How to Calculate Shock Loads

Hello AL51, Is what your asking about the jack stand breaking or just the stand bouncing and the paw releasing and the extended support dropping to a lower notch position?

I would think you would need to know was the stands under the suspension or solid frame. Springs do odd things. Also with one end of the vehicle on the ground the location of the jack stands from the other end, weight distribution front to back and over the stands. The velocity in the four inches the vehicle falls assuming no drag from the jack.

The cast iron parts spring, the stamped steel frame and the surface the stands are sitting on. ie concrete, asphalt, dirt, wood.....all have a different effect on bounce.

This sounds like it could be some type of homework question or has it happen and you are trying to figure out why.

if first, study.

if second, look at your picture and see the safety pin that is to stop the release paw from doing what you are describing. If your stands dont have the safety pin you can tie the releace lever down. not as safe as a pin although but better than none.

As to the jack dropping to fast I would check the hydraulic oil level, could be low.

And last use stands that have enough capacity plus for vehicle.

Better safe than sorry!

Charles

ps i know this did not answer your question the way you wanted.

I think I found the equations (someone posted a link to the formula's elsewhere).

Yes, know all the ways things can go wrong with jacks and jack stands, but for sake of the problem (not my homework or job), I wanted to keep it simple ie: no springs, no bouncing, no soft ground, no slow let down of the floor jack (then not a shock load), etc. It could be a very complicated problem if done to the Nth degree; type of ductile steel, investment cast vs not, temperature of the iron rack bar at the time of the shock load, (more brittle the colder it gets), angle it strikes at, vertical versus horizontal, etc,etc. Was just looking for some rough numbers ie: how much force is generated when 6000 lbs fall 4 inches and stop suddenly.

Also, yes, I could get a 20 ton pair of jack stands for a 3 ton vehicle, but that would be a waste of money and energy lugging them around.

I will most likely be using either a 6 ton pair of stands (that aren't made with a locking pin, most do not have the pin), and a 12 ton pair that do have a locking/safety pin (Torin) along with the pawl. Also, most pawls are not spring loaded, only one company (Norco) make a jack stand with a spring loaded pawl to my knowledge, but no safety pin. The weight of the vehicle is what keeps the pawl in place. That is where the safety pin comes into play, but the pin itself could also shear if the force was great enough.

One of the last true American made Jack Stands (US Jack) have a double pawl (catch two teeth on the rack bar), most of their stands are apparently sold to Snap On and the US military and are very expensive, like over $400 a pair for the 12 ton version, but theirs are rated at 12 tons for each stand, while most other jack stands are rated per pair.

If anyone else comes up with some numbers I would appreciate it

That is what I was looking for, thanks. I tried the calculator but got some very high results which seemed too high, will have to try again tomorrow. In my experience when a vehicle with frame rails like an SUV comes down it hits hard if the jack stands are under the frame rails and it may bounce slightly, which according to the calculator would drastically increase the force of impact. I also noticed that as you decrease the distance traveled after the impact, the forces go way up quickly. If the jack stands are under the lifted axle when it comes down the springs obviously will attenuate the forces. My two concerns are immediate failure versus fatique failure ie: after X number of impacts small microscopic fractures become big fractures leading to sudden failure.

Just how many times have you seen a dropped SUV bounce off of stands?!?!!?!?

Where do you work so that I can avoid it like the plague?

Due to the mass involved it is the time factor that is critical. If you increase the time of acceleration or decrease the time of deceleration it affects the force dramatically. As one of the guests noted, the right way to do it is to let the car rest on the stands that way there will be little or no impact on them.

In the military we used the expensive jack stands you spoke of for our 6000 gallon fuel trucks for changing tires. I must admit we never settled the truck onto the stands. I did see one young airman release the jack way too quick and let the truck slam down on the stands which were set on the frame rails. It fell about 3 inches, but the stands took the load without complaint leaving some nasty chips in the concrete. We were not mechanics, just fuel operators and were lucky the military purchased the good jack stands.

