Abstract:
A spindle drive unit includes a spindle having a spindle axis and an external circumferential surface and a nut embracing the spindle and being in threaded engagement with the spindle. In an axial end portion of the nut a wiper assembly is removably fixed to the nut. The wiper assembly has at least one sealing ring in wiping engagement with the external circumferential surface of the spindle. The wiper assembly is removably connected to an adapter in a rotationally fixed manner, the adapter in turn being directly and removably connected to the nut in an axially and rotationally fixed manner. The use of the adapter enables the wiper assemblies of various constructions or with different numbers of sealing rings to be attached to a conventional nut.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a spindle drive unit comprising a spindle having a spindle axis and an external circumferential surface, a nut embracing said spindle and being in threaded engagement with said spindle and at least one wiper assembly comprising at least one wiper member in wiping engagement with said external circumferential surface of said spindle, said wiper assembly being removably fastened to said nut in an axial end portion thereof. 
     2. Description of the Prior Art 
     A spindle drive unit like this is known from the DE 28 22 560 A1. In this known solution the wiper assembly comprises a sealing ring attached to the internal circumference of a sleeve. The sealing ring sealingly abuts on the back of an external thread of the spindle. The sealing ring is permanently riveted to the sleeve. The wiper assembly consisting of the sealing ring and the sleeve may be fixed to the nut when the nut is mounted on the spindle. For this purpose an axially projecting tubular socket is welded on the nut. The sleeve of the wiper assembly supporting the sealing ring is pushed on said tubular socket. Then the wiper assembly is manually adjusted in the axial direction with respect to the tubular socket until the sealing ring is optimally positioned on the back of the external thread of the spindle. After adjusting the wiper assembly, it is axially and circumferentially fixed to the tubular socket by means of pins and a clamp strap. 
     For this known solution an adaptation of the nut is necessary to be able to mount the wiper assembly. The nut has to be subjected to a welding process to attach the tubular socket to it. If a conventional nut, which can be obtained as a component of mass production, is to be used, it is often completely impossible to weld a tubular socket from the outside on the nut. Often the material of the nut or the components connected therewith do not allow welding of a tubular socket, for example due to high sensitivity to heat, great danger of deformation or poor welding properties. This especially applies to precision spindle drive units, which are used in machine tools or measuring devices. Moreover, welding is a comparatively extensive and difficult process as the tubular socket has to be positioned exactly in the radial direction in order to guarantee uniform abutment relations of the sealing ring at the back of the spindle thread across the entire circumference. Even in conventional nuts often no suitable place for welding the tubular socket is found for reasons of design or space. Finally, the tubular socket is no longer removable from the nut after welding so that the nut is assigned to a certain purpose of application and can no longer be used for other applications in which a wiper assembly axially attached from the outside is under certain circumstances not necessary anymore. In this case the welded tubular socket would be disturbing and require unnecessary space. 
     From U.S. Pat. No. 3,643,521 there is known a further spindle drive unit having a spindle and a nut being in threaded engagement with the spindle. In a mounted state of the nut a sealing ring can be attached to the nut from axially outside. The sealing ring engages with a threaded groove in the external circumferential surface of the spindle and is screwed on the spindle from one end of the spindle. A plurality of axially oriented peg receiving holes are provided in an axial end face of the nut distributed along the circumference. Similar peg receiving holes are also provided in the sealing ring. If the sealing ring, when mounted on, is turned into its desired axial position relative to the nut, the peg receiving holes in the sealing ring and the nut are aligned and then equipped with pegs which keep the sealing ring to the nut without being able to be turned. By means of a pretensioning member supported on the nut the sealing ring is axially biased away from the nut against the turn of the spindle thread. 
     Even in the solution according to DT 20 01 558 a conventional and commercially available nut cannot readily be used to attach the sealing ring. It is rather that the nut has to be purposefully machined to create the preconditons for mounting the sealing ring. For the peg receiving holes have to be drilled which causes an additional expenditure of work. Moreover the mounting of the sealing ring is extraordinarily laborious since the sealing ring has to be held against the action of the pretensioning member relative to the nut to be able to insert the individual pegs into the peg receiving holes which have been brought in alignment. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to enable simple mounting of a wiper assembly on a nut in a spindle drive unit, largely avoiding special measures of adaptation at the nut. 
     To achieve this object the present invention provides a spindle drive unit, comprising a spindle having a spindle axis and an external circumferential surface, a nut embracing said spindle and being in threaded engagement with said spindle and at least one wiper assembly comprising at least one wiper member in wiping engagement with said external circumferential surface of said spindle, said wiper assembly being removably fastened to said nut in an axial end portion thereof, wherein said wiper assembly, for its fastening to the nut, is removably connectable with an adapter in a rotationally fixed manner, said adapter being removably connectable with said nut in an axially and rotationally fixed manner. 
     The adapter free of wiping tasks forms an interface member between the nut and the wiper assembly. The adapter is a component physically distinct from the nut and the wiper assembly—or a constructional unit. Said component or constructional unit may on the one hand be optimally adapted to the fastening possiblities of the nut, preferably of a conventional nut. This means that the adapter can be designed in such a way that it may be fixed to the nut without specially machining the nut, for example providing a welded tubular socket or bores. Thus, the nut can also be used for applications in which the wiper assembly and the adapter are not desired or not necessary. On the other hand, the adapter may be optimally adapted to the fastening possiblities present at the wiper assembly. For it has to be considered that in designing the wiper assembly first of all its wiping funciton is of importance. Therefore in fixing the wiper assembly it always has to be considered that its wiping function is optimally guaranteed. The adapter solves the obvious conflict of goals between a simple, however operatively secure fixing of the wiper assembly with at the same time an optimally ensured wiping function and the desired intactness of the nut so that a commercially available nut can be used requiring no subsequent treatment. 
     With respect to mounting it is favourable when at least the wiper member may be mounted on and dismounted from said nut in a mounting state of said nut in which said nut is mounted on said spindle. Then the spindle does not have to be dismounted from its spindle bearings in order to be able to fix the wiper member to the nut with interposition of the adapter. It is advisable that said adapter, in said mounting state of said nut, may also be mounted on or dismounted from said nut. 
     The mounting of the spindle drive unit is further simplified when at least a part of said wiper assembly including the wiper member and said adapter may be premounted to form a constructional unit and may be attached to said nut in the form of said constructional unit. In this case it is again favourable when the premounted constructional unit which comprises said wiper member and said adapter may be mounted on and dismounted from said nut in a mounting state of said nut in which said nut is mounted on said spindle. 
