Counter balanced boom crane dynamic load test

[mattheweng]

Hi All
We are the manufactures of a counterbalanced boom used for the film industry with tilt and pan mobility.
my current safety rating is 2.5 to compensate for the dynamic load that the boom faces while mounted to a vehicle in tracking which is restricted to 80km/h for now.
this is the standard which I have found in most documentation I've gone through.

Notes:
Does not carry persons.
tilt and pan are operated manually operated by a person and will be automated at a later stage.

picture of boom attached

My Questions:
1)Is this safety rating enough in this application, as I cant seem to find much info on counterbalanced cranes strapped to the roof of cars?
2)suggestions on how to apply a load test that would in most part test the dynamic load in all the axis that the boom is taking strain?
3)Any recommendations on documentation would be appreciated.

thanks in advance.

 

[RolMec]

I'd like to post some perhaps obvious things, so pls. forgive me any commonplaces being beneath your engineering level.
I don't have a ballpark figure, and perhaps there's none. Similar applications where an analogy might be found are firefighting vehicles or fastgoing crane trucks, however the lifting applications will be fastened during vehicle movement. Don't now for what kind of films your device shall be used but it seems pretty fast to run 50 mph with a swivelling arm like this one.
On the long run:
Basically, you have a car it should be possible to analyze the forces imparted on this car and anything attached to it from a street car or offroad car design knowledgebase. If you in a first step have the kinematic model and the dynamic accelerations to be applied from movement etc., you then can make up your proper kinematic model which would allow you to put the appropriate dynamic forces onto your application. The strain etc. derived you then can compare to permitted strains etc. as per common engineering practise.
I did not get your meaning concerning the manual / automated control, however if there's persons involved / onboard the vehicle and you alter some base vehicle to your purposes, you should imo run a risk analysis (overturning?).
In the end (1st): There's corporations which provide safety analyis' / quality assessments to new products and by contact should be able to at least point you into some direction. E.g.: TUV group (no rel.)
In the end (2nd): Even if up until now there's only been this "number" of 2.5, I don't think it's appropriate to just follow it... unless it's written somewhere "this value applies everywhere and for everything".
In th end (3rd): one remarks your wording quote "restricted to 80km/h for now" unquote, this seems to be fishing in dark water. Imo its better to get the numbers straight from the beginning... perhaps they'll use it to some F1 race film one day...;-)
Regards
R.

RSVP

 

[chicopee]

A reminder to statement "...or fastgoing crane trucks..." Crane trucks or any other mobile crane lifting a load while travelling will be done at a walking pace or commonly termed "walking the load. Lifting a load while driving the crane is dangerous and is not done.
A 2.5SF at 80 km/hr seems to be. I suggest that you research some of the ANSI standards that would compare to your situation. The movie industry may also be an area of research.

 

[BEMPE16524]

I've design an almost similar things but for offshore industries to operate at sea state 6 (wave height at 4 to 6 mtrs).
For my situation, the most conservative s.f was used which is 3 s.f. eventhough in most design document, s.f 1.6 (or sometimes called dynamic factor) is stated.
I personally think 2.5 is more than enough provided your truck doesn't fall into a hole.
I suggest you test your system by using drop test method (hang your load to the crane by using a sling or similar, an drop it from certain height).

 

[drawoh]

mattheweng,

Safety factors actually are factors of ignorance. Are your calculations absolutely rigorous and accurate? How carefully are they checked? Did you take fatigue into account? How accurate are your loads? How disciplined are the people who operate this thing? Will they really stay within 80kph (50mph)?

How acceptable is failure? When it breaks, will the movie shoot be held up half an hour while a studio worker does a complete, safe repair? Will an expensive, hard to replace piece of equipment be damaged or destroyed? Will any people be hurt or killed by the failure?

Are you trying to reduce weight or cost? Weight reduction gets expensive, especially if you get into exotic materials, and high quality fabrication.

Can you inspect this thing and see impending failures? Are the users willing to?

Aside from the obvious static load, how do you anticipate this thing getting loaded? I would be comfortable assuming a normal car will not accelerate or decelerate more than 1.5g on a smooth road. I do not know what road bumps do. Accelerations go in all directions.

Are you willing to test these one of things to destruction?

--
JHG

 

[chicopee]

I reread my response and noted part of my statement"... A 2.5SF at 80 km/hr seems to..." s/h been "...2.5SF at 80 km/hr seems to be low..." Other factor of safeties mentioned by various responders are basically for reasonable conditions, however, in your case that does not appear to be so. The response "Safety factors actually are factors of ignorance..." may be so ,however, you will note that standards and regulations published by ASME/ANSI and OSHA will use the term "safety factor" as long as material properties are stated.
I personally would look into higher numbers such as five and above due to the dynamic effects that you are proposing and at 80km/hr, there will be a few bumps here and there. Just remarking: have you ever questioned yourself how is it that truck axles particularly of those trucks use on construction sites and quarries seldom have broken axles and that is part due to very high safety factors.

 

[mattheweng]

Hi All thanks very much for your input.
this was the first time posting here and the feedback was immensely helpful.
So at the end of it all I just destroyed our Prototype which was extremely informative,
a) incremental load and unload test, measuring deviation from return to normal.
I got up to 8x working load before seeing signs boom displacement not returning to zero.
b) failure was at approximately 12x working load due to a weld on face of one of the booms breaking.
which was suspected to be the first point of failure all along.

these were higher than what I had worked on my simulation by almost double, will look at that again.
will go with safety factor of 5 to suit under hook lifting equipment in USA for the time being.

as questioned above, yes someone could get hurt or killed with this, its up to 9m long and weighs 450kgs fully loaded.

as for fatigue that is still something I'm working on and I'm sure ill get back to you.

@drawoh
You hit the nail on the head have acceleration in all directions, very hard to calculate
think of it. Two masses in balance over a pivot which can rotate and then its strapped to the roof of a car. I like making my life difficult it seems.
thanks for your help will look into that fatigue also!

FYI pic attached is the boom in action