Shock Absorbing System

Thanks,

I can't go into too much detail at the moment.....although by the look of the opening post...haha

Anyway, lets have a theoretical system.

Mass = 300kg

Initial drop = 250mm

Stroke length of cylinder is not a major issue and as I said originally, things such as air resistance etc can be ignored.

In your answer, you state the force would be 1962N for what you describe. That does not take into account the distance dropped though. Surely the distance dropped would take a calculated amount of time, thereby increasing the KE in the system at the point of the cylinder starting to extend.

Meh...I rushed my calculations and neglected to account for everything...upon re-reading this looks like garbage :(

If you want, I can try to run some more numbers later when I have more time...

Drew K

If you have time and would like to run a few calcs that would be great and a fantastic help. As nick_name has said though it looks as though the calculation process needed to design and manufacture my own would be huge.

Being able to go to a supplier knowing what I am asking about would be a start.

Kev

The problem is not trivial so best is to look at Shock dampers and make a catalog choice you will be better served by a device developed for this scope.

The problem is not trivial since the orifice has to change according to stroke or piston velocity and the pressure drop is proportional to velocity square.

Hi Nick Name,

Yeah, I've been wracking my brains for a few days regarding this. I am being asked to look in to designing a system to work.

Why design a new device when on the market already available of the shelf are to be found ?

You should only match stroke and energy to be dissipated it is easier than to make all computations and still not be 100% sure so that tests are required and several adjustments. I know what I speak about since I had to design years ago a piloted shock absorber and I VERY well know the difficulties due to the non -linearity the system has.

Is it any special reason not to use a commercial design ?

In the worse case I would help you but it is a LOT of work you will have to do.

I realise it is a lot of work and I am now thinking of using a standard unit.

Unfortunately the mass is in the region of 300Te which is extremely large. I have spotted some units though which seem to man enough for the job.

I'm now thinking along the lines of using a standard unit but incorporating it into a structure whereby I retract the cylinder instead of extending it. Can obtain higher loads that way.

If stroke is not a problem, you could put several shock absorbers in series and parallel thus distributing your load.

For large loads, sometimes I have seen a special sling used that stretches to absorb shocks...I don't know if it is rated for your loads but might be worth checking.

Drew K

That's great information. Many thanks for that.

I'm happy to see so many recognisable users helping me out.

I should visit these forums more often.....I've no excuse really.

Sounds a lot like the coil springs on the back of my truck.

Most "normal" shock absorbers work in compression, some also in extension. You want one to work in extension.

How will you 'reset" this shock absorber after it is extended?

Magic?

No, was thinking of charging it or taking the weight up once again to compress it.

Maybe divine intervention?

Fine. It was just a thought. You will have to either have an retraction spring on it or something to re-compress it.

Now, I wish to put a hydraulic cylinder in between the arm and the mass so as to create a damping mechanism and allow the mass to come to rest when it is dropped....i.e. the cylinder will be extending. I do not wish the mass to bounce or oscillate. I need it to come to a controlled stop.

Why?

I don't understand why you can't leave the cable as is, and place your dampening mechanism underneath the weight...like a platform with shock absorbers underneath it. When you lift the weight again, the shocks will return to their original position...waiting for the next drop.

It is initially resting on something which could move and thus make the mass drop.

would this slow your mass?

Thinking outside the box is good, I think for the mass quoted by the OP your shutes would not be sufficient. You did make me think of magnets though.

I have heard about electro-magnetic shock absorbers that might work. According to Wikipedia, they are comparable to standard shock absorbers. You would have to find a supplier willing to make you some rated for your load but the advantage is you could control the dampening effect electrically and set it to your load.

Drew K

Let's get really outside the box.

Ferrofluid suspension

Magnetorheological fluid - Wikipedia, the free encyclopedia

Not that simple.

Putting the shock between the cable and the weight is better.

Unless all the dimensions are worked out perfectly, when the shock hits the plate there's a good chance that the whole assembly will topple over and that might not be good.

Granted if there were guides to keep everything aligned, or the plate were big enough, it might work.

I vote for cable/absorber/weight.

But, it's not our design and that wasn't the question.

[edit] Or see above response.

Gee, you haven't aged at all....

Looks to me like you need to talk to Tony Welling at Ace Controls. If anybody can come up with the right answer, he can .

Just had a look at their website.....the information is identical to that posted by Solar Eagle a few posts back (the Parker PDF).

You're not giving us enough information.

Is your mass fixed or variable?

Is the velocity of your "mass drop" fixed or variable (is gravity the only actor)?

Why are you fixated on an extension type of damper (which as pointed out by others is odd)?

Unless the "mass" is less than 0.001 gram, obviously the air resistance is minuscule, and can be neglected (unless we're talking about a balloon).