     A laborious axial positioning of the adapter relative to the nut is avoided when first cooperating axial abutment means are provided on said adapter and on said nut, respectively, and define an axial position of said adapter relative to said nut. Preferably said wiper member is connectable with said adapter in an axially fixed manner. The mounting of the wiper member on said adapter may be facilitated by the fact that second cooperating axial abutment means are provided on said wiper assembly and on said adapter, respectively, and define an axial position of said wiper member relative to said adapter. 
     It is advisable that separate fixing members are provided for fixing said adapter to said nut and for fixing said wiper assembly to said adapter. Thus, for different nuts or/and different wiper assemblies the suitable fastening members can be chosen, respectively. 
     If the wiper assembly comprises a wiper member which engages with a threaded groove at the external surface of the spindle the wiper assembly can usually not be axially pushed on the spindle for mounting purposes. Rather it has to be screwed on the spindle. A simple mounting of the spindle drive unit without the expense of adjusting can be guaranteed by the fact that fixing members are provided for fixing said adapter to said nut, said fixing members allowing fixation of said adapter independent from a rotary position of said adapter relative to said nut. 
     It is conceivable that for different wiper assemblies individual types of adapters are present. An essential idea of the invention is, however, to supply a universal adapter which is adapted for selectively fastening different wiper assemblies. The difference of the wiper assemblies can lie in a different number or/and design of their wiper members. The adapter can have different attachment locations for different wiper assemblies. Preferably, however, it is provided that for attaching a wiper assembly said adapter has attachment locations which are equal for different wiper assemblies. 
     The wiper assembly can comprise at least two physically distinct wiper members. At least a part of said wiper members can be in axially consecutive wiping engagement with said external circumferential surface of said spindle. The wiper members can take over different functions so that a highly effective wiper assembly can be realized, perfectly sealing the nut against the outside. Therefore it is conceivable that two wiper members in axially consecutive wiping engagement with said external circumferential surface of said spindle differ with respect to their wiping effect. It is expedient that a wiper member which is in wiping engagement with said external circumferential surface of said spindle more remote from an axial center portion of said nut forms a secondary wiper with a coarse wiping effect and a wiper member which is in wiping engagement with said external circumferential surface of said spindle closer to said axial center portion of said nut forms a primary wiper with a fine wiping effect. 
     For enhancing the sealing of the nut there can be provided that in said axial end portion of said nut at least one additional wiper member separate from said wiper assembly is fixed or may be fixed to said nut. It is suitable when said additional wiper member will be or may be fixed to said nut closer to an axial center portion of said nut than said wiper assembly. For accommodating said additional wiper member a wiper member receiving groove can be formed on said nut. In standard nuts the wiper member receiving groove is often formed as an annular groove into which a sealing ring as an additional wiper member may be inserted. Thus it is left to the user of the spindle drive unit to mount an additional sealing ring on the nut in addition to the the wiper assembly or to leave it out. 
     In a preferred embodiment there is provided that said adapter has a mounting surface axially directed away from said nut for fixing said wiper assembly. A plurality of axial fixation bores can be formed in said mounting surface for accommodating fastening bolts serving for the fixation of said wiper assembly to said mounting surface of said adapter, said fixation bores being distributed along the circumferential direction with respect to the spindle axis. At least a part of said fixation bores can be formed as blind holes. Alternatively or additionally at least a part of said fixation bores may axially traverse said adapter enabling to insert fastening bolts into said fixation bores from both sides of the adapter. 
     A fixation of the adapter to the nut in the radial direction can be achieved in that said adapter has at least one support projection substantially directed in the axial direction of said spindle towards said nut. The support projection serves for centering the adapter on the nut. If desired, such a support projection can also be provided for the fixation of the adapter in the circumferential direction. The support projection can be formed as an annular rib or rib segment. Expediently, said nut has at least one support seat for accommodating said support projection. Said support seat can be radially defined by an internal circumferential surface of said nut. In standard nuts a ring-shaped gap in the area of their axial ends is often present between the external circumferential surface of the spindle and the internal circumferential surface of the nut. The support projection of the adapter can be inserted into this radial gap, so that the support projection is in supporting engagement with the internal circumferential surface of the nut. 
     Preferably the adapter substantially entirely embraces the spindle, wherein the adapter can suitably be formed as a ring member. A simple and quick mounting of the adapter on the nut is rendered possible in that said adapter is circumferentially separated by a slot, the width of said slot being variable by means of width variation means, wherein said adapter is clampable to said nut by varying the width of said slot. Modifications at the nut in the form of subsequent machining of the same are not necessary in this type of mounting since the adapter, when suitably designed, can substantially be clamped to any surface of the nut. Said width variation means can be formed as slot constricting means. Thus, the adapter can be clampable against an external circumferential surface of said nut. It is also possible that said width variation means are formed as slot enlarging means and said adapter is clampable against an internal circumferential surface of said nut. As a rule an external or internal circumferential surface suited to clamp the adapter is present in conventional nuts. 
     In a preferred solution, characterized by a small number of components and reliable and simple handling, the width variation means comprise a width variation screw which is insertable into a bore in said adapter, which bore opens into a slot defining surface of said slot. Since under certain circumstances it can be difficult to cut a thread into the bore, for example because the wall thickness around the bore is too thin, there is preferably provided that in said slot a physically distinct threaded nut is retained which may be brought into threaded engagement with said width variation screw. For exact positioning of the threaded nut it is advisable that said threaded nut is arranged in a recess in a slot defining surface of said slot. 
     An alternative type of mounting the adapter on the nut can be that said adapter may be hooked to said nut by means of a hook arrangement. Said hook arrangement preferably comprises at least two fixation hooks cirumferentially distributed with respect to said spindle axis. Said fixation hooks are suitably adapted for engaging with a hooking recess of said nut. Favourably, the hooking recess can be formed of a circumferential groove in an internal circumferential surface or an external circumferential surface of said nut. Such a circumferential groove is often present in conventional nuts and can be used without additional working measures for hooking the adapter. 
     In a preferred embodiment the fixation hooks are formed as pivot hooks being supported by said adapter and pivotable between a release position and a hook position. For moving said pivot hooks from their release position into their hook position said adapter can be provided with a pivot arrangement acting on a pivot end opposite to a hook end of each of said pivot hooks. A hooking of the adapter on the nut which is readily releasable, but is nevertheless safe, can be guaranteed in that said pivot arrangement comprises a pivot screw for each of said pivot hooks which is insertable into a threaded bore of said adapter and acts on said pivot end of the respective pivot hook. 
