DE1243114B - Device for looms to keep the chain tension constant - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. CL:

D03d

German class: 86 c-18/01

Number: 1243 114

File number: Z 7420 VII a / 86 c

Filing date: July 14, 1959

Open date: June 22, 1967

Chain discharge devices in their entirety serve the following purposes:

a) Keeping the chain tension constant when weaving the chain from the full chain beam to to the empty chain tree;

b) Guarantee of the chain discount by always the same amount of shot per shot;

c) as complete as possible compensation of the chain tension fluctuations, which arise through the movement of the shafts or through the shedding.

Keeping the warp tension constant is extremely important, both for the weaving process itself as well as for the evenness in the appearance of the fabric. If in the course of specialist training high static and especially high dynamic warp tensions during weaving occur, there is a risk of chain thread breakage extraordinarily increased, whereby the efficiency of the loom as a result of frequent downtimes in sensitive Dimensions is reduced.

A disadvantage of many known chain let-off devices is that this only supplies the chain for Submit the weft entry, but not the temporary additional warp required for each compartment opening, most of which is returned when the compartment is closed. The chain discount is applied in these devices essentially by the elasticity of the warp threads, whereby these are increasingly subjected to alternating tensile loads.

There are also known arrangements with spring-loaded string trees. These have the disadvantage that As a result of the oscillating masses, the compensation usually takes place too late, which under certain circumstances instead of the desired compensation - due to the dynamic stresses that occur - even there is an increase in the chain tension. There are also loom constructions, for example known that the changing chain need as a result of the shedding by a synchronous, with compensate the shedding oscillating drive of the backrest, whereby the additional strain the warp threads are largely eliminated. Such backrest drives, however, cause considerable structural effort and expense are mostly not able to weave the entire chain from the beginning to the End to effect full compensation.

There are various devices in the weft insertion to guarantee the chain discount Use. Depending on the type of loom or the device for looms

to keep the chain tension constant

Applicant:

Zellweger A. G. Apparatus

u. Uster machine works. Uster (Switzerland)

Representative:

Dr.-Ing. RK Löbbecke, patent attorney,
Berlin 37, Neue Str. 6

Named as inventor:

Hans Locher, Uster (Switzerland)

Claimed priority:
Switzerland from October 22, 1958

weaving material, positive or negative chain dump devices are used. The negative Chain release devices based on the principle of friction have the disadvantage that the chain tree must be brought into forced vibrations, whereby the dynamic Fluctuations are greatly increased. It is known that a suitable one acts with this negative chain drain Braking device on the chain boom. However, there is now the risk that this will only occur when Exceeding a certain chain tension due to a torque overcoming the braking torque the chain tension is rotated by a certain amount, so that the chain tension for the duration of one or more weaving cycles decreases again.

4.0 The positive chain indulgence works in such a way that the chain tree is mechanically driven to the extent that the chain required for tissue formation is automatic is subsequently delivered. For this purpose, for example, stepping mechanisms with which the chain tree is inevitably rotated by a certain amount with each shot. The positive chain drain device does not have the disadvantages of the negative-acting chain let-off device. On the other hand, in In this case, the reduction in the chain tree diameter when weaving a chain is taken into account will. Relatively complicated arrangements are required for this.

709 607/171

3 4

Chain release devices in which the chain is timed as a function of the tension due to mechanical arrangements constant compartment opening, d. H. held by the temporary chains will, all have the common additional need and chain surplus, which the permanent disadvantage, that after a loom standstill, the additional warp demand due to the increase in or after a manipulation on the warp, in which 5 weave lengths are superimposed, to be fed. It also offers the chain tension has been adjusted arbitrarily, the electrical-mechanical control system (e.g. German several shots, possibly ten or even patent specification 945 098) the possibility of one more more, are required until the chain tension fold means to change the chain tension, such as by the mechanical regulating elements again on this, for example when the loom is at a standstill the original or the new stable Mit- io is required, and the operating value is the chain value is brought. The consequence of this is that this tension already resumes with the first weft insertion first fabric strips to set the target value as a result of changed spacing, or the chain will with each weft inserted weft threads in the randomly let off until sagging and again the entire tissue image is clearly visible. Tighten this.

