Abstract:
The invention relates to a friction brake which can be used for a shaft and a bearing part that can be rotated in relation to each other about a rotational axis ( 3 ). The friction brake comprises two brake elements: the first brake element has a first friction surface ( 24 ) and has to be mechanically connected to the bearing part, and the second brake element has a second friction surface ( 25 ) and has to be mechanically connected to the shaft. The connectible and disconnectable dynamic effect of an actuation mechanism forces the two brake elements against each other during a braking action or stop in such a manner that the two friction surfaces ( 24, 25 ) rest one on the other and contact each other in the area of a contact surface ( 29, 30 ). The contact surface ( 29, 30 ) covers only a part of every friction surface ( 24, 25 ) at any point in time during a braking action or stop. The relative position of the contact surface ( 29, 30 ) in relation to any of the two friction surfaces ( 24, 25 ) changes during a braking action.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is the U.S. National Stage of International Application No. PCT/EP2008/051435, filed Feb. 6, 2008, which designated the United States and has been published as International Publication No. WO 2008/095949 and which claims the priority of German Patent Application, Ser. No. 10 2007 006 164.3, filed Feb. 7, 2007, pursuant to 35 U.S.C. 119(a)-(d). 
     BACKGROUND OF THE INVENTION 
     The invention relates to a friction brake for a shaft and a mounting part which can rotate in relation to one another about a rotational axis, comprising two brake elements, the first brake element of which has a first friction surface and has to be mechanically connected to the bearing, and the second brake element has a second friction surface and has to be mechanically connected to the shaft, and an activation mechanism whose force effect, which can be switched on or off, presses the two brake elements against one another during a braking process or stopping process in such a way that the two friction surfaces bear against one another and are in contact in the region of a contact surface. 
     Such a friction brake is known, for example, from DE 100 46 903 C2. It is an emergency stop brake or stopping brake of an electric drive. The friction surfaces of these friction brakes which act against one another are of equal size and both annular. They each form in their entirety the given contact surface during the braking/stopping process. In a mechanically excited and controlled friction brake of this type, said contact surface should, on the one hand, not be too large in order to maximize the magnetic flux density passing through here, and therefore to maximize the magnetic cohesion which can be achieved. On the other hand, in order to conduct away heat a surface which is as large as possible is desirable. The compromise which always has to be made in practice in this regard can lead, in particular in the case of strong or long braking processes, to an accumulation of heat at the friction surfaces. The resulting overheating may reduce the braking effect or stopping effect. 
     In other known friction brakes such as, for example in the case of disk brakes which are used in a motor vehicle, the braking behavior may be impaired as a result of excessive generation of heat at one of the friction surfaces which are involved. 
     In order to avoid overheating, internally ventilated brake disks are also used. Because of the poor thermal conductivity of the steel of the brake disks, this measure does not have any effect in the case of strong, brief braking. 
     In the case of two-wheels, perforated brake disks have been used in order to reduce the vapor pressure occurring when there is moisture on the friction surface. This also is not capable of preventing overheating of the friction surfaces in all cases. 
     SUMMARY OF THE INVENTION 
     The object of the invention is therefore to specify a friction brake of the type designated at the beginning which also operates reliably in the case of strong or long braking processes. 
     This object is achieved by a friction brake for a shaft and a mounting pall which can rotate in relation to one another about a rotational axis, with the friction including a) two brake elements, the first brake element of which has a first friction surface and has to be mechanically connected to the mounting part, and the second brake element has a second friction surface and has to be mechanically connected to the shaft ( 4 ), and b) an activation mechanism whose force effect, which can be switched on or off, presses the two brake elements against one another during a braking process or stopping process in such a way that the two friction surfaces bear against one another and are in contact in the region of a contact surface, wherein the two friction surfaces are configured in such a way that c) the contact surface engages with only a part of each of the two friction surfaces at any point in time during a braking process or stopping process, and d) the relative position of the contact surface with respect to each of the two friction surfaces changes during a braking process. 
     The friction brake according to the invention is distinguished by the fact that the two friction surfaces are configured in such a way that the contact surface engages with only a part of each of the two friction surfaces at any point in time during a braking process or stopping process, and the relative position of the contact surface with respect to each of the two friction surfaces changes during a braking process. 
