Method and device for press overload protection

A method and a device for protecting against a press overload is disclosed whereby, when an excessive pressure which exceeds a specified hydraulic pressure is transmitted to a pressure chamber of a press cylinder, this excessive pressure activates the punching of a shear plate by a shear piston and the excessive pressure is released by the shear piston.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and a device for protecting 
against a press overload to prevent damage to a press machine when the 
press machine receives an overload. 
In a press machine such as a punch press or the like, when a striker 
strikes a punch to cause that punch to descend, if the punch is unable to 
descend, for example, as the result of a galling phenomenon of a punch and 
a die, an excessive load is produced. Conventionally, the following types 
of devices are known as press overload prevention devices for preventing 
damage to the punch or the striker or the like. 
Specifically, for a mechanical press, Japanese Examined Patent Publication 
(KOKOKU) 52-27393 discloses a structure in which a shear plate is set 
directly in the striker so that, when an excessive load occurs, this shear 
plate is punched by the striker, and the punch is unable to perform the 
punching operation. 
In addition, the following methods are known for use with hydraulic press, 
or a mechanical press with a cylinder built into the ram. 
(A) A safety accumulator or a safety valve is provided on the pressurized 
side of the circuit. 
(B) An emergency halt is applied to the impacting operation when the 
generation of excessive hydraulic pressure is detected on the pressurized 
side of the circuit. 
(C) The above two methods are combined. 
The method by which a shear plate is directly set in the striker is not 
suitable with respect to the structure of a hydraulic press. In addition, 
the method by which a safety accumulator or a safety valve is provided on 
the pressurized side of the circuit, or the method by which an emergency 
halt is applied to the impacting operation when the generation of 
excessive hydraulic pressure is detected on the pressurized side of the 
circuit, requires a rather large amount of time for the application of 
hydraulic pressure to the accumulator, the release of hydraulic pressure 
by the safety valve, or the application of the emergency halt to the 
impacting operation. Accordingly, the prevention of damage to the striker, 
punch or the like cannot always be said to be adequate. Furthermore, 
because these devices such as a safety valve or the like must operate when 
a high pressure occurs from excessive hydraulic pressure, they must have a 
large capacity and are therefore high-priced devices. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide, with due consideration to 
the drawbacks of such conventional devices, a method and a device for 
protecting against a press overload wherein the impacting operation is 
quickly halted when an excessively high pressure occurs, accomplished by a 
device with a simple configuration. 
This object is achieved in the present invention by the provision of a 
method for protecting a press from an overload, comprising the steps of: 
punching a shear plate by means of a shear piston when the hydraulic 
pressure exceeds the specified pressure; and releasing the excessive 
pressure by the movement of the shear piston. 
This object is further achieved in the present invention by the provision 
of an overload protection device for a press comprising a press cylinder 
provided on a striker with free vertical movement, for use with a punch 
and a die which operate in conjunction; a shear piston which receives 
pressure introduced to a pressure chamber of the press cylinder; a shear 
plate which is punched by the shear piston when the pressure exceeds a 
specified pressure; and a release section which releases the excess 
pressure introduced to the pressure chamber according to the position of 
the shear piston which has performed the punching operation. 
When, for example, as the result of the phenomenon of galling of a punch 
and a die, the punch is unable to descend so that an excessively high load 
is produced, and a pressure in excess of the specified pressure is 
transmitted to the pressure chamber of a press cylinder, a shear piston 
punches a shear plate as a result of this excessive pressure, and the 
shear piston releases the excess pressure through this punch transfer 
operation. Accordingly, the excessive pressure produced in the pressure 
chamber of the press cylinder is released from the pressure chamber with 
almost no delay when the shear plate of the shear piston is punched, and 
damage to the punch or the like is satisfactorily prevented. 
The excessive pressure produced in the pressure chamber is released through 
a release section which functions according to the position of the shear 
piston which has performed the punching action, so that the release 
structure for the excess pressure is extremely simple.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Other features of this invention will become apparent in the course of the 
following description of exemplary embodiments which are given for 
illustration of the invention and are not intended to be limiting thereof. 
The present invention will now be explained with reference to the drawings 
with respect to an embodiment applied to a turret punch press. FIG. 3 is a 
front elevation of a conventional turret punch press 1 provided with a 
turret device 11 comprising a disk-shaped upper turret 5 provided with a 
plurality of punches 3, 3 . . . on its periphery, and a disk-shaped lower 
turret 9 provided with a plurality of dies 7, 7 . . . on its periphery, 
each die 7 being positioned to correspond with a punch 3. Between the 
punches 3, 3 . . . and the dies 7, 7 . . . , a workpiece W can be moved 
both longitudinally and laterally by a clamp device 13. A striker 15 is 
moved vertically through pressure transmitted to a press piston which has 
been omitted from the drawing. Each punch 3 can be moved vertically in 
this vertical operation, and the workpiece W is punched between the punch 
3 and the die 7. 
