Hydraulic closing unit

A hydraulic closing unit has a ring-shaped hydraulic cylinder that generates a closing force, a locking bushing, an assisted drive for swivelling the locking bushing from a first angular position to a second angular position, and a pressure bar that extends through the hydraulic cylinder and the locking bushing in the axial direction. Outer teeth on the pressure bar and inner teeth in the locking bushing allow the pressure bar to be axially pushed through the locking bushing in the first angular position of the locking bushing, and an axial force to be transmitted in the second angular position of the locking bushing. The hydraulic cylinder is designed as a double action, ring-shaped compression cylinder. The piston of the hydraulic cylinder is secured against rotation and screwed by a rotary thread into the rotary locking bushing.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a hydraulic closing unit such as used, for
 instance, in injection molding machines.
 2. Related Art
 The closing unit of an injection molding machine receives the injection
 mold. It carries out the movements necessary for the closing and opening
 of the injection mold and produces the forces necessary for the locking
 and opening of the injection mold. The main components of each closing
 unit are a stationary plate on the injection side (hereinafter referred to
 as the injection plate), a movable closure plate, as well as a locking
 device. One part of the injection mold is clamped on the stationary
 injection plate while the complementary part of the injection mold is
 clamped on the movable closure plate. By locking device there is to be
 understood the device which, upon the injection, produces the necessary
 closing force for keeping the injection mold closed. Both mechanical
 locking devices with lever mechanisms and hydraulic locking devices with
 hydraulic cylinders are known. The present invention relates to a closing
 unit with hydraulic locking.
 From International Patent Application WO-A-93/16828, a closing unit for an
 injection molding machine is known in which the movable closure plate can
 be displaced between the stationary injection plate and an end plate which
 is also stationary by two displacement cylinders. The movable closure
 plate is provided with a central push rod which is guided in an annular
 insert in the stationary end plate. On this end plate a force cylinder is
 arranged which has a single-acting annular piston passed through axially
 by the push rod. A locking ring is guided in a cylindrical guide tube
 which is screwed onto the end plate. If the force cylinder is acted on by
 pressure, the annular piston of the force cylinder is advanced in the
 direction towards the movable closure plate. The front surface of the
 annular piston thereby comes against the facing end surface of the locking
 ring, the latter being pushed axially in its cylindrical guide tube in the
 direction towards the movable closure plate. In order to lock the locking
 ring on the push rod, the push rod has an outer toothing and the locking
 ring has a complementary inner toothing. The inner toothing and the outer
 toothing are divided into several rows of teeth by longitudinal grooves.
 The locking ring can be turned into first and second angular positions by
 means of a positioning cylinder. In the first angular position, the rows
 of teeth of the outer toothing can be passed through axially by
 longitudinal grooves of the inner toothing and the rows of teeth of the
 inner toothing by longitudinal grooves of the outer toothing, so that the
 push rod slides without substantial resistance through the locking ring.
 In the second angular position, the tooth of the outer toothing, on the
 other hand, can engage behind the teeth of the inner toothing. In this
 position, the locking ring is locked on the push rod. The annular piston
 of the force cylinder can exert a pressing force on the push rod via the
 locking ring and thus transmit the necessary closing force to the movable
 closure plate.
 Upon the opening of the mold, the locking ring initially remains locked to
 the push rod. The two displacement cylinders produce a short rearward
 stroke in order to open the mold. By this short rearward stroke, the
 locking ring and the annular piston are moved backward simultaneously.
 Thereupon, the lock between the push rod and the locking ring is opened so
 that the push rod can slide through the locking ring when the closure
 plate is pulled back at high speed by the two displacement cylinders.
 It may be emphasized that it would be desirable to produce the opening
 force for the opening of the injection mold also by the force cylinder.
 However, this is not possible in a closing unit in accordance with
 WO-A-93/16828 since the force cylinder is designed as a single-acting
 cylinder and furthermore it cannot be seen how a pulling force can be
 transferred in a simple manner from the piston via the turnable locking
 ring to the push rod.
 SUMMARY OF THE INVENTION
 The object of the present invention is therefore to create a closing unit
 in which the closing and opening forces are produced by the sane hydraulic
 cylinder. This object is achieved by a closing unit in accordance with
 claim 1.