     A space-saving and protected accommodation of the fixation hooks can be that each of said fixation hooks is accommodated in a respective hook pocket of said adapter and projects from said hook pocket with a hook end. 
     For reasons of strength and for easy machinability the adapter is preferably made of metal, wherein a particularly light weight of the adapter is guaranteed by the use of aluminum. It can be seen, however, that the adapter can also be made of plastics material, for example as an injection moulded part. 
     Possible leaks between the adapter and the nut can be eliminated in that an abutment surface of said adapter provided for abutment on said nut is at least partly coated with a cover material. The cover material can consist of vulcanized rubber or injected plastics material and preferably has a certain elasticity, so that it can completely fill the leaks between the adapter and the nut. If the adapter has a slot it is advisable that also said slot is at least partly filled with a sealing material, preferably with a deformable sealing material, to prevent the penetration of contaminations through the slot. If in such a slit embodiment of the adapter the width of the slot for clamping the adapter is varied, it is favorable when the sealing material is able to adapt to the width variation, i.e. in case of a constriction of the slot the material is elastically compressible and in case of an enlargement of the slot it can extend into the additional volume of the slot. 
     According to a further embodiment of the invention there is provided that said wiper assembly comprises a wiper ring as a wiper member which is in seating engagement with said external circumferential surface of said spindle substantially over its entire internal circumference. A good sealing effect can be obtained when said wiper ring is mounted with a pretension radially biassing said wiper ring against said external circumferential surface of said spindle. A structurally simple type of the radial pretension of the wiper ring is that said wiper ring is circumferentially separated by a separation and may be biased by pretensioning means constricting said separation. Preferably there is provided that said separation is formed by a substantially axially oriented separation slit or separation cut being inclined relative to the radial direction. The inclination of the separation slit or separation cut relative to the radial direction has the effect in that the cirumferentially oppposite surfaces of the separation slit or separation cut slide along each other like wedge surfaces when the separation is constricted by means of the pretensioning means, thus reinforcing the pretension. 
     Said pretensioning means can comprise a pretensioning member having a pretensioning recess into which said wiper ring is insertable in order to be pretensioned. It can be provided that said pretensioning member is formed physically distinct from said adapter and comprises a pretensioning sleeve into which said wiper ring is insertable. As an alternative it can be provided that said pretensioning member is formed by said adapter and said pretensioning recess is formed on said adapter. 
     The slit embodiment of the wiper ring not only allows its radial pretension but also the exchange of the wiper ring without dismounting the spindle from its spindle bearings. For the wiper ring can be bent up at its separation and then be drawn off the spindle at the thus emerging gap. A new wiper ring can be mounted in the same way in bending it up and slipping it on the spindle. 
     Plastics material is recommendable as a material for the wiper ring as it is sufficiently wear resistant, on the one hand, and has the necessary flexibility in order to bend up the wiper ring for mounting and dismounting it in the previously described manner, on the other end. 
     Not only for carefully removing contaminations from the external surface of the spindle but also to optimally seal the nut against the outside is it advisable that when said external circumferential surface of said spindle is formed with a threaded profile, said wiper ring has a complementary profile at its internal circumference engaging with said threaded profile. 
     Mounting is facilitated when said wiper ring may be fixed to said adapter by means of fastening members and support formations are assigned to said adapter and to said wiper ring allowing that said wiper ring is supported on said adapter even before attaching said fastening members. It can be provided that said support formations comprise at least two support members which, circumferentially distributed with respect to the spindle axis, are arranged or may be arranged at one of the members, i.e. wiper ring and adapter, and which project in the axial direction towards the respective other member and which are axially insertable into an opposing support recess in the respective other member. It is particularly favourable when said wiper ring is slit and said support members and said support recesses are localized on both members in the circumferential direction on both sides of the slot of said wiper ring such that said wiper ring may be supported on said adapter with radial constriction. A pretension of the wiper ring is achieved by means of said radial constriction. This pretension keeps the wiper ring to the adapter as long as the fastening member are not yet attached. 
     A simple way of fastening the wiper member to the adapter is that said wiper member is axially traversed by at least two through bores for accommodating fastening bolts serving for fixing said wiper member to said adapter, said through bores being circumferentially distributed with regard to the spindle axis. It is conceivable to first of all fix the adapter to the nut and then to attach wiper members according to one&#39;s wishes to the adapter by means of the fastening bolts. If the wiper assembly comprises a plurality of wiper members the fastening bolts can traverse all of these wiper members. If the the fastening bolts in the final mounting state of the spindle drive unit are accessible from the outside, it is possible to subsequently add further wiper members to the wiper assembly or remove individual wiper members from the wiper assembly or replace them by other wiper members without removing the adapter from the nut. 
     For reasons of sealing and costs the wiper ring will often be made of plastics material which, however, is in danger of being pinched if the fastening bolts are overtightened. If the wiper member is made of material endangered of being pinched, in particular plastics material, it is favourable when in the through bores of which a spacer sleeve, preferably made of metal, is inserted which on the one end is supported by said adapter and on the other end is adapted for receiving clamping forces exerted by the respective fastening bolt. 
     Besides a sealing ring said wiper assembly can also comprise a wiper brush as a wiper member substantially embracing said spindle. The wiper brush brushes off the external surface of the spindle with a plurality of brush bristles. Further said wiper assembly can comprise a groove scraper as a wiper member which has at least one scraping projection engaging with a threaded nut in said external circumferential surface of said spindle. The wiper brush as well as the groove scraper primarily serve for brushing rough contaminations off the external surface of the spindle. Advantageously, they are supplemented by a primary wiper which also removes fine contaminations from the external surface of the spindle. Said primary wiper can be part of the wiper assembly or can be directly attached to the nut as a sealing member separate from the wiper assembly. 
     Further, the wiper assembly can comprise at least one cover member for said at least one wiper member. The cover member can be formed as a cover disk or cover plate, preferably made of sheet metal. The cover member serves to protect the wiper member(s) from hot chips which can accumulate in machine tools during machining workpieces. Further the cover member prevents the collection of other contaminations, such as dust, on the wiper member or the wiper members, and also offer proctection from aggressive chemicals, coolants or lubricants. 