FIG. 1 is a vertical section of a front elevation showing the principal 
parts of the present invention. A striker 15 is linked to a press piston 
17. A pressure section 19 of the press piston 17 is intimately inserted 
into a pressure chamber 23 of a press cylinder 21. 
A fluid passage 25 and a exhaust passage 27 are connected to the pressure 
chamber 23, and a return passage 31 is connected to a return chamber 29 
provided on the opposite side of the pressure chamber 23. The fluid 
passage 25 is connected to a fluid pressure supply device (omitted from 
the drawing) through a check valve 33. The return passage 31 is also 
connected to this fluid pressure supply device. 
From this configuration, when pressure is transmitted to the pressure 
chamber 23 through the check valve 33, the press piston 17 descends 
together with the striker 15. When pressure from the return passage 31 is 
transmitted to the return chamber 29, the press piston 17 is elevated 
together with the striker 15. 
The exhaust passage 27 is connected to a pressure chamber 39 of a shear 
cylinder 37 into which a shear piston 35 is intimately inserted. An 
exhaust section 43 which is an exhaust passage communicating with an 
exhaust tank 41 opens into the shear cylinder 37. 
The shear piston 35 is mounted on a shear plate 47 installed on a back-up 
die 45 which opposes the shear piston 35. In this state the exhaust 
section 43 is covered by the shear piston 35. When the pressure 
transmitted to the pressure chamber 23 becomes excessive, exceeding a 
specified pressure P1 (a normal hydraulic pressure during a specified 
punching operation by the punch and die), the shear plate 47 is punched by 
the shear piston 35, and the shear piston 35 moves as indicated in FIG. 2. 
The position of the exhaust section 43 is set so that the exhaust section 
43 communicates with the pressure chamber 39 in this state. 
If the cross sectional area of the pressure section 19 of the press piston 
17 is taken as A1 and the cross sectional area inside the pressure chamber 
39 of the shear piston 35 is taken as A2, when the specified pressure P1 
is applied, a hydraulic pressure P2 applied to the shear piston 35 becomes 
equal to P1.multidot.A2/A1. If this value of A2/A1 is sufficiently small 
the punching action by the shear piston 35 is reliably carried out when an 
excessive pressure occurs. Specifically, the shear piston 35 should be 
formed with a diameter as small as possible with respect to the pressure 
section 19 of the press piston 17. The provision of a small diameter for 
the shear cylinder 37 and the shear piston 35 results in low fabrication 
costs. 
Next, the operation of the device with the above-described configuration 
will be explained. When pressure from the fluid pressure supply device 
(omitted from the drawing) is transmitted to the pressure chamber 23 
through the check valve 33, the press piston 17 descends together with the 
striker 15, and the workpiece W is punched, as specified, from the action 
of the punch 3 and the die 7. In addition, the pressure applied to the 
pressure chamber 23 is further transmitted to the pressure chamber 39 of 
the shear cylinder 37 from the exhaust passage 27. Because the pressure at 
this time does not exceed the specified pressure, the punching action by 
the shear piston 35 is not performed. Accordingly the exhaust section 43 
remains blocked by the shear piston 35. 
When the press piston 17 and the striker 15 descend together, and, for 
example, because of the galling phenomenon of the punch 3 and the die 7, 
the punch 3 cannot descend, an excessive pressure exceeding the specified 
pressure is produced in the pressure chamber 23. This excessive pressure 
is transmitted from the exhaust passage 27 to the shear cylinder 37, and 
the shear piston 35 moves to punch the shear plate 47. The exhaust section 
43 then communicates with the pressure chamber 39 as shown in FIG. 2. When 
this occurs, the excessive pressure in the pressure chamber 23 is relieved 
through the pressure chamber 39 and the exhaust section 43 from the 
exhaust passage 27 and is quickly dissipated to the exhaust tank 41, thus 
reducing the excess pressure in the pressure chamber 23. As a result, when 
the galling phenomenon or the like is produced at the punch 3 and the die 
7, the undesired descent of the press piston 17 and the striker 15 is 
halted and damage to the punch 3 or the like is avoided. 
The above explanation was made for an embodiment of the present invention 
applied to a turret punch press. However, the present invention can also 
be applied equally well to a punch press or any press machines other than 
a turret punch press in the same manner. 
Because the method and device of the present invention for protecting 
against a press overload has the above-described configuration, when, for 
example, as the result of the phenomenon of galling of a punch and a die, 
the punch is unable to descend so that an excessively high load is 
produced, and a pressure in excess of the specified pressure is 
transmitted to the pressure chamber of a press cylinder, a shear piston 
punches a shear plate as a result of this excessive pressure, and the 
shear piston releases the excess pressure through this punch transfer 
operation. Accordingly, the excessive pressure produced in the pressure 
chamber of the press cylinder is released from the pressure chamber with 
almost no delay when the shear plate of the shear piston is punched, and 
damage to the punch or the like is satisfactorily prevented. 
The excessive pressure produced in the pressure chamber is released through 
a release section which functions according to the position of the shear 
piston which has performed the punching action, so that the release 
structure for the excess pressure is extremely simple.