Case Study : Robot Assembly of Pneumatic Valves (1/3)

This is a quick feasibility study that I was asked to carry out for a Swedish world leading manufacturer of compressors, generators, construction and mining equipment, industrial tools and assembly systems.  They wanted a swift appraisal of the economics for the robot assembly of their pneumatic valves.

INTRODUCTION

The client company has committed to capital investment in a two-armed robot for the assembly of its range of pneumatic cylinders at one of its Swedish manufacturing plants.  The robot will not be fully utilised and another product is required to economically justify the installation.

The manufacturing plant currently assembles two product families :

1) Pneumatic cylinders
2) Pneumatic valves

The feasibility of assembling a model of pneumatic valve is investigated for the client company.

VOLUME REQUIREMENTS OF THE SPECIFIC PNEUMATIC VALVE MODEL

The pneumatic valve annual production volume is 30 000 units.  The valve is currently assembled manually and the client company assumes that demand for the valve will increase to 40 000 within 2 years.  Three workers are currently required for product assembly.  The product has a total of 63 separate parts, of which, 31 are unique parts.

MANUAL ASSEMBLY


The manual assembly of the valve has been studied to create individual times for the 97 operations.  The manual assembly worksheet, shown below, gives the sequence of operations and their corresponding operation times.  The worksheet shows that the cycle time for the complete assembly is 389 seconds.  One worker can assemble 11 786 valves in one year, with single shift working at a labour efficiency of 70 percent :

225 working shifts per annum (single shift working)
= 5 940 000 working seconds per annum (440 minutes / shift)
= 4 158 000 working seconds per year at 70 percent labour efficiency
= 10 689 units assembled per annum.    .

ROBOT ASSEMBLY

Certain assembly operations can be executed by the robot without a re-design of the pneumatic valve.  The assembly sequence for the robot assembly is given at the end of this article and an estimate for the robot capital expenditure is :

(a) Cost of the robot and controller = 50 000 euros.

(b) Turret and eight grippers = 5 000 euros

(c) Fixture number 32 = 3 000 euros, fixture number 33 = 2 000 euros, fixture number 34 = 2 000 euros, fixture number 35 = 3000 euros, fixture number 36 = 2 000 euros, fixture number 37 = 500 euros, fixture number 38 = 500 euros, fixture number 39 = 2 000 euros

(d) Arm-2 0-ring tools (6 off), including tool holder = 1 800 euros.  Arm-2 screwdriver bit and friction screwdriver bit = 200 euros.

(e) Greasing station = 2 000 euros

( f) Labelling station = 5 000 euros

(g) Cleaning station = 2 000 euros

(h) Eight vibratory linear feeders at 3 000 euros each = 24 000 euros

Total = 105 000 euros

CYCLE TIME

It is estimated that the cycle time for the robot and manual assembly of the valve would be 456 seconds.  Using this estimate, one robot can assemble 5210 valves in one year, with single shift working at a robot efficiency of 80 percent :

225 working shifts per annum (single shift)
= 5 940 000 working seconds per annum (440 minutes/shift)
= 4 752 000 working seconds per year at 80% robot efficiency
= 10 421 units assembled per year (single shift)

The robot can assemble approximately the same number of products per year as one worker, considering single shift working.  However, certain operations (using the existing product design) must be executed manually.

ANNUAL COST SAVINGS

The annual cost saving of using the robot is one worker per year.  If the annual cost of an operator is 50 000 euros per year (including taxes, social charges, pension contributions, overhead contribution, etc.) then the cost saving would be approximately 50 000 euros per year.

PAYBACK PERIOD

The payback period for using the robot is 2 years, for the assembly of 10 421 units per year.

VALVE SPECIFICATIONS

If the valve is to be re-designed then it must have the following performance characteristics :

(a) It must achieve a flow rate of 2.2 litres per second for 10 000 000 cycles of operation, without leakage from port to port.

(b) The upper sealing gasket must not drop off when the body sub-assembly is transported between operations.

