DE102008045847A1 - Cogwheel has wheel hub and sprocket with running toothing, and connection of hub and sprocket occurs by catch mechanism which fixes sprocket in direction opposite to hub, where part of catch mechanism forms form-fit connection - Google Patents

Abstract

The cogwheel (01) has a wheel hub (02) and a sprocket (03) with a running toothing (04). The connection of the hub and the sprocket occurs by a catch mechanism which fixes the sprocket in the direction opposite to the hub. A part of the catch mechanism forms a form-fit connection in a peripheral direction between the hub and the sprocket. A pass toothing (07) is arranged on an outer surface (05) of the hub facing the sprocket, where pass toothing is arranged on an inner surface (06) of the sprocket facing the outer surface.

Description

The The invention relates to a gear according to the preamble of claim 1 or 3.

One Gear consists essentially of a can be arranged on a shaft or arranged wheel hub with a circular outer contour and one arranged over the circumference, as a running gear designated toothing.

at Spur wheels and screws, as well as partially with worm wheels the circular outer contour is cylindrical. The toothing is arranged on the front side of the wheel hub on the circumference of the cylinder.

at Bevel gears is the circular outer contour conical. The toothing is on the lateral surface arranged the cone.

at Crown wheels is the circular outer contour cylindrical. The gearing is not like the spur gears at the front, but arranged on the wheel surface.

at Worm wheels can be the circular outer contour in addition to the shape of a cylinder also in the form of a single-walled hyperboloid exhibit. The toothing is on the circumference on the lateral surface of the Hyperboloids arranged.

gears are often integrally formed from one material shaped. But there are also so-called composite gears known, which consists of two or more firmly interconnected Elements made of different materials.

By the EP 0 751 246 A1 a gear is known, consisting of a separately prepared base body made of plastic, on the circumference of which an outer toothing forming band of wear-resistant material, for example of metal, is fixed by means of an adhesive layer. The main body consists of a hub, which is connected by means of a disc provided with recesses with a rim, which is provided at its periphery with a toothing. The band covers the body in such a way that it largely covers the rim in the area of the teeth. The gear fulfills the tasks of having as possible by molding or spraying of the body due to molding error.

By the DE 698 04 263 T2 is a gear with a hub and a hub connected to the ring gear known. The wheel hub consists of a sandwich-type intermediate arrangement of a disc of a high-resistance, lightweight material between two discs of carbon or glass fiber or a metallic film. The ring gear is formed by a circular metallic toothing, which is applied along the circumference of the disc made of a high-resistance, lightweight material. The connection between the discs as well as the connection with the metallic teeth is ensured by a synthetic resin. The gear satisfies the contrasting demands for resistance to wear, low weight, dimensional stability and high rigidity.

By the DE 24 38 141 A1 is a gear with a hub made of metal and a ring gear made of plastic known. This gear has the noise behavior and the damping properties own a plastic gear. At the same time its hub sits securely on the shaft and can accommodate a feather key. In addition, there is no thermal expansion difference between shaft and hub.

By the DE 101 27 224 A1 a gear is known, with a metal disc having a central opening for a shaft and with an annular plastic element, which is arranged on the circumference of the metal disc with this force and positively connected. The plastic element has a molded sprocket and is cast on the metal disc. The gear has a low weight and is inexpensive to produce.

By the DE 101 49 759 A1 is a plastic gear with an injected bushing for mounting on a shaft known.

By the DE 697 04 941 T2 is a gear known, consisting of a rigid body portion of z. B. composed of a plurality of metal plates or discs having teeth on its circumference. A first layer of a thermoplastic material, such as. Nylon, is disposed on a first side of the rigid body portion and a second layer of the thermoplastic material on an opposite side. Both the first and the second layer each have at their periphery thermoplastic tooth portions adjacent to the teeth of the body portion. The gear reduced compared to other composite gears, the rolling or gear noise but allows under emergency conditions, the inclusion of high load torque without damage.

adversely on the gears of the prior art is the Low load capacity on the one hand and their high tool technical Effort to maintain low tolerances, as well as their low Endurance load capacity and its sometimes difficult production on the other hand.

When An object of the invention can therefore be considered a To develop gear, which requires a low production cost has, has a low noise when rolling, and which is highly resilient.

