GB2270290A

GB2270290A - Assembly line system.

Info

Publication number: GB2270290A
Application number: GB9218893A
Authority: GB
Inventors: David John Cockayne
Original assignee: Modular Automation Ltd
Current assignee: Modular Automation Ltd
Priority date: 1992-09-07
Filing date: 1992-09-07
Publication date: 1994-03-09
Also published as: GB9218893D0

Abstract

An assembly line system 1 around which pallets 3 can be directed in sequence to perform work operations comprises a main line or loop 2a with four branch lines 2b. Pallets 3 can be conveyed along the assembly line with options at A to D for transfer from the main line 2a to an associated branch line 2b, return to the main line being at a respective spaced location E to H from the respective entry location A to D. Thus pallets 3 can be guided around the assembly line, on and off branch lines, whilst having a constant leading edge. The assembly line system 1 is made up of a number of modules or cells which can be reconfigured. <IMAGE>

Description

ASSEMBLY LINE SYSTEM This invention relates to an assembly line system more particularly in which a series of pallets or platens are conveyed to work stations along an assembly line at which various operations can be performed on workpieces carried by said pallets/platens.

It is believed that present assembly line systems employing such pallets tend to be disadvantageous in that they may be more limited in format than need necessarily be the case. Where an assembly line comprising a plurality of work stations is provided, the system tends to be relatively poorly adapted to accommodate a change or changes in the configuration of the assembly line (for example to perform additional work operations on the workpieces). Whenever the configuration of the assembly line needs to be changed for example by adding an additional portion into the assembly line, the time taken to complete the additional operation in the additional assembly line portion may be unnecessarily long and out of proportion for the particular task to be undertaken.

Additionally, the installation time for the additional assembly line portion may be longer than necessary and the Vi actual installation may be more difficult than need be the case. Of course, the time taken in completing a series of operations to be performed on workpieces conveyed along an assembly line is extremely important and the shorter the amount of time involved the more cost effective the ~exercise. What is required is a highly efficient assembly line system which can be quickly and easily varied to accommodate any change or changes required in the assembly line, which change/s themselves might only be transient.

Additionally, it is believed that the accessibility to the workpiece carried on the pallets may be improved in relation to current systems in addition to providing an improvement in implementing reconfiguratidn of the assembly line itself and improved transfer options of the pallets along said assembly line, leading to a reduction in overall time taken for successfully performing operations on the workpieces carried by the pallets.

It is an object of the present invention to provide an assembly line system which is improved in at least some respect and/or which at least alleviates the aforementioned, or other, disadvantages of assembly line systems.

According to the present invention there is provided an assembly line system of modular construction, said assembly line system having a reconfigurable assembly line made up from a plurality of cells or units provided with sections of the assembly line, the arrangement being such that, in use of the assembly line system, pallets can be conveyed to work stations on the assembly line by conveyor means, said assembly line having a main line and one or more branch lines, the pallets being able to run selectively along the main line or along the branch line or branch lines, said pallets having a constant leading edge whilst travelling along the assembly line and being returned to the main line from a, or the, branch line at a spaced location from the entry location onto said branch line from the main line.

Preferably, the arrangement is such that transfer to and from the main line on and off the branch line or branch lines can be undertaken at a generally constant speed so that a delay time is not introduced into a system.

Preferably, transfer from the main line to the branch line or branch lines can be undertaken in one continuous movement of the pallet, preferably without lifting of the pallet during said transfer or with the pallet remaining substantially at the same height relative to the branch line as to the main line during said transfer and/or preferably with the same drive being used to transfer the pallet on and off the main line as along a section of the main line adjacent to the associated branch line.

Preferably, the assembly line comprises a plurality of modules or cells each including a main line section and a branch line section. Preferably, each cell or module has its own drive for its main line and branch line sections.

Further according to the present invention there is provided an assembly line system of modular construction, said assembly line system having a reconfigurable assembly line made up from a plurality of cells or units provided with sections of the assembly line, the arrangement being such that, in use of the assembly line system, pallets can be conveyed to work stations along the assembly line by conveyor means, said assembly line having a main line and one or more branch lines, the pallets being able to run selectively along the main line or along the branch line or branch lines, the arrangement being characterised by one or more of the following features: a) The pallets having a constant leading edge while travelling along the assembly line; b) The pallets being returned to the main line from a, or the, branch line at a spaced location from the entry location onto said branch line from the main line; c) Transfer from the main line to and from a, or the, branch line being capable with the pallets remaining at a constant speed and/or with each pallet remaining substantially at the same height relative to said branch line as to the main line during said transfer or being capable without lifting of the pallets or movement of the pallets in a direction other than along the main line or branch line; d) Each of the reconfigurable cells or units comprising part of the main line and part of an associated branch line such that the same drive can be utilised to transfer pallets along a section of the main line and along the associated branch line; e) The pallets having pucks mounted offset to one side of the pallet such that the main area of the pallet overhangs the conveyor means in use.

