1. Field of the Invention
The invention relates to double safety valves for stamping presses and the like which are used to control the clutch-brake of a press. In such normally closed valves, the two valves are actuated simultaneously to pressurize an outlet port which actuates the clutch and disengages the brake, operating the press. Shifting of the valves to their exhaust position will disengage the clutch and actuate the brake. Discrepant positions between the two valves is intended to result in inability to operate the clutch. The double valve is often monitored in addition, in order to shut off the pressure or electrical supply to the double valves. One form of monitoring is position monitoring which is sensing the actual positions of the valve stems. A more desirable monitoring arrangement in some cases is to sense discrepant pressures between corresponding locations of the double valve. This not only senses a stuck valve regardless of degree of movement but will also monitor poppet seal leaks with certain double valve arrangements.
2. Description of the Prior Art
One double valve arrangement is shown by DiTirro U.S. Pat. No. 2,906,246 in which the flow between the inlet valves and the outlet port is in parallel, as is the flow from the outlet port to the exhaust valves. A disadvantage of this arrangement is that if one valve is stuck in a partially or fully opened position, the valve which moves to its closed or exhaust position must dump not only air from the press clutch motor but also from the stuck main valve. This might result in residual pressure remaining at the outlet port which could delay the setting of the clutch/brake. Restricted inlets are often engineered for this condition.
Another arrangement has the main valves in series; with this system there is a positive supply cutoff when one valve fails in an open position and the other moves to its closed position. However it has been found that if the valve which fails in the open position is the downstream valve, the exhausting of the outlet port is sometimes not fast enough because it must follow a tortuous flow path through the inlet of the downstream valve.
A third arrangement is exemplified by Mahorney U.S. Pat. No. Re. 28,520 and Sweet U.S. Pat. No. 3,757,818. In this arrangement the flow from the inlet port to the outlet port is in series through two inlet valves but the flow from the outlet port to the exhaust port is in parallel through two exhaust valves. While this arrangement overcomes disadvantages of those previously described, it is difficult to monitor discrepant valve positions by measuring asynchronous pressures, as is done with a spool element in the above mentioned DiTirro patent. Instead, one must utilize position monitoring, that is, sensing the relative positions of the valve stems themselves as shown in the Mahorney patent.
East German Pat. No. 38,890 dated Aug. 25, 1965 and the article by Morgenstern in Maschinenbautechnik 14 (1965) Heft 6 show a double valve in which each valve stem has two inlet poppet valves, with cross passages between the upstream poppet valve of one valve stem and the downstream poppet valve of the other stem. This construction has advantages of the series-parallel arrangement shown in the Sweet and Mahorney patents but because of its balanced arrangement is amenable to monitoring by the sensing of discrepant pressures. A disadvantage of this construction is that each valve stem has two poppet valves closing in the same direction which is a difficult construction to manufacture while avoiding leakage problems. Efficient sealing could only be accomplished by providing very precise dimensions in the valve body and similarly precise dimensions controlling the relative closure positions. With less careful dimensions, differences could be compensated for by flexible sealing surfaces or elements, and spring loaded closures. However, these methods would require a longer stem stroke to allow for the additional take-up distance required by the closures.
Herion Werke KG of Fellbach, West Germany has a "sivex" press safety valve with cyclic pressure monitoring. In this valve the balanced series-parallel arrangement of the Morgenstern construction is preserved but spool valves are substituted for the upstream poppet valves. Although this construction avoids the above-mentioned multiple poppet sealing difficulties, it could create other problems with respect to wear and monitoring. In the Herion construction the valve stem is radially supported at one end by the piston and at the other end by the spool valve itself. This type of design has been found to create considerable side pressure and wear on the spool valves during continued operation in handling pressurized air to and from a relatively large volume such as that of a press clutch-brake construction. As a result, if a discrepant position occurs between the two valve stems after some service, some line pressure could pass through a worn spool valve causing a delay in the setting of the clutch/brake. While a monitor would sense discrepant positions between the two valve stems after they occur, it would not sense wearing of the spool valves before the discrepant condition. Furthermore, because of the fact that the upstream inlet valve in the Herion construction is the spool valve, the pressures in the conduits between the upstream and downstream inlet valves is constant. If constant line pressures are used for spool sensing purposes, it has been found that the spool type monitors are not as likely to be reliable when needed.