US2547656A

US2547656A - Hydraulic hold-down

Info

Publication number: US2547656A
Application number: US29508A
Authority: US
Inventors: Frederick E Munschauer
Original assignee: Niagara Machine and Tool Works
Current assignee: Niagara Machine and Tool Works
Priority date: 1948-05-27
Filing date: 1948-05-27
Publication date: 1951-04-03
Anticipated expiration: 1968-04-03

Description

April 3, 1951 F. E. MUNSCHAUER HYDRAULIC HOLD-DOWN 4 Sheets-Sheet 2 Filed May 27, 1948 Fig.5

gwg Qfluw ATTORNEYS F. E. MUNSCHAUE HYDRAULIC HOLD-DOWN 'Aprifi 3, E91

' Filed May 27, 1948 4 Sheets-Sheet 4 INVENTOR M3 M6 fieder/c/f E. Mwz q/zauer ATTORNEYJ Patented Apr. 3, 1951 antes HYDRAULIC HOLD-DOWN Frederick E. Munschauer, Buffalo, N. Y., assignor to Niagara lilachine & Tool Works, Bufialo,

Application May 27, 1948, Serial No. 29,508

(Cl. 16i-52 20 Claims. 1

This invention relates to hydraulic hold-down means for power shears and analogous machines. Conventional hold-down means for holding metal or other sheets to a work table during shearing or the like involve mechanically driven reciprocating means, usually spring-cushioned. Such hold-down devices are simple, inexpensive and free of trouble, as compared with conventional hydraulic hold-down devices presently known and available. Also the energy put in to compress the springs in mechanical hold-downs is returned to the system on the up stroke.

7 Nevertheless,advantages are to be derived from the use of hydraulically operated hold-down means, despite the fact that all the prior means oiiered to provide such a device have been subject to various objections, among them the cyclical action heretofore considered inherent in the prololem, matters of objectionable leakage, and the amount of energy required to displace considerable quantities of liquid and develop the nose sary pressure therein.

According to the present invention hydraulic hold down means are provided which provide a satisfactory and practical solution of the various problems which have heretofore militated against the successful and practical employment of hydraulic pressure for hold-down purposes in place of the usual mechanically driven spring cushioned hold-downs.

According to the present invention the hydraulic pressure generating means is continuous in operation, not cyclical or intermittent as in prior proposals, and the arrangement is such that full hold-down pressure is applied almost instantaneously when desired, rather than with a material time lag. Further, the arrangement of the present invention is such that the work of operating the hold-down and providing holddown pressure is removed from the machine drive proper which reduces the strength requirements of the machine and simplifies the construction thereof.

The almost complete elimination of moving parts in the hold-down provides a very quite operation and a small pump is adequate to provide full-hold-down pressure requirements. If desired the pump may be driven from one of the gears in the conventional shear drive gear box instead of by a separate electric motor, but in either event the advantage of low power requirement is present. Further, the provision of hydraulic pressure means which are independent of the shear proper permits the hold-down. to be operated at will in motion, and this is of particular advantage when cutting a sheet without using a fixed, positive sack gauge, as cutting along a line or to a measured dimension.

Since the main supporting bar for the hydraulically operated. hold-down feet is stationary, it serves as a rigid structural tie between the end housings of the shear and thus strengthens the machine and minimizes the need of other crossbracing components between the end housings.

Of course, the provision of a stationary hold-down bar also eliminates all of the usual guides and slide-hearing surfaces normally associated with mechanically reciprocating shear hold-downs and makes for a safer and less hazardous machine.

In the hold-down means of the present invention the hold-down feet are in contact with the work, resting thereon with the force of gravity only, prior to the application of hydraulic holddown pressure. This prevents marring the surface of the sheet which often results when various impact hold-down means are employed. This is of particular importance when operating upon polished sheets of aluminum or stainless steel. This arrangement also eliminates the possibility of a workman accidentally getting his fingers under the hold-down feet.

Various embodiments of the principles of the present invention are illustrated in the accompanying drawings and described in detail in the ensuing specification by way of example. However, many modifications may be made within the spirit and scope of the invention and without departing from the principles thereof, the invention being limited only as defined in the appended claims.