(c) The inner sleeve 0-rings must be stable during assembly of the inner sleeve sub-assembly to the valve body.

(d) The activation time of the unit must be better than 0.02 seconds.

(e) The operating air pressure for the double acting valve should be lessthan 1.2 kg/cm2 and less than 2.5 kg/cm2 for the spring return valve.

(f) The customer should have the option of achieving flow rates between 0 and 50 percent of the maximum and between 50 and 100 percent of the maximum, using a convenient design feature.

Case Study : Robot Assembly of Pneumatic Valves (2/3)

RECOMMENDATIONS FOR A RADICAL RE-DESIGN OF THE PNEUMATIC VALVE TO REDUCE THE NUMBER OF PARTS IN THE ASSEMBLY

The following design changes are recommended to reduce the cost of the assembly of the pneumatic valve :

(1) Eliminate the choke screw housing (6) by providing an internal thread in the valve body, where the choke screw housing sub-assembly is currently situated.  This would involve the use of a choke screw and O-ring only, thus eliminating two parts.    .

(2) Eliminate the gasket (2) and top cover (23) by moulding the airway into the integral body and top cover. This would eliminate two parts but would require the bodies to be stocked in two styles to accommodate single acting and double acting valves.

(3) Eliminate the piston sleeve (9) by reducing the bore of the body at this point so that the piston is guided by the body, instead of the sleeve.

(4) Eliminate the three sleeves (9), (12), (14) and integrally mould the three sleeves as one part.

(5) Eliminate the piston by integrally moulding it with the spool piece, for single acting valves.

(6) Eliminate the spool piece O-rings / sealing rings and locate them on the spool piece sleeves.

(7) Eliminate one end piece and integrate it with the valve body.

(8) Eliminate the indicator and integrate it with the spool piece.

(9) Eliminate the label and print it directly onto the valve.

(10) Eliminate the cover screws and incorporate (a) a bayonet fitting or, (b) a screw thread between the body and end cover.

(11) Eliminate the short spring by changing the following design features of the long springs :
(a) spring wire gauge
(b) number of turns per inch
(c) spring material
(d) external diameter of spring

CONSIDERATIONS FOR THE RE-DESIGN OF THE PNEUMATIC VALVE

There are many factors that must be considered when re-designing the pneumatic valve for assembly.  The effect of changing one design feature of a part may have an effect on the design of the other parts.  The performance of the valve can be reduced by adverse design changes, or there may be an increase in the manufacturing costs of the product parts, due to new tooling costs.  A number of factors must be considered when the re-designed valve is being evaluated :

(1) A capital investment has been made by the client company in mould tooling for the valve body.  If other part features are to be integrated within the body, or if the body is to be split into more than one component, then there will be an investment required for the new mould tooling.

(2) If the choke screw housing feature is to be integrated within the valve body then the body tooling modification cost, and the scrapped choke screw housing tooling cost, must be considered.

(3) The assembly of the O-ring seals to the spool piece presents the problem of expanding the O-rings over the part and then allowing them to contract into the o-ring groove.  The task can be simplified by re-designing the joints between the spool piece sleeves.  Unfortunately, the current design of joints has been carefully chosen to avoid the possibility of an O-ring passing over a joint between two sleeves.  It would be very difficult to achieve this same performance, so that the valve would operate for more than 3 000 000 cycles without a loss in performance.

(4) The spool piece is surrounded by sleeves having a multitude of holes in them.  It would be logical to eliminate the sleeves and to direct the flow of air from one port directly to another port.  However, these sleeves are required to provide an even flow path around the spool piece to maintain the required flow rate.  Larger ports could be moulded into the valve body, but this could cause the  O-rings to be damaged as they passed over the ports.