Based on the information discovered here, from now on I will always settle the vehicle on the stand before working and I will try to get this information to some of my previous colleagues. If you cannot afford the over engineered stands make sure your co-workers always settle the load before working, it is the only way you can add safety controls to the situation.

Drew

DREW K: Interesting story about the fuel trucks (I'm still in, Reserves). Yes to everyone that says it is best practice to gently lower the vehicle to the jackstands.

In an ideal world we all would drive the speed limit and never be distracted by a cell phone or the hornet that just flew in the window. The purpose of the question was to try to determine from an engineering viewpoint exactly what the forces are when the mass is suddently dropped, either due to faulty equipment or an untrained operator, or just someone who isn't paying attention. Any floor jack that the AF uses would be of very high quality, just as the jack stands were. The high quality high dollar floor jacks (thousands each) are much more reliable in how they release ie: you can more easily determine when it will let go, and it lets go slowly. Some models of floor jacks will not even allow the load to drop suddenly, no matter what the operator does. I'm sure that one of the main purposes of this soft release mechanism is to avoid shock loading. However, the majority floor jacks that we civilians buy cost anywhere from $50 to $200, and are no where near as reliable as the expensive ones. A high quality 100% true American made 4 ton floor jack will run you at least $1000 (US Jack, Weaver, Milwaukee Hydraulics); not many do-it-yourselfers are going to spend that for a floor jack.

Again, not the purpose of my question to get into that background, just trying to clean up some of the comments. We all agree that it is proper technique to gently lower the vehicle down to the jack stands. The purpose of the original question was to determine the shock loads when things are not done right, either due to equipment failure, operator error, etc.

There are no simple calculations for what you want, you would have to carry out some highly monitored experiments, or use some some fancy FEA.

The only thing that you will be able to do with simple calculations is reverse engineer the things. That is, you will guess a deceleration (stopping distance), find the force involved, then say to yourself "That doesn't seem right, it seems a little high/low" - then you will adjust your deceleration (stopping distance) to try and match the force that you feel is "about right". So it is all academic, and not accurate.

Good answer.

Jack stands tend to be (seemingly) over-designed by large factors because of the uncertainties in the way they are applied. The greater the rigidity of the load points on the vehicle and on the ground, the greater the impact force -- the numbers can get extremely high.

From now on, I will be setting my jack stands ac close to the bottom of the car as I can, and including some hard foam or soft wood to slow the impact.

I think you probably know that you should never use jack stands in an unloaded condition, (because of these design uncertainties associated with impact) to avoid jack stand failure and to avoid damage the the vehicle's support points, if the primary support fails. You should also load them as gradually as reasonable possible. Too many mechanics just drop the vehicle onto the jack stands without much thought to the impact loads, and too many shade tree mechanics use the stands as a backup to the jack, with the jack holding the load and the stands only coming into play should the jack leak or fail -- which could cause the sort of impact loads to be avoided.

All of this is probably obvious to you and most of the thread audience, but one never knows who may be reading; we wouldn't want to give the impression that work can begin with unloaded jack stands.

Only vaguely related, but something to think about when flying on an airliner: An ordinary production spam can aerobatic airplane like I used to fly is certified to 6G positive load (a load tending to bend the wings upward). An airliner is built to only 2.1 or 2.2, if I recall correctly. In emergency maneuvers such as a descent after pressurization failure, the bank angle can be 60%, which in level flight corresponds to a 2G wing load. The safety margin is 50% (beyond the design load), so in an emergency descent (or severe turbulence) your are uncomfortably close to bending the wings.

In the event of a shock load, hard foam would be almost completely useless except to absorb oil and soften the audible clang of impact (unless your talking about some incredibly hard foam... I hear they can make foamed aluminum now...) and a piece of wood isn't going to do much more than help prevent scratches on your lift point; if it doesn't become a projectile.

Your best bet is to avoid shock loads. In my business any piece of equipment that has seen a shock load in excess of it's SWL (Safe Working Load) is discarded, usually after being made un-usable so that no one comes along later and says... 'Oo, look at that nice piece of chain... I bet I can hang my car from it... snap, ping, crash... dead guy'.