 The solution in accordance with the invention consists essentially therein
 that the force cylinder is developed as a doubling annular pressure
 cylinder having a first pressure chamber for producing an closing force
 and a second pressure chamber for producing an opening force, the piston
 of this hydraulic force cylinder being secured against turning and coupled
 by a screw thread to the locking bushing. In this way, both a pressing
 force and a pulling force can be transmitted from the piston which is
 fixed against rotation via the rotatable locking bushing to the push rod,
 and thus to the closure plate. The screw thread is a simple, extremely
 compact, and low-stress solution for turnably connecting together the
 piston and the locking bushing. By turning the locking bushing from the
 first angular position into the second angular position, the locking
 bushing naturally experiences an advance X relative to the piston.
 However, this is not disturbing since this advance can even be used in an
 extremely advantageous manner in order to distribute the axial play
 between the inner toothing and the outer toothing which is necessary for
 their engagement in such a manner that inner toothing and outer toothing
 already lie substantially without play against each other prior to the
 transmission of the force.
 In order to assure a dependable engagement of the inner toothing into the
 outer toothing, a relatively large axial flank clearance should actually
 be present upon the engagement. A large flank clearance, however, also has
 substantial disadvantages. Thus, for instance, the operating stroke of the
 force cylinder is increased, and thus the consumption of energy of the
 closing unit. The flows through the force cylinder are considerably
 greater, so that the hydraulic system of the closing unit must also be
 designed larger. Furthermore, the locking bushing is relatively strongly
 accelerated upon overcoming a large flank clearance, so that the teeth of
 the inner toothing strike with great momentum against the outer toothing.
 WO-A-93/16828 proposes developing the outer toothing on the push rod and
 the inner toothings in the locking ring as a thread, the disturbing flank
 clearance, in accordance with WO-A-93/16828, being eliminated by the
 turning of the locking ring. However, this means that the toothing
 necessarily has, in the direction of rotation of the locking ring, a
 negative pitch in the direction of the force to be transmitted, and that
 furthermore, the pitch of this thread is necessarily determined by the
 flank clearance and the angle of rotation of the locking ring. In this
 way, the designer, however, is subjected to substantial constraints in
 development with respect to the toothings, which constraints, for
 instance, prevent functionally correct, load-resistant optimizing of the
 toothings in many cases.
 In accordance with the present invention, it is unimportant whether the
 teeth of the inner and outer toothings are arranged annularly, or
 helically with positive pitch or helically with negative pitch. The
 advance produced by the screw thread upon the turning of the locking
 bushing permits in each case the axial flank clearance S to be distributed
 between inner toothing and outer toothing in such a manner that, in the
 locked position between the tooth flanks which are to transmit the force,
 no substantial axial flank clearance is present any longer. If,
 furthermore, the actuator for the turning of the locking bushing is so
 designed that it can place the locking bushing into a second angular
 position both by counterclockwise rotation and by clockwise rotation, then
 the flank clearance S between the inner and outer toothings is distributed
 on the one hand on the left side and on the other hand on the right side
 depending on direction of rotation. The toothings are accordingly
 automatically free of clearance for the transmission of the closing force
 in the first direction of turning and automatically free of clearance for
 the transmission of the opening force to the closing plate in the second
 direction of turning.
 One very advantageous embodiment of the actuator will be described inter
 alia in the following detailed description.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS
 The general construction of a closing unit 10 in accordance with the
 invention will be xplained with reference to FIGS. 1 and 2, which show a
 hydraulic closing unit of an injection molding machine. An injection plate
 14 having a central injection opening 16 for the injection head of the
 injection molding machine is rigidly mounted on a base 12. Four columns 17
 connect the four corners of the injection plate 14 mechanically to the
 four corners of an end plate 24 which is mounted, also rigidly, on the
 other end of the base 12. The base 12 forms a guide bed 18 for a movable
 closure plate 20. The complementary halves of an injection mold (not
 shown) are clamped onto the injection plate 14 and the closure plate 20.
 The movable closure plate 20 is displaceable on the guide bed 18 via a
 displacement device which comprises, for instance, laterally arranged
 displacement cylinders 25. This displacement device 25 has the task of
 moving the complementary halves of the mold together and apart with
 relatively high speed by displacement of the closure plate 20 relative to
 the injection plate 14. It will be noted that the housing of the
 displacement cylinder 25 is fastened on the stationary end plate 24 so
 that the displacement cylinders 25 can have a rigid hydraulic connection
 on the fixed end plate 14.