     The solution in accordance with the invention is preferably used for a spindle drive unit, in which said spindle has at least one threaded groove in its external circumferential surface extending helically about said spindle axis. When forming the spindle drive unit as a rolling element screw drive unit, in particular ball screw drive unit, there is provided according to a preferred embodiment that in an internal circumferential surface of said nut at least one rolling-element-containing nut groove is formed which extends helically about said spindle axis and comprises two end portions, said nut groove together with said external circumferential surface of said spindle defining a helical rolling element path extending between said two end portions, said helical rolling element path being supplemented by a rolling element return path in said nut extending between said two end portions to form a closed rolling element circulating path, said rolling element circulating path being equipped with an endless row of rolling elements which on the one hand engage with said nut groove and on the other hand with a helical track surface of said external circumferential surface of said spindle. 
     According to a further aspect the invention relates to a kit for retrofitting a spindle drive unit, said spindle drive unit comprising a spindle having a spindle axis and an external circumferential surface as well as a nut embracing said spindle and being in threaded engagement with said spindle, wherein said kit comprises: at least one adapter which is removably connectable with said nut in an axial end portion of said nut in an axially and rotationally fixed manner, and at least one wiper assembly which is removably connectable with said adapter in a rotationally fixed manner and comprises at least one wiper member for wipingly engaging with said external circumferential surface of said spindle. This kit enables to subsequently equip a conventional spindle drive unit with a wiper assembly, wherein the adapter may be adapted to the nut with respect to its design, so that the nut does not have to be machined for attaching the wiper assembly. Different adapters or/and different wiper assemblies can be provided, so that for any desired application of the spindle drive unit and for any size and design of the spindle drive unit a suitable kit for retrofitting can be provided. The adapter and the individual members of the wiper assembly can be formed in the way described above. 
     According to a further aspect the invention relates to a motion system comprising at least two members being movable relative to each other, at least one wiper assembly being retained on at least one of said members, said wiper assembly being in wiping engagement with the respective other member. 
     In accordance with the invention in such motion system there is provided that said wiper assembly being retained on said one of said members by means of an adapter, said adapter being on the one hand fixedly, however, removably connectable with said one of said members and on the other hand fixedly, however, removably connectable with said wiper assembly. 
     The interconnection of an adapter may not only be applied with advantage in a motion system in which the two members are linearly movable relative to each other, as is for example the case in a linear guide device comprising a carriage guided for longitudinal movements on a guide rail, in particular profile rail, or comprising a ball bush guided lengthwise or in spherical liners guided lengthwise on a shaft. In the same way the two members can be rotable relative to each other, in particular screwable, as is for example the case in the above described spindle drive unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a perspective exploded view of a first embodiment of the spindle drive unit according to the invention; 
     FIG. 2 shows a spindle and a longitudinal section of a nut of the spindle drive unit of FIG. 1 mounted on the spindle; 
     FIG. 3 shows a view of the mounting of the spindle drive unit of FIG. 1 in a view similar to that of FIG. 2; 
     FIG. 4 shows a perspective view of an adapter of the spindle drive unit of FIG. 1; 
     FIG. 5 shows a view of the adapter of FIG. 4 in the line of sight of an arrow V, wherein a threaded nut is inserted into a slot of the adapter and the slot is filled with a sealing material; 
     FIG. 6 shows a section along line VI—VI of FIG. 5; 
     FIG. 7 shows a sealing ring and pretensioning clamp of the spindle drive unit of FIG. 1; 
     FIG. 8 shows a modification of a sealing ring and an adapter; 
     FIG. 9 shows a perspective exploded view of a further embodiment of the spindle drive unit according to the invention; 
     FIG. 10 shows a further embodiment of the spindle drive unit according to the invention in a view similar to that of FIG. 3; 
     FIG. 11 shows a perspective exploded view of a further embodiment of the spindle drive unit according to the invention; 
     FIG. 12 shows a longitudinal section of a portion of the spindle drive unit of FIG. 11; 
     FIG. 13 shows a perspective exploded view of a further embodiment of a wiper assembly and an adapter; 
     FIG. 14 shows a front view of the adapter of FIG. 13 from its side remote from the nut; 
     FIG. 15 shows an enlarged view of the portion designated with XV in FIG. 14; and 
     FIG. 16 shows a section along line XVI—XVI of FIG.  14 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a ball screw drive unit generally designated  10 , a nut  12 , a threaded spindle  14  comprising a spindle axis  16 , an adapter  18  which can be fixed to the nut  12  with an axial end, and a wiper assembly generally designated  20  which can be fixed to the adapter  18  with its side axially remote from the nut  12 . As regards the design of the threaded spindle  14  and the nut  12  being in threaded engagement therewith, reference is made to FIG.  2 . The threaded spindle  14  has a screw thread  24  worked into the external circumferential surface  22  of the spindle  14 . In the illustrated example the screw thread  24  is a single thread and formed by an external thread groove  26  extending helically around the spindle axis  16 . It goes without saying that the screw thread  24  of the threaded spindle can also be a multiple thread, for example a double thread. The nut  12  which annularly embraces the threaded spindle  14  also has at least one helically extending nut groove  30  in its internal circumferential surface  28 . The nut groove  30  has, just as the thread groove  26  in the external circumferential surface  22  of the spindle, a substantially semi-circular cross-section. The nut groove  30  and the thread groove  26  together form a ball screw path which is supplemented by an axial return channel, which is not shown, in the nut  12  to form a closed ball circulating path with an endless row of balls  32  circulating therein. 
     Adjacent to an axial front surface  34  of the nut  12  there is a profiled recess  36  worked into its internal circumferential surface  28 . Said profiled recess  36  comprises an annular groove  38  closer to the front surface  34  and an annular groove  40  further away from the front surface. The annular grooves  38 ,  40  are separated by an annular web  42 . The annular groove  38  is delimited by an annular web  44  towards the front surface  34 . The annular groove  40  serves for accommodating a circlip which is not shown and which can snap into the annular groove  40  and which secures a reversing member, which is also not shown, on the nut  12 , which reverses the balls  32  in an axial end portion of the nut  12  between the ball screw path and the ball return channel. 