(5) The piston is guided by the piston sleeve.  This piston guide design feature could be integrated within the valve body.  The inside diameter of the piston guide must be such that the air pressure required to operate the valve is no greater than that already required.  Additionally, it must still be possible to insert internal parts to the valve body.  If the piston sleeve is integrated into one half of the body only, for spring return valves, then all of the parts associated with the spool piece can be inserted from one end of the of the valve.  This option would, of course, require there to be two valve bodies for the product range.

(6) The thrust from a new single spring must be such that it can overcome the action of the fluid pressure and the friction between the spool piece O-rings and the sleeves.

(7) The sleeves are moulded as separate components because, as an integral part, it would be difficult to get the correct distribution of plastic in the mould.  If the sleeve must be split for this reason then it would be advantageous to situate spool piece O-ring seals between the sleeves.

(8) It must be impossible to inadvertently unscrew the choke screw out of the body.  If the valve was to be re-designed so that the choke screw could be inserted into the body after assembly of the cover, or into a body with an integral top cover, difficulties may arise.  If the choke screw can be inserted after the cover, or cover feature, then it could also be removed by screwing.  The addition of a retaining part would be counter-productive and, therefore, a stamping operation would be more efficient.  The tops of the choke screw holes would be deformed after insertion of the choke screw, thus retaining it.

(9) During manual assembly of the spool piece sub-assembly to the valve body, special tools are required to assist the operator.  The outside diameter of the sealing ring is much larger than the inside diameter of the spool piece sleeve.  A special tool is required to contract the rings before insertion into the valve body.  The operation is so complex that it may not be efficient to carry it out by a robot, in its current state of design.

(10) The operation of inserting the spool piece sub-assembly into the valve body is so complex that it is not feasible for it to be done by the robot.

(11) The piston and lip seal can only be inserted into the sleeve in one direction.  This is because the lip of the seal has a larger diameter than the inside of the sleeve.  The piston could be inserted in both directions if the seal was an O-ring.

(12) If one of the end pieces were to be integrally moulded with the body then all parts could only be inserted into the body from one direction.  The sealing of the indicator would create special problems.  If the seal is inserted by the robot then it cannot be sufficiently located.  Otherwise, the robot would not be able to assemble the seal.  During movement of the indicator, the seal may be removed from its housing.

(13) Integration of the top cover would make it impossible to change the routing of the signal air because the gasket would no longer be present.

(14) The current pneumatic valve design currently has two springs to generate the required thrust.

(15) The pin is required for the stability of the long spring, during operation.

(16) For aesthetics, the top cover and end covers must be of aluminium, to give the impression of robustness to the product.

(17) The dimensions of the inlet / outlet ports must be kept the same for compatibility with complimentary and substituted products.

Case Study : Robot Assembly of Pneumatic Valves (3/3)

MANUAL ASSEMBLY WORKSHEET

1 = Part identification number
2 = Number of times that the operation is carried out consecutively
3 = Two digit manual handling code
4 = Manual handling time per part
5 = Two-digit manual insertion code
6 = Manual insertion time per part
7 = Operation time in seconds (2) x [(4)+(6)]
8 = Operation cost, centimes 0.4 x (7)
9 = Figures for the estimation of the theoretical minimum number of parts

