Automation steers valve assembly

Anglian Automation Systems has developed some special-purpose robotic systems for use by French-based TRW in the production of components for automotive power steering assemblies. Contributing editor Rex Narraway went along to see how they did it

Power steering mechanisms rely on a hydraulic spool valve as the prime actuation element. The valve itself comprises two major components: a honed sleeve and a ground hollow stem. The stem has a series of six scallops machined into the outside diameter, three of which contain a single orifice to allow the fluid to flow to the central bore. The sleeve, with three oil grooves machined around its periphery, runs between the central stem and an external housing. Each of the grooves has a series of ports bored through to allow hydraulic fluid to flow into scallops located on the stem. These oil grooves are separated by a further set of four identical grooves, designed to house Shamban glass-filled oil sealing rings.

To facilitate the handling of the special features on the two components of the spool valves, Anglian Automation Systems was contracted to supply TRW with seven machines. Three similar machines, each equipped with a Sankyo 8437 SCARA horizontally articulated robots, were developed for loading the stem to one of three 6-axis cylindrical grinding machines. Another pair of machines, also fitted with SCARA robots, assemble the sealing rings into the grooves of the sleeve. The final two machines test the performance of the assembled components using standard automotive hydraulic fluid as the operating medium. These pass or reject the assembled components and classify those selected into one of 16 grades.

The valve stems are produced to size as turned-and-ground blanks. They are then transferred to a cylindrical grinder for the finishing operation. This entails grinding scallops into the outside diameter and calibrating the orifices by grinding a slight chamfer on the edges of each scallop (see: Grinding the stem).

As well as designing loading machines for the stem grinding operation, another machine has been designed to automatically place four Shamban sealing rings in the grooves machined in the sleeve of the valve. Not as easy as it sounds, since the characteristics of the seal material had to be taken into account when designing the assembly machine. The material of the seal, for example, contains glass fibre, making it is relatively inelastic. To produce an accurate fit in the groove, it was necessary to expand the ring by means of a hollow conical dolly. This enables each ring to clear the machined features on the surface of the sleeve whilst it is being placed.

What is more, the material retracts very slowly after stretching, so, once a seal has been placed in position on the sleeve, it has to be squeezed back to ensure a tight fit in the groove. In practice, this was achieved by means of a Parker Pneumatic 3D linear unit fitted with a special clutch attachment which can be forced over the diameter of the seal.

Novel twist on air gauging

Each sealing ring assembly machine comprises an indexing table fitted with four stations that are fed by a SCARA robot. They are capable of being adapted to handle two sizes of sleeve using only a 10min changeover time. The sleeves are fed to and from the robot by means of input and output conveyors.

Before entering the sealing ring assembly table, individual components are placed on an air gauging station to determine whether or not the bore has been honed; if not, it is rejected. The system of air-gauging is based on an SMC Series ISA air-catch sensor. Normally, this sensor is used for non-contact sensing applications on flat surfaces to confirm the presence of a work-piece; as such, it is capable of reliably detecting gaps below 10 m. For example, one of its main applications is in checking the gap at certain points located around the machined edge of an engine block and a surface table. However, for this application it has been modified to allow it to be used in similar fashion to that of a conventional air gauge for precision holes, but at one fifth of the price.

All that was required was to machine two mandrels with diameters to fit each size of sleeve, together with a pair of accurate holes to provide a graduated air flow, then mount them adjacent to the index table of the ring assembly machine. SMC supplied two air catch sensor modules, individually tuned to suit the maximum and minimum limits of each size of sleeve. When a sleeve is placed over the mandrel and air is fed via the two holes, the air-catch sensor detects if the bore is too large or too small.

The robot places the accepted sleeves on the first of the location fixtures mounted at four stations on the indexing table. The fixtures use pins of the same diameter as the stems and are made to float with respect to the table by using a spring-loaded mounting arrangement. The table then indexes to position the first sleeve at the second station, while the next sleeve is being mounted at the first station.

At the second station, a scissors-type gripper swings into place and closes around the component to form a platform just below the first sealing ring groove. This acts as a datum for a mechanical probe which is brought into the groove and used to force down the component, plus its fixture, against the compressive action of the spring until the probe bottoms on the platform created by the gripper.

