US3001511A - Indexing mechanism for machine parts - Google Patents

Description

Sept. 26, 1961 w. F. POYNER INDEXING MECHANISM FOR MACHINE PARTS 5 Sheets-Sheet 1 Filed June 13, 1960 Sept. 26, 1961 w. F. POYNER INDEXING MECHANISM FOR MACHINE PARTS 5 Sheets-Sheet 2 Filed June 15, 1960 Sept. 26, 1961 w. F. POYNER 3,001,511

INDEXING MECHANISM FOR MACHINE PARTS Filed June 15, 1960 5 Sheets-Sheet s Sept. 26, 1961 w. F. POYNER INDEXING MECHANISM FOR MACHINE PARTS 5 Sheets-Sheet 4 Filed June 13, 1960 Zia Sept. 26, 1961 w. F. POYNER INDEXING MECHANISM FOR MACHINE PARTS 5 Sheets-Sheet 5 Filed June 13, 1960 3,001,511 $IIIIEXING MECHANISM FOR MACHINE PARTS 1 Brain Francis Poyner, Blackburn, England, assignor to ester, Yates & Thorn Limited, Blackburn, England CE Filed June 13, 1960, Ser. N 0. 35,717

aims priority, application Great Britain June 12, 1959 17 Claims. (Cl. Ill-38) I This invention relates to indexing mechanisms for machme P s enabling such parts to be displaced angularly g firiltiltleergisellznd loctited in a series of positions having the invenfigm tr or one: rclatlonship. It 15 the ob ect of above purpose (15531? c an mu laroved mechamsm for the heavy machine arts 51 harly suitable for indexing accelerated and retarded i hr ugil ii sr nzi l t ii s l eiz2131:; without undesirable shock. p

fiuii filcgordinng to the invention, indexing is efiected by a perated cylinder and piston arrangement including piston areas for actlon in opposite directions, crank means operable thereby over 180 through either dead centre position and having a drive connection with the machine part to be indexed, and means for controlling the supply of pressure fluid to either or both of said piston areas. In a preferred construction the cylinder and piston arrangement includes a double acting piston for operating a single crank. According to a further feature of the invention there is provided a directional valve arrangement actuated in accordance with an indexing movement to open a first piston area to a controlled connection from a pressure fluid supply while opening the second piston area to a controlled connection to an exhaust during the first part of an indexing movement, subsequently to open both said piston areas to both said connections simultaneously, and subsequently during the latter part of the movement to open said first piston area to the controlled connection to exhaust and to open the second piston area to the controlled connection to the pressure fluid supply.

According to a further feature of the invention, the means for controlling the supply of pressure fluid comprises a main control valve operable to connect the pressure fluid supply with a duct leading to one piston area while connecting a duct from the other piston area to exhaust.

In a preferred construction according to the invention the cylinder arrangement includes an oscillating cylinder having its axis of oscillation near the cylinder end remote from the crank, the cylinder being carried by trunnions, one of which is formed as a directional valve controlling the fluid flow to and from the respective piston areas.

According to a further feature of the invention, means is provided for dissipating the energy of the rotary part or parts as they are brought to rest and for preventing excessive pressure in the cylinder or cylinders. In one construction this is achieved by relief valves interconnecting the respective cylinder spaces and arranged for operation in opposite directions In another and preferred construction for this purpose, piston areas are afforded by piston members displaceable longitudinally relative to the piston rod away from abutment means thereon, and each engageable with fixed stop means when the mechanism reaches an indexed position, the arrangement being such that further movement of the piston rod tends to disengage the other piston member from its fixed stop, and if such disengagement takes place the latter piston member is subject to unbalancedfluid pressures to return the piston rod to indexed position. The piston members are preferably formed as sleeves slidable on a cylindrical member on the piston rod. Preferably the space between said piston members communicates with an atmospheric relief Ila-1W1 Indexing is preferably effected when the crank is at 90 from each of its dead centre positions.

Operation of the main control valve is effected in accordance with movement of the rotary part or parts by limit switches or equivalent means.

Other features of the invention are defined in the claims.