In the drawings:

Fig. l isa front elevational view of a power shear showing one form of hold-down bar and hydraulic cylinder means for-holding a sheet to the work table of the shear;

Fig. 2 is a general schematic View ofthe hydraulic circuit in one form of the present invention Fig. 3 is anether general schematic View of another adaptation of the hydraulic circuit of the present. invention having a modified form of control means;

Fig. 4 is a fragmentary elevational view of a hcld down bar provided with one form of hydraulic hold-down means;

Fig. 5 is a cross-sectional view taken on the line without setting the shearing or other mechanism 55 V-V of Fig. 4;

Fig. 6 is a cross-sectional view taken on the line VIVI of Fig. 4;

Fig. 7 is a view similar to Fig. 4 but showing another form of hydraulic hold-down means;

Fig. 8 is a cross-sectional view taken on the line VIIIVIH of Fi '7;

Fig. 9 is a cross-sectional view taken on the line IXIX of Fig. '7;

Fig. 10 is another view similar to Fig. 4 but showing still another form of hydraulic holddown means;

Fig. 11 is a cross-sectional view taken on the line XI-XI of Fig. 10; and

Fig. 12 is a cross-sectional view taken on the line XII,XII of Fig. 10.

Throughout the several figures of the drawings like characters of reference denote like parts and, referring to Fig. l, the power squaring shear there shown comprises a pair of end housing members I5 and I6 which support therebetween a bed H having a work receiving table l8.

The shear includes a main drive shaft l9 having eccentric or crank connection with the crosshead in conventional manner, the latter carryinga shear blade 2|. In Fig. 1 the numeral 24 designates a hold-down bar which is rigidly fixed at its opposite ends to end housings |5 and i6 as at 25 and 26. Hold-down bar 24 includes a series of hydraulic cylinders 21, each of which has a work clamp foot 28 at its lower end.

The details of construction and operation of the hydraulic cylinders and their associated hold-down feet will be described in detail later herein in several forms. Fig. 1 shows schematically the hold-down bar 24 and hydraulic cylinders 21. In Fig. 2, a liquid reservoir or supply is indicated schematically at 30V and a continuous operating pump, which may be of relatively low displacement, has its intake port in free communication with the interior of the reservoir as by means of conduit 32. Pump 3| has an outlet conduit 33 which leads to a branch conduit 34 leading back to the reservoir and also a branch conduit 35 leading to hold-down bar 24.

At hold-down bar 24 branch conduit 35 communicates with a fluid passage 36 which is associated with the hold-down bar and communicates freely with all of the hydraulic cylinders 21 by means of branch passages 39. Another branch conduit 40 from conduit 35 leads back to the interior of reservoir 30 and includes a manually operable valve 42 and a second control valve 43. Control valve 43, which is normally open, may be conveniently attached to the frame of the shear as shown in Fig. l and includes a valve closing element 44 adapted to be actuated by a cam 45 located upon drive shaft I9. Manual valve 42 is selectively closable to operate the hold-down independently of the shear crosshead for placing work to be cut to a line or to a measured dimenmen.

In Fig. 2 the numeral designates a pressure limiting valve which determines the maximum pressure that pump 3| delivers to conduit 35. Valve 50 normally prevents flow into conduit 34 from conduit 33 but any excessive pressure in conduit 33 causes pressure limiting valve 50 to open and relieve the excess pressure by exhausting through conduit 34 back to the reservoir. Passage 36 of hold-down bar 24 is provided, at the end opposite the connection with conduit 35, with a capillary tube 52 or any tube having a very small inside diameter lead-ing therefrom back to reservoir 30. When the shear is in condition for operation, but is not executing a shearing stroke,

valves

42 and 43 are open, and the liquid displaced by pump 3| and flowing through conduit 35 bypasses back to the reservoir through passage 43, at the same time maintaining the passage 36 and hydraulic cylinders 21 full of liquid.

Attention will now be given to Figs. 4, 5, and 6, which show one embodiment of hold-down bar 24, hydraulic cylinders 21 and hold-down feet 28. In Fig. 4 the hold-down bar 24 comprises upper and lower bar portions 53 and 5| which are welded into an integral unit as at 63 and 64. Beforewelding, the upper surface of bar 6| is milled or otherwise machined to provide a groove 65 extending from near one end of bar 6| to the other, stopping just short of the end of the bar in each instance. When bars 60 and 6| are joined by welding as just described, groove 65 cooperates with the under side of bar 60 to form the liquid passage 36 of Fig. 2. Communicating with this passage is an entrance passage 61 which is connected to

conduits

35 and 40.