Why not compress the cylinder?

I am obviously confused as well.

Fixed mass

Gravity is the only actor on the mass.

Not fixated on extension only now......thinking of compression cylinders as well.

Ignore air resistance.

The mass is huge (300T) so that I suppose it is not suspended on only 1 cable. It would be of help to have a simple sketch how the mass is connected to the arm. Due to the dimension of your mass only several ACE/Parker shock absorbers in parallel could be a solution. However the whole system could be cumbersome. A simple "mechanical inverter" can be used to put at work compressive devices as those in the catalogues. It is possible -if you still stay for a specific design- to make a "tension absorber" with a gaz (N2) spring return as in some hydraulic accumulators designs. The rod must be -in this case working under tension- object of a very thorough fatigue analysis due to the threads which could be source of failure (high notch effect).

The free fall stroke will be the factor to define which type of damper you should use.

No, thinking purely in a compression manner now.

I had looked at a tension system but the general nature of an 'off-the-shelf' damping cylinder has it operating in compression. I can design my structure around this so that I am using them in compression.

As for the cable, that is not my concern as I am only tasked with looking at a damping system. The associated cabling is man enough for the job at hand.

A few of the absorbers in parallel is the current method I am looking at. It's just at a concept stage at the moment.

The cable works also as a spring and this you should consider in your model for the design of the combined damper system.

I am picturing the Op as having a heavy load that needs to be lifted off of a moving object like a boat in high seas. You could have unanticipated shock loads as the ship the load is on dips in the sea but the crane on a platform moves at a different speed.

A shock absorbing system would prevent severe shock loads to the cable and prevent the load from damaging the boat as much.

In thinking about that situation, couldn't you control the shock loading by putting a load limiter on the winch? Have it auto slip or wind in the same manner as a shock would? If the bottom drops from underneath the load, the cable could slip a little or take up the slack if the load moves up?

This could also be done with a hydraulic ram with a pulley on the end. I can't upload a drawing at the moment but picture a section of cable pulled out of line from the vertical linear line by a rig with 2 pulleys above and below a hydraulic ram with a pulley on the end. As the ram extends it draws up the cable and as it retracts it lets it back out. It could be installed anywhere on the cable and would only require a hydraulic system to operate.

Drew K

Is this hypothetical or for real?

Controlled decent of cable(rope) suspended masses is normally done with winching systems.

Does it have to be hydraulic? How about an air bag system? Maybe an actuator?

Will a hydraulic system not give a greater 'cushioning' force than air for the same volume?

Hydraulic systems don't cushion anything by themselves. They work by passing a non-compressive fluid through a tortuous path that slows and restricts the flow. Air is squishy and compressable and can be sort of related to a spring. Take a large cylinder full of air and compress it with a piston, the air will compress until the pressure exerted on the piston is equal to the force pushing the piston down.

The problem with an air cylinder is that at high pressures a fialure turns it into a pipe bomb whereas a hydraulic cylinder failure is usually less devastating.

Drew K

I doubt he is interested in "cushioning" only. A cushion is a spring so that if he has a spring (of course with air it will be quite big) there is a risk to have the load jump up and down. Think about a 300T load dancing at the cable end !

If the mass hits the cushion with an initial speed it has a kinetic energy which with the cushion "spring" will play kinetic - potential till it will be destroyed.

You are right in writing that a hydraulic system does not cushion but what it does is destroy the kinetic energy and I think this is EXACTLY what is needed.

If the problem is as one mentioned : a load hung on a crane on a barge and which has to be placed on an other barge then 2 (two) systems have to be used i.e. one for the cable to avoid dynamic overloads and one for the contact zone at the load bottom toward the second barge to avoid shocks and possible local destruction.

For 300T???...I wouldn't want to be anywhere near it when it drops.

An Engineer with common sense in action. Shock suppression is a science in and of itself. I do applications every day in this arena, and find that it is best to let my vendor be involved, as it is ultimately his equipment I use. I use Ace Controls out of Michigan, and they really know their stuff. Why waste my time trying to re-invent the wheel? The application being bantered around has more dynamics involved than meets the eye. Why not ask the professionals, that is what they do for a living, and the cost is zero.

I agree with you 100%.

The supplier has a vested interest in making sure that any application that uses his products will be totally successful.

If you (OP) spec a product, and it fails, the supplier is not to blame.

If he specs a product, and it fails, he will be looking for a new job in another field.

You have 300 kg dropping 250mm, and you want a soft landing, right? As long as the distances stay the same, all you need is a really big storm door closer or two, with the spring taken out. It will allow the load to descend, and then slow the stop, just like the latest kitchen cabinet drawers. Also common in commercial doors.

