Pulling stumps

RobS · May 31, 2001

Re: Pulling stumps/Chain Energy

That's exactly the thought I was having on my way to work this morning DDL. A force applied to a cable or chain will only store energy if the chain/cable is stretched. No stretch, no stored energy.

Now the variables become the modulus of elasticity for the materials and the geometry of large section link shapes versus small section wound strands.

Back to the books tonight /w3tcompact/icons/smile.gif

Richard · May 31, 2001

I'm with you MJB, gotta love the backhoe if it can be had/rented.

Last Sunday, I pulled up about 10 stumps, all pine. Three of them were "multi" tree stumps, in other words, I had one stump maybe 3 1/2 feet diamater that (several feet above ground) split into two or three individual trees These were some of the trees I was earlier in year, unable to push over. (actually didn't try pushing the "multi tree's" over)

I have dozer dude coming out soon to grade yard and I figured, Hmmm, I can let HIM take them out and spend a couple of hours ($$) doing it, or "I" can take them out and get some "hoe time". Took me roughly 6 hours to take them all out. Some of these root balls were maybe 6 feet deep and 3 feet wide. I'm gonna let dozer dude push them into pile and I'll have another one of my (yet to be world famous) weenie roasts! (aka huge fires)

Richard

Is the following a compliment??

Imagine fiance/wife/other getting on your backhoe and successfully learning to operate it. Ok, so NOW imagine sitting around with a bunch of friends cooking out, and making the comment that "Yep, (insert female's name here) was out working in the yard other day (referring to backhoe work), she has GOT to be the best 'hoer in the state" /w3tcompact/icons/wink.gif

Of course, this compliment isn't on my top 10 recommended list but, I had the father of an old girlfriend once make that comment and he just DOUBLED over howeling with laughter (as in fact did she...family had demented sense of humor) it WAS funny at the time though.../w3tcompact/icons/crazy.gif

Anonymous Poster · May 31, 2001

Re: Pulling stumps/Chain Energy

...the variables become the modulus of elasticity for the materials and the geometry of large section link shapes versus ...

Robbie,

Whew... come up for a breath of air... sometimes speak in laymen's terms for guys like me... /w3tcompact/icons/shocked.gif

RPM · May 31, 2001

Re: Pulling stumps/Chain Energy

Here's the catch when thinking about breaking chains vs. continuous cables or straps - the chain links would most likely break asymmetrically. I would suspect that typically a chain link will fracture along one side rather than at an end. On small chain there is obviously a gap there - the links are formed by bending. On larger chain there would be a few likely spots:

- Along the heat affected zone by the weld. The hardening caused by the weld increases the likelihood of fracture.
- At any point of damage to a link
- Points of corrosion
- Points of metallurgical weakness

Since the chain link will probably break on one side before the other, the link will then be subjected to an asymmetrical load. i.e. the load is now all carried on one side of the link. Final separation of the link now happens in one of two ways:

- The remaining intact side of the link fractures due to the shock load
- One of the (now free) ends of the link bends and releases the next intact link.

Either way, the load is released off-center / asymmetrically. (That's a difficult condition to reproduce in a test - the example of releasing tension on a come-along does not mirror the conditions of breaking since tension is just released along the axis of the links).

The shock-loading of the chain as it breaks and the asymmetric nature of the fracture are enough to send a whiplash effect through the chain. I believe the analogy of snapping a towel holds well.

The whiplash effect would work as follows (you have to think on the scale of each individual link):

As the broken link separates asymetrically you have to consider the effect on the two adjacent links. Both these links are released off-center to the direction of the chain.

Each link has a certain quantity of stored elastic energy (they all stretch however minimally). This stored elastic energy converts to kinetic energy as the chain link contracts as the tension is released. The link also picks up the momentum (in an off-axis direction) imparted by the breaking link. The combined velocity vector is still off-axis -- resulting in an off-axis force applied to the next link and so on.

When this 'ripple' or whiplash works its way down the chain you encounter another condition. Both ends of the chain are constrained - one on the tractor and the other on what was attempting to be pulled. This sends the wave back down the chain, again off-axis. When it reaches the free end of the chain once again you get a nice whip-crack effect.

Don't forget that the tractor jerking forward (still under load from the engine) - and the object being pulled now falling - add a nice extra whip to each end of the chain at this point.

You don't want to be around when that happens!!

This all happens very quickly. The energy dissipates much faster in a chain than in a cable - BUT - it still has to dissipate and I believe that the above is the most probable failure mode. Don't forget that typically chain is heavier than the equivalent rated cable so there is more total momentum to deal with also.

By the way - cable separation is also asymmetric. That's why they really snake around for a while as the individual strands break. Ask anyone that has worked on a ship for a while and tried to avoid a snaking cable on deck as it whips around! This can go on for a while - it's a very nasty, very scary situation - particularly if the object on the other end of the cable is falling (like an anchor). Plenty of limbs and lives lost due to this.

I'm open to being corrected on this though - just what I think is the most probable explanation based on thinking about it and anecdotal evidence.

RobS · May 31, 2001

Re: Pulling stumps/Chain Energy

OK John, just for you...

Modulus of Elasiticity, E, (also called Young's modulus) is the ratio of unit stress to unit strain within the proportional limit of a material in tension or compression. (From Machinery's Handbook 25th edition).

Basically, it tells how much a material "gives" when loaded. More give means more energy stored in our example. Brittle materials only give a little before bending or breaking while ductile materials will give quite a bit before bending permanently. Higher numbers mean more give, but since it also accounts for the overall strength of the material a cast iron has a higher modulus than a ductile aluminum. For similar strength materials (cast versus wrought aluminum), the higher the number, the more ductile (or "bendable") the material.

