Mechanical Lock for Scissor Lift

A scissor lift can be built with safety stops that take the weight and cannot be removed unless you raise the lift a little to enable the stops to be released as they are made so you cannot release them under load or you would get a collapse.

If you go to a place that rents scissor lifts they will show you the safety features

Yes.

DOGS.

normally the hydraulic cylinder would incorporate a counter-balance/ over-center valve to hold the pressure inside in case of a power failure. that will hold the load while working and prevent it from falling off suddenly.

no, they use fixed machanical stops, or "dogs" as someone calls them, in case the cylinder cracks and the fluid pressure drops and the load drops.

Like jacking up your car, you placed blocks under the car in case the jack fails

From all the recent posts on scissor lifts, could it be that someone is attempting to design one, bit by CR4 bit?

I've been wondering the same thing.

You can use faile safe circuit for this.

can any one explain me the mechanism of dogs in scissor lift.Are they similar to ratchet and pawl mechanism

yes, similar. In a hoist by cable or hydraulic, what happens if the hydraulic fails and the pressure drops, or if the cable breaks?? The dogs as one guy calls them, are a mechanical stop that is applied when you reach the desired lift. You apply the stop and then release the hydraulc pressure or cable, and the load is now on the mechanical stop and no failure of the hydraulics or electric cable will let the lift fall on you underneath it.

http://www.google.ca/search?hl=en&safe=off&q=jack+%2Bsafety+%2Bstop&btnG=Search

http://www.google.ca/search?hl=en&safe=off&q=hoist+%2Bsafety+&btnG=Search