 A push rod 22 extends from the movable closure plate 20 through the
 stationary end plate 24. The push rod 22 is rigidly fastened to the
 closure plate 20. On the stationary end plate 24, a force cylinder 26 is
 associated with the push rod 22, the housing 27 of the cylinder being
 rigidly attached to the end plate 24. In FIG. 2 and in FIG. 9 it can be
 seen that this force cylinder 26 comprises an annular piston 28. This
 annular piston 28 is secured against turning, for instance by a lengthwise
 spline 30 in a longitudinal groove. A locking bushing 34 is mechanically
 connected for rotation with the annular piston which is fixed against
 rotation. In particular, the locking bushing 34 can be coupled for turning
 via a thread 36 (hereinafter called the screw thread 36) to the annular
 piston 28; it can, for instance, be threaded by means of the screw thread
 36 into the free end of the annular piston 28. An actuator 37 permits the
 locking bushing 34 to turn in two directions and thus adjust its angular
 position relative to the push rod 22.
 An outer toothing 40 is provided on the push rod 22 along the rear section
 of the rod. The locking bushing 34 has an inner toothing 46 on its inner
 side. This outer toothing and inner toothing are developed complementary
 to each other, in such a manner that in a first angular position of the
 turnable locking bushing 34, the push rod 22 can be passed axially through
 the locking bushing 34 and that in at least one second angular position,
 the inner toothing 46 of the locking bushing 34 engages into the outer
 toothing 40 of the push rod for the transmission of an axial force. In the
 first angular position of the locking bushing 34, the push rod 22 can
 accordingly slide, without substantial resistance axially through the
 annular piston 28 of the hydraulic cylinder 26. In its second angular
 position, on the other hand, the locking bushing 34 which is screwed onto
 the annular piston 28 can transmit considerable force to the push rod 22.
 The force cylinder 26 has within its housing 27 a first pressure chamber 30
 in which the annular piston 28 forms a front-side pressure surface. If,
 after locking the locking bushing 34 onto the push rod 22, the first
 pressure chamber is placed under pressure, then the annular piston 28
 exerts a closing force on the closure plate 20 via the locking bushing 34
 which is locked on the push rod 22.
 In a second pressure chamber 31 of the housing 27, the annular piston 28
 forms a substantially smaller pressure surface as shoulder surface. If
 this second pressure chamber 31 is placed under pressure and the first
 pressure chamber relieved, the annular piston 28 exerts, via the locking
 bushing 34 locked on the push rod 22, an opening force in the direction
 opposite the closing force described above, on the closure plate 20. This
 opening force serves to open the injection mold after the molding.
 For the bringing together of the mold halves by displacement of the closure
 plate 20 by means of the displacement cylinders 25, the locking bushing 34
 is in the first angular position. In this first angular position, the push
 rod 22 slides axially through the locking bushing 34 upon the displacement
 of the closure plate 20. When the closure plate has reached its end
 position, the locking bushing 34 is locked on the push rod 22 by turning
 into the second angular position. The force cylinder 26 can now transmit
 the required closing force via the push rod 22 to the closure plate 20.
 One advantageous further embodiment of the locking means will be described
 in further detail with reference to FIGS. 2 to 8. The locking means on the
 push rod 22 advantageously comprise (see FIGS. 2 and 5) an outer toothing
 40 which is divided by longitudinal grooves 42 into, for instance, four
 axial rows of teeth 40.sub.1, 40.sub.2, 40.sub.3, 40.sub.4. In these rows
 of teeth 40.sub.1, 40.sub.2, 40.sub.3, 40.sub.4, the teeth of the outer
 toothing are arranged in each case parallel to and at equal distance from
 each other. The locking bushing 34 (see FIGS. 9 and 6) comprises a
 complementary inner toothing, which is also divided by longitudinal
 grooves 44.sub.1, 44.sub.2, 44.sub.3, 44.sub.4 into three axial rows of
 teeth 46.sub.1, 46.sub.2, 46.sub.3, 46.sub.4. The longitudinal grooves
 42.sub.i in the outer toothing of the push rod 22 are somewhat wider than
 the tooth 46.sub.i of the locking bushing 34, and the longitudinal grooves
 44.sub.i in the inner toothing of the locking bushing 34 are somewhat
 wider than the teeth 40.sub.i of the push rod 22.
