Patent Publication Number: US-2023133937-A1

Title: Printing system

Description:
BACKGROUND 
     Certain printing systems utilize one or more print carriages that carry elements such as print heads that can deliver substances to a printing area of the printing system. So that such elements have access to the printing area, the print carriage is arranged to transverse the printing area. The print carriage can be guided along the desired path by a guide such as a rod. 
     In three-dimensional (3D) printing systems, a print carriage may be deployed to deposit layers of build material onto the printing area and/or deposit certain printing agents, such as a fusing agent in the form of an image representing a portion of layer to be solidified. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various features of the present disclosure will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate features of the present disclosure, and wherein: 
         FIG.  1    is a schematic diagram of an example printing system comprising a carriage, a bearing housing, and a carriage guide. 
         FIG.  2    is a cross-section through the bearing housing and carriage guide of  FIG.  1   . 
         FIG.  3    is a is plan view of another example printing system comprising a carriage, bearing modules, and a carriage guide. 
         FIG.  4    is a cross-section through the bearing housing and carriage guide of  FIG.  3   . 
         FIG.  5    is a is a schematic diagram of another example printing system comprising a carriage, a bearing housing, and a carriage guide. 
         FIG.  6    is a cross-section through the bearing housing and carriage guide of  FIG.  5   . 
         FIG.  7    is diagram illustrating an example of the angular positions of a pair of bearings, relative to a carriage guide. 
         FIG.  8    is diagram illustrating an example of the angular distance between first and second guide contact positions. 
         FIGS.  9   a ,  9   b ,  10   a , and  10   b    illustrate an example of a printing system in which a carriage guide is movable relative to a supporting structure of the printing system. 
         FIGS.  11   a  and  11   b    illustrate an example of a printing system in which at least one bearing is movable relative to a printer carriage. 
         FIG.  12    is a flow chart of an example method of relocating at least one bearing laterally around the profile of a longitudinally extending carriage guide. 
     
    
    
     DETAILED DESCRIPTION 
     One example of a printing system  10  employing a printer carriage  12  is shown in  FIG.  1   . The carriage guide  30  extends in a longitudinal direction. In some examples, the carriage guide  30  extends in in a substantially straight line. The printing system  10  includes a carriage guide  30 . The printer carriage  12  is selectively movable along the carriage guide  30 . That is the printer carriage can be moved to any chosen position along the carriage guide  30 . The carriage guide  30  directs the motion of the printer carriage  12 . In certain examples, the carriage guide  30  acts to support the printer carriage  12 . The printer carriage  12  carries elements that can perform a printing function. For instance, print agent depositing print heads may be located in the printer carriage  12 . 
     In the example shown in  FIG.  1   , the carriage guide  30  is linearly straight and directs the motion of the printer carriage  12  in an X-direction so that the printer carriage  12  is translatable across a print area A that extends in the X-direction. Hence, in this example, the longitudinal direction of the carriage guide  30  is substantially aligned with the X-direction. The printer carriage  12  is movable in a back and forth motion along the carriage guide  30 . The print area A also extends in a Y-direction. The printer carriage  12  may extend in the Y-direction to cover the full extent of the print area A. 
     The print area A may provide space for print media to be located so that the elements located in the printer carriage  12 , such as print heads, can deposit print agents upon the print media, in use. 
     In examples where the printing system is a three-dimensional (3D) printing system, the print area A extends in a Z-direction. For example, the print area A may be defined by a build platform that is movable in the Z-direction so that successive layers of a build material may be deposited in the print area A. The print area A may also be referred to as a print bed or a build area. The build material may be a powder comprising metal or other material. Each layer may have print agents, for example a fusing agent or a binding agent, applied from the printer carriage  12  selectively passing along the carriage guide  30 . The printer carriage  12  may deploy other functionality when translating over the print area A such as applying heat or infrared radiation to fuse portions of build material or evaporate liquid components of the applied agents. The printer carriage  12  may also function to deposit a new layer of build material into the print area A. Each of these functions may be performed by the printer carriage  12  or additional carriages may be provided that perform specific functions. 
