Tightening device for the gripping jaws of a plate stretcher

A tightening device for the gripping jaws of a plate stretcher comprises one push rod for each pair of gripping jaws sliding in guides in the gripping head in the stretching plane, and having hinged thereto two link rods which are also hinged to the gripping jaws, the gripping jaw moving synchronously rearwards for the closing operation and forwards for the opening operation, in two guides disposed in V-formation to each other. The link rods are coupled to the gripping jaws in a disengageable manner to avoid damage to the link rods and tightening device if tearing of the plate takes place during the stretching operation, or on account of any difference in the gripping of the plate by the upper and lower gripping jaw causing generation of very large but different forces on the two link rods. The link rods are in the form of angle levers each having a long and short arm pivotally connected to the hydraulically mobile push rod. Each short arm is pivotally connected to a separate hydraulic cylinder disposed on the push rod, and the long arm is releasably coupled by way of mouth-shaped openings to entrainment cams on the gripping jaws so that they can be uncoupled therefrom by swivelling.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a tightening device for the gripping jaws of a 
plate stretcher, comprising, per pair of gripping jaws, one hydraulically 
slidable push rod on which there are swivel-mounted two link rods which 
are each hinged to one gripping jaw, and these gripping jaws move 
synchronously forwards for closing and backwards for opening in two guides 
disposed in V formation relative to each other. 
2. Description of the Prior Art 
Hydraulic tightening devices are used for pressing the gripping jaws 
against the plate to be gripped, in which a hydraulic cylinder is 
connected to a push rod on which there are hinged two rods which 
themselves are connected by joint bolts to two opposing gripping jaws 
guided in guides of V formation. This is necessary in order to ensure 
synchronous sliding of the two opposing gripping jaws. This tightening 
device operates under the following conditions: 
1. If the plate becomes torn, the gripping jaws experience a reaction due 
to the torn plate, which imparts a temporary acceleration to the gripping 
jaws of more than 100 g. Because of the inertia of the mass of the 
tightening device, very high forces arise in the connection bolts, which 
can lead to damage to the hinges. 
2. It happens that the toothed gripping surfaces of the gripping jaws, and 
particularly the lower gripping jaw, become partly covered by shaved-off 
pieces of the stretched plates, thus resulting in a considerable reduction 
in the friction coefficient, i.e. the plate is gripped initially or mainly 
only by one gripping jaw, whereas the other gripping jaw remains behind or 
falls back. Thus because of the different gripping jaw movement, opposing 
high forces arise in the rods, which frequently lead to damage of the 
tightening device. 
In addition, wrinkle formation or tearing occur in the plate to be 
stretched, i.e. the plate either cannot be used or must be newly 
straightened, heat-treated and stretched. 
In order to prevent damage in these two cases, spring elements have been 
inserted into the joints between the push rods and the two link rods. By 
this means, in case 1, the mass of the accelerated rod is indeed reduced, 
but not eliminated. Experience shows that even with this solution, the 
joint bolts must be very frequently replaced due to deformation. The high 
spring force required because of the necessary pressing force and the 
large spring deflection required in case 2 considerably limit the 
effectiveness of this measure, both in case 1 and in case 2. This measure 
is also made more difficult by the fact that such devices are accessible 
only with great difficulty, and long idle periods must be assumed in the 
case of trouble. 
BRIEF SUMMARY OF THE INVENTION 
The object of the invention is therefore to provide a device in which 
1. The high acceleration forces in the rod of the tightening device are 
prevented, 
2. In the case of unequal friction coefficients at the gripping surfaces, 
the leading gripping jaw can exert no destructive force on the rod, 
3. The accessibility to the joint between the link rod and gripping jaw is 
improved, and the susceptibility to trouble in the region of the joint 
between the push rod and link rods is reduced, 
4. A device is provided which indicates when there is any lead of one 
gripping jaw relative to the other, and if the degree of lead exceeds a 
determined value, cuts out the stretching cylinder in order to prevent any 
damage to the plate to be stretched. 
This object is attained according to the invention in that in the case of 
the initially described tightening device, the link rods are connected to 
the gripping jaws in a disengageable manner. 
For this purpose, according to a further characteristic of the invention, 
the link rods are in the form of angle or crank levers with their fulcrum 
at the push rod hinge, the long lever arms of the angle levers comprising 
mouth-shaped openings at their free end, and the gripping jaws comprising 
entrainment cams which engage in the openings, and the short arms of the 
angle levers are each hinged independently of each other by way of tie 
rods to a respective hydraulic cylinder disposed on the push rod. 