My thoughts are you shouldn't shock the stands AT ALL. They are not meant to be. Proper use is to jack the load up, locate the stands, and then slowly release the load onto the stands. (and please don't say something to the effect of - "That's nice, but in the real world ........)

You are going to find it really difficult to estimate the shock loads, because as you have discovered, it is HIGHLY dependant on the deceleration.

I read this thread with interest. When I was a teenager a man I personally knew in town jacked his car up to work on the transmission. Apparently he needed the entire car up because I guess he was working on the drive shaft that went from the front to the back so what does he do? He drags out large pieces of firewood and stands them on end. Positions one under each of the 4 corners of the car. Don't worry, he did things right. He let the load down gently on the pieces of firewood. No need to worry whether they could stand the shock or not. He than crawled underneath the car and proceed to yank and jerk on the transmission. Well as you can imagine at some point one of the logs tipped over. Down came the car on top of him! His wife hearing the crash came running out and while his head was out from under the car and he asked her to jack it back up she didn't know how to operate the lousy jack! So instead she ran down to the road and flagged someone down. By the time they got back up there and jacked the car back up he was dead. Crushed his chest to the point where he couldn't breathe. Don't know if he would have survived had she immediately jacked it back up but he was breathing and talking when she left him although I am sure he was breathing with great difficulty. After that incident and our family did attend his funeral it made a big impact on me and the realization of how important safety really is. I mean my uncles constantly drummed safety into my head but I never took it seriously until that happened. Not saying I ignored my uncles or safety. Just didn't really realize how absolutely important it was and that you could actually die if you weren't safe. True story. I know probably a bit off topic. I never really gave any thought as to whether the floor jacks I was using were safe though. If some yo yo has dropped cars on them several times previous to my using them than maybe I'm not safe afterall crawling under a car even if I have properly lowered the car onto them. If they are getting ready to fail? Of course you can usually tell if they have been abused or not. I'm not a mechanic and the only ones I use are the pair I bought for my personal use. And yes as we have all agreed I let the car down onto them before working on it. But in the real world... and yes I'm going to say it! Some people use firewood. And others drop the load onto the jacks without thinking because they haven't been trained in safety.

I have two formulas for calculating shock loads, either works:

Shock Load = (W)(1 + D_F/D_S) (From Entertainment Rigging by Harry Donovan)

Shock Load = [(W x D_F) ÷ D_S] + W (From Stage Rigging Handbook by Jay Glerum)

Where:

W= the weight that is dropped

D_F is the freefall distance

D_S is the stopping/braking distance (for your purposes D_S should be a number just above zero (like 0.01) since there is almost no deceleration time before the stop at the end, and we all know what happens when you divide by zero)

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In this instance there are some other factors we need to consider. You say the weight of the vehicle is 6000 lbs, but that would not be the weight that is falling as some of that weight is supported by the ground since only one end is jacked up; probably in the neighborhood of a a bit more than half of the weight is on the ground if you assume the weight of the car is evenly distributed (we know it's not... I can get you the formulas for figuring out the actual mass that is moving, but it is probably way more complicated then you need or want to get involved with).

We also need to determine what the load rating is based on and if there is a safety factor included in that rating, among other things, if you want to really get specific.

Your best bet if you want an accurate figure is to contact the munfacturer.

I forgot the most important part... DON'T SHOCK LOAD YOUR JACK STANDS! As many other respondants have commented this is not proper or safe use. ALWAYS follow the manufacturers instructions unless you are prepared to be held personally and legally responsible for whatever consequences may follow (remember these consequences could include someone's death or worse).

As an intellectual problem this is interesting, as a real life problem it is potentially deadly as shock loading may cause damage that is invisible to the naked eye yet sufficient to cause complete spontaneous failure the next time the stand is used, even if it is gingerly loaded every time after it receives a shock load it could fail without warning.

And let's forget about the jack stands for a minute and remember that whichever part of the vehicle impacts the jack receives the same shock load as the jack.