Further according to the present invention there is provided an assembly line system of modular construction having an assembly line comprising a main assembly line and one or more branch lines, the arrangement being such that, in use, pallets can be conveyed along the assembly line to work stations and may be guided on and off the main line, said main line having a conveyor means to carry the pallets along the assembly line and the conveyor means in the main line comprising a plurality of aligned sections of conveyor belt or chain loops so that the main line conveyor means is modular, and preferably the arrangement having one or more of the following:a) the pallets having a constant leading edge while travelling along the assembly line; b) the pallets being returned to the main line from the or each branch line at a spaced location from the entry location onto said branch line from the main line; c) the pallets being maintained at a constant speed during transfer on and off the main line; d) the assembly line consists of a number of reconfigurable cells or units in which each cell or unit comprises part of the main line and part of the branch line, the same drive being utilisable to transfer pallets along a section of the main line and along an associated branch line.

Still further according to the present invention there is provided an assembly line system comprising a number of modules or cells each having a conveyor means section which can be or which is combined with conveyor means sections of neighbouring modules or cells to form a main assembly line, and preferably the arrangement having one or more of the following:a) the pallets having a constant leading edge while travelling along the assembly line; b) the pallets being returned to the main line from a branch line at a spaced location from the entry location onto said branch line from the main line; c) the pallets being maintained at a constant speed during transfer on and off the main line; d) each module or cell comprises part of the main line and part of a branch line, the same drive being utilisable to transfer pallets along a section of the main line and along an associated branch line.

In one embodiment of the assembly line system, nine cells or modules are provided comprising three end cells, three work cells, two straight cells and one link cell.

Preferably, each end cell carries an open U-shaped track, each work cell carrying an open straight track section and adjoining U-shaped track section, each straight cell having an open ended straight track section and each. link cell having an open ended straight length track section joined to an H-shaped track section. Each module or cell may have a base unit supporting a tabletop or worktop in which a track or slot is provided for pallet pucks to run along on the conveyor means. Advantageous features of the modules or cells will be apparent from the following description and drawings.

Preferably, the assembly system includes pallets having pucks mounted off-set to one side of the pallet so that the main area of the pallet overhangs the conveyor means. Preferably, the pallets are supported by castors (which may be of a type as shown in our co-pending application of even date the content of the specification of said application now being incorporated into this specification by reference). Preferably, the castors are shrouded by substantially rigid cups limiting the relative downward movement of the pallet to the table-top.

Further according to the present invention there is provided a method of conveying pallets along an assembly line comprising a main line and one or more branch lines, said method comprising transferring the pallets from the main line onto a, or the, branch line and returning the pallet back onto the main line at spaced location from the entry location onto said branch line from the main line, and maintaining a constant leading edge of the pallet at least during said transfer.

Further advantageous features of the present invention will be apparent from the following description and drawings.

An embodiment of an assembly line system in accordance with the present invention will now be described, by way of example only, with reference to the accompanying simplified drawings in which: FIGURE 1 shows a much simplified diagrammatic representation of an assembly line of the assembly line system, which assembly line is easily reconfigurable; FIGURE 2 shows a lower section of the embodiment depicted in FIGURE 1, in greater detail; FIGURE 3 shows a view similar to FIGURE 1 in which the assembly line has been reconfigured into an alternative configuration and additionally shows the modular construction of the assembly line; FIGURE 4 shows a plan view of a pallet and workpiece at a work station; FIGURE 5 shows a part-sectional side view of a pallet at the work station, looking along the direction of assembly line conveyor means;; FIGURE 6 shows a diagrammatic perspective view of a central base unit; FIGURE 7 shows a similar view of a work cell base unit; FIGURE 8 shows a view similar to FIGURE 1 but incorporating a larger number of modules to give a larger assembly line; FIGURE 9 shows a more detailed view of a work cell; FIGURE 10 shows a more detailed view of a straight cell; and FIGURE 11 shows a schematic representation of a modified embodiment of an assembly line system.

