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Gin poles for the amateur

06/06/2007 1:53 PM

Having been graced with a small piece of land, I now am using the building of a barn as an excuse to educate myself and acquire new toys!

Questions fall into many categories, I'll try to be organized but was unsure whether this was large enough for Civil, academic enough for Mechanical; and so find myself back in General.

The subject is truck mounted gin poles, an "A" frame device commonly used in the oilfield and other locations for light lifting. Usually of a fixed angle with respect to the vehicle they are mounted on, with a winch cable run through the apex of the A frame. Just wanted to make sure we were all talking about the same device! Not too different from the arrangement of a tow truck/recovery vehicle, but generally with much longer extension of the A frame.

1. Is there a general relationship between the weight of the vehicle you mount this device on and the weight it can lift? Can this be adjusted by the use of outriggers? What about changing the boom angle?

2. Are the rubber tires a consideration as they flex under load, and can this be offset with outriggers?

3. Does it make ANY difference at all WHERE you mount the base of the A frame? I have seen them mounted on rear bumpers, front bumpers, rear of the bed and front of the bed. Does the weight distribution of engine-in-front matter at all or can the vehicle be analysed as a uniform block?

4. What happens to the back of the envelope calculations if you want to lift a load and then move the vehicle?

5. How do you size the A arm materials? What kind of specification makes sense to a seller of piping? Or me, for that matter.

6. Is there a practical limit to how high you can lift a given weight? Does it vary with respect to the vehicle footprint and degree of level?

Sounds like straight-up trigonometry, but I keep talking meself in circles. Expert advice deeply appreciated,

Emmett

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Pathfinder Tags: boom crane erection gin pole lift load Truck winch
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#1

Re: Gin poles for the amateur

06/07/2007 2:32 AM

Emmett,

Congratulations on having been graced. I am not a gin pole expert but I can provide some information relating to your questions.

1. Yes, generally heaver vehicles can be used to lift heavier loads. Yes, outriggers can broaden the footprint of the truck improving the righting moment. Yes, changing the boom angle will move the center of gravity which affects the overturning moment.

2. Yes, when the tires (and suspension) flex the angle of the suspended load will change causing the center of gravity to change. Yes, Outriggers can help to offset the flex.

3. Yes, it makes a difference where you mount the base. Yes, weight distribution does matter. No, the vehicle can not be analyzed as a uniform block (unless the center of gravity is at the exact center which is not likely).

4. To experience an example of what happens, place a tall lamp in a wagon then move the wagon.

5. Materials should be sized to withstand the maximum stresses that they will be subjected to without failure, plus a generous safety factor. Pipe sellers understand manufacturing method, material, size and schedule.

6. Yes, it would not be practical to lift a given weight higher than the apex of the gin poles. However once the load is suspended and stability can be confirmed, if all other factors remain constant the load may be raised as far as clearance will allow and stability will not be affected.

Additional comments.

When a crane is loaded it exerts an overturning moment on the truck. The overturning moment works to tip the truck, while the weight of the truck, equipped with outriggers, works to keep it upright. A comparison of the tipping moment to the righting moment forms the basis of stability. If the gross center of gravity (truck and load) is outside the footprint, stability will be lost. The potential consequences of an unstable truck could be undesirable. On the bright side if stability is lost the center of gravity and footprint will be automatically reconfigured for you by gravity to restore stability (truck tips over).

There are no circles in trigonometry.

Be careful and good luck.

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#4
In reply to #1

Re: Gin poles for the amateur

06/07/2007 6:55 PM

Excellent and thorough answer!

An addendum to your #6: Although you don't intentionally try to create pendulums while doing this stuff, you sometimes do by accident. Dynamic stability can be quite different than static stability.

There are no circles in trigonometry.

Wait just a gull dern minute! There are two circles... one after the "g" one ofter the "n".

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#2

Re: Gin poles for the amateur

06/07/2007 7:05 AM

Having been a crane operator for many years, I would suggest you look into (as in google is your best friend) crane operation and operating characteristics. This will give you a better idea of what forces are involved before you make even one mount. You don't want any part failing under load, could get you or a bystander hurt or killed rapidly even though the loads and lifts aren't going to be very high.

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#3

Re: Gin poles for the amateur

06/07/2007 9:49 AM

I have never worked with gin poles on vehicles but have errected wind turbines using gin poles. With respect to question 6. The wind turbines were lifted from the ground to vertical by using the gin poles. The biggest load is when you start lifting and as the angle with respect to vertical decreases the load decreases. We used a 6 tonne digger and pulleys to lower and raise the wnd turbines but it was quite amusing when the generator was near vertical, you could lower and raise the whole thing by pulling the wire with one hand.

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#5

Re: Gin poles for the amateur

06/07/2007 7:00 PM

I suggest you look at derrick crane designs, which the gin pole is a basic form of.