     The printing system  10  comprises at least one bearing. The at least one bearing is to direct motion of the carriage along the carriage guide  30 . In certain examples, the printing system  10  comprises a single bearing. In other examples, more than one bearing is provided. For example, a single friction bearing or pad may be provided to slide along the carriage guide  30 . In other examples, one or more friction pads may be provided. In certain examples, one or more roller bearings may be provided.  FIG.  2   , which is a schematic diagram of a cross-section in a Y-Z plane through the printing system  10  of  FIG.  1   , is another example of more than one bearing being used. In the example shown, the printing system  10  comprises two bearings  20   a ,  20   b . The bearings  20   a ,  20   b  each comprise roller bearings that roll along the surface of the carriage guide  30 . Other bearing types may be used. 
     In the example shown in  FIG.  2   , the bearings  20   a ,  20   b  are retained in a separate bearing housing  14 , although the bearings  20   a ,  20   b  could instead be housed in the bulk of the carriage  12 . The bearing housing  14  may be described as a bearing module and may contain any suitable number of bearings, such as the pair of roller bearings illustrate din  FIG.  2   . The bearing housing  14  may be connected to the printer carriage  12  or to a carriage support that connects the bearing housing  14  to the printer carriage  12 . In some examples, the printer carriage  12  is to be supported on the bearings  20   a ,  20   b  in addition to being directed along the carriage guide  30  by the bearings  20   a ,  20   b.    
     To guide and direct the motion of the printer carriage  12  along the carriage guide  30 , in use, the at least one bearing contacts the carriage guide  30  at a contact position. The contact between the carriage guide  30  and the at least one bearing maintains the position of the printer carriage  12  relative to the carriage guide  30 . The contact position is a point around a profile of the carriage guide  30  at which the at least one bearing contacts the carriage guide  30  at any one location along the length of the carriage guide  30 . The profile of the carriage guide  30  is the outline, or boundary, of the carriage guide  30  formed on a plane passing through the carriage guide  30  and oriented substantially laterally, or in a transverse direction, to the longitudinal direction of the carriage guide  30 . Thus, the plane is substantially perpendicular to the longitudinal direction of the carriage guide  30 . Put another way, the contact position is the point along the perimeter, or boundary, of a cross-section through the carriage guide  30  where the at least one bearing contacts the carriage guide  30 . 
     In certain examples, the carriage guide  30  has a fixed cross-sectional profile along the entire length of the carriage guide  30 . This can help provide reliable positioning of the print carriage  12 , for example in the X, Y, and Z-directions as shown in  FIG.  2   . In such examples, the contact position is at the same relative point around the carriage guide  30  profile for each location along the length of the carriage guide  30 . In the example shown in  FIGS.  1  and  2   , the carriage guide  30  is straight and substantially aligned with the X-direction; hence, the two bearings  20   a ,  20   b  fix the position in the Y-direction and the Z-direction and allow for movement in the X-direction. The carriage guide  30  may comprise a metal, for example a carbon steel, which is easily machined. The metal may be plated to provide a higher surface hardness. For example, the carbon steel may be chrome plated. 
     In the example shown in  FIGS.  1  and  2   , the carriage guide  30  has a substantially circular profile  32  and thus comprises a cylindrical rod. The two bearings  20   a ,  20   b  contact the carriage guide  30  at contact positions  40   a ,  40   b . Thus, in this instance, the contact position of the bearings  20   a ,  20   b  can be defined as located along the circumference of a cross-sectional circular profile of the carriage guide  30 . It will be understood that other carriage guide shapes are possible. 
