The patented Stelron RWB base is a synchronous, linear, cam actuated assembly chassis. The design of the RWB is unique in the marketplace. The product is either fixtured in or attached to the pallet. The pallets are moved in a rectangular pattern around the table top by synchronized cam actuated walking beams. This walking beam engages each pallet with dowel pins and the pallets ride in tool steel guideways. The walking beam indexes the pallets horizontally one pallet pitch. Before the walking beam dowel pins dis-engage, a mechanically synchronized locking beam engages each pallet with two low clearance shot pins.
Design & Dimensions:
The pallets are fabricated from self lubricating (impregnated with molybdenum) and resilient Delrin®. The calculated and actual experienced wear factor is extremely low. In fact based upon manufacturer’s data, at a wear rate of approx. 55 x 10 to the minus 10th power cubic inches per hour, a wear of .001 inches would occur over
100,000 or more hours of run time.
As mentioned before, the pallets can serve as nests or have fixtures mounted to them. An entire set of pallets can easily be changed out in about 15 minutes. This allows rapid product change-overs and inexpensive retooling for future products.
The chassis accuracy of +/- .0015 is available at all stations including the ends but excluding all four corners. Users report long term accuracy and repeatability to be very stable. They have not found it necessary to re-locate stations as is the case on link, chain or band systems.
These other systems are subject to drift as the link pivots wear and the individual link/pivot tolerances begin to accumulate. Also the same chain or link system that indexes the load must also provide the final accuracy whereas in the RWB the locking beams only function is to provide shot pin accuracy.
The indexing mass of the RWB chassis is very low due to the light weight Delrin® pallets. The coefficient of friction of the pallets is very low at approximately .16 to .2. Each leg of the chassis indexes 90 degrees out of phase from all the others, avoiding the “everything at once” indexing of competing designs.
The walking and locking beams, actuated by the same cam and connected by a rocker arm, are balanced to each other. All of the above results in high speed capability, minimal input power requirement / fluctuation and low wear. The walking beams are driven by Stelron T-400 modified Translators which incorporate oversized ball bearings and cam followers thereby providing long life. Typical RWB applications require the T-400 to exert 10 to 20 lbs per unit during the index period. Whereas the T-400 has a thrust rating in excess of 200 lbs. This operation in the 5 to 10 percent range explains the long life experienced by RWB users. Standard index periods as low as 90 degrees (or less) are available due to the large diameter cams employed in the T-400. The Stelron Translator also provides true modified sine cam motion directly to the indexing mass. Competing chain, link and band systems impart the index motion through linkages, levers or sprockets. These indirect systems do not provide true “in line” modified sine motion.
The RWB offers considerable application flexibility. The pallets can be accessed from all sides and from underneath at certain locations. An anvil can also be included under the pallet to accommodate pressing operations. Pallet sizes are available from 4 through 8 inches. Pallet and table top thickness can also be varied. A wide range of table lengths (up to thirty feet or longer) and widths are available since modules can be combined. The flat table top and linear transfer design simplifies tooling layout. Ample space between pallet tracks allows pick & place, tooling and other mechanisms to be mounted inside the tracks. This improves operator and maintenance personnel access and allows for more efficient use of a given size chassis.
The chassis’ are supplied with a corner overload feature including a proximity sensor at each corner. These proximity sensors must be wired into the e-stop circuit of the machines control system to provide for immediate shut down in the event of an overload in one of the corners. It’s purpose is to protect various parts of the machine from severe damage due to a jam situation. This option is not and should not be considered an operator safety device. The machine builder / user is obligated to provide proper protection in accordance with state and local occupational safety regulations.
This illustration shows the timing sequence for a walking beam with a pick and place unit and a pair of grippers. The walking beam will pick-up and index the product along a pair of rails. The index time will consume 150° of the cycle leaving 210° of dwell to perform the desired operation with the pick and place device. In this case the pick and place will pick-up from a dead nest, during the index of the walking beam, and place into the product at the mid-point of the walking beam return stroke. The grippers will be actuated precisely at the mid-point of the pick-up and de-actuated at the mid-point of the placement or insertion into the product. Thus, synchronous timing is easily achieved between the cam operated devices and the PLC by means of an encoder which can be attached to the main drive shaft.
Modified Sine Curve
The Modified Sine curve (below) has become the most widely used profile as its characteristics tends to have the most benefit in indexing heavy, pure inertia (minimal friction or work) loads.
Walking Beam Cross Section
This is a cross section of the walking beam system, showing optional anvils to accommodate pressing operations.