     In the friction brake according to the invention, the contact surface at which friction heat occurs at a particular time is always smaller than the two friction surfaces. Furthermore, the contact surface changes its position, in particular continuously, with respect to each friction surface. This is brought about, in particular, when the two friction surfaces do not have any rotational symmetry or circular symmetry with respect to the rotational axis. The two friction surfaces are therefore preferably rotationally asymmetrical or have circular asymmetry with respect to the rotational axis. Furthermore, the two friction surfaces differ from one another, in particular, in their geometric shape. As well as the position, the shape and/or the size of the contact surface can preferably change during the braking process. Consequently, only a part of each of the two friction surfaces is ever involved in the braking friction at a particular point in time. The other regions of the two friction surfaces which are not involved at this point in time can irradiate thermal energy until they are engaged again by the migrating contact surface and involved in the braking friction. During a relative rotation through 360° between the two friction surfaces, each region of the friction surfaces is preferably part of the contact surface at least once. 
     As a result, the friction heat which is produced is distributed over the friction surfaces which are relatively large compared to the contact surface. Furthermore, the radiation which occurs at the regions of the friction surface which are not engaged by the contact surface leads to cooling. In addition, the medium which surrounds the friction brake, that is to say for example the ambient air, can also cool the regions of the friction surfaces which do not form the contact surface at that particular time and, in particular, are freely accessible. This also contributes to the comparatively low operating temperatures. Overall, a relatively low energy density results, so that the influence of temperature is perceived less. 
     Above all, in the case of short but strong braking processes, adiabatic conditions can occur during which all of the thermal energy which is produced at the contact surface collects at the friction surfaces. The thermal capacity of the involved components of the brake elements plays a subordinate role here. In contrast, the conditions at the friction surfaces are decisive. 
     Owing to the relatively large available friction surface and since in the friction brake according to the invention the contact surface moves over the relatively large friction surfaces, the accumulation of heat between the two friction surfaces, which otherwise threatens to occur when the contact surface remains the same, either no longer occurs or only occurs when the brake loads are very much greater. In the friction brake according to the invention, a larger surface is involved in the friction process. As a result, the increase in temperature which occurs overall owing to the friction heat is reduced. The lower temperatures ensure a good braking force effect when the brake materials which are currently customary are used, and said lower temperatures also lead to less wear. 
     With the friction brake according to the invention it is possible, despite the friction surfaces which preferably have circular asymmetry, to achieve a braking torque which is substantially constant in the circumferential direction. This applies, in particular, if the contact surface is arranged distributed in a tangential direction (that is to say in the circumferential direction) with respect to the rotational axis. 
     The friction brake according to the invention can be used in various applications, for example in an electric drive, in a car or in a two-wheeled vehicle. 
     A variant in which at least one of the two friction surfaces is composed of a plurality of partial friction surfaces which are separated from one another is favorable. This promotes self-cleaning of the friction surfaces. 
     Furthermore, one of the two friction surfaces can preferably be composed of a plurality of strip-shaped partial friction surfaces which are distributed uniformly in a circumferential direction which is specified with respect to the rotational axis. This geometric shape is simple. It can easily be fabricated. 
     According to another preferred variant, the strip-shaped partial friction surfaces are each directed radially outward. In particular, the partial friction surfaces may extend radially outward as straight strip segments. This produces particularly good ventilation and cooling of the friction surface or surfaces. 
     This effect is increased further by inclining the strip-shaped partial friction surfaces, in particular, in each case with respect to the radial direction. Furthermore, this also has a favorable effect on the self-cleaning. Particles of dust are then removed particularly effectively from the friction surfaces. 
     Furthermore, the strip-shaped partial friction surfaces are preferably each embodied in a curved fashion, in particular as annular segments. They therefore have, for example, the cross-sectional shape of a fan or blade. This provides further advantages in terms of cooling. 
     In a further favorable configuration, at least one of the two friction surfaces has a strip shape which is nonannular, enclosed and, in particular, periodic in the circumferential direction. It then has, for example the shape of a rounded polygon or of an annulus with symmetrically distributed protrusions and depressions. In particular it is possible for in each case three protrusions and depressions to be provided in an alternating sequence distributed over the circumference of the circle. 