       FIGS.  3  and  4    further schematically illustrate an example printing system  10 . In  FIG.  3   , the printer carriage  12  is relatively large, such as may be deployed in a 3D printing system in comparison with a 2D printing system, the printer carriage  12  is supported on a carriage guide  30  by left and right bearing modules  14 L,  14 R at a first end of the printer carriage  12 , which may each comprise a bearing housing  14  as described above with respect to  FIGS.  1  and  2   . At a second end opposing the first end, the printer carriage  12  is carried on a third bearing module  90  that is in contact with a second carriage guide  80 . The second carriage guide  80  may only act to support the printer carriage  12  and assist with positioning the printer carriage  12  in the Z-direction or may function in a similar manner to the any of the carriage guides  30  described herein. Detail of a second end of the printer carriage  12  has been excluded from  FIG.  1   . 
     In certain examples, the carriage guide  30  is mounted to a structure of the printing system  10  to provide suitable rigidity and therefore accurate printing from the elements carried on the printer carriage  12 . For example, the carriage guide  30  may be mounted on a supporting beam  50  that runs substantially parallel to the carriage guide  30 . Supporting beam  50  is located under carriage guide  30  in  FIG.  3    and can be seen in a schematic cross-section through the bearing module  14 R in  FIG.  4   . The beam  50  comprises a carriage support  52  on which the carriage guide  30  is mounted. The carriage support  52  may comprise a supporting rib that extends the length of the carriage guide  30  or may comprise a plurality of individual supports distributed along the length of the beam  50 . The carriage support comprises a suitable surface to mate with the carriage guide  30 . The carriage guide  30  is fixed to the supporting beam  50  using fasteners, in this case machine screws, distributed along the length of the carriage guide  30 . 
     Movement of the printer carriage  12  along the carriage guide  30  traces out a bearing contact path  42 , as shown in  FIG.  1   . The traced contact path  42  passes through each of the contact positions along the length of the carriage guide  30 . As shown in  FIG.  1   , the contact path  42  may be substantially straight when the carriage guide  30  is substantially straight and has a fixed cross-sectional profile along the entire length of the carriage guide  30 . Over the lifetime of a printing system  10 , the printer carriage  12  will traverse backwards and forwards along the carriage guide  30  many thousands of times. The carriage guide  30  will therefore be subject to surface wear and degradation in a region in and around the contact path. Both the bearing(s) and the carriage guide  30  can be worn during the repetitive motion of the carriage  12 , but, in certain examples, the carriage guide  30  tends to wear out faster because the surface of the carriage guide  30  is not as hard as the surface of the bearings(s). For instance, the wearing may progress to the extent that a substantially flat surface is formed along the contact path  42 . In some examples, the carriage guide  30  may be plated, such as with a chrome plating, and the plating will be worn off the carriage guide  30  along the contact path  42 . The wear or degradation may only involve the removal of microns of depth off the surface of the carriage guide  30  surface and still cause issues with the functioning of the printing system. For instance, plating on the carriage guide  30  may only be a few tens of microns thick and the service life of the carriage guide  30  may be considered over when that plating has been fully eroded along the contact path  42 . Such wear can cause the useful service life of the carriage guide  30  to end because the positional accuracy of the printer carriage  12  can no longer be guaranteed due to the worn contact path. In particular, the Z-direction positional accuracy of the printer carriage  12  can be lost and this impacts the print quality delivered from the printer carriage  12 ; for instance, print head nozzles carried by the printer carriage can be undesirably closer to the print bed than intended thereby causing a degradation in print quality. The print heads themselves can also then suffer a shortened service life in such circumstances. 
     In certain printing systems, a carriage guide that has reached the end of its service life is difficult to replace without significant disassembly of many components of the printing system. For instance, a carriage guide is usually firmly attached to the supporting structure, such as the beam described above, and this structure often requires specific tooling and calibration processes that are normally only found in a factory setting. Accordingly, replacing a carriage guide is considered a difficult and expensive maintenance task. 
     The Applicant has also found that the shortening of the service life of a carriage guide due to the factors described above can be exacerbated in two-dimensional and three-dimensional printers that are deployed into industrial environments where such printers experience intensive and heavy use. In some three-dimensional printer applications, the printer carriages are comparatively large because such carriages may need to hold a large number of print heads and/or have additional sealing requirements to prevent ingestion of powdered build material into the carriage. In some applications, the removal of printing system components requires the use of a lifting crane before a carriage guide can be replaced. 