Because of the fact that the link rods are in the form of angle levers, the 
long arms of the angle levers can be released from the gripping jaw 
entrainment cams by swivelling them by loading the hydraulic cylinder. By 
this means, if the plate to be stretched is ripped, the acceleration force 
is no longer transmitted from the gripping jaws to the angle lever-shaped 
link rods, and consequently to the pivot of these levers. 
According to a further characteristic of the invention, deflection plates 
are selectively disposed at the rear of the gripping jaws and/or on the 
base of the gripping head mouth. These deflection plates, which are 
replaceable, now absorb the mass of the gripping jaws which becomes 
released during the stretching operation if the plate is ripped. Because 
of the fact that the arm of the angle lever becomes uncoupled from the 
gripping jaws, the rebound velocity is much higher than in the case of the 
fixed connection between the gripping jaws and link rods which has been 
known up to the present time, in which the recoil was damped by clyinders 
by way of the fixed link rods, but serious danger to the rod and its 
joints still existed. 
According to a further characteristic of the invention, the long arms of 
the angle levers comprise flat sliding surfaces on the same side as the 
mouth-shaped openings. By this means, the entrainment cams of the gripping 
jaws can slide on these surfaces if the push rod with the unengaged angle 
levers has moved further forwards and the ends of the angle levers lie 
between the gripping jaws. 
According to a further characteristic of the invention, the mouth-shaped 
openings comprise sliding surfaces, and the angle .alpha. between the 
peripendicular to this sliding surface and the connection line between the 
mouth-shaped opening and the fulcrum of the angle lever is 
40.degree.-60.degree.. 
If one of the two gripping jaws remains behind the other on tightening and 
stretching the plate, the angle lever connected to the lagging gripping 
jaw by way of the coupling is forcefully thrust, with the other gripping 
jaw being simultaneously pulled forwards by the plate to be stretched. In 
order to enable the angle lever coupled to the forward gripping jaw to 
move aside due to the fact that it is in danger by virtue of high traction 
force, the mouth-shaped opening of the angle level is provided with a 
sliding surface in such a position that the angle .alpha. is substantially 
greater than the angle of friction .rho..sub.1. 
According to a further characteristic of the invention, the tie rods 
between the short arms of the angle levers and the hydraulic cylinders are 
provided with guide pulleys, about which there is led a cable, one end of 
which is fixed to the push rod and the other end to a spring element, the 
spring element being operationally connected to a potentiometer or a limit 
switch. 
During the build-up of the stretching force, this measuring device 
indicates any lead of one of the two gripping jaws, and by way of the 
potentiometer or limit switch issues a signal for switching off the 
stretching operation, when a determined difference in the lead between the 
gripping jaws is exceeded. This provides further protection for the plate 
to be stretched and for the tightening device.

DETAILED DESCRIPTION 
In a gripping head 1 of a plate stretcher, which is not shown in greater 
detail, there is provided a gripping head mouth 2 in which gripping jaws 
5, 6 are slidably guided in guides 3, 4. At the rear ends of the gripping 
jaws 5, 6 there are disposed striker plates 7, 8. These striker plates can 
also be provided on the base of the gripping head mouth 2. Gripping wedges 
9, 10 are provided on the gripping side of the gripping jaws 5, 6, and 
their toothed surface 11, 12 grips a plate 13 to be stretched. 
In order to slide the gripping jaws 5, 6 in their mutual V-formation guides 
3, 4 so that they grip the plate 13 to be stretched, a push rod 15 sliding 
in guides 14 is provided in the gripping head 1 along the stretching axis. 
The push rod 15 is connected by way of a piston rod 16 and piston 17 to a 
hydraulically fed sliding cylinder 18, which is fixed to the gripping head 
1. Hydraulically operated swivel cylinders 20, 21, which operate 
independently of each other, are disposed on a rear cross member 19 of the 
push rod 15, respectively above and below the stretching axis. 
A push rod hinge 22 acting as a pivot for link rods in the form of angle 
levers 23, 24 disposed on the push rod hinge, is provided at the front end 
of the push rod 15. The angle levers 23, 24 are each joined with their 
short arm by means of connecting hinges 25, 26 to tie rods 27, 28, which 
are hinged to the piston rods 20a, 21a of the swivel cylinders 20, 21. The 
angle or crank levers 23, 24 are each provided at the free end of their 
long arm, which stretch in the direction of the gripping jaws 5, 6, with a 
mouth-shaped opening 29, 30. Entrainment cams 31, 32 fixed to the rear of 
the gripping jaws 5, 6 engage in these mouth-shaped openings 29, 30. 