This is all kind of interesting for me since I'm re-learning stuff I didn't learn very well the first time around. Thanks for the questions /w3tcompact/icons/smile.gif

RobS · May 31, 2001

Re: Pulling stumps/Chain Energy

Good input RPM, I haven't even begun thinking about the specific failure machanisms yet. I suspect you are right about the asymetrical loading.

I'm still stuck on the potential energy contained in the chain. Each link only has so much, and it can't be much as chain tends not to stretch too much. The energy will be converted into motion (a miniscule amount into the sound of the break). Chain links are heavy so the acceleration will be slow and short. Once the energy is dissipated, the individual links become a free body and will slow down and/or react with each other.

I guess the hypothesis I'm suggesting is that the amount of potential energy stored in the chain link will not accelerate it to a very high velocity. I'm basing this on my assumption that the chain will not stretch much and thus will not store much energy.

Interesting thought also about the whipping effect. I can see a wave riding down the chain, but as it is not continuous I'm not sure it would return to the free end. I'll have to think in terms of side loading, not along the length.

Keep those thoughts coming, we can turn this into a Phd thesis yet /w3tcompact/icons/smile.gif

RPM · May 31, 2001

Re: Pulling stumps/Chain Energy

So ... another take from a kinematic perspective:

First I'll make an assumption - that the chain is initially horizontal (to simplify things). Having it vertical changes the behavior slightly due to the weight of the chain.

1. Chain link breaks - gives an off-center push to the links next to it.
2. Each subsequent link in the chain receives an off-center jolt in turn - link by link. Don't forget that the chain links in front of the 'ripple' don't yet 'know' the chain is broken - they are still acting as if under tension.
3. As the ripple reaches each link, they give up some portion their elastic/potential energy - contributing to the momentum of the ripple.
4. When the ripple reaches the tractor (or object being pulled) - the tractor suddenly sees it's load lighten. This will jerk the tractor forward.
5. The chain is still in motion due to the momentum it acquired from the release of all that elastic/potential energy in the chain links.
6. The combined jerk forward and the fact that the chain has just 'discovered' that it's end is tethered results in the snap back along the chain.

The net result for most people is the sound of a chain whizzing past them.

Re: the weight of the links. It's all relative. The links may be heavy, but the stresses in the chain are high. The net result is high momentum (product of velocity and mass).

Re: the amount of stretching. Stretching is not the only mechanism for storage of energy. You also get contact-face stresses (Hertzian stresses) where the links contact each other. The compression of the material there also needs to be taken into account.

Good example of stored elastic energy in steel is to drop a small ball-bearing onto a hard surface (like a smooth concrete floor). It will bounce nicely.

RobS · May 31, 2001

Re: Pulling stumps/Chain Energy

I'm trying to follow the snap back, but I can't get beyond the notion of "pushing on a chain". Maybe a beer or two will help /w3tcompact/icons/smile.gif

I agree with the momentum though in my hypothesis the velocity will be low due to the energy required to overcome the zero velocity state at time of failure. The high stresses in the links won't contribute to stored energy unless there is elastic deformation. Push on a building all you want but you haven't done any work on it until it moves. Work is force times distance and power is work per unit time.

Good call on the other energy stored. I'll add that to my "list". Safe to assume these are pretty small?

I do have to question the ball bearing example as one for elastic energy. The ball bearing falls as a result of potential energy (mass times gravitational constant times height) being converted to kinetic energy (mass times velocity squared). The reason for the bounce is the IN-elastic collision with the hard surface. No (or very little) energy is transmitted to the surface so the ball bearing rebounds with nearly all the energy it had before the collision. The rebound height will be proportional to the amount of deformation/displacement of the bodies in the collision (ignoring air resistance and sound). It's the same as the building move. If the ball bearing doesn't move the surface, it hasn't done any work on it and will keep it's energy level.

Now what does all this have to do with pulling stumps again /w3tcompact/icons/smile.gif

BTW for all, I'm not suggesting that a chain is necessarily safer than a cable or strap, only that they may behave differently when they break. We must all continue to be safe operators in all circumstances /w3tcompact/icons/smile.gif

Edited by RobS on 05/31/01 02:53 PM (server time).

RPM · May 31, 2001

Re: Pulling stumps/Chain Energy

OK - I'll give you the point on the ball-bearing!! Rushing off the last post didn't help. Coefficient of restitution and all that good stuff comes into play there ...

I guess to stand back one step further what you are looking at is the mechanics of a wave in a medium. Granted this is a peculiar one, but the basics apply.

There just aren't that many good illustrations of how chains behave in everyday life. Next closest example - I know it's got its problems - would be how a swing behaves when you (or your kids or grandkids!!) jump off it. The chains will bounce around quite a bit for a couple of seconds before slowing down.

Jeez - now I'm tempted to go down to Home Depot and get a length of light chain on the way home from work to play with tonight. As if my wife doesn't already think I'm bordering on the crazy side ...!

Of course, the best way to get rid of stumps is to put a block of C-4 under them. Way easier than chains. You can even adjust the launch trajectory to a certain degree.

huckflynn · May 31, 2001

Re: Pulling stumps/Chain Energy

Never studied the stuff -- but you guys have me thinking. Ouch!

Suppose you had a chain with a big bucket attached hanging from a steel girder (assume the weakest link in the chain is at the point where the bucket is attached). And suppose you kept adding 100 lb weights to the bucket until the chain finally broke. Would the chain recoil?

Or does the recoil come from the sudden lurch of the vehicle pulling on the chain when the chain breaks?

Huck

Edited by huckflynn on 05/31/01 05:59 PM (server time).