     Disclosed herein is a printing system comprising a carriage guide, a print carriage, and at least one bearing. The carriage guide extends in a longitudinal direction and the print carriage is selectively movable along the guide, in use. The at least one bearing is to direct motion of the print carriage along the carriage guide. The at least one bearing is movable from a first carriage guide contact position to a second carriage guide contact position. In moving the from the first carriage guide contact position to the second carriage guide contact position, the at least one bearing moves, relative to the carriage guide, laterally to the longitudinal direction of the carriage guide. The lateral movement is movement in a transverse direction to the longitudinal direction. In moving the at least one bearing, from the first carriage guide contact position to the second carriage guide contact position, and relative to the carriage guide is to relocate the at least one bearing laterally around the profile of the longitudinally extending carriage guide. 
     Moving the at least one bearing relative to the carriage guide in a lateral direction moves the contact position to a new contact position where the surface of the carriage guide is unworn and not degraded. Hence, performed as a maintenance procedure, this rearranging of the relative positions of the at least one bearing and the carriage guide allows the printing system to be returned to a substantially as new refurbished state, excepting any existing wear on the at least one bearing (which may also be replaced separately in some instances). In the case where this relative rearrangement is performed once, the service life of the carriage guide can be doubled. In some examples, the Applicant envisions that this procedure could be performed several times such that the at least one bearing is moved laterally relative to the carriage guide over service life periods to third or fourth positions—or more positions—thereby increasing the service life of the carriage guide by three or four times. The number of times that the procedure could be performed depends on the profile geometry of the carriage guide. The rearrangement procedure is also easier than a maintenance procedure where a new carriage guide is installed in the printing system thereby reducing the operating downtime during which the printing system is being maintained. 
     In certain examples, the carriage guide is moved relative to a supporting structure of the printing system on which the carriage guide is mounted whilst the at least one bearing remains in the same position relative to the carriage. In one example described further below, the carriage guide may be rotated to a new position on the support structure on which it is mounted thereby providing an unworn portion of the surface of the carriage guide as a new contact position for the at least one bearing. 
     In other examples, the at least one bearing may be moved relative to the printer carriage on which the bearing is mounted, such as in the bearing housing described herein, whilst the carriage guide remains fixed to the structure of the printing system on which it is mounted. For example, as described further below, a bearing housing holding the at least one bearing may be shifted to a different mounting location on a structure of the print carriage so that the at least one bearing runs on or contacts the surface of the carriage guide in a new contact position. 
     Moving the at least one bearing laterally to the longitudinal direction of the carriage guide involves moving the at least one bearing around the profile of the carriage guide so that a new point on the profile is where the at least one bearing contacts the carriage guide. Put another way, moving the at least one bearing laterally to the longitudinal direction of the carriage guide involves moving the at least one bearing along the perimeter of a cross-sectional area of the carriage guide so that a new point on the perimeter is where the at least one bearing contacts the carriage guide. Thus, where the carriage guide has a fixed cross-sectional profile along the entire length of the carriage guide, the new contact position is at the same relative point around the carriage guide profile for each location long the length of the carriage guide and, where the carriage guide is substantially straight, movement of the carriage along the carriage guide will trace out a substantially straight bearing contact path. 
       FIGS.  5  and  6    illustrate an example of a printing system  10  comprising a carriage guide  30 , a carriage  12 , and two bearings  20   a ,  20   b . It will be understood that any suitable number of bearings could be employed instead. As with  FIGS.  1  and  2   ,  FIGS.  5  and  6    illustrate the carriage guide  30  having a substantially circular profile  32 , and thus comprising a cylindrical rod. The two bearings  20   a ,  20   b  contact the carriage guide  30  at first contact positions  40   a ,  40   b . Movement of the printer carriage  12  along the carriage guide  30  traces out bearing contact paths, of which only the path  42  traced out by bearing  20   a  is shown in  FIG.  5   . As shown in  FIG.  5   , the contact paths  42  are substantially straight. 