so-called Anwebstellen occur depending on the type of tissue, 15 In the drawing is the subject of the invention

Material, binding, etc. after each weaving machine shown on the basis of several exemplary embodiments,

stand still and affect the fabric quality in the it shows

bad sense. Certain already known systems F i g. 1 schematically shows the essential parts of a

of chain discharge devices represent a closed loom with the device according to the invention

this control loop, in which a mechanical 20 device,

Measured size and then again for the in fi gure. 2 a measuring element with its path-voltage position a setpoint of the same mechanical diagram,

Size is used. Since such mechanical advantages. 3 a first variant of the measuring element with directions can have no amplification, its path-voltage diagram,
the regulation of the setpoint follows at the expense of 35 F i g. 4 and 5 show the relationship between onetime or the positioning accuracy. output signal and output signal each one electrical

The invention is based on the task of providing amplifier,

a device for looms for constant- F i g. 6 is a schematic representation of an electrical chain tensioner in such a way that electric-mechanical transducers and
probably an automatic chain releasing device for 30 F i g. 7 is a diagram showing the time course of the weft insertion from a backrest tree machine pivoted against the action of the shaft movement and the warp tension of a weaving spring during a full weaving cycle,
is controlled, as well as a device for dec- In F i g. 1, the essential parts of the additional warp requirement due to the shedding of a loom known per se are shown schematically. In is provided. 35 a frame 27, a chain tree 1 is mounted from

In the case of a device of the type mentioned above, which is a chain 2, which is achieved via a backrest 5 essentially in that it is tensioned and drawn into the shafts 18, this is carried out for the scanning which is lost due to the chain tension. The shafts 18 create a movement of the backrest which is formed by a compartment 19, in the front compartment 19 'of which the chain 2 for the known electrical-mechanical control circuit is woven in advance. Next, the chain 2 can be seen, consisting of a chest tree 23 and around a goods peeling tree 24

a) a measuring element, in which the movement of the is placed, from which a tree of goods 26 is the ver-string tree in a first electrical quantity wove chain 2 accommodates. The loom has formed, with one of the setpoint value of the chain, also a main drive motor 29, which a voltage corresponding, adjustable second 45 crankshaft 20 drives. Move their cranks electrical variable, for the generation of which a sley 22 in a known voltage source is generated by means of the connecting rod 21 back and forth with an adjusting element of the previous type. An eccentric shaft 14 is of can be seen, compared and if there is a discrepancy between the crankshaft 20 by gears, which in the two electrical quantities from each other a F i g. 1 are not shown, driven and used third electrical quantity is formed, 50 movement of the shafts 18.

b) an electrical amplifier for the amplifying machine and all the equipment for the operator third electrical quantity and movement thereof are also not shown, since

c) an electro-mechanical converter for explaining the invention in detail Reshaping the third reinforced electrical is not required.

Size in rotational movements of the chain tree 55 The device according to the invention is based on the

of appropriate size and direction, closed loop principle in which

and that in addition one of the weaving chain tension P is provided at a suitable point between

Machine forcibly controlled impulse generators see the chain tree 1 and the goods tree 26

direction, measured by the third electrical variable and in a measuring element 9 in an electrical

additional electrical impulses of predetermined size 5o size is transformed. This electrical quantity becomes

and polarity and with adjustable and changeable amplified in an amplifier 40 and then one

Advance or delay to the timely correct electrical-mechanical converter 10 is supplied. The-

Introduction of the shedding caused by the ser electro-mechanical converter 10 calls for example

A change in the direction of the additional chain requirement indicates a movement of the chain tree 1, where-

are storable. 65 is more or less tensioned by the chain 2,

Compared to the known mechanical control loops, there is always a setpoint value for the chain tension P

sen (ζ. B. German patent specification 869 324) enables automatic adjustment. With a movement of the prank

the device according to the invention in the control loop tree 5 could theoretically do the same thing, for example

Result can be achieved. Due to the sufficiently large amplification possibility of the electrical variable the setting accuracy and the reaction speed of the control process are so high be that in each case the chain tension P is kept constant within very narrow limits.