If the distances change, you will need a feedback system, to tell the system when to hit the brakes. I like hydraulics, you can free fall at a controlled speed, and then do a nice controlled stop, with a final bleed for a real soft landing. A proximity sensor can trigger the stop system.

Being just a tad more than familiar with hydraulics, I agree. I wasn't following the thread that close. A 640 lb. load will get your attention. On the face of things, why would you drop that weight and then worry about shock? OSHA would be all over you like white on rice. I have probably done too many transfer lines to be in this conversation. It looks like the present process wasn't well thought out in advance. It's only dropping the load 10 inches, build yourself a platen that can handle the load, move the platen with hydraulic cylinders, and call it a day. Why make it complicated?

Am I wrong or 300 T correspond to 300,000 kg ?

Maybe I misread, I saw 300kg. Which is 640 lbs based on 1 kilo equals 2.2 lbs. Either way, too much weight there to play with, in my humble opinion.

Sorry, I started with the small numbers in an attempt to work the equations. The OP is working with some very heavy loads.

Some very good advice of going to a vendor and giving them the problem to solve is the best way to go. Or you could hire an engineering firm for some consulting, they will be able to crunch the numbers and provide you a solution that will suit your needs. It might cost some money, but better that than putting together a solution that doesn't work and then going for help.

Drew K

300 x 2.2 = 660 or am I wrong ?

At some time in my career I had to deal with weights up to 4,000 T= 4 E6 kg ! The mass is huge as I said but it is not as big as this one.

Of course using manufacturer's skills is very good but it is also recommendable to have an own opinion since some body has to accept proposed solution and due to communication difficulties the seller is not always able to "get the point". His calculations have to be understood and verified this being as well valid for an outsourced engineering counsel. I saw too many times solutions which were good for the salesman but not for the user.

True, salesmen are often only looking for the sale. There are good ones out there who are experts in their field and want you to get the product they need because they know that you will go back to them for more products in the future (and tell your friends).

This forum is a good way to test the waters and 'crowdsource' some thinking and we can offer advice on what the salesman says too. But at the end of the day, it is best to use sources that are credible and liable.

Drew K

Too bad it's not an elevator...you could just use a counterweight and call it a day.

"The cable is slack at this point.....the mass is allowed to drop a distance before the cable would become taught"

So...300T will yank on the cable after it drops and then you want it to decelerate? Am I understanding this correctly?

The 300T yanking on a cable bothers the hell out of me for some reason...

It is common job in off-shore platform assembly on spot with modules weighing much more up to 3000 To. There is a dutch company specialized in heavy lifting with barge cranes simple and double hooks handling such weights.

In on-shore the weight is a lot lower but 100 to 200 To are common too. What was not said is that such masses have also big dimensions 30x30x30 m are not unusual. They are lifted with 4 cables and there are preparations compulsory as weighing and COG computations in order to have the right cable length for each corner so that the hook will be at COG's vertical.

Many years ago I designed and realized such a weighing system which after control offered a repetitivity of 1/10000 and an accuracy of 1/1000. We were able to demonstrate that weight is an invariant.

I'm familiar with heavy lifting of that nature but you're referring to static vertical loading which is not unusual. The OP states he wants to DROP 300T hanging from a slack cable until it's taught. Its the DROPPING of the load that has me wondering. I can't imagine what kind of cables can withstand such enormous shock loads. There's no way any crane operator would attempt that on his crane.

It depends how much the load can drop. Slack is a VERY qualitative notion it says nothing in fact. If "slack" means a big stroke then even the holding structure will fail not only the cable!

However the energy the dampers have to dissipate will be very important and I am afraid that this could be the problem since from one side the fall has to be small and from the other the damper efficiency is proportional to the velocity. I will have a look at the ACE dampers (which I did not indicate since I thought that the OP can do by himself a google search) to see what "monster" the solution could be.

As mentioned I had a look at the possibility to get a solution. To have a base to estimate I assumed that the free fall before the damper becomes active is ONLY 0.05m=2".

The results is that about 12 to 16 ACE biggest dampers have to be considered dampers.

I forgot in my previous analysis to mention that the mass energy is NOT only the one it had at first "touch" but it is the one which it will as well harvest during its "damping" stroke. And this stroke is about 0.16m The damper has to be computed with this aspect in mind the mass moves between two potential surfaces which are at a distance equal to the free stroke + the damping stroke and ALL this energy has to be destroyed by the damper. As you see from above numbers if for only 0.05m one needs 12 to 16 how many will be required for a stroke 4x bigger ? I let you estimate.

As a conclusion even if a solution can be found it will be practically not a solution. The best is not to let the mass fall.

Hi Kev,

ACE Controls International in Newton-le-Willows, they are all about absorbing shock.

I have dealt with them for over 40 years, their rep's should be able to advise you.

Best regards,

John