 In a first angular position of the push rod, shown in FIG. 3, the teeth
 46.sub.i of the outer toothing of the rack (sic) 22 lie in the
 longitudinal grooves 44.sub.i of the locking bushing 34. In this angular
 position the push rod 22 can be pushed through the locking bushing 34, the
 teeth 40.sub.i of the outer toothing being guided by the longitudinal
 grooves 44.sub.i of the inner toothing and the teeth 46.sub.i of the inner
 toothing being guided by the longitudinal grooves 42.sub.i of the outer
 toothing. FIG. 7 shows, in a cross section along the section line A--A of
 FIG. 3, the teeth of the inner toothing in the longitudinal grooves of the
 outer toothing.
 In a second angular position--see FIG. 4--after the turning of the locking
 bushing 34 by an angle .gamma.=180.degree./n (n=number of longitudinal
 grooves or of rows of teeth), the teeth 46.sub.i of the locking bushing 34
 are located axially between the teeth 40.sub.i of the push rod 22. In this
 second angular position, therefore, the rows of teeth of the inner
 toothing engage into the rows of teeth of the outer toothing for the
 transmission of the necessary closing force.
 FIG. 8 shows a section along the section line B--B of FIG. 4. It can be
 seen that the teeth of the outer and inner toothings have a trapezoidal
 cross section. The toothings can be developed helically, i.e. the teeth
 arranged along a helical line, and the toothings accordingly form a thread
 having a pitch P. The toothings can, however, also be annular, i.e. the
 teeth can form parallel rings which are arranged in each case at a
 distance P apart (also called pitch P).
 In order that the inner toothing can engage into the outer toothing upon
 the turning of the locking bushing 34, the teeth 46.sub.i of the inner
 toothing must, of course, be axially between the teeth 40.sub.i of the
 outer toothing in the first angular position of the locking bushing 34. In
 order that small errors in position of the movable closure plate 20 do not
 prevent engagement of the inner toothing into the outer toothing, a
 relatively large axial flank clearance is desired between the inner
 toothing and the outer toothing.
 From FIG. 8 it can be seen that P=2D+S in which:
 P=pitch;
 D=average tooth width;
 S=axial flank clearance.
 In practice, it has proven to be advantageous for S to be equal to 0.5 D,
 and therefore P to be equal to 2.5 D.
 The locking bushing 34 transmits extremely high closing forces via the push
 rod 22 to the movable closure plate 20. In addition, the frequency in
 actual practice of the closings and openings is very high. The material of
 the locking bushing and of the push rod is accordingly subject to
 substantial static and dynamic loads. This can lead to permanent
 deformations of the toothings which impair the function of the locking
 device. In order to reduce the negative effects of such permanent
 deformations on the locking function, the following measures can
 advantageously be taken:
 a) The locking bushing 34 is so fastened to the piston 28 that it is under
 compressive stress upon transmission of the very high closing force to the
 push rod 22. In this way, the result is obtained that the push rod 22 and
 the locking bushing 34 are deformed similarly.
 b) The cross sections of the locking bushing 34 and the push rod 22 should
 be such that, upon transmission of the closing force, they are subjected
 to approximately the same maximum stresses, i.e. their minimum cross
 section should, if possible, be of the same size.
 c) With the same tooth geometry, the base of the teeth of the outer
 toothing should be approximately equal to the base of the teeth of the
 inner toothing so that the stress maxima at these critical places are
 approximately the same. This means, for instance, the arc length (in
 degrees) of the teeth of the outer toothing is greater than the arc length
 of the teeth of the inner toothing.
 d) The teeth of the outer toothing should be of a greater hardness than the
 teeth of the inner toothing. In addition, the flank surface of the teeth
 of the outer toothing should be larger than the flank surface of the teeth
 of the inner toothing so that an imprint of the teeth of the outer
 toothing on the softer teeth of the inner toothing is avoided.
 e) The elastic limit of the push rod 22 should be about 20% higher than the
 elastic limit of the locking bushing 34. In this way, in combination with
 measures b) and c), the result is obtained that plastic deformations
 occur, in particular, on the locking bushing 34 and less so on the push
 rod 22. Plastic deformations on the push rod 22 are far more disturbing,
 since they namely destroy the axial homogeneity of the outer toothing,
 which can lead to inaccuracies in the positioning of the closure plate 20
 if different size molds are used. Furthermore, the replacement of the push
 rod 22 is far more expensive than the replacement of the locking bushing
 34.
