MOSS Davit Procedure

[roca]

In the MOSS book davit procedure for a manway davit design I believe that type 2 design should be used.
One supplier has used type 1 which doubles up the load and moments and the davit fails – even though his spreadsheet calc shows the davit as passing…!!
Out of interest in the Type 1 design why are there two fv loads?

 

[jte]

The key is in the free body diagram. With the Type 1, the rope/cable is in tension equivalent to the weight being hoisted - and if the winch side is approaching vertical, then the reaction at the pulley is 2F.

With the Type 2, presumably the chain/ratchet hoist is geared, likely with a greater than 10:1 ratio. Thus, someone pulling on the chainfall will exert less than 1/10 of the weight being lifted. To be more exact, perhaps the formula should be modified to account for the load on the chainfall side... but in reality, we're talking about a few percent so the load is considered neglegible and the simplification is made.

This is a good example of what can go wrong: A designer states on his drawing that the davit is a "Type 2" and designs away. The davit is installed in the field and the maintenance crew rig it up with a ground based winch - do you really expect the maintenance crew to a) have the drawing, b) review the drawing, and c) look up "Type 2" in Moss' book?

I'd design for a Type 1 in every case. Its much harder to mis-rig it in the field in a manner which would cause a problem.

jt

 

[roca]

Thanks jte
In my case it is not a column davit.
Just a simple manway davit -no chain or hoist.
So in my opinion Type 2 is sufficient.

 

[jte]

roca-

Yes, with a typical manway davit, the free body diagram would indicate that the davit sees only the weight of the blind.

jt