FIGURE 1 shows a schematic representation of a programmed assembly line system 1 having an assembly line 2 around which pallets 3 (only one shown in FIGURE 1) can be directed in sequence in order to perform relevant work operations thereon. The assembly line 2 comprises a main line or loop 2a with four branch lines 2b as shown. The main line 2a comprises the central generally rectangular loop with two generally square U-shaped, branch lines 2b being provided on one side of the main line 2a and two opposed branch lines being provided on the other side of the main line 2a (at the top of the FIGURE). Pallets 3 are able to start at some point along the assembly line 2 to be conveyed to various work stations at which operations can be performed on workpieces (not shown) carried by the pallets 3 in a generally known manner.The arrows 4 show the direction of travel of the pallets 3 along the assembly line and it will be noted that at locations A to D there exists an option for transfer from the main line 2a to an associated one of the branch lines 2b, return to the main line from the associated branch line being at a spaced location E to H from the respective entry location A to D of the pallet onto the associated branch line 2b. In this way, pallets 3 can be guided around the main assembly line 2a as many times as necessary and around any of the branch lines 2b in a selective manner, whilst having a constant leading edge, in order to perform work operations on the workpieces (not shown) carried by the pallets 3. The work operations will generally be undertaken on the branch lines during which the pallets 3 themselves are stopped or clamped at a work station.The pallets 3 are carried along or around the assembly line by conveyor means 5 (see FIGURE 2) which may, for example, be in the form of a rubber belt means or cardanic chain means. Importantly, the conveyor means 5 is designed so that there may be a smooth quick transfer from the main line 2a onto a particular branch line 2b and back onto the main line, whilst a constant leading edge of the pallet is maintained.

In prior art assembly lines arranged to convey pallets to work stations, the main line may comprise a single conveyor belt or. chain and any branch line provided may comprise a further belt or chain arranged to provide a single entry and exit point for conveying pallets on and off the main line. Thus, the assembly line may consist of a main loop belt with its own drive means and branch line loops each having a single contact point with the main line loop. It is believed that this system tends to have various disadvantages and it is not possible to "break into" the main loop line to fit a new branch line, pallets being conveyed on and off the branch line at a point location.In an alternative prior art arrangement consisting of a double track assembly line, in order to "break into" the main assembly line loop, pallets are lifted from the main line over the main line section onto the branch line in order to perform a work operation thereon after which the pallets have to be lifted back up onto the main line section. Such an arrangement may introduce a significant delay time into processing of the workpieces and the pallets do not have a constant leading edge during transfer on and off the main line. Separate drives are required for the main line and branch lines.

As will become evident from the following description of the assembly line system 1, the main line loop 2a does not consist of a single loop belt driven by a single motor but rather it is modular in nature comprising a number of conveyor sections which align to give the form of main loop shown in FIGURE 1.

FIGURE 2 shows a view of the lower portion of FIGURE 1 in greater detail. The movement of a pallet or pallets 3 is shown in a diagrammatic form along the assembly line.

The assembly line itself is made up of a number of individual modules or cells (cells 6,7 and 8 shown in FIGURE 2) and this will be discussed in further detail with reference more particularly to FIGURE 3. Each module or cell 6,7,8 has its own motor and drive means for driving its own section 5a,5b of conveyor means 5. Each module 6,7 has an upper, straight line, open-ended conveyor means length section 5a forming part of the main line loop 2a and is aligned in end-to-end relationship with adjacent main line loop sections. Each module 6,7 is provided with a U-shaped branch line conveyor means section Sb. FIGURE 8 represents another view of module 6 or module 7 showing the drive means for the conveyor means sections 5a,5b.

FIGURE 3 shows a view similar to FIGURE 1 in which the assembly line 2 has been reconfigured by the addition or substitution of various modules or cells in a manner which should now be self-apparent. The modular assembly line system 101 shown in FIGURE 3 has a main line loop 102 and is comprised of nine cells or modules 103 to 110, each having its own motor and drive means and conveyor means section. The modules 103 to 110 can be easily joined together in a variety of configurations possibly with other such cells in order to create a very flexible assembly line system to cater for changing needs along the assembly line. The main line 102 is the central part of the system and controls the overall sequence of the assembly operations.The branch line 111 is a subassembly line along which a series of sub-assembly operations can be performed before returning the pallets onto the main line 102. As shown, the branch line 111 is arranged at 900 to the main line. The modules 103 to 110 are divided into three basic types, namely straight cells 110, work cells 103,105 and 109, link cells 106 (only one shown). These are transfer cells. Additional work cells 104,107 and 108 are provided and these may be manual or automatic cells. A work cell has a U-shaped assembly track loop L which diverts a pallet from the main line to a work station and returns it to the main line. More detailed views of a work cell, a link cell and straight cell are shown in FIGURES 8 to 10.