     There is furthermore also provision in an advantageous manner that the nonannular friction surfaces are produced by superimposing a sine function on a circular function. This provides in a particularly simple fashion periodicity in the circumferential direction which can also be manufactured easily, for example by means of a milling process. 
     According to a further favorable embodiment, the two friction surfaces each have a strip shape which is nonannular, enclosed and periodic in the circumferential direction, wherein the tangential periodicity of the first friction surface differs from that of the second friction surface. A deviation in the circumferential periodicity is easy to fabricate. Furthermore, differences in the geometric shape of the two friction surfaces promote the advantageous variation of the contact surface during a braking process. 
     It is preferably also possible for each of the two friction surfaces to be embodied in a strip shape and for a strip width of the first friction surface to differ from that of the second friction surface. The strip width is a further parameter which, given a different specification for the respective friction surface, causes the contact surface to vary during a braking process. 
     A variant in which the friction brake is embodied with magnetic excitation, and the first brake element has a first and a second magnetic pole, which are formed by partial friction surfaces, separated from one another, of the first friction surface which is embodied in a plurality of parts, is also favorable. In particular, the magnetic circuit of a permanently excited and electromagnetically switched friction brake can therefore be implemented particularly easily. In particular, in the case of the magnetically excited variant of the friction brake, it is also possible to provide that a magnetic flux passes through the contact surface. This provides a particularly good conduction of flux. 
    
    
     
       BRIEF DESCRIPTION OF THE BRAWING 
       Further features, advantages and details of the invention emerge from the following description of exemplary embodiments with reference to the drawing, in which: 
         FIGS. 1 and 2  show two exemplary embodiments of friction brakes with a contact surface formed between two friction surfaces, and with one or two contact-forming magnetic pole(s), 
         FIGS. 3 and 4  show exemplary embodiments of a combination of a single-part friction surface and a multi-part friction surface of a friction brake, 
         FIG. 5  shows an exemplary embodiment of a combination of two multi-part friction surfaces of a friction brake, and 
         FIGS. 6 and 7  show exemplary embodiments of a combination of a single-part friction surface and a multi-part friction surface of a friction brake. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Parts which correspond to one another are provided with the same reference symbols in  FIGS. 1 to 7 . 
       FIG. 1  shows an exemplary embodiment of a friction brake  1  which has permanent-magnet excitation and can be activated electromagnetically, for an electric drive  2  in the form of an electric motor. The drive  2  comprises a shaft  4  which can be driven in rotation about a rotational axis  3  and which is rotatably mounted, by means of a bearing  5 , in a stator  6  of the drive  2  (of which only a detail is shown schematically). 
     The friction brake  1  is composed substantially of two brake elements  7  and  8 , the first brake element  7  of which comprises an external pole body  9 , a permanent magnet  10 , an internal pole body  11  and a solenoid coil  12  (=activation mechanism) which can be switched on and off and is arranged between the external pole body  9  and the internal pole body  11 . The external pole body  9  and the internal pole body  11  each have, as a main component, a cylinder which is arranged concentrically with respect to the rotational axis  3 , wherein a radius R 2  of the cylinder of the external pole body  9  is larger than a radius R 1  of the cylinder of the internal pole body  11 . The first brake element  7  is securely mechanically connected to the stator  6 , and therefore also to the bearing  5 , by means of an attachment or connection flange of the internal pole body  11 . On axial end sides, the external pole body  9  and the internal pole body  11  each have a partial friction surface  13  and  14 , respectively, which together form the friction surface  15  of the first brake element  7 . 
     The second brake element  8  is embodied as an armature disk or yoke. It is connected, by means of a spring mechanism  16  and a securing body  17  (only illustrated schematically) to the shaft  4  in a manner secured against rotation, and is therefore also connected to a rotor (not shown in more detail) of the electric drive  2 . This connection permits axial movement in the direction of the rotational axis  3  and in relation to the shaft  4 . The spring mechanism  16  is embodied, for example, as a flat spring. On the end side facing the first brake element  7 , the second brake element  8  has a friction surface  18 . 
     The method of operation of the friction brake  1  will be described in the text which follows. 