The The problem is solved with the features of claim 1 or with the features of claim 3.

to Solution of the problem is therefore a gear with a hub and one connected to the wheel hub, separately producible or manufactured annular ring gear with a preferably integral running toothing provided, d. H. with a running gear the z. B. by incorporation, such as by subsequent Milling or by injection molding or the like one-piece component of this ring gear or ring is. The wheel hub can be made of a first, preferably easy to process material consist, preferably of a plastic. The separately manufacturable or produced ring gear or ring consists of a heavy duty, wear-resistant second material, for example a high-quality, wear-resistant, z. B. fiber reinforced Plastic or a metal.

According to one The first subject of the invention is the connection of wheel hub and sprocket via a latch, which the sprocket fixed in the axial direction relative to the wheel hub, wherein at least a part of the catch simultaneously a form-fitting, in particular play-free connection in the circumferential direction between the wheel hub and sprocket forms.

According to one second object of the invention is between the sprocket and the wheel hub, more precisely on a bearing the running gear teeth Sprocket facing outer surface of the wheel hub, and on one of these outer surface facing Inner surface of the sprocket one as spline, Serration or splined as a splined shaft profile arranged.

The Gearing consists of one on the sprocket facing Outside surface of the wheel hub arranged and a first part of the spline forming external hub teeth, and one facing the outer surface of this Inner surface of the ring gear arranged and with the hub outer teeth corresponding, the second part of the splines forming Ring gear internal gear.

advantages The invention over the prior art arise among other things, that by the separately producible or manufactured and the ring gear forming ring of wear-resistant more expensive material in which incorporated the running gear is, a high load capacity of each tooth of the running gear especially compared to gears with a running gear from less wear-resistant and therefore cheaper Plastic or a running gear with by means of a metal sheet covered teeth made of plastic is. Furthermore, by the running gear bearing ring and the arranged between the ring and the hub gear teeth over the wheel hub weiterzuleitendes torque all teeth of the splines over the circumference the wheel hub is distributed so that it also in the area of the splines too no overuse of the individual teeth of the Gearing comes. In addition, the spline teeth between wheel hub and ring a higher load capacity, as for example a simple adhesive bond between cylindrical Surfaces. In addition, in the inventive Gear wheel hub can be easily manufactured compared to the known from the prior art sandwich-type wheel hubs.

Further advantages of the invention will become apparent, for example, by a high functionality by the modular system resulting from the separate production of wheel hub and sprocket while high load capacity of the connection between the wheel hub and sprocket. Such a modular principle or such a modular solution is often referred to as a so-called mechatronic system solution. Compared to the prior art, there is an increase. The durability and strength, avoiding flutter, since both the hub, and the sprocket can be produced with the optimal manufacturing process at low cost. Furthermore, results from the now high-strength tooth flanks of the running gear an improvement in efficiency. Compared to also a running gear of high-strength or high-strength and wear-resistant material results in addition z. B. by the very easily executable wheel hub a significant weight saving. According to the prior art, gears with a ring gear made of high-strength and wear-resistant material have a wheel hub made in sandwich technology including metal, which is considerably heavier than in a gear according to the invention. Furthermore, in comparison with the prior art, taking into account a comparable load capacity, a volume saving is achieved for the highly resilient and wear-resistant material required for producing a gear according to the invention. Due to the optimal separate production possibility of sprocket and hub also results in a significant improvement in quality at least the running teeth over the prior art technology.

According to one advantageous embodiment of the invention is between the wheel hub and Sprocket arranged a layer of an elastically deformable material. As a result, on the one hand finds a noise insulation in operation of the gear, and on the other hand, a start-up and / or Abbremsruckdämpfung between the hub and the sprocket instead of.

According to one Another advantageous embodiment of the invention are at least in the circumferential direction between the teeth of the spline elements arranged from elastically deformable material. As a result finds at least one start-up and / or Abbremsruckdämpfung between the wheel hub and the ring gear instead.

The Wheel hub and the ring gear can be glued together or be welded together, z. B. by ultrasonic welding or laser welding or with a heating wire.

alternative or in addition to this, the wheel hub and sprocket are preferably by a latching in the axial direction with each other connected.

The Locking can be arranged from in the axial direction on the wheel hub Latching noses and arranged in the sprocket recesses, which recesses with the locking lugs corresponding Hintergreifungen exhibit.