In the schematic view, Fig. 2, conduit 36 is shown as declining to the left as there viewed. In Fig. 4 the same substantive effect is attained by disposing passage 61 at a higher elevation than passage 36. By reason of this arrangement passage 36 will be maintained full of liquid by pump 3| even though bypass 4|) is freely open to bypass excessive liquid back to reservoir 30. This also maintains the several hydraulic cylinders 21 in a filled condition.

In the form shown in Figs. 4, 5, and 6, each hydraulic cylinder 21 has a threaded upper end 13 screwed into the under side of bar 6| and suitably packed as indicated at 1|. Hydraulic cylinder 2'! has a fluid chamber 12 and a passage !3 leading upwardly therefrom in registry with a passage 15 which is formed in bar 5| and extends downwardly from groove 65, there being a separate passage 15 in the hold-down bar 24 for each hold-down cylinder 21. Thus, passage 35 is in free communication with the fluid chamber I2 in each of the hydraulic cylinders 21. Each of the hold-down feet 28 has an upper piston portion I? fixed thereto and disposed for vertical sliding movement in fluid chamber 12 of cylinder 21.

In the form shown in Figs. 4 through 6, the hold-down foot proper is generally rectangular, as viewed in plan and the bottom of cylinder 21 is bifurcated as at to receive hold-down foot 28 and hold the same against rotation. The forward lower edge of each hold-down foot 23 is rounded or beveled as indicated at 8| to facilitate moving a metal sheet or the like thereunder as at 83 in Figs. 4 and 5. The hold-down feet 28 are normally in a lowered position against the work table I8 but under no pressure, resting there by gravity alone, so that a sheet may simply be pushed beneath the several hold-down feet, whereupon they move upwardly a distance equal to the thickness of the sheet and rest lightly on the upper surface of the sheet, the oil from the cylinder being displaced by way of passage 40.

As shown in Fig. 5, the inner wall of cylinder 2! has an encircling oil collecting groove 85 and

passages

81 and 88 lead therefrom to a leakage collecting conduit 89 which may communicate in common with all of the several hydraulic cylinders and have gravity flow back to reservoir 30 at 93 in Fig. 2.

As the crosshead 20 begins to descend in commencing a shearing stroke, cam 45 engages operator 44 to close the normally open hold-down operating bypass valve 43. Since pump 3| is in continuous operation and the passage 36 and the passages leading therefrom are all in a liquidfilled condition, pressure against hold-down feet 28. through the medium of pistons 11 is substantially immediate and direct. There is no necessity for any movement of the piston 11 in cylinders 21 andan immediate and secure hold down action results.

Each piston 11 fits rather closely in cylinder 21 both above and below the oil collecting groove 86. Accordingly, such liquid as passes downwardly about the piston to the oil collecting groove, because of the considerable pressure in chamber 12 during a hold-down operation, flows into the substantially pressureless collecting groove and thence outwardly through

passages

31 and 88 to leakage conduit 89 and back to the reservoir. Since it requires a considerable pressure to move liquid past piston 11 at any point along cylinder 21.; there will be substantially no tendency for liquid to pass below the collecting groove 86.

Because there is only the minute axial movement of piston 11 in cylinder 21 equal to the thickness of the metal being sheared, there is relatively no tendency for a iilm of oil to be continuously moved downwardly between piston I1 and the interior wall of cylinder 21, as might be the case if sliding movement of greater magnitude were nec'essary in operating the hold-down.

The relatively minute but substantially continuous flow of liquid from the left-hand end of passage 36 through capillary tube 52 whenever passage 36 is under-pressure carries off any air which may be entrained with the liquid in passage 35.

Reference will now be had to the modified hydraulic hold-down system shown schematically in Fig; 3. In this form of the invention, as in the case of the embodiment of Fig. 2, there is a stationary hold-down bar 95 having a plurality of hydraulic hold-down cylinders 96 and a ccmmon pressure supply passage 61. In Fig. 3 the pressure generating pump is designated I613 and its outlet passage IIH is connected with a conduit I62 leading to the cylinder supply passage 91.

Interposed between the outlet passage IDI and conduit IE2 is a pressure limiting valve I63 with aconduit I65 for returning liquid to a reservoir I65 when the pressure output of pump I66 exceeds the pressure setting of valve ms. The intake jconduit of pump I60 is designated I51 and leads from reservoir I65.