Since the chain boom 1 is to be stored as low as possible due to its high weight, the backrest 5 is necessary, which, as is known, serves to guide the chain 2 from the chain boom 1 to the level of the shafts 18. The backrest 5 must therefore absorb the entire force which is caused by the change in direction of the chain. It can therefore be designed in such a way that it is pressed against an elastic element, for example against a spring 8, under the influence of the horizontal component P of the chain tension P ' acting entirely on it.

The use of the chain tension P acting in the horizontal direction as a component of the total chain tension P ' acting on the backrest 5 is therefore preferred to other options because at the location of the weft entry and the reed stop, i.e. in the front compartment 19', only the chain tension P is decisive for the formation of the fabric is. The total chain tension P ' acting on the backrest 5 does not exactly correspond to the horizontally acting chain tension P due to the friction of the chain 2 on the backrest 5.

Changes in the chain tension P change the spring pressure of the spring 8 and result in a change in the position of the backrest 5 relative to the frame 27. For this reason, the backrest 5 is rotatably supported in backrest arms 6 in the bearings 7. The springs 8 acting on both sides of the backrest arms 6 give the backrest S a specific position for each chain tension, which position is given by the force-displacement diagram of the springs 8 mentioned.

The position of the backrest 5 is transmitted to the measuring element 9, which forms a first electrical quantity U _{1 corresponding to the above-mentioned position at every moment.} This can be done, for example, by an inductance that can be influenced, consisting of a fixed yoke 42 and a movable armature 41. Both parts together form a magnetic circuit with a variable air gap. The yoke 42 carries an electrical coil through which an alternating current flows. This creates a magnetic flux in the magnetic circuit. A displacement of the armature 41 by the distance 5 (see FIG. 2) causes a change in the air gap and thus a change in the inductance. The alternating current J _n flowing through the coil of the yoke 41 changes its amplitude, as a result of which voltage changes occur on the coil. In Fig. 2 shows a corresponding arrangement. The armature 41 is force-locked against the backrest arm 6 by a return spring 39. The alternating voltage occurring at the coil of the yoke 42 is fed to a rectifier 44 with associated filter means 48, which rectifies this alternating voltage and supplies a first electrical variable U ₁ , which thus also corresponds to the chain voltage P.

If both the relationship between the first electrical variable U ₁ and the position of the backrest 5 and the spring diagram of the spring 8 - spring travel as a function of the spring force - are known, the first electrical variable CZ ₁ can be expressed directly in units of force of the chain tension P, for example in kilograms. The movably mounted backrest 5, which is supported against springs 8, is very suitable for this measurement. The measuring element 9 can, however, also act on an organ 5 'which is pressed against the chain 2 at any point on the chain 2 between the chain tree 1 and the goods tree 26. The setpoint of the

ίο Chain tension P is also shown as an electrical quantity. A voltage source 34 with an adjusting element 38 in the form of a potentiometer supplies a second electrical variable U ₂ , the magnitude of which corresponds to the setpoint value of the chain voltage P on the Sireichbaum 5. With the aid of the potentiometer 38, this setpoint can be set within the desired limits. The voltage source 34 is now, for example, with its negative Po! the more negative. Pole of rectifier 44. Thus, the difference between the first electrical variable U ₁ and the second electrical variable U ₂ , namely a third electrical variable U ₃ , occurs between the positive pole of the rectifier 44 and the tap of the potentiometer 38. This third electrical variable U ₃ can thus be positive, zero or negative, depending on the magnitude of the first electrical variable EZ ₁ compared to the second electrical variable U ₂ . In the diagram of FIG. 2 shows the course of the first electrical variable U ₁ as a function of the displacement of the backrest tree + S , as well as the second electrical variable EZ ₂ , which is a straight line parallel to the abscissa for a certain position of the potentiometer 38. Their difference, the third electrical variable CZ ₃ , has a profile that is shifted parallel to the first electrical variable EZ _1. A change in the second electrical variable U ₂ by means of the potentiometer 38 by the amount J CZ ₂ changes the distance between the first electrical variable CZ ₁ and the third electrical variable CZ ₃ by Δ U ₃ in the diagram, so that the

«To curve EZ ₃ + A EZ ₃ cuts off a distance Δ S on the abscissa +5. Such a distance J 5, however, means a change in position of the backrest tree 5, which change in position corresponds to the new setpoint value of the chain tension P given _{by the changed second electrical variable CZ 2} + Δ U _2.