In the accompanying drawings,

FIGURE 1 is a side elevation, in section, of an indexing mechanism for machine parts constructed and operating in accordance with one form of the invention;

FIGURE 2 is a half plan view of FIGURE 1;

FIGURE 3 is a half end elevation of FIGURE 1 in the direction of the arrow X;

FIGURE 4 is a cross section of a rotary trunnion valve;

FIGURE 5 is a diagram showing the arrangement and connections of a choke valve and main control valve;

FIGURE 6 is a side elevation, in section of another form of indexing mechanism for rotary machine parts;

FIGURE 7 is a sectional plan view of the cylinder, piston and piston rod of FIGURE 6 showing three different positions of the crank;

FIGURE 8 is a diagram showing a main control valve for the indexing mechanism of FIGURE 6.

FIGURE 9 is an enlarged view of the rotary trunnion valve of FIGURE 4.

In carrying the invention into effect according to one convenient mode, described by way of example with reference to FIGURES 1 to 5, there is provided a housing 1 in which is mounted an oscillating cylinder 2 by means of trunnions 3 which are pivotally mounted in bearing members 4 forming part of the housing 1. A double acting piston 4 slidable in the cylinder is carried on a rod 5 slidable in glands 6, and one end of the rod 5 is connected to a crank 7 on a vertical shaft 8 carried in bearings in the housing 1. A gear wheel 9 is secured to the upper end of the shaft 8 and is adapted to mesh with another gear or rack (not shown) forming part of the mechanism to be indexed. Such mechanism may comprise for example a rotary or reciprocating table carrying a plurality of moulds which are required to be presented successively to an injecting device for plastic materials. In such an arrangement the rotary table rotates about a central vertical axis and carries on its underside a gear wheel of suitable size meshing with the gear 9.

The trunnions 3 of the oscillating cylinder 2 are arranged near the outer end of the latter so that the piston is at approximately half stroke at indexed position. The piston 4 occupies an intermediate position OIII'LS rod 5, and provides two oppositely directed and equal annular piston areas 10 and 11. The lower cylinder trunnion 3 (see FIGURES 1 and 4) is formed as a directional valve and includes two vertical passages 12 and 13. The passage 12 communicates directly with the piston area 10 and the passage 13 is in communication with the piston area 11 by way of a passage 14. A circumferential groove 15 in the trunnion communicates with diametrically opposed ports 16 and 17 in the trunnion bearing (see FIG- URE 4) which serve as supply and exhaust ports. Also two diametrically opposed radial passages 18 and 19in the trunnion afford communication between the circum-' ferential groove 15 and the vertical passages 12 and 13. The trunnion also carries two aligned radial valve members 20 and 21 which project outwardly into the radial groove 15 and are arranged so as to obstruct the said groove. The outer ends of the valve members 20, 21 are flow from the port 16 in one direction only in the circumferential groove 15, i.e. to the radial port 18 and thence into the cylinder passage 12. At the same time the cylinder passage 19 is placed in communication with the port 17. When the crank is at 180 to the above position the valve members 20, 21 occupy positions respectively on opposite sides of the ports 16, 17 so that the cylinder connections are reversed. Also when the trunnion occupies a position corresponding to either of the two dead centre positions of the crank, is. as shown in full lines in FIGURE 4, the taperedouter ends of the valve members 20 and 21 afford communication between the circumferential groove and the ports 16 and 17.

To prevent excessive fluid pressure in either of the cylinderspaces there are provided two adjustably loaded relief valves controlling passages in the piston interconnecting the cylinder spaces and arranged to open in opposite directions. One of these valves is shown at 22 in FIG- URE 1 and controls a passage 23. The second valve, not shown, controls a similar passage but acts in the opposite direction.

The ports 16 and 17 of the tunnion valve are controlled by a choke valve, indicated at 24 in FIGURE 1, actuated by a cam 25 on the crankshaft 8 so that both the supply of pressure fluid and the exhaust of fluid to a sump or drain can be simultaneously and progressively reduced or increased by motion of the cam as the crankshaft is angularly displaced. Referring to FIGURE 5, the duct 16a communicates with the trunnion port 16 and the duct 17a communicates with the trunnion port 17. The valve comprises piston valve members 25a and 26 controlling the aforesaid ducts and is loaded by a spring. The supply of pressure fluid from a pump or other source is controlled by a main control valve 27 having two valve pistons 28 and 29 and is actuated by solenoids 3t) and 31. Pressure fluid enters at 32, and the spaces 33 and 34 are connected to a sump or drain. In the position shown,

pressure fluid from the inlet 32 is admitted by ducts 35 and 36 to a space 37 in the choke valve 24 and thence by the duct 17a to the trunnion port 17. At the same.