As in the previous instance, the pressure supply passage '91 of the hydraulic cylinders terminates in communication with a capillary tube I I6 which leads'back to reservoir I05 in the same manner and for the same purpose set forth in connection with the description of Fig. 2. In Fig. 3 the leakagereturn conduit similar to that designated 89 in Fig. 2 is designated I I I.

In addition to pressure passage 91 for the hydraulic cylinders, conduit I62 communicates with a bypass conduit I I2 which leads back to reservoir I05 and has interposed therein a solenoid controlled valve Ii3 which is normally open and is closa-ble upon energization of an electromagnet H4. In this form of the invention the single valve H3 is operated either manually or automatically. Manual operation is provided for operating the hold-down independently of the shear crosshead and automatic operation accompanics reciprocating movement of the shear crosshead.

A supply circuit for energizing electromagnet l'l is designated I I5 and has connected in parallel therein'a manual: normally open treadle operated switch H6 and a second normally open switch II! which replaces the valve 43 in Fig. 1 so that cam 44' of the drive shaft of the machine acts upon an operator H8 of the switch to automatically close the same for predetermined periods during the reciprocation of the shear crosshead. It is believed that the operation of the electrically controlled hydraulic hold-down system of the embodiment of Fig. 3 will be clear from the foregoing.

A second embodiment of the hold-down bar and hydraulic cylinder means per se is illustrated. in Figs. 7, 8, and 9. In this formthe pres sure and leakage conduits for the cylinders are independent of the hold-down bar proper and the entire cylinder and conduit assembly is removable from the stationary hold-down bar.

Referring to Figs. 7 and 8 the numeral I20 designates a hold-down bar similar to the bar 24 of Figs. 1, 2, 4 and 5 and one of a series of hydraulic cylinder elements is designated I21. As in the preceding instance, cylinder I2I has a pressure chamber I22, a piston I23 vertically slidable therein, and an oil collecting groove I24 about its inner periphery. Piston I23 is provided at its lower end with a hold-down foot I26 similar to that previously described and the lower portion of cylinder IZI is bifurcated as at I28 to guide hold-down foot I26 against rotative movement.

Cylinder I2I is secured against the under side of hold-down bar I26 by means of screws I36 which extend upwardly through lugs I3! which project from opposite sides of cylinder I2I.

Cylinder I2! has a forwardly offset upper extension I34 and the several upward extensions I34 of a series of hydraulic cylinders are connected by pressure conduits I35 and leakage return conduits I36. Pressure conduits I35 apply operating pressure to pressure chamber I22 by means of passages I31, I38, and I39 formed in each of the upward extensions I34 of the hydraulic cylinders I2 I. As appears best in Fig. '7, the liquid collecting groove I24 communicates with leakage return conduits I36 by means oi passages I II, I42, and I43 formed in the body portion of cylinder I2I and upward extension I34.

In the further embodiment of the hydraulic cylinder means per se illustrated in Figs. 10

through 12, the numeral I designates one of a series of hydraulic cylinders having lateral lugs I5I at its upper end for securement to the under side of a hold-down bar I52 by means of screws I53.

Hydraulic cylinder I50 has a pressure chamber for receiving a piston I55 and the latter has a hold-down foot I51 at its lower end as in the.

previously described embodiments. Cylinder I56 is bifurcated at its lower end as at I58 to prevent rotation of the hold-down foot. The inner periphery of the pressure chamber I55 of cylinder I56 is provided with a leakage collecting groove I66 as in the previously described embodiments.

A guard plate I depends from a bar [16 to which it is fixed by welding or the like and bar it is secured to hold-down bar I52 by means of screws I11. Guard plate 115 extends downwardly behind cylinders I50 to a point just above work table [8 to prevent an operator from reaching under the hold-down bar between a pair of spaced cylinders I50. Similar guard plates are intended to be employed in conjunction with the previously described hydraulic cylinder embodiments, the guard plate of the first described embodiment being designated 80 in Fig. 4.

The operation of the several forms of hydraulic cylinder arrangements illustrated herein is the same in function and theory as described generally in connection with the schematic hydraulic circuit, Fig. 2, and obviously any of the cylinder arrangements of Figs. 4 through 12 may be used in the systems of either Figs. 2 or 3.