1. The vehicle weight is part of your counter balance. The whole structure is considered as a see-saw, the three important dimensions are, the COG (Center of Gravity) position of the load to be lifted, the COG position of the counter and the tipping point position all to be considered on a single horizontal plane. Using the basic see-saw model you want to balance it out for your lifting capacity. To determine stability lift capacities (Neglecting Structural Now) you must consider your loads as moments, a force multiplied by a distance in metric Nm imperial ftlbs. The distance of the Counter Balance COG to the piviot point x by its mass is one moment (it will force the see sae to rotate in one direction) the distance from the Load COG to the piviot point x the mass of the load is an opposing moment, it wants to force the see saw it the oposite directions. If the Counter Balance distance is fixed, by verying the distance of the load you can determine a basic stability load chart, i.e. the further you move the load from the pivot point the less load you can lift with the fixed counter balance. Lowering the boom angle increase that distance.

In your case picking up on the tyres, the wheels become you pivot point positioning out riggers in front of the wheels would increase your capacity through stability, but the amount would depended on how it affected your pivot point.

2. Out riggers have the advantage of not puncturing like tyre (and I have experienced outrigger seals or hoses break a few time) and they are stiffer and bounce the load less even than soild tyres. If your loads lift spans are low and within the load ranges of the tyres the tyres should be okay to lift on I would keep within as SF of 5 though.

3. The mounting position will effect your span/reach and the structural strength of what you are mounting to must also be considered. As before for capacity based on stability, if you know the mass of your vehicle and the position of the COG to your pivoit point you can consider the vehicle as a block (but eliminate the uniform from your thinking), the important componets are COG's and Pivot points, if you cannot find the COG on any specs, you can determine it by axle weights if you have selected to pick up on the tyres, if you dont have axle weights put the truck on a weigh bridge one axle on the weigh bridge the other axle off and swop over, when you have these weights you can find the balancing point between the two which is your COG.

4. Now you are looking at dynamic loads even with the stability of the truck determined for the above I would use a 25% allowance for dynamic loading. But when pick an carrying there is a lot more to consider as a standard for rough terrain cranes which have the abillity to pick and carry on rubber, the first speed would be creep less than 1mph and the capacity would be reduced by 1/4 for the second speed 2.5mph would be reduced by 1/3rd. But this is for level terrain. But this is for professionally engineered cranes.

5. Size depends on the stresses that will be exerted on to the boom, now you are entering the structural capacity, lifting equipment is to be designed to a SF of 5, if you want to learn how to design basic mechanisims like this start reading up on mechanics of materials, Derricks are common examples, try Engineering on a disk + claymore in the google search bar, it is a good basic free 874 page pdf file, but just keep searching along these lines if you want to learn more. When you test your derrick you need to proof test to 110-125% SWL.

6. Lift height will be a combination of structural and stability factors, the vehicle being unlevel will alter distances to pivoit points and your basic stability for falling over sideways, wind is also an important factor on high lifts, the load and the boom become problems. You need to learn about the mechanics of everything first before atempting too much, what you have is a basic heath robinson crane. I have seen a fair few amongst the old school barn erectors, I remember a guy who came to my friends farm with an old fergie T20 with an I beam joist attached to the front extenting 20ft this was guyed back to the counter weight, which was a metal frame with every bit of scrap steel he could find piled into it, he had even run out of space in the basket and had started to weld bits on, there were old style clothes irons welded all around it.

Keep learning but remember lifting operations are dangerous, air on the side of caution, you are pretty much building a trebuche if anything goes wrong.

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#6

Re: Gin poles for the amateur

06/07/2007 7:17 PM

Other things to be aware of, if you are not already:

If there are failures in gin pole types of structures, they are often bucking failures: once a pole starts to bend (gets out of column) the stresses can quickly get high. Long poles can be stabilized with stays (for example, four cables running from one end to the other of the pole, through the corners of a square frame in the middle of the pole).

I like to test this kind of thing before use, if I can. Given some hydraulic jacks, a large tree of stump, or big rock, etc, you can get a feel for what might happen when the thing is under load. (Of course, you don't want to have the test be the thing that kills you -- but cautious jury rigging of a test can be helpful to see what might happen when the real load is in the air.)

Never put yourself, or anyone else under a suspended load. Don't fall off the roof. (I actually wear a fall arrest harness (about $39) and have several tie-off points on my roof, so when I clean the gutters, I don't' have to worry about plummeting to my death.) (If you set it up so you can't go off the edge, then the tie offs don't need to be anywhere as strong as the type that is intended to catch you after you've left the roof (which are rated for 5000#.)

Enough mothering.

Sounds like a fun project.

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#7
In reply to #6

Re: Gin poles for the amateur

06/07/2007 8:56 PM

I think that a DIY A-frame boom is going to buckle under it's own weight if you are thinking of jacks to lift the boom. Notice that hydraulic crane booms which are lifted from low on the boom, are solid, heavy structures.