In Fig. 3 shows a variant of the measuring element 9, which has features similar to that of FIG. 2. It consists of a differential transformer with a fixed yoke 42 and a movable armature 41. The fixed yoke 42 contains two electrical primary coils connected in series which a current / ^ flows, and also two galvanically separated secondary coils. Each of these secondary coils is connected to a rectifier 44, 47. As long as the movable armature 41 is in the middle between the two secondary coils, the voltages induced in the secondary coils are the same. After rectification in the rectifiers 44, 47 has taken place, the first electrical variable U ₁ and the second electrical variable EZ ₂ are therefore also equal to one another, and the third electrical variable EZ ₃ is zero. If the movable armature 41 is now moved together with the backrest 5, for example from the center to the right, the ratio of the air gaps in the fixed yoke 42 changes, as a result of which, for example, the first electrical variable CZ ₁ increases and the second electrical variable CZ ₂ becomes smaller. the

third electrical variable U ₃ thus assumes a positive value. If the movable armature 41 is shifted to the left, the third electrical variable U ₃ becomes negative.

The mutual dependence of the first electrical variable U ₁ and the second electrical variable U ₂ makes it necessary that the setpoint value of the chain tension P is made adjustable in that the fixed yoke 42 of the measuring element 9 in the frame 27 of the loom with relatively simple means, for example by means of an adjusting screw, can be shifted in the direction of movement of the movable armature 41. This possibility of displacement is advantageously carried out in such a way that, in a known force-displacement diagram of the springs 8, certain positions of the measuring element 9 are provided for certain mean chain tensions P.

The third electrical variable U _a then passes through a network 30 with a potentiometer 31, a capacitor arrangement 32 and a resistor 33. With the aid of the potentiometer 31, the size of the input signal U ₃ ′ supplied to a subsequent electrical amplifier 40 can be adjusted. The capacitor arrangement 32 contains a number of capacitor stages with the aid of which the correct phase of the input signal U ₃ ' obtained from the third electrical variable U ₃ for the amplifier 40 can be set according to the working conditions of the loom on the one hand and the device according to the invention on the other. The adjustment of the phase of the input signal U ₃ is necessary because the reciprocal effect between shaft movement and weft insertion on the one hand and the warp requirement on the other hand shows small differences in time. With the help of the phase setting on the capacitor arrangement 32, these time differences can be compensated, which ensures the optimal effect of the chain tension regulation.

F i g. 4 and 5 show the relationship between the input signal U ₃ and the amplified signal U _i of a specially designed electrical amplifier 40. In FIG. 4, the amplified signal U _{4 is} directly proportional to the input signal U ₃ '. In Fig. 5, the characteristic of the electrical amplifier 40 behaves in such a way that when a positive input signal U _{3 occurs,} an amplified signal f / ₄ 'is applied to a first output terminal pair and when a negative input signal Uζ occurs, an amplified signal Z7 ₄ "is applied to a second output terminal pair, while the amplified signals U [, U " in the case of an input signal U ₃ = O are only very small and equal to one another.

The amplified signal U ₄ is fed to the electrical-mechanical converter 10. An example of such an electrical-mechanical converter 10 is shown in FIG. 6. Mechanical power is supplied to this. The same is either taken from the main drive motor 29 of the weaving machine or a separate motor drive with constant speed is provided for this purpose. An output shaft 67 of the electrical-mechanical converter 10 acts with its revolutions n _e in both directions of rotation via a worm gear 3, 4 on the chain tree 1 and thus releases the chain depending on the speed and direction of rotation or it winds up the chain.