What claimed is:

1. A hold-down for shears like machines, said hold-down comprising a rigid support, hydraulic pressure actuated hold-down means supported thereby, a pressure conduit for energizing said pressure actuated hold-down means by application of hydraulic pressure thereto, a continuously operating pressure generator in free continuous communication with said pressure conduit, a normally open relief conduit between said pressure generator and said pressure conduit to relieve the pressure conduit, valve means in said relief conduit closable to prevent fiow therein and thus cause the pressure generator to act directly against the pressure conduit to activate the hold-down means, and means operable in timed relation with the operation of the machine proper for automatically closing and opening said valve at predetermined points in the cycle of operation of the machine.

2. A hold-down for shears and like machines, said hold-down comprising a rigid support, hydraulic pressure actuated hold-down means supported thereby, a pressure conduit for ener izing said pressure actuated hold-down means by application of hydraulic pressure thereto, a continuously operating pressure generator in free continuous coim; unication with said pressure conduit, a normally open relief conduit between said pressure generator and said pressure conduit to relieve the pressure conduit, and valve means in said relief conduit closable to prevent flow therein and thus cause the pressure generator to act directly against the pressure conduit to activate the hold-down means, means operable in timed relation with the machine proper for automatically closing opening said valve at predetermined points in the cycle of operation of the machine, and manual means for selectively closing said relief conduit to actuate the hold-down means independently of machine operation.

3. A hold-down for shears and like machines, said hold-down comprising a rigid support, a hydraulic pressure actuated hold-down means s1; ported thereby, a pressure conduit for energizing pressure actuated hold-down means by application of hydraulic pressure thereto, a continuously operating pressure generator in free continuous communication with said pressure conduit, a normally open relief conduit in open communication with said pressure conduit to relieve the pressure conduit, and means for closing said relief conduit automatically for operating said hold-down in timed relation with the operation of the machine proper and manually for operating the hold-down independently of machine operation.

4. A hold-down for shears and like machines, said hold-down comprising a rigid support, hydraulic pressure actuated hold-down means supported thereby, a pressure conduit for energizing said pressure actuated hold-down means by application of hydraulic pressure thereto, a continuously operating pressure generator in free continuous communication with said pressure conduit, a normally open relief conduit in open communication with said pressure conduit to relieve the pressure conduit, means for closing said relief conduit automatically in timed relation with the operation of the machine proper to activate the hold-down means, and means for closing the relief conduit by manual operation to activate the hold-down means independently.

5. A hold-down for shears and like machines, said hold-down comprising a rigid support, bydraulic pressure actuated hold-down means supported thereby, a pressure conduit for energizing said pressure actuated hold-down means by application of hydraulic pressure thereto, a continuously operating pressure generator in free continuous communication with said pressure conduit, a normally open relief conduit between said pressure generator and said pressure conduit to relieve the pressure conduit, and valve means in said relief conduit automatically closable in timed relation with the operation of the machine proper to cause the pressure generator to act directly against the pressure conduit to activate the hold-down means.

6. A hold-down for shears and like machines, said hold-down comprising a rigid support, hydraulic pressure actuated hold-down means supported thereby, a pressure conduit for energizing said pressure actuated hold-down means by application of hydraulic pressure thereto, a continuously operating pressure generator in free continuous communication with said pressure conduit, a normally open relief conduit between said pressure generator and said pressure conduit to relieve the pressure conduit, and valve means in said relief conduit automatically closable in timed relation with the operation of the machine proper to cause the pressure generator to act directly against the pressure conduit to activate the hold-down means, said valve means being manually closable to activate the holddown means independently of the operation of the machine proper.

'7. A hold-down for shears'and like machines, said hold-down comprising a rigid support, hydraulic pressure actuated hold-down means supported thereby, a pressure conduit for energizing said pressure actuated hold-down means by application of hydraulic pressure thereto, a continuously operating pressure generator in free continuous communication with. said pressure conduit, a normally open relief conduit between pressure generator and said pressure conduit to relieve the pressure conduit, valve means in said relief timed relation with the operation of the machine proper to cause the pressure generator to act directly against the pressure conduit to activate the hold-down means, and other valve means in said relief conduit selectively closable to activate the hoiddown means independently of the operation of the machine proper.