A cable crane uses a lattice structure boom, which is very light, and won't buckle without cable-block or a boom strike causing a kink. Thr secret of the lattice boom is that the only load on the boom is directly linear, with no side loads or vertical loads other than the weight of the boom itself. Amazing to watch a light-duty lattice lift a thousand tons several hundred feet into the air.

The secret of the lattice crane is the gantry system. A rear mounted winch through a 10 part line to a pair of cables (usually heavy threaded rod sections with coupling nuts) running to the boom tip. They are best mounted to the pulley pivot. The gantry usually has a pivoting frame holding the crossbar above the cab to allow raising and lowering the boom from the ground. Rods go up to boom tip, multi-part cables down to winches, slip joint nesting pipes allow the gantry to lift at higher boom angles while keeping alignment for boom lowering.

That might be the easiest way to guage your boom requirements.


RichH

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#9
In reply to #6

Re: Gin poles for the amateur

06/07/2007 9:33 PM

Remember when hanging suspended from your fall arrest harness you have about 10 minutes before the lack of blood flow to your extremeties causes permanent damage. Always have a means of extracting yourself from your impromtu swing should things go awry.

As for the gin pole .. cut the bed off of whatever truck you have available and build your pivot point on the rear axle near the wheel bearings. This uses the engine and transmision as counter balance. Your max reach will need to be from halfway down the roof truss on your barn to the ground. The most weight you will need to lift to that height is the weight of a roof truss (prob not more than 100lbs). Build your rig so that when it is over loaded it lifts the front of the truck off of the ground. Testing to failure of the gin pole is unnecessary (provided it lifts the truck nose off of the ground). Don't attempt to move the truck very much with a load suspended. Very small indentations in the ground are multiplied many times at the tip of the boom causing the load to swing and the rig to become unstable.

Good luck with the new property. Sounds like you are going the right way for some real enjoyment.

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#8

Re: Gin poles for the amateur

06/07/2007 9:29 PM

So many answers, so little time...

Let me thank you all generally, especially Guest; for his excellent "rules of thumb" for safety margins.

As I have never (well, almost never) squished a relative or friend I appreciate all the "mothering" I can get. Used to have a bunch of friends worked the oilfield who could have cobbled this in an afternoon without resorting to the back of an envelope, but they had a lot more experience than I, so believe me when I say all warnings will be heeded - especially the suggestions for load testing before we go in situ.

As always this forum and it's members (and visitors, Guest) are more than a delight, but a godsend.

I'll start hitting the books - especially like the idea of outriggers to help share the load under heavy weight. I used to have a set of dandy chocks and land-anchors I used for vehicle recovery, similar concept. Along with front and rear winches on that pickup truck. Used to dazzle people with what I could do when tire friction was no longer the determining factor.

As the barn I am looking at is pre-fab steel truss, getting reasonable weights for the load components should be easy. I'll work backwards from there.

Speaking of winches...I should probably start a new thread but:

Given that I get the load rating correct, there are still a dazzling number of options on winches! Especially if I expand beyond the normal truck sized ones of Warn, Ramsey, MileMarker, etc. into Tulsa and other industrial rated winches.

How about some suggestions of desirable features given the stated purpose of this device? I am referring here to the lifting winch, since for cost I will probably set boom angle and anchor it with chain.

Issues I foresee:

What should it do if you lose power?

What kind of motive power - electric, PTO, hydraulic.

What specific drive design - worm, radial, hydraulic.

And just to be completely silly, has anyone ever seen a powered windlass on a pickup?

Thanks again, and if the moderator chooses to slap me for thread drift it is all my fault.

Emmett

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#10
In reply to #8

Re: Gin poles for the amateur

06/08/2007 12:58 AM

Gin poles are nice, cumbersome but effective.

Depending on your wereabouts, here in grain country (with a little bit of oil) gin poles have been taken over by either crane mounted picker trucks or mobile (trailer) bin cranes used for lifting grain bin componentry.

They are easy to use, move, and already come equipped with hoist (sometimes electric).

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Anonymous Poster
#11
In reply to #8

Re: Gin poles for the amateur

06/08/2007 8:14 AM

Successfull cobbling of things together without calculations is usually over engineering, which is what it has to be in such cases, where there are so many unknowns.

For the winch what is you max load to be lifted? You need this to determine your wire rope size, winch size and no of lines/sheaves, to keep it simple I would stick to a 1 or 2 part line to maintain a good hoisting speed. A worm geared winch would be your safest option. Hydrualic winches will are very reliable and do not over heat, but an electric one has the advantage of being able to take a power source separate fom the truck more easily.

What to do when it fails? That depends how it fails, the important thing is to not take risks with human lives,

For the chain as a guy, try to run through example calculations of derricks to deterimine what you need, you could always post it on CR4.

For some further reading on cranes look at the NAVFAC P307 it is a free crane code supplied by the US navy

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