The vehicle used for the example is 6000lbs with ~ 60:40 weight distribution, front being heavier. I actually did talk to one manufacturer of high quality high dollar stands, and one individual said they do test to a certain standard. I'm betting that most would not reveal anything about their test results other than they passed for liability reasons. I also would assume that most manufacturers would design their jack/garage stands to withstand more than their stated tonnage, but how much more is the question. One jack stand I am aware of failed with much less than it's rated tonnage, so the question is, was it improperly designed, defective production, or had it been abused such that it eventually failed without warning, hence the original question. And this one stand that failed, it was the pawl that failed, not the legs or base. The pawl area seems to me to be the weakest link, but I have no engineering data to back that up. I've only found two manufacturers that have a double pawl or double lock of some sort, Torin which uses a pawl and a safety pin, and US Jack which uses a double pawl which catches two teeth on the ratchet bar. The US Jack garage stands are also the only ones I've found that rate the tonnage that each stand can support separately, where everyone else rates them as a pair.

On the one hand, it seems that even one hard impact of say dropping a 6000 lb vehicle (4000lbs of it at least), a few inches onto a jack stand that is sitting on concrete could well exceed the rated tonnage. On the other hand, we all know that many jack stands are used and abused for years, but relatively few actually fail.

What someone needs to do is to set up a testing fixture with a ram that would strike a jack stand with a specified force over and over until it failed or showed signs of impending failure (cracks or bending) then repeat that with say 30 of the same brand/style of stands from the same production lot to get a large enough N to have some useful numbers. It would be nice to know if someone has already done this.

If the pawl is cast and was not x-rayed then there could be voids or other casting flaws that caused it to fail... or it might just be that it is the weakest link...

The majority of pawls I have looked at are cast. Just last week I looked at a new pair of name brand stands, and was unimpressed by the quality of the casting of the pawl on one of the stands, definitely substandard by the eyeball test. I sent them back. My guess is that virtually no one x-rays any part of their jack stands, unless maybe as part of a rare quality test performed by an outside testing lab, but even that may be a stretch. One company apparently hasn't changed the design of their jack stands for over half a century, even while the company has changed hands a few times during that same time period, they just keep turning out the old design. On the other hand, there probably isn't much you can do to change the design. Almost all brands look like copies of one another.

I imagine some jack manufacturers will x-ray a small percentage of their cast parts to check for casting flaws, but you're right, they would be in the minority (unless you're looking at one of those $1000 jacks, in which case it probably doesn't have any cast parts anyway). Many of the ones that appear cast may be forged, forging pounds out the 'air bubbles' and greatly reduces the chances of inconsistencies and flaws resulting in a much stronger finished product.

The parts may look the same, the designs may even be identical, but a poorly manufactured jack is just that. For my money, I want something that I trust isn't going to fail and and maim me... I would look for those tell tell signs of superior craftsmanship, like detailed specifications regarding the materials used, certificates of load testing and the like. And whenever possible try not to use cast components, you'll pay a little more for forged but you'll get a lot more, so I'd say it's a fair trade

Check out this link. It provides a method of comparing the stress from impact to the static stress from the object resting on the support.

If the load is released suddenly with zero drop height, the stress is doubled. If it is dropped from a greater height, the stress in the support is magnified much more.

Anyone out there who can do the math? Plug in 4000 lbs (~1800kg) with a height drop of say 3cm onto a relatively immovable object (the jack stands)??

A relatively immovable object is not specific enough. What is the deflection of the support under a load of 4000#? If it is zero, then the impact force is infinite.

What you need to know is the spring constant of your support.

Don't know what the deflection would be other than a very small number. The jack stands themselves would weigh about 20 lbs each and are designed to be a very rigid structure. The first piece that it struck is made of ductile iron, that force would be transmitted to a pawl, also made of ductile iron, from there the force would be transmitted down a four legged base made of steel, then to a concrete floor. I have seen vehicles come down onto jack stands hard when they were inadvertantly let down too fast, and there is often a very small bounce or jostle of both the vehicle and the jack stands. That's not a number that can be plugged into the equation however.