8. A hold-down f shears and like machines, said hold-down comprising a rigid support, hydraulic pressure actuated hold-down means conduit automatically closable in 9 supported thereby, a pressure conduit for energizing said pressure actuated hold-down means by application of hydraulic pressure thereto, a continuously operating pressure generator in free continuous communication with said pressure conduit, a normally open relief conduit between said pressure generator and said pressure conduit to relieve the pressure conduit, and a normally open valve in said relief conduit, means for automatically closing said valve in timed relation with the operation of the machine proper to cause the pressure generator to act directly against the pressure conduit to activate the holddown means, and other means manually operable for closing said'valve to activate the holddown means independently 40f machine operation.

9. Hold-down means for shears and like machines, said hold-down means comprising a rigid support, a plurality of hydraulic hold-down cylinders supported thereby, piston means in said cylinders adapted to be urged downwardly against a work piece upon application of hydrau lic pressure to said cylinders, a common pressure conduit for said cylinders, a continuously opcrating pressure generator in free continuous communication with said common pressure iconduit, a relief conduit between said pressure generator and said common pressure conduit to relieve the pressure conduit, and valve means in said relief conduit closable to prevent bypass of said pressure conduit and thus cause the pressure generator to act directly against the common pressure conduit.

10. A hold-down for shears and like machines, said hold-down comprising a ri id support, .a plurality of hydraulic pressure actuatedv holddown means supported thereby, a common pressure conduit for energizing-said pressure actuated hold-.downmeans, a continuously operating'pressure generator in free continuous communication with said common pressure conduit, a relief conduit between said pressure generator and said common pressure conduit'to relieve the pressure conduit, and valve means in said relief conduit closable to prevent bypass of said'pressure conduit and thus cause the pressure generator to act directly against the common pressure conduit.

ll. Hold-down means for shears and like machines, said hold-down means comprising a rigid support, a plurality of pairs of piston and cylinder elements including fluid pressure chambers, one of each of said pairs of elements being fixed to the support and the other normally resting on the work table under the force of gravity whereby a work piece may he slipped thereunder, a common pressure conduit in communication with the several fluid pressure chambers, a continuously operating pressure generator in free continuous communication with said common pre sure conduit, a relief conduit between said pressure generator and said common pressure conduit to relieve the pressure conduit, and valve means in said relief conduit closable to prevent bypass of said common pressure conduit and thus cause the pressure generator to act directly against the common pressure conduit and the several fluid pressure chambers and thus cause said other of the pairs of piston and cylinder elements to be urged downwardly against the work piece.

12. Hold-down means for shears and like machines, said hold-down means comprising a rigid support, a plurality of pairs of piston andcylinder elements, said cylinder elements being fixed to the support and having "fluid pressure chambers, the piston elements normally resting .on the work table under the force of gravity whereby a work piece may be slipped thereunder, a common pressure-conduit in communication with the several fluid pressure chambers, a continuously operating pressure generator in free continuous communication with said common pressure conduit, a relief conduit between said pressure generator and said common pressure con duit to relieve the pressure conduit, and valve means in said relief conduit .closable to prevent bypass of saidcommonpressure conduit and thus cause the pressure generator to act directly against the common pressure conduit and the several Jfiuid pressure chambers and thus cause said other of the pairs .of piston and cylinder elements to be urged downwardly against the Work piece;

13. Hold-down means for shears and lilzelmachines, said hold-down means comprising a rigid support, 'a pair of :piston and cylinder elements including a fluid pressure chamber, one of said pair of elements being fixed to the support and the other normally resting on the work table under the force of gravity whereby a workpiece may be slipped thereunder, a continuously operating pressure generator in free continuous communication with said pressure chamber, a relief conduit between said pressure generator and said pressure chamber to relieve the pressure chamber, and valve means in said relief conduit closable to prevent bypass :of said pressure chamber and thus cause the pressure genorator to act directly against said other of the pair of piston and cylinder elements to urge the same downwardly against :the workpiece.

1,4. Hold-downmeansfor shears and like me.- chines, said hold-down means comprising a rigid support, a pair of piston andcylinoler elements, saidcylinder elements beingfixed to the support and having a fluid pressure chamber and the piston element normally resting on the work table under thefo-rce of gravity whereby a work piece may be slipped thereunder, a continuously operating pressure generator in :free continuous communication with said fluid pressure chamber, :a relief conduit between said pressure generator and said pressure chamber 'to relieve the pressure chamber, and valve means in said relief conduitclosable to prevent bypass of said pressure chamber and. thus cause the pressure generator to act directly against the piston element and thus cause said piston element to be urged downwardly against the work piece.

