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Why a Backmounted Harnass works.....


RobVanGelder

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i guess, with fewer words, that is what i disagree with.

i think that you operate under and up to the tipping point. but not at the tipping point.

 

 

 

but more to the point :) , i was trying to say that the cg would not be in the same place in the drawing if you moved the ladder.... the load chart for a truck crane is very different for picks made over the front than over the rear (except for some of the newest that balance the undercarriage differently)... your drawings would suggest the load chart would be the same...

 

 

likewise i think that when you lean back, as in your example in the last post, you are also changing the CG...

 

 

fundamentally- i am not trying to say that you are right or wrong about much of anything - i am just trying to say that the drawings are not completely showing all of the forces, and they dont show the shifts in the cg when the ladder is moved from the front to the back.... likewise they dont show what is happening as you lean back (this could be likened to using a crane on a steep incline)= much more mechanical stess failure than tipping likelihood...

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Interesting stuff..

 

I'd think that you woudl always be exactly in ballance when operating if you where pivoted on a point. but as yoru boot essentially has 2 pivot points your overall CG must be bbahind the front of your foot (or you woudl fall forward) and forward of the back of your foot (or you would fall backwards).

So if your foot was very thin, then only woudl you be exaclty in ballance..

Now where the 'optimum' operating position is between the front and back of your foot i guess is a matter of oppinion?

 

 

I think that when you lean back you are chainging your CG because you are of course moving the mass of your body.

 

But i dont' see how moving the ladder's attactment point has any effect.. It's still only the center of masses that matter. (Pun intended) I can see that if the mass ont eh end of the ladder moved when you move the ladder (which woudl make sence in real life) THEN of course mass would be moving and changing the CG. But with for example a extension ladder that allows the mass to remain in position when the attachment point moves wouldn't effect teh CG.

 

Just my ponderments on teh issue.

 

- Mikko

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Mikko,

 

........"I'd think that you would always be exactly in ballance when operating"

 

Yes. If you are standing still and all the masses are at rest, then there is perfect balance. Must be right, Newton's First Law says so!

 

........" i dont' see how moving the ladder's attachtment point has any effect"

 

You're right, it doesn't

 

........"with for example an extension ladder that allows the mass to remain in position when the attachment point moves wouldn't effect the CG."

 

Right again, anything else would be rewriting Physics.

 

Mikko, I think I'll bow out of this thread and let you take over. Good luck!

 

Paul

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........" i dont' see how moving the ladder's attachtment point has any effect"

 

You're right, it doesn't

 

 

Mikko, I think I'll bow out of this thread and let you take over. Good luck!

 

Paul

 

my apologies if i have offended you Paul.... You spent a good deal of time with the drawings and your concept is basically correct... i am only talking about a very smallshift in the CG within the center of the body...

 

 

 

the ladder has mass.... moving an object from one side of a balanced teeter totter to the other changes the CG position.

 

imagine a teeter totter.... and it is balanced with a ladder in the seat of one side.

in order to put the ladder on the other side and have it be balanced you would have to change where the teeter totter attached to the main board.

 

to use the drawings as an example, if you could balance the truck with the rear ladder on the head of a pin you would find a point in the center of the truck where all four wheels came off of the ground. if you then moved the ladder to the front of the truck the cg would now shift and the pin would have to be moved to balance the truck.

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Paul,

I'm not goign to take over the thread by any means. I don't feel i'm near qualified to.

But i'm happy to join in the chatter! :-)

 

David,

Oh you are refering to the mass *Of the ladder itself*... ...well yeah, i agree that this would definatly cause a slight shift - just as the weight of the arm does effect operating.. (Someone who can easily compare 2 arms,, (Say a Master and Geo arm) confirm this please!)

 

But for general calculations it's irrelevent. - For example (back to the original topic) The arm is still there anyway when you connect to a BM vest as opposed to a FM vest.

 

Just as Dymnamic ballance can be measured and calculated: "Measure the main components: Camera, battery, monitor and any other accesories" and do the math, easy! Now i dont' knwo anyone who actually then weighs the post of the sled, and the monitor mounting rods, and the battery rods.. The forces are so small it's irrelevent.

 

Same with the firetruck..

I agree that these forces no wear near include all the forces in the equasion, but they are only to demonstrate the main principles. I mean yeah you could as the weight of the ladder. But then there's the friction of the tires on the road, then the fact that the ground isnt' flat but curved (eath is round) then there's the position of the moon and it's gravety (strong enough to effect tides, so must be on the drawing...)

And then there's the fact that if the truck tiped it would roll on it's wheels which would affect the pivot point slightly beacuse they would act as a cam...

 

I apologize how absurd that sounded.. But i was just making the point that I dont' think that the ladder counts in our calculations.

 

I do however agree with you that the truck doesn't have to be in perfect balance to keep the ladder up. The 'balace/tipping point' (overall CG) must be somewhere between the wheels. This goes back to my analogy of the length of our feet.