     The permanent magnet  10  generates a magnetic field which exits the external pole body  9  and internal pole body  11  perpendicularly at the partial friction surfaces  13  and  14 , respectively. Magnetic poles are therefore formed at the partial friction surfaces  13  and  14 . The magnetic field which exits here brings about magnetic attraction forces acting in the axial direction on the yoke of the second brake element  8 . When the solenoid coil  12  is currentless, the yoke of the second brake element  8  is pressed with its friction surface  18  against the partial friction surfaces  13  and  14  of the first brake element  7  because of these attraction forces. 
     Contact surfaces (not designated in more detail in  FIG. 1 ), are then produced between the two brake elements  7  and  8 . In the case of the friction brake  1 , the contact between the involved friction surfaces  15  and  18  therefore occurs precisely at the location of the two magnetic poles which are formed by the partial friction surfaces  13  and  14 . 
     Owing to the pressure between the friction surfaces  15  and  18  of the two brake elements  7  and  8 , frictional forces are produced which bring about a braking torque as a function of the location of their point of action, that is to say approximately at the distance R 1  and R 2  from the rotational axis  3 . 
     If, on the other hand, current flows through the solenoid coil  12 , a solenoid magnetic field which counteracts the magnetic field of the permanent magnet  10  is generated. The two magnetic fields substantially cancel one another out. The spring force of the spring mechanism  16  then pulls the yoke of the second brake element  8  away from the first brake element  7 , with the result that an axial gap  19  is produced between the friction surfaces  15  and  18 , and there is no longer any braking effect. 
     A further exemplary embodiment of a friction brake  20  which has permanent-magnet excitation and can be activated electromagnetically which is shown in  FIG. 2  is of similar design to the friction brake  1 . A significant difference is that the yoke of the second brake element  8  is in contact only with the partial friction surface  13  of the external pole body  9 , that is to say just with one magnetic pole, during the braking process. The friction brake  20  therefore also has only a single-part friction surface  21  on its first brake element  7 , which friction surface  21  is formed exclusively by the partial friction surface  13 . The latter again determines the contact surface which makes contact with the yoke of the second brake element  8  during the braking process. 
     The first brake element  7  has a slightly modified internal pole body  22  whose cylinder is somewhat longer than in the case of the friction brake  1 , and which extends into a central recess of the yoke of the second brake element  8 . Between the cylinder end, which extends into the recess, and the yoke of the second brake element  8  a radial gap  23  is provided which has to be bridged by the magnetic flux both in the braked and in the unbraked states. 
     On the other hand, the braking torque behavior improves since only the external radius R 2  is decisive, instead of the two effective radii R 1  and R 2 . As a result, fluctuations in the braking torque which otherwise occur owing to different erosion of material at the surfaces which are determined by the two radii R 1  and R 2 , that is to say at the partial friction surfaces  13  and  14 , can be avoided. 
     In  FIG. 1 and 2 , the friction surfaces  15 ,  18  and  21  and the partial friction surfaces  13  and  14  are only indicated schematically. They can assume different shapes. Exemplary embodiments thereof are shown in  FIGS. 3 to 7 . 
     In the exemplary embodiments of combinations of friction surfaces of the first and the second brake elements  7  and  8  which are shown in  FIGS. 3 to 7 , the two friction surfaces are each always shaped in such a way that the contact surface, formed between the first and second brake elements  7  and  8  during a braking process, engages with only a part of each friction surface at any point in time during a braking or stopping process, and the relative position of the contact surface with respect to the two friction surfaces and, if appropriate, also the shape and/or size of the contact surface change continuously during a braking process. This is achieved, in particular, by virtue of the fact that the friction surfaces are each shaped differently and they both have no rotational symmetry or circular symmetry with respect to the rotational axis  3 . 
     The contact surface at a particular time is therefore always smaller than any friction surface. As a result, in each case only a part, which varies over time during the braking process, of the two friction surfaces contributes to the braking friction. During the braking process, the contact surface passes over all the regions of the friction surfaces if the two friction surfaces are rotated once in relation to one another about the rotational axis  3 . The locations on the two friction surfaces which are not involved in the braking process at a particular time become cooler owing to the irradiation of thermal energy and the interaction with the surrounding air. As a result, a very favorable temperature behavior is obtained. Overall, the temperature is kept at a relatively low level. This is advantageous for the braking effect. 