The electrical-mechanical converter 10 is controlled by the electrical signal U ₄ , U ₄ " by means of electrically controllable brakes 11, 12 with rotors 1Γ, 12 '. A gear 61 connected to a drive wheel 13 drives a differential wheel 62 of a first epicyclic gear 60 a cage 63 and a planet gear 64 with an essentially constant speed Zt _1. On the left side of the cage 63 is the drive shaft 67 with a sun gear 65. The output shaft 67 is connected to the worm gear 3, via a further reduction gear 71, 72. 4. On the right side of the cage 63 is a second output 68 rotating at a speed n ₆ with a sun gear 66 and a gear 69. A second epicyclic gear 50 contains a differential gear 52 which is driven at an essentially constant speed ny The same drive source as for the first epicyclic gear 60 or a further motor can be used for this earth. The differential gear 52 of the second epicyclic gear 50 contains a cage 53 and a planet gear 54. On the left side of the cage 53 there is a first output 57 rotating at a speed n _s with a sun gear 56, on which output 57 the first electrically controllable brake 11 attacks. The second electrically controllable brake 12 is firmly connected to an output shaft 58 rotating at a speed n _% , a gear 59 and a sun gear 55. The gear 59 of the second epicyclic gear 50 and the gear 69 of the first epicyclic gear 60 mesh with one another and form an intermediate gear 70. The rotors 11 'and 12' of the electrically controllable brakes 11, 12 can be adjusted by applying a suitable variable voltage i7 ₄ ', U ₄ "are continuously braked in their speed to a standstill. The combination of the epicyclic gears 50 and 60 and the electrically controllable brakes 11 and 12 result, provided that the speeds n ₂ and / Z _{1 are} equal and opposite and the speed« ₅ = -2n _{4 is} twice as large and also opposite to n ₄ , the speeds and directions of rotation compiled in the table below (in particular for the output shaft 67).

Brake U open, brake 12 to brakes 11 and 12 open

Brake 12 open.
Brake 11 closed

Differential gear 62

Differential gear 52

Output 57 (rotor 11 '): « ₃ =

Output 58 (rotor 12 '): n ₄ =

Output 68: n ₅ =

Output 67: n "-

In ₂ = -2 / Z ₁ 0
0

"i

«2 =

n ₂ =

-In ₂ =

"ι
/ I ₂ ⁼ - / I ₁

2 / ζ ₂ = -2 / Z ₁
- 4 / ΐ ₂ = + 4 / Z ₁
-2 / line ₁

9 10

By only partial excitation of the electrical control an arbitrary one; Such a division is, however, the brakes 11, 12 which can be used at any speed at least approximately in most shaft drives between the values +2 n _x and -In ₁ at the Ab-.

drive shaft 67 of the first epicyclic gear 60. The chain, which varies with the shaft change, is adjusted. The speed ratio H _1: n ₂ is the differential voltage P 5 also affects the measuring element 9, which rentialräder 62 and 52 can also be different from a result in the manner described above the Ketten-- 1 be chosen, provided that at the same time the rotary Bauml to. reciprocating rotary movement number ratio n ₄ : n _{5 of} the intermediate gear 70 by which - and thus the same amount is changed for draining and tightening. In this way chain - induced. It is obvious, however, that the rotors 1Γ, 12 'of the electrical control unit can be driven to cover the additional chain demand controlled by the chain tension P brakes 11, 12 at high speeds. As a result, you need - as a result, can not work properly, since the chain drain in the small torques ■ - ■ only small increased sequence of the inertia of the moving parts signals t / ₄ to brake them to a standstill. It only begins to act when the chain converter 10 already has a significant increase in chain transducer 10 in the manner described, has caused counter-tension. Likewise - in relation to other possible types of electrical-mecha-sequence of inertia - a chain suit would only frill converters, a number of different advantages would arise if the chain tension was already on. When electrically controllable brakes 11, 12 can reach a value that is far too low. The phase shifter, for example, purely electrically acting brakes, 20 in network 30, which electrically influences the third electrical variable electrically-magnetically acting brakes, can trigger the reversal of the mechanically acting brakes or electrical chain tree 1, which always acts hydraulically before the occurrence Brakes are used. a change in chain tension is required, in many cases the use can only be fulfilled imperfectly. Magnetic particle braking as an electrically controllable braking phase shifter can namely, quite generally, 11, 12. These are characterized by the fact that they speak, only periodically have very small rotating masses in the steady state, so that processes can be driven a time advance of the output variable at a high speed n ₃ , « ₄ compared to the input variable applied to it for their excitement to act to generate one. When an input-specific braking torque is applied for the first time, only a small electrical magnitude is applied to a phase shifter, its output powers are required. Such small electrical magnitudes still deviate from the static power for a few periods, in turn, resulting in a simple general mode of operation. In the present case construction of the electrical amplifier 40. The small one corresponds to a weaving cycle, which includes a chain draining rotating masses, furthermore a high self-tensioning and a chain suiting, a single peripheral frequency of the entire electro-mechanical 35 or the steady state. Converter 10, whereby the required very fast. A loom with the previously used