time the trunnion port 16 is in communication through the duct 16:: and a duct 38 with the exhaust space 33. When the control valve is moved to the opposite or left hand position, the connections to the trunnion ports 16.

and 17 are reversed. When the control valve occupies an intermediate or neutral position, the valve pistons 28, 29 close both the ducts 35 and 38 and thus seal off both the trunnion ports 16 and 17. 1

The arrangement of the choke valve 24 is such that movement of the valve pistons 25, 26 to the left in FIG: URE progressively chokes flow to fluid through the duct 16a and the trunnion port 16 and also chokes the flow from the trunnion port 17 through the duct 17a and thence through space 37 to exhaust or drain. The form of the cam 25 is such that with the mechanism in an indexed position, both supply and exhaust are reduced to a small amount, and this is increased and subsequently decreased as the crank 7 rotates through 180.

The main control valve 27 is operated to open position for. initiating an indexing movement and to closed position for bringing the mechanism to rest in the required angular position. In this example the required movements of'this main control valve are effected by solenoids, but obviously other actuating means could be employed. The solenoids are actuated from a power supply and are controlled by limit switches actuated by movement of the rotary part or parts to be indexed so that the crank is stopped in the required positions. These limit switche are of conventional kind.

The arrangement is such that with the gear wheel 9 in. a position corresponding to an indexed position of the. mechanism. or part, eg. a rotary table, to which. it is operativelyconnected, the crank 7 lies midwaybetwecn its two dead centre positions, and the trunnion valve 3 is approximately at one of its extreme angular positions, the piston 4 being approximately at mid stroke. The main control valve then occupies a mid position in which both the ducts 35 and 38 are closed. The position of the choke valve is such thatboth supply and exhaust passages are open to a small extent only. To eflect an indexing operation the main valve 27 is opened, for example by displacing the valve to the right as shown in FIGURE 5, thereby connecting the pressure supply 32 through ducts 35 and 17a to the trunnion port 17, the trunnion port 16 being simultaneously connected through ducts 16a and 38 to exhaust. Pressure fluid accordingly passes through the annular trunnion passage 15 and into one of the vertical passages 12 or 13 according to the angular position of the trunnion, and thence to the piston area which is in communication with said passage. The opposite piston area is in communication with exhaust through the other vertical passage. The piston is accord ingly displaced and actuates the crank. The choke valve is progressively opened so as to increase the amount of fluid entering and leaving the respective cylinder spaces. The piston accelerates, and during this acceleration the mechanical advantage of the piston driving the crank is approximately constant. As the piston reaches its maximum velocity the velocity ratio between the piston and the crank increases, and the rotating parts continue to accelerate provided that the piston moves at the same rate. As the crank nears either of'its dead centre positions, angular movement of the trunnion valve-3 causes both cylinder spaces to be placed in communication with the passages 16 and 17 simultaneously as seen in the full line position in FIGURE 4. The supply of motive fluid accordingly ceases and the piston will slow down, stop, and reverse by the action of the crank, the kinetic energy of the parts driven by the gear wheel 9 supplying the motive power for this part of the movement. When the crank leaves its dead centre position in continuing its motion, the trunnion valve 3 reverses the connections to the cylinder spaces, thereby supplying fluid to the opposite side of the piston 4 to provide energy for the final movement to the next index position. During this movement the piston has an increasing mechanical advantage relative to the crank which assists in absorbing the energy required to slow down the indexed part as it nears the index position. This is effected by actuation of the choke valve 24 by the cam 25 which progressively restricts the supply and exhaust of fluid to the cylinder spaces and so retards the piston movement. Stopping 0f the indexed part at the index position is effected by closing the main control valve 27 'to its mid position to seal the exhaust and supply ducts 35 and 38, so that an indexed part stops in the angular position require It will be seen that since the velocity of the moving parts is very small at this point, the cutting off of the pressure fluid supply should not result in any undesired shock. The same sequence of operations takes place when the crank is moved through its opposite dead centre position for a further indexing movement.

The loaded relief valves previously described prevent any build up of undesirably high fluid pressure in the system.