Simplified views of a central base unit and a work cell base unit are shown in FIGURES 6 and 7 which will be discussed after the construction of the pallet itself shown in FIGURES 4 and 5. FIGURES 4 and 5 show plan and part-sectional side views of a pallet at a work station along the assembly line. The pallet 3 has a generally rectangular plate 3a provided with two spring-loaded rotatable buffers or pucks 3b mounted on the underside of the plate 3a and at two corners thereof. The pucks 3b ride along the conveyor means 5 (not shown in FIGURES 4 or 5) by the frictional engagement of the pucks on said conveyor means.The construction of the spring-loaded pucks 3b is generally known (save for the sleeve P1 advantageously freely rotatable on puck body P2 in order to reduce friction on contact with side walls of the slot S - see our copending patent application of even date previously referred to) and therefore will not be further described. However, the location of the pucks along one side of the pallet 3 (right hand side of the pallet as shown in FIGURE 4) is advantageous rather than the pucks being provided along the central line of the pallet.

Positioning the pucks 3b on the pallet 3 in this manner is believed to be advantageous since it leaves virtually all of the pallet area (and workpiece when mounted thereon) overhanging the conveyor means 5 itself so that the workpiece is more readily accessible at a work station in which the pallet is clamped for either an automatic work operation or manual work operation to be performed on said workpiece. Positioning the buffers 3b as shown is advantageous in assisting the smooth movement of the pallet 3 selectively along the main line 2a or along the branch lines 2b as should be apparent from FIGURE 2. The pucks 3b frictionally engage the conveyor means 5 whilst being constrained to move in a slot S (see FIGURE 5) in the support surface X (see chain-dotted lines in FIGURE 5) in which the pallet buffers 3b run.In this embodiment, the pallet 3 is advantageously provided with eight castors 3c mounted in rigid, cylindrical, inverted cups 3d attached to the underside of the plate 3a. The castors 3c themselves are of the form as described in our co-pending application of even date. The underside of the plate 3a is provided with an electronic platen code label 3e to enable the pallet 3 to pass through the assembly sequence along the assembly line 2 in a given order of steps. The code label 3e records the number of assembly steps that have been completed on the workpiece carried by a particular pallet 3 and as the pallet 3 enters a work cell the number is rewritten to the next assembly step. The code label data can be updated at any work cell. FIGURES 4 and 5 show a read/write sensor Y (which is fitted near the entry to a work cell).The sensor Y reads the data already on the code label 3e to determine whether the pallet 3 should go into that work station or continue along the main line. If the pallet is diverted into a work station, the code label 3e is updated to indicate that the assembly operation has been completed. A pallet 3 can be taken off the assembly line at any time simply by lifting it from the assembly line. FIGURES 4 and 5 additionally show pneumatic clamp means 9 and 10 of a generally known type including downwardly moveable generally V-shaped locating heads 9a and 10a which seat in receiving bushes 3f on the upper surface of the pallet 3.

When a downward force is applied on the pallet 3, resilient castor wheels 3'c compress until the cylindrical cups 3d engage the surface of the work table X, thus firmly holding the pallet whilst a work operation can be performed on the workpiece carried by the pallet. Thus, it is not necessary to provide vertically reciprocable means engaging the underside of the pallets and lifting the buffers or pucks 3b free of the conveyor means 5 whilst simultaneously clamping the pallet 3 from above as in known systems. Reference is made again to our copending application of even date.

Referring more particularly to FIGURES 3, 6 and 7, each module or cell is constructed as a box section, table supported by a base unit which in some cases is an integral part of the cell. FIGURES 6 and 7 show two such tables which are made from two plates of mild steel which are bolted one on top of the other. All drives, gates, stops, sensors and valves are fitted between the two plates (see FIGURES 8, 9 and 10) so that the surface of the table is clear of obstruction. The top plate has side pieces so that when it is bolted on the bottom plate it completes the box section. The main assembly line 102 and branch line 111 are formed by a track or slot in the top plate.Some cells have one track (end cells 103, 105, 109), the conveyor means 5 runs along the track and, as shown more particularly in FIGURES 8,9 and 10, a motor driving the conveyor means (in the form of rubber belts is fitted to the underside of the table.