     The adapter  18  serving as carrier of the wiper assembly  20  is clampable on the nut  12  in any desired position of the rotational angle relative to the nut  12 . In a mounting state in which it is clamped on the nut  12  the adapter  18  is fixed to the nut  12  in the axial direction as well as in the circumferential direction with respect to the spindle axis  16 . Reference is made to FIGS. 4 to  6  for an explanation of the adapter  18 . The adapter  18  is formed as an annular part the internal diameter of which is slightly larger than the external diameter of the spindle  14 . Its external diameter almost corresponds to the external diameter of the nut  12  in its axial end portion adjacent to the front surface  34 . Thus the adapter  18  in its mounting state is on the outside substantially even with the nut  12 . On its side axially facing the nut  12  the adapter  18  has an axially directed, ring-shaped plane surface  46  which is determined for plane abutment on the axial front surface  34  of the nut  12 . The plane surface  46  and the front surface  34  together form cooperating axial abutment means defining the axial position of the adapter  18  relativ to the nut  12 . A ring-shaped support rib  48  projects from the plane surface  46  substantially in the axial direction. The ring-shaped support rib  48  fits into the recess  36  of the nut  12  when mounting the adapter  18  on the nut  12  and centers the adapter  18  at the internal circumferential surface  28  of the nut  12 , i.e. at the internal circumferential surface of the annular web  44 . 
     At its side axially facing away from the nut  12  the adapter  18  has a ring-shaped plane mounting surface  50  serving for attaching the wiper assembly  20 . From the side of the adapter axially facing away from the nut  12  several, in the illustrated example four, axial fixation bores  52  are drilled into the mounting surface  50 . The fixation bores are formed as threaded blind holes and are distributed about the mounting surface  50  at almost identical angular distances. They serve for accommodating fastening screws  54 , shown in FIG. 1, and for fixing the wiper assembly  20  to the adapter  18 . 
     The adapter  18 , preferably made of aluminum, optionally of plastics material, is circumferentially separated by a slot  56  radially extending from the internal circumference to the external circumference of the adapter. The slot  56  axially traverses the adapter  18 . It is defined by two opposing slot defining surfaces  58  and  60  in the circumferential direction. A recess  62  is worked into one of the slot defining surfaces, here into the slot defining surface  58 , which extends through the adapter  18  in the axial direction and radially reaches from the external circumference of the adapter  18  to a web  64  circumferentially projecting into the slot  56 . At the same time a bore  66  extending orthogonal to the slot defining surface  58  opens into the slot defining surface  58 . The bore  66  is worked into the adapter  18  almost tangentially with respect to the circumferential direction from the external circumference. 
     For clamping the adapter  18  to the nut  12  a threaded nut  68  is inserted into the recess  62  so that it rests on the web  64  and abuts on the recessed part of the slot defining surface  58 . The threaded nut  68  comprises a threaded hole  70  which is in alignment with the bore  66  when the threaded nut  68  is inserted. The volume of the slot which remains after the nut  68  has been inserted into the recess  62  is then filled with an at least limitedly elastical sealing material  72 . The sealing material  72  prevents contaminations from penetrating into the interior of the adapter  18  from outside the adapter  18  and retains the nut  68  in the recess  62 . Preferably the sealing material  72  is injected around the nut  68  so that the nut  68  is not only retained at the slot defining wall  58  through the sealing material  72  but is also radially and axially secured against losening. At the same time with filling the slot  56 , the plane surface  46  is at least radially outside the support rib  48  also coated with a sealing material  74  which, in the mounting state of the adapter  18 , forms a sealing coat between the plane surface  46  and the axial front surface  34  of the nut  12 . Said sealing coat seals the connecting point formed between the adapter  18  and the nut  12  against the penetration of dirt and against the leakage of lubricant and compensates for irregularities in the plane surface  46  or/and in the axial front surface  34 . The slot filling  72  and the sealing coat  74  are formed during one working process, if desired continuously. The sealing and/or filling material used in the this process can be a rubber material, applied and vulcanized on the adapter  18 . However, it is also conceivable to apply a plastics material in an injection molding process. 
     The adapter  18  thus prepared is axially applied to the front surface  34  of the nut  12 . The support rib  48  engages—as has already been described—with the recess  36  at the internal circumference of the nut  12 . Then an attachment screw  76  (s. FIG. 1) is inserted into the bore  66 , which has no thread, and screwed into the threaded nut  68 . The attachment screw  76  is screwed into the threaded nut  68  so far that it sinks in the slot filling  72  with its end screwed in at first. If the attachment screw  76  is further screwed into the treaded nut  68 , which is retained in the recess  62  so that it cannot be turned, the fastening screw  76  finally presses against the slot defining surface  60  to such an extent that the adapter  18  widens. It is possible that the fastening screw  76  cuts through the material of the slot filling  72  thus getting in direct contact with the slot defining surface  60 . It is also possible that the fastening screw  76  presses into the slot filling  72  without affecting it. Due to the radial widening of the adapter  18  the support rib  48  is radially pressed against the top surface, designated  78 , of the annular web  44  until finally a friction-tight engagement is obtained between the support rib  48  and the annular web  44  which not only fastens the adapter  18  to the nut  12  in the circumferential direction but also prevents an axial removal of the adapter  18  from the nut  12 . 
     The radial widening of the adapter  18  is accompanied by an enlargement of the width of the slot  56 , i.e. the volume of the slot increases. The sealing material used for the slot filling  72  preferably has an elasticity which compensates for the increase in volume of the slot  56  and which enables it to expand into the additional volume of the slot arising when the adapter  18  widens. This is advantageous in preventing contaminations to penetrate or lubricant from leaking through the additional volume of the slot. Further the threaded nut  68  is always tightly kept in the recess  62 . 
     In order to remove the adapter from the nut  12  it is sufficient to release the attachment screw  76  which, in turn, again radially constricts the adapter  18  since it was only elastically and not plastically deformed by the previous radial widening. This loosens the press fit between the support rib  48  and the annular web  44  allowing to axially remove the adapter  18  from the nut  12 . It can easily be seen that the adapter  18  when the nut  12  is mounted on, i.e. when it is screwed onto the spindle  14 , can be mounted on and dismounted from the nut  12  as the adapter  18  can simply be axially applied on the nut  12  from the outside. When the adapter  18  has a suitable design, e.g. when the adapter  18  can be opened by means of a joint or consists of a sufficiently elastically deformable material, it can be conceivable to not only push the adapter  18  axially from one spindle end of the spindle  14  onto the spindle  14  but also to radially slip it on the spindle  14  or remove it from the spindle  14 . 