     The bearings  20   a ,  20   b , are movable, laterally to the longitudinal direction of the carriage guide  30 , from the first contact positions  40   a ,  40   b  to second contact positions  41   a ,  41   b  where the bearings  20   a ,  20   b  each contact the carriage guide  30  in a new location. Moving the bearings  20   a ,  20   b  laterally to the longitudinal direction of the carriage guide  30  involves moving the bearings  20   a ,  20   b  around a profile  32  of the carriage guide  30  so that the bearings  20   a ,  20   b  contact the carriage guide  30  at new points on the profile  32 . 
     In the example shown in  FIGS.  5  and  6   , the first contact positions  40   a ,  40   b  of the bearings  20   a ,  20   b  can be defined as the location along a circumference of the circular profile  32 . Thus, in this example, moving the bearings  20   a ,  20   b  laterally to the longitudinal direction of the carriage guide  30  involves moving the bearings  20   a ,  20   b , relative to the carriage guide  30 , in a circumferential direction C of the carriage guide  30  from the first contact positions  40   a ,  40   b  to the second contact positions  41   a ,  41   b . For example, the bearings  20   a ,  20   b  may be rotated by an angle δ, about an axis of the carriage guide  30 , from the first contact positions  40   a ,  40   b  to the second contact positions  41   a ,  41   b . Once the bearings  20   a ,  20   b  are arranged in the second contact positions  41   a ,  41   b , movement of the printer carriage  12  along the carriage guide  30  traces out second bearing contact paths, of which, for the sake of clarity, only the path  43  traced out by bearing  20   a  is shown in  FIG.  5   . 
     The lateral movement of the bearings  20   a ,  20   b  relative the carriage guide  30  may be performed by moving the bearings  20   a ,  20   b  whilst the carriage guide  30  remains fixed in position on the structural support of the printing system  10  or performed by moving the carriage guide  30  whilst the bearings  20   a ,  20   b  remain fixed in position relative to the printer carriage  12 . For example, the carriage guide  30  may be rotated by angle δ about an axis of the carriage guide  30  and fixed again to the supporting structure of the printing system  10  in the new location whilst the two bearings  20   a ,  20   b  remain in position relative to the carriage  12 . Alternatively, the bearings  20   a ,  20   b  may be moved relative to the printer carriage  12  to which they are attached, in use, and rotated by angle δ about an axis of the carriage guide  30  and then fixed in a new position relative to the printer carriage  12  whilst the carriage guide  30  remains fixed to the supporting structure of the printing system  10 . 
     In the arrangements described herein, the angle between the pair of bearings is designed to ensure no bearing will lose contact with the carriage guide during motion of the printer carriage along the carriage guide. Accordingly, when moving the at least one bearing laterally around the profile of a carriage guide in a printing system, it is important to consider the dynamic effects of a printer carriages carried on the carriage guide so that this functionality is not lost. 
       FIG.  7    schematically illustrates an example of the angular positions of a pair of bearings  20   a ,  20   b , such as those described herein, relative to a carriage guide  30 . The angles α and β show the angular position (P 0 ) of the bearings  20   a ,  20   b  if there is no requirement to account for moving the bearings  20   a ,  20   b  around the profile  32  of the carriage guide  30  to new guide contact positions. Angles α and β may be equal or different sized angles. Angles θ 1  and θ 2  illustrate the angular offset from angular position P 0  respectively to angular position P 1  for the first contact position  40   a  and to angular position P 2  for the second contact position  41   a . Angles θ 1  and θ 2  can be equal or different sized angles. Setting angles θ 1  and θ 2  at angular offsets can be arranged that do not compromise the dynamics of a printer carriage that is to be carried on the carriage guide  30 . The desired angles θ 1  and θ 2  may depend on, for example, mass, acceleration rates, geometry, application point of impelling forces, and centre of gravity. For best performance, it is desirable that the bearings  20   a ,  20   b  are held in firmly in the desired angular positions during use. 