_1    2    _3    ___4    _5    ___6    _____7    ___8    9
23    1    30    01.95    00    01.5    003.45    01.38    1    PICK UP TOP COVER
                                   99    12.0    012.00    03.00        CLEAN THE TOP COVER
30    1    88    06.35    33    05.0    011.35    04.54        PEEL OFF LABEL
06    2    11    01.80                       003.60    01.44        PICK CHOKE S. HOUSING
05    2    03    01.69    30    02.0    007.38    02.95        PICK UP O-RING
04    2    10    01.50    00    01.5    006.00    02.40        PICK UP CHOKE SCREW
                                                                                      AUTO. SCREW CHOKE
07    2    03    01.69    30    02.0    007.38    02.95        PICK UP O-RING
01    1    30    01.95    00    01.5    003.45    01.38    1    PICK UP VALVE BODY
29    2    33    02.51    40    04.5    014.02    05.60        PICK UP BLANKING PC.
01    1    98    09.00                       009.00    03.60        TURN BODY UP/DOWN
        2    30    01.95    31    05.0    013.90    05.56        PICK UP CH. SCREW S/A
02    1    38    03.34    43    07.5    010.84    04.34        INSERT GASKET
        1                         02    02.5    002.50    01.00        INSERT COVER TO BODY
19    1    10    01.50                       001.50    00.60        PICK UP PISTON
18    1    10    01.50    30    02.0    003.50    01.40        INS. LIP SEAL TO PISTON
                                                                                      PICK UP O-RING TOOL
16    1    03    01.69    00    01.5    003.19    01.28        INSERT O-RING TO TOOL
17    1    00    01.13    30    02.0    003.13    01.25        INS. SPOOL TO O-RING
                                                                                      PICK UP MIDDLE O-RING
16    2    03    01.69    00    01.50    006.38    02.55        INS. O-RING TO TOOL
17    2    00    01.13    30    02.00    006.26    02.50        INS. O-RING TO SPOOL
15    3    03    01.69    44    08.50    030.57    12.23        SEAL RING TO SPOOL
24    2    10    01.50                         003.00    01.20        PICK UP INDICATOR
20    2    03    01.69    30    02.00    007.38    02.95        INSERT O-RING TO IND.
22    2    30    01.95    30    02.00    007.90    03.16        INSERT ENDPIECE
09    2    10    01.50                         003.00    01.20        PICK UP PISTON SLEEVE
08    4    03    01.69    30    02.00    014.76    05.90        INS. 0-RING TO SLEEVE
10    2    10    01.50    30    02.00    007.00    02.80        INS. SEAL TO SLEEVE
12    2    10    01.50    30    02.00    007.00    02.80        INS. SEAL TO PISTON
                                                                                        PICK UP O-RING TOOL
11    2    03    01.64    30    02.00    007.38    02.95        INS. O-RING TO SLEEVE
        1    10    01.50    00    01.50    003.00    01.20        SLEEVES TO FIXTURE
14    1    00    01.13                         001.13    00.45        PICK UP HALF SLEEVES
13    1    03    01.69    30    02.00    003.69                   INS. 0-RING TO HALF-SLV
                                   00    01.50    001.50                    INSERT HALF PISTON
        1    00    01.13                         001.13                    PICK UP VALVE BODY
                                                                                      INS. TOOL TO VLV. BODY
                                    30    02.00    002.00                  INSERT VALVE BODY
                                                                                      PICK UP O-RING TOOL
13    1    03    01.69    30    02.00    003.69                   INS. O-RING TO TOOL
        1                         12    05.00    005.00                   INS. O-RING TO VALVE
        1    10    01.50    30    02.00    003.50                   INS. SLEEVE TO BODY
        1    10    01.50    30    02.00    003.50                   INS. PISTON TO SLEEVE
        2    00    01.13                         002.26                   PICK UP END PIECE
21    2    23    02.36    43    07.50    019.72                  GASKET TO ENDPIECE
        2                         02    02.50    005.00                  ENDPIECE TO BODY
25    8    11    01.80    00    01.50    026.40                  INS. COVER SCREW
        8                         92    05.00    040.00    16.00     FASTEN COV. SCREWS
        1    98    09.00                         009.00    03.60     TURN VALVE BODY
        1    99    12.00                         012.00                  LUBRICATE VLV. BODY
                                                                                     PICK UP SPOOL TOOL
        1    00    01.13    31    05.00    006.13            PICK UP SPOOL PIECE
                                                                               PUT TOOL DOWN
26    1    00    01.13    02    02.50    003.63            INS. SPRING TO SPOOL
27    1    00    01.13    02    02.50    003.63            INS. SPRING TO SPOOL
28    1    10    01.50    02    02.50    004.00            INS. PIN TO SPRING
        1    00    01.13    00    01.50    002.63            INS. VALVE TO FIXTURE
02    1    23    02.36    43    07.50    009.86            INS. GASKET TO BODY
                                                        389.22

ASSEMBLY SEQUENCE FOR THE ROBOT ASSEMBLY OF THE PNEUMATIC VALVE, WITH MANUAL ASSISTANCE

Note - Operations marked with an asterisk (*) are carried out manually.