     Again reference is made to FIG.  1 . The wiper assembly comprises a sealing ring  80  engaging with the thread  24  of the spindle  14 , a cover plate  82  protecting the sealing ring  80  for example from hot chips as well as a groove scraper  84  roughly prepurifying the thread  24  of the spindle  14 . The components sealing ring  80 , cover plate  82  and groove scraper  84  may be fixed to the mounting surface  50  of the adapter  18  by means of the fastening screws  54 . The sealing ring  80  comprises a set of through bores  86 , the number of which corresponds to the number of the blind bores  52  of the adapter  18  and which are distributed about the circumferential direction just as the blind bores  52 , so that they may be axially aligned with the blind bores  52 . The through bores  86  do not have a thread. Even the cover plate  82  has a set of through bores  88  which may be axially aligned with the through bores  86  and the fixation bores  52 . Eventually the groove scraper  84  comprises receiving holes  90  which are elongated in the circumferential direction and may be axially aligned with the through bores  88  of the cover plate  82 . As in the example illustrated in FIG. 1 the groove scraper  84  only has a semi-circular basic design, it only has two receiving holes  90 , whereas both the cover plate  82  and the sealing ring  80  each have four through bores  88  and  86 , respectively. For mounting the wiper assembly two of the fastening screws  54  are inserted through respective through bores  88  in the cover plate  82  and through respective through bores  86  in the sealing ring  80  and screwed into respective fixation bores  52  in the adapter  18 . For attaching the groove scraper  84  two other fastening screws  54  are additionally inserted through the receiving holes  90  thereof and screwed into the remaining two fixation bores  52  of the adapter  18 . 
     The sealing ring  80  comprises an internal circumferential surface  92  which has a threaded profile  94  being complementary to the thread  24  of the spindle  14 . On its side axially facing the adapter  18  the sealing ring  80  has an axially directed abutment surface  96  (cf. FIG. 3) for abuting to the mounting surface  50  of the adapter  18 . On its side axially facing away from the adapter  18  the sealing ring  80  has an axially directed front surface  98  into which a receiving recess  100  adapted to the contour of the cover plate  82  is worked. The cover plate  82  can be positively inserted into said receiving recess  100  so that it fits evenly into the part of the front surface  98  of the sealing ring  80  recessed by the recess  100 . The cover plate  82  is formed as a segment ring, but it can also have any other shape which sufficiently protects the sealing ring  80  from damage. The sealing ring  80  is formed of plastics material, e.g. injection moulded. Preferably the cover plate  82  consists of sheet steel, however, if desired, it can also consist of other metal, e.g. brass or aluminum, or of a suitable shock- and heat-resistant plastics material. 
     The sealing ring  80  is slit. In this context reference is made to FIG. 7 which shows a somewhat simplified top view of the sealing ring  80 . A separating slot  102  extending diagonally with respect to the radial direction separates the sealing ring  80  at a circumferential location. Substantially the separating slot  102  is oriented in the axial direction. As the slot  102  is constricted a radial pretension of the sealing ring  80  is obtained radially biassing the sealing ring  80  in its mounted state against the external circumferential surface  22  of the spindle  14 . It turned out that radial pretension considerably improves the sealing and/or wiping effect of the sealing ring  80 . In order to bias the sealing ring  80  the slot  102  has to be constricted and thus the sealing ring  80  is radially constricted. For constricting the separating slot  102 FIGS. 1 and 7 show a pretensioning clamp  104  which extends like a bow in the circumferential direction over the separating slot  102  and, with its bent up ends  106  of the bow, is insertable in insert holes  108  arranged on both sides of the separating slot  102  with respect to the circumferential direction. The mutual distance of the insert holes  108  in the circumferential direction is chosen in such a way that when the pretensioning clamp  104  is inserted the separating slot  102  is constricted or even closed to such an extent that the desired pretension of the sealing ring  80  is obtained. The diagonal course, i.e. with respect to the radial direction, of the separating slot  102  is of advantage as the two cutting edges defining the separating slot  102  and lying opposite of each other in the circumferential direction act as wedge surfaces when the separating slot  102  is completely closed by means of the pretensioning clamp  104  or any other suitable pretensioning means. The wedge angle of the wedge surfaces is chosen so that is does not lie in the range of self-locking. When closing the separating slot  102  the two wedge surfaces slide along each other which leads to the fact that a slot edge portion  110  of the sealing ring  80  which in FIG. 7 is shown on the right side radially moves towards the inside relative to a slot edge portion  112  of the sealing ring which in FIG. 7 is shown on the left side, thus intensifying the pretension of the sealing ring  80 . 
     Due to the radial constriction of the sealing ring  80  during its pretension the through bores  86  in the sealing ring  80  are offset transversely with respect to the axial direction by a small degree relative to the fixation bores  52  in the adapter  18 . Therefore it is advisable that the through bores  86  in the sealing ring  80  have a somewhat larger cross-section than the threaded portions of the fastening screws  54 , so that a clearance is formed between the fastening screws  54  and the through bores  86 . It turned out that usual tolarances which cannot be avoided when drilling holes into plastic or metal parts are often sufficient to guarantee the desired small clearance between the fastening screws  54  and the through bores  86 . 
     The groove scraper  84  is preferably formed as a bent sheet metal part. It has a semi-circular fixation portion  114  including the receiving holes  90  and at least one axially bent up scraper portion  116  at one end of the fixation portion  114 . For a single thread  24  of the spindle  14  one scraper portion  116  is enough. The scraper portion  116  is formed with an almost crescent-like scraper projection  118  which is adapted to the cross sectional contour of the threaded groove  26  of the spindle  14 . In the example illustrated in FIG. 1 the groove scraper  84  has two scraper portions  116  the scraper projections  118  of which are determined to engage in a double thread of a spindle and which diametrically lie opposite each other. The design of the receiving holes  90  as long holes enables to adjust the groove scraper  84  relative to the adapter  18  in the circumferential direction in order to achieve optimum engagement of the scraper projections  118  with the thread  24  of the spindle  14 . 