     Here, a friction surface is understood to be a surface of the first or second friction element  7  or  8  which can basically be involved in the friction during a braking process. The surface in this context may be a single coherent surface or else a plurality of partial surfaces which are separated from one another at least in the friction plane. On the other side of the friction plane, the regions which form the partial surfaces can, however, also be connected to one another mechanically. In contrast, the contact surface is understood to be the intersection of the friction surfaces of the first or second friction elements  7  or  8 . The contact surface can also be in a single part or multiple parts. While the friction surfaces are formed by the selected design and also remain unchanged, the contact surface generally changes its position, shape and size during a braking process. 
     The size of the contact surface of the respective friction brake depends on the requirements made of the pressing force between the two brake elements  7  and  8 . In order to increase the conduction away of heat, the friction surfaces can also be enlarged further without at the same time changing the contact surface and therefore the pressing force and the braking effect. Relatively large friction surfaces also lead to less erosion. 
       FIG. 3  shows a combination of a single-part, enclosed, nonannular friction surface  24  of the external pole body  9  and a multi-part friction surface  25  of the yoke of the second brake element  8 . The two friction surfaces  24  and  25  are used in a friction brake which is comparable to that according to  FIG. 2 . In this example, contact is made with just one magnetic pole during a braking/stopping process. 
     The friction surface  24  is formed by superimposing a sine shape on an annular shape. Overall three sine periods are provided distributed uniformly over the length of the circumference. The friction surface  24  has three protrusions  26  extending outward and three depressions  27  extending inward. The friction surface  24  in the exemplary embodiment has approximately the shape of a rounded triangle. 
     The friction surface  25  is composed of a plurality of strip-shaped partial friction surfaces  28  (six in the example) arranged distributed uniformly in the circumferential direction. The partial friction surfaces  28  are each straight strip segments which extend radially outward and are arranged in a star shape. The radial outer ends of the partial friction surfaces  28  are rounded. 
     The friction surfaces  24  and  25  can, for example, be formed by axially protruding webs on the axial end sides, facing one another, of the external pole body  9  and of the yoke of the second brake element  8 . Furthermore, the assignment of the friction surfaces  24  and  25  to the first or second brake element  7  or  8  can also be interchanged. 
     During the braking process, a multi-part contact surface  29 , which is composed of a total of six partial contact surfaces  30  in the exemplary embodiment according to  FIG. 3 , is present between the two brake elements  7  and  8 . The contact surface  29  is formed here by the intersection of the friction surfaces  24  and  25  at that particular time. The partial contact surfaces  30  change their respective position in relation to the two friction surfaces  24  and  25  during a relative rotation of the friction surfaces. This is indicated in  FIG. 3  by the directional arrows on one of the partial contact surfaces  30 . 
     Owing to the specific configuration of the friction surfaces  24  and  25 , in each case two of the partial contact surfaces  30  lie opposite one another in relation to the rotational axis  3 . Their central distance is twice the radius R 2  of the circle  31  which is also represented in  FIG. 3  by means of dashed lines. The total braking torque of such partial contact surfaces  30  which lie opposite one another is therefore practically independent of the current relative rotational angle between the friction surfaces  24  and  25 . 
     The partial contact surfaces  30  change their respective radial distance from the rotational axis  3  as a function of the current relative rotational angle between the friction surfaces  24  and  25 . This results in a cleaning effect for the friction surfaces  24  and  25 . Particles of dirt are taken up during a relative rotational movement and conveyed in the direction of the outer or inner edge of the friction surfaces and therefore out of the zone in which friction takes place. Since the contact surfaces always engage with changing portions of the friction surfaces  24  and  25  during the friction process, water vapor can also escape very well from the friction surfaces  24  and  25 . In particular, this does not require any perforation of the friction surfaces  24  and  25 . Contamination and water films therefore do not have any effect, or only a very small effect, on the braking effect. 
     The circumferential periodicity of three, which is provided for the friction surface  24 , permits easy fabrication of the friction surface  24  itself and furthermore also of the friction brake as a whole. The friction surface  24  can be manufactured with little complexity, for example by means of a milling process. If the friction surface  24  is provided on the yoke of the second brake element  8 , the abovementioned circumferential periodicity furthermore permits very easy mounting of the spring mechanism  16 , embodied as a flat spring arrangement, by means of six rivets, three of which are located in the region of the protrusions  26  on the yoke, and the other three of which are located on the securing body  17 . 