 It should be pointed that measures a), b) and c) of the above enumeration
 have advantageous effects on the distribution of force also in the normal
 case of elastic deformation. The elastic deformation of the locking
 bushing and the elastic deformation of the push rod are caused by these
 measures to take place in the sale direction and be of the same order of
 magnitude so that the force to be transmitted is distributed uniformly
 over all interangaging teeth of the outer and inner toothings.
 In the description of FIG. 8, it was pointed out that substantial flank
 clearance has the advantage that small inaccuracies in the positioning of
 the closure plate 20 do not prevent engagement of the inner toothing of
 the locking bushing 34 into the outer toothing of the push rod 22.
 However, a substantial axial flank clearance also has essential
 disadvantages. First of all, the relatively small force stroke of the
 piston 28 in the case of large flank clearance S increases percentually
 more, as a result of which the consumption of oil and energy of the force
 cylinder becomes greater. Secondly, with large flank clearance S, the
 locking bushing 34 is imparted a high acceleration when acted on with
 pressure by the force cylinder 28, so that the teeth of the inner toothing
 strike strongly against the teeth of the outer toothing. For this reason
 it is advantageous to reduce or eliminate the flank clearance in the
 direction of the transmission of force.
 In the present invention, the reduction in play or elimination of play in
 the direction of the transmission of force takes place automatically.
 Since the locking bushing 34 is connected via the screw thread 36 with the
 annular piston 28 and the latter is secured against turning, the locking
 bushing 34 will experience an advance X=(.gamma./345.degree.)P' relative
 to the annular piston 28 if it is turned by an angle .gamma., P' being the
 pitch of the screw thread 36.
 The turning of the locking bushing 34 is effected via a turning device 54
 which is driven by the actuator 37. This turning device 54 comprises a
 housing 56 which is, for instance, flanged onto the end plate 24. A
 toothed bushing 58 is arranged, turnable in a ball bearing 60, within the
 housing 56. The toothed bushing 58 is placed on the free and of the
 locking bushing 34 and so connected with such end via a tooth or
 spline-shaft connection that a moment of rotation is transmitted in
 form-locked manner, but that at the same time an axial displacement of the
 locking bushing 34 in the toothed bushing 58 is possible. The angular
 position of the toothed bushing, and thus the angular position of the
 locking bushing 34, can be adjusted via the actuator 37 (see also FIG. 10)
 which engages into the outer toothing 64 of the toothed bushing 58. It
 should be pointed out that a pure moment of rotation is transmitted to the
 locking bushing 34. All radial forces which act on the toothed bushing 58
 are transmitted directly by the ball bearing 60 to the housing 56. In this
 way assurance is had that the screw thread 36 is not stressed further by
 setting forces.
 FIG. 10 shows an advantageous development of an actuator 37 for the turning
 device 54. This actuator 37 comprises a rack 72, the toothing 74 of which
 can mesh in the toothed bushing of the turning device 54. The rack 72 is
 arranged in a housing tube 73. In each nd of the rack 72 there is a
 cylindrical bore 76', 76". Pistons 78', 78" are introduced, sealed off,
 into the respective cylinder bores 76', 76". These pistons 78', 78" are
 advantageously developed as plunger pistons and are flanged axially onto
 the two ends of the housing tube 73. The rack 72 is displaceable back and
 forth axially in the housing tube 73 between the two pistons 78', 78". A
 guide shoe 80 takes up the radial forces of reaction which are transmitted
 by the toothed bushing 58 to the rack 72. In FIG. 10 the rack is shown
 resting against the right-hand piston 78'.
 Both pistons 78', 78" have an axial connecting channel 82', 82" for a
 pressure fluid. Via these connecting channels 82', 82", the cylinder bores
 76', 76" can be acted on optionally with the pressure fluid behind the
 piston 78', 78" so that two oppositely acting pressure cylinders are
 developed for the displacement of the rack 72. It should be pointed out
 that the actuator is so designed that the push rod 22 can be turned in
 each case to the right and to the left out of the first angular position.
 From FIG. 9 it can be seen that both an the entrance side on the housing 56
 of the rotary drive 54 and on the housing 27 of the force cylinder, an
 axial guide device 90 for the push rod 22 is provided. Each of these guide
 devices 90 comprises four slide shoes 96. The four longitudinal grooves 42
 on the push rod 22 are developed as guide surfaces for these slide shoes
 96 and are extended beyond the rod section having the outer toothing. The
 push rod 22 is centered in the locking bushing by these two guide devices
 90.