15. A hold-down for shears and like machines, said hold-down comprising a rigid support, hydraulic pressure actuated hold-down means supported thereby, a pressure conduit for energizing said pressure actuated hold-down means by application of hydraulic pressure thereto, a continuously operating pressure generator in free continuous communication with said pressure conduit, a normally open relief conduit between said pressure generator and said pressure conduit leading therefrom at an elevated point relative to the pressure conduit to relieve the pressure conduit of iiuid pressure without permitting liquid drainage therefrom, and valve means in said relief conduit closable to cause the pressure generator to act directly against the pressure conduit to activate the hold-down means.

16. A hold-down for shears and like machines, said hold-down comprising a rigid support, hydraulic pressure actuated hold-down means supported thereby, a pressure conduit for energizing said pressure actuated hold-down means by application of hydraulicpressure thereto, a continuously operating pressure generator in free continuous communication with said pressure conduit, a normally open relief conduit between said pressure generator and said pressure conduit to relieve the pressure conduit, valve means in said relief conduit automatically closable in timed relation with the operation of the machine proper to cause the pressure generator to act directly against the pressure conduit to activate the hold-down means, and a capillary tube leading from said pressure conduit beyond said hydraulic pressure actuated hold-down means to carry off air which may be entrained with the hydraulic medium in said pressure conduit.

'17. A hold-down for shears and like machines, said hold-down comprising a rigid support, hydraulic pressure actuated hold-down means supported thereby, a pressure conduit for energizing said pressure actuated hold-down means by application of hydraulic pressure thereto, a continuously operating pressure generator in free continuous communication with said pressure conduit, a normally open relief conduit between said pressure generator and said pressure conduit leading therefrom at an elevated. point relative to the pressure conduit to relieve the pressure conduit of fluid pressure without permitting liquid drainage therefrom, valve means in said relief conduit closable to cause the pressure gen erator to act directly against the pressure conduit to activate the hold-down means, and a capil lary tube leading from said pressure conduit beyond said hydraulic pressure actuated hold-down means to carry on air which may be entrained with the hydraulic medium in said pressure conduit.

18. Hold-down means for shears and like machines, said hold-down means comprising a rigid support, a pair of inter-sliding piston and cylinder elements including a fluid pressure chamber, one of said pair of elements being fixed to the support and the other normally resting on the work table under the force of gravity whereby a work piece may be slipped thereunder, the fixed element having an encircling groove at its point of inter-sliding engagement with said other element, and a conduit for draining leakage liquid from said groove, a continuously operating pressure generator in free continuous communication with said fluid pressure chamber, a relief conduit between said pressure generator and said pressure chamber to relieve the pressure chamber, and valve means in said relief conduit closable to prevent bypass of said fluid pressure chamber and thus cause the pressure generator to act directly against said other of the pair of piston and cylinder elements to urge the same downwardly against the work piece.

19. Hold-down means for shears and like machines, said hold-down means comprising a rigid support, a plurality of hydraulic hold-down cylinders supported thereby, piston means in said cylinders adapted to be urged downwardly against a work piece upon application of hydraulic pressure to said cylinders, a common pressure conduit for said cylinders, a continuously operating pressure generator in free continuous communication with said common pressure conduit, a bypass conduit between said pressure generator and said common pressure conduit to relieve the pressure conduit, and valve means in said bypass conduit closable to prevent bypass and thus cause the pressure generator to act directly against the common pressure conduit, said cylinders having internal peripheral grooves adjacent to said piston means for collecting liquid leaking past said piston means, and a common conduit for draining leakage liquid from said grooves.

20. A hold-down for shears and like machines, said hold-down comprising a rigid support, a plurality of hydraulic pressure actuated hold-down means supported thereby, a common pressure conduit for energizing said pressure actuated hold-down means, a continuously operating pressure generator in free continuous communication with said common pressure conduit, a relief conduit and valve means in said relief valve conduit closable to cause the pressure in said pressure conduit from said generator to act directly against said hydraulic pressure actuated hold-down means.

FREDERICK E. MUNSCHAUER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 616,801 McNutt Dec. 27, 1898 2,323,770 Hazelton July 6, 1943