       FIG. 4  shows a slightly modified exemplary embodiment of a combination of the single-part friction surface  24  and of a multi-part friction surface  32 . The friction surface  32  also has partial friction surfaces  33  in the form of outwardly extending strip segments. However, the latter have a curvature which is directed outward, (i.e. in the direction of the outer edge of the friction surface  24 ), and an incline in relation to the radial direction. Said partial friction surfaces  33  are shaped in the manner of a fan cross section or blade cross section. The partial friction surfaces  33  also form together with the friction surface  24  partial contact surfaces  34 , which change at least in their relative position during the friction process, of an in turn multi-part contact surface (not designated in more detail). 
     The subdivision of the friction surfaces  24  and  32  into the partial friction surfaces  28  and  33  brings about additional cooling of the friction surface  24  during a rotational movement of the yoke of the second brake element  8 . The specific shape of the partial friction surfaces  28  and in particular of the partial friction surfaces  33  promotes an additional supply of air, which is also brought about by an impeller wheel. 
       FIG. 5  shows an exemplary embodiment, which is also slightly modified compared to  FIG. 3 , of a combination of the multi-part friction surface  25  with a further multi-part friction surface  35 . The friction surfaces  25  and  35  of this combination are intended for use in a friction brake, in a way which is comparable to that according to  FIG. 1 . During the braking/stopping process, both the external pole body  9  and the internal pole body  11  are therefore involved in the formation of contact with the yoke of the second brake element  8 . 
     The friction surface  35  which is assigned to the first brake element  7  therefore comprises two enclosed, nonannular partial friction surfaces  36  and  37 . The latter each have a shape which is comparable to that of the friction surface  24  according to  FIG. 3 . The partial friction surface  37  of the internal pole body  11  is located completely within the partial friction surface  36  of the external pole body  9 . During the braking process, a plurality of locally changing partial contact surfaces  38  are again formed on the respective intersection points between the strip-segment-shaped partial friction surfaces  28  of the friction surface  24  and the enclosed, but not annular, partial friction surfaces  36  and  37  of the friction surface  35 . 
       FIG. 6  shows an exemplary embodiment of a combination of the single-part friction surface  24  with a further single-part friction surface  39 . The friction surfaces  24  and  39  of this combination are intended for use in a friction brake which is comparable to that according to  FIG. 2 . The friction surface  39  is, like the friction surface  24 , formed by superimposing a sine shape on an annular shape. However, in the case of the friction surface  39 , five sine periods are provided distributed uniformly over the length of the circumference, instead of three. Accordingly the friction surface  39  has five outwardly extending protrusions  40  and five inwardly extending depressions  41 . In the exemplary embodiment, the friction surface  39  is approximately in the shape of a rounded star. During the braking process, a plurality of locally changing partial contact surfaces  42  are again formed on the respective intersection points between the friction surfaces  24  and  39 . 
       FIG. 7  shows a further exemplary embodiment of two single-part friction surfaces  43  and  44 . Both friction surfaces  43  and  44  have an enclosed, but not annular, form. The friction surface  43  is embodied as a rectangle and the friction surface  44  as an ellipse, which rectangle and ellipse each have a different strip width B 1  and B 2 , respectively, in the exemplary embodiment according to  FIG. 7 . Between them, a plurality of partial contact surfaces  45 , which change in position and here also additionally in size and shape, are again formed on the respective intersection points during the braking process. 
     The favorable cooling and cleaning effects which were described in conjunction with the combination according to  FIG. 3  are also provided in a similar way with the combinations according to  FIGS. 4 to 7 . 
     In order to assist the cooling effect, the yoke of the second brake element  8  can have an impeller wheel structure on a side facing away from the friction surface in all the exemplary embodiments above. Furthermore, the respective friction brake can preferably be arranged in the region of a winding head of an electric winding which is provided in the stator  6  of the electric drive  2 . The cooling effect which is achieved owing to the specific configurations of the friction brakes can at the same time also be used to cool the winding head which is otherwise usually uncooled. 
     The favorable configurations of the friction surfaces and contact surfaces described above can basically also be used in other types of brake, that is to say in friction brakes which are not magnetically excited. The applications are likewise not restricted to electric machines. They can basically also be used in a motor vehicle or motorcycle.