and only very little delayed movement changing device for chain tension regulation of the chain tree 1 can only be realized. tion switched on again after a standstill, see above

It is known that for most weaving machines, the setpoint of the warp tension P must already be advantageous if their warp release device does not reach the moment of the first reed stop, which only provides the movement of the sley 22 through only the warp requirement for the weft insertion, but in each case also the preconditioned by the shedding that the chain beam 1 already covers the additional chain demand that arises over the course of time. Sequence of the first weaving cycle, ie before the first This cover of the additional chain demand for the specialist period of the chain tension changes, in truth formation occurs with every shaft change and includes 45 term rotation and around the correct amount one chain reduction and one chain suit per must be controlled. The phase shifter in the net weaving cycle. For the size and point in time of the related mechanism 30, however, according to what has been said in the first mole of the chain reduction component, the characteristic is not at all able to take into account a correct movement of the shafts 18 over time. leading input signal U ₃ ' to the electrical F i g. At the top, FIG. 7 shows the time output 50 amplifier 40 as a diagram. For the operation of the shaft stroke H, in the middle the course of the loom, however, the correct chain tension P as a function of time is particularly important, in the event that the chain tension P is at the moment of the application, that no or no sufficient chain slack occurs before first weft entry, from the decisive hand. At point A are the shafts 18 of importance.

in closed shed position, in point B already in 55 To cover the additional chain demand as a result of the open shed. In this position they always remain at the correct point in time until the shedding is in point C, where the shaft change begins again to guide, therefore in the device according to the invention through the closed shed position D into the next tang, a pulse generator device 14 to 17 is in front of the open shed position E-i ^{7 to} switch over. The chain - seen, which the acceleration of the chain tree 1 voltage P runs accordingly from a mini 60 in strict dependence on the shaft movement so malwert in the point to its maximum while controls that the chain drain when opening the compartment of the time interval BC, falls during the Compartment 19 or the chain suit when closing the compartment alternately to a minimum at point D and 19 has already started. This pulse generator reset to the maximum value during the time interval consists in two switches 16, 17 ( FIG. 1), vails EF. The time intervals ABCDEFG which are actuated by a cam 15 speak in this case to the movement characteristics of the cam 15 with which the shaft mechanism is driven. The 18 controlling eccentric shaft 14 made in FIG. 7 is connected. The division of a shaft cycle into three times 120 ° is switches 16 and 17 are in a circuit with