In carrying the invention into effect according to another mode, described by way of example as illustrated in FIGURES 6, 7 and 8, there is provided a housing 39 in which there is mounted an oscillating cylinder 40 by means of trunnions 41 pivotal in bearing ports 42 forming part of the housing 39. A double acting piston is carried on a rod 43 slidable in glands 44 in the cylinder ends, and one end of the rod 43 is connected to a crank 45 on a vertical shaft 46 carried in bearings 46a in the housing 39. A gear wheel 47 is secured to the upper end of the shaft 46 and is adapted to mesh with another gear or rack (not shown) forming part of-. or in. driving connection with the mechanism or machine part to be indexed. Thelatter may comprise for example a rotary table carrying a series of moulds and adapted to be rotated to present such moulds to an injection moulding machine. The arrangement so far described is similar to that of the preceding example, including the lower trunnion 41 acts as a directional valve as previously described. The piston occupies an intermediate position on its rod 43 and includes a central cylindrical part 48 having at its mid point a radial flange 49. Slidably mounted on the cylindrical part 43 are two sleeves 50 and 51 which occupy the space between the cylindrical port 48 and the central portion 52 of the cylinder bore. On each side of the said bore portion 52 are enlarged bores 53 and 54, and the sleeves 5t) and 51 have end flanges 55 and 56 which can engage with the shoulders formed by the enlarged bores 53 and 54 at the positions shown in FIGURES 6 and 7. The cylinder space 57 communicates directly with a vertical passage 58 in the lower trunnion valve 41 which corresponds to the passage 12 in the trunnion valve 3 of FIGURE 1. The cylinder space 59 communicates by a duct 60 with a vertical trunnion valve passage 61 corresponding to the passage 13 of FIGURE 1. Also a duct 62 leads from the space 63 between the piston sleeves 50 and 51 to a spring loaded atmospheric relief valve 64. The trunnion valve 41 has radial ports 65, 66 which correspond to the radial ports 16 and 17 of FIGURE 4, the construction of the present valve being the same.

The supply of pressure fluid to the cylinder spaces is controlled by a main control valve shown in FIGURE 8. The movable valve member has two valve pistons 68 and 69 and can be displaced in either direction by solenoids 7i) and 71 controlled in the manner described inthe previous example. Pressure fluid from a pump or other source is admitted at '72 between the two valve pistons, and in the mid position of the valve as shown, fluid is admitted to ducts 73 and 74 and thence to the ports 65 and 66 of the trunnion valve 41. Pressure fluid is admitted to both the opposed cylinder spaces 57 and 59 when the control valve is in the FIGURE 8 position. Referring to FIGURE 7, 75 indicates an indexed position of the crank, the other indexed position being at 180 from this as indicated by the arrowed arcs '76.

The arrangement is such that assuming the gear wheel 47 and the crank 45 to be in an indexed position and the control valve 67 to be in mid position as shown in full lines in FIGURE 8, an indexing operation is efiected by displacing the control valve in the required direction. The movement of the control valve to the right, for example, so that the valve pistons occupy the positions shown in broken lines at 68a, 69a connects the duct '73 to an exhaust space 77 and admits pressure fluid to the duct 74 and thence to the port 17 (FIGURE 4) of the trunnion valve 41. If the valve members 20, 21 of the trunnion valve occupy the broken line positions in FIG- URE 4, pressure fluid is admitted to the trunnion passage 61 and thence through the duct 63 to the cylinder space 59, thereby driving the piston and crank, and exhausting fluid from the cylinder space 57 through the duct 73. The sleeve 51 acts on the piston, and when this reaches the indexed position this sleeve seats on the shoulder of the reduced cylinder bore. It will be understood that bringing the piston to the indexed position is effected by returning the control valve 67 to its mid position so as to open both ducts 73 and 74, and the piston is thereby.

retarded as in the previous example. When the piston reaches the position aforesaid, any further movement will tend to unseat the opposite sleeve 50 from its engagement with the shoulder of the reduced lbore as shown in the lower part of FIGURE 7. Should there be sufficient energy in the moving parts to eflect such unseating, the sleeve 56 will be moved to the left against the pressure of the fluid in the cylinder space 57, thereby absorbing the energy of the moving parts and bringing them to rest. The sleeve 50 is now out of fluid balance and thereby forces the piston in the opposite direction against the indexed position. Since the initial unseating of the sleeve 50 absorbs a considerable amount of energy, the piston will oscillate a very small number of times over a progrmsively diminishing stroke, and will be brought to rest with the crank in indexed position without requiring limit switches or other control operations. Also the positioning of the piston in this way is not afiected by variations in the friction of the moving parts. The subsequent indexing operation is effected by moving the control valve 67 as before. It will be seen that the provision of the displaceable sleeve piston members operating in the manner described above avoids the necessity for the regulating or choke valve of the previous example.

