| 摘要 |
A safety device which prevents any one-sided lowering motion of a jacking system including a plurality of jacks which are designed for simultaneous operation, and a mechanism connecting the jacks for their simultaneous operation. Each jack includes a first hydraulic or pneumatic cylinder for raising and lowering a table, a substantially vertically disposed rack mechanism, a pawl mechanism which is normally engaged with the rack mechanism to hold the table in a raised position, and a second pneumatic or hydraulic cylinder for disengaging the pawl mechanism from the rack mechanism. The safety device includes a plurality of valves each provided in one of the jacks and operable by a pushbutton to produce a fluid presure signal when the pushbutton has been depressed. A mechanical actuator is provided in each jack for depressing the pushbutton when the pawl mechanism has been disengaged from the rack mechanism. An electrical control circuit for the first cylinder includes at least one pressure switch which is operable in response to the signal for allowing the first cylinder to lower the table only when the pawl mechanisms in all of the jacks have been disengaged from the rack mechanisms.
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