FIGURE 6 shows a central base unit which supports straight cells (see cells 110) forming the two sides of the main line. Each control base unit has a complete power supply system and can be operated alone if required.

The tracks may be in a straight cell, a link cell or work cell. The central base unit 200 has two leg frames 201, 202 table support limbs 203, 204 and the work table portion of a straight cell 110 is shown in chain-dotted lines in the FIGURE 6. Each table is provided with four adjustable feet 205 and a pneumatic line 206 for the assembly line system is carried through the table. A reset switch 207 is provided connected to the power supply. A cable box is supported lengthwise between the leg frames 201, 202 in the power supply cabinet 208 that carries all the power supply equipment including the motor for driving the conveyor means. The control system including a programmable logic controller for the individual work cell is fitted in the cabinet 208.

FIGURE 7 shows a view similar to FIGURE 6 of a work cell 300. The work cell has its own base unit comprising a table support 301, two leg frames 302 and power supply cabinet 303. The work tabletop for the work cell is shown at 304. A footswitch control 305 is provided for operating the electrical equipment in the supply cabinet 303. Similar base units may be provided for a link cell and an end cell although it is thought unecessary to show same in detail here.

FIGURES 8 and 9 show views of a work cell and link cell respectively. Code sensors are shown at Y and a read/write head is shown at Z. Stops at various positions along the track are shown at 400 and gates for directing the pallets 3 along the main line or the branch line are shown at G. Each module or cell is independent in its logic functions so that as a pallet enters the cell, the cell reads the pallet code from the electronic code label 3e. FIGURES 8 and 9 show the rubber belt drive system employed for the work cell and link cell respectively. In FIGURE 8 the motor (not shown) drives main pulley 401 which drives a loop rubber belt section as depicted by the arrows 402.A second rubber belt drive including jockey pulley 403 is taken off pulley 404 of the main drive system giving a separate belt loop 405 driven by the same motor, said belt loop 405 bridging the gap across the top of the U-shaped branch line. The belt drive system shown in FIGURE 9 is similar to that of FIGURE 8 with the main loop rubber belt section being depicted by arrows 402'. A chird belt drive system linked to the main drive is shown at 406. FIGURE 10 shows a straight cell in greater detail than the cells shown in FIGURES 8 and 9. A single belt drive D is provided.

FIGURE 11 shows a schematic representation of a further developed assembly line system having a main line 500 (closed loop) and main branch lines 501, 502, additional branch lines being provided. Figures F are shown to indicate manual work cells.

Reference is made to technical work manual number MF AS/T/1992-1 of Modular Automation Ltd which is included in this specification by way of an appendix.

It is to be understood that the scope of the present invention is not to be unduly limited by the particular choice of terminology and that a specific term may be replaced by any equivalent or generic term where sensible.

Further it is to be understood that individual features, method or functions related to the assembly line system or cells might be individually patentably inventive. In particular, any disclosure in this specification of a range for a variable or parameter shall be taken to include a disclosure of any selectable or derivable subrange within that range and shall be taken to include a disclosure of any value for the variable or parameter lying within or at an end of the range. The singular may include the plural and vice versa.

Claims

1. An assembly line system of modular construction, said assembly line system having a reconfigurable assembly line made up from a plurality of cells or units provided with sections of the assembly line, the arrangement being such that, in use of the assembly line system, pallets can be conveyed to work stations on the assembly line by conveyor means, said assembly line having a main line and one or more branch lines, the pallets being able to run selectively along the main line or along the branch line or branch lines, said pallets having a constant leading edge whilst travelling along the assembly line and being returned to the main line from a, or the, branch line at a spaced location from the entry location onto said branch line from the main line.

2. A system as claimed in Claim 1 which transfer to and from the main line on and off the branch line or branch lines can be undertaken at a generally constant speed.

3. A system as claimed in Claim 1 or Claim 2 in which transfer from the main line to the branch line or branch lines can be undertaken in one continuous movement of the pallet.

4. A system as claimed in Claim 3 in which said transfer can be undertaken without lifting of the pallet or with the pallet remaining substantially at the same height relative to the branch line as to the main line.

5. A system as claimed in Claim 3 or Claim 4 in which said transfer can be undertaken with the same drive being used to transfer the pallet on and off the main line as along a section of the main line adjacent to the associated branch line.