 FIGS. 11 to 14 show various embodiments of a push rod 22 as well as various
 arrangements of the slide shoes 96 and embodiments of the guide surfaces
 for the slide shoes 96. In accordance with an embodiment, shown in FIG.
 11, which is intended primarily for closing units with relatively small
 closing force, the push rod 22 comprises two longitudinal grooves
 42.sub.1, 42.sub.2 which divide the outer toothing into two rows of teeth
 40.sub.1, 40.sub.2. The slide shoes 96.sub.1, 96.sub.2 are guided in guide
 channels in the longitudinal grooves 42.sub.1, 42.sub.2. Corresponding to
 the embodiment shown in FIG. 12, the push rod comprises three longitudinal
 grooves 42.sub.1, 42.sub.2, 42.sub.3 which divide the outer toothing into
 three rows of teeth 40.sub.1, 40.sub.2, 40.sub.3. The guide surfaces for
 the slide shoes 96.sub.1, 96.sub.2, 96.sub.3 are developed as flat
 surfaces which are at an angle of 120.degree. to each other. The
 embodiment in accordance with FIG. 13 differs from the embodiment of FIG.
 12 in the manner that the push rod 22 has four guide surfaces 42.sub.1,
 42.sub.2, 42.sub.3, 42.sub.4 which are at an angle of 90.degree. to each
 other. In accordance with FIG. 14, the outer toothing is divided by six
 longitudinal grooves into six rows of teeth; however only every second
 longitudinal groove is developed as guide surface for a slide shoe
 96.sub.1, 96.sub.2, 96.sub.3. It is self-evident that larger closing units
 require more rows of teeth and slide shoes than smaller closing units do.
 On the basis of FIGS. 15 to 20, the design of the pitch of the thread 36
 for the taking up of the axial flank clearance S will be explained in
 further detail. These figures show in each case a 135.degree. development
 of the outer and inner toothings of FIGS. 3 and 4. There can be noted two
 of the four rows of teeth of the outer toothing of the push rod 22 and one
 of the four rows of teeth of the inner toothing of the locking bushing 34.
 The teeth of the inner toothing are shown hatched. The following
 designations are used in the drawings andlor the following description:
 P: pitch of the outer toothing on the push rod 22, or of the inner toothing
 on the locking bushing 34;
 D: average tooth width;
 S: axial flank clearance between inner toothing and outer toothing;
 P': pitch of the screw thread 36 between push rod 22 and piston 28.
 FIGS. 15, 17 and 19 show the position of the inner toothing before and
 after a 45.degree. rotation of the locking bushing 34 in counterelockwise
 direction. Before the 45.degree. rotation, the teeth of the inner toothing
 lie in a first angular position in the annular grooves between the rows of
 teeth of the outer toothing. After this 45.degree. rotation in
 counterclockwise direction, the teeth of the inner toothing lie in a
 second angular position with their left flanks against the tooth of the
 outer toothing and can transmit a force to the left without play from the
 locking bushing to the push rod. FIGS. 16, 18, and 20 show the position of
 the inner toothing before and after a rotation of the locking bushing by
 an angle of 45.degree. in clockwise direction. Before the 45.degree.
 rotation, the teeth of the inner toothing lie in a first angular position
 in the longitudinal grooves between the rows of teeth of the outer
 toothing. After this 45.degree. rotation in clockwise direction, the teeth
 of the inner toothing lie in a second angular position with their right
 flank against the teeth of the outer toothing and can transmit a force to
 the right without play from the locking bushing to the push rod. For the
 designing of the pitch of the thread 36 for the taking Up of the axial
 flank clearance S, it is assumed that, in the starting position, before
 the turning of the locking bushing, the rows of teeth of the inner
 toothing are in each case angularly precisely in the center between the
 rows of teeth of the outer toothing, and that the axial flank clearance S
 between inner toothing and outer toothing is divided equally on both
 sides.
 In the general case, the pitch of the screw thread is so designed that by
 turning the locking bushing from the first angular position into the
 second angular position, the existing flank clearance S between inner and
 outer toothings is distributed unilaterally in such a manner that no
 essential flank clearance is present any longer between the tooth flanks
 which are to transmit force.
 FIGS. 15 and 16 refer to the case of an annular toothing. The pitch of the
 screw thread 36 is so designed that by turning the locking bushing from
 the first angular position into the second angular position, the advance
 of the locking bushing corresponds approximately to half of the flank
 clearance S between inner and outer toothings, i.e.:
EQU P/8=0.5 S or P'=4 S;
 for the special case that S=0.5 D, i.e. S=P/5, we have accordingly:
EQU P'=0.8 P.