 (1) Pick up the cover (23) from the pallet and insert into fixture (32).
 (2) Automatically clean the cover.
 (3) Automatically feed and insert the label (30) to top cover.
*(4) Pick up the choke screw housing (6).
*(5) Pick up the O-ring seal (5) and insert onto choke screw housing.
*(6) Pick up the choke screw (4) and insert into the choke screw housing.
*(7) Fasten choke screw in choke screw housing by friction screwdriver.
*(8) Pick up O-ring seal (7) and insert into choke screw housing.
*(9) Pick up valve body (1).
*(10) Pick up gasket and assemble to valve body.
 (11) Pick up valve body and insert into fixture (33).
 (12) Automatically feed and insert the blanking piece (29) into the valve body.
 (13) Rotate the body in the fixture by 180 degrees.
 (14) Pick up the choke screw sub-assembly and insert into the valve body.
 (15) Pick up the top cover and insert into the valve body.
 (16) Automatically feed the piston (19) and insert into fixture (34).
 (17) Automatically feed the lip-seal and insert into the piston.
*(18) Pick up O-ring tool.
*(19) Pick up O-ring (16) and insert onto O-ring tool.
*(20) Pick up spool piece (17) and insert into tool.
*(21) Insert O-ring into spool piece.
*(22) Pick up O-ring tool.
*(23) Pick up O-ring and insert into O-ring tool.
*(24) Pick up spool piece and insert into tool.
*(25) Insert O-ring into spool piece.
*(26) Pick up O-ring tool.
*(27) Pick up O-ring and insert into tool.
*(28) Insert O-ring into spool piece.
*(29) Pick up sealing ring (15) and insert onto spool piece.
*(30) Pick up end piece.
*(31) Pick up end piece gasket (21) and insert into end piece.
*(32) Insert end piece sub-assembly into magazine.
 (33) Pick up end piece and insert into fixture (35).
 (34) Automatically feed and pick up end piece O-ring (20) and insert end piece.
 (35) Automatically feed and pick up indicator (24) and insert into end piece.
 (36) Automatically feed the piston sleeve (9) and insert into fixture (36).
 (37) Automatically feed the O-ring (8) and insert into piston sleeve.
 (38) Automatically feed the lip-seal (10) and insert into piston sleeve.
 (39) Automatically feed the sleeve (12) and insert into piston sleeve.
 (40) Automatically feed O-ring (11) and insert into sleeve.
 (41) Pick up piston sleeve and sleeve and insert into fixture (37).
 (42) Automatically feed middle sleeve O-ring (13).
 (43) Automatically feed middle sleeve (12) and insert into O-ring.
 (44) Insert middle sleeve and O-ring into sleeve.
 (45) Pick up the body and insert onto sleeves.
*(46) Insert O-ring into valve body.
 (47) Pick up sleeve and piston sleeve and insert into valve body.
 (48) Pick up piston and insert into piston sleeve.
 (49) Pick up end piece and insert into valve body.
 (50) Automatically feed end piece screw and insert into valve body.
 (51) Fasten end piece screw into valve body.
 (52) Rotate the valve body by 180 degrees.
 (53) Lubricate the valve.
 (54) Insert spool piece tool into body.
 (55) Insert spool piece into body.
 (56) Remove spool piece tool.
 (57) Automatically feed and insert long spring (26) into spool piece.
 (58) Automatically feed and insert short spring (27) into spool piece.
 (59) Automatically feed and insert pin (28) into spring.
 (60) Repeat operations 49 to 51.
 (61) Pick up the valve and place onto the test station.
 (62) Pick up the gasket (3) and insert into the valve body.