     The spindle drive unit of FIG. 1 is assembled in the following way: First the nut  12  is mounted on the spindle  14  as is already shown in FIG.  1 . Then the individual components of the wiper assembly  20  are attached to the adapter  18  by means of the fastening screws  54 . At that time the pretensioning clamp  104  does not yet have to be attached to the sealing ring  80 , however, this may not be excluded. The premounted constructional unit consisting of adapter  18 , sealing ring  80 , cover plate  82  and groove scraper  84  is then mounted on the nut. Said constructional unit is screwed onto the spindle which is necessary as the threaded profile  94  of the sealing ring  80  engages with the thread  24  of the spindle  14 . The constructional unit is screwed so close to the nut  12  until the adapter  18  abuts with its plane surface  46 , i.e. the sealing coat  74 , on the front surface  34  of the nut  12 . In tightening the attachment screw  76  the adapter  18  and the wiper assembly  20  are fixed to the nut  12 . Afterwards the pretensioning clamp  104  is mounted in order to bias the sealing ring and the spindle  14  is mounted to respective spindle bearings at its spindle ends. The spindle drive unit  10  is dismounted in the reverse order. 
     An alternative to assemble the spindle drive unit  10  is to fix the adapter  18  on the nut  12  mounted on the spindle  14 , and then to insert the spindle  14  in its spindle bearings before the wiper assembly  20  is mounted on the adapter  18 . In effect, the wiper assembly  20  can be mounted on when the spindle  14  is already mounted. Because of the separating slot  102  the sealing ring  80  can be bent apart and radially applied on the spindle  14 . By manual alignment the engagement of the threaded profile  94  with the thread  24  of the spindle  14  can be achieved easily. Since the cover plate  82  is formed as a partial ring, it can radially be put on the spindle  14  with ease. The same applies to the semicircular groove scraper  84 . The components of the wiper assembly are fixed to the adapter  18  by means of the fastening screws  54 . The adapter  18  itself is fixed to the nut  12 . The spindle drive unit  10  can be disassembled in the reverse order. In particular when only the sealing ring  80  is worn-out and is to be exchanged this last procedure is advisable as neither the spindel  14  has to be removed from it, spindle bearings nor the adapter  18  has to be dismounted from the nut  12 . FIG. 3 shows the assembled state of the spindle drive unit  10 . 
     As regards the description of FIGS. 8-16 the same reference numerals, supplemented by a small letter, are used as in FIGS. 1 to  7 , as far as the same components or components having the same effect are addressed. In order to avoid repetitions substantially only the differences as regards the embodiment of FIGS. 1-7 are described. Apart from that reference is made to the foregoing description of FIGS. 1 to  7 . 
     FIG. 8 shows an alternative of pretensioning the sealing ring  80   a . It is not pretensioned, as is the case in the embodiment of FIGS. 1 to  7 , by a separate pretensioning member, i.e. in this case the pretensioning clamp  104 , but is inserted in a circular pretensioning recess  120   a  recessed in the mounting surface  50   a  of the adapter  18   a . The diameter of said pretensioning recess  120   a  is somewhat smaller than the external diameter of the sealing ring  80   a  in its relaxed state. In order to insert the sealing ring  80   a  into the pretensioning recess  120   a  of the adapter  18   a  it is compressed by hand and then pushed into the pretensioning recess  120   a . Within the pretensioning recess  120   a  the sealing ring  80   a  is fixed in both the radial and the axial direction. 
     FIG. 9 shows a further embodiment in which the wiper assembly  20   b  only comprises the sealing ring  80   b , however, no groove scraper. This is possible without further ado as long as no contaminations of the spindle  14   b  are to be expected which require a secondary wiper. In this embodiment the pretension of the sealing ring  80   b  is carried out in that the sealing ring  80   b  is inserted into a sleeve portion  122   b  which axially connects to the cover sheet  82   b . The sleeve portion  122   b  can be of one piece with the cover plate  82   b , e.g. in the form of a bent sheet metal part, but may also be fixed to the cover plate  82   b  as a separately manufactured component, e.g. by glueing or welding. The internal diameter of the sleeve portion  122   b  is again somewhat smaller than the external diameter of the sealing ring  80   b  in the relaxed state, so that the separating slot  102   b  of the sealing ring  80   b  is constricted and the sealing ring  80   b  is radially pretensioned when the cover plate  82   b  together with the sleeve portion  122   b  is placed on the sealing ring  80   b . A pretensioning member separate from the cover plate  82   b , such as the pretensioning clamp  104  in FIGS. 1 and 7, is not necessary in the embodiment of FIG.  9 . The spindle drive unit  10   b  of FIG. 9 is assembled and disassembled analogously to the embodiment of FIGS. 1 to  7 . 
     FIG. 10 shows a spindle drive unit  10   c  in which the wiper assembly  20   c  has no sealing ring but only a groove scraper  84   c . The groove scraper  84   c  is fixed to the adapter  18   c  by means of two fastening screws as can easily be seen in FIG.  1 . Thus two fixation bores of the adapter  18   c  remain vacant. Since the groove scraper  84   c  only has the effect of a secondary wiper and only removes rough contaminations from the spindle thread  24   c , a sealing ring  124   c  separated from the wiper assembly  20   c  is inserted into the annular groove  38   c  of the nut  12   c  in order to achieve sufficient sealing of the nut  12   c  with regard to the spindle  14   c . The sealing ring  124   c  is axially fixed in the annular groove  38   c  by the annular webs  42   c  and  44   c . The sealing ring  124   c  preferably consists of an elastomer material and has a sealing lip  126   c  radially projecting to the inside. The sealing lip  126   c  has a lip edge  128   c  which sealingly abuts at the external circumferential surface  22   c  of the spindle  14   c . The sealing lip  126   c  may axially flexibly deflected so that it optimally adapts to the external circumferential surface  22   c  of the spindle  14   c . The sealing ring  124   c  is preferably mounted with radial pretension to improve the sealing effect. Attention has to be paid to the fact that the mounting of the adapter  18   c  on the nut  12   c  is not impeded by the sealing ring  124   c  since the support rib  48   c  of the adapter  18   c  does not axially extend beyond the annular web  44   c  of the nut  12   c.    