       FIG.  8    schematically illustrates that, in certain examples where more than one bearing is present, it is desirable that the angular distance between position P 1  for the first contact position  40   a  and to angular position P 2  for the second contact position  41   a  are such that the worn portions of the carriage guide  30  surface at each of the first and second contact positions  40   a ,  41   a  are, at their maximum extent, sufficiently separated from each other. That is, the sufficient separation, measured angularly or linearly around the profile of the carriage guide  30 , should be determined based on the condition of the carriage guide  30  surface when the first and second contact positions  40   a ,  41   a  are worn. For example, if the worn portion of the carriage guide  30  is removed thickness t from the radius R of the carriage guide  30 , then angle γ is the angular measurement of the worn portion at P 2  and the minimum angular amount θ 1 +θ 2  that the bearings  20   a ,  20   b  be rotated from P 1  to P 2 —to provide the necessary separation between the worn portions of the carriage guide  30 —should be greater than angle γ. 
       FIGS.  9   a ,  9   b ,  10   a , and  10   b    illustrate one example of a printing system  10  in which a carriage guide  30  is movable relative to a supporting structure of the printing system on which the carriage guide  30  is mounted whilst the at least one bearing remains in the same position relative to a printer carriage  30 . 
     The printing system  10  comprises a longitudinally extending beam  50  on which the carriage guide  30  is mounted. The beam  50  may be an aluminum extrusion. In this example, the beam  50  comprises a carriage support  52  on which the carriage guide  30  is mounted. The carriage support  52  may comprise a supporting rib that extends the length of the carriage guide  30  or may comprise a plurality of individual supports distributed along the length of the beam  50 . The carriage support comprises a suitable surface to mate with the carriage guide  30 . 
     The printing system  10  comprises at least one fastener  54  to hold the carriage guide  30  to the beam  50 . A plurality of fasteners may be distributed along the length of the carriage guide  30 . In the example shown in  FIGS.  9   a  and  9   b   , the one or more fasteners comprise a shaft  54  fixed to the carriage guide  30  where the, or each, fastener passes through a corresponding elongate slot  58  provided in the beam  50 . The fastener is provided with a head that is larger than the slot  58  so that a force can be applied between the fastener and the beam  50  thereby retaining the carriage guide  30  on the beam  50  carriage support  52 . 
     The elongate slot  58  permits the one or more fasteners to move with the carriage guide  30  as the carriage guide  30  is moved relative to the beam  50  from a first carriage guide contact position to a second carriage guide contact position. In the example shown in  FIGS.  9   a  and  9   b   , the carriage guide  30  has a circular profile and carriage guide  30  and the fasteners rotate on the carriage support  52  to move from the first carriage guide contact position to the second carriage guide contact position. To aid the motion of the fastener along the slot  58 , the fastener may comprise a low friction washer  56  that rests on the beam  50  around the slot  58 . The washer may comprise PTFE, for example. To apply the force between the fastener and the beam  50 , a biasing means  57  may located between the washer  56  and the underside of the fastener head. For example, the biasing means  57  may comprise a coil spring placed in compression between the fastener head and the washer  58 . The biasing means allows the carriage guide  30  to be retained against the supports at both the first and second carriage guide contact positions, as well as during the transition between the guide contact positions, and permit rotational motion of the fastener and carriage guide  30 . The fastener may comprise a cap screw, screw, bolt, or threaded rod and nut, for example. The one or more fasteners may each be mounted in a tapped blind hole  36  in the carriage guide  30 . 