It will be appreciated that an advantage of the indexing mechanism of the above example is that a machine part may be indexed in steps in either direction without the necessity of reversing direction at each indexing operation and without disconnecting the mechanism from the indexed part as when employing a pawl or ratchet. Indexing may be effected in either a clockwise or an anti-clockwise direction by energizing the appropriate solenoid 70 or 71 (FIGURE 8). During automatic operation of the mechanism the appropriate solenoid is intermittently energizing, and the table or other part is indexed in one direction for each cycle of the mechanism. In operation, when pressure fluid is fed to the parts 58 and 60, the piston is maintained at indexing position. For a single indexing movement it is only necessary to exhaust either passage 58 or 66 until the piston is moved to a top dead centre position when both passages 58 and 60 will be connected to the fluid supply. The differential area of the piston sleeve 51 is approximately the same as the differential area of the piston 48, and therefore if pressure fluid is supplied at the rate of say 1 gallon per minute to the differential piston area of the piston 48 at one side of the cylinder, there will be displaced one gallon per minute from the diflerential area of the piston 48 on the other side of the cylinder together with one gallon per nnuute from the diflerential area of the piston 51. The speed of displacement will be at the rate of one gallon per minute to one piston area. After leaving the top or bottom dead centre position it is necessary to supply fluid at the rate of two gallons per minute to move both piston areas back to the indexed position at the same speed. One gallon per minute is provided by the fluid supply and one gallon per minute is supplied from the balancing diflerential area of the piston 48. Thus there is provided the same power to return to indexed position as to leave the indexed position, and this requires the same quantity of fluid from the power source.

The relief valve 64 maintains a small back pressure in the cylinder for lubrication purposes. It will be understood that the motions of the crank when moving from one indexed position to the next will be generally the same as in the previously described example, together with the advantages achieved thereby.

The operating fluid employed may be compressible or incompressible, e.g. air, oil, or other liquid. The use of oil has the advantage that it may also be employed to lubricate the bearings by providing suitable take-01f and distributing passages in the moving parts. Such oil ducts are shown in FIGURES 1 and 6.

The invention is not restricted to the above examples, since details of construction may obviously be widely varied. Thus, for example, whilst a double acting piston has the advantage of compactness, the same mechanical result may be achieved by two single acting pistons suitably arranged.

By this invention there is provided an indexing mechanism which may be embodied as a self contained unit and affords the most eifective use of the piston action,

Also the piston or pistons are in continuous driving con nection with the part or parts to be indexed and there is no idle stroke such as occurs for example when a Geneva mechanism is employed. The arrangements described in the above examples permit a wide range of adjustments, and it is not possible for the timing sequence to be out of phase.

Whilst the invention can be applied with particular advantage to rotary mould carrying tables for injection moulding machines, for example as described in British patent specification No. 831,193, it is also applicable to a wide range of indexing mechanism, for example in the machine tool industry, especially where heavy machine parts or the like are to be indexed.

I claim:

1. In an indexing mechanism for indexing machine parts, fluid operated cylinder and piston means including first and second piston areas for action in opposite directions, crank means operatively connected with said piston means, a machine part to be indexed having a driving connection with said crank means, means for controlling the supply of pressure fluid to said piston means, and directional valve means having an actuating connection with said cylinder and piston means, said directional valve means being arranged so that upon actuation thereof from a position corresponding to an indexed position of the mechanism the first said piston area is opened to said supply controlling means and the said second piston area is opened to exhaust during the first part of an indexing movement, and on subsequent further movement both said piston areas are opened to supply and exhaust simultaneously, and subsequently during the latter part of the movement the said second piston area is opened to the pressure fluid supply, the said crank means being arranged to be operable over 180 through one of the dead centre positions of said crank during such movement of the mechanism.

2. Mechanism according to claim 1, in which the cylinder and piston means comprises a double acting piston, and the crank means comprises a single crank.

3. Mechanism according to claim 1 in which the means for controlling the supply of pressure fluid comprise a main control valve and ducts leading from said main control valve to the respective piston areas, said main control valve being arranged to be operable to connect the pressure fluid supply with a duct leading to one piston area while connecting the other piston area to exhaust.