The sliding cylinder 18 comprises cylinder compartments 33 and 34. The 
cylinder compartments of the upper swivel cylinder 20 are indicated by the 
reference numerals 35 and 36, and the cylinder compartments of the lower 
swivel cylinder 21 are indicated by 37 and 38. 
On the same side as the mouth-shaped openings 29, 30, the long arms of the 
angle levers 23, 24 comprise flat sliding surfaces 39, 40, on which the 
entrainment cams 31, 32 can slide. The mouth-shaped openings 29, 30 also 
comprise special slide surfaces 41 and 42, on which the entrainment cams 
31, 32 under certain conditions can slide out of their engagement with the 
mouth-shaped openings 29, 30. 
The operation of the tightening device is described in detail hereinafter 
with reference to normal operation and to disturbed operation. 
The basic idea of the invention is that during the operating stage in which 
there is the danger of the plate becoming torn, i.e. when the stretching 
force F rises more than about 50% of the rated or nominal value, the 
tightening device is disengaged from the gripping jaws. 
This is attained in that the angle levers 23 and 24 are able to be 
swivelled about the push rod hinge 22 by means of separate independently 
operating cylinders 20 and 21, by way of tie rods 27 and 28 and connecting 
hinges 25 and 26, and the connecting hinges between the angle levers and 
gripping jaws are releasable in the form of entrainment cams 31 and 32 on 
one side and mouth-shaped openings 29 and 30 on the other side. The normal 
position before sliding in a plate 13 to be stretched is the coupled 
position of the angle levers 23 and 24, with the gripping jaws 5 and 6 
drawn back, i.e. the pressure in the cylinder compartments 36 and 38 of 
the swivel cylinders 20 and 21 is p.sub.1 and p.sub.2, and the pressure in 
the cylinder compartment 34 of the sliding cylinder 18 is p. 
The push rod 15 is guided in the normal manner in guides 14 in the gripping 
head 1, and is connected to the piston rod 16 of the sliding cylinder 18, 
which is fixed in its turn to the gripping head 1. The gripping jaws 5, 6 
disposed in the gripping head mouth 2 and guided in the gripping jaw 
guides 3 and 4 of hammer-head type lie by way of their deflection plates 7 
and 8 against the base of the gripping head mouth 2 (FIG. 1)(initial or 
normal position). 
After sliding in the plate 13 to be stretched, the cylinder compartment 34 
is unloaded, and the cylinder compartment 33 is loaded with a pressure p. 
The push rod 15 then moves forward, and slides the gripping jaws 5 and 6 
forward by way of the angle levers 23 and 24, by which the angle levers 
swivel slightly inwards corresponding to the V arrangement of the gripping 
jaw guides 3 and 4, and force hydraulic oil out of the cylinder 
compartments 36 and 38 by pulling on the tie rods 27 and 28, which are 
hinged to the piston rods of the swivel cylinder 20 and 21 (FIG. 2). 
By virtue of the sychronous movement of the two gripping jaws 5 and 6, the 
plate 13 lying for example on the gripping surface 12 of the lower 
gripping jaw 6 is necessarily lifted into the middle of the gripping head, 
until both gripping surfaces 11 and 12 press against the surface of the 
plate (FIG. 3). 
The pressing force produced by the sliding cylinder 18 enables the teeth of 
the gripping surfaces 11 and 12 to be forced so far into the plate 
material that when the pulling force F is subsequently induced in the 
plate 13, the frictional connection by way of the teeth of the gripping 
surfaces 11 and 12 is so great that the gripping jaws 5 and 6, initially 
supported by the sliding cylinder 18, are further drawn into the 
V-formation gripping jaw guides 3 and 4, and the teeth of the gripping 
surfaces 11 and 12 are able to press deeper into the plate. 
As the traction force F further increases, support by the sliding cylinder 
18 is no longer necessary, and the cylinder compartment 33 can be 
unloaded, after which by unloading the cylinder compartments 36 and 38 and 
loading the cylinder compartments 35 and 37 of the swivel cylinders 20 and 
21, the angle levers 23 and 24 are swivelled inwards, i.e. disengaged, the 
push rod 15 then being moved forward into its end position with the aid of 
the sliding cylinder 18 (FIG. 4). 