     In certain examples, the carriage guide  30  is shaped at one or both ends to engage with a tool so that the tool can exert a torque on the carriage guide  30  and cause it to move from the first carriage guide contact position to the second carriage guide contact position. For instance, as shown in  FIG.  10   b   , one end  34  of the carriage guide  30  has parallel flat faces to allow engagement with a positioning tool  60 . The end  34  may be machined to a square or hexagonal form to engage with a corresponding positioning tool. In some examples, the positioning tool  60  can be locked in place to hold the carriage guide  30  in the selected guide contact position. For instance, at the opposing end of the positioning tool  60  from the carriage guide  30  engaging end, a screw  62  may be used to lock the positioning tool to a component of the supporting structure of the printing system  10 . One or both ends of the carriage guide  30  may be shaped to engage with a corresponding positioning tool. The positioning tool  60  and, by extension, the carriage guide  30  may be rotatable manually by hand. The biasing means  57  can be set to exert enough force to retain the carriage guide  30  in place on the beam  50  yet allow manual rotation of the positioning tool  60 . The provision of a positioning tool and a corresponding engagement portion on the carriage guide allows the carriage guide to be rotated without having to access the fasteners, which, in some printing systems, can only be accessed with difficulty from under the printing system  10  in a narrow space formed by the particular geometry of the supporting beam  50 . 
       FIGS.  11   a  and  11   b    illustrate one example of a printing system  10  in which at least one bearing  20   a  is movable relative to a printer carriage on which the at least one bearing  20   a  is mounted whilst the carriage guide  30  remains fixed to the supporting structure of the printing system  10 . 
       FIG.  11   a    shows an exploded perspective view of a bearing housing  14  that is mountable to a carriage support  70 . The bearing housing  14  comprises a bearing receptacle  16  in which a pair of roller bearings  20   a ,  20   b  are held on respective shafts. A lid  18  is fixed with screws  19  to the bearing receptacle  16  to hold the bearings  20   a ,  20   b  in place. 
     In the illustrated example, the bearing housing  14  is mountable to the carriage support  70  in more than one position. By repositioning the bearing housing  14  in the carriage support  70  the bearings  20   a ,  20   b  are movable from a first mounting position corresponding to a first carriage guide contact position to a second mounting position corresponding a second carriage guide contact position as described above. The bearings  20   a ,  20   b  may therefore be rotatable, in the bearing housing  14 , about the carriage guide  30 .  FIG.  8    illustrates two possible mounting positions about the carriage guide  30 , which is supported on a longitudinally extending beam  50  in a manner similar to the arrangement described above.  FIG.  11   b    also illustrates alternate alignment/mounting holes  15   a ,  15   b  on the carriage support, which correspond to the first and second mounting positions, through which screw  72 , shown in  FIG.  11   a   , can be inserted to fix the bearing housing  14  in position on the carriage support  70 . It will be understood that any suitable number of mounting positions may be provided—as permitted by the geometry of a carriage support, bearing housing, carriage guide, and other components—corresponding to respective carriage guide contact positions, and the 
     With reference to  FIG.  12   , a method of relocating at least one bearing laterally of a longitudinally extending carriage guide to maintain a printing system will now be described. At block  1202 , the method comprises relocating at least one bearing laterally around the profile of a longitudinally extending carriage guide from a first guide contact position to a second guide contact position. The at least one bearing is to direct motion of a printer carriage that is movable along the carriage guide. 
     In certain examples, at block  1204 , the method, in relocating the at least one bearing, comprises moving the carriage guide relative to a support structure on which the carriage guide is mounted. In certain examples, the at least one bearing remains fixed relative to the carriage whilst the carriage guide is moved. 
     In certain examples, at block  1206 , the method, in relocating the at least one bearing, comprises moving the at least one bearing relative to the printer carriage, wherein the at least one bearing is mounted to the printer carriage. In certain examples, the carriage guide remains fixed relative to a support structure on which the carriage guide is mounted whilst the at least one bearing is moved. 
     The preceding description has been presented to illustrate and describe examples of the principles described. This description is not intended to be exhaustive or to limit these principles to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is to be understood that any feature described in relation to any one example may be used alone, or in combination with other features described, and may also be used in combination with any features of any other of the examples, or any combination of any other of the examples.