4. Mechanism according to claim 1 in which the means for controlling the supply of pressure fluid comprises a main control valve and ducts leading from said main valve to the respective piston areas, said main control valve being arranged to be operable to connect the pressure fluid supply to a duct leading to one piston area while connecting the other piston area to exhaust, said main control valve also having an intermediate position in which both said ducts are closed.

5. Mechanism according to claim 1 in which the means for controlling the supply of pressure fluid comprises a main control valve and ducts leading from said main control valve to the respective piston areas, said main control valve being arranged to be operable to connect the fluid supply to a duct leading to one piston area while connecting the other piston area to exhaust, said main control valve having an intermediate position in which both said ducts are connected to the pressure fluid supply.

6. Mechanism according to claim 1 in which the cylinder and piston means includes, an oscillating cylinder.

7. Mechanism according to claim 1 in which the cylinder and piston means includes an oscillating cylinder having its axis of oscillation near an end of the cylinder remote from the crank means.

8. Mechanism according to claim 1 in which the cylinder and piston means includes an oscillating cylinder carriedby trunnions, and in which one of said trunnions is formed as said directional valve means.

9. Mechanism as claimed in claim 1 in which thecylin der and piston means includes an oscillating cylinder carried by trunnions, and in which one of said trunnions is formed as said directional valve means, said trunnion including longitudinally disposed passages in communication with the respective piston areas, a circumferential groove having ports communicating with said passages, a trunnion bearing having ports arranged in a plane other than the plane of said ports in the circumferential groove of the trunnion and being adapted for the supply and exhaust of pressure fluid, said trunnion havingrvalve members projecting into said circumferential groove and arranged to afford a communication from each of the piston areas to one of said passages when the crank means is inclined to a dead centre position of said crank means, and to afford a communication between both said piston areas when said crank means occupies a dead centre position.

10. Mechanism according to claim 1 including regulating or choke valve means in the pressure fluid supply, and means for actuating said choke valve means in accordance with an indexing movement of the mechanism when the part to be indexed is near its next indexed position, whereby to restrict fluid flow through said valve.

11. Mechanism according to claim 1 comprising regulating or choke valve means in the pressure fluid supply, and cam means operatively driven from the rank means for actuating said choke valve means to restrict fluid flow through the said choke valve means when the part to be indexed is near its next indexed position.

12. Mechanism according to claim 1 comprising a rotary part to be indexed and means for dissipating the energy of rotation of said part as it is brought to reset when indexing.

l3. Mechanism according to claim 1 comprising connections between the respective piston areas and a relief.

valve in each of said connections and arranged to operate in opposite directions for dissipating the energy or" the part to be indexed when said part is brought to reset.

l4. Mechanism according to claim 1 in which means is provided for dissipating the energy of the part being indexed when said part is brought to rest, said means comprising a piston rod, abutment means on said rod, piston members aflordingthe first and second piston areas slidable upon said rod away from said abutment means, and fixed stop means for each of said piston members arranged so that one of said stop means is engaged by one of said piston members when the mechanism reaches an indexed position, the arrangement being such that further movement of the piston rod beyond said indexed position tends to disengage the other said piston member from its fixed stop means, and if such disengagement takes place the latter piston member is subject to unbalanced fluid pressure to return the piston rod to a position corresponding to an indexed position of the mechanism.

15. Mechanism according to claim 1 in which means is provided for dissipating the energy of the part being indexed when said part is brought to rest, said means comprising a piston rod, abutment means on said rod, sleeves having radial projections and slidable upon said rod away from said abutment means and affording the first and second piston areas, enlarged bores in the outer ends of the cylinder means forming shoulders constituting fixed stop means for each of said sleeves engageable with said radial projections when the mechanism reaches an indexed position, the arrangement being such that further movement of the piston rod beyond said indexed position tends to disengage the other said sleeve from its fixed stop means, and it such disengagement takes place the latter piston member is subject to unbalanced fluid pressure to return the piston rod to a position corresponding to an indexed position of the mechanism.

16. Mechanism according to claim 1 arranged so that 9 10 an indexed position is attained when the crank means References Cited in the filc of this patent is at 90 to its dead centre positions.

17. Mechanism according to claim 1, comprising means UNITED STATES PATENTS for actuating the means for controlling the supply of 2,326,494 Ratzburg Aug. 10, 1943 pressure fluid to the piston means in accordance with 5 2,763,481 Hackett Sept. 18, 1956 the position of the part to be indexed. 2,900,964 Liebmann Aug. 25, 1959

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