 FIGS. 17 and 18 refer to the case of a helical toothing which ascends in
 direction of rotation of the locking bushing in the direction of the force
 to be transmitted. If it is assumed that the pitch P' of the screw thread
 is ascending also in the direction of rotation of the locking bushing in
 the direction of the force to be transmitted, then the advance of the
 locking bushing must correspond approximately to half of the flank
 clearance S between inner and outer toothings plus 1/8 of the pitch P of
 the toothing, i.e.:
EQU P'/8=0.5 S+P/8 or P'=4 S+P.
 For the special case of S=P/5, ie. S=0.5 D, we have accordingly:
EQU P'=1.8 P.
 FIGS. 19 and 20 refer to the case of a helical toothing which has a
 negative pitch in the direction of turning of the locking bushing in the
 direction of the force to be transmitted. Furthermore, in FIGS. 19 and 20,
 the toothing is developed with a double thread, i.e. S=0.5 P-2 D. If one
 proceeds from the basis that the pitch P' of the screw thread has a
 positive pitch, then the advance of the locking bushing must correspond
 approximately to half of the flank clearance S between inner and outer
 toothings minus one eighth of the pitch P of the toothing, i.e.:
EQU P'/8=0.5 S-P/8 or P'=4 S-P;
 for the special case of S=P/10, i.e. D=P/5, we have:
EQU P'=-0.6 P.
 The minus sign in this case means that the screw thread 36 must also have a
 negative pitch.
 FIG. 21 shows an advantageous device for optimally adapting the closing
 unit 10 to the length of the injection mold. The closure plate 20 is
 provided with a position sensor 105. A position sensor 108 is also
 associated with the piston 28 of the force cylinder 26. The position
 sensors 106 and 108 supply the input signals I1 and I2 of a digital axis
 control 106. Reference numeral 110 shows an input unit for the length "L"
 of the injection sold, i.e. the axial distance between closure plate 20
 and injection plate 14. A first control unit in the axis control 106, with
 I1 as input signal, supplies, with O1, a control signal for the control
 hydraulics 110 of the displacement cylinders 25 of the closure plate 20.
 This control hydraulics 110 positions the closure plate 20 at a distance
 "L" from the fixed injection plate 14.
 Before the turning of the locking bushing 34 from the first angular
 position into the second angular position, the teeth of the inner toothing
 of the locking bushing 34 should be positioned precisely axially between
 the teeth of the outer toothing of the push rod in order to permit the
 proper engagement of the inner toothing into the outer toothing. In order
 to make this axial positioning of the toothings possible independently of
 the length "L" set, the position of rest of the annular piston 28 as a
 function of the length "L" set is established hydraulically within a
 region [-0.5 P;+0.5 P] around a predetermined reference position. In other
 words, the locking bushing 34 is displaced axially, relative to a
 reference point, by an amount y, in which connection -0.5 P&lt;y&lt;+0.5 P. For
 this, a calaulating unit calculates the position of rest of the piston 28
 as a function of the piston of the mating plate 20 in such a manner that,
 before the engagment of the inner toothing of the locking bushing into the
 outer toothing of the push rod, the teeth of the inner toothing lie
 axially between the teeth of the outer toothing. A second control unit in
 the axis control 106, with I2 as input signal, supplies, with O2, a
 control signal for the control hydraulics 110 of the force cylinder 26.
 This control hydraulics 110 positions the piston 28 in the calculated
 position of rest. The device described makes it possible, at little
 expense, to adjust the length "L" regardless of the pitch of the inner and
 outer toothings.
 Referring to FIGS. 1 and 2, an advantageous embodiment of an ejection
 device 200 will now be described. This ejection device 200 comprises a
 base plate 202 which is mounted displaceably on the front end of the push
 rod 22 and is guided at its four corners on the four columns 17. This base
 plate 202 is displaceable by an ejection cylinder 204, which is integrated
 in the push rod 22, along this front end of the push rod 22 from a
 withdrawn position up to against the closure plate 20. It has several
 ejection pistons 206 which protrude from corresponding openings 208 (see
 FIG. 1) in the closure plate 20 when the plate 202 is displaced by the
 ejection cylinder 204 in the direction of the closure plate 20.