AL51,

The deflection under static load is often a very small number if the support is an axially loaded member. If the support consists of a member with a known area, a known length and a known modulus of elasticity, the deflection under static load can be easily calculated. The static stress can also be calculated as P/A where P is the load (4000# in your case) and A is the area of the member.

I do not profess any particular knowledge on the subject of car jacks, but it seems to me that you could improve their performance by adding a spring or shock absorber to make it a little less rigid.

sir how can we calculate theoratically the deflection in shock absorber(oleo pneumatic struts) of an aircraft at the vary instant of landing ? what formulae are available ?

Yikes! A shock absorber built into the floor jack! Not sure I would want a pair like that. If I'm under the car working on it and for whatever reason the lift fails and drops the car I don't want it having a large range of motion down even if after it travels down it re-springs itself back up. By that time my chest is crushed! Yes, the floors jacks may have been protected from severe damage but what about me? Under the car. It is me they are ultimately supposed to be keeping safe! This of course is simply my 2 cents worth! Of course I suppose if I looked at it from the perspective that I'm always going to settle the car onto them first before crawling under it than the absorbers would protect the jacks from fatal injury when incompetent employees dropped cars onto them. I guess that makes sense. Personally I can't get over the idea of mushy floor jacks. That would definately bother me!

The flexibility of the chassis is an important factor in determining the impact load. Dropping the rear end of a car on two jack stands is not quite the same as dropping a 2000# steel ball on each stand. The steel ball is rigid whereas the chassis deflects slightly under static load.

If the chassis deflects 1/4" under static load and the rear end is dropped 4", the impact factor is approximately 7, so a 2000# load would produce an impact of about 14,000# on each stand.

If the chassis deflects 1/8" under static load, then for a 4" drop the impact factor is a little over 9 and the impact force is about 18,000#.

If the vehicle bounces up off the stands, then all bets are off.

Lots of variables here; if the jack stands are placed under the axles, then the vehicles springs come into play, or if placed under a vehicle with a uni-body I could see how some deflection or deformation of the thin metal might occur. On the other hand, if the stands are placed under a vehicle that has a (relatively rigid) ladder frame like a pick up truck or SUV, I would assume that any deflection would be extremely small. Regarding spring loaded jack stands; not saying it would be easy or cheap, but I could see how the ratchet bars and pawl mechanism could be redesigned to incorporate some short very stiff springs that would allow a short stroke, like a max of a couple of cm, which would decrease the impact forces. On the other hand, they would not be very practical, and then you would introduce a few more potential areas for failure; the springs and their attachment points. With our hypothetical spring loaded jack stands no one would be injured as they would not be under the vehicle while it is being lowered, unless of course they were trying to win the Annual Darwin Award.

Going back to the point of the original problem; to calculate potential impact loads on jack stands. Once we have that number (we have some now), we can then better decide on what size/tonnage rating to use. In other words, if you have a three ton vehicle (two tons lifted) you would be safer to use a 6 ton or even 12 ton rated jack stand. However, if you have a ten ton dump truck, you better have very very stout set jack stands that could withstand the occasional inadvertent shock loading. One practical problem with using the higher weight rated jack stands is their minimum height ie: some 12 ton jack stands are 18 inches tall at their lowest setting (and up to 30 inches at their highest). That would mean you would first have to jack the car up to at least 19 inches before you could push the 12 ton jack stand under the jack point, and many hydraulic floor jacks used for cars do not go that high. On the other hand there are some very pricey high tonnage stands that go as low as 10-12 inches up to 20 inches tall but you could pay up to $1000 for a pair, which is ~10 times the cost of a set of 6 ton stands.

Ok, so back to my idea of high density foam or rubber pads on the top of the stands. As I recall, it is common to see rubber pads on the big car lifts.

Perhaps a solution would be to make your own jack stands. This brings in its own problems, but at least you can be sure to overengineer them to withstand abuse.

