Tuesday, 27 June 2006

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


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


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.

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