Claims (1)

a direct current source 46 and a potentiometer 45. By closing the switches 16, 17 electrical pulses U5 ', U5 "are given to the amplifier 40. The amplitude of these pulses U5, U5" is such that they represent the third electrical variable Us or the input signal U3 at the amplifier 40 predominate under all circumstances. Their duration τ and the temporal moments of their occurrence are chosen so that the electrical-mechanical converter 10 pushes the chain tree 1 in one or the other direction of rotation so far that the chain drain or the chain suit in points A and C respectively to the correct one Time and with the correct amplitude. The further control of the chain tree 1 from the decay of the previous pulse until the arrival of the next pulse is left to the third electrical variable U3 corresponding to the chain voltage P. The use of the pulses U5 ', U5 "takes place by the lead angle φ in front of points A and C, so that the speeds of the masses involved in the chain tree rotation in points Λ, C, D etc. have already reached the required values Interval of AC 240 ° and the interval of CD is 120 °, the switches 16, 17 are to be attached offset by the same angular amounts with respect to the cam 15. This means that the switches 16, 17 are actuated by the cam 15 one after the other In cases in which the time sequence of the shaft movement per weaving cycle is not divided into three times 120 °, it is necessary that the switches 16, 17 are also mounted in a different angular position executes, the cam 15 contains two lugs offset by 180 °, and the switches 16, 17 are only offset from one another by = 60 °. The lead angle φ can thereby be made adjustable, that the cam 15 can be fixed in any position relative to the eccentric shaft 14. The size of the lead angle φ must be determined experimentally for each loom. The mode of operation of the loom requires that, in the event of loom stoppages, the warp tension P is reduced to a lower value than is necessary for weaving. For this purpose, there is a switch 35 (FIG. 3) in the circuit of the second electrical variable U2, which switch 35 is connected to the parking mechanism of the loom. A potentiometer 38 enables the amount of lowering of the chain tension P to be set. The second electrical variable U.2 and thus the setpoint value of the chain tension P is reduced when the switch 35 is closed, so that the first electrical variable Ux also assumes a smaller value as a result of a decreasing chain tension P. When the loom starts up again, the switch 35 is first opened, so that the second electrical variable U2 and thus the setpoint value of the chain tension P again assume the original value. As a result, the chain tree 1 is rotated so far that the first electrical variable U1 also assumes the original value. This chain tightening when the loom drive is switched on again has to be carried out so quickly that the correct chain tension P is restored by the sley 22 at the latest when the reed is struck for the first time. Another requirement for the operation of the loom is that the weaver must be able to lower the warp 2 so far that not only the warp tension P assumes the value zero, but that the warp 2 sags. For this purpose, there is a manually operated switch 36 in the manner of a step switch 36 ', 36 "with three switch positions in the circuit of the first electrical variable U1 and in the circuit of the amplified signal t / 4. The switch part is in a first of the three positions 36 'open, the switch part 36 "closed, and these do not affect the operation of the electrically controlled chain lowering device. In a second position of the step switch 36, by closing the switch part 36 ', such a first electrical variable U1 is formed that it is not compensated for by any value of the second electrical variable U.2. As a result, a third electrical variable Us different from zero is always present at the electrical amplifier 40, so that the electrical-mechanical converter 10 receives an amplified signal U1 as long as the switch 36 is in the second position. If the chain is lowered far enough, the weaver can cut off the amplified signal Ui from the electro-mechanical converter 10 by switching on a third position of the step switch 36, ie by opening the switch part 36 ″, so that the chain slack is stopped and the chain slack This is maintained as long as the step switch 36 is left in the third position. Before weaving any further, the weaver must now return the step switch 36 to the first position, which restores the correct setpoint of the chain tension F and the chain beam 1 the deflated one Chain 2. The step switch 36 is advantageously designed in such a way that the first position is its rest position, while the second position must be held by hand as long as the chain descent is to last 36 useful for whether the warp tension required for weaving is available or n ot. Anything that goes beyond the content of the claims in the description is not the subject of the invention. Patent claims: 1. Apparatus for weaving machines to keep the chain tension constant with an automatic chain releasing device for the weft entry, which is controllable by a backrest pivoted against the action of a spring, and a device to cover the additional chain demand due to the shedding, characterized in that for scanning the a known electrical-mechanical control circuit is provided, consisting of a) a measuring element (9) in which the movement of the backrest (5) is converted into a first electrical variable (U ₁ ) , with an adjustable second electrical variable (U ₂ ) corresponding to the setpoint value of the chain tension, for the generation of which a voltage source (34) with an adjusting element (38) is provided