This can be tested as simply as: imagine the mass on the end of the ladder moves towards the truck (Say a firefigter climbs down the ladder) the truck won't fall over backwards. In fact the truck would never fall over backwards untill the overall CG got behind the *back* wheels.

 

The only time perfect ballance is required is if the backwards pivot point is the same as the forward pivot point! ...This is why we operate standing on our feet properly, and not on our toes! (that and we'd all have broken toes by now!)

The wider teh base between pivot points (eg: wider wheelbase), the more stable an object (the more range for the CG to be in untill it corsses that magic boundry) This is why when you drive on a hillside that horizontal space between the wheels gets smaller and you become less stable.

 

I agree with you that the drawings dont' illustrate all the forces, but I feel that the ommited foreces are irrelevent to the discussion (at least regarding tipping forward)

And I agree that the system doesn't have to be perfectly in ballance to remain stable. Only that the overall CG (all that matters) is suppoted on both sizes to prevent it from falling.

 

Ok.. so now i'm not sure who i agree with! :unsure: Myself i guess.. ;)

 

- Mikko

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*steps up the the plate*

 

I'll give it a shot at least..

 

Going by the diagram, when the boom is off front of the truck it's "attached" near the back. Due to the length of the boom this would place more of the truck under the boom and move that pivot point (front wheels) closer to the mass on the boom.

 

When the boom is off the back (like it is in the diragram) the attachment point is in the center of the truck. This is closer to the new pivot point (the back wheels) than with the boom facing forward. So this woudl reduce the capaciy of the boom.

 

So how can the boom actually hold more when it's facing backwards?

 

My guess is that the answer lies in the CG of the base of the truck. In the previous theoretical drawings the CG was in the center. However for the boom to be more stable, the CG woudl have to actually be closer to the FRONT of the truck. And i think it is. Most of that truck is a empty bed, the crane operator's cab moves witht eh boom, so that has no effect.

So here's my vote: The engine!

 

I think the Engine moves the CG of the truck so far forward that it can better ballance against the boom's load witht he boom off the back.

 

Anyone care to do a fancy Drawing?

 

- Mikko

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Hmm, 31 is interesting too.. That little block under teh front of the tracks moves that pivot over just slightly - sometimes enough to make a difference.

 

 

And 11 continues to state that the front is unsuitable for use as a boom direction... I can only presume that the engine actually pulls the CG of the truck *forward* of the front wheels! (or very close to..)

 

 

- Mikko

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Mikko....

 

 

you got it... is is because of the weight of the engine....

 

the "pivot" point actually is where the outriggers touch the ground, not the front tires...

 

for the calculations the CG is the pivot point...

 

 

THE CG is critical to all load calcualtions....

 

 

if you move the mass of a crane boom (thousands of pounds), a fire ladder (at least a few thousand lbs), or even an engine (+- 1000 lbs) from one side to the other of the cg it makes a huge difference.

 

 

if you dont overcomplicate it with lots of math and therotical physics it becomes easier to understand--- ie, we have the real world practical physics lessons from a playground when we were children.... the seesaw or teeter totter.....

 

fatkid on one end, skinny kid on the other... everything pivots about the CG...

(you move the fat kid closer to the CG to give the skinny kid a leverage advantage to achieve balance)

everthing is pivoting about the CG....

 

ladder in front/ ladder in back.... either way, everything pivots about the CG...

arm in front/ arm in back.... either way, everything pivots about the CG...

 

 

so, in my opinion, the two most important factors on how the point of attachment effect the human body are:

1. where is the cg and how does it effect comfort and wear/tear...

2. Method of attachment- the rear mount seems to pull on the hips and push on the back...(sort of like a good easy chair?)

 

you can think and think and think and think, but just ask and see which one people like better and try and see which one you like better.....

 

.... then watch, to make sure that no long term ill effects are observed with whichever type you choose....

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Aww come on David... there arn't any Outriggers in that picture in the PDF! (thoguh they are mentioned below i guess... :P

 

 

I'm still of the oppinion that the attachment point has no effect on the CG of the system ...except the effect of the Connecting hardware of course!

Imagine having *both* ladders (like an 'A' frame arrangement...) but only having a connection at the back point first. calculate ballance, then connect eh front and disconnect the back. (heck, even keep the bolts there, just pull them out!) the system's ballance wont' change at all!

 

However I do very much agree that the internal stresses within the attachement systems do of course have an effect on each other!

- Not on the overall system: you can stay in ballance jsut as well with a FM or a BM vest. But within the system yes, the pressure on yoru body will vary depending on how the vest is worn.

 

But the CG of the system between a FM and a BM vest (of the same weight) *doesn't change*, just the way you feel them will be different. And to each his or her own prefference!

 

 

- Mikko

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