After this has occurred, a rodless pneumatic cylinder comes in below the fixture to lock it in the new position, with the spring still compressed. At that point, a hollow dolly with a conic upper end is automatically fitted over the sleeve by the action of a gripper mounted on a 3D linear unit.

Meanwhile, the glass-filled sealing rings are transferred from a bowl feeder by means of a flexible chute and are stacked in a magazine located above the dolly. When the gripper that positions the dolly over the sleeve has been withdrawn, a sealing ring is released from the magazine using a pneumatically controlled shuttle mechanism. The ring is deposited over the conic end of the dolly and is forced onto the major diameter by the action of a spring-loaded 3-fingered collet mounted at the end of pneumatic ram. The sprung jaws of the collet allow the three fingers to maintain contact with the face of the dolly over the whole length and forces the ring down until it snaps into the groove. At this point, the collet is removed and the gripper that positions the dolly is raised.

Placing the ring into the next groove is performed in a similar manner after the entire ring-fitting assembly – including the scissors gripper, mechanical probe and ring feeding system – is raised under the action of a servomotor-controlled ball-screw. The distance by which the table is moved is exactly the same as the pitch between the sealing grooves and actual movement is determined by a cam. This elevates the scissors platform to the next groove, the fixture is released to allow further compression of the spring, and the mechanical probe relocates the sleeve in the next sealing ring position.

The dolly is raised by the 3D linear unit while the spring fixture is again locked. It is then returned to the next datum for fitting the second seal. The same sequence continues until all four seals are in place. Overall, the action of placing all four seals in their respective grooves on the sleeve is accomplished in a phenomenal 12s. Finally, the dolly is lifted away from the fixture to allow the table to be indexed to the next position.

The Parker 3D linear unit was selected to provide all the vertical movements of the dolly because it was the only standard unit that offered a variable mid-position stop. This feature allowed accurate lengths to be set over two movements of travel, thereby enabling the gripper to pick up the dolly in any of the set positions. In practice, the unit proved to be extremely versatile and accurate while, at the same time, providing easy adjustment. A similar unit without the mid-position stop is used at the fourth and final station to apply another gripper to squeeze the ring to size.

A system of fibre optic sensors is used at the third station to detect that rings have been fitted to each groove and that they have not fractured during the process. Since the machine has been designed to accommodate two different sleeves, two sets of four sensors have been fitted in order to meet the requirements of both sizes.

After checking, each sleeve is transferred to the fourth station where the outside faces of the sealing rings are gripped to the correct diameter, and then returned back to the loading and unloading position at the first station. The SCARA robot places any faulty assemblies, typically amounting to 1/1000, in the reject conveyor while the accepted components are placed in another conveyor which transports them to the one of the two hydraulically operated grading machines.

Here, they are loaded onto a master stem where they are subjected to the same hydraulic pressure that would be applied in service. The stem is rotated first one way then the other, by means of the servodrive, to measure the pressure drop as the ports open. From these figures, each part is categorised as falling into one of 16 grades for selective assembly with its mating component.

Figure 1: The sealing ring assembly machine showing the load/unload station, bowl feeder, input conveyor & SCARA robot with the air gauging system beneath

Figure 2: The system of air-gauging is based on an SMC Series ISA air-catch sensor that has been modified to allow it to be used in similar fashion to that of a conventional air gauge for precision holes, but at approximately one fifth of the price

Figure 3: When the gripper that positions the dolly over the sleeve has been withdrawn, a sealing ring is deposited over the conic end of a dolly and forced down by the action of a spring-loaded 3-fingered collet mounted at the end of pneumatic ram. The sprung jaws of the collet allow the three fingers to maintain contact with the face of the dolly over the whole length and forces the ring down until it snaps into the groove. At this point, the collet is removed and the gripper that positions the dolly is raised

Figure 4: The ram for pushing the seals over the dolly. The dolly is attached to the gripper on the Parker Pneumatic 3D linear unit

Figure 5: Double gripper attached to the SCARA robot arm for loading and unloading the valve sleeves

{{TRW FranceTel: France (33) 3 80 632 000Enter 470

Anglian Automation SystemsTel: Ipswich (0473) 611 990Enter 471

Parker HannifinTel: Cannock (01543) 456000Enter 472

SMCTel: Milton Keynes (01908) 265247Enter 473

ASCO JoucomaticTel: Skelmersdale (01695) 724270Enter 474

Busak and ShambanTel: Nottingham (01602) 411866Enter 475