The Locking can also be at least in the area of the teeth the splines on the wheel hub and / or on the sprocket a groove, and at least in the area of the interdental spaces of the splines on the sprocket and / or on the wheel hub corresponding to the groove Include footbridge. There are various embodiments conceivable. For example, the latching a circumferential groove on a first surface of the splines, for example on the inside of the ring gear, and a circumferential, protruding Web on a second surface of the splines, for example at the wheel hub outer surface. Likewise is conceivable that, for example, only certain teeth of the spline of the ring gear and / or the hub in the circumferential direction z. B. in the middle have a groove, and corresponding gaps the splines of the wheel hub and / or the ring gear in the circumferential direction z. B. have a web in the middle.

The Locking can continue a first, on the outer circumference of the wheel hub arranged circumferential groove, a second, on the inner circumference of the sprocket arranged circumferential groove, as well as an insertable in the two grooves Include spring washer.

The Wheel hub is preferably made of a plastic. This saves weight and is sufficient thanks to the splines between the sprocket and Wheel hub to be transmitted torques.

Of the Sprocket is preferably made of a plastic or a Metal.

Of the Plastic can fillers and support inserts, such as As metal rings, long glass fibers, or other the strength contain increasing materials.

From Advantage is between the hub and sprocket a layer of one arranged elastically deformable material. Thereby one finds a noise reduction in the operation of the gear instead, and On the other hand, there is a start-up and / or Abbremsruckdämpfung between the wheel hub and the sprocket instead.

Further Advantages, special features and expedient further education The invention results from the further subclaims or their sub-combinations.

The Invention will be described below with reference to the drawings Embodiments explained in more detail. Show it:

1 a schematic representation of a first embodiment of a gear according to the invention in a perspective exploded view.

2 a view of the gear 1 from the opposite side.

3 a schematic representation of a second embodiment of a gear according to the invention in a perspective exploded view.

4 a view of the gear 3 from the opposite side.

5 a schematic representation of a third embodiment of a gear according to the invention in a perspective exploded view.

6 a view of the gear 5 from the opposite side.

7 a schematic representation of a fourth embodiment of a gear according to the invention in a perspective exploded view.

8th a view of the gear 7 from the opposite side.

9 a schematic representation of a fifth embodiment of a gear according to the invention in a perspective view, wherein the sprocket is shown in this illustration in the X-ray view.

10 a schematic representation of one of the embodiment 9 in a perspective exploded view.

11 a view of the gear 10 from the opposite side.

12 a schematic representation of a sixth embodiment of an assembled gear according to the invention in a perspective view.

13 the gear out 12 in an exploded view from a first viewing direction.

14 the gear out 12 in an exploded view from a second viewing direction.

15 a schematic representation of a seventh embodiment of a gear according to the invention in a perspective exploded view.

16 a view of the gear 15 from the opposite side.

The like numerals in the figure designate the same or the same acting elements.

In the drawings 1 to 16 are gear wheels according to the invention designed as face or worm wheels with a cylindrical, circular outer contour 01 shown.

Each in the 1 to 16 illustrated gear 01 consists of a wheel hub 02 and one with the wheel hub 02 connected, separately producible or produced annular ring gear 03 with an integral running gear 04 , Between the sprocket 03 and the wheel hub 02 , more precisely on one of the running teeth 04 wearing sprocket 03 facing outer surface 05 the wheel hub 02 , as well as on one of these outer surfaces 05 facing inner surface 06 of the sprocket 03 is also referred to as a spline, serration or as a splined spline toothing 07 arranged. The gearing 07 consists of one on the sprocket 03 facing outer surface 05 the wheel hub 02 arranged and a first batch of the splines 07 forming wheel hub external toothing 08 , as well as one on the outer surface 05 facing inner surface 06 of the sprocket 03 arranged and with the hub outer teeth 08 corresponding, the second part of the splines 07 forming sprocket internal teeth 09 ,

The wheel hub 02 and the sprocket 03 in the 1 and 2 illustrated gear 01 are glued together or z. B. welded together by ultrasonic welding or laser beam welding.