If before reaching the predetermined rated stretching force there occurs a 
tear in the plate, which generally takes place suddenly, the plate flies 
in the direction of the base of the gripping head opening 2 with the 
kinetic energy which was stored in the elastic deformation of the torn-off 
piece of the plate 13 before its tearing, and is received thereat (FIG. 
7). During this, the gripping jaws 5 and 6 are likewise accelerated in the 
rearward direction, until the teeth of the gripping surfaces 11 and 12 
come loose. This takes place under a stretching force F which lies only 
slightly below the predetermined stretching force, in a stage in which the 
angle levers 23 and 24 are already uncoupled, i.e. the tightening device 
remains unaffected by the reaction of the gripping jaws 5 and 6. The 
gripping jaws 5 and 6 strike with high energy against the base of the 
gripping head mouth 2. During this, they can be flung to and fro, by which 
the energy is dissipated by the deformation of the replaceable deflection 
plates 7 and 8 and the surface of the plate 13. The deflection plate can 
be fixed selectively on the gripping jaws 5 and 6 or on the base of the 
gripping head mouth 2. 
After a normal stretching operation, the stretching force is eliminated and 
the gripping head is moved partly forwards, after which the cylinder 
compartments 35 and 37 of the swivel cylinder 20 and 21 are unloaded and 
the cylinder compartments 36 and 38 loaded, i.e. the angle levers 23 and 
24 are swivelled outwards by way of the tie rods 27 and 28, whereupon 
their sliding surfaces 39 and 40 strike against the entrainment cams 31 
and 32 of the gripping jaws 5 and 6, these cams lying in an undefined 
position (FIG. 5). On subsequent withdrawal of the push rod 15 by 
switching-over the sliding cylinder 18, the mouth-shaped openings 29 and 
30 of the angle levers 23 and 24 engage independently of each other with 
the entrainment cams 31 and 32, and pull the gripping jaws further into 
their normal position (FIGS. 6 and 1). 
In the case of different penetration of the teeth of the gripping surfaces 
11 and 12 into the plate 13 during the tightening procedure on account of 
shavings from the plate material attaching to the gripping surfaces, and 
in particular the gripping surface 12 of the lower gripping jaw 6, the 
lower angle lever 24 becomes forceably pushed or even pushed back, and the 
upper gripping jaw 5 is pulled forwards through the plate 13 under 
uncontrolled high force. In order in this case to enable the angle lever 
23, which is in danger from the high force, to move away, both the 
mouth-shaped opening 29 of the angle lever 23 and the mouth-shaped opening 
30 of the lower angle lever 24 is provided with a sliding surface 41 and 
42 respectively (FIG. 8), of such a length that the angle .alpha. (FIG. 
9), between 40.degree. and 60.degree. greater than the angle of friction 
.rho..sub.1. The occurring force F.sub.A then has a maximum value of Fs 
1/L, where Fs is the traction force of the swivel cylinder 20 connected to 
the angle lever, for an overflow pressure of p.sub.1. On the other hand, 
the angle .alpha. is substantially smaller than the angle .beta. (FIG. 
10), so that when the upper gripping jaw 5 is withdrawn, the angle lever 
23 does not move away under the force F.sub.R, i.e. it does not slide away 
by way of its opening 29 from the cam 31 of the gripping jaw 5. The angle 
.beta. is the angle which lies between the direction of the force vector 
F.sub.R and the connection line between the mouth-shaped opening 29 and 
the fulcrum 22 of the angle lever 23. F.sub.R is the force vector 
associated with pulling back the gripping jaw 5, and deriving from its own 
weight G and the guide bearing force R. 
As a further measure for protecting the plate to be stretched, on each 
tightening device there is installed a measuring device (FIGS. 11a and 
11b) which during the stage in which the stretching force is being built 
up gives warning of any lead of one of the gripping jaws, and gives a 
signal for stopping the stretching operation if a determined value is 
exceeded. 
For this purpose, the tie rods 27 and 28 and push rod 15 are provided with 
guide pulleys. A traction cable 43 is fixed at one end 43a to the push rod 
15 and at the other end 43b in a spring element 44 disposed on the push 
rod 15. The spring element 44 acts on a lever 45 of a potentiometer 46. 
The traction cable 43 is guided over guide pulleys 47 and 48 mounted on 
separate tie rods, and 49 and 50 mounted on the push rod. The path taken 
by the traction cable 43 around the guide pulleys 49 and 50 is such that 
the spring element 44 and potentiometer 46 can be disposed in a position 
accessible from the outside. A limit switch can be used instead of the 
potentiometer 46 and lever 45.