I always thought it would be good to have some spring loaded wheels like on those small round step stools (the ones that are only strong enough to lift the weight of the stool). That way they would be easier to place and remove. As long as you will not be selling them you should be able to copy the design of your favorite. Just keep in mind that you cannot reproduce some of the factory welds, your own will have to be perfect. I might even make one and crush it to see what kind of load it will take.

Just be careful making your own because it puts the lifes of anybody else who uses them in your hands.

Drew

Here is an example of a 20 ton jack stand, list price about $400, each.

Here is a 12 ton example, these run between $75-150 a pair.

This is the correct way to do the job!

Chas

I don't think so. The sloping wood supports are not positively anchored at the bottom, so they could slip out, endangering the worker under the vehicle.

wait...is he welding on the gas tank!!!?

Drew

I think you are right, Drew K. Why worry about a couple of props slipping out when you can get blown to bits by welding beside the gas tank? Maybe the guy has a death wish.

That picture is interesting. Of course if you carefully observe the shadows you can see there is a person standing out of the frame to the left that I am sure is ready to leap forward at a moments notice and keep the truck from coming down were one of the poles to slip. lol. I'm still undecided as to whether the poles themselves are casting proper shadows or not i.e. perhaps it was photoshopped? But I'm willing to believe it is real. I would guess taken in a third world country somewhere.

This photo has traveled the internet for some time, taken in Central America where methods such as this are common sight in that part of the world, a very dangerous practice but a bit off topic as no jack stands at all were being used.

It has definitely made the rounds, I posted it in the last couple weeks in another thread...

I think an argument could be made that he is using a 'rustic wooden jack stand'...

I have a stack of 2x12s cut to 24". After jacking the car up I slip as many as needed under the axle. Wood is horizontal and axle is centered on wood. This is only done on a level paved driveway/carport/garage. After lowering the car onto the wood I work under the car with no fear. The solid wood 12" x 24" block is not going to fail under the weight of the car. And, if last time I lowered the car too fast I still can trust that the solid block of wood will not fail this time. This isn't an answer to your question, but it is a VERY SAFE suggestion.

I like it.

I tried to say this before, perhaps it didn't work.

The formulae that hairlesssimian, in #14, posted, you will see that the impact load is the weight that is dropping, multiplied by the distance fallen, divided by the distance stopping. If your weight drops 4" and, without springs or shock absorber, the jack stand deflects say 1/100th of an inch (strain), the multiplier is 400. It is probably less than 1/100th of an inch but I know of no way to calculate it, the manufacturer might have a number for strain under load.

Because your trucks have springs and shock absorbers, only the weight of the wheels and axle are stopped that quickly, then the shock absorber kicks in. You need to measure the normal height over the wheels of the truck when just standing, and to drop it from your four or six inches and measure how much it sinks below the free standing position. The four or six inches, or whatever amount you drop it,gives you the Df and the amount it sinks is gives you the Ds. All provided that the absorber doesn't bottom, if it does, all bets are off.

I think the shock load should be in the region of [Weight of unsprung parts (wheel and axle,etc.)]x[distance dropped]/ [distance stopping (strain in jack stand)] + [weight of sprung parts]x[weight of sprung parts]/[distance stopping (shock absorbers)]

The shocks, springs, and tires would not come into play in the original scenario, ie; calculating shock loads on jack stands when the already raised vehicle is dropped accidentally onto the jack stands which have been positioned under the vehicles frame.

Of course the shocks, springs, and tires would come into play if you were attempting to determine shock loads on the vehicles frame, but that is not the problem.

They would also come into play if you positioned the jacks under the axles instead of the frame, this would add shock load protection for the jacks as the springs, shocks etc will absorb much of the impact... Of course the added springiness could cause the dropped load to bounce off the stands...

Then it is hard to see why the stands haven't collapsed or punched holes in the floor. I have to assume that the trucks are not dropped but just not lowered gently.

The basics of hairlesssimian's formulas are accurate, there must be some elastic bending of the frame somewhere, something to increase the length of Ds.