diiferential pressure means responsive to the pressure within said cylinder and calibrated to effect movement of said latching means to said release position when the pressure within said cylinder is above a predetermined value which is greater by a predetermined amount than the pressure equivalent of said control spring means to effect movement of said latching means to said latching position when the pressure within said cylinder is between said predetermined value and said pressure equivalent of said control spring means.

2. In a control system for the blades of an adjustable pitch propeller having a drive engine and. a rotary hub structure including adjustable supporting means for said blades, a hydraulic motor within said hub structure including a cylinder and piston, means connecting said piston and said supporting means to increase and decrease the pitch of said blades in response to movement of said piston inwardly and outwardly of said cylinder respectively, means defining the minimum pitch limit position for said outward movement of said piston, control spring means arranged to bias saidpiston inwardly of said cylinder, means for supplying hydraulic pressure It is: also a. special i fluid to said cylinder to move said piston against said control spring means, and selectively operable valve means for relieving said pressure fluid supply, the combination of a stop mechanism, for selectively blocking increase in the pitch of said blades beyond a predetermined low pitch, comprising latching means supported on said hub structure for movement relative thereto be tween a release position offering no resistance to the movement of said piston and a latching position blocking said piston from movement inwardly of said cylinder beyond an intermediate position corresponding to said predetermined low blade pitch, means for biasing said latching means to said latching position, actuating means responsive to the hydraulic pressure within said cylinder for moving said latching means to said release position in opposition to said biasing means, and said biasing means having a pressure equivalent which is greater than the pressure equivalent of said control spring means to effect latching of said latching means while said piston is in said minimum pitch position after tie drive engine is shut off.

3. The combination defined in claim 1 comprising a supplemental cylinder connected to receive pressure fluid from the first named said cylinder, a supplemental piston movable in said supplemental cylinder, means responsive to movement of said supplemental piston inwardly and outwardly of said supplemmental cylinder for effecting movement of said latching mean to said latching and release positions respectively, and mean biasing said supplemental piston inwardly of said supplemental cylinder, said biasing means having a pressure equivalent which is greater than the pressure equivalent of said control SPIlIlg means.

4. The combination defined in claim 3 wherein said latching means includes a latching member mounted on the first named said piston, and wherein said supplemental cylinder is formed within said first named piston.

5. In a control system for the blades of an adjustable pitch propeller having a drive engine and a rotary hub structure including adjustable supporting means for said blades, a hydraulic motor within said hub structure including a main cylinder and piston, means connecting said piston with said supporting means to increase and decrease the pitch of said blades in response to movement of said piston inwardly and outwardly of said piston respectively, control spring means arranged to bias said piston inwardly of said cylinder, means for supplying hydraulic pressure fluid to said cylinder to move said piston against said control spring means, and selectively operable valve means for relieving said pressure fluid supply, the combination of a stop mechanism for selectively blocking increase in the pitch of said blades beyond a predetermined low pitch, comprising a stop defining the minimum pitch limit position for said outward movement of said main piston, a keeper fixed with respect to said stop, a latching member movable on said piston into latching position in engagement with said keeper wherein said piston is in engagement with said stop to block movement of said piston inwardly of said cylinder, means defining a supplemental cylinder in said main piston connected to receive pressure fluid from said main cylinder, a supplemental piston in said supplemental cylinder, means responsive to movement of said supplemental piston inwardly and outwardly respectively of said supplemental cylinder for effecting movement of said latching member into said latching position and into a release position with respect to said keeper, means biasing said supplemental piston inwardly of said supplemental cylinder, and said biasing means having a pressure equivalent which is greater than the pressure equivalent of said control spring means but les than the normal operating pressure within said main cylinder to effect latching of said latching member while said piston is in engagement with said stop after the drive engine is shut off.

6. The combination defined in claim 5 comprising means forming an orifice connection between aid supplernental cylinder and said main cylinder for metering the flow of pressure fluid from said supplemental cylinder after the drive engine is shut olT to maintain the pressure within said main cylinder in excess of the pressure equivalent of said control spring mean until said latch ing member is in said latching position.

References Cited in the file of this patent UNITED STATES PATENTS

Claims (1)

1. IN A CONTROL SYSTEM FOR THE BLADES OF AN ADJUSTABLE PITCH PROPELLER HAVING A DRIVE ENGINE AND A ROTARY HUB STRUCTURE INCLUDING ADJUSTABLE SUPPORTING MEANS FOR SAID BLADES, A HYDRAULIC MOTOR WITHIN SAID HUB STRUCTURE INCLUDING A CYLINDER AND PISTON, MEANS CONNECTING SAID PISTON WITH SAID SUPPORTING MEANS TO INCREASE AND DECREASE THE PITCH OF SAID BLADES IN RESPONSE TO MOVEMENT OF SAID PISTON INWARDLY AND OUTWARDLY OF SAID CYLINDER RESPECTIVELY, MEANS DEFINING THE MINIMUM PITCH LIMIT POSITION FOR SAID OUTWARD MOVEMENT OF SAID PISTON, CONTROL SPRING MEANS ARRANGED TO BIAS SAID PISTON INWARDLY OF SAID CYLINDER, MEANS FOR SUPPLYING HYDRAULIC PRESSURE FLUID TO SAID CYLINDER TO MOVE SAID PISTON AGAINST SAID CONTROL SPRING MEANS, AND SELECTIVELY OPERABLE VALVE MEANS FOR RELIEVING SAID PRESSURE FLUID SUPPLY, THE COMBINATION OF A STOP MECHANISM FOR SELECTIVELY BLOCKING INCREASE IN THE PITCH OF SAID BALDES BEYOND A PREDETERMINED LOW PITCH, COMPRISING LATCHING MEANS SUPPORTED ON SAID HUB STRUCTURE FOR MOVEMENT RELATIVE THERETO BETWEEN A RELEASE POSITION OFFERING NO RESISTANCE TO THE MOVEMENT OF SAID PISTON AND A LATCHING POSITION BLOCKING SAID PISTON FROM MOVEMENT INWARDLY OF SAID CYLINDER BEYOND AN INTERMEDIATE POSITION CORRESPONDING TO SAID PREDETERMINED LOW BLADE PITCH, AND DIFFERENTIAL PRESSURE MEANS RESPONSIVE TO THE PRESSURE WITHIN SAID CYLINDER AND CALIBRATED TO EFFECT MOVEMENT OF SAID LATCHING MEANS TO SAID RELEASE POSITION WHEN THE PRESSURE WITHIN SAID CYLINDER IS ABOVE A PREDETERMINED VALUE WHICH IS GREATER BY A PREDETERMINED AMOUNT THAN THE PRESSURE EQUIVALENT OF SAID CONTROL SPRING MEANS TO EFFECT MOVEMENT OF SAID LATCHING MEANS TO SAID LATCHING POSITION WHEN THE PRESSURE WITHIN SAID CYLINDER IS BETWEEN SAID PREDETERMINED VALUE AND SAID PRESSURE EQUIVALENT OF SAID CONTROL SPRING MEANS.

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