6. A system as claimed in any one of the preceding claims in which the assembly line comprises a plurality of modules or cells each including a main line section and a branch line section.

7. A system as claimed in Claim 6 in which each cell or module has its own drive for its main line and branch line sections.

8. An assembly line system substantially as herein described with reference to FIGURES 1 to 10 or as modified substantially as shown in FIGURE 11.

9. An assembly line system of modular construction, said assembly line system having a reconfigurable assembly line made up from a plurality of cells or units provided with sections of the assembly line, the arrangement being such that, in use of the assembly line system, pallets can be conveyed to work stations along the assembly line by conveyor means, said assembly line having a main line and one or more branch lines, the pallets being able to run selectively along the main line or along the branch line or branch lines, the arrangement preferably being characterised by one or more of the following features: a) The pallets having a constant leading edge while travelling along the assembly line; b) The pallets being returned to the main line from a, or the, branch line at a spaced location from the entry location onto said branch line from the main line; c) Transfer from the main line to and from a, or the, branch line being capable with the pallets remaining at a constant speed and/or with each pallet remaining substantially at the same height relative to said branch line as to the main line during said transfer or being capable without lifting of the pallets or movement of the pallets in a direction other than along the main line or branch line; d) Each of the reconfigurable cells or units comprising part of the main line and part of an associated branch line such that the same drive can be utilised to transfer pallets along a section of the main line and along the associated branch line; e) The pallets having pucks mounted offset to one side of the pallet such that the main area of the pallet overhangs the conveyor means in use.

10. An assembly line system of modular construction having an assembly line comprising a main assembly line and one or more branch lines, the arrangement being such that, in use, pallets can be conveyed along the assembly line to work stations and may be guided on and off the main line, said main line having a conveyor means to carry the pallets along the assembly line and the conveyor means in the main line comprising a plurality of aligned sections of conveyor belt or chain loops so that the main line conveyor means is modular, and preferably the arrangement having one or more of the following:a) the pallets having a constant leading edge while travelling along the assembly line; b) the pallets being returned to the main line from the or each branch line at a spaced location from the entry location onto said branch line from the main line; c) the pallets being maintained at a constant speed during transfer on and off the main line; d) the assembly line consists of a number of reconfigurable cells or units in which each cell or unit comprises part of the main line and part of the branch line, the same drive being utilisable to transfer pallets along a section of the main line and along an associated branch line.

11. An assembly line system comprising a number of modules or cells each having a conveyor means section which can be or which is combined with conveyor means sections of neighbouring modules or cells to form a main assembly line, and preferably the arrangement having one or more of the following:a) the pallets having a constant leading edge while travelling along the assembly line; b) the pallets being returned to the main line from a branch line at a spaced location from the entry location onto said branch line from the main line; c) the pallets being maintained at a constant speed during transfer on and off the main line; d) each module or cell comprises part of the main line and part of a branch line, the same drive being utilisable to transfer pallets along a section of the main line and along an associated branch line.

12. A system as claimed in Claim 11, comprising nine cells or modules comprising three end cells, three work cells, two straight cells and one link cell.

13. A system as claimed in Claim 12 in which each end cell carries an open U-shaped track, each work cell carries an open straight track section and adjoining Ushaped track section, each straight cell having an open ended straight track section and the link cell having an open ended straight length track section joined to an H shaped track section.

14. A system as claimed in any one of Claims 11 to 13 in which each module or cell has a base unit supporting a tabletop or worktop in which a track or slot is provided for pallet pucks to run along on the conveyor means.

15. A system as claimed in any one of Claims 11 to 14 including pallets having pucks mounted off-set to one side of the pallet so that the main area of the pallet overhangs the conveyor means.

16. A system as claimed in Claim 15 in which the pallets are supported by castors.

17. A system as claimed in Claim 16 in which the castors are resilient or resiliently mounted.

18. A system as claimed in Claim 17 in which the castors are shrouded by substantially rigid cups limiting the relative downward movement of the pallet to the table-top.

19. A method of conveying pallets along an assembly line comprising a main line and one or more branch lines, said method comprising transferring the pallets from the main line onto a, or the, branch line and returning the pallets back onto the main line at spaced location from the entry location onto said branch line from the main line, and maintaining a constant leading edge of the pallets at least during said transfer.

20. A method as claimed in Claim 19 and substantially as herein described.