     FIGS. 11 and 12 show a spindle drive unit  10   d  in which the adapter  18   d , unlike in FIGS. 1 to  10 , is clampable to the nut  12   d  by means of a hook arrangement  130   d . In the illustrated embodiment the hook arrangement  130   d  comprises two diametrically opposing fixation hooks  132   d  of which only one can be seen in FIG.  11 . The fixation hooks  132   d  are each inserted into an accommodating pocket  134   d  of the adapter  18   d  from the side of the adapter  18   d  axially facing the nut  12   d . In the region of the accommodating pocket  134   d  a radial threaded bore  138   d  extending from the external circumferential surface  136   d  of the adapter  18   d  to the accommodating pocket  134   d  is worked in, which serves to accommodate a headless screw  140   d . FIG. 12 shows that the headless screw  140   d  may be screwed in the threaded bore  138   d  so that it engages with a recess  142   d  of the fixation hook  132   d  which lies in the area of a pivot end  144   d  of the fixation hook  132   d  which axially opposes a hook end  146   d  of the fixation hook  132   d . The fixation hook  132   d  has a pivot portion  148   d  which rests on a bottom surface  150   d  of the accommodating pocket  134   d . By means of further screwing in of the headless screw  140   d  into the threaded bore  138   d , the fixation hook  132   d  is pivotal from a release position shown in FIG. 12 into a hook position in which the fixation hook  132   d  with its hook end  146   d  is hooked behind the annular rib  44   d  which is adjacent to the front surface  34   d  of the nut  12   d . Then, the adapter  18   d  is axially fixed to the nut  12   d  in this hook position. To remove the adapter  18   d  from the nut  12   d  the headless screw  140   d  is released which leads to the fact that the fixation hook  132   d  tips back into its release position shown in FIG.  12 . Then, the adapter  18   d  can be axially removed from the nut  12   d.    
     In FIG. 11 it can be seen that the fixation bores  52   d  of the adapter  18   d  axially traverse the adapter  18   d  and the fastening screws  54   d  are inserted from the side of the adapter  18   d  axially facing the nut  12   d.    
     In the spindle drive unit  10   d  of FIGS. 11 and 12 the wiper assembly  20   d  comprises a wiper brush  152   d , made up of a two-part brush housing  154   d  and a brush insert  156   d . The ring-shaped brush housing  154   d  may be assembled of two semi-circular housing halves  158   d  and has threaded bores  160   d  which may be axially aligned with the fixation bores  52   d  of the adapter  18   d  and may be screwed into the fastening screws  54   d  in order to attach the wiper brush  152   d  to the mounting surface  50   d  of the adapter  18   d . The brush insert  156   d  is insertable into a ring-shaped brush accommodation  162   d  in the brush housing  154   d . It is slit as is indicated in FIG.  11 . Thus, the brush insert  156   d  can be applied on the spindle  14   d  by bending it apart. Thereafter, the two housing halves  158   d  are closed around the brush insert  156  to form the brush housing  154   d . FIG. 12 shows that the adapter  18   d  has a ring-shaped centering projection  166   d  on its side axially facing the wiper brush  152   d  which serves to center to brush housing  154   d.    
     The spindle drive unit  10   d  of FIGS. 11 and 12 can be assembled such that the adapter  18   d  is premounted with the wiper brush  152   d  to form a constructional unit, said constructional unit is then pushed onto the spindle  14   d  from one end of the spindle. Afterwards the spindle  14   d  together with the nut  12   d  attached to it is installed in the spindle bearings at its spindle ends. Alternatively it is also possible to apply the adapter  18   d  on the spindle  14   d , however, without fixing it to the nut  12   d . Then the spindle  14   d  is installed in its spindle bearings whereupon the brush insert  156   d  is radially applied on the spindle  14   d  and the brush housing  154   d  is attached. The wiper brush  152   d  is then screwed to the adapter  18   d , this constructional unit is then axially pressed on the nut  12   d  and, by means of screwing in the headless screws, hooked to the nut  12   d . The spindle drive unit  10   d  can be disassembled in the reverse order. 
     Now reference is made to the embodiment of FIG. 13 to  16 . There, in the adapter  18   e  the attachment screw  76   e  is not screwed into a nut separately inserted into the slot  56   e  but into a screw thread  168   e  formed in the tangential bore  66   e . A sink recess  170   e  is formed in the slot defining surface  60   e . When the attachment screw  76   e  is screwed into the screw thread  168   e  of the tangential bore  66   e  to clamp the adapter  18   e  to the nut, the leading end of the attachment screw  76   e  bridging the slot  56   e  is supported in the sink recess  170   e , which leads to a widening of the slot  56   e  when the attachment screw  76   e  is screwed in further. 
     FIG. 16 shows that the blind bores  52   e  in the adapter  18   e  are formed as step bores and each have a step-like extension  172   e  opening into the mounting surface  50   e . In these step-like extensions  172   e  distance sleeves  174   e  (s. FIG. 13) are inserted, the length of which is larger than the depth of the step-like extensions  172   e . When the distance sleeves  174   e  are inserted in the step extensions  172   e  they project from the mounting surface  50   e  of the adapter  18   e . The diameter of the through bores  86   e  in the sealing ring  80   e  corresponds to the external diameter of the distance sleeves  174   e . In the relaxed state of the sealing ring  80   e  the circumferential distance of the two through bores  86   e  located on both sides of the separating slot  102   e  is somewhat larger than the circumferential distance of the two opposing step extensions  172   e  of the adapter  18   e . For mounting the wiper assembly  20   e  on the adapter  18   e  first of all the sealing ring  80   e  can be applied on the distance sleeves  174   e  inserted in the adapter  18   e . Thereby the sealing ring  80   e  is slightly compressed in the region of the separating slot  102   e . This leads to a radial inherent pretension of the sealing ring  80   e  which keeps it at the adapter  18   e  without falling off when being released from the person responsible for mounting. This facilitates the mounting process insofar as the person responsible for mounting has both hands free to attach the sealing ring  82   e  and the groove scraper  84   e  to the fastening bolts  54   e  and screw the latter ones through the sealing ring  80   e  and the distance sleeves  174   e  into the adapter  18   e . The radial constriction when applying the sealing ring  80   e  onto the distance sleeves  174   e  can be so large that the desired radial pretension of the sealing ring  80   e  with regard to the spindle is achieved. 
     The length of the distance sleeves  174   e  is preferably measured so that they slightly project into the recess  100   e  of the sealing ring  80   e  into which the cover plate  82   e  is inserted. When mounting the wiper assembly  20   e  the cover plate  82   e  is pressed against the distance sleeves  174   e  which, in turn, are supported by the step extensions  172   e  of the adapter  18   e . When tightening the fastening screws  54   e  the sealing ring  80   e  which is preferably made of plastics material is not axially pinched. As the adapter  18   e , the distance sleeves  174   e  and the cover plate  82   e  are preferably made of metal which is not endangered of being pinched the fastening screws  54   e  can nevertheless be tightened to such an extent that they do not loosen even when subjected to strong operational vibrations. Further when determining a suitable tightening torque for the fastening screws  54   e  the dependence on temperature of the material hardness of the sealing ring  80   e  does not have to be taken into account.