The wheel hub 02 and the sprocket 03 in the 3 and 4 illustrated gear 01 are by means of a heat wire 10 welded together, preferably as a Verlierteil in the gear 01 after welding the wheel hub 02 with the sprocket 03 remains.

Alternatively or additionally, the wheel hub and the ring gear are preferably as in the 5 to 9 represented by a catch in the axial direction to be connected or become.

The locking can be like in the 5 and 6 shown in the axial direction of the wheel hub 02 arranged locking lugs 11 and out in the sprocket 03 arranged recesses 12 exist, which recesses 12 with the locking lugs 11 have corresponding Hintergreifungen.

At least part of the catch, for example, in the recesses 12 protruding catches 11 at the wheel hub 02 facing (formed in the example) ends simultaneously form a positive connection in the circumferential direction between the wheel hub and sprocket, via which positive connection at least part of a between hub 02 and sprocket 03 can be transmitted to transmit torque. In addition, the catch can 11 be resilient, so that between sprocket 03 and wheel hub 02 on the one hand a noise reduction in the operation of the gear 01 takes place, and on the other hand, a start-up and / or Abbremsruckdämpfung between the wheel hub and the sprocket is made possible.

In the 7 and 8th a variant is shown in which the blocking in the axial direction by a in a groove 14 of the sprocket 03 lying footbridge 16 the wheel hub 02 he follows. This is the wheel hub 02 produced in a second manufacturing step by casting / spraying, whereby after hardening of the material a positive connection between the wheel hub 02 and sprocket 03 established.

The groove runs in the teeth 13 the spline 07 on the sprocket 03 in front preferably arranged centrally in the circumferential direction. web 16 and groove 14 can also be reversed than in the 7 and 8th be shown, ie the web 16 can also be on the sprocket 03 located and the groove 14 at the wheel hub 02 , Likewise, it is basically conceivable that webs 16 and grooves 14 alternate each other on both components.

It is also conceivable to configure the web latched in the groove. The latching can alternatively also a first, on the outer circumference or on the outer surface 05 the wheel hub 02 arranged circumferential groove, a second, on the inner circumference or on the inner surface 06 of the sprocket 03 arranged circumferential groove, as well as a insertable in the two grooves spring ring.

A lock between wheel hub 02 and sprocket 03 can also be like in 9 be shown, wherein a Varrastung on corresponding projections 22 and 23 which takes place on at least individual interdental spaces 15 the wheel hub 02 are provided.

Will the gear 01 or its wheel hub 02 and / or its sprocket 03 Made of plastic, can be like in 9 exemplary for the sprocket 03 represented fillers and support inserts, such. B. metal rings 17 / 18 , Long glass fibers, or other strength-increasing materials contained in the plastic.

The 10 and 11 show the embodiment 9 in an exploded view, each from a different direction.

At one in the 12 . 13 and 14 illustrated gear 01 are at least in the circumferential direction between the teeth 19 the spline 07 elements 20 arranged from elastically deformable material. As a result, at least one start-up and / or Abbremsruckdämpfung takes place between the wheel hub and the sprocket. The teeth 19 For this purpose, preferably perpendicular to the outer surface 05 the wheel hub 02 or inner surface 06 of the sprocket 03 standing tooth flanks.

The 15 and 16 show a gear 01 with a combination of at least circumferentially between the teeth 19 the spline 07 arranged elements 20 made of elastically deformable material and a reinforcement 21 , This example, designed as a resilient metal ring reinforcement 21 supports the gear 03 from the inside by the reinforcement in the context of an injection molding process in the example shown in the example preferably made of plastic gear 03 is introduced.

It is important to emphasize that the invention provides the wheel hub 02 and the example performed as a worm gear ring gear 03 Produced separately with the optimal technology and then connect both elements form-fitting, non-positive or frictional with each other. A combination of the types of connection is possible according to the invention. For the elements sprocket 02 and wheel hub 03 Depending on the shape, material properties and geometry of the parts, the optimum production methods are selected with regard to strength, tolerance requirements, costs and efficiency for the selected material.

Also, a three-component (3-K) embodiment of the gear according to the invention is possible, for example so that the elements ring gear 03 and wheel hub 02 Under load limited against each other are rotatable.

For example, as in the 10 to 16 represented a third elastic material between the wheel hub 02 and sprocket 03 inserted, can the wheel hub 02 and the sprocket 03 be rotated when applying a torque against each other, so that a damping, for example, one between wheel hub 02 and sprocket 03 to be transmitted startup or deceleration torque and thus a startup or Abbremsrucks occurs. The gear according to the invention 01 is dampened by it.

A two-component version (2-K) or a two-component technology can also be used in the inventive gears 01 Find application. The 2-component elements can fillers and support inserts z. B. metal rings 17 . 21 , Long glass fibers or other strength-increasing materials.

It can plastics with plastics and also all the usual ones metallic parts are paired with plastics. Likewise different metals are paired. This will be a for a high stability of the running gear necessary, higher quality or cost-increasing material used only where the requirements are also to be met, or where they are high. At the same time Cost savings and functionality and reliability increased compared to a gear wheel according to the prior art.

The production of a sprocket 03 made of high quality plastic can preferably be carried out by extrusion or injection molding, with any teeth, such as straight teeth and helical gears are possible.

In a gear according to the invention 01 becomes the wheel hub 02 because of their complicated shape preferably by injection molding in plastic materials and metallic materials preferably produced by sintering.

The wheel hub 02 can reasonably be made of cost-effective materials, if the specification requirements are lower there.

Preferably, for the wheel hub 02 and the sprocket 03 possible materials, such as polyolefins, polyamides, polyacetates, polyesters and polyether ketones, in all their modifications and combinations conceivable.

• – Polypropylen (PP);
• – Polypropylenlangglasfaser (PPLGF)
• – Polyamid (PA)
• – Polybutylenterephthalat (PBT).

For the production of a wheel hub 02 made of plastic preferably the following materials can be used:

• - polypropylene (PP);
• - Polypropylene long fiberglass (PPLGF)
• - polyamide (PA)
• - Polybutylene terephthalate (PBT).

The mentioned materials can z. B. with glass or carbon fibers or other fillers.

• – Polyoxymethylen (POM), auch Polyacetal oder Polyformaldehyd genannt;
• – Polyamid 12 (PA 12);
• – Polyetheretherketon (Peek).

For producing a sprocket 03 made of plastic preferably the following materials can be used:

• - Polyoxymethylene (POM), also called polyacetal or polyformaldehyde;
• - polyamide 12 (PA 12);
• - Polyetheretherketone (Peek).

Also These materials can z. B. with glass or Carbon fibers or other fillers to be enriched.

The connection between wheel hub 02 and sprocket 03 preferably takes place positively with injection-molded form-fitting (groove 14 and footbridge 16 ), Locking elements, for example with locking lugs 11 and corresponding recesses, or arranged between the teeth of the splines corresponding Verrastungselementen 22 and 23 ,

The axial fixation can also be achieved by welding z. B. ultrasonic welding or laser welding or with heat wire 10 or by gluing, whereby the parts are joined together.

The Invention is particularly suitable for use, for example with worm wheels in gearboxes of automotive engineering, the previously one-piece and made of only one material, for example Made of plastic. Such worm wheels are so far mainly manufactured in one piece by injection molding. As a result, so far due to the given geometry a high tool technical effort required to the required tolerances observed. Besides, they endure in one piece for example, by injection molding produced worm wheels because of the required low cost and sprayable Materials in the gearing the necessary endurance load not or only risky. In addition, such worm wheels tend Under continuous load to deform and cause synchronized fluctuations.

one-piece Worm wheels from z. B. high-strength plastics are due the material properties of high-strength plastics or not only difficult to produce.

The Invention provides a cost effective solution for the disadvantages described.

The Invention is particularly in the field of production of gears, preferably of gears, as z. For example electric actuators are provided in motor vehicles, commercial applicable.

01

gear

02

wheel hub

03

sprocket

04

running gear

05

outer surface

06

palm

07

pass teeth

08

Hub outer teeth

09

Gear internal gear

10

heat wire

11

locking lug

12

recess

13

tooth

14

groove

15

Interdental

16

web

17

metal ring

18

metal ring

19

tooth

20

element made of elastic material

21

reinforcement

22

latching

23

latching

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• EP 0751246 A1 [0008]
• - DE 69804263 T2 [0009]
• DE 2438141 A1 [0010]
• - DE 10127224 A1 [0011]
• - DE 10149759 A1 [0012]
• - DE 69704941 T2 [0013]

Claims (16)

Gear ( 01 ) with a wheel hub ( 02 ) and one with the wheel hub ( 02 ), separately producible annular ring gear ( 03 ) from opposite the gear ( 01 ) higher than the gear ( 01 ) stronger, wear-resistant material with a running gear ( 04 ), characterized in that the connection of wheel hub ( 02 ) and sprocket ( 03 ) via a catch ( 11 . 12 ), which the sprocket ( 03 ) in the axial direction relative to the wheel hub ( 02 ), wherein at least a portion of the latch simultaneously a positive connection in the circumferential direction between the wheel hub ( 02 ) and sprocket ( 03 ). Gear according to claim 1, characterized in that on one of the running teeth ( 04 ) supporting ring gear ( 03 ) facing outer surface ( 05 ) of the wheel hub ( 02 ), as well as on one of these outer surfaces ( 05 ) facing inner surface ( 06 ) of the sprocket ( 03 ) a spline ( 07 ) is arranged. Gear ( 01 ) with a wheel hub ( 02 ) and one with the wheel hub ( 02 ), separately producible annular ring gear ( 03 ) made of a heavy-duty, wear-resistant material with a running gear ( 04 ), characterized in that on one of the running teeth ( 04 ) supporting ring gear ( 03 ) facing outer surface ( 05 ) of the wheel hub ( 02 ), as well as on one of these outer surfaces ( 05 ) facing Innenflä surface ( 06 ) of the sprocket ( 03 ) a spline ( 07 ) is arranged. Gearwheel according to claim 3, characterized in that the connection of wheel hub ( 02 ) and sprocket ( 03 ) via a catch ( 11 . 12 ), which the sprocket ( 03 ) in the axial direction relative to the wheel hub ( 02 ), wherein at least a portion of the latch simultaneously a positive connection in the circumferential direction between the wheel hub ( 02 ) and sprocket ( 03 ). Gearwheel according to one of claims 1 to 4, characterized in that between wheel hub ( 02 ) and sprocket ( 03 ) a layer of an elastically deformable material is arranged. Gear according to one of claims 2 to 5, characterized in that at least in the circumferential direction between the teeth ( 19 ) of the splines ( 07 ) Elements ( 20 ) are arranged made of elastically deformable material. Gear wheel according to one of claims 1 to 4, characterized in that the wheel hub ( 02 ) and the sprocket ( 03 ) are glued together. Gear wheel according to one of claims 1 to 4, characterized in that the wheel hub ( 02 ) and the sprocket ( 03 ) are welded together. Gear according to one of claims 1, 2 or 4 to 8, characterized in that the latching in the axial direction on the wheel hub ( 02 ) arranged latching noses ( 11 ) and in the sprocket ( 03 ) arranged recesses ( 12 ) with the locking lugs ( 11 ) corresponding Hintergreifungen. Gear according to one of claims 1, 2 or 4 to 9, characterized in that the latching at least in the region of the teeth ( 13 ) of the splines ( 07 ) at the wheel hub ( 02 ) and / or on the sprocket ( 03 ) a groove ( 14 ), and at least in the area of the interdental spaces ( 15 ) of the splines ( 07 ) on the sprocket ( 03 ) and / or at the wheel hub ( 02 ) one with the groove ( 14 ) corresponding bridge ( 16 ). Cogwheel according to one of claims 1, 2 or 4 to 10, characterized in that the latching a first, on the outer circumference ( 05 ) of the wheel hub ( 02 ) arranged circumferential groove, a second, on the inner circumference ( 06 ) of the ring gear arranged circumferential groove, and comprises an insertable in the two grooves spring ring. Gear wheel according to one of the preceding claims, characterized in that the wheel hub ( 02 ) consists of a plastic. Gear according to one of the preceding claims, characterized in that the sprocket ( 03 ) consists of a plastic. Gearwheel according to claim 12 or 13, characterized in that the plastic strength-increasing materials ( 17 . 18 . 21 ) contains. Gearwheel according to one of claims 1 to 12, characterized in that the sprocket ( 03 ) consists of metal. Gearwheel according to claim 15, characterized in that the wheel hub ( 02 ) consists of a plastic which enhances the strength-increasing materials ( 17 . 18 . 21 ) contains.