hydraulic in free fallgreenspun.com : LUSENET : Elevator Problem Discussion : One Thread
a stupid question... what are the means of safety if a hydraulic elevator piston were to blow? in the case of older hydraulic models, is there no safety installed? and my last question is... how effective is the safety buffer on the floor of the elevator shaft if the car falls one floor? also are all hydraulic elevators (or all elevators for that matter) equipped with the buffer?
-- Jobocop (firstname.lastname@example.org), August 19, 2004
to start you have to understand there are two types of hydro lifts, firstly roped hydros.. these have safety gear assemblies the same as all traction lifts , secondlly direct acting hydros.. these have rupture valves on the jacks. to answer your last question 99.9% of lifts have buffers, but i dont think you understand the reason for them, they arent to buffer a lift in free fall, because lifts dont free fall(except in movies or under certain exceptional circumstances). hope this helps
-- paul (email@example.com), August 19, 2004.
the reason hydraulics don't blow is that they are engineered to stand pressures far in excess of that which the pump can deliver. Hydraulics pressurise their system before starting lift movement, as soon as the lift moves, in real terms the hydraulic pressure drops slightly, so the time when the cylinder is under max pressure the lift is stationary. There is also a reduction in torque but this is acceptable as the lift now has momentum. hope this clears this up
-- geoff judge (firstname.lastname@example.org), August 20, 2004.
There are no requirements for ALL hydraulic elevator to have a Safety with the exception of single-bottom Direct-Acting Hydraulic elevators (those manufacturered before 1972). Cathodic protections have been traditionally relied on to prevent electrolysis a very bad effect, one which commonly requires replacement of 5 to 10% of the present inventory every year. This effect has caused fatalities and severe injuries almost every year. Present Code has addressed single- bottom direct-acting jacks with the adoption of retrofit requirements of replacement of the jacks or addition of plunger grippers; but the lack of an equivalent traction (electric) elevator type Safety requirement still potentially allows for uncontrolled movement due to other failures of the pressure system. For instance, though equiped with rupture valves, the differential pressures to cause them to close will allow slight overspeed, full speed and slower uncontrolled movements, those typically experienced with a failure of the jack due to electrolysis along the sides of the jack and victaulic fitting failures. THough a completer line break will caused these valves to stop the fall, there still are many unprotected failure modes.
All elevators are required to have buffers (or bumpers) and the buffers of all elevators are only designed for a slight overspeed at full load impacts. Free falls will only temporarily slow down the elevator during the bending, deflection and destruction of the buffer. It is not designed for this impact.
Measurements of one hydraulic whose fall from 6 feet generated a speed of over 1,200 fpm (6 m/s) before it impacted. This was with only a 2 sq. in. ball valve opened to atmosphere in a study I carried out. I hope this answers your questions.
-